NEURAL PATH TAKEN BY AFFERENT STREAMS IN STRIATE CORTEX OF THE CAT

The type of thalamic input received by different types of striate neurons in paralyzed cats, anesthetized with N2O/O2, was assessed by eletrical stimulation from stimulating electrodes positioned in the optic chiasm (OX) and in the optic radiations (OR1) just above the dorsal lateral geniculate nucleus (dLGN). The difference between latencies to electrical stimulation in the optic chiasm (OX latency) and in the optic radiations (OR1 latency) was used to place striate neurons into one of two groups - group I when the neuron's OX-OR1 latency difference was smaller than 1.4 ms, and group II when the OX-OR1 latency difference was longer than 1.7 ms. The OX-OR1 latency difference was measured for 46 dLGN axons isolated within the striate cortex. These axons were classified as brisk sustained (BS) or brisk transient (BT) according to their receptive-field properties. All dLGN axons with an OX-OR1 latency difference lower than 1.4 ms were BT axons, and all those with OX-OR1 value longer than 1.7 ms were BS axons. This suggests that striate neurons belonging to group I receive an excitatory input from BT (or Y) dLGN cells and that those belonging to group II receive an excitatory input from BS (or X) dLGN cells. The type of afferent input was then determined for 93 striate neurons classified as S, S(h), C, B, and cells with nonoriented or concentric fields (N-O/conc), following the scheme of Henry. Nearly all C- and B-cells belonged to group I and all S(h)- and most N-O/conc cells to group II. On the other hand, the population of S-cells was divided almost equally between the two groups. A comparison was made between the functional characteristics of S-cells belonging to groups I and II. Apart from a tendency for groups I S-cells to exhibit larger receptive-field sizes and to respond to higher stimulus velocities than group II S-cells, the response properties of cells belonging to both groups were similar. Furthermore, the distribution of S-cells among different ordinal positions was similar in the two groups and it appears the group I and group II S-cells may perform parallel roles in the cat striate cortex. The results obtained in this study suggest that the segregation of fast and slow streams in the retinogeniculate pathway is preserved in the striate cortex and that each afferent stream branches into several cortical tributaries, the fast stream giving rise to the S and C tributaries and the slow stream to the S, S(h), and N-O/conc tributaries.