UC San Diego

The core function of the brain is to make meaningful decisions based on environmental stimuli. While the basic response properties of sensory regions have largely been agreed upon in recent times, a driving force of neuroscience research is how sensory information is integrated with biases, motivations, and assessments of internal state, and then transformed into robust choices and movements.

The primary motor cortex (M1) has long been known as the main controller of movement for some time, but the exact role of M1 and how it communicates with movement preparation-associated areas, like the secondary motor cortex (M2) and posterior parietal cortex (PPC), are still being elucidated. Together, these three areas form the parietal motor network, and their study is essential to understanding the sensorimotor transformation.

In the first experiment, mice were trained in a novel two-alternative forced-choice visual discrimination task, the requirement of these areas was probed through optogenetic inactivation. M1 inactivation caused animals to default choice to their more stereotyped direction. M2 inactivation caused similar deficits in discrimination, and also impaired hold performance. PPC was found to be non-essential for the task.

PPC is known to be involved in aspects of decision-making, motor coordination, and analysis of value. In the second experiment, mice trained to perform a visually-guided memory task developed a reward-choice history-dependent bias revealed through mathematical modeling. Optogenetic perturbation of PPC activity during the pre-stimulus, but not during the post-stimulus period, altered the internal bias. As a result, the model’s predictive capability was significantly lowered in inactivation trials.