The Same-Different task presents two stimuli in close succession and participants must indicate whether they are completely identical or if there are any attributes that differ. While the task is simple, its results have proven difficult to explain. Notably, response times are characterized by a fast-same effect whereby Same responses are faster than Different responses even though identical stimuli should be exhaustively processed to be accurate. Herein, we examine a little more than a quarter million response times (N = 255,744) obtained from 327 participants who participated in one of 14 variants of the task involving minor changes in the stimuli or their durations. We performed distribution fitting and analyzed estimated parameters stemming from the ex-Gaussian, lognormal, and Weibull distributions to infer the cognitive processing characteristics underlying this task. The results exclude serial processing of the stimuli and do not support dual-route processing. The fast-same effect appears only through a shift of the entire response time distributions, a feature impossible to detect solely with mean response time analyses. An attentionmodulated process driven by entropy may be the most adequate model of the fast-same effect.
Public Significance StatementThe ability to detect a lack of difference between successive stimuli is performed easily and without effort thousands of times every day. Yet, despite considerable research over the last 50 years, very little is known to explain how humans perform these decisions. Herein, we reexamine over a quarter million data from more than 300 participants, which lead to a new hypothesis involving dynamic modulation of attention.