The Pupil and Stimulus Probability

Abstract

ABSTRACT Three studies were conducted to examine the relationship of the task‐evoked pupillary response (TEPR) to the probability of the eliciting stimuli in Bernoulli sequences. Subjects counted target tones in sequences containing two types of tones differing in pitch. For a given probability level, both targets and non‐targets elicited pupillary dilations that were similar in amplitude. The dilation to both targets and non‐targets was inversely related to probability at low probability levels. This relationship appeared to break down at high probability levels, at least for non‐targets. When tone omissions were used as targets they elicited a larger dilation than tonal targets at very low target probability. The amplitude of dilations produced by omitted targets was also affected by probability. The data converge to indicate that the TEPR in random stimulus sequences is sensitive to the surprise value of events, whether involving a physical stimulus or not, and not to the target designation of these events.