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1The Laboratory of Sensorimotor Research, National Eye Institute, Bethesda, Maryland; 2Department of Neurology, Georgetown University School of Medicine, Washington, DC; 3Mahoney Center for Brain and Behavior, Center for Neurobiology and Behavior, Columbia University College of Physicians and Surgeons, and the New York State Psychiatric Institute, New York; and 4Departments of Neurology and Psychiatry, Columbia University College of Physicians and Surgeons, New York, New York
Submitted 11 August 2005; accepted in final form 1 December 2005
We examined the activity of neurons in the lateral intraparietal area (LIP) during a task in which we measured attention in the monkey, using an advantage in contrast sensitivity as our definition of attention. The animals planned a memory-guided saccade but made or canceled it depending on the orientation of a briefly flashed probe stimulus. We measured the monkeys' contrast sensitivity by varying the contrast of the probe. Both subjects had better thresholds at the goal of the saccade than elsewhere. If a task-irrelevant distractor flashed elsewhere in the visual field, the attentional advantage transiently shifted to that site. The population response in LIP correlated with the allocation of attention; the attentional advantage lay at the location in the visual field whose representation in LIP had the greatest activity when the probe appeared. During a brief period in which there were two equally active regions in LIP, there was no attentional advantage at either location. This time, the crossing point, differed in the two animals, proving a strong correlation between the activity and behavior. The crossing point of each neuron depended on the relationship of three parameters: the visual response to the distractor, the saccade-related delay activity, and the rate of decay of the transient response to the distractor. Thus the time at which attention lingers on a distractor is set by the mechanism underlying these three biophysical properties. Finally, we showed that for a brief time LIP neurons showed a stronger response to signal canceling the planned saccade than to the confirmation signal.
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