Surprise leads to noisier perceptual decisions

Abstract

Surprising events in the environment can impair task performance. This might be due to complete distraction, leading to lapses during which performance is reduced to guessing. Alternatively, unpredictability might cause a graded withdrawal of perceptual resources from the task at hand and thereby reduce sensitivity. Here we attempt to distinguish between these two mechanisms. Listeners performed a novel auditory pitch-duration discrimination, where stimulus loudness changed occasionally and incidentally to the task. Responses were slower and less accurate in the surprising condition, where loudness changed unpredictably, than in the predictable condition, where the loudness was held constant. By explicitly modelling both lapses and changes in sensitivity, we found that unpredictable changes diminished sensitivity but did not increase the rate of lapses. These findings suggest that background environmental uncertainty can disrupt goal-directed behaviour. This graded processing strategy might be adaptive in potentially threatening contexts, and reflect a flexible system for automatic allocation of perceptual resources.

Keywords: attention; change detection; oddballs; prediction; sensitivity; uncertainty.

Figure 1.

Experimental design and task schematic. Participants performed a two-alternative forced-choice perceptual discrimination task where they had to report which of two, high or low pitch, sounds lasted the longer, in every trial. In oddball blocks (surprise condition) both sounds occasionally changed their loudness. In control blocks (predictable condition) the loudness was kept constant. Blue and orange rectangles correspond to high and low pitch sounds played alone, and grey to the superposition of the two.

Figure 2.

Surprised-induced noise slows down sensory decisions. Reaction time data for surprising and predictable conditions in median, softer, and louder sound levels. The thick segments indicate standard errors and the lines correspond to standard deviation.

Figure 3.

Example psychophysical data from one listener. Probability of responses for the surprising (S), and predictable (P), conditions in the three matched perceived loudness levels (standard = 70dB SPL, soft = 60dB SPL, and loud = 80 dB SPL). Crosses show data points and smooth lines show psychometric functions fitted to the data.

Figure 4.

Surprised-induced noise in sensory decisions. Scatter plot of the slopes (or inverse variance) estimated for individual listeners. The dashed line represents equal slopes for the surprising and predictable conditions; that is, no effect of surprise. For most listeners, variance is higher (slopes are lower) in the surprising than in the predictable context, for median, softer, and louder stimuli. There is no significant evidence for an effect on the lapse rate.