Mistaking imagination for reality: Congruent mental imagery leads to more liberal perceptual detection

Abstract

Visual experiences can be triggered externally, by signals coming from the outside world during perception; or internally, by signals from memory during mental imagery. Imagery and perception activate similar neural codes in sensory areas, suggesting that they might sometimes be confused. In the current study, we investigated whether imagery influences perception by instructing participants to imagine gratings while externally detecting these same gratings at threshold. In a series of three experiments, we showed that imagery led to a more liberal criterion for reporting stimulus presence, and that this effect was both independent of expectation and stimulus-specific. Furthermore, participants with more vivid imagery were generally more likely to report the presence of external stimuli, independent of condition. The results can be explained as either a low-level sensory or a high-level decision-making effect. We discuss that the most likely explanation is that during imagery, internally generated sensory signals are sometimes confused for perception and suggest how the underlying mechanisms can be further characterized in future research. Our findings show that imagery and perception interact and emphasize that internally and externally generated signals are combined in complex ways to determine conscious perception.

Keywords: Mental imagery; Perception; Reality monitoring; Signal detection theory.

Fig. 1

Experimental paradigm. At the start of each block, participants were instructed whether they would be detecting left or right tilted gratings and whether they would have to imagine those gratings during this block. Trials consisted of 200 ms fixation followed by 2 s of either a grating ramping up in noise (50%) or pure noise (50%). Participants had to indicate whether a grating was present or not. After each block participants were asked if they did or did not imagine the grating during this block to check whether they followed instructions.

Fig. 2

Main effect of imagery on SDT measures in Experiment 1. Top row: values per condition. Grey is without imagery and blue is with imagery. Bottom row: without imagery – with imagery. Positive difference indicates greater values during imagery and negative difference indicates smaller values during imagery compared to no imagery. Dots represent individual participants. Boxplot lines represent the range of data excluding outliers. **** p < 0.0001. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

Fig. 3

Main effect of imagery on SDT measures in Experiment 2. Top row: values per condition (imagery x expectation). Grey is without imagery and blue is with imagery; a base-rate of 20% represents an expectation of absence and a base-rate of 80% represents an expectation of presence. Bottom row: difference between imagery and no-imagery blocks for the two expectation conditions. Positive difference indicates greater values during imagery and negative difference indicates smaller values during imagery compared to no imagery. ** p < 0.01, *** p < 0.001, *** p < 0.0001, **** p < 0.00001. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

Fig. 4

Main effect of imagery on SDT measures in Experiment 3. Top row: values per condition. Grey is without imagery; blue is with congruent imagery and red is with incongruent imagery. Bottom row: difference between imagery and no-imagery blocks for congruent and incongruent imagery. Positive difference indicates greater values during imagery and negative difference indicates smaller values during imagery compared to no imagery. * p < 0.05 ** p < 0.01, *** p < 0.001, *** p < 0.0001, **** p < 0.00001. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)