The posterior parietal cortex does more than map space
The posterior parietal cortex has long been taught as the brain’s spatial awareness and action-planning hub. That’s true, but incomplete. A University of Chicago team led by David Freedman, Ph.D., with postdoctoral researcher Yang Zhou, published in Science and reported by Neuroscience News on July 11, 2019, shows the PPC, and specifically the lateral intraparietal area, plays a causal role in visual decision making by making sense of what’s seen.
Traditionally this part of the brain has been thought to be involved in controlling spatial attention and planning actions. There has been less attention paid to how much of a role this brain area plays in processing the visual stimuli themselves, Freedman said. Here we were able to show that it plays an important role in making sense of the things we see, perhaps even more so than its role in planning your next action or directing your attention.
If a region long tied to where to look is actually weighing what you’re looking at, models of perception-action need updating.
How the dot-motion game split seeing from pointing
Freedman and Zhou trained monkeys on a computer game that decouples sensory evaluation from motor output. On each trial a pattern of dots moved across the screen, either up and to the left or down and to the right. The animal reported its choice by shifting gaze to a green target for one direction and a red target for the opposite.
The sensory judgment is which way are the dots moving. The motor response is look at green or red. By holding the mapping constant, the team could ask whether a brain area contributes to reading the stimulus or to executing the eye movement.
They focused on the lateral intraparietal area, LIP, a subregion of the PPC. Is LIP involved in guiding these decisions?
Silencing LIP hurts the judgment, not the movement
To test causality, the researchers administered a drug that temporarily halted neural activity in LIP, then ran the same dot-motion task. While the drug was active, the monkeys’ decisions about the visual patterns they viewed were impaired. Once the drug wore off, their decisions returned to normal.
That transient loss is the key. It’s a reversible knock-out that demonstrates a causal role in the moment-by-moment process of making sense of visual input.
The researchers also recorded activity in the same pool of neurons once the drug had worn off and found activity in that area was indeed strongly correlated with the same kinds of decisions which had been impaired during the experiments. Neural coding and behavior lined up.
Critically, inactivation specifically impaired the monkeys’ decisions about the visual patterns, the sensory component, while leaving motor planning intact. The animals could still generate the appropriate eye movements toward the correct color-coded target. They just couldn’t use the visual information correctly to guide those movements. When LIP activity was restored, decision accuracy recovered.
The paper abstract, “Posterior parietal cortex plays a causal role in perceptual and categorical decisions,” states inactivation affected both motor and sensory aspects of behavior, but preferentially impaired decisions when visual stimuli, rather than motor response targets, were in the inactivated visual field. This demonstrates a causal role for PPC in decision-making, with preferential involvement in evaluating attended task-relevant sensory stimuli compared with motor planning.
Why the 2016 Nature null result actually fits
This finding gives new context to a puzzling earlier report. A 2016 Nature study reported that deactivating parts of LIP seemed not to have any impact on decision making. The difference is domain.
That study only examined LIP’s role in motor planning, such as the decision about whether to look leftwards or rightwards. In contrast, the current work shows LIP is more involved in making sense of the visual images the subjects are viewing, rather than deciding which actions they should take next.
All the neuronal data we examined in our past experiments gave us the impression that this area of the brain was involved in processing the meaning of visual images during decision making, Freedman said. Now we find that indeed when we temporarily shut the activity down in that part of the brain it really does affect the sensory parts of decisions.
The earlier null result wasn’t wrong. It was testing the wrong leverage point. LIP does contribute to motor planning, but its primary, preferential role is sensory evaluation.
What changes for models of visual categorization
Freedman argues the study forces a rethink of fundamental brain mechanisms. If a brain area previously associated with spatial attention and action planning is instead or also critical for interpreting visual evidence, models of decision-making circuitry must account for this dual role.
The work also has implications for visual categorization. Parietal cortex helps determine not just where to look but what the seen information means. That’s an upgrade from a waypoint for attention to an evaluator of evidence.
These results show that the brain’s parietal cortex is an important hub for guiding decisions, so now we’re even more motivated to move ahead and try to work out the details of neural circuits in this part of the brain that actually carries out these cognitive functions, he said.
From monkeys to clinical motivation
Beyond basic science, the study points toward translational relevance. A deeper understanding of how the brain evaluates perceived information could inform treatments for disorders that affect decision making, ranging from certain neurodegenerative diseases to conditions involving impaired sensory integration.
Freedman is motivated to map the neural circuits in PPC that carry out these cognitive functions, with the long-term goal of translating this knowledge into therapeutic strategies. Understanding this process in detail will be critical for developing new treatments for brain-based diseases and disorders which affect decision making.
The posterior parietal cortex is more than a waystation for spatial attention or motor commands. It’s an active evaluator of visual evidence, a gatekeeper that determines which sensory inputs shape our choices. By teasing apart LIP’s role in sensory evaluation from its role in motor planning, Freedman and Zhou give a clearer picture of how the brain turns seeing into deciding.
Source: https://neurosciencenews.com/decision-making-spatial-awareness-14464/
