The role of GABAergic inhibition in the response properties of neurones in cat visual area 18.

Bicuculline methiodide was iontophoretically applied to single neurones in cat area 18 to investigate how removal of gamma-aminobutyrate mediated inhibition affects the visual response properties. Moving sinusoidal gratings were used to study spatial and temporal response characteristics. Orientation sensitivity and spatial and temporal frequency tuning curves were determined with and without iontophoretically applied bicuculline. In most neurones, orientation sensitivity and spatial frequency tuning remained largely unaffected, whereas temporal frequency tuning was very much broadened. It is suggested that the dominant excitatory input to area 18 cells is a spatially organized input from area 17 and local inhibition in area 18 sharpens primarily temporal selectivity. An alternative explanation of our results would be that the distribution of synapses mediating temporal tuning in area 18 is fundamentally different from that mediating spatial frequency and orientation tuning, which may be located at sites distant from the cell body and relatively inaccessible to the drug application.