Correlated variability in primate superior colliculus depends on functional class

Correlated variability (spike count correlations, rSC) in a population of neurons can constrain how information is read out, depending on behavioral task and neuronal tuning. Here we tested whether rSC also depends on neuronal functional class. We recorded from populations of neurons in macaque superior colliculus (SC), a structure that contains well-defined functional classes. We found that during a guided saccade task, different classes of neurons exhibited differing degrees of rSC. "Delay class" neurons displayed the highest rSC, especially during the delay epoch of saccade tasks that relied on working memory. This was only present among Delay class neurons within the same hemisphere. The dependence of rSC on functional class indicates that subpopulations of SC neurons occupy distinct circuit niches with distinct inputs. Such subpopulations should be accounted for differentially when attempting to model or infer population coding principles in the SC, or elsewhere in the primate brain.

Task context modulates feature-selective responses in area V4

ABSTRACTFeature selectivity of neuronal responses in primate visual cortex is typically measured while animals fixate a small dot on the screen and a stimulus is presented in the near-periphery. This paradigm allows the efficient exploration of feature space, but it provides only a partial view of selectivity by failing to characterize how cognitive factors influence neuronal tuning. Here we focus on primate area V4, known to be influenced by cognitive processes, and ask how neuronal tuning is modulated by task engagement. We compared the tuning for shape and color in 83 well-isolated V4 neurons measured during passive fixation and during active engagement in a shape discrimination task. In both tasks, animals saw the same set of objects—shape x color combinations—but while neither stimulus feature was relevant during the fixation task, shape identity was relevant for behavior during the discrimination task. Consistent with attentional studies, V4 responses during the discrimination task showed a stimulus-independent gain scaling relative to passive fixation, but this was only in a minority of neurons (21/83). For the rest (62/83), response modulations during discrimination depended on stimulus identity: on stimulus shape in neurons more strongly tuned to shape, and on color in neurons more strongly tuned to color. Overall, this resulted in broader tuning for stimulus color, but not shape, during active task engagement. These results suggest that task context can influence the shape and color selectivity of V4 neurons, and in some neurons this effect is consistent with a change in the width of feature tuning.