The posterior parietal cortex encodes the first and second goal in sequential reaching arm movements

The parietal reach region (PRR) has been found to be involved in the preparation of goal-directed arm movements to single targets. In this study we explored whether the reach planning activity in this region encodes only the target of the very next movement or, alternatively, also a subsequent goal in a fast reaching sequence. We trained two macaque monkeys to perform sequential reaches to peripheral positions without breaking eye fixation. While the monkey initially positioned his hand at a circle in the center of the touch screen a small triangle and square were presented in the periphery for 400 ms and cued the first and second movement goal, respectively. After these cues disappeared the animal had to memorize the locations of the two targets for 600 ms and to prepare for a rapid double reach sequence. Once the central circle turned off (GO-signal) the animals had to reach in a predetermined order first to the location where the little square had been presented and then immediately to the location where the triangle had been. The movement goals were arranged such that either the target for the first reach or the target for the second reach was inside the response field of an isolated neuron. We analyzed the neural activity of single cells in PRR during the last 400 ms of the memory period between cue offset and the GO-signal. Most cells in PRR encoded the first as well as the second reach of the planned reaching sequence in advance of movement initialization, although the response for the second target is usually less than for the first. Among the 43 neurons recorded so far 31% were only active when the first target was inside the cell's response field and 10% only if the second, final goal location lay in the cell's preferred direction. The majority of cells (59%), however, showed significantly elevated activity during the late delay period for either target in the response field. The results indicate that the posterior parietal cortex is involved in the spatial planning of more complex action patterns and represents immediate and subsequent movement goals.