VISUAL PROCESSING IN THE PRIMATE BRAIN

National Eye Institute

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Abstract

Rapid or saccadic eye movements are those that move the eyes quickly from one part of the visual scene to another. The areas of the cerebral cortex of non-human primates (an excellent animal model of the human saccadic system) related to the generation of these movements have been identified, and it is generally assumed that activity in these cortical areas is transmitted to brainstem areas that then generate the movement. We have investigated this pathway and found that the activity is not only projected to the brainstem, but is projected back to cortex as well. By stimulating the superior colliculus on the roof of the brainstem, neurons in the thalamus (medialis dorsalis nucleus) were activated orthodromically showing that these neurons received input from the superior collculus. The same thalamic neurons were antidromically activated from the frontal cortex (the frontal eye field) which indicated that the neurons projected to the frontal cortex. The neurons that were in this pathway from superior colliculus to cortex had activity that was similar to that in the colliulus; they responded to visual stimuli and they increased their activity before the onset of saccadic eye movements. The time of transmission of information from colliculus to cortex was frequently less than three msec so that the information was transmitted to the cortex with great rapidity. The significance of both the type of signal conveyed (visual and eye movement related) and its speed is that when examining the activity of the cerebral cortical neurons, it is not possible to determine whether their activity is going to the colliculus or coming from it. The only way to address this issue is to open this "loop" between cortex and colliculus and examine the activity in the open loop condition and this work is now underway.