Population response characteristics of neurons in anterior motorsensory cerebral cortex (field 6) of the domestic cat

Patterns in the brain. Neuronal population coding in the somatosensory system

The aim of this article is to review some basic principles of neural coding, with an emphasis on mechanisms of stimulus representation in ensembles of neurons. The theory of "across-neuron response patterns" (ANRPs), first suggested by Thomas Young (1802) and fully developed by Robert Erickson (1963-2000), is summarized and applied to the problem of coding in primary afferent fibers and cortical neurons of the somatosensory system. The basic premise of the theory is that precise information about stimulus features cannot be encoded by single neurons, but is encoded by patterns of activity across populations of neurons. Different stimuli produce uniquely different patterns of ensemble activity (ANRPs)-discrimination between two stimuli is based on the absolute difference in total amount of activity (neural mass difference) of the ANRPs for those stimuli. Review of the literature shows that ANRPs and related population codes can accurately represent and differentiate among various stimulus parameters that cannot be distinguished by single neurons alone. Finally, the behavior of neuronal ensembles can be used to account for the sensory-perceptual changes associated with plasticity of thalamocortical circuits following selective sensorimotor deprivation or experience.

Raccoon forelimb motorsensory cortex: II. Somatosensory inputs to single neurons

Somatosensory input to 431 neurons in MsI has been studied in unanesthetized, paralyzed raccoons (Procyon lotor). The type of sensory input to neurons in lateral sigmoid gyrus (cytoarchitectonic area 4) and in posterior sigmoid gyrus (areas 4 and 3a) was not significantly different. Of these neurons, 36% were activated by superficial cutaneous stimulation (touch, tap or hair deflection) and 26% by deep stimulation (pressure or joint movement). Mute neurons (not driven by any form of peripheral stimulation tested, or vaguely driven) comprised 38% of the sample. Only 4% of anterior sigmoid gyrus (area 6) neurons responded to superficial or deep stimulation; 96% were mute. The majority of MsI neurons had small (less than or equal to 20 cm2) peripheral receptive fields (PRFs). There was a statistically significant trend for PRF size to decrease along the proximal-distal axis of the forelimb. The area of MsI digit PRFs was significantly larger than the area of SmI digit PRFs. Comparing the data for raccoon MsI with information from the literature for cats and monkeys suggests that the type and amount of somesthetic afferent input to forelimb MsI is related to the behavioral uses to which each animal puts the forelimb.

Three-dimensional plotting of neuronal population response patterns

A computer technique is described for generating three-dimensional plots of the distribution of spike activity in neuronal populations. Data obtained from single neurons in the sensorimotor cerebral cortex of cats are used to plot spike density with respect to depth in the cortex and time following a cutaneous stimulus. The computer programs include a masking (hidden line) subroutine and produce a cross-hatched plot showing the rise and fall of neuronal activity in both space and time. By manipulation of the data matrix and by axis rotations, the plot can be viewed from any desired perspective. This method constitutes a powerful technique for analyzing the spatio-temporal response patterns of neuronal populations. The significance of neuronal population responses for encoding stimulus information and predicting behavior is discussed. Note: Details concerning the computer programs developed for this plotting technique may be obtained from the Division of Systems and Computer Services, Medical College of Georgia, Augusta, GA 30901.