Responses of Intraparietal Neurons to Saccadic Targets and Visual Distractors

Current evidence suggests that neuronal activity in the lateral intraparietal area (LIP) reflects sensory-motor processes, but it remains unclear whether LIP activation participates directly in the planning of future eye movements or encodes data about both sensory events and the behavioral significance of those sensory events. To examine this issue, 31 intraparietal neurons were studied in awake, behaving monkeys trained to perform two tasks that independently controlled the location of a saccadic target and the location and behavioral relevance of a visual distractor. In both of these tasks, two eccentric light-emitting diodes (LEDs) were illuminated yellow, one above and one below a fixation stimulus. Shortly after the eccentric LEDs were illuminated, a change in the color of the fixation stimulus indicated which of these LEDs served as the saccadic goal and which served as a visual distractor. In the first or distractor-irrelevant task, fixation offset indicated that the subject must initiate a saccade shifting gaze to the saccadic goal. In the second or distractor-relevant task, distractor offset served as the saccade initiation cue. Intraparietal neurons responded more strongly in association with an LED that served as a saccadic target than in association with the same LED when it served as a visual distractor. Neuronal responses in association with either target or distractor stimuli on distractor-relevant and distractor-irrelevant blocks of trials were statistically indistinguishable. When the location of either the target or the distractor was varied across trials, the response of each neuron in association with a particular stimulus location was always greater for targets than for distractors and the magnitude of this response difference was independent of distractor relevance; however, distractors were nearly always associated with some intraparietal neuronal activity. A target/distractor selectivity index was computed for each neuron as the difference between responses associated with targets minus responses associated with distractors divided by the sum of these values. When the selectivity of each neuron on the distractor-relevant task was plotted against the selectivity of the same neuron on the distractor-irrelevant task, activity in the population of intraparietal neurons was found to be independent of distractor relevance. These data suggest that LIP neuronal activation represents saccadic targets and, at a lower level of activity, visual distractors, but does not encode the relevance of distractor stimuli on these tasks.