CiteULike: rsekuler's library [188 articles]

The statistics of natural hand movements

James Ingram — 2008-06-19T19:38:25-00:00

Experimental Brain Research, Vol. 188, No. 2. (12 June 2008), pp. 223-236.

Abstract Humans constantly use their hands to interact with the environment and they engage spontaneously in a wide variety of manual activities during everyday life. In contrast, laboratory-based studies of hand function have used a limited range of predefined tasks. The natural movements made by the hand during everyday life have thus received little attention. Here, we developed a portable recording device that can be worn by subjects to track movements of their right hand as they go about their daily routine outside of a laboratory setting. We analyse the kinematic data using various statistical methods. Principal component analysis of the joint angular velocities showed that the first two components were highly conserved across subjects, explained 60% of the variance and were qualitatively similar to those reported in previous studies of reach-to-grasp movements. To examine the independence of the digits, we developed a measure based on the degree to which the movements of each digit could be linearly predicted from the movements of the other four digits. Our independence measure was highly correlated with results from previous studies of the hand, including the estimated size of the digit representations in primary motor cortex and other laboratory measures of digit individuation. Specifically, the thumb was found to be the most independent of the digits and the index finger was the most independent of the fingers. These results support and extend laboratory-based studies of the human hand.

The Representation of Simple Ensemble Visual Features Outside the Focus of Attention

Alvarez — 2008-04-11T05:42:28-00:00

Psychological Science, Vol. 19, No. 4. (April 2008), pp. 392-398.

Prefrontal Cortex Function in the Representation of Temporally Complex Events

Philip Browning — 2008-04-09T21:16:04-00:00

J. Neurosci., Vol. 28, No. 15. (9 April 2008), pp. 3934-3940.

The frontal cortex and inferior temporal cortex are strongly functionally interconnected. Previous experiments on prefrontal function in monkeys have shown that a disconnection of prefrontal cortex from inferior temporal cortex impairs a variety of complex visual learning tasks but leaves simple concurrent objectreward association learning intact. We investigated the possibility that temporal components of visual learning tasks determine the sensitivity of those tasks to prefrontaltemporal disconnection by adding specific temporal components to the concurrent objectreward association learning task. Monkeys with crossed unilateral lesions of prefrontal cortex and inferior temporal cortex were impaired compared with unoperated controls at associating two-item sequences of visual objects with reward. The impairment was specific to the learning of visual sequences, because disconnection was without effect on objectreward association learning for an equivalent delayed reward. This result was replicated in monkeys with transection of the uncinate fascicle, thus determining the anatomical specificity of the dissociation. Previous behavioral results suggest that monkeys represent the two-item serial compound stimuli in a configural manner, similar to the way monkeys represent simultaneously presented compound stimuli. The representation of simultaneously presented configural stimuli depends on the perirhinal cortex. The present experiments show that the representation of serially presented compound stimuli depends on the interaction of prefrontal cortex and inferior temporal cortex. We suggest that prefrontaltemporal disconnection impairs a wide variety of learning tasks because in those tasks monkeys lay down similar temporally complex representations. 10.1523/JNEUROSCI.0633-08.2008

Motor Adaptation as a Process of Reoptimization

Jun Izawa — 2008-03-22T02:22:48-00:00

J. Neurosci., Vol. 28, No. 11. (12 March 2008), pp. 2883-2891.

Adaptation is sometimes viewed as a process in which the nervous system learns to predict and cancel effects of a novel environment, returning movements to near baseline (unperturbed) conditions. An alternate view is that cancellation is not the goal of adaptation. Rather, the goal is to maximize performance in that environment. If performance criteria are well defined, theory allows one to predict the reoptimized trajectory. For example, if velocity-dependent forces perturb the hand perpendicular to the direction of a reaching movement, the best reach plan is not a straight line but a curved path that appears to overcompensate for the forces. If this environment is stochastic (changing from trial to trial), the reoptimized plan should take into account this uncertainty, removing the overcompensation. If the stochastic environment is zero-mean, peak velocities should increase to allow for more time to approach the target. Finally, if one is reaching through a via-point, the optimum plan in a zero-mean deterministic environment is a smooth movement but in a zero-mean stochastic environment is a segmented movement. We observed all of these tendencies in how people adapt to novel environments. Therefore, motor control in a novel environment is not a process of perturbation cancellation. Rather, the process resembles reoptimization: through practice in the novel environment, we learn internal models that predict sensory consequences of motor commands. Through reward-based optimization, we use the internal model to search for a better movement plan to minimize implicit motor costs and maximize rewards. 10.1523/JNEUROSCI.5359-07.2008

Motor Planning, Imagery, and Execution in the Distributed Motor Network: A Time-Course Study with Functional MRI

Takashi Hanakawa — 2008-03-21T13:45:42-00:00

Cereb. Cortex (20 March 2008), bhn036.

Activation of motor-related areas has consistently been found during various motor imagery tasks and is regarded as the central mechanism generating motor imagery. However, the extent to which motor execution and imagery share neural substrates remains controversial. We examined brain activity during preparation for and execution of physical or mental finger tapping. During a functional magnetic resonance imaging at 3 T, 13 healthy volunteers performed an instructed delay finger-tapping task either in a physical mode or mental mode. Number stimuli instructed subjects about a finger-tapping sequence. After an instructed delay period, cue stimuli prompted them either to execute the tapping movement or to imagine it. Two types of planning/preparatory activity common for movement and imagery were found: instruction stimulusrelated activity represented widely in multiple motor-related areas and delay period activity in the medial frontal areas. Although brain activity during movement execution and imagery was largely shared in the distributed motor network, imagery-related activity was in general more closely related to instruction-related activity than to the motor executionrelated activity. Specifically, activity in the medial superior frontal gyrus, anterior cingulate cortex, precentral sulcus, supramarginal gyrus, fusiform gyrus, and posterolateral cerebellum likely reflects willed generation of virtual motor commands and analysis of virtual sensory signals. 10.1093/cercor/bhn036

Neural Underpinnings of Gesture Discrimination in Patients with Limb Apraxia

Mariella Pazzaglia — 2008-03-20T13:00:43-00:00

J. Neurosci., Vol. 28, No. 12. (19 March 2008), pp. 3030-3041.

Limb apraxia (LA), is a neuropsychological syndrome characterized by difficulty in performing gestures and may therefore be an ideal model for investigating whether action execution deficits are causatively linked to deficits in action understanding. We tested 33 left brain-damaged patients and 8 right brain-damaged patients for the presence of the LA. Importantly, we also tested all the patients in an ad hoc developed gesture recognition task wherein an actor performs, either correctly or incorrectly, transitive (using objects) or intransitive (without objects) meaningful conventional limb gestures. Patients were instructed to judge whether the observed gesture was correct or incorrect. Lesion analysis enabled us to evaluate the relationship between specific brain regions and behavioral performance in gesture execution and gesture comprehension. We found that LA was present in 21 left brain-damaged patients and it was linked to frontal and parietal lesions. Moreover, we found that recognition of correct execution of familiar gestures performed by others was more impaired in patients with LA than in nonapraxic patients. Crucially, the gesture comprehension deficit correlated with damage to the opercular and triangularis portions of the inferior frontal gyrus, two regions that are involved in complex aspects of action-related processing. In contrast, no such relationship was observed with lesions centered on the inferior parietal cortex. The present findings suggest that lesions to left frontal regions that are involved in planning and performing actions are causatively associated with deficits in the recognition of the correct execution of meaningful gestures. 10.1523/JNEUROSCI.5748-07.2008

Characterizing observers using external noise and observer models: assessing internal representations with external noise.

ZL Lu — 2008-02-14T13:21:35-00:00

Psychol Rev, Vol. 115, No. 1. (January 2008), pp. 44-82.

External noise methods and observer models have been widely used to characterize the intrinsic perceptual limitations of human observers and changes of the perceptual limitations associated with cognitive, developmental, and disease processes by highlighting the variance of internal representations. The authors conducted a comprehensive review of the 5 most prominent observer models through the development of a common formalism. They derived new predictions of the models for a common set of behavioral tests that were compared with the data in the literature and a new experiment. The comparison between the model predictions and the empirical data resulted in very strong constraints on the observer models. The perceptual template model provided the best account of all the empirical data in the visual domain. The choice of the observer model has significant implications for the interpretation of data from other external noise paradigms, as well as studies using external noise to assay changes of perceptual limitations associated with observer states. The empirical and theoretical development suggests possible parallel developments in other sensory modalities and studies of high-level cognitive processes.

Interactions between visual working memory and visual attention.

CN Olivers — 2008-03-07T17:48:12-00:00

Front Biosci, Vol. 13 (2008), pp. 1182-1191.

Visual attention is the collection of mechanisms by which relevant visual information is selected, and irrelevant visual information is ignored. Visual working memory is the mechanism by which relevant visual information is retained, and irrelevant information is suppressed. In addition to this overlap in definition, a strong overlap in brain areas active during attention and working memory tasks is found. The present paper reviews the behavioral evidence for and against the hypothesis that visual working memory and attention are best regarded as one and the same cognitive function, with the same capacity, the same control processes, and the same representational content. The data are best explained by a unified model in which multiple representations can be maintained, but only one receives the current focus of attention. Task circumstances then determine how successful this central representation can be prioritized over its mnemonic competitors.

Detection of unexpected events during spatial navigation in humans: bottom-up attentional system and neural mechanisms.

Giuseppe Iaria — 2008-02-19T20:36:57-00:00

Eur J Neurosci (13 February 2008)

Navigation is a complex cognitive ability requiring the processing and integration of several different types of information extracted from the environment. While navigating, however, an unexpected event may suddenly occur, which individuals are required to detect promptly in order to apply an appropriate behavioural response. The alerting mechanism that is integral to the detection of unexpected events is referred to as the bottom-up attentional system. Using event-related functional magnetic resonance imaging, we investigated the neural basis of bottom-up detection of unexpected events while individuals moved within a virtual environment. We identified activation within a right fronto-temporo-parietal network in response to unexpected events while navigating in this virtual environment. Furthermore, when an unexpected event requires an adjusted behavioural response, a region of the right ventrolateral pre-frontal cortex (areas 45 and 47/12) is selectively activated. Our data replicate earlier findings on the neural mechanisms underlying visual attention and extend these findings to the more complex real-life ability of spatial navigation, thereby suggesting that these neural mechanisms subserve the bottom-up attentional systems that are crucial for effective locomotion in real surroundings.

Aging disrupts the neural transformations that link facial identity across views.

C Habak — 2008-02-08T03:16:26-00:00

Vision Res, Vol. 48, No. 1. (January 2008), pp. 9-15.

Healthy human aging can have adverse effects on cortical function and on the brain's ability to integrate visual information to form complex representations. Facial identification is crucial to successful social discourse, and yet, it remains unclear whether the neuronal mechanisms underlying face perception per se, and the speed with which they process information, change with age. We present face images whose discrimination relies strictly on the shape and geometry of a face at various stimulus durations. Interestingly, we demonstrate that facial identity matching is maintained with age when faces are shown in the same view (e.g., front-front or side-side), regardless of exposure duration, but degrades when faces are shown in different views (e.g., front and turned 20 degrees to the side) and does not improve at longer durations. Our results indicate that perceptual processing speed for complex representations and the mechanisms underlying same-view facial identity discrimination are maintained with age. In contrast, information is degraded in the neural transformations that represent facial identity across views. We suggest that the accumulation of useful information over time to refine a representation within a population of neurons saturates earlier in the aging visual system than it does in the younger system and contributes to the age-related deterioration of face discrimination across views.

Aging and visual processing: Declines in spatial not temporal integration.

GJ Andersen — 2008-02-08T03:15:16-00:00

Vision Res, Vol. 48, No. 1. (January 2008), pp. 109-118.

Age-related declines in vision are well documented in the literature. In the present study we examined whether changes in spatial or temporal integration contribute to this decline. Younger (mean age of 21) and older (mean age of 745) subjects were asked to identify 2D shapes based on kinetic occlusion information---the accretion and deletion of texture during motion. The results of the first experiment indicated age-related decrements in spatial but not temporal integration. In the second experiment we manipulated the lifetime of motion stimuli to more directly examine temporal integration. The results indicated no differential effect of age on temporal integration. The results considered together suggest age related changes in recovering 2D shape from occlusion are the result of spatial but not temporal integration. Age-related changes in neural inhibition and ACh for regulating spatial integration are proposed as possible mechanisms for this decline.

Multidigit Movement Synergies of the Human Hand in an Unconstrained Haptic Exploration Task

Pramodsingh Thakur — 2008-02-06T17:32:48-00:00

J. Neurosci., Vol. 28, No. 6. (6 February 2008), pp. 1271-1281.

Although the human hand has a complex structure with many individual degrees of freedom, joint movements are correlated. Studies involving simple tasks (grasping) or skilled tasks (typing or finger spelling) have shown that a small number of combined joint motions (i.e., synergies) can account for most of the variance in observed hand postures. However, those paradigms evoked a limited set of hand postures and as such the reported correlation patterns of joint motions may be task-specific. Here, we used an unconstrained haptic exploration task to evoke a set of hand postures that is representative of most naturalistic postures during object manipulation. Principal component analysis on this set revealed that the first seven principal components capture >90% of the observed variance in hand postures. Further, we identified nine eigenvectors (or synergies) that are remarkably similar across multiple subjects and across manipulations of different sets of objects within a subject. We then determined that these synergies are used broadly by showing that they account for the changes in hand postures during other tasks. These include hand motions such as reach and grasp of objects that vary in width, curvature and angle, and skilled motions such as precision pinch. Our results demonstrate that the synergies reported here generalize across tasks, and suggest that they represent basic building blocks underlying natural human hand motions. 10.1523/JNEUROSCI.4512-07.2008

Hue-specific colour memory impairment in an individual with intact colour perception and colour naming

LS Jakobson — 2008-02-01T21:32:24-00:00

Neuropsychologia, Vol. 46, No. 1. (2008), pp. 22-36.

Cases of hue-selective dyschomatopsias, together with the results of recent optical imaging studies [Xiao, Y., Casti, A. R. R., Xiao, J., & Kaplan, E. (2006). A spatially organized representation of colour in macaque primary visual cortex. Perception, 35, ECVP Abstract Supplement; Xiao, Y., Wang, Y., & Felleman, D. J. (2003). A spatially organized representation of colour in macaque cortical area V2. Nature, 421, 535-539], have provided support for the idea that different colours are processed in spatially distinct regions of extrastriate cortex. In the present report, we provide evidence suggesting that a similar, but distinct, map may exist for representations of colour in memory. This evidence comes from observations of a young woman (QP) who demonstrates an isolated deficit in colour memory secondary to a concussive episode. Despite having normal colour perception and colour naming skills, and above-average memory skills in other domains, QP's ability to recall visually encoded colour information over short retention intervals is dramatically impaired. Her long-term memory for colour and her colour imagery skills are also abnormal. Surprisingly, however, these impairments are not seen with all hues; specifically, her ability to remember or imagine blue shades is spared. This interesting case contributes to the literature suggesting that colour perception, naming, and memory can be clinically dissociated, and provides insights into the organization of colour information in memory.

Reduced implicit and explicit sequence learning in first-episode schizophrenia

Anya Pedersen — 2008-02-01T21:29:42-00:00

Neuropsychologia, Vol. 46, No. 1. (2008), pp. 186-195.

A high prevalence of deficits in explicit learning has been reported for schizophrenic patients, but it is less clear whether these patients are impaired in implicit learning. Deficits in implicit learning indicative of a fronto-striatal dysfunction have been reported using a serial reaction-time task (SRT), but the impact of typical neuroleptic medication and chronicity remains controversial. The present study compared 37 patients with first-episode schizophrenia treated with atypical neuroleptics and 37 healthy matched control participants on two sequence learning tasks: a modified SRT for implicit sequence learning and a serial generation task (SGT) for explicit sequence learning. The two tasks were designed to be procedurally equivalent, in order to provide better comparability between implicit and explicit performance. Although unaffected in global cognitive functioning, schizophrenic patients were significantly impaired in implicit and explicit sequence learning. Deficient sequence learning in schizophrenic patients was neither related to psychopathology nor to chlorpromazine equivalent daily dosage. As performance was impaired even though patients were exclusively treated with atypical neuroleptics, the present findings concur with converging evidence of a sequence learning deficit inherent in schizophrenia. This deficit would be consistent with a fronto-striatal dysfunction and might constitute a crucial factor for the acquisition of new information.

The Neural Coding of Stimulus Intensity: Linking the Population Response of Mechanoreceptive Afferents with Psychophysical Behavior

Michael Muniak — 2008-01-30T17:47:22-00:00

J. Neurosci., Vol. 27, No. 43. (24 October 2007), pp. 11687-11699.

How specific aspects of a stimulus are encoded at different stages of neural processing is a critical question in sensory neuroscience. In the present study, we investigated the neural code underlying the perception of stimulus intensity in the somatosensory system. We first characterized the responses of SA1 (slowly adapting type 1), RA (rapidly adapting), and PC (Pacinian) afferents of macaque monkeys to sinusoidal, diharmonic, and bandpass noise stimuli. We then had human subjects rate the perceived intensity of a subset of these stimuli. On the basis of these neurophysiological and psychophysical measurements, we evaluated a series of hypotheses about which aspect(s) of the neural activity evoked at the somatosensory periphery account for perception. We evaluated three types of neural codes. The first consisted of population codes based on the firing rate of neurons located directly under the probe. The second included population codes based on the firing rate of the entire population of active neurons. The third included codes based on the number of active afferents. We found that the response evoked in the localized population is logarithmic with stimulus amplitude (given a constant frequency composition), whereas the population response across all neurons is linear with stimulus amplitude. We conclude that stimulus intensity is best accounted for by the firing rate evoked in afferents located under or near the locus of stimulation, weighted by afferent type. 10.1523/JNEUROSCI.1486-07.2007

ROLE OF NEUROGENESIS IN AGE-RELATED MEMORY DISORDERS.

Elodie Drapeau — 2008-01-30T17:45:30-00:00

Aging Cell (23 January 2008)

Neuroplasticity is characterized by growth and branching of dendrites, remodeling of synaptic contacts and neurogenesis, thus allowing the brain to adapt to changes over time. It is maintained in adulthood but strongly repressed during aging. An age-related decline in neurogenesis is particularly pronounced in the two adult neurogenic areas, the subventricular zone and the dentate gyrus. This age-related decline seems to be attributable mainly to limited proliferation, associated with an age-dependent increase in quiescence and/or a lengthening of the cell cycle, and is closely dependant on environmental changes. Indeed, when triggered by appropriate signals, neurogenesis can be reactivated in senescent brains, thus confirming the idea that the age-related decrease in new neuron production is not an irreversible, cell-intrinsic process. The co-evolution of neurogenesis and age-related memory deficits - especially regarding spatial memory - during senescence supports the idea that new neurons in the adult brain participate in memory processing and that a reduction in the ability to generate new neurons contributes to the appearance of memory deficits with advanced age. Furthermore, the age-related changes in hippocampal plasticity and function are under environmental influences that can favor successful or pathological aging. A better understanding of the mechanisms that regulate neurogenesis is necessary to develop new therapeutic tools to cure or prevent the development of memory disorders that may appear during the course of aging in some individuals.

Habituation Reveals Fundamental Chromatic Mechanisms in Striate Cortex of Macaque

Chris Tailby — 2008-01-30T17:42:40-00:00

J. Neurosci., Vol. 28, No. 5. (30 January 2008), pp. 1131-1139.

Prolonged viewing of a chromatically modulated stimulus usually leads to changes in its appearance, and that of similar stimuli. These aftereffects of habituation have been thought to reflect the activity of two populations of neurons in visual cortex that have particular importance in color vision, one sensitive to redgreen modulation, the other to blueyellow, but they have not been identified. We show here, in recordings from macaque primary visual cortex (V1), that prolonged exposure to chromatic modulation reveals two fundamental mechanisms with distinctive chromatic signatures that match those of the mechanisms identified by perceptual observations. In nearly all neurons, these mechanisms contribute to both excitation and to regulatory gain controls, and as a result their habituation can have paradoxical effects on response. The mechanisms must be located near the input layers of V1, before their distinct chromatic signatures diffuse. Our observations suggest that the fundamental mechanisms do not give rise to two distinct LM and S chromatic pathways. Rather, the mechanisms are better understood as stages in the elaboration of chromatic tuning, expressed in varying proportions in all cells in V1 (and beyond), and made accessible to physiological and perceptual investigation only through habituation. 10.1523/JNEUROSCI.4682-07.2008

Primary Motor Cortex Tuning to Intended Movement Kinematics in Humans with Tetraplegia

Wilson Truccolo — 2008-01-30T17:41:21-00:00

J. Neurosci., Vol. 28, No. 5. (30 January 2008), pp. 1163-1178.

The relationship between spiking activities in motor cortex and movement kinematics has been well studied in neurologically intact nonhuman primates. We examined the relationship between spiking activities in primary motor cortex (M1) and intended movement kinematics (position and velocity) using 96-microelectrode arrays chronically implanted in two humans with tetraplegia. Study participants were asked to perform two different tasks: imagined pursuit tracking of a cursor moving on a computer screen and a "neural cursor center-out" task in which cursor position was controlled by the participant's neural activity. In the pursuit tracking task, the majority of neurons were significantly tuned: 90% were tuned to velocity and 86% were tuned to position in one participant; 95% and 84%, respectively, in the other. Additionally, velocity and position of the tracked cursor could be decoded from the ensemble of neurons. In the neural cursor center-out task, tuning to direction of the intended target was well captured by a log-linear cosine function. Neural spiking soon after target appearance could be used to classify the intended target with an accuracy of 95% in one participant, and 80% in the other. It was also possible to extract information about the direction of the difference vector between the target position and the instantaneous neural cursor position. Our results indicate that correlations between spiking activity and intended movement velocity and position are present in human M1 after the loss of descending motor pathways, and that M1 spiking activities share many kinematic tuning features whether movement is imagined by humans with tetraplegia, or is performed as shown previously in able-bodied nonhuman primates. 10.1523/JNEUROSCI.4415-07.2008

Modulation of motor cortex excitability by physical similarity with an observed hand action.

MC Désy — 2007-12-01T04:02:55-00:00

PLoS ONE, Vol. 2, No. 10. (2007)

The passive observation of hand actions is associated with increased motor cortex excitability, presumably reflecting activity within the human mirror neuron system (MNS). Recent data show that in-group ethnic membership increases motor cortex excitability during observation of culturally relevant hand gestures, suggesting that physical similarity with an observed body part may modulate MNS responses. Here, we ask whether the MNS is preferentially activated by passive observation of hand actions that are similar or dissimilar to self in terms of sex and skin color. Transcranial magnetic stimulation-induced motor evoked potentials were recorded from the first dorsal interosseus muscle while participants viewed videos depicting index finger movements made by female or male participants with black or white skin color. Forty-eight participants equally distributed in terms of sex and skin color participated in the study. Results show an interaction between self-attributes and physical attributes of the observed hand in the right motor cortex of female participants, where corticospinal excitability is increased during observation of hand actions in a different skin color than that of the observer. Our data show that specific physical properties of an observed action modulate motor cortex excitability and we hypothesize that in-group/out-group membership and self-related processes underlie these effects.

The Effect of Real-World Personal Familiarity on the Speed of Face Information Processing

Benjamin Balas — 2007-12-01T03:43:20-00:00

PLOS One, Vol. 2, No. 11. (2007), e1223.

Background Previous studies have explored the effects of familiarity on various kinds of visual face judgments, yet the role of familiarity in face processing is not fully understood. Across different face judgments and stimulus sets, the data is equivocal as to whether or not familiarity impacts recognition processes. Methodology/Principal Findings Here, we examine the effect of real-world personal familiarity in three simple delayed-match-to-sample tasks in which subjects were required to match faces on the basis of orientation (upright v. inverted), gender and identity. We find that subjects had a significant speed advantage with familiar faces in all three tasks, with large effects for the gender and identity matching tasks. Conclusion/Significance Our data indicates that real-world experience with a face exerts a powerful influence on face processing in tasks where identity information is irrelevant, even in tasks that could in principle be solved via low-level cues. These results underscore the importance of experience in shaping visual recognition processes.

Measuring visual clutter

Ruth Rosenholtz — 2007-11-30T18:48:33-00:00

Journal of Vision, Vol. 7, No. 2. (2007), pp. 1-22.

Visual clutter concerns designers of user interfaces and information visualizations. This should not surprise visual perception researchers because excess and/or disorganized display items can cause crowding, masking, decreased recognition performance due to occlusion, greater difficulty at both segmenting a scene and performing visual search, and so on. Given a reliable measure of the visual clutter in a display, designers could optimize display clutter. Furthermore, a measure of visual clutter could help generalize models like Guided Search (J. M. Wolfe, 1994) by providing a substitute for “set size” more easily computable on more complex and natural imagery. In this article, we present and test several measures of visual clutter, which operate on arbitrary images as input. The first is a new version of the Feature Congestion measure of visual clutter presented in R. Rosenholtz, Y. Li, S. Mansfield, and Z. Jin (2005). This Feature Congestion measure of visual clutter is based on the analogy that the more cluttered a display or scene is, the more difficult it would be to add a new item that would reliably draw attention. A second measure of visual clutter, Subband Entropy, is based on the notion that clutter is related to the visual information in the display. Finally, we test a third measure, Edge Density, used by M. L. Mack and A. Oliva (2004) as a measure of subjective visual complexity. We explore the use of these measures as stand-ins for set size in visual search models and demonstrate that they correlate well with search performance in complex imagery. This includes the search-in-clutter displays of J. M. Wolfe, A. Oliva, T. S. Horowitz, S. Butcher, and A. Bompas (2002) and Bravo and Farid (2004), as well as new search experiments. An additional experiment suggests that color variability, accounted for by Feature Congestion but not the Edge Density measure or the Subband Entropy measure, does matter for visual clutter.

Do masks terminate the icon?

H Smithson — 2006-12-14T22:53:27-00:00

Q J Exp Psychol (Colchester), Vol. 59, No. 1. (January 2006), pp. 150-160.

Iconic memory is operationally defined by part-report experiments (Sperling, 1960). If a mask is presented after the target, the mask is thought to be superposed on the target in the iconic representation, or to displace it from the representation. But could a cue presented after a pattern mask still allow selection within the target array? A target array of letters was followed by a checkerboard mask. We compared two target-mask interstimulus intervals (ISIs; 0 and 100 ms), and six cue delays. At ISI = 0 ms, performance was at chance, for part report and whole report. At ISI = 100 ms, with the shortest cue delay, observers demonstrated a part-report advantage of 25-30%. As cue delay increased the part-report advantage decreased. These results are inconsistent with an iconic memory that is automatically displaced or overwritten by new information. We consider two alternatives: a second-stage store, which represents letters in terms of their high-level features and which the mask cannot penetrate, or a four-dimensional store that preserves separately the representations of the target and its aftercoming mask. We discuss the implications of our results for studies that use backward masking to "terminate the icon".

EPS Mid-Career Award 2004: brain mechanisms of attention.

J Duncan — 2007-11-30T17:40:13-00:00

Q J Exp Psychol (Colchester), Vol. 59, No. 1. (January 2006), pp. 2-27.

There are many varieties of "attention", to some extent separate yet working together to produce coherent perception, thought, and behaviour. Using data from human behaviour, functional neuroimaging, and single-cell recording in the behaving monkey, I consider different levels of attention and their basis in physiological mechanisms of biased competition. Beginning with visual attention, I suggest that processing is competitive in many brain systems that code visual input. Competition is biased towards stimuli that match task requirements and is integrated between systems coding different object properties. The result is flexible, object-based attentional selection. In the second part of the paper, I describe recent experiments on attentional competition within and between sensory modalities. Though competition is often modality specific, more global levels of interference are also easy to demonstrate. In the third part of the paper, I move to frontoparietal cortex and to a pattern of similar brain regions recruited by many different cognitive demands. This multiple-demand (MD) pattern, I suggest, reflects neurons with highly flexible response properties, adapting to represent the information and events of many different tasks. Biased competition in MD regions may play a central role in broad attentional capacity limits and attentional focusing. More generally, I suggest that biased competition is characteristic of many different cognitive domains and brain systems. Coherent "attention" develops as different systems converge to work on related cognitive content.

The selective disruption of spatial working memory by eye movements.

BR Postle — 2007-11-30T17:27:48-00:00

Q J Exp Psychol (Colchester), Vol. 59, No. 1. (January 2006), pp. 100-120.

In the late 1970s/early 1980s, Baddeley and colleagues conducted a series of experiments investigating the role of eye movements in visual working memory. Although only described briefly in a book, these studies have influenced a remarkable number of empirical and theoretical developments in fields ranging from experimental psychology to human neuropsychology to nonhuman primate electrophysiology. This paper presents, in full detail, three critical studies from this series, together with a recently performed study that includes a level of eye movement measurement and control that was not available for the older studies. Together, the results demonstrate several facts about the sensitivity of visuospatial working memory to eye movements. First, it is eye movement control, not movement per se, that produces the disruptive effects. Second, these effects are limited to working memory for locations and do not generalize to visual working memory for shapes. Third, they can be isolated to the storage/maintenance components of working memory (e.g., to the delay period of the delayed-recognition task). These facts have important implications for models of visual working memory.

Memory for the search path: evidence for a high-capacity representation of search history.

CA Dickinson — 2007-11-30T17:13:26-00:00

Vision Res, Vol. 47, No. 13. (June 2007), pp. 1745-1755.

Using a gaze-contingent paradigm, we directly measured observers' memory capacity for fixated distractor locations during search. After approximately half of the search objects had been fixated, they were masked and a spatial probe appeared at either a previously fixated location or a non-fixated location; observers then rated their confidence that the target had appeared at the probed location. Observers were able to differentiate the 12 most recently fixated distractor locations from non-fixated locations, but analyses revealed that these locations were represented fairly coarsely. We conclude that there exists a high-capacity, but low-resolution, memory for a search path.

Colour constancy.

A Hurlbert — 2007-11-23T18:17:56-00:00

Curr Biol, Vol. 17, No. 21. (6 November 2007)

Impairment of actions chains in autism and its possible role in intention understanding

Luigi Cattaneo — 2007-11-07T13:22:34-00:00

Proceedings of the National Academy of Sciences, Vol. 104, No. 45. (6 November 2007), pp. 17825-17830.

Experiments in monkeys demonstrated that many parietal and premotor neurons coding a specific motor act (e.g., grasping) show a markedly different activation when this act is part of actions that have different goals (e.g., grasping for eating vs. grasping for placing). Many of these "action-constrained" neurons have mirror properties firing selectively to the observation of the initial motor act of the actions to which they belong motorically. By activating a specific action chain from its very outset, this mechanism allows the observers to have an internal copy of the whole action before its execution, thus enabling them to understand directly the agent's intention. Using electromyographic recordings, we show that a similar chained organization exists in typically developing children, whereas it is impaired in children with autism. We propose that, as a consequence of this functional impairment, high-functioning autistic children may understand the intentions of others cognitively but lack the mechanism for understanding them experientially. 10.1073/pnas.0706273104

Cognitive and biomechanical influences in pianists finger tapping

Loehr — 2007-05-15T20:09:10-00:00

Experimental Brain Research, Vol. 178, No. 4. (April 2007), pp. 518-528.

When practice does not make perfect: well-practiced handwriting interferes with the consolidation phase gains in learning a movement sequence.

Meirav Balas — 2006-12-07T16:46:38-00:00

Exp Brain Res (8 November 2006)

Practice on a novel sequence of movements can lead to two expressions of procedural memory consolidation: delayed performance gains evolving hours after training, and a decrease in the susceptibility of the training-related gains to interference by subsequent experience. It has been assumed that behavioral interference occurs only if a critical overlap between the representations of the two tasks exists, and that such overlap is more likely when the two tasks are novel, competing for general resources for their execution. We investigated whether the delayed gains in the simple finger-opposition sequence (FOS) learning task are more prone to interference by well practiced than by less practiced complex hand movements. Participants were trained on the FOS task in a baseline (no interference) and an interference training condition. In the Interference condition, after FOS practice, participants wrote Hebrew common words in Hebrew (native script) or a Latin script (Heblatin). Native script writing but not the less practiced Heblatin, interfered with FOS learning, with significantly reduced delayed gains. Our results show that interference can occur even when two tasks share little or no kinematic or dynamic features and indicate that the representation of complex but well-practiced movement sequences may overlap with the representation of simpler ones. This result is in line with the notion that well-practiced complex movement sequences come to be represented as simpler ones in long-term motor memory.

Microstimulation of frontal cortex can reorder a remembered spatial sequence.

MH Histed — 2007-05-11T12:53:39-00:00

PLoS Biol, Vol. 4, No. 5. (May 2006)

Complex goal-directed behaviors extend over time and thus depend on the ability to serially order memories and assemble compound, temporally coordinated movements. Theories of sequential processing range from simple associative chaining to hierarchical models in which order is encoded explicitly and separately from sequence components. To examine how short-term memory and planning for sequences might be coded, we used microstimulation to perturb neural activity in the supplementary eye field (SEF) while animals held a sequence of two cued locations in memory over a short delay. We found that stimulation affected the order in which animals saccaded to the locations, but not the memory for which locations were cued. These results imply that memory for sequential order can be dissociated from that of its components. Furthermore, stimulation of the SEF appeared to bias sequence endpoints to converge toward a location in contralateral space, suggesting that this area encodes sequences in terms of their endpoints rather than their individual components.

Encoding of Movement Fragments in the Motor Cortex

Nicholas Hatsopoulos — 2007-05-09T17:03:37-00:00

J. Neurosci., Vol. 27, No. 19. (9 May 2007), pp. 5105-5114.

Previous studies have suggested that complex movements can be elicited by electrical stimulation of the motor cortex. Most recording studies in the motor cortex, however, have investigated the encoding of time-independent features of movement such as direction, velocity, position, or force. Here, we show that single motor cortical neurons encode temporally evolving movement trajectories and not simply instantaneous movement parameters. We explicitly characterize the preferred trajectories of individual neurons using a simple exponential encoding model and demonstrate that temporally extended trajectories not only capture the tuning of motor cortical neurons more accurately, but can be used to decode the instantaneous movement direction with less error. These findings suggest that single motor cortical neurons encode whole movement fragments, which are temporally extensive and can be quite complex. 10.1523/JNEUROSCI.3570-06.2007

The effects of aging on orientation discrimination.

Lisa R Betts — 2007-05-09T14:01:51-00:00

Vision Res (25 April 2007)

The current experiments measured orientation discrimination thresholds in younger (mean age approximately 23 years) and older (mean age approximately 66 years) subjects. In Experiment 1, the contrast needed to discriminate Gabor patterns (0.75, 1.5, and 3c/deg) that differed in orientation by 12deg was measured for different levels of external noise. At all three spatial frequencies, discrimination thresholds were significantly higher in older than younger subjects when external noise was low, but not when external noise was high. In Experiment 2, discrimination thresholds were measured as a function of stimulus contrast by varying orientation while contrast was fixed. The resulting threshold-vs-contrast curves had very similar shapes in the two age groups, although the curve obtained from older subjects was shifted to slightly higher contrasts. At contrasts greater than 0.05, thresholds in both older and younger subjects were approximately constant at 0.5deg. The results from Experiments 1 and 2 suggest that age differences in orientation discrimination are due solely to differences in equivalent input noise. Using the same methods as Experiment 1, Experiment 3 measured thresholds in 6 younger observers as a function of external noise and retinal illuminance. Although reducing retinal illumination increased equivalent input noise, the effect was much smaller than the age difference found in Experiment 1. Therefore, it is unlikely that differences in orientation discrimination were due solely to differences in retinal illumination. Our findings are consistent with recent physiological experiments that have found elevated spontaneous activity and reduced orientation tuning on visual cortical neurons in senescent cats (Hua, T., Li, X., He, L., Zhou, Y., Wang, Y., Leventhal, A. G. (206). Functional degradation of visual cortical cells in old cats. Neurobiology Aging, 27(1), 155-162) and monkeys (Yu, S., Wang, Y., Li, X., Zhou, Y. & Leventhal, A. G. (2006). Functional degradation of visual cortex in senescent rhesus monkeys. Neuroscience, 140(3), 1023-1029; Leventhal, A. G., Wang, Y., Pu, M., Zhou, Y. & Ma. Y. (2003). GABA and its agonists improved visual cortical function in senescent monkeys. Science,300 (5620), 812-815).

Time-variant processing in V1: from microscopic (single cell) to mesoscopic (population) levels.

HR Dinse — 2007-05-01T22:44:44-00:00

Trends Neurosci, Vol. 24, No. 4. (April 2001), pp. 203-205.

Visual causes versus correlates of attentional selection in dynamic scenes

Ran Carmi — 2007-05-01T22:37:02-00:00

Vision Research, Vol. 46, No. 26. (December 2006), pp. 4333-4345.

What are the visual causes, rather than mere correlates, of attentional selection and how do they compare to each other during natural vision? To address these questions, we first strung together semantically unrelated dynamic scenes into MTV-style video clips, and performed eye tracking experiments with human observers. We then quantified predictions of saccade target selection based on seven bottom-up models, including intensity variance, orientation contrast, intensity contrast, color contrast, flicker contrast, motion contrast, and integrated saliency. On average, all tested models predicted saccade target selection well above chance. Dynamic models were particularly predictive of saccades that were most likely bottom-up driven-initiated shortly after scene onsets, leading to maximal inter-observer similarity. Static models showed mixed results in these circumstances, with intensity variance and orientation contrast featuring particularly weak prediction accuracy (lower than their own average, and approximately 4 times lower than dynamic models). These results indicate that dynamic visual cues play a dominant causal role in attracting attention. In comparison, some static visual cues play a weaker causal role, while other static cues are not causal at all, and may instead reflect top-down causes.

Deployment of visual attention before sequences of goal-directed hand movements.

D Baldauf — 2007-05-01T21:51:07-00:00

Vision Res, Vol. 46, No. 26. (December 2006), pp. 4355-4374.

We examined the allocation of attention during the preparation of sequences of manual pointing movements in a dual task paradigm. As the primary task, the participants had to perform a sequence of two or three reaching movements to targets arranged on a clock face. The secondary task was a 2AFC discrimination task in which a discrimination target (digital 'E' or '3') was presented among distractors either at one of the movement goals or at any other position. The data show that discrimination performance is superior at the location of all movement targets while it is close to chance at the positions that were not relevant for the movement. Moreover, our findings demonstrate that all movement-relevant locations are selected in parallel rather than serially in time, and that selection involves spatially distinct, non-contiguous foci of visual attention. We conclude that during movement preparation--well before the actual execution of the hand movement--attention is allocated in parallel to each of the individual movement targets.

Role of focal attention on latencies and trajectories of visually guided manual pointing.

JH Song — 2007-05-01T21:49:48-00:00

J Vis, Vol. 6, No. 9. (2006), pp. 982-995.

Previous studies have shown that an odd-colored target among uniformly colored distractors can be rapidly detected and localized using broadly distributed attention over an entire display. In the current study, we show that such a broadly distributed attentional allocation is not sufficient for seemingly effortless goal-directed manual pointing. Latencies and movement durations of manual pointing in odd-colored search tasks become shorter and curved trajectories decreased as the number of distractors increase or target color repetitions increases. Because these manipulations have been shown to facilitate the deployment of narrowly focused attention to a target but not for distributed attention, this adds further support to the view that focal attention is necessary for goal-directed action. In addition, the presence of highly curved movement trajectories, directed first to a distractor then to the target reflects ongoing changes in focal attentional deployment and target selection.

The effects of perceptual history on memory of visual objects.

S Preminger — 2007-04-30T22:32:25-00:00

Vision Res, Vol. 47, No. 7. (March 2007), pp. 965-973.

We investigated how the recognition and perception of memory-stored visual objects are influenced by cumulative experience with similar stimuli. The memory of a face was established by training observers to identify a set of faces as either "friends" or "non-friends". Subsequently, for multiple daily sessions, observers continued to perform this identification task, in which presented faces included a sequence of morphed faces, gradually transforming from a friend face (source) to another initially distinguishable non-friend face (target), interleaved with other faces. Initially observers identified only the first part of the morph sequence as "friends". In experimental conditions for which the initial "friends" portion was at least 54% of the sequence, this portion increased along repeated daily practice, until eventually most of the sequence was identified as "friends". After this practice, perceived similarity between source and target faces was much higher than the average similarity between the other face images. These effects did not occur when the morph images were shown in random order using a similar protocol. In addition, corresponding recognition confusions between source and target faces were found. Our findings suggest that memories of objects can be changed as a result of exposure to similar stimuli and show the dependency of these changes on the order in which stimuli are presented and on their level of similarity.

Saccades and covert shifts of attention during active visual search: Spatial distributions, memory, and items per fixation.

BC Motter — 2007-04-30T22:29:54-00:00

Vision Res, Vol. 47, No. 10. (May 2007), pp. 1261-1281.

Target detection during active visual search was examined. The chance corrected spatial distribution of target detection was found to be symmetrically distributed around the point of fixation and, unexpectedly, was independent of the proximity of fixations to the display boundaries. Memory was found to play a very limited role in target detection, but a significant role in the guidance of eye movements. A model of covert shifts was used to estimate the number and spatial distribution of shifts required to explain observed performance. An increase from one to five shifts per fixation across increasing array set size as estimated by two different methods was inconsistent with unchanging fixation durations, suggesting that multiple covert shifts are not occurring during the fixations in active search.

Gaze bias both reflects and influences preference.

S Shimojo — 2006-07-18T00:21:29-00:00

Nat Neurosci, Vol. 6, No. 12. (December 2003), pp. 1317-1322.

Emotions operate along the dimension of approach and aversion, and it is reasonable to assume that orienting behavior is intrinsically linked to emotionally involved processes such as preference decisions. Here we describe a gaze 'cascade effect' that was present when human observers were shown pairs of human faces and instructed to decide which face was more attractive. Their gaze was initially distributed evenly between the two stimuli, but then gradually shifted toward the face that they eventually chose. Gaze bias was significantly weaker in a face shape discrimination task. In a second series of experiments, manipulation of gaze duration, but not exposure duration alone, biased observers' preference decisions. We thus conclude that gaze is actively involved in preference formation. The gaze cascade effect was also present when participants compared abstract, unfamiliar shapes for attractiveness, suggesting that orienting and preference for objects in general are intrinsically linked in a positive feedback loop leading to the conscious choice.

Early interactions between orienting, visual sampling and decision making in facial preference.

C Simion — 2007-04-30T22:13:04-00:00

Vision Res, Vol. 46, No. 20. (October 2006), pp. 3331-3335.

Decision making has been regarded as the last stage before action in the human information processing, certainly subsequent to sensory sampling and perceptual integration. Our latest study showed that orienting contributes to preference decision making, by integrating preferential looking and mere exposure in a positive feedback loop leading to the conscious choice. Here, we introduce a gaze-contingent window method of stimulus presentation into our experimental paradigm, to completely block holistic stimulus processing while preserving piecemeal sampling through the gaze-contingent "peephole". This effectively zooms the visual processing in time domain, allowing us to show that orienting and decision making can interact long before the actual conscious choice. The finding also suggests that this interaction is independent of holistic properties of face stimuli and can be totally memory-driven.

Dynamical evolution of motion perception.

R Kanai — 2007-04-30T22:11:37-00:00

Vision Res, Vol. 47, No. 7. (March 2007), pp. 937-945.

Motion is defined as a sequence of positional changes over time. However, in perception, spatial position and motion dynamically interact with each other. This reciprocal interaction suggests that the perception of a moving object itself may dynamically evolve following the onset of motion. Here, we show evidence that the percept of a moving object systematically changes over time. In experiments, we introduced a transient gap in the motion sequence or a brief change in some feature (e.g., color or shape) of an otherwise smoothly moving target stimulus. Observers were highly sensitive to the gap or transient change if it occurred soon after motion onset (< or =200 ms), but significantly less so if it occurred later (> or = 300 ms). Our findings suggest that the moving stimulus is initially perceived as a time series of discrete potentially isolatable frames; later failures to perceive change suggests that over time, the stimulus begins to be perceived as a single, indivisible gestalt integrated over space as well as time, which could well be the signature of an emergent stable motion percept.

Large-scale organization of preferred directions in the motor cortex. II. Analysis of local distributions.

T Naselaris — 2007-04-15T19:53:15-00:00

J Neurophysiol, Vol. 96, No. 6. (December 2006), pp. 3237-3247.

The spatial arrangement of preferred directions (PDs) in the primary motor cortex has revealed evidence for columnar organization and short-range order. We investigated the large-scale properties of this arrangement. We recorded neural activity at sites on a grid covering a large region of the arm area of the motor cortex while monkeys performed a 3D reaching task. Sites were projected to the cortical surface along anatomically defined cortical columns and a PD was extracted from each site with directionally tuned activity. We analyzed the resulting 2D surface map of PDs. Consistent with previous studies, we found that any particular reaching direction was re-represented at many points across the recorded area. In particular, we determined that the median radius of a cortical region required to represent the full complement of reaching directions is at most 1 mm. We also found that for the majority of regions of this size, the distribution of PDs within them exhibits an enrichment for the representation of forward and backward reaching directions (see companion paper). Finally, we found that the error of a population vector estimate of reaching direction constructed from neural activity within these regions is small on average, but varies significantly across different sections of the motor cortex, with the highest levels of error sustained near the fundus of the central sulcus and lowest levels achieved near the crown. We interpret these findings in the context of two well-known features of motor cortex, that is, its highly distributed anatomical organization and its behaviorally dependent plasticity.

Large-scale organization of preferred directions in the motor cortex. I. Motor cortical hyperacuity for forward reaching.

T Naselaris — 2007-04-15T19:52:14-00:00

J Neurophysiol, Vol. 96, No. 6. (December 2006), pp. 3231-3236.

We used statistical methods for spherical density estimation to evaluate the distribution of preferred directions of motor cortical cells recorded from monkeys making reaching movements in 3D space. We found that this distribution, although broad enough to represent the entire 3D continuum of reaching directions, exhibited an enrichment for reaching forward from the body and, to a lesser degree, for reaching backward toward the body. The distribution of preferred directions of cells in the motor cortex may have important implications for motor cortical function and for the decoding of arm trajectories from population activity.

Chunking in Task Sequences Modulates Task Inhibition

Iring Koch — 2006-03-22T05:31:59-00:00

Psychological Science, Vol. 17, No. 4. (April 2006), pp. 346-350.

Forward and backward recall: different response time patterns, same retrieval order.

JG Thomas — 2007-03-07T16:04:02-00:00

Psychol Sci, Vol. 14, No. 2. (March 2003), pp. 169-174.

How do people retrieve information in forward and backward recall? To address this issue, we examined response times in directional recall as a function of serial position and list length. Participants memorized lists of four to six words and entered responses at the keyboard. Recall direction was postcued. Response times exhibited asymmetry in terms of direction. In forward recall, response times peaked at the first position, leveling off for subsequent positions. Response times were slower in backward recall than in forward recall and exhibited an inverse U-shaped function with an initial slowdown followed by a continuous speedup. These asymmetries have implications for theoretical models of retrieval in serial recall, including temporal-code, rule-based, and network models. The response time pattern suggests that forward recall proceeds in equal steps across positions, whereas backward recall involves repeated covert cycles of forward recall. Thus, retrieval in both directions involves a forward search.

Bend it like Beckham: embodying the motor skills of famous athletes.

P Bach — 2007-03-01T18:46:48-00:00

Q J Exp Psychol (Colchester), Vol. 59, No. 12. (December 2006), pp. 2033-2039.

Observing an action activates the same representations as does the actual performance of the action. Here we show for the first time that the action system can also be activated in the complete absence of action perception. When the participants had to identify the faces of famous athletes, the responses were influenced by their similarity to the motor skills of the athletes. Thus, the motor skills of the viewed athletes were retrieved automatically during person identification and had a direct influence on the action system of the observer. However, our results also indicated that motor behaviours that are implicit characteristics of other people are represented differently from when actions are directly observed. That is, unlike the facilitatory effects reported when actions were seen, the embodiment of the motor behaviour that is not concurrently perceived gave rise to contrast effects where responses similar to the behaviour of the athletes were inhibited.

Feature-based attention in visual cortex.

JH Maunsell — 2007-03-01T17:52:53-00:00

Trends Neurosci, Vol. 29, No. 6. (June 2006), pp. 317-322.

Although most studies of visual attention have examined the effects of shifting attention between different locations in the visual field, attention can also be directed to particular visual features, such as a color, orientation or a direction of motion. Single-unit studies have shown that attention to a feature modulates neuronal signals in a range of areas in monkey visual cortex. The location-independent property of feature-based attention makes it particularly well suited to modify selectively the neural representations of stimuli or parts within complex visual scenes that match the currently attended feature. This review is part of the TINS special issue on The Neural Substrates of Cognition.

Prefrontal Neural Correlates of Memory for Sequences

Bruno Averbeck — 2007-03-01T02:22:58-00:00

J. Neurosci., Vol. 27, No. 9. (28 February 2007), pp. 2204-2211.

The sequence of actions appropriate to solve a problem often needs to be discovered by trial and error and recalled in the future when faced with the same problem. Here, we show that when monkeys had to discover and then remember a sequence of decisions across trials, ensembles of prefrontal cortex neurons reflected the sequence of decisions the animal would make throughout the interval between trials. This signal could reflect either an explicit memory process or a sequence-planning process that begins far in advance of the actual sequence execution. This finding extended to error trials such that, when the neural activity during the intertrial interval specified the wrong sequence, the animal also attempted to execute an incorrect sequence. More specifically, we used a decoding analysis to predict the sequence the monkey was planning to execute at the end of the fore-period, just before sequence execution. When this analysis was applied to error trials, we were able to predict where in the sequence the error would occur, up to three movements into the future. This suggests that prefrontal neural activity can retain information about sequences between trials, and that regardless of whether information is remembered correctly or incorrectly, the prefrontal activity veridically reflects the animal's action plan. 10.1523/JNEUROSCI.4483-06.2007

In praise of artifice

Nicole Rust — 2005-11-23T18:52:21-00:00

Nature Neuroscience, Vol. 8, No. 12. (23 November 2005), pp. 1647-1650.

Aspects of complexity in visual-spatial working memory: indication for the application of strategies?

C Zoelch — 2007-02-19T20:37:19-00:00

Cogn Process, Vol. 7 Suppl 1 (September 2006)

Visual-spatial working memory is commonly assumed to consist of two systems: One for spatial-dynamic information and one for visual-static information. Four experiments were carried out to disentangle aspects of complexity in visual-spatial working memory using the Corsi Block Test and the Matrix Task. In Experiment 1 adult subjects were asked to memorize Corsi-sequences and Matrix-patterns of different complexity each consisting of a fixed amount of items. Results show that complexity in the Corsi Test is defined by the absolute length of the sequence-path, the number of path crossings and the number of blocks that were crossed by the imagined path but actually not involved in the sequence. Additionally it was found that sequences which form a path on basis of Gestalt criteria are easier to remember. Complexity in the Matrix Task is defined by the degree of symmetry of the pattern and the possibility to integrate items to an overall structure. Based on these findings a span procedure for both tasks was constructed in Experiment 2. Aspects of complexity were found to be relevant in both tasks, showing that complexity lowers the overall span in visual and spatial working memory. Experiment 3 investigated the role of visual and spatial complexity in children aged from 6 to 10 years by using the span procedures of Experiment 2. Results reveal reliable complexity effects within the Corsi and the Matrix Task across all age groups. Next to clear age effects, different developmental trends were found for the visual-static and the spatial-dynamic task. In Experiment 4 the serial demand within the Corsi Tasks was examined by varying the speed given to encode spatial location: Adult subjects had to memorize sequences consisting of a fixed amount of blocks. Next to the aspects of complexity the presentation time for the sequences was varied. The memorization of easy sequences was not influenced by the reduction of presentation time, whereas memory for complex sequences showed a significant decline. Generally our findings indicate that the processing demand of visual-static and spatial-dynamic information varies with task complexity. Particularly the application of strategies, like spatial rehearsal or chunking, seems to be influenced by different aspects of complexity. The fact that this effect can be found in younger children points towards an early acquisition of visual-spatial strategies. Serial demands within the Corsi Task are not yet clear, but it can be assumed that they play an important role in the processing of spatial-dynamic information.