DELAY-PERIOD ACTIVITY IN THE PRIMATE PREFRONTAL CORTEX ENCODING MULTIPLE SPATIAL POSITIONS AND THEIR ORDER OF PRESENTATION

To investigate whether prefrontal neurons temporarily retain informati on regarding multiple spatial positions, single-neuron activity was re corded while monkeys performed a delayed sequential reaching task, in which they needed to remember two cue positions out of three and their temporal order of presentation. Most neurons were also tested on a co nventional delayed reaching task, in which they needed to remember one cue position during the delay. Among 72 neurons that exhibited signif icant delay-period activity, one group of neurons (n=19) exhibited del ay-period activity only when a visual cue was presented at one of the three positions (position-dependent). Of these, 6 neurons exhibited th is activity when a cue was presented at that position independent of t he temporal order, whereas 13 neurons exhibited this activity only whe n a cue was presented at that position in a particular temporal order (e.g., as the first cue or the second cue). Another group of neurons ( n=39) exhibited delay-period activity only when visual cues were prese nted at two positions out of three (pair-dependent). Of these, 7 neuro ns exhibited pair-dependent activity independent of the temporal order of cue presentation. However, 32 neurons exhibited this activity only when two cues were presented in a particular temporal order. The rema ining 11 neurons exhibited non-differential activity during the delay period and 3 neurons exhibited miscellaneous activity. These results s how that a single prefrontal neuron can retain information regarding t wo spatial positions, and that, to retain two spatial positions and th e temporal order of cue presentation, new types of delay-period activi ty emerged; i.e., pair-dependent activity and temporal order-dependent activity. Both types of activity could be a mechanism for simultaneou sly retaining two items of spatial information and for effectively com bining multiple spatial information by a single neuron. In addition, t he presence of delay-period activity with position-dependency, pair-de pendency and temporal order-dependency suggests that the dorsolateral prefrontal cortex plays an important role in planning sequential behav iors.