2 Sulfaro et al. contrast to feedforward connections that stereotypically drive, rather than modulate, action potentials (Koenig-Robert and Pearson 2021 ). However, feedback to early visual regions can drive action potentials depending on local neurochemistry ( Aru et al. 2020 ). Furthermore, neither of these differences between feedforward and feedback processes can explain the unique expe-rience of mental imagery without a framework specifying what it means for a mental image to be a quasi-sensory experience to begin with. A more readily interpretable explanation is that mental images appear impoverished simply because the features of the images they depict are distorted relative to those of real images. That is, mental images might merely have less-precise image statistics (e. g. spatial frequency, location, and size) compared to real images, but are otherwise just as visible. Breedlove et al. (2020) provide an account that allows for this possibility, finding that represen-tations of mental images in early visual cortices are deficient in detailed sensory information compared to real images. As hierar-chical feedforward processes combine multiple pieces of sensory information (e. g. edges) to form fewer pieces of more abstract information (e. g. a shape), reconstructing sensory information using retrohierarchical feedback requires extrapolating from a rel-atively information-deficient source material. This process should distort imagined stimuli in predictable ways such that neural pop-ulations that represent mental images in early visual cortices have lower spatial frequency preferences, more foveal receptive fields, and larger receptive fields than when responding to real images (Breedlove et al. 2020). Ultimately, many factors may distinguish between veridical and mental percepts given that the former are externally gen-erated while the latter are internally generated. However, any complete explanation of the unique appearance of mental images must also account for how the quasi-sensory experience of mental imagery might be differentiated from other internally generated, yet unambiguously sensory experiences, such as which occur in hallucinations, many dreams, and eidetic imagery. Regardless of the image statistics of the internally generated content being depicted, these latter phenomena tend to involve a sensory expe-rience of internally generated content that is entirely equivalent to the subjective experience of externally generated content (Chiu 1989 ; Ffytche et al. 1998) except that the content experienced is non-veridical and is not typically experienced as part of normal waking life. For the purposes of this article, we refer to the inter-nally generated, unambiguously sensory experience common to each of hallucinations, dreams, and eidetic imagery as hallucina-tory imagery, or a hallucination generally, regardless of whether such imagery is voluntary or involuntary, regardless of whether it is believed to be real or unreal, and regardless of where exactly within the visual field it is perceived to be located. Crucially, while mental imagery is quasi-sensory, seemingly seen yet unseen, hal-lucinatory imagery is definitively visible. This should require that non-veridical, internally generated sensory content or features replace veridical sensory content or features, overwriting them in a given region of the visual field. For instance, hallucinations may involve seeing a cat on this page instead of text, or seeing the text on this page as moving instead of still, or seeing any-thing at all during a dream instead of merely the back of our eyelids. Here, we propose that imagined, veridical, and hallucinated percepts can be clearly distinguished by considering perception as a competitive, hierarchical process. While mental image gen-eration can be modelled without competition from veridical per-ception (e. g. Breedlove et al. 2020), mental imagery almost never occurs in isolation. Instead, mental images are invariably gener-ated while we are awake and receiving sensory input from the external world. If mental imagery and veridical perception share neural resources, then both processes should interact such that neural representations of imagined and real stimuli interfere or compete with one another. Interactions between real and imagined stimuli have been previously demonstrated, lending support to the idea that percep-tion may be a competitive process, although the exact outcome and nature of this interaction seems to vary. For instance, men-tal images can constructively interfere with real image content, depending on stimulus congruency (Dijkstra et al. 2022), and can induce priming or adaptation to veridical stimuli, depending on the individual ( Keogh and Pearson 2017 ; Dijkstra et al. 2019b, 2021). Increasing environmental luminance can also impede the priming effects of mental imagery ( Keogh and Pearson 2017 ), while real modality-specific distractors reduce the recall of modality-specific information (Wais et al. 2010 ; Vredeveldt et al. 2011 ), potentially reflecting competitive interference. We sought to predict whether perceptual interference could account for the quasi-sensory, rather than hallucinatory, experi-ence of mental imagery. Yet, any valid explanation of the quasi-sensory experience of mental imagery must first clearly specify what it means to have a quasi-sensory experience at all. Such an experience may be possible under a framework where per-ceptual experience itself is considered as a multifaceted phe-nomenon, composed of both sensory and abstract experiences at any given time. In vision, this entails having an experience of low-level features of an image, such as its hues and edge ori-entations, while also having an experience of high-level features of what the image actually depicts, such as recognizing that an image is depicting a duck. Usually, low-and high-level features of an image seem intertwined such that it is unintuitive to con-sider them as separate components of perceptual experience. Yet, these aspects become clearly separable when we consider bistable percepts (e. g. the rabbit-duck illusion), where the same low-level information can give rise to different abstract experi-ences. These abstract experiences feel perceptually distinct from one another even though the spatial pattern of light in our visual field, and our purely low-level visual experience of it, remains unchanged. To anchor these experiences to physical processes, we assume that the sensory and abstract aspects of experience are dependent on neural systems encoding each respective type of information in the brain, such as those along the visual ventral stream. However, as neural representations of externally and internally generated stimuli are not explicitly sensory or abstract, but distributed on a spectrum, so too may perceptual experience itself be composed of a spectrum of sensory and abstract aspects. Given that internally generated perceptual experiences also seem to use similar neural resources to their externally generated counterparts, including in a feature-specific manner (Ffytche et al. 1998), then mental and hallucinatory imagery processes may also be experienced with a spectrum of abstract and sensory features. Under this framework, the quasi-sensory experience of mental imagery can be clearly conceptualized as the experience arising from the activation of high-level to mid-level, but not low-level, feature representations of internally generated content, regardless of the content itself. Hallucinatory imagery, in contrast, may invoke low-level sen-sory representations, with or without more abstract components, while veridical percepts should at least invoke low-level featural representations given their bottom-up entryway into the visual system. Downloaded from https://academic. oup. com/nc/article/2023/1/niad018/7248966 by guest on 25 August 2024 