ORDINAL POSITION OF NEURONS IN CAT STRIATE CORTEX

The ordinal, or serial, position of different types of striate neurons was assessed from their latencies to electrical stimulation of the optic radiations in paralyzed cats anesthetized with N2O/O2(70/30%). The distribution histogram for response latencies of striate neurons to electrical stimulation in the optic radiations shows three broad peaks. These peaks become sharper if the stimulating site is moved from a low point (OR1) to a high point (OR2) in the optic radiations. It is argued that each peak represents a population of cells lying at a different ordinal, or serial, position within the cortical pathway, i.e., receiving a mono-, di-, or polysynaptic excitatory input from the thalamus. The duration separating the peaks, of the order of 1.0 ms, is then taken as the transmission time for conduction of the nervous impulse from one cortical cell to the next. Confirmatory evidence for this value of the cell-to-cell transmission time comes from the measurement of interspike intervals in cortical cells that responded with multiple spikes to electrical stimulation. From the peaks in the distribution of OR2 latencies, boundaries have been established to arrive at an ordinal position for 131 cells classified as S, S(h), C, B, or cells with nonoriented or concentric receptive field (N-O and conc), following the terminology of Henry. Four ordinal groups were distinguished: group M comprised cells receiving an exclusive monosynaptic input from the thalamus, group C(1+n) consisted of cells exhibiting a convergence of excitatory inputs, one of which was monosynaptic; group D was composed of cells in which the lowest order input was disynaptic. Cells were placed in group P if they were driven from the thalamus through three or more synapses. Examination of the distribution of different cell types for these various ordinal groups revealed that no particular cell type was exclusively associated with a single ordinal position. In particular, a large proportion of S-, S(h)-, C-, and N-O and conc cells showed a monosynaptic input from the thalamus (groups M and C(1+n). For the three classes of cell, S, S(h), and C, with sufficient numbers in groups M, C(1+n), and D, a search was made for differences in response properties associated with the various ordinal groups. The properties examined included ocular dominance, direction selectivity, orientation specificity, and receptive-field size. With the exception of the C-cell, which showed a difference of direction selectivity in groups M and C(1+n), we observed no systematic changes in the response properties with differences in ordinal position. For most cell types in cat striate cortex there are examples of neurons receiving a monosynaptic input. It is difficult to reconcile these findings with a hierarchical model for the processing of visual information. It is proposed that these cells are first-order elements of different streams running in parallel within the cortex and that this multiplicity of streams may be associated with the diversity of outputs leaving the striate cortex.