A re-evaluation of the mechanisms underlying simple cell orientation selectivity.
Following from evidence supporting GABA as a putative inhibitory transmitter in the visual cortex, we have iontophoretically applied the GABA antagonist N-methyl bicuculline (Nmb) to simple cells in order to block the inhibitory inputs acting on them. We found that under these conditions previously sharply-tuned simple cells responded equally to all orientations. Moreover receptive field dimensions, judged by the response to stimuli at the optimal and orthogonal orientations, equated best with that expected from a single dLGN cell input. It seems thus, that asymmetries in the excitatory input are not a significant factor in the generation of simple cell orientation selectivity. The asymmetry underlying orientation selectivity rather originates from the operation of an intracortical inhibitory mechanism.
D000818:Animals; D001640:Bicuculline / Q000494:pharmacology; D002415:Cats; D005071:Evoked Potentials / Q000187:drug effects; D005829:Geniculate Bodies / Q000166:cytology; D009433:Neural Inhibition / Q000187:drug effects; D009435:Synaptic Transmission / Q000187:drug effects; D014793:Visual Cortex / Q000166:cytology*; D014795:Visual Pathways / Q000166:cytology; D005680:gamma-Aminobutyric Acid / Q000378:metabolism
D005680:gamma-Aminobutyric Acid; D001640:Bicuculline