CiteULike: oamg's McCarthy

Face processing without awareness in the right fusiform gyrus.

James P Morris — 2007-07-25T00:38:25-00:00

Neuropsychologia (13 June 2007)

We investigated brain activity evoked by faces which were not consciously perceived by subjects. Subdural electrophysiological recordings and functional neuroimaging studies have each demonstrated face-specific processing in the fusiform gyrus (FFG) of humans. Using pattern masks, a stimulus can be presented but not consciously perceived, and thus can be used to assay obligatory or automatic processes. Here, using event-related functional magnetic resonance imaging and pattern masking, we observed that masked faces but not masked objects activated the right FFG. Other regions activated by consciously perceived unmasked faces were not activated when faces were masked. These data provide strong evidence for an automatic face-processing region in the right FFG.

Grasping the intentions of others: the perceived intentionality of an action influences activity in the superior temporal sulcus during social perception.

KA Pelphrey — 2007-04-13T17:43:18-00:00

J Cogn Neurosci, Vol. 16, No. 10. (December 2004), pp. 1706-1716.

An explication of the neural substrates for social perception is an important component in the emerging field of social cognitive neuroscience and is relevant to the field of cognitive neuroscience as a whole. Prior studies from our laboratory have demonstrated that passive viewing of biological motion (Pelphrey, Mitchell, et al., 2003; Puce et al., 1998) activates the posterior superior temporal sulcus (STS ) region. Furthermore, recent evidence has shown that the perceived context of observed gaze shifts (Pelphrey, Singerman, et al., 2003; Pelphrey et al., 2004) modulates STS activity. Here, using event-related functional magnetic resonance imaging at 4 T, we investigated brain activity in response to passive viewing of goal- and nongoal-directed reaching-to-grasp movements. Participants viewed an animated character making reaching-to-grasp movements either toward (correct) or away (incorrect) from a blinking dial. Both conditions evoked significant posterior STS activity that was strongly right lateralized. By examining the time course of the blood oxygenation level-dependent response from areas of activation, we observed a functional dissociation. Incorrect trials evoked significantly greater activity in the STS than did correct trials, while an area posterior and inferior to the STS (likely corresponding to the MT/ V5 complex) responded equally to correct and incorrect movements. Parietal cortical regions, including the superior parietal lobule and the anterior intraparietal sulcus, also responded equally to correct and incorrect movements, but showed evidence for differential responding based on the hand and arm (left or right) of the animated character used to make the reaching-to-grasp movement. The results of this study further suggest that a region of the right posterior STS is involved in analyzing the intentions of other people's actions and that activity in this region is sensitive to the context of observed biological motions.

Evidence for a refractory period in the hemodynamic response to visual stimuli as measured by MRI.

SA Huettel — 2007-02-20T23:04:00-00:00

Neuroimage, Vol. 11, No. 5 Pt 1. (May 2000), pp. 547-553.

We investigated the effects of paired presentations of visual stimuli upon the evoked hemodynamic response of visual cortex measured by magnetic resonance imaging (MRI). Stimuli were identical 500-ms high-contrast checkerboard patterns, presented singly or with an interpair interval (IPI) of 1, 2, 4, or 6 s (onset-to-onset), followed by an intertrial interval of 16-20 s. Images were acquired at 1.5 Tesla using a gradient-echo echoplanar imaging sequence sensitive to blood-oxygenation-level dependent (BOLD) contrast. Single checkerboards evoked a hemodynamic response from visual cortex characterized by a rise at 3 s, peak activation at 5 s, and return to baseline by 10 s. We subtracted subjects' single-stimulus hemodynamic response from their paired-stimulus responses to isolate the contribution of the second stimulus. If the hemodynamic responses were fully additive, the residual should be a time-shifted replica of the single stimulus response. However, the amplitude of the hemodynamic response to the second checkerboard was smaller, and the peak latency was longer, than for the first. Furthermore, the amplitude decrement was dependent upon IPI, such that the response to the second stimulus at 1 s IPI was only 55% of that to a single stimulus, with recovery to 90% at a 6 s IPI. Peak latency was similarly dependent upon IPI with longer latencies observed for shorter IPIs. These results demonstrate an extended refractory period in the hemodynamic response to visual stimuli consistent with that shown previously for neuronal activity measured electrophysiologically.

The effects of single-trial averaging upon the spatial extent of fMRI activation.

SA Huettel — 2007-02-20T23:02:54-00:00

Neuroreport, Vol. 12, No. 11. (8 August 2001), pp. 2411-2416.

We examined effects of trial averaging upon spatial extent, spatial topography, and temporal properties of fMRI activation. Two subjects participated in an event-related visual stimulation design. There was an exponential relation between number of trials and spatial extent, such that additional trials identified, on average, a constant proportion of the remaining voxels. At values typical of fMRI experimentation (e.g. 50 trials) only about 50% of eventually active voxels were significant; asymptotic values were approached by 150 trials. The variability of the estimated hemodynamic response decreased with signal averaging, becoming stable across samples of > or = 25 trials. Therefore, group or condition differences may result from differences in voxelwise noise exacerbated by averaging small numbers of trials.

Dissociating the neural mechanisms of visual attention in change detection using functional MRI.

SA Huettel — 2007-02-20T23:02:33-00:00

J Cogn Neurosci, Vol. 13, No. 7. (1 October 2001), pp. 1006-1018.

We investigated using functional magnetic resonance imaging (fMRI) the neural processes associated with performance of a change-detection task. In this task, two versions of the same picture are presented in alternation, separated by a brief mask interval. Even when the two pictures greatly differ (e.g., as when a building is in different locations), subjects report that identification of the change is difficult and often take 30 or more seconds to identify the change. This phenomenon of "change blindness" provides a powerful and novel paradigm for segregating components of visual attention using fMRI that can otherwise be confounded in short-duration tasks. By using a response-contingent event-related analysis technique, we successfully dissociated brain regions associated with different processing components of a visual change-detection task. Activation in the calcarine cortex was associated with task onset, but did not vary with the duration of visual search. In contrast, the pattern of activation in dorsal and ventral visual areas was temporally associated with the duration of visual search. As such, our results support a distinction between brain regions whose activation is modulated by attentional demands of the visual task (extrastriate cortex) and those that are not affected by it (primary visual cortex). A second network of areas including central sulcus, insular, and inferior frontal cortical areas, along with the thalamus and basal ganglia, showed phasic activation tied to the execution of responses. Finally, parietal and frontal regions showed systematic deactivations during task performance, consistent with previous reports that these regions may be associated with nontask semantic processing. We conclude that detection of change, when transient visual cues are not present, requires activation of extrastriate visual regions and frontal regions responsible for eye movements. These results suggest that studies of change blindness can inform understanding of more general attentional processing.

Perceiving patterns in random series: dynamic processing of sequence in prefrontal cortex.

SA Huettel — 2005-05-19T23:30:18-00:00

Nat Neurosci, Vol. 5, No. 5. (May 2002), pp. 485-490.

We demonstrate that regions within human prefrontal cortex develop moment-to-moment models for patterns of events occurring in the sensory environment. Subjects viewed a random binary sequence of images, each presented singly and each requiring a different button press response. Patterns occurred by chance within the presented series of images. Using functional magnetic resonance imaging (fMRI), we identified activity evoked by viewing a stimulus that interrupted a pattern. Prefrontal activation was evoked by violations of both repeating and alternating patterns, and the amplitude of this activation increased with increasing pattern length. Violations of repeating patterns, but not of alternating patterns, activated the basal ganglia.

Deterministic and stochastic features of fMRI data: implications for analysis of event-related experiments.

MJ McKeown — 2007-02-20T23:01:50-00:00

J Neurosci Methods, Vol. 118, No. 2. (30 August 2002), pp. 103-113.

As the limits of stimuli presentation rates are explored in event-related fMRI design, there is a greater need to assess the implications of averaging raw fMRI data. Selective averaging assumes that the fMRI signal consists of task-dependent signal, random noise, and non-task dependent brain signal that can be modeled as random noise so that it tends to zero when averaged over a practical number of trials. We recorded a total of four fMRI data series from two normal subjects (subject 1, axially acquired; subject 2, coronally acquired) performing a simple visual event-related task and a water phantom with the same fMRI scanner imaging parameters. To determine which fraction of the fMRI data was deterministic as opposed to random, we created different data subsets by taking the odd or even time points of the full data sets. All data sets were first dimension-reduced with principal component analysis (PCA) and separated into 100 spatially independent components with independent component analysis (ICA). The mutual information between best-matching pairs of components selected from full data set-subset comparisons was plotted for each data set. Visual inspection suggested that 45-85 components were reproducible, and hence deterministic, accounting for 79-97% of the variance, respectively, in the raw data. The reproducible components exhibited much less trial-to-trial variability than the raw data from even the most activated voxel. Many (22-47) of reproducible components were significantly affected by stimulus presentation (P < 0.001). The most significantly-stimulus-correlated component was strongly time-locked to stimulus presentation and was directly stimulus correlated, corresponding to occipital brain regions. However, other spatially distinct task-related components demonstrated variable temporal relationships with the most significantly-stimulus-correlated component. Our results suggest that the majority of the variance in fMRI data is in fact deterministic, and support the notion that the data consist of differing components with differing temporal relationships to visual stimulation. They further suggest roles for restricting interpretations of the spatial extent of activation from event-related designs to a specific region of interest (ROI) and/or first separating the data into spatially independent components. Averaging the time courses of spatially independent components time-locked to stimulus presentation may prevent possible biases in the estimates of the spatial and temporal extent of stimulus-correlated activation and of trial-to-trial variability.

What is odd in the oddball task? Prefrontal cortex is activated by dynamic changes in response strategy.

SA Huettel — 2007-02-20T23:01:32-00:00

Neuropsychologia, Vol. 42, No. 3. (2004), pp. 379-386.

In the "oddball" target detection task, subjects respond to target stimuli that occur infrequently and irregularly within a series of standard stimuli. Although detection of these targets reliably evokes transient activity in prefrontal cortical regions, it has not been established whether this activity is due to selection of an infrequent response or to changes in response strategy. We investigated this issue using a novel variant of the oddball task that incorporated the Simon effect, while measuring hemodynamic brain activity in prefrontal cortex using functional magnetic resonance imaging (fMRI). Subjects viewed a series of circles and squares that required left and right button presses, respectively. On 90% of trials ("standard" trials), the stimuli were presented in the same visual hemifield as the hand of response, but on 10% of trials ("strategy-change" trials) they were presented in the opposite visual hemifield. Significant activation to the infrequent strategy-change trials was found in the anterior middle frontal gyrus (MFG), the posterior inferior frontal gyrus (IFG) and adjacent insular cortex, and in the anterior cingulate gyrus (ACG). These regions, which correspond to previous reports of oddball-related activation, were consistent across subjects. Behavioral results supported our interpretation that subjects potentiated a position-based response strategy, which was inhibited on the strategy-change trials. Activity within the MFG and ACG was much greater on error trials than on correct trials, while IFG activity was similar between error and correct trials. We conclude that the dorsolateral prefrontal cortex (dlPFC) is associated with dynamic changes in the mapping of stimuli to responses (e.g. response strategies), independently of any changes in behavior.

Linking hemodynamic and electrophysiological measures of brain activity: evidence from functional MRI and intracranial field potentials.

SA Huettel — 2007-02-20T22:57:57-00:00

Cereb Cortex, Vol. 14, No. 2. (February 2004), pp. 165-173.

We investigated the relation between electrophysiological and hemodynamic measures of brain activity through comparison of intracranially recorded event-related local field potentials (ERPs) and blood-oxygenation level dependent functional magnetic resonance imaging (BOLD fMRI). We manipulated the duration of visual checkerboard stimuli across trials and measured stimulus-duration-related changes in ERP and BOLD activity in three brain regions: peri-calcarine cortex, the fusiform gyrus and lateral temporal-occipital (LTO) cortex. ERPs were recorded from patients who had indwelling subdural electrodes as part of presurgical testing, while BOLD responses were measured in similar brain regions in a second set of subjects. Similar BOLD responses were measured in peri-calcarine and fusiform regions, with both showing monotonic but non-linear increases in hemodynamic amplitude with stimulus duration. In sharp contrast, very different ERP responses were observed in these same regions, such that calcarine electrodes exhibited onset potentials, sustained activity over the course of stimulus duration and prominent offset potentials, while fusiform electrodes only exhibited onset potentials that did not vary with stimulus duration. No duration-related ERP or BOLD changes were observed in LTO. Additional analyses revealed no consistent changes in the EEG spectrum across different brain sites that correlated with duration-related changes in the BOLD response. We conclude that the relation between ERPs and fMRI differs across brain regions.

Dynamic and strategic aspects of executive processing.

SA Huettel — 2006-01-20T16:49:15-00:00

Brain Res, Vol. 1000, No. 1-2. (12 March 2004), pp. 78-84.

Executive cognitive functions have been postulated to include both dynamic behavioral selection and strategic goal-setting or response preparation. To investigate the relation between these aspects of executive processing, we embedded an event-related oddball paradigm within a blocked design. Subjects responded to infrequent targets presented within a series of standard stimuli that required no response; this task alternated with a visually similar nontask condition. Using functional magnetic resonance imaging (fMRI), we found that a set of brain regions including dorsolateral prefrontal cortex (dlPFC), insular cortex, cingular cortex, and the basal ganglia demonstrated transient activation both to target stimuli and to the onset of task blocks. Within the parietal cortex, there was a dissociation such that the supramarginal gyrus exhibited greater activity to the target stimuli than to block onsets, while the converse pattern was observed in the intraparietal sulcus. Sustained positive activity during task blocks was present in the caudate and supplementary motor area, while sustained negative activity was present in the precuneus and medial parietal cortex. We conclude that dlPFC and related brain regions mediate both dynamic and strategic processing, through the preparation and selection of rules for behavior.

Decisions under uncertainty: probabilistic context influences activation of prefrontal and parietal cortices.

SA Huettel — 2005-05-19T20:12:31-00:00

J Neurosci, Vol. 25, No. 13. (30 March 2005), pp. 3304-3311.

Many decisions are made under uncertainty; that is, with limited information about their potential consequences. Previous neuroimaging studies of decision making have implicated regions of the medial frontal lobe in processes related to the resolution of uncertainty. However, a different set of regions in dorsal prefrontal and posterior parietal cortices has been reported to be critical for selection of actions to unexpected or unpredicted stimuli within a sequence. In the current study, we induced uncertainty using a novel task that required subjects to base their decisions on a binary sequence of eight stimuli so that uncertainty changed dynamically over time (from 20 to 50%), depending on which stimuli were presented. Activation within prefrontal, parietal, and insular cortices increased with increasing uncertainty. In contrast, within medial frontal regions, as well as motor and visual cortices, activation did not increase with increasing uncertainty. We conclude that the brain response to uncertainty depends on the demands of the experimental task. When uncertainty depends on learned associations between stimuli and responses, as in previous studies, it modulates activation in the medial frontal lobes. However, when uncertainty develops over short time scales as information is accumulated toward a decision, dorsal prefrontal and posterior parietal contributions are critical for its resolution. The distinction between neural mechanisms subserving different forms of uncertainty resolution provides an important constraint for neuroeconomic models of decision making.