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Two major processing streams in the brain are involved in the neural control of visually guided eye movements, neuroscientist Peter H. Schiller reported.
Schiller, the D.W. Poitras Professor of Brain and Cognitive Sciences, organized the Feb. 17 AAAS symposium titled "How the Brain Selects Objects and Commands Movement." Emilio Bizzi, the E. McDermott Professor of Brain and Cognitive Sciences, also reported on separate results that provide evidence for single-cell plasticity in the primary motor cortex of primates.
Bizzi and colleagues found that a subpopulation of brain cells in the part of the cortex that controls movements acquires novel firing patterns while an animal learns a new set of voluntary movements.
The data indicate that the nervous system constantly reorganizes itself to deal with new motor acts needed in a new environment. Any time the nervous system encounters a new environment, a subpopulation of neurons change to accommodate the new conditions. This "suggests that neural plasticity is the rule rather than the exception," the researchers said.
A quick glance
An important part of perceptual activity is the numerous eye movements that must be made during even the shortest task. Most of the photoreceptors in the eye are in a small region in the center of the retina called the fovea. Often, the eyeball has to move so that the image of an object that requires detailed perceptual analysis projects onto this high-resolution area. The method by which this is done is known as a saccade.
Numerous cortical areas are involved in the generation of saccadic eye movements. Unlike the slow, smooth eye movements that follow a steadily moving target, saccadic eye movements are rapid and reflex-like.
Schiller and colleagues looked at the functions of the various areas of the brain that generate visually guided eye movements. The researchers applied several methods to monkeys that were trained to do tasks involving making eye movements to visual targets. They found that two major processing streams are involved in visually guided saccadic eye movements: the anterior and the posterior.
The posterior system, originating in the occipital and parietal cortices, sends its prime signal for saccade generation through the superior colliculus. That structure, located in the midbrain, plays a role in helping orient the head and eyes to all types of sensory stimuli.
The anterior system, originating in the frontal and medial eye fields, has direct access to the brainstem. It receives profuse projections from many other cortical areas, including the parietal lobe. Interconnections among the structures of the anterior and posterior systems assure that eye movements are generated cohesively, Schiller said.
A version of this article appeared in MIT Tech Talk on February 27, 2002.