We examined the activity of neurons in 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 upon 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 2 equally active regions in LIP, there was no attentional advantage at either location. This time, the crossing point, differed in the 2 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 3 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.