Functional heterogeneity of macaque lateral intraparietal neurons.

The macaque lateral intraparietal area (LIP) has been implicated in many cognitive processes, ranging from saccade planning and spatial attention to timing and categorization. Importantly, different research groups have used different criteria for including LIP neurons in their studies. While some research groups have selected LIP neurons based on the presence of memory-delay activity, other research groups have used other criteria such as visual, presaccadic, and/or memory activity. We recorded from LIP neurons that were selected based on spatially selective saccadic activity but regardless of memory-delay activity in macaque monkeys. To test anticipatory climbing activity, we used a delayed visually guided saccade task with a unimodal schedule of go-times, for which the conditional probability that the go-signal will occur rises monotonically as a function of time. A subpopulation of LIP neurons showed anticipatory activity that mimicked the subjective hazard rate of the go-signal when the animal was planning a saccade toward the receptive field. A large subgroup of LIP neurons, however, did not modulate their firing rates according to the subjective hazard function. These non-anticipatory neurons were strongly influenced by salient visual stimuli appearing in their receptive field, but less so by the direction of the impending saccade. Thus, LIP contains a heterogeneous population of neurons related to saccade planning or visual salience, and these neurons are spatially intermixed. Our results suggest that between-study differences in neuronal selection may have contributed significantly to the findings of different research groups with respect to the functional role of area LIP.