FROM WALKING TO REACHING: SHARED NEURAL SUBSTRATES IN FREELY MOVING PRIMATES

In 1989, Georgopoulos and Grillner proposed that reaching evolved from walking, with motor cortical circuits for precise forelimb placement during locomotion later adapted for goal-directed hand movements. Despite its impact, this hypothesis has lacked direct empirical support in primates. In contrast, prevailing motor control theories posit functional segregation within the premotor cortex, assigning reaching and grasping to ventral regions and rhythmic locomotion to dorsomedial areas.
Here, we recorded single-unit activity (n = 470) from the ventral motor and premotor cortex of two freely moving monkeys during flat-ground walking and reaching-to-grasp food from the floor. Additional conditions included walking on an elevated structure requiring increased balance and postural control and reaching–grasping of suspended food captured mid-air using finger prehension.
To estimate the relative prevalence of walking- and reaching-related responses under comparable conditions, we examined sessions providing adequate sampling of all four behaviors for statistical evaluation (7 sessions, 321 single units). Among neurons significantly modulated during at least one action (n = 104), 36 were selective for reaching–grasping, 34 for walking, and 34 were active during both behaviors, showing either similar or behavior-specific response patterns. Incorporating kinematic variables showed that activity in neurons responsive to both behaviors could not be fully explained by movement kinematics alone, indicating involvement in higher-order neurobehavioral processes common to walking and reaching.
Together, these findings provide direct experimental evidence for shared cortical mechanisms underlying locomotion and voluntary reaching, supporting the hypothesis that reaching control may have evolved from locomotor circuits.