Abstract
The dorsolateral prefrontal (dlPFC) and posterior parietal cortex (PPC) represent visuospatial information and are both activated during tasks requiring orienting of attention; however, the distinct roles of PFC and PPC in visual bottom-up attention are still controversial. We used three behavioral tasks that orient attention based on bottom-up factors and compared the time course of neuronal activity of area 46 of dlPFC and areas LIP and 7a of PPC. In the first task, monkeys were trained to identify a salient stimulus surrounded by distractor stimuli on the screen (color and location varied randomly from trial to trial) and to release a lever when another stimulus appeared at the same location. Analysis was performed on 222 dlPFC neurons, 104 LIP neurons, and 90 7a neurons with significant responses to visual stimuli. Surprisingly, the average time of target discrimination was earlier in dlPFC neurons (120–130 ms) than in LIP neurons (140 ms) and 7a neurons (160 ms), even though visual response latencies of PPC neurons were no longer than PFC latencies. The results indicate that salient stimuli defined by bottom-up factors are represented first in the activity of prefrontal than parietal neurons in this task. In order to investigate further how the timing of neuronal activity in prefrontal and parietal cortex determines the animals' behavioral responses, monkey's reaction times were tested in the second task by requiring them to release the lever as soon as they found the salient stimulus in an array. We varied the task difficulty by adjusting the color (hue) of distractor stimuli. The average lever releasing time increased as a function of task difficulty. In the third task, monkeys naïve to visual search tasks passively viewed stimulus sets. Neural and behavioral results from these experiments will provide insights into the role of prefrontal and parietal cortex in orienting of bottom-up attention.