UCSF

For many behaviors we rely on our senses, which inform the brain about the

world around us, such as the spatial location of an object we would like to grasp or the

motion of a target that we intend to follow with our eyes. In order to execute an

appropriate movement, neural responses in sensory areas must be read out and

transformed into signals meaningful to premotor and motor areas so an accurate motor

command can evolve. I used smooth pursuit eye movements to study the sensory-motor

transformation of visual signals about object motion into an appropriate motor command

for accelerating the eyes. I found that fluctuations in single neuron responses in visual

area MT are predictive of deviations in eye speed during pursuit initiation, which

suggests that responses in MT contribute to noise in the motor output. Further, the

relationship between the sensory and motor variability revealed constraints about the

neural mechanisms underlying the transformation of MT signals into a command for eye

speed: if downstream noise is low, a modified vector averaging computation involving

opponent signals between oppositely tuned neurons could explain the relationship

between responses of single neurons in area MT and eye speed at the initiation of pursuit.

In addition to smooth pursuit eye movements, we studied MT responses during

drifts in eye movements of fixation. I found that deviations in sensory responses not only

predict variation in pursuit initiation, but that deviations in eye velocity during fixation

also modulate the neural responses in MT. I showed that firing rates in MT are modulated because tiny changes in eye velocity generate image motion on the retina. These results demonstrate that image motion due to miniature eye movements is large enough to affect the responses of visual neurons beyond the retina or early cortical visual areas.

This thesis has underscored how tightly sensory and motor systems are

interwoven; visual area MT, in particular, forms an important link between the processing

of visual signals and the generation of motor commands for smooth pursuit eye

movements.