Illusions and delusions: relating experimentally-induced false memories to anomalous experiences and ideas

Abstract

The salience hypothesis of psychosis rests on a simple but profound observation that subtle alterations in the way that we perceive and experience stimuli have important consequences for how important these stimuli become for us, how much they draw our attention, how they embed themselves in our memory and, ultimately, how they shape our beliefs. We put forward the idea that a classical memory illusion - the Deese-Roediger-McDermott (DRM) effect - offers a useful way of exploring processes related to such aberrant belief formation. The illusion occurs when, as a consequence of its relationship to previous stimuli, a stimulus that has not previously been presented is falsely remembered. Such illusory familiarity is thought to be generated by the surprising fluency with which the stimulus is processed. In this respect, the illusion relates directly to the salience hypothesis and may share common cognitive underpinnings with aberrations of perception and attribution that are found in psychosis. In this paper, we explore the theoretical importance of this experimentally-induced illusion in relation to the salience model of psychosis. We present data showing that, in healthy volunteers, the illusion relates directly to self reported anomalies of experience and magical thinking. We discuss this finding in terms of the salience hypothesis and of a broader Bayesian framework of perception and cognition which emphasizes the salience both of predictable and unpredictable experiences.

Keywords: false memory; prediction error; psychosis; salience.

Figure 1

The relationship between DRM confidence and odd beliefs/experiences: Scatter-plot of subject's confidence in DRM errors and their self-rated positive schizotypy scores.

Figure 2

The correlation between criterion and positive schizotypy: Scatter-plot of subject's criterion values and their susceptibility to odd beliefs. A low criterion or negative criterion corresponds to a liberal acceptance criterion or expectancy bias.

Figure 3

Signal Detection Theory, Memory and Psychosis. (A) The role of familiarity or fluency in decision making about memory. Subjects make inferences about whether or not they have experienced an event by comparing the fluency/familiarity they experience with these distributions. In green, the familiarity of events that truly occurred, in red, that of events that did not take place. The area under the curves in blue, represents the region of highest ambiguity. (B) The criterion subjects use to make inferences reflects their decision bias, subjects with a low criterion endorse more experiences that did not actually take place. The correlation we observed between low criterion and positive schizotypy suggests that the perceptual aberrations and odd beliefs that subjects reported were related to this criterion value. Subjects with a low criterion on the DRM task would be more likely to report confident illusory false memories, we would also expect them to imbue ambiguous or uncertain percepts (like the image of rocks from the surface of Mars) with meaning, concluding that they represented a face.