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CNBC researchers investigate the neural basis of perceptual stability

Carol L. Colby
CNBC Faculty Webpage
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Laura Heiser, Eli Merriam, Becky Berman

 

CNBC researchers investigate the neural basis of perceptual stability

Carol Colby investigates a perceptual puzzle that occurs even as you are reading this text: each time you move your eyes, a new image appears on the retina. Despite this constantly changing input the world remains still. What are the brain mechanisms that give rise to this perceptual stability? Earlier investigations by Dr. Colby and her colleagues demonstrated that neurons in parietal cortex construct a dynamic spatial representation that is updated when the eyes move. Parietal neurons fire when an eye movement brings the receptive field onto a previously stimulated location after the eye movement.

 

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Recent studies in Dr. Colby's lab use the complementary techniques of functional neuroimaging and single-unit recording to shed light on the neural circuitry that underlies this transfer of visual information. Using functional magnetic resonance imaging, Colby and Merriam observed for the first time the transfer of visual information in humans (Figure 1). Neurons in the right hemisphere, which encode the initial retinal location of the stimulus, are active prior to the eye movement. When the eyes move, neurons in the left parietal cortex become active, even though no physical stimulus ever appeared in the right visual field (Figure 1B, green arrow). This result shows that spatial updating occurs in human parietal cortex.

What is the neural pathway for this transfer of visual information? In the example shown above, the stimulus location is updated from one visual field to the other, indicating that visual information was transferred from one hemisphere to the other. Colby, Berman, and Heiser asked whether this transfer depends on the forebrain commissures, the dense fiber bundles that interconnect the cerebral hemispheres. They measured the activity of parietal neurons in a monkey whose forebrain commissures were surgically transected (Figure 2). Parietal neurons were active when a stimulus was updated within a visual hemifield, as expected. But what do these neurons do when a stimulus is updated across visual hemifields, requiring a transfer of information across hemispheres? Remarkably, parietal neurons also responded in across-hemifield conditions. This unexpected finding demonstrates that updated information can reach parietal cortex even in the absence of the forebrain commissures, and suggest that subcortical structures, such as the superior colliculus, are part of the neural circuit that keeps the world still.

 

 

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Merriam, E.P., Genovese, C.R., Colby, C.L. (2003). Spatial updating in human parietal cortex. Neuron. 39, 361-373.