The afferent connexions and laminar distribution of cells in area 18 of the cat.

1. The receptive field properties, laminar distribution and afferent connectivity of cells in area 18 of the cat are described. 2. Testing with both moving and stationary stimuli revealed three main receptive field types which have been termed S, C and B, respectively (cf. Henry, 1977; Henry, Lund & Harvey, 1978). All three classes may show end-zone inhibition and units exhibiting this property have been designated SH, CH and BH. 3. S cells can be divided into spatially separate lights and/or dark edge response regions when tested with moving edges and usually have separate ON and/or OFF areas when tested with stationary flashing stimuli. They are the most commonly encountered cell type in area 18 and occur most frequently in laminae IIIb, IVa and VI. 4. Both C and B cells have spatially coincident light and dark edge response regions and give mixed ON and OFF discharges when tested with stationary flashing stimuli. Compared to B cells however, C cells have large receptive fields, they are broadly tuned for stimulus orientation and generally have a relatively high rate of spontaneous activity. C cells are more common than B cells and are encountered most often in laminae IVb and V. 5. Electrical stimulation of the optic chiasm (OX) and optic radiation (OR) was used to examine the afferent connectivity of parastriate neurons. Cells driven from both OX and OR have been divided into two main groups and it is argued that group 1 cells are directly, and group 2 cells are indirectly, excited by rapidly conducting afferent fibres. Group 1 cells are found most often in laminae IIIb, IVa, IVb and VI, and their distribution closely follows the anatomically defined laminar disposition of geniculocortical afferent terminals. Group 2 neurones predominate in laminae II-IIIa, IIIA and V. 6. The majority of S and SH cells are directly driven, whereas most C and CH cells have OX and OR latencies suggestive of indirect activation by thalamic afferents. 7. The intrinsic organization and possible functional role of area 18 is discussed in the light of these results.