Figure 1.

Psychophysical and physiological methods. A, Schematic of the motion direction change detection task. Once the monkey fixated a central spot, three small Gabor stimuli synchronously flashed on for 200 ms and off for a randomized interval between 200 and 400 ms. Two stimuli were positioned inside the joint receptive fields of the V1 and MT units that we recorded and a third was positioned in the opposite hemifield. After an unsignaled and randomized number of stimulus presentations (picked from an exponential distribution, minimum two, mean six, maximum 14 stimulus presentations), one stimulus moved in a different direction. The monkeys were cued in blocks of 50–100 trials to detect changes in (and therefore attend to) one of the stimuli and ignore motion direction changes in the other stimuli. The monkey was rewarded for making a saccade to the attended stimulus within 500 ms of the stimulus change. Responses to distractor changes were never rewarded. The two stimuli in the MT receptive field moved in opposite directions (the preferred and null directions of the MT cell under study) and which of the two stimuli moved in the preferred direction varied randomly from trial to trial. The third stimulus, when present, moved in an orthogonal direction. B, Receptive fields and visual stimuli from an example experimental session. We recorded simultaneously from a 96-channel chronically implanted microelectrode array in area V1 and either a single electrode or a movable 24-channel probe in area MT. We selected MT units with receptive fields (black dashed circle) that overlapped the envelope of receptive fields of the units that we recorded in V1 (centers denoted by the gray dots). The locations and approximate sizes of the visual stimuli are denoted by solid black circles. Spatial receptive fields were estimated by rapidly presenting a single, small Gabor stimulus at a range of locations while the monkey fixated.