Cholinergic enhancement of direction selectivity in the visual cortex of the cat.

Acetylcholine and acetyl-beta-methacholine were applied iontophoretically to single cells in the feline striate cortex. The directional bias of the visual response to an optimally oriented stimulus was assessed quantitatively, before and during drug application. For the great majority of the cells that were affected by the drugs, selectivity was either unchanged (28/60, 47%) or increased (21/60, 35%). In particular, directional bias increased for 36% (12/33) of the cells that were facilitated by acetylcholine or acetyl-beta-methacholine and 43% (nine out of 21) of those that were inhibited, as compared with 9% (three out of 33) and 24% (five out of 21) for which the bias decreased. Six additional cells, of which three showed a reduced selectivity, were apparently excited by the drugs, in that the background discharge level was greatly increased with a concomitant decrease in signal-to-noise ratio. It is known that cholinergic input has the potential to enhance cortical function, by facilitating both the excitatory and the inhibitory components of the neuronal circuit. Our data show that this combination of effects can precipitate an enhancement of selectivity as well as of response magnitude.