Order of operations for decoding superior colliculus activity for saccade generation

Abstract

Katnani HA, Gandhi NJ. Order of operations for decoding superior colliculus activity for saccade generation. J Neurophysiol 106: 1250-1259, 2011. First published June 15, 2011 doi:10.1152/jn.00265.2011To help understand the order of events that occurs when generating saccades, we simulated and tested two commonly stated decoding models that are believed to occur in the oculomotor system: vector averaging (VA) and center-of-mass. To generate accurate saccades, each model incorporates two required criteria: 1) a decoding mechanism that deciphers a population response of the superior colliculus (SC) and 2) an exponential transformation that converts the saccade vector into visual coordinates. The order of these two criteria is used differently within each model, yet the significance of the sequence has not been quantified. To distinguish between each decoding sequence and hence, to determine the order of events necessary to generate accurate saccades, we simulated the two models. Distinguishable predictions were obtained when two simultaneous motor commands are processed by each model. Experimental tests of the models were performed by observing the distribution of endpoints of saccades evoked by weighted, simultaneous microstimulation of two SC sites. The data were consistent with the predictions of the VA model, in which exponential transformation precedes the decoding computation.sequence of decoding; dual microstimulation; vector averaging; vector summation; center-of-mass THE SUPERIOR COLLICULUS (SC) is a critical, subcortical hub that plays a major role in converting sensory information into a motor command for saccade generation (Gandhi and Katnani 2011;Sparks 1986). Visual information in the SC is organized topographically in a logarithmic manner. A disproportionately large amount of SC tissue is allocated for (para-) foveal space and small-amplitude saccades, whereas a compressed region is attributed for the visual periphery and large-amplitude saccades (Cynader and Berman 1972;Robinson 1972). With the presentation of a stimulus, a population of neurons becomes active at a locus that represents the vector between the stimuli and line of sight (Wurtz and Goldberg 1972). Under ordinary circumstances, a comparable population discharges a highfrequency premotor burst to produce a saccade of the appropriate displacement (Sparks et al. 1976). To make certain that the saccadic eye movement lands near the desired location, two critical operations must be performed. One, the population response must be decoded using standard decoding mechanisms such as averaging or summation. Two, the SC output must undergo an exponential transformation to convert the saccade vector into visual coordinates (Ottes et al. 1986), the inverse transformation used to map visual space into SC coordinates. Studies have implemented the order of these two operations in different fashions. To the best of our knowledge, no attempt has been made to distinguish the order of these operations.In one scheme, the exponential transformation is applie...

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