External noise methods and observer models have been widely used to characterize the intrinsic perceptual limitations of human observers and changes of the perceptual limitations associated with cognitive, developmental, and disease processes by highlighting the variance of internal representations. The authors conducted a comprehensive review of the 5 most prominent observer models through the development of a common formalism. They derived new predictions of the models for a common set of behavioral tests that were compared with the data in the literature and a new experiment. The comparison between the model predictions and the empirical data resulted in very strong constraints on the observer models. The perceptual template model provided the best account of all the empirical data in the visual domain. The choice of the observer model has significant implications for the interpretation of data from other external noise paradigms, as well as studies using external noise to assay changes of perceptual limitations associated with observer states. The empirical and theoretical development suggests possible parallel developments in other sensory modalities and studies of high-level cognitive processes.Keywords: signal detection theory, internal representation, observer model, perceptual template, internal noise Human decisions are based on internal representations of information. Understanding how stimuli are represented internally is one of the classic problems in psychology. This article examines how modifying an external stimulus with external noise can provide insight into how the stimulus is processed by the human observer. We conducted a systematic and comprehensive review of the external noise paradigms and observer models widely used in characterizing the internal response properties of human observers. The observer approach builds on the broadly applicable framework of signal detection theory (SDT) by elaborating the relationships between external stimuli and the internal response distributions that form the basis for decision. The empirical tests introduce external noise-either masking noise or variation in the relevant stimulus dimension-to provide a reference for characterizing and quantifying the limiting factors in perceptual sensitivity. The current review, analysis, and empirical test focus on visual perception. Some of the model properties, especially the empirical findings, may be modality specific. Still, this framework and the findings could serve as an example for parallel development of the empirical methods and theoretical models in other sensory modalities. The model development and testing also have major implications for applications of the external noise paradigms in understanding the mechanisms underlying changes of perceptual sensitivity in different cognitive, disease, and/or developmental states.
Internal Response DistributionsSDT provides a general framework for analyzing human decision making in perceptual and cognitive tasks (Green & Swets, 1966;Macmillan & Creelman, 1991). In a ...