Effect of stimulus uncertainty on the pupillary dilation response and the vertex evoked potential

Abstract

Scalp recordings of slow baseline shifts (CNV) preceding auditory stimuli, as well as pupillary dilations and evoked potentials to these stimuli, were recorded and averaged in awake adult humans. The experimental situation manipulated the degree of advance knowledge by the subject as to which of two auditory stimuli would be presented next. In the certain condition the subject was told prior to each stimulus which of the two stimuli would be presented next; in the uncertain condition the subject made a guess prior to each stimulus and the occurrence of the stimulus confirmed or disconfirmed the subject's guess. In both the certain and uncertain conditions, the relative probability of occurrence of the two stimuli was varied in different blocks of trials so that each of the two stimuli occurred at 20, 40, 60 and 80% probabilities. Pupillary dilation was found to be larger and the CNV was found to be more negative in the uncertain condition than in the certain condition. Of the evoked potential components, P 3 was larger and N 2 was smaller in the uncertain condition. Somewhat less consistently, P 2 was larger and P 1 and N 1 were smaller in the uncertain condition. P 3 amplitude and peak dilation amplitude decreased monotonically as a function of the rareness of event occurrence (as measured by either stimulus or outcome probability). This relationship held for P 3 in both certain and uncertain conditions, but only in the uncertain condition for pupillary dilation. The P 2 component also showed similar relationships to probability, but less strongly than did the P 3 component. CNV did not vary as a function of stimulus probability in either condition. The results are consistent with the interpretation that the uncertain condition is characterized by a higher arousal level associated with greater task involvement on the part of the subject. The similarity between the pupillary dilation relationship to probability and the P 3 relationship to probability is consistent with an interpretation of P 3 as a “cerebral correlate of the orienting response”. But this would require a broadening and redefinition of the orienting response concept in “informational” terms and in any case would fit all the available data on P 3 only with difficulty. A less ambitious formulation at this time would simply suggest that any operation which increases the “salience” of the stimulus to the subject tends to increase the amplitude of P 3.