This functional Magnetic Resonance Imaging study investigated high and low suggestible people responding to two visual hallucination suggestions with and without a hypnotic induction. Participants in the study were asked to see color while looking at a grey image, and to see shades of grey while looking at a color image. High suggestible participants reported successful alterations in color perception in both tasks, both in and out of hypnosis, and showed a small benefit if hypnosis was induced. Low suggestible people (...) could not perform the tasks successfully with or without the hypnotic induction. The fMRI results supported the self report data, and changes in brain activity were found in a number of visual areas. The results indicate that a hypnotic induction, although having the potential to enhance the ability of high suggestible people, is not necessary for the effective alteration of color perception by suggestion. (shrink)

We administered suggestions to see a gray-scale pattern as colored and a colored pattern in shades of gray to 30 high suggestible and eight low suggestible students. The suggestions were administered twice, once following the induction of hypnosis and once without an induction. Besides rating the degree of color they saw in the stimuli differently, participants also rated their states of consciousness as normal, relaxed, hypnotized, or deeply hypnotized. Reports of being hypnotized were limited to highly suggestible participants and only (...) after the hypnotic induction had been administered. Reports of altered color perception were also limited to high suggestibles, but were roughly comparable regardless of whether hypnosis had been induced. These data indicate that suggestible individuals do not slip into a hypnotic state when given imaginative suggestions without the induction of hypnosis, but nevertheless report experiencing difficult suggestions for profound perceptual alterations that are pheonomenologically similar to what they report in hypnosis. (shrink)

We tested Collerton et al.'s model of visual hallucinations by re-examining a data set for correlations between visual hallucinations and measures of attentional function in schizophrenia patients. These data did not support their model. We suggest that cortical hyperexcitability plays an important role in hallucinations, and propose an alternative model that links evidence for cortical hyperexcitability with abnormal neural dynamics.

This chapter presents findings on a study of neuropsychological aspects of visual hallucinations with a focus on hallucinations in Parkinson's disease. The results of the study suggest that VHs in PD are a complex multifactor effect of different risk factors, primarily the dysfunctions of the visual system and the system that regulates rapid eye movement sleep and arousal. The study reported here also investigated whether the same risk factors are implicated in proneness to VHs as they are in (...) VHs in PD. The results suggest that the predisposition to VHs in the healthy young population is a multifactor effect of strikingly similar risk factors to those found in PD. However, in addition to the dysfunctions of the visual and arousal system, proneness to hallucinations in the normal population is also predicted by a specific personality profile. (shrink)

Applying Behrendt & Young's (B&Y's) model of thalamocortical synchrony to complex visual hallucinations in neurodegenerative disorders, such as dementia with Lewy bodies and progressive supranuclear palsy, leads us to propose that the primary pathology may be cortical rather than thalamic. Additionally, the extinction of active hallucinations by eye closure challenges their conception of the role of reduced sensory input.

As many as two million people in the United Kingdom repeatedly see people, animals, and objects that have no objective reality. Hallucinations on the border of sleep, dementing illnesses, delirium, eye disease, and schizophrenia account for 90% of these. The remainder have rarer disorders. We review existing models of recurrent complex visual hallucinations (RCVH) in the awake person, including cortical irritation, cortical hyperexcitability and cortical release, top-down activation, misperception, dream intrusion, and interactive models. We provide evidence that these (...) can neither fully account for the phenomenology of RCVH, nor for variations in the frequency of RCVH in different disorders. We propose a novel Perception and Attention Deficit (PAD) model for RCVH. A combination of impaired attentional binding and poor sensory activation of a correct proto-object, in conjunction with a relatively intact scene representation, bias perception to allow the intrusion of a hallucinatory proto-object into a scene perception. Incorporation of this image into a context-specific hallucinatory scene representation accounts for repetitive hallucinations. We suggest that these impairments are underpinned by disturbances in a lateral frontal cortex–ventral visual stream system. We show how the frequency of RCVH in different diseases is related to the coexistence of attentional and visual perceptual impairments; how attentional and perceptual processes can account for their phenomenology; and that diseases and other states with high rates of RCVH have cholinergic dysfunction in both frontal cortex and the ventral visual stream. Several tests of the model are indicated, together with a number of treatment options that it generates. Key Words: Blindness; Charles Bonnet; cholinergic; cortical release; delirium; dementia; dream intrusion; hallucination; Perception and Attention Deficit (PAD) model; schizophrenia. (shrink)

A “multifactorial” model should accommodate a psychological perspective, aiming to relate the phenomenology of complex visual hallucinations not only to neurobiological findings but also an understanding of the patient's psychological problems and situation in life. Greater attention needs to be paid to the role of the “lack of insight” patients may have into their hallucinations and its relationship to cognitive impairment.

Commentators agree that the Perception and Attention Deficit (PAD) model is a promising model for accounting for recurrent complex visual hallucinations (RCVH) across several disorders, though with varying detailed criticisms. Its central tenets are not modified, but further consideration of generative models of visual processing and the relationship of proto-objects and memory systems allows the PAD model to deal with variations in phenomenology. The commentaries suggest new ways to generate evidence that will test the model.

The dual-deficit model of visual hallucinations (Collerton et al. target article) is compared with the dual-deficit model of auditory hallucinations (Waters et al., in press). Differences in cognitive mechanisms described may be superficial. Similarities between these models may provide the basis for a general model of complex hallucinations extended across disorders and modalities, involving shared (overlapping) cognitive processes.

There is little to refute in Collerton et al.'s argument that recurrent complex visual hallucinations involve multiple physiological mechanisms, and the target article's proposed PAD model implicitly incorporates this concept, advancing the field. The novel concept in this model is the intrusion of hallucinatory proto-objects into relatively preserved scenes. The weakness of the model is the lack of physiological detail for this mechanism.

Recent neural models clarify many properties of mental imagery as part of the process whereby bottom-up visual information is influenced by top-down expectations, and how these expectations control visual attention. Volitional signals can transform modulatory top-down signals into supra-threshold imagery. Visual hallucinations can occur when the normal control of these volitional signals is lost.

In this paper, I argue that the indirect realists’ recourse to mental representations does not allow them to account for the possibility of hallucination, nor for the presentational character of visual experience. To account for the presentational character, I suggest a kind of intentionalism that is based on the interdependency between the perceived object and the embodied perceiver. This approach provides a positive account to the effect that genuine perception and hallucination are different kinds of states. Finally, I offer a (...) tentative suggestion as to how a hallucinatory experience may still be mistaken for a genuine perceptual experience. (shrink)

Collerton et al. propose that one and the same mechanism (PAD) underlies recurrent complex visual hallucinations (RCVH) in various disorders, including schizophrenia, dementia, and eye disease. The present commentary offers an alternative account of RCVH and other recurrent complex hallucinations specific to schizophrenia and related disorders only. The proposed account is consistent with the bias of schizophrenic RCVH contents toward animate, socially active entities.

Synesthesia literally means a “union of the senses” whereby two or more of the five senses that are normally experienced separately are involuntarily and automatically joined together in experience. For example, some synesthetes experience a color when they hear a sound, although many instances of synesthesia also occur entirely within the visual sense. In this paper, I first mainly engage critically with Sollberger’s view that there is reason to think that at least some synesthetic experiences can be viewed as truly (...) veridical perceptions, and not as illusions or hallucinations. Among other things, I explore the possibility that many forms of synesthesia can be understood as experiencing what I will call “second-order secondary properties,” that is, experiences of properties of objects induced by the secondary qualities of those objects. In doing so, I shed some light on why synesthesia is typically one-directional and its relation to some psychopathologies such as autism. (shrink)

REM sleep behaviour disorder is a REM sleep-related parasomnia which may be considered a “dissociated state of wakefulness and sleep”, given that conflicting elements of REM sleep and of wakefulness coexist during the episodes, leading to motor and behavioural manifestations reminiscent of an enacted dream. RBD has been reported in association with α-synucleinopathies: around a third of patients with Parkinson’s disease have full-blown RBD.Recent data indicate that PD patients with RBD are more prone to hallucinations than PD patients without (...) this parasomnia. However it is still not clear why RBD in PD is associated with an increased prevalence of VHs.Data exist which suggest that visual hallucinations in PD may be the result of untimely intrusions of REM visual imagery into wakefulness. RBD, which is characterised by a REM sleep dissociation pattern, might be a condition that particularly favours such intrusions. However, other hypotheses may be advanced. In fact, deficits in attentional, executive, visuoperceptual and visuospatial abilities have been documented in RBD and found to occur far more frequently in PD with RBD than in PD without RBD. Neuropsychological deficits involving visual perception and attentional processes are thought to play an important role in the pathophysiology of VHs. On this basis, RBD in PD could be viewed as a contributory risk factor for VHs. (shrink)

Hypnagogic and hypnopompic experiences (HHEs) accompanying sleep paralysis (SP) are often cited as sources of accounts of supernatural nocturnal assaults and paranormal experiences. Descriptions of such experiences are remarkably consistent across time and cultures and consistent also with known mechanisms of REM states. A three-factor structural model of HHEs based on their relations both to cultural narratives and REM neurophysiology is developed and tested with several large samples. One factor, labeled Intruder, consisting of sensed presence, fear, and auditory and visual (...) hallucinations, is conjectured to originate in a hypervigilant state initiated in the midbrain. Another factor, Incubus, comprising pressure on the chest, breathing difficulties, and pain, is attributed to effects of hyperpolarization of motoneurons on perceptions of respiration. These two factors have in common an implied alien ''other'' consistent with occult narratives identified in numerous contemporary and historical cultures. A third factor, labeled Unusual Bodily Experiences, consisting of floating/flying sensations, out-of-body experiences, and feelings of bliss, is related to physically impossible experiences generated by conflicts of endogenous and exogenous activation related to body position, orientation, and movement. Implications of this last factor for understanding of orientational primacy in self-consciousness are considered. Central features of the model developed here are consistent with recent work on hallucinations associated with hypnosis and schizophrenia. (shrink)

Two experiments investigated the reality attributed to hypnotic suggestion through subtle projection of a visual image during simultaneous suggestion for a visual hallucination that resembled the projected image. In Experiment 1, high and low hypnotizable participants were administered either a hypnotic induction or wake instructions, given a suggestion to hallucinate a shape, and then the projected image was subsequently introduced. Although highs in both conditions rated the projected image more vividly than lows, highs in the hypnosis condition made comparable reality (...) ratings when the projected image was absent and present. In Experiment 2, high hypnotizable participants were administered a suggestion to see a shape on a wall. For half the participants the suggested image was projected on the wall and then removed, and for half the projection was initially absent and then introduced. Participants who had the projection absent and then present reported comparable reality and vividness ratings when the projection was absent and present. These findings indicate that elevated hypnotizability and hypnosis are associated with attributions of external reality to suggested experiences. (shrink)

This paper is a new defense of the view that visual hallucinations lack content. The claim is that visual hallucinations are illusory not because their content is nonveridical, but rather because they seem to represent when they fail to represent anything in the first place. What accounts for the phenomenal character of visual experiences is not the content itself (content-representationalism), but rather the vehicle of content (vehicle-representationalism), that is, not the properties represented by visual experience, but rather the (...) relational properties of experience (or of the brain) of representing singular contents, namely particular instantiations of properties. I argue that the Russellian particular-involving proposition is the only appropriate model for the representational content of visual experience and hence that visual hallucinations are just like failed demonstrations. (shrink)

Waking hallucinations suggest mechanisms of dream initiation and maintenance. Visual association cortex activation, yielding poorly attended-to, visually ambiguous dream environments, suggests conditions favoring hallucinosis. Attentional and visual systems, coactivated during sleep, may generate imagery that is inserted into virtual environments. Internally consistent dreaming may evolve from successive, contextually evoked images. Fluctuating arousal and context-evoked imagery may help explain dream features.

Externalist representationalists claim that the phenomenal character of a visual perceptual experience is determined by the representational content of that experience. Their deployment of the idea that perceptual experience is transparent shows that they account for representational content with reference to the properties which are represented – the properties out there in the world. I explain why this commits the externalist representationalist to objectivism and realism about colour properties. Colour physicalism has proved to be the position of choice for externalist (...) representationalists. However, my aim in this paper is to demonstrate that the proponent of the view which combines externalist representationalism with colour physicalism is unable to account for the phenomenal character of colour hallucination. (shrink)

This paper is a new defense of the view that visual hallucinations lack content. The claim is that visual hallucinations are illusory not because their content is nonveridical, but rather because they seem to represent when they fail to represent anything in the first place. What accounts for the phenomenal character of visual experiences is not the content itself, but rather the vehicle of content, that is, not the properties represented by visual experience, but rather the relational properties (...) of experience of representing singular contents, namely particular instantiations of properties. I argue that the Russellian particular-involving proposition is the only appropriate model for the representational content of visual experience and hence that visual hallucinations are just like failed demonstrations. (shrink)

Comparing the phenomenology, neurochemical pathology, and psychopharmacology of hallucinations and dreaming is limited by the available data. Evidence to date reveals no simple correspondence between the two states. Differences in the phenomenology of visual hallucinations and the visual component of dreams may reflect variations in visual context acting on the same underlying mechanism – the minimal visual input during dreaming contrasts with the more substantial perceived context in hallucinations. Variations in cholinergic, dopaminergic and serotonergic neurotransmitter function during (...) sleep and during hallucinations in Lewy body dementias, together with relevant drug effects suggest that, on the whole, different, potentially opposite, changes characterise the two states. A similar analysis of other psychotic features in Lewy body dementia and other disorders suggests that, in contrast to hallucinations, there may be more convincing parallels between dreaming and delusional states. (shrink)

This book provides the first full-length treatment of disjunctivism about visual experiences in the service of defending a naive realist theory of veridical visual perception. It includes detailed theories of hallucination and illusion that show how such states can be indistinguishable from veridical experiences without sharing any common character.

Flickering light induces visual hallucinations in human observers. Despite a long history of the phenomenon, little is known about the dependence of flicker-induced subjective impressions on the flicker frequency. We investigate this question using Ganzfeld stimulation and an experimental paradigm combining a continuous frequency scan with a focus on re-occurring, whole percepts. On the single-subject level, we find a high degree of frequency stability of percepts. To generalize across subjects, we apply two rating systems, a set of complex percept (...) classes derived from subjects’ reports and an enumeration of elementary percept features, and determine distributions of occurrences over flicker frequency. We observe a stronger frequency specificity for complex percept classes than elementary percept features. Comparing the similarity relations among percept categories to those among frequency profiles, we observe that though percepts are preferentially induced by particular frequencies, the frequency does not unambiguously determine the experienced percept. (shrink)

I argue that there are no plausible non-representational explanations of episodes of hallucination. To make the discussion more specific, I focus on visual hallucinations in Charles Bonnet syndrome. I claim that the character of such hallucinatory experiences cannot be explained away non-representationally, for they cannot be taken as simple failures of cognizing or as failures of contact with external reality—such failures being the only genuinely non-representational explanations of hallucinations and cognitive errors in general. I briefly introduce a recent (...) computational model of hallucination, which relies on generative models in the brain, and argue that the model is a prime example of a representational explanation referring to representational mechanisms. The notion of the representational mechanism is elucidated, and it is argued that hallucinations—and other kinds of representations—cannot be exorcised from the cognitive sciences. (shrink)

When viewed from a distance, visual hallucinations fall into one of two symptom patterns, a dichotomy which poses a problem for theoretical models treating them as a single entity. Such models should be broadened to allow for two distinct but overlapping syndromes – one likely to relate to visual de-afferentation, the other to Perception and Attention Deficit (PAD) cholinergic pathology.

A comprehensive theory of the structure and cognitive function of the human imagination, and its relationship to perceptual experience, is developed, largely through a critique of the account propounded in Colin McGinn's Mindsight. McGinn eschews the highly deflationary (and unilluminating) views of imagination common amongst analytical philosophers, but fails to develop his own account satisfactorily because (owing to a scientifically outmoded understanding of visual perception) he draws an excessively sharp, qualitative distinction between imagination and perception (following Wittgenstein, Sartre, and others), (...) and because of his fatally flawed, empirically ungrounded conception of hallucination. In fact, however, an understanding of perception informed by modern visual science will enable us to unify our accounts of perception, mental imagery, dreaming, hallucination, creativity, and other aspects of imagination within a single coherent theoretical framework. (shrink)

A theory of the structure and cognitive function of the human imagination that attempts to do justice to traditional intuitions about its psychological centrality is developed, largely through a detailed critique of the theory propounded by Colin McGinn. Like McGinn, I eschew the highly deflationary views of imagination, common amongst analytical philosophers, that treat it either as a conceptually incoherent notion, or as psychologically trivial. However, McGinn fails to develop his alternative account satisfactorily because (following Reid, Wittgenstein and Sartre) he (...) draws an excessively sharp, qualitative distinction between imagination and perception, and because of his flawed, empirically ungrounded conception of hallucination. His arguments in defense of these views are rebutted in detail, and the traditional, passive, Cartesian view of visual perception, upon which several of them implicitly rely, is criticized in the light of findings from recent cognitive science and neuroscience. It is also argued that the apparent intuitiveness of the passive view of visual perception is a result of mere historical contingency. An understanding of perception (informed by modern visual science) as an inherently active process enables us to unify our accounts of perception, mental imagery, dreaming, hallucination, creativity, and other aspects of imagination within a single coherent theoretical framework. (shrink)

If perception is constructed, what keeps perception from becoming mere hallucination unlinked to world events? The visual system has evolved two strategies to anchor itself and correct its errors. One involves completing missing information on the basis of knowledge about what most likely exists in the scene. For example, the visual system fills in information only in cases where it might be responsible for the data loss. The other strategy involves exploiting the physical stability of the environment as a reference (...) frame with respect to which the eyes and body can move. (shrink)

The visual brain consists of several parallel, functionally specialized processing systems, each having several stages (nodes) which terminate their tasks at different times; consequently, simultaneously presented attributes are perceived at the same time if processed at the same node and at different times if processed by different nodes. Clinical evidence shows that these processing systems can act fairly autonomously. Damage restricted to one system compromises specifically the perception of the attribute that that system is specialized for; damage to a given (...) node of a processing system that leaves earlier nodes intact results in a degraded perceptual capacity for the relevant attribute, which is directly related to the physiological capacities of the cells left intact by the damage. By contrast, a system that is spared when all others are damaged can function more or less normally. Moreover, internally created visual percepts-illusions, afterimages, imagery, and hallucinations-activate specifically the nodes specialized for the attribute perceived. Finally, anatomical evidence shows that there is no final integrator station in the brain, one which receives input from all visual areas; instead, each node has multiple outputs and no node is recipient only. Taken together, the above evidence leads us to propose that each node of a processing-perceptual system creates its own microconsciousness. We propose that, if any binding occurs to give us our integrated image of the visual world, it must be a binding between microconsciousnesses generated at different nodes. Since any two microconsciousnesses generated at any two nodes can be bound together, perceptual integration is not hierarchical, but parallel and postconscious. By contrast, the neural machinery conferring properties on those cells whose activity has a conscious correlate is hierarchical, and we refer to it as generative binding, to distinguish it from the binding that might occur between the microconsciousnesses. (shrink)

Salvador Dalí’s oilpainting Hallucination partielle. Six apparitions de Lénine sur un piano (1931) has been considered to be one of the most difficult works to interpret. O. Zaslavskii has analyzed it, using the sound of the words in title and the items depicted on the masterpiece, “the phonetic subtext”. Obviously, Zaslavskii’s interpretation is based on Osip Mandelstam’s poem “Grand piano” (1931), that in the context of Russian language associates the piano ( ) with the French Revolution. Nevertheless, Zaslavskii’s final conclusion (...) of the connections between Dalí’s painting and the French Revolution turns to be accurate, because it is possible to find iconographic parallels between Dalí’s “Partial hallucination…” and Jacques-Louis David’s “The death of Marat” (1793). On at least four most significant oil paintings from the beginning of Dalí’s surreal period we can observe his “emblem of love and death” as the combination of fellatio and bleeding. Obviously, he understood in the same code also Marat’s murdering by the knife of a woman. This allows us to insist, that Dalí was inspired to paint“Partial hallucination…” by “The death of Marat”. The shadow of a grand piano on his painting “Diurnal illusion: the shadow of a grand piano approaching” (1931) directly bears the meaning of “terror” and “fear”. In such motif combination and graphic parallel, the complex cultural metaphoric relations of these two paintings can be viewed. This complex can be considered as rhetorical in the sense of Juri Lotman’s conception. But it is evidently a case of “pure visual metaphor”, not an illustration of verbal metaphors. (shrink)

Salvador Dalí’s oilpainting Hallucination partielle. Six apparitions de Lénine sur un piano (1931) has been considered to be one of the most difficult works to interpret. O. Zaslavskii has analyzed it, using the sound of the words in title and the items depicted on the masterpiece, “the phonetic subtext”. Obviously, Zaslavskii’s interpretation is based on Osip Mandelstam’s poem “Grand piano” (1931), that in the context of Russian language associates the piano ( ) with the French Revolution. Nevertheless, Zaslavskii’s final conclusion (...) of the connections between Dalí’s painting and the French Revolution turns to be accurate, because it is possible to find iconographic parallels between Dalí’s “Partial hallucination…” and Jacques-Louis David’s “The death of Marat” (1793). On at least four most significant oil paintings from the beginning of Dalí’s surreal period we can observe his “emblem of love and death” as the combination of fellatio and bleeding. Obviously, he understood in the same code also Marat’s murdering by the knife of a woman. This allows us to insist, that Dalí was inspired to paint“Partial hallucination…” by “The death of Marat”. The shadow of a grand piano on his painting “Diurnal illusion: the shadow of a grand piano approaching” (1931) directly bears the meaning of “terror” and “fear”. In such motif combination and graphic parallel, the complex cultural metaphoric relations of these two paintings can be viewed. This complex can be considered as rhetorical in the sense of Juri Lotman’s conception. But it is evidently a case of “pure visual metaphor”, not an illustration of verbal metaphors. (shrink)

The Perception and Attention Deficit (PAD) model of visual hallucinations is as limited in generality as other models. It does, however, raise an interesting hypothesis on the role of attentional biases among proto-objects. The prediction that neither impaired attention nor impaired sensory activation alone will produce hallucinations should be addressed in future studies by analysing partial correlations between putative causes and hallucinatory effects.

En 1928 la théorie des form constants par Klüver catégorisait les hallucinations visuelles en quatre grandes catégories. Alors que la notion de form constants venait à s’appliquer virtuellement à toutes les figures entoptiques, comme l’avait prévu son auteur, dont les photismes synesthésiques, Cytowic a décrit et Carol Steen représenté ce que voient réellement des synesthètes visuels. L’une des caractéristiques d’œuvres de peintres synesthètes serait justement la présence de ces formes classées par Klüver. L’anthropologie avec la thèse de l’externalisation a (...) montré que certaines cultures ont repris les form constants perçues dans le premier stade de l’intoxication pour en faire les patterns de leur art. Les modèles physiologico-mathématiques permettent de localiser en V1 l’origine de production des photismes et d’expliquer leur morphogénèse. Mais une explication des circuits neuraux empruntés pour passer des différents types de déclencheur sensoriel à l’apparition de photismes synesthésiques reste aujourd’hui au stade de l’hypothèse. La synesthésie est l’objet de recherches alliant les sciences les plus contemporaines dont la génomique. Il est possible que les études sur la synesthésie transforment son statut de phénomène marginal en un élément essentiel dans la compréhension du système physiologique sous-jacent à l’esprit humain. In 1928, Klüver’s theory of form constants placed visual hallucinations into four broad categories. Whereas, as predicted by its author, the notion of form constants was virtually applicable to all entoptic figures, including synesthetic photisms, the very notion of entoptics was definitively extended. Cytowic has described and Carol Steen has represented what visual synesthetes actually see. The presence of these forms, as they have been classified by Klüver, is precisely one of the characteristics of the works of synesthetic painters. With the thesis of externalization, the field of Anthropology has shown that some cultures have adopted the constant forms perceived in the first stage of intoxication as patterns for their art. Physiological-mathematical models allow us to locate the origin of photism production in V1, and to explain these photisms’ morphogenesis. But, even today, an explanation of the neural pathways used to pass from different types of sensory triggers to the appearance of synesthetic photisms remains at the hypothesis stage. Synesthesia is a research topic that combines some of the most contemporary sciences, including genomics. Studies on synesthesia may just transform its status as that of a marginal phenomenon into that of an essential element in understanding the physiological system underlying the human mind. (shrink)

In defence of naïve realism, Fish has advocated an eliminativist view of hallucination, according to which hallucinations lack visual phenomenology. Logue, and Dokic and Martin, respectively, have developed the eliminativist view in different manners. Logue claims that hallucination is a non-phenomenal, perceptual representational state. Dokic and Martin maintain that hallucinations consist in the confusion of monitoring mechanisms, which generates an affective feeling in the hallucinating subject. This paper aims to critically examine these views of hallucination. By doing so, (...) I shall point out what theoretical requirements are imposed on naïve realists who characterize hallucinations as non-visual-sensory phenomena. (shrink)

At the very heart of the causal theory of perception are the peculiar examples sometimes called veridical hallucinations. These examples originate with Grice, who used them to prove ‘conclusively’ that when we say, for example, ‘Jane saw John’, we mean that John is the cause of certain visual experiences or impressions had by Jane.

In this paper, I attempt to consider Jewish philosophy in opposition to the anti-ocularcentrism that defined the German Jewish philosophical tradition after Kant, namely the idea that Judaism—or at least its philosophical expression in Maimonidean philosophy—is aniconic and cognitively abstract. I do so by attempting to rethink the epistemic-veridical place of the imagination and visual experience in the Guide of the Perplexed . Once the imagination has been disciplined by reason, is there any cognitive status to an image or sound (...) that the eye or the ear perceives, and to that mental faculty that combines and recombines such impressions? Is the sight or sound of revelation a hallucination or just a mere figure of speech? Does it bear any relation to a spiritual reality external to the human mind and finite physical existence? To address these questions I explore the visual images, both iconic and aniconic-abstract, that distinguish the Guide . There is no getting past the visual imagination, although I am not sure Maimonides would have recognized it as such. Even when he leaves behind figurative visual cues such as the false image-work of the undisciplined imagination or the appearance of angels and images of God found in lower grades of prophecy, he turns to another visual register, namely the “abstract art“ of pure, dazzling light. In regard to these questions, Maimonides was more Greek than German, ascribing, cautiously, penultimate cognitive status to the visual imagination. (shrink)

In a recent study, Becker and Elliott [Becker, C., & Elliott, M. A. . Flicker induced color and form: Interdependencies and relation to stimulation frequency and phase. Consciousness & Cognition, 15, 175–196] described the appearance of subjective experiences of color and form induced by stimulation with intermittent light. While there have been electroencephalographic studies of similar hallucinatory forms, brain activity accompanying the appearance of hallucinatory colors was never measured. Using a priming procedure where observers were required to indicate the presence (...) of one of eight target colors we compared electrophysiological correlates of hallucinatory color with brain states associated with other visual phenomena. Different target colors were accompanied by different patterns of EEG activation. However, in general, we found that the appearance of hallucinatory colors is preceded by a power decrease in the lower alpha band alongside an increase in gamma band frequencies. We argue that decreasing activity in the lower alpha band acts as a gating mechanism, inducing a switch in perception between different colors. The increasing gamma activation may correlate with the formation of a coherent conscious percept. (shrink)

I set out two theses. The first is Lynn Robertson’s: (a) spatial awareness is a cause of object perception. A natural counterpoint is: (b) spatial awareness is a cause of your ability to make accurate verbal reports about a perceived object. Zenon Pylyshyn has criticized both. I argue that nonetheless, the burden of the evidence supports both (a) and (b). Finally, I argue conscious visual perception of an object has a different causal role to both: (i) non-conscious perception of the (...) object, and (ii) experience, e.g. hallucination, that may be subjectively indiscriminable from, but is not, perception of the object. (shrink)

red and round. According to common sense, the red, round thing we see is the tomato itself. When we have a hallucinatory vision of a tomato, however, there may be present to us no red and round phys- ical object. Still, we use the words 'red' and 'round' to describe that situation as well, this time applying them to the visual experience itself. We say that we have a red, round visual image, or a visual experience of a red disk, (...) or some such. Because we see physical objects far more often than we hallucinate, we apply terms for color and shape to physical objects far more often than to visual experiences. Moreover, different theories of perception explain in different ways the applications such terms have to physical objects and to visual experiences. But whatever their frequency and explanation, it seems clear that both sorts of application occur. (shrink)

This paper proposes a classificatory framework for disjunctivism about the phenomenology of visual perceptual experience. Disjunctivism of this sort is typically divided into positive and negative disjunctivism. This distinction successfully reflects the disagreement amongst disjunctivists regarding the explanatory status of the introspective indiscriminability of veridical perception and hallucination. However, it is unsatisfactory in two respects. First, it cannot accommodate eliminativism about the phenomenology of hallucination. Second, the class of positive disjunctivism is too coarse-grained to provide an informative overview of the (...) current dialectical landscape. Given this, I propose a classificatory framework which preserves the positive-negative distinction, but which also includes the distinction between eliminativism and non-eliminativism, as well as a distinction between two subclasses of positive disjunctivism. In describing each class in detail, I specify who takes up each position in the existing literature, and demonstrate that this classificatory framework can disambiguate some existing disjunctivist views. (shrink)

According to the predictive coding theory of psychosis, hallucinations and delusions are explained by an overweighing of high-level prior expectations relative to sensory information that leads to false perceptions of meaningful signals. However, it is currently unclear whether the hypothesized overweighing of priors represents a pervasive alteration that extends to the visual modality and takes already effect at early automatic processing stages. Here, we addressed these questions by studying visual perception of socially meaningful stimuli in healthy individuals with varying (...) degrees of psychosis proneness. In a first task, we quantified participants’ prior for detecting faces in visual noise using a Bayesian decision model. In a second task, we measured participants’ prior for detecting direct gaze stimuli that were rendered invisible by continuous flash suppression. We found that the prior for detecting faces in noise correlated with hallucination proneness as well as delusion proneness. The prior for detecting invisible direct gaze was significantly associated with hallucination proneness but not conclusively with delusion proneness. Our results provide evidence for the idea that overly strong high-level priors for automatically detecting socially meaningful stimuli might constitute a processing alteration in psychosis. (shrink)