MECHANISMS UNDERLYING DIRECTION SELECTIVITY OF NEURONS IN THE PRIMARYVISUAL-CORTEX OF THE MACAQUE

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.