10 Sulfaro et al. imagined in a given region of the visual field. In comparison, con-sider inner speech, a type of auditory imagery specifically related to the imagination of oral sound content like words. If audi-tory imagery can be expressed as inner speech, such as through converting imagined sounds to onomatopoeia, then real or sim-ulated vocal movements could produce a stream of temporally precise sensory information not just informing on when a par-ticular imagined sound should be heard but also informing on what the imagined sound content should actually be. In essence, using motor systems, the brain could have a greater capacity to simulate sensory features of imagined sounds compared to images, provided that the imagined sounds are somewhat possi-ble to replicate vocally. Yet, even when not readily imitable, sounds could still be imagined as some form of onomatopoeia, sacrificing real-world accuracy in order to boost the sensory quality of the imagery experience. Consequently, we expect that auditory men-tal imagery may often be a more compelling sensory experience than visual mental imagery. Recent work supports these predic-tions, finding that auditory imagery is generally superior to visual mental imagery across a number of self-report metrics, although experiences seem to be quite diverse overall (Sulfaro et al., 2023). Limitations and future directions The primary intention of this article is to highlight that competi-tion from externally generated sensory input could be a major fac-tor that explains the unique phenomenology of mental imagery. Our modelling is mainly intended merely to provide a simple example of how treating perception as a competitive process can explain aspects of imagery, supporting the larger theoret-ical argument that mental imagery should always be consid-ered in tandem with co-occurring veridical perceptual processes. Nonetheless, both our argument and our suggested model rely on some assumptions. Our most crucial restraint is that we require that internally and externally generated perceptual processes utilize the same neural substrates such that interference can occur. Our cur-rent understanding is that these processes do overlap ( Ffytche et al. 1998 ; Dijkstra et al. 2017, 2020 ), down to the laminar cortical ( Lawrence et al. 2018; Bergmann et al. 2019 ), if not cel-lular ( Aru et al. 2020), level. However, differences between each process may mean that overlap might vary across the visual hierarchy (e. g. Lawrence et al. 2018), influencing the degree or nature of interference. Yet, even if these processes do over-lap, imagined representations may not necessarily propagate smoothly along a serial, retrohierarchical cascade. If imagery utilizes direct feedback connections from associative cortices to primary sensory cortices, rather than via a series of interme-diate cortical units, interference may have radically different outcomes. We also rely on assumptions about how neural activity trans-lates into subjective experience. Namely, we assume that lower and higher levels of processing, respectively, contribute to sen-sory and abstract features of perceptual experience. Yet, encoding along the visual hierarchy may only correlate with the degree to which semantic content of a perceptual experience is sim-ple or complex, rather than the degree to which such content is experienced in a sensory and modality-specific, rather than non-sensory, way at all. Of course, ascertaining the neural substrates of subjective experience is an immense area of investigation. Although we make broad predictions about the decaying influ-ence of internally generated stimuli towards primary sensory regions, the exact balance of this influence, and its dynam-ics, would likely vary substantially with more neurologically plausible methods of simulating interference beyond the simple weighted-averaging rule used to approximate interference in our model. Also, note that our model does not comment on any of the non-linear transformations necessary to encode a representa-tion within a layer but only makes assumptions about how such representations, once formed, may be transmitted and combined. Modelling a more neurologically plausible system would be a log-ical next step for investigating mental imagery as a competitive process. Furthermore, it is entirely plausible that other mecha-nisms aside from competition could account for the segregated distributions of internally and externally generated content in the visual system. We merely show that simple models of competi-tion can account for these distributions and many other imagery phenomena. Our simulations also use retinal inputs (and generally imag-ined stimulus inputs) that are static and constant, so our sim-ulations cannot account for the effects of neural adaptation on perceptual interference. In reality, retinally stabilized images tend to induce a reversible blindness, greying out the visual field. How-ever, such blindness is suspected to be a very early, low-level effect, at or near the retina (Martinez-Conde et al. 2004). Within our model, this state should then be roughly equivalent to feed-ing in a homogenous field as a real stimulus input, such that the same logic regarding the imagery improvements (or lack thereof) associated with eye closure and the perception of a dark room would also apply to the perceptual greying out that occurs with retinally stabilized images. However, neural adaptation could have many more consequential effects on mental imagery which future studies could explore. Our model also assumes that feedforward and feedback pro-cesses are computationally equivalent. However, Koenig-Robert and Pearson (2021) argue that while feedforward signals initiated from visual input may drive action potentials in the visual cor-tex, feedback signals terminating in early visual areas tend to be more modulatory, guiding but not overriding activity induced by sensory input. They suggest that this asymmetry could account for the apparent difference in vividness between veridical and mental imagery. However, weak feedback could also be a logical consequence of competition within our model. Our simulations predict that the influence of internally generated stimulus infor-mation should gradually diminish towards lower levels in the presence of bottom-up competition. At some point, this declining influence could fall below a threshold such that it can no longer drive action potentials. Yet, our model still uniquely predicts that feedback should remain influential when external competition is not present. Evidence supports this prediction, as feedback can drive action potentials in the visual cortex of neurotypical individuals during dreaming (Aru et al. 2020 ), including in neu-rons that are arguably crucial for manifesting conscious sensory perception (Aru et al. 2019). Hence, it is plausible that weak feedback follows from perceptual competition. Our model may then supplement the account of Koenig-Robert and Pearson (2021) which could otherwise be interpreted as precluding the possi-bility of purely top-down hallucinations. Asymmetries in feed-forward and feedback processing can still be operationalized in our model by manipulating bottom-up and top-down weightings, but our model distinctly predicts that imagined percepts should be experienced in a less sensory manner than veridical percepts, even if feedforward and feedback processes are computationally equivalent, due to the interference caused by competing sensory input. Overall, vindicating or falsifying competition as the origin of the quasi-sensory experience of mental imagery will ultimately Downloaded from https://academic. oup. com/nc/article/2023/1/niad018/7248966 by guest on 25 August 2024 