Vector Averaging for Smooth Pursuit Eye Movements Initiated by Two Moving Targets in Monkeys

Abstract

The visual input for pursuit eye movements is represented in the cerebral cortex as the distributed activity of neurons that are tuned for both the direction and speed of target motion. To probe how the motor system uses this distributed code to compute a command for smooth eye movements, we have recorded the initiation of pursuit for 150 msec presentations of two spots moving at different speeds and/or in different directions. With equal probability, one of the two spots continued to move at the same speed and in the same direction and became the tracking target, whereas the other disappeared and served as a distractor. We measured eye acceleration in the interval from 110 to 206 msec after the onset of spot motion, within both the open-loop interval for pursuit and the interval during which eye motion was affected by the two spots. Our results demonstrate that weighted vector averaging is used to combine the responses to two moving spots. We found only a minute number of responses that were consistent with either vector summation or winner-take-all computations. In addition, our data show that it is difficult for the monkey to defeat vector averaging without extended training on the use of an explicit cue about which spot will become the target. We argue that our experiment reveals the computations done by the pursuit system in the absence of attentional bias and that vector averaging is normally used to read the distributed code of image motion when there is only one target.

Middle Temporal Visual Area Microstimulation Influences Veridical Judgments of Motion Direction

“…In a perceptual discrimination task, Salzman and Newsome (1994) found evidence favoring a winnertake-all readout of MT activity. In a second approach, Groh et al (1997) microstimulated MT during the initiation phase of smooth pursuit eye movements and found clear evidence of a vectoraveraging readout of MT, a conclusion that was supported by subsequent behavioral studies of pursuit initiation (Lisberger and Ferrera, 1997).…”

Temporal dynamics of motion integration for the initiation of tracking eye movements at ultra-short latencies

“…Most of these studies used dots (either single spot or random-dots flowfields) moving in different directions across different parts of the visual field. Lisberger and Ferrera (1997) assumed that "the pursuit system uses the same computation to combine information from the two spatial locations [we used] as it does for a single location" (page 7500). The present study indicates that a vector combination strategy is used to recover the 2D direction of object motion.…”

How Is a Sensory Map Read Out? Effects of Microstimulation in Visual Area MT on Saccades and Smooth Pursuit Eye Movements

“…A wide variety of studies have found evidence for vector averaging in the saccadic system using both physiological and behavioral techniques (Robinson and Fuchs, 1969;Robinson, 1972;Becker and Jurgens, 1979;Schiller et al, 1979;Findlay, 1982;Schiller and Sandell, 1983;du Lac and Knudsen, 1987;Lee et al, 1988;du Lac, 1989;van Opstal and van Gisbergen, 1990;McIlwain, 1991;Glimcher, 1993). Recently, Lisberger and Ferrera (1996) have examined the pursuit responses of monkeys when two moving visual targets are presented. They found evidence for both vector averaging (Lisberger and Ferrera, 1996) and winner-take-all mechanisms (Ferrera and Lisberger, 1995) depending on the conditions of the task.…”

“…Recently, Lisberger and Ferrera (1996) have examined the pursuit responses of monkeys when two moving visual targets are presented. They found evidence for both vector averaging (Lisberger and Ferrera, 1996) and winner-take-all mechanisms (Ferrera and Lisberger, 1995) depending on the conditions of the task.…”