H. Sato et al., MECHANISMS UNDERLYING DIRECTION SELECTIVITY OF NEURONS IN THE PRIMARYVISUAL-CORTEX OF THE MACAQUE, Journal of neurophysiology, 74(4), 1995, pp. 1382-1394
1. We studied the effects of blocking intracortical inhibition by micr
oiontophoretic administration of bicuculline methiodide (BMI), a selec
tive antagonist for gamma-aminobutyric acid-A receptors, on direction
sensitivity of 103 neurons in the primary visual cortex (V1) of anesth
etized and paralyzed monkeys. 2. The direction selectivity index (DSI)
of each cell was calculated for the control response and response dur
ing the BMI administration at the optimal stimulus orientation to asse
ss the directionality of an individual cell. 3. The averaged direction
tuning of visual responses of cells was sharp in layers IVa and IVb,
moderate in both interblob and blob regions of layer II/III and layers
V and VI, and poor in layers IVc alpha and IVc beta. 4. Iontophoretic
administration of BMI uncovered or facilitated responses to stimuli m
oving in the nonpreferred direction, and reduced DSIs of cells to a va
rying extent in all the layers except layer VT. Responses to stimuli m
oving in the preferred direction were also facilitated so that a sligh
t bias of response toward the originally preferred direction remained
during BMI administration in most cells. 5. Most of the cells in layer
s II/III (both blobs and interblobs) and IVb that receive inputs from
layers IVc alpha and IVc beta showed a clear reduction of direction se
lectivity during BMI administration. This result suggests that intraco
rtical inhibition plays an important role in the elaboration of direct
ion selectivity at the second stage of information processing in V1. 6
. The direction selectivity of cells in layer VI was most resistant to
the effects of BMI, suggesting that it is dependent on excitatory inp
uts that are already direction selective, even though the sample size
of this layer was small. 7. In direction-selective cells outside layer
VI, responses to a stimulus moving in the preferred direction were en
hanced in a way that was linearly related with those in the nonpreferr
ed direction as the BMI dose was increased. This suggests that various
amounts of inhibition interact linearly with directionally biased exc
itatory inputs to raise the firing threshold to various levels so as t
o produce various degrees of directionality. 8. These results suggest
that, in most of the directionally sensitive cells except for those in
layer VI, there are excitatory inputs which are bidirectional but sli
ghtly biased to one direction, and that the intracortical inhibition r
aises a threshold level of responses to excitatory inputs so that the
response become direction selective.