Abstract
To understand how humans utilize visual information up to the moment of making a decision, the present study applied a reverse-correlation analysis to behavioral responses in a reaction-time (RT) task for stochastic orientation stimuli. Each stimulus consisted of a dynamic texture (15 Hz frame rate) composed of Gabor elements whose orientation varied randomly according to Gaussian noise sampled in both the space and time domains. During stimulus presentation, observers pressed a button as soon as possible and indicated whether average spatiotemporal orientation appeared tilted clockwise or anti-clockwise. RTs ranged from ~500 to 2000 ms, and we calculated logistic-regression coefficients between observer RTs and spatial mean-orientation for each temporal frame. The analysis revealed a sharp peak in regression coefficients at 400-500 ms prior to the motor response. The peak was less pronounced for trials with longer RTs. Subsequent analysis showed that, within a temporal period around this peak, spatial mean-orientation was larger (i.e., more tilted) and the spatial variance of orientation was narrower than the other periods. Results indicate that, at least under time-constrained situations, human perceptual decision making is dominated by stimulus information presented 400-500 ms before the motor response. Qualitatively, this behavior can be accounted for by a standard drift-diffusion mechanism that receives delayed sluggish inputs of ensemble orientation signals from the early visual system.