ACTIVITY OF INFERIOR TEMPORAL NEURONS DURING ORIENTATION DISCRIMINATION WITH SUCCESSIVELY PRESENTED GRATINGS

We recorded from inferior temporal (IT) cells in three monkeys while they performed an orientation discrimination task with successively presented gratings. Histological reconstruction of two monkey brains indicated that most recordings were from TE3. The task was exactly the same as the one used in a previous V1 study, allowing direct comparison. One quarter of IT cells were responsive to the grating. Response strength and variability of the cells excited by the grating (n = 341) were similar to those in V1, whereas response latency was on average 40 ms longer than in V1. In one third of the responsive cells orientation had a significant effect. Sensitivity for orientation was captured by the orientation sensitivity index, which ranged from 0 to 0.95 with a median of 0.23. Orientation sensitivity of IT cells was on average much less than that of V1 cells. More cells preferred horizontal and vertical than oblique orientations. The differences in responsiveness and orientation sensitivity among individual animals could be accounted for by the anterior/ posterior difference in recording position. Task-related behavioral effects were examined in 283 cells that were responsive to the visual stimuli. Order effects were examined in ''same'' trials and occurred in half of the IT cells in each of the three monkeys. In these cells there was on average a threefold difference in response between the first stimulus(S1) and the second stimulus in same trials(S2same). In two monkeys the average response to S2same was less than that to S1, whereas in the third monkey the average response to S1 was smaller than that to S2same. IT cells also exhibited same/different effects, whereby response to a physically identical second stimulus differed between same and ''different'' trials. Generally cells responded more to the second stimulus in different trials than to S2same but the opposite also occurred. For a subset of these cells we could show that the correct/error type of the trial had no effect and that the response occurred before the saccadic response. These context effects were modulations of the gain of the orientation tuning curves for the different types of responses. The tuning for orientation of these different responses was similar. 8. One third of the responsive IT cells exhibited an increased activity in the interstimulus interval(ISI), fitting the definition of delay cells. This activity was maintained when the ISI was lengthened from 300 to 800 ms. The orientation tuning of this response was similar to that for S1 responses. These results show that visual and contextual factors interact in IT cells even for simple spatial stimuli. The role of these effects in visual short-term memory and learning are discussed.