Neural Control of Movement Vision gives rise to both the perception of what we are looking at, and an action (eye movements to the target). This project is devoted to understanding the nature of neuronal and muscular mechanisms required for clear vision. Our interest in normal behavior is refined by a focus on problems that lead to clinical eye movement disorders, and misalignment of the two eyes (strabismus). Recent studies have shown that systems for vision and action interact, and thus a fuller understanding of our visual system requires study of both motor and sensory systems. We are looking into the network of areas that provide for action and perception to understand how they may be coupled. Eye-Head Coordination Gaze saccades (coordinated eye and head movements) could be controlled by a single gaze controller, with eye and head movements sharing a common motor drive. Alternatively, gaze could be controlled by coupling together separate controllers for the eye and the head. However, experimental evidence has shown that eye and head trajectories can be decoupled by changes in initial eye and head position. This evidence has led to us to propose a new model of gaze saccades based on two separate controllers, one for gaze and one for head, but without a separate controller for the eye. Clinical Eye Movement Disorders We have looked at several clinical eye movement disorders. Our recent models of brain stem neurons include the biophysical properties of voltage- and ligand-dependent ion channels. This has enabled us to model many eye movement disorders. In the past, we have used models of neurons that represented their state as a continuous variable (i.e., the cell's membrane potential). Now, we are using spiking neurons that communicate more like real neurons. This may be key to understanding the role of the brain's mathematical integrator. Visual Perception Strabismus, the abnormal alignment of the two eyes, is an important clinical problem. How the brain knows when the eyes are aligned requires matching the same features in the images from each eye. This is not easy to do, because each visual neuron sees only a tiny part of the whole image. We have been carrying out experiments in human subjects that show this feature matching does not occur only through a bottom-up comparison, but also requires a temporally slower feedback component. Another important aspect of vision is the time it takes to perceive a new object. When the new object occurs in a peripheral location it is common for an eye movement to bring the image of the object onto the fovea (the area of highest visual acuity). It seems obvious that perception and action should be tightly coupled in time. There is no point in looking at an object you cant perceive, and vice versa. Nonetheless, the pathways for vision and action are different, and whether or how they may be coupled remains unknown. We have been studying the saccadic reaction time, the decision reaction time, and the content of the percept in human subjects. Surprisingly, we find that all three share a common time course, suggesting that they may be sharing the same trigger for their response.
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