Delay-related activity in the primate prefrontal cortex during sequential reaching tasks with delay

To investigate how prefrontal neurons store multiple spatial locations simultaneously, neuronal activity during delay was analyzed while a monkey performed a delayed sequential reaching (DSR) task, in which the monkey was required to store both locations and the order of two sequentially presented targets during delay, and a delayed reaching (DR) task with single target. 38 out of 61 task-related neurons showed delay-related activity and were classified into three types. Type 1 (58%) showed delay-related activity depending upon the direction of the first reaching movement. Type 2 (21%) showed delay-related activity depending upon whether the reaching cue was presented at either left or right. Type 3 (21%) showed delay-related activity nonspecifically during all trials of both DSR and DR. Neurons with delay-related activity only in DSR trials were not observed. These results suggest that each prefrontal neuron may not hold information for the whole sequence of a complex movement during delay, but may hold partial information of that movement, such as one target location or one movement direction.