Receptive-field transformations between LGN neurons and S-cells of cat-striate cortex.

1. To determine the functional transformation in the processing step between lateral geniculate nucleus (LGN) and striate cortex, the receptive-field characteristics of 50 directly driven, lamina 4 S-cells were compared with those of a similar number of LGN neurons. Experiments were performed on paralyzed and anesthetized adult cats. 2. A signature was established for the responses of LGN neurons by observing the responses to thin light and dark moving bars and, after allowing for latency, by comparing these responses with the flashing-bar receptive field. Evidence of this signature was then sought in the responses of S-cells to similar stimuli. 3. When stimulated with moving bars, on-center LGN neurons responded with a strong discharge to a light bar followed, in spatial position, by a discharge to a dark bar. When compared to the flashing-bar receptive field, the light-bar discharge corresponded with the near side of the receptivefield center and the dark-bar discharge with the far side. Off-center LGN neurons generally gave a complementary pattern to the one above. Brisk sustained (BS) and brisk transient (BT) LGN units showed similar results. 4. By identifying the signature of LGN neurons in the responses of S-cells it was possible to recognize at least two and possibly three arrangements in the inputs to these cells. Individual S-cell responses could be explained as arising from: a) a single type (onor off-center) of LGN neuron, b) two types (onand off-center) of LGN neurons with their receptive fields placed side by side along the line orthogonal to the optimal orientation, c) three types of LGN neuron. Only a small number of S-cells were placed in the c category. There is doubt about the existence of the tripartite input, since in certain respects the responses of these cells resembled those of cells receiving a single or dual input. In all three groups, S-cells were found receiving inputs from either the fast (BT) or slow (BS) stream. 5. In the population of dual-input S-cells, the separation between the centers of onand off-responses distributed over a range similar to that found for the diameters of the receptive-field centers in parent LGN neurons. This was the case for cells in either the fast (BT) or slow (BS) streams. 6. Along the line parallel to the optimal orientation the receptive-field length of Scells, measured by the method of length summation, ranged over a distance twice that of the receptive-field center diameters in the parent LGN neurons. 7. An estimate was made of the number of LGN neurons contributing to the receptive fields of single S-cells in lamina 4. After making certain assumptions about the distributions of inputs to the S-cell receptive field, it was estimated that the response in each discharge region came from 1 to 20 like LGN neurons. It is argued that the receptive-field midpoints of these afferents are scattered over a circular area, whose diameter is equal to the receptive-field center diameter of one of the afferent LGN neurons (0.4” for BS and 0.8O for BT neurons).