Recent single neuron studies offer support for information accumulation type model with a threshold-crossing mechanism during simple choice RT tasks. In a previous study of random-dot motion-discrimination task (Roitman and Shadlen, 2002), it is shown that neuronal firing rate of the monkey's lateral intraparietal cortex (LIP) accumulates during each trial up until monkey's behavioral response, and the accumulation ("buildup") rate is monotonically related to the strength of the stimulus. Their data analysis, however, was unable to distinguish the sensorimotor role of the neurons during the stimulus-response association task. Here, we apply the technique of Locus Analysis (Zhang et al, 1997) to this data set in order to quantitatively characterize the processing "locus" of the neuronal activity along the sensorimotor continuum. Locus analysis provides an index of how a neuron's firing activity is differentially related to stimulus identification and to response preparation in a trial-specific manner. Our analysis shows that this so-called differential activity of the LIP neuronal population is essentially zero in the beginning of a trial, then increases and finally reaches peak right before the saccadic response is made. High differential activity is associated with motoric coding as opposed to sensory coding in LIP neurons. Further, differential activity peak is much tighter under response-locked analysis compared with stimulus-locked analysis, further supporting the claim that LIP buildup activity do not encode sensory information but rather encode intended movement. (Single cell data kindly made available by Michael Shadlen). |