Single Units in the Posterior Parietal Cortex Encode Patterns of Bimanual Coordination

Abstract

Bimanual coordination is critical for a broad array of behaviors. Drummers, for example, must carefully coordinate movements of their 2 arms, sometimes beating on the same drum and sometimes on different ones. While coordinated behavior is well-studied, the early stages of planning are not well understood. In the parietal reach region (PRR) of the posterior parietal cortex (PPC), the presence of neurons that modulate when either arm moves by itself has been taken as evidence for a role in bimanual coordination. To test this notion, we recorded neurons during both unilateral and bimanual movements. We find that the activity that precedes an ipsilateral arm movement is primarily a sensory response to a target in the neuron's visual receptive field and not a plan to move the ipsilateral arm. In contrast, the activity that precedes a contralateral arm movement is the sum of a movement plan plus a sensory response. Despite not coding ipsilateral arm movements, about half of neurons discriminate between different patterns of bimanual movements. These results provide direct evidence that PRR neurons represent bimanual reach plans, and suggest that bimanual coordination originates in the sensory-to-motor processing stream prior to the motor cortex, within the PPC.

Spatial eye–hand coordination during bimanual reaching is not systematically coded in either LIP or PRR

“…PRR cells, in contrast, showed greater modulation for contralateral compared with ipsilateral arm reach (25.55 ± 1.87 versus 11.34 ± 1.37 sp/s, significant at P < 0.05 for 69% of the 89 individual cells) or compared with saccade-only trials (13.26 ± 1.57 sp/s, significant for 73% of 89 cells). Thus, as previously described, LIP encodes primarily the presence or absence of a saccade into the RF, while PRR primarily encodes a contralateral arm reach into the RF, with about half as much modulation for effectors other than the contralateral arm ( 6 ).…”

“…Previous studies show that PRR cells preferentially encode upcoming arm movements compared with eye movements and that the majority of cells preferentially encode movements of the contralateral rather than the ipsilateral arm ( 4 , 6 , 7 , 42 ). If PRR is involved in eye–hand coordination, then we might expect that PRR cells will reflect whether a saccade accompanies one arm or the other during a bimanual-apart reach or whether the saccade will be made into or out of the RF.…”

“…In monkeys, cells in different parts of the PPC encode spatial locations of interest to particular effectors. For example, the parietal reach region (PRR), situated at the posterior end of the intraparietal sulcus (IPS) and overlapping portions of the medial intraparietal area (MIP) and V6a, contains cells that encode the direction or endpoint of an upcoming reach ( 3 – 5 ), particularly for the contralateral arm ( 6 , 7 ). Similarly, the lateral intraparietal area (LIP), located midway along the lateral bank of the IPS, contains cells that encode the direction or endpoint of an upcoming saccade ( 8 – 13 ).…”

Maintained representations of the ipsilateral and contralateral limbs during bimanual control in primary motor cortex

“…The parietal reach region (PRR) also displays neural representations related to motor actions of both limbs (Kermadi et al, 2000; Chang et al, 2008; Mooshagian et al, 2018). PRR is primarily involved with controlling the contralateral limb (Chang et al, 2008; Yttri et al, 2013), however neurons in PRR respond prior to movements of the contralateral and ipsilateral limbs as well as upcoming saccades (Chang et al, 2008; Chang and Snyder, 2012).…”

“…PRR is primarily involved with controlling the contralateral limb (Chang et al, 2008; Yttri et al, 2013), however neurons in PRR respond prior to movements of the contralateral and ipsilateral limbs as well as upcoming saccades (Chang et al, 2008; Chang and Snyder, 2012). However, this ipsilateral activity is predominantly related to a sensory response to the visual target, whereas responses for the contralateral limb are related to both the sensory event and motor planning (Mooshagian et al, 2018).…”