FUNCTIONAL-PROPERTIES OF NEURONS IN MACAQUE AREA V3

We investigated the functional properties of neurons in extrastriate a rea V3. V3 receives Inputs from both magno- and parvocellular pathways and has prominent projections to both the middle temporal area (area MT) and V4. It may therefore represent an important site for integrati on and transformation of visual signals. We recorded the activity of s ingle units representing the central 10 degrees in anesthetized, paral yzed macaque monkeys. We measured each cell's spatial, temporal, chrom atic, and motion properties with the use of a variety of stimuli. Resu lts were compared with measurements made in V2 neurons at similar ecce ntricities. Similar to area V2, most of the neurons in our sample (80% ) were orientation selective, and the distribution of orientation band widths was similar to that found in V2. Neurons in V3 preferred lower spatial and higher temporal frequencies than V2 neurons. Contrast thre sholds of V3 neurons were extremely low. Achromatic contrast sensitivi ty was much higher than in V2, and similar to that found in MT. About 40% of all neurons showed strong directional selectivity. We did not f ind strongly directional cells in layer 4 of V3, the layer in which th e bulk of V1 and V2 inputs terminate. This property seems to be develo ped within area V3. An analysis of the responses of directionally sele ctive cells to plaid patterns showed that in area V3, as in MT and unl ike in V1 and V2, there exist cells sensitive to the motion of the pla id pattern rather than to that of the components. The exact proportion of cells classified as being selective to color depended to a large d egree on the experiment and on the criteria used for classification. W ith the use of the same conditions as in a previous study of V2 cells, we found as many (54%) color-selective cells as in V2 (50%). Furtherm ore, the responses of V3 cells to colored sinusoidal gratings were wel l described by a linear combination of cone inputs. The two subpopulat ions of cells responsive to color and to motion overlapped to a large extent, and we found a significant proportion of cells that gave relia ble and directional responses to drifting isoluminant gratings. Our re sults show that there is a significant interaction between color and m otion processing in area V3, and that V3 cells exhibit the more comple x motion properties typically observed at later stages of visual proce ssing.