Maintained activity of single neurons in striate and non-striate areas of the cat visual cortex

Abstract

A study was made of the spontaneous impulse activity generated by single neural elements in striate and non-striate areas of the cat visual cortex in a state of assumed steady conditions. Microelectrode recordings were made using the technique of the closed chamber, under continuous control of several physiological parameters. The maintained activity of each neuron was examined by considering the mean firing rate and the interspike intervals. Neurons in areas 17 and 18 show similar discharge frequencies (6–7 spikes/sec), whereas neurons in area 19 display a lower frequency (2.76 spikes/sec) of discharge. The interspike interval analysis shows that the spontaneous activity of neurons in areas 18 and 19 is different from that in area 17 for its dominating pattern (grouped activity) of spike discharge. This pattern, whenever present, is responsible for the L-shape shown by the histogrammatic representation of these intervals. Therefore, neurons of area 18, although they have a firing rate similar to that of striate neurons, show with respect to the latter a different pattern of activity. Neurons of area 19 are closer to cells of area 18 than to those of area 17. An analysis of the relation between mean firing rate and cortical depth led to the conclusion that the discharge frequency of a cortical visual neuron is a probable function of cell size. When tested for the property of independence on systematic factors, the spontaneous activity of neurons of the visual cortex turned out to be dependent in 81% of the units ( N= 124). Examination of the units in different layers of the visual cortex showed that the percentage of dependent cells is somewhat lower in deep than in superficial layers. The differential and common properties of neurons of striate and non-striate areas of the visual cortex were examined in some detail and are commented upon.