Variance Signatures of Decision Formation in the Superior Colliculus

Perceptual decision-making involves sampling sensory evidence over time. The process of evidence accumulation is thought to be reflected in the ramping activity of neurons in sensorimotor regions of the brain such as area LIP. Ramping neuronal activity occurs in many other brain areas too and whether this reflects the same underlying process as in LIP remains unknown. Here, we use measures of variability to parse the signals arising from superior colliculus neurons while monkeys perform two different perceptual decision tasks: discrimination and detection. We found that spike trains from superior colliculus neurons behave in a manner consistent with an initial process of evidence accumulation followed by a switch to a committed decision state during performance of a discrimination task. During performance of a detection task, the activity of neurons in the superior colliculus behaved in a manner consistent with a single, rapid switch in processing from an uncommitted to a committed decision state. Simulations of ramp-like, step like, and vacillating spike trains confirmed the findings from the actual spiking data. Taken together, our results support the hypothesis that the colliculus plays an important role in perceptual decision-making and therefore could compensate for an impaired LIP during evidence accumulation.