On an Internal Reference for Perceptual Judgments

Abstract

Contrary to classic psychophysical models, one typically observes in psychophysical experiments that the judgment of sensory stimuli is not a direct function of the physical stimulus attributes. Rather, perceptual judgment also depends on the temporal position of a stimulus in various ways. For example, discrimination sensitivity is typically higher when a constant standard precedes rather than follows a variable comparison (Type B Effect). Furthermore, stimulus judgment is often biased towards past stimuli (sequence effects and central tendency effects). Potentially, such assimilatory history biases are the signature of a mechanism which integrates present sensory information with prior knowledge based on past sensory information in order to provide perceptual stability. For example, the Internal Reference Model (IRM, Dyjas et al., 2012) postulates that human judgment relies on an internal reference, which constitutes a continuously updating conglomerate of past and present stimulus instances. Aim of the present work was to investigate the scope of this model and to clarify the nature of the internal reference. Study 1 establishes IRM as a general psychophysical model, since a specific prediction of IRM (Type B Effect) was observed for discrimination tasks across various modalities and stimulus attributes. The formation of an internal reference henceforth appears as a general component of human perceptual performance. Study 2 demonstrates that humans rely less on the stimulus history in the form of the internal reference when the presentation of past stimuli is temporally more distant. The internal reference thereby appears as a perceptual short term memory representation which is prone to decay over time. Study 3 suggests that humans form and maintain two separate internal references for two distinct discrimination tasks they alternate between. However they isolate and integrate the task-relevant stimulus attribute across different stimuli into a single internal reference in case of a single discrimination task with different stimuli, suggesting a feature-based coding of the internal reference. In summary, the present work establishes IRM as a general, valid and theoretically rich formal model of a mechanism which potentially underlies the integration of present and past sensory information in order to optimize perceptual stability; since the world is relatively stable across short time intervals, it might be adaptive to rely on the recent past in order to reduce uncertainty about the presence.