CiteULike: Tag homunculus

Computational Role of Large Receptive Fields in the Primary Somatosensory Cortex.

Guglielmo Foffani — 2008-04-21T15:03:48-00:00

Journal of neurophysiology (9 April 2008)

Computational studies are challenging the intuitive view that neurons with broad tuning curves are necessarily less discriminative than neurons with sharp tuning curves. In the context of somatosensory processing, broad tuning curves are equivalent to large receptive fields. To clarify the computational role of large receptive fields for cortical processing of somatosensory information, we recorded ensembles of single-neurons from the infragranular forelimb/forepaw region of the rat primary somatosensory cortex while tactile stimuli were separately delivered to different locations on the forelimbs/forepaws under light anesthesia. Using single-trial analyses of many, single-neuron responses, we obtained two main results.(1)The responses of even small populations of neurons recorded from within the same estimated column/segregate can be used to discriminate between stimuli delivered to different surround locations in the excitatory receptive fields.(2)The temporal precision of surround responses is sufficiently high for spike-timing to add information over spike-count in the discrimination between surround locations. This surround spike-timing code (i) is particularly informative when spike-count is ambiguous, e.g. in the discrimination between close locations or when receptive fields are large, (ii) becomes progressively more informative as the number of neurons increases, (iii) is a first-spike code and (iv) is not limited by the assumption that the time of stimulus onset is known. These results suggest that even though large receptive fields result in a loss of spatial selectivity of single-neurons, they can provide as a counterpart a sophisticated temporal code based on latency differences in large populations of neurons, without necessarily sacrificing basic information about stimulus location.

When the Brain Loses Its Self: Prefrontal Inactivation during Sensorimotor Processing.

II Goldberg — 2006-05-05T16:38:59-00:00

Neuron, Vol. 50, No. 2. (20 April 2006), pp. 329-339.

A common theme in theories of subjective awareness poses a self-related "observer" function, or a homunculus, as a critical element without which awareness can not emerge. Here, we examined this question using fMRI. In our study, we compared brain activity patterns produced by a demanding sensory categorization paradigm to those engaged during self-reflective introspection, using similar sensory stimuli. Our results show a complete segregation between the two patterns of activity. Furthermore, regions that showed enhanced activity during introspection underwent a robust inhibition during the demanding perceptual task. The results support the notion that self-related processes are not necessarily engaged during sensory perception and can be actually suppressed.

Spectral Changes in Cortical Surface Potentials during Motor Movement

Kai Miller — 2007-03-21T04:15:19-00:00

J. Neurosci., Vol. 27, No. 9. (28 February 2007), pp. 2424-2432.

In the first large study of its kind, we quantified changes in electrocorticographic signals associated with motor movement across 22 subjects with subdural electrode arrays placed for identification of seizure foci. Patients underwent a 5-7 d monitoring period with array placement, before seizure focus resection, and during this time they participated in the study. An interval-based motor-repetition task produced consistent and quantifiable spectral shifts that were mapped on a Talairach-standardized template cortex. Maps were created independently for a high-frequency band (HFB) (76-100 Hz) and a low-frequency band (LFB) (8-32 Hz) for several different movement modalities in each subject. The power in relevant electrodes consistently decreased in the LFB with movement, whereas the power in the HFB consistently increased. In addition, the HFB changes were more focal than the LFB changes. Sites of power changes corresponded to stereotactic locations in sensorimotor cortex and to the results of individual clinical electrical cortical mapping. Sensorimotor representation was found to be somatotopic, localized in stereotactic space to rolandic cortex, and typically followed the classic homunculus with limited extrarolandic representation. 10.1523/JNEUROSCI.3886-06.2007