Different processing strategies underlie voluntary averaging in low and high noise

An underlying assumption of the external noise paradigm is that the same processing strategy operates whether the dominating noise source comes from the observer (i.e., internal) or the stimulus (i.e., external). Here, we challenged this noise-invariant processing assumption for a particular variant of the external noise paradigm-the voluntary averaging paradigm-where processing is characterized by the efficiency of averaging across the samples. The task consisted in discriminating the mean orientation of four distinctly perceived Gabors, and the external noise corresponded to orientation-jitter added to these Gabors. The averaging efficiencies for the four-sample case were measured by comparing discrimination thresholds of the average orientation of four Gabors to a baseline with a single Gabor. In high noise, orientation discrimination thresholds were better when 4 Gabors rather than 1 were presented, showing efficient averaging. But in absence of external noise, presenting four identically oriented Gabors rather than one did not improve performance, showing that subjects no longer averaged the individual estimates. We conclude that the averaging process operating in high noise does not operate in low noise because there is no reason to voluntarily average stimuli that appear identical. This conclusion implies that the processing strategy can change depending on the external noise level, an implication which violates the noise-invariant processing assumption underlying the averaging paradigm.