Although mental imagery is often studied as a visual phenomenon, it can occur in any sensory modality. Given that mental images may recruit similar modality-specific neural systems to those which support veridical perception, the properties of mental images may be constrained by the modality in which they are experienced. Yet, little is known about how mental images are experienced at all, let alone how such experiences may vary depending on the modality in which they occur. Here we explored how mental images are experienced in different modalities using an extensive questionnaire. Mainly focusing on visual and auditory mental imagery, we surveyed participants on if and how they experienced their thought content in a sensory way when thinking about the appearance or sound of the letter "O". Specifically, we investigated temporal properties of imagined content (e.g. onset latency, duration), as well as spatial properties (e.g. apparent location), effort (e.g. ease, spontaneity, control), dependence on body movements (e.g. eye movements), interactions between real and imagined content (e.g. inner speech during reading), the perceived normality of imagery experiences, and how participants labeled their own experiences. Participants also ranked their mental imagery experiences in the five traditional sensory modalities and reported on the involvement of each modality during their thoughts, imagination, and dreams. Confidence ratings were taken for every answer recorded. Overall, visual and auditory experiences tended to dominate mental events relative to other sensory modalities. However, most people reported that auditory mental imagery was superior to visual mental imagery on almost every metric tested, except with respect to spatial properties. Our findings suggest that mental images are restrained in a similar matter to other modality-specific sensory processes in the brain. Broadly, our work also provides a wealth of insights and observations into how mental images are experienced by individuals, acting as a useful resource for future investigations.
Mental imagery can be understood as the process by which a thought becomes experienced with sensory characteristics. Yet, it's not clear why mental images have diminished sensory features compared to veridical percepts, nor if and how mental images are phenomenologically distinct from hallucinations, another type of non-veridical sensory experience. Current evidence suggests that externally- and internally-generated percepts both depend on similar neural systems. If so, it's plausible that imagined and veridical stimuli should interfere with one another. Here we propose that considering how externally-generated stimuli (i.e. sensory input) and internally-generated stimuli (i.e. thoughts) might compete could be sufficient to differentiate veridical, imagined, and hallucinated percepts. To predict the perceptual consequences of competition, we constructed a computational model of a serially-connected, hierarchical system with bidirectional information flow, emulating the primate visual system. We simulated mental imagery as top-down generative feedback in our model, both with and without competition from bottom-up sensory input. Without competition, imagined stimuli formed the dominant representation at each level of the hierarchy. Yet, when competing sensory input was present, imagined stimuli were outcompeted at lower levels while remaining dominant at higher levels, with an equilibrium point in between. We interpret our findings under a framework whereby low-level, high-dimensional stimulus information (e.g. in the early visual cortex) contributes most to the sensory aspects of perceptual experience, while high-level, low-dimensional stimulus information (e.g. towards temporal regions) contributes most to the abstract, amodal aspects of perceptual experience. Our findings suggest that ongoing bottom-up sensory input during waking life may prevent imagined stimuli from outcompeting and overriding our sensory experience of veridical stimuli. In contrast, dampening or eradicating sensory input could cause internally-generated stimuli to dominate sensory experience. We further discuss how our model can explain individual differences in mental imagery quality, account for phenomena such as daydreaming and hallucinations, and model mental imagery in multiple sensory modalities.
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