RESPONSES OF NEURONS IN THE PARIETAL AND TEMPORAL VISUAL PATHWAYS DURING A MOTION TASK

The visual cortex of macaque monkeys has been divided into two functio nal streams that have been characterized in terms of sensory processin g (color/form vs motion) and in terms of behavioral goals (object reco gnition vs spatial orientation). As a step toward unifying these two v iews of cortical processing, we compared the behavioral modulation of sensory signals across the two streams in monkeys trained to do a visu al short-term memory task. We recorded from individual neurons in area s MT, MST, 7a, and V4 while monkeys performed a delayed match-to-sampl e task using direction of motion as the matching criterion. This task allowed us to determine if sensory responses were modulated by extrare tinal signals related to the direction of the remembered sample. We so rted neuronal responses as a function of the remembered direction and calculated a modulation index, MI = (maximum response - minimum respon se)/(maximum response + minimum response). In the motion pathway, we f ound virtually no extraretinal signals in MT (average MI = 0.11 +/- 0. 01 SE, 66 cells), but progressively stronger extraretinal signals in l ater stages, that is, MST (average MI = 0.17 +/- 0.01 SE, 57 cells) an d 7a (average MI = 0.23 +/- 0.02 SE, 46 cells). In contrast to MT, str ong extraretinal signals for direction matching were found in V4 (aver age MI = 0.28 +/- 0.02 SE, 94 cells), a relatively early stage of the color/form pathway, even though this pathway is not generally viewed a s playing a major role in motion processing. Some cells in V4 were als o tested while the animals performed a color matching task. These cell s showed memory-related modulation of their response when either color or direction was used as the matching criterion. We conclude that ext raretinal signals related to the match-to-sample task may be stronger in the temporal (color/form) pathway than in the parietal (motion) pat hway, regardless of the stimulus dimension involved. Furthermore, our results indicate that the temporal pathway is capable of making a sign ificant contribution to motion processing in tasks where motion can be considered as a cue for the identification of object attributes.