Forward prediction embodied in cortical preparatory dynamics

Prevalent dynamical systems view posits that cortical preparatory activity seeds temporal evolution to generate movement, but it remains unclear the neural computation that preparatory activity develops to initialize movements in dynamic sensorimotor contingency, such as interception of motion targets. In the present study, we analyzed neural population dynamics recorded from motor cortex via Utah array while two macaques performed a delayed reach toward a target either static or moving circularly. Principal component analysis (PCA) showed that the directional tuning of preparatory latent factors was stable in the static condition but continuously updating in the motion condition. Then, we determined a 2-dimensional auxiliary subspace by two steps: 1). Constructed a 2-dimensional subspace from the latent factors in static condition; 2). Rotated the subspace so that latent factors matched the corresponding target in each direction in static condition. Leveraging the auxiliary subspace, the projected latent factors shared the same angle coordinate with the visual target in both conditions. We defined the neural angle as the function atan2(PC2, PC1) in the auxiliary subspace and compared it to target direction. Intriguingly, the neural angle was ahead of the target angle in motion direction. We calculated the correlation metrics, which measured the correlation between the target angle and shifted neural angles, as a function of the time shift in neural angles. We determined the tau value, which corresponds to the peak of the metrics, was approximately the total time that reaction time plus movement time. Our finding suggested that motor cortex is involved in an iterative neural computation of forward prediction to compensate the sensorimotor delays in dynamic sensorimotor transformation. The iterative prediction embodied in cortical preparatory activity might reflect forward model that encapsulates knowledge of the external world.