Posterior Parietal Cortex Guides Visual Decisions in Rats

Abstract

Neurons in putative decision-making structures can reflect both sensory and decision signals, making their causal role in decisions unclear. Here, we tested whether rat posterior parietal cortex (PPC) is causal for processing visual sensory signals or instead for accumulating evidence for decision alternatives. We disrupted PPC activity optogenetically during decision making and compared effects on decisions guided by auditory versus visual evidence. Deficits were largely restricted to visual decisions. To further test for visual dominance in PPC, we evaluated electrophysiological responses after individual sensory events and observed much larger response modulation after visual stimuli than auditory stimuli. Finally, we measured trial-to-trial spike count variability during stimulus presentation and decision formation. Variability decreased sharply, suggesting that the network is stabilized by inputs, unlike what would be expected if sensory signals were locally accumulated. Our findings suggest that PPC plays a causal role in processing visual signals that are accumulated elsewhere. Defining the neural circuits that support decision making bridges a gap between our understanding of simple sensorimotor reflexes and our understanding of truly complex behavior. However, identifying brain areas that play a causal role in decision making has proved challenging. We tested the causal role of a candidate component of decision circuits, the rat posterior parietal cortex (PPC). Our interpretation of the data benefited from our use of animals trained to make decisions guided by either visual or auditory evidence. Our results suggest that PPC plays a causal role specifically in visual decision making and may support sensory aspects of the decision, such as interpreting the visual signals so that evidence for a decision can be accumulated elsewhere.

Dynamic Reorganization of Neuronal Activity Patterns in Parietal Cortex

“…Similar results were obtained weeks after the mouse achieved plateau performance, suggesting that PPC activity was necessary for performing the task even in the post-learning phase. These results were consistent with our earlier pharmacological inactivation experiments and other studies showing a role for rodent PPC in visual decision tasks (Goard et al, 2016; Harvey et al, 2012; Licata et al, 2017; Raposo et al, 2014). We note that although inactivation was centered on PPC, such activity manipulations may have effects that spread beyond PPC (Otchy et al, 2015).…”

“…Also, because PPC activity was not required during the delay period, it seems unlikely that PPC had an essential short-term memory of the cue or upcoming action. Collectively the results here and from previous work support the hypothesis that PPC functions in the visual-to-motor transformation (Goard et al, 2016; Harvey et al, 2012; Licata et al, 2017; Raposo et al, 2014). This hypothesis is consistent with PPC’s connectivity in which it receives multisensory input, has recurrent connections with frontal regions, and has outputs to motor-related structures (Harvey et al, 2012).…”

Strategic and Dynamic Temporal Weighting for Perceptual Decisions in Humans and Macaques

“…This is of possible broader interest, for example, in linking to rodent work (Erlich et al, 2015, Scott et al, 2015, Morcos and Harvey 2016, Pinto et al, 2017, Odoemene et al, 2017, Licata et al, 2017). …”

“…In other instances, however, “late weighting” has been observed, where choices were primarily influenced by sensory evidence presented in late stimulus epochs (Tsetsos et al, 2012; Cheadle et al, 2014; Bronfman et al, 2016; Carland et al, 2016). In rodents, a mixture of either early or flat weighting profiles has been reported (Erlich et al, 2015; Scott et al, 2015; Pinto et al, 2017; Licata et al, 2017). …”

Models, movements, and minds: bridging the gap between decision making and action

“…47 Furthermore, the idea that LIP plays a causal role in evidence accumulation is being reevaluated in light of recent experiments implementing pharmacological or optogenetic LIP inactivation, which fail to show corresponding deficits in decision making. [48][49][50] These challenges to LIP-based decision models give rise to the idea that perhaps options are represented and selected elsewhere in the brain, but at the same time do not invalidate the bounded evidence accumulation mechanism.…”