Neurons in area 5 of the posterior parietal cortex in the cat contribute to interlimb coordination during visually guided locomotion: a role in working memory.

We tested the hypothesis that area 5 of the posterior parietal cortex (PPC) contributes to interlimb coordination in locomotor tasks requiring visual guidance by recording neuronal activity in this area in three cats in two locomotor paradigms. In the first paradigm, cats were required to step over obstacles attached to a moving treadmill belt. We recorded 47 neurons that discharged in relationship to the hindlimbs. Of these, 31/47 discharged between the passage of the fore- and hindlimbs (FL-HL cells) over the obstacle. The activity of most of these neurons (25/31) was related to the fore- and hindlimb contralateral to the recording site when the contralateral forelimb was the first to pass over the obstacle. In many cells, discharge activity was limb-independent in that it was better related to the ipsilateral limbs when they were the first to step over the obstacle. The other 16/47 neurons discharged only when the hindlimbs stepped over the obstacle with the majority of these (12/16) discharging between the passage of the two hindlimbs over the obstacle. We tested 15/47 cells, including 11/47 FL-HL cells, in a second paradigm in which cats stepped over an obstacle on a walkway. Discharge activity in all of these cells was significantly modulated when the cat stepped over the obstacle and remained modified for periods of ≤ 1 min when forward progress of the cat was delayed with either the fore- and hindlimbs, or the two hindlimbs, straddling the obstacle. We suggest that neurons in area 5 of the PPC contribute to interlimb coordination during locomotion by estimating the spatial and temporal attributes of the obstacle with respect to the body. We further suggest that the discharge observed both during the steps over the obstacle and in the delayed locomotor paradigm is a neuronal correlate of working memory.