Podcast with Jon Kaas on motor cortex and posterior parietal cortex

What if the motor cortex does not just encode movements but organizes entire behavioral repertoires, reaching, grasping, defending, across three interconnected cortical stages? Neuroanatomist Jon Kaas describes how long-duration electrical stimulation reveals a modular architecture for goal-directed action in primates that challenges standard views of motor control. Subscribe for more from the Convergent Science Network podcast series. Jon Kaas joins Paul Verschure and Tony Prescott at the BCBT summer school to present his research on the functional organization of the primate motor system, spanning prosimian galagos, New World monkeys, and macaques. Using half-second electrical stimulation pulses, Kaas and colleagues discovered that specific behavioral patterns , hand-to-mouth movements, defensive gestures, reaching, grasping , can be evoked from small, corresponding regions in posterior parietal cortex, premotor cortex, and primary motor cortex. These three stages form a hierarchical but parallel system where posterior parietal cortex integrates high-level sensory information, premotor cortex contributes executive and motivational inputs, and motor cortex provides the critical output. The discussion explores how this organization differs from the standard population-vector model of motor encoding and how it relates to subcortical control. Cooling experiments demonstrate that motor cortex is required for the other stages to produce movements, confirming a hierarchical dependency. Tracer injections reveal that corresponding behavioral zones across all three cortical stages converge on the same regions of the basal ganglia, suggesting a role for subcortical structures in learning and modulating these cortical action modules. Kaas argues that posterior parietal cortex expanded dramatically in primate evolution, adding cortical control over behaviors that were previously managed subcortically. Key topics include how long-duration stimulation reveals behavioral organization invisible to standard mapping, why posterior parietal cortex is a primate innovation with multimodal sensory inputs, how inhibitory connections between behavioral zones enable competition and action selection, what the scaling challenges are from galago to human motor repertoires, and whether the modular organization of stereotyped behaviors can accommodate the arbitrary, learned action sequences that characterize human performance. Part of the Convergent Science Network podcast series from the BCBT Summer School.