Population coding of saccadic eye

Neurobiology Research Center, Department of Physiology and tions and amplitudes2,7,8 . These neurons form a map of motor Biophysics, University of Alabama, Birmingham, (saccadic) space as they are topographically organized on the Alabama 35294, USA basis of their movement fields2,7,9 . The high-frequency burst of spike activity generated by collicular neurons is tightly coupled The deeper layers of the superior colliculus are involved in the temporally to saccade onset10, but spatially the movement fields initiation and execution of saccadic (high velocity) eye movements' . are large and coarsely tuned 2 . Consequently a large population A large population of coarsely tuned collicular neurons is active of cells is active before each saccade . Here we reversibly deactibefore each saccade . The mechanisms by which the signals that vated a small subset of the active population with lidocaine . precisely control the direction and amplitude of a saccade are Figure 1 outlines the rationale of the experiment and illustrates extracted from the activity of the population are unknown . It has the main predictions of the population-averaging hypothesis . It been assumed'` that the exact trajectory of a saccade is determined is assumed that the population of neurons discharging before by the activity of the entire population and that information is a saccade occupies a symmetrical area 3 . For example, the not extracted from only the most active cells in the population at hypothetical region of neurons active before a 5° rightward a subsequent stage of neural processing. The trajectory of a saccade saccade is shown schematically by the stippled region in panel could be based on vector summation of the movement tendencies a. Neurons in the middle of this active population (A) discharge provided by each member of the population of active neurons° or maximally before the intended saccade (a straight right saccade be determined by a weighted average of the vector contributions 5° in amplitude) . Referring to the motor map, neurons located of each neuron in the active population2. Here we present the at point B discharge maximally before upward (20° angle) results of experiments in which a small subset of the active populasaccades 4° in amplitude and neurons at point C discharge maximally before downward (-20° angle) saccades 9° in ampli` Present addresses : Department of Psychology, Seoul National University' Seoul, Korea (5, tude. The population-averaging hypothesis suggests that for each Department of Anatomy, University of Medicine and Dentistry of New Jersey, PO Box 55S Piscataway, New Jerse y 08854, USA (W.H .R .) . subset of active neurons (B) tending to produce movements t To whom correspondence should be addressed . other than the programmed saccade, there will be another subset