Coarse orientation discrimination is impaired by microstimulation of macaque posterior inferior temporal cortex.

We have recently identified an orientation selective region in the posterior inferior temporal (PIT) cortex of monkeys with response properties that are affected by practicing a coarse orientation discrimination task (Adab et al., 2014). In this task, subjects discriminate two orthogonal gratings that are masked by noise (variable signal to noise ratio; SNR). To assess whether there is a causal link between PIT activity and the monkey’s decision, we applied microstimulation in PIT while the monkeys were performing the coarse orientation and a control, color discrimination task. In the latter task the noisy gratings were presented at the same location as in the orientation task but were irrelevant for the task performance. The color patch was presented ipsilateral to the stimulated hemisphere at 10.3 deg eccentricity. Microstimulation consisted of bipolar, cathodal phase first, current pulses of 650 uA delivered at 300 Hz. Each pulse was 0.25 ms in duration with 0.01 ms between the cathodal and anodal phase. Stimulating pulses were delivered for 250 ms (equal to grating duration) with a 50 ms delay following onset of the grating. The stimulation (50% of the total number of trials) and no-stimulation trials were randomly interleaved. Percent correct performance as a function of SNR in the orientation discrimination task was fitted with a cumulative normal function for stimulation and no-stimulation conditions, separately, estimating 75% correct thresholds. Microstimulation of PIT increased the thresholds dramatically (median percent threshold increase: Monkey M: 195%, monkey P: 281%). The behavioral impairment was also obtained with currents as low as 100 uA. Microstimulation after the stimulus presentation but before the saccadic response had no effect. Performance in the color discrimination task was affected but less so (median percent decrease in z-transformed percent correct: monkey M: 6%, monkey P: 9%), suggesting that the microstimulation effect is specific for the properties of the discrimination task. To test for visual area specificity, we microstimulated 3 regions 6 mm anterior to PIT of monkey P: the upper bank of the superior temporal sulcus (USTS), the lower bank of the STS (LSTS) and the lateral convexity of inferotemporal (LCIT) cortex. Whereas no significant effect of stimulation on the threshold in orientation discrimination was found in the USTS (median: 1.7%), significant but only moderate to weak effects were present in the LSTS (median: 139%) and LCIT (median: 40%). Overall, these data support the hypothesis that PIT is the part of the network of areas that underlie performance in coarse orientation discrimination.status: publishe