Online research in philosophy

There are strong resemblances between the neurobiological characteristics of hallucinations occurring in the particular case of schizophrenia and the hallucinatory activity observed during the rapid-eye-movement (dreaming) sleep stage: the same prefrontal dorsolateral deactivation; forebrain disconnectivity and disinhibition; sensory deprivation; and acetylcholine, monoamine, and glutamate modifications.

This paper explores how the Generalized Continuum Hypothesis (GCH) arose from Cantor's Continuum Hypothesis in the work of Peirce, Jourdain, Hausdorff, Tarski, and how GCH was used up to Gödel's relative consistency result.

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.

Hallucinations occur in a wide range of organic and psychological disorders, as well as in a small percentage of the normal population According to usual definitions in psychology and psychiatry, hallucinations are sensory experiences which present things that are not there, but are nonetheless accompanied by a powerful sense of reality. As Richard Bentall puts it, “the illusion of reality ... is the sine qua non of all hallucinatory experiences” (Bentall 1990: 82). The aim of this paper is to find out what lends an experience ‘a sense of reality’: what features are required for an experience to feel ‘real’, in the relevant sense? I will investigate the claim that phenomenological features are largely responsible for a sense of reality, and will find this claim wanting. My suggestion is that a sense of reality is created and sustained by the larger nexus of the subject's beliefs.

It is common both in philosophy and in the cognitive sciences (broadly understood as ranging from, say, neuroscience to developmental or evolutionary psychology) to distinguish between two kinds of hallucinations.1 What differentiates them is whether they are subjectively indistinguishable from genuine perceptions and therefore mistaken by us for the latter. While perceptual (or ‘true’) hallucinations cannot, from the subject’s perspective, be told apart from perceptions, non-perceptual (or ‘pseudo’) hallucinations can and usually are. Sometimes, when subjects, say, auditorily hallucinate someone else calling their name or commenting on their behaviour, they are able to realise, on the basis of how they subjectively experience their episode of hallucination, that they are not perceiving real speech: their hallucination is non-perceptual. This happens, for instance, when subjects suffering from schizophrenia or other illnesses hear ‘inner voices’ speaking to them. But in other circumstances, the subjects concerned are - even under conditions of proper mental health and rationality - in no position to recognise the hallucinatory status of their experience without the help of some external evidence: their hallucination is perceptual. Wrongly hearing the phone ringing while taking a shower, say, is a good example of this kind of hallucination.2 When philosophers speak or write about hallucinations, they usually concentrate on perceptual hallucinations. One reason for this is the fact that philosophers tend to address the topic of hallucination, not for its own sake, but only in the context of some wider issues. Thus, when they are discussing hallucinations, they are primarily interested in other topics, such a how - or whether - we are able to acquire knowledge about the external world, in which sense our mental states are directed at objects and properties, how best to account for what our experiences are subjectively like, which features suffice for something to count as a conscious experience, and so on..

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.

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.

Behrendt & Young's (B&Y's) novel “unifying model” of hallucinations, although comprehensive, fails to incorporate research into the possible role of 5-HT2A receptors in the mode of action of novel “atypical” antipsychotic drugs (which treat hallucinations effectively), and into the role of such receptors, which are located in thalamocortical circuits, in mediating drug-induced hallucinations.

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.

This commentary challenges the argument that the diathesis for hallucinations is equivalent to that for schizophrenia. Evidence against this comes from data on the prevalence of hallucinations in schizophrenia, their nonspecificity, and their relationships with moderating variables. We also highlight, however, the manner in which the Behrendt & Young (B&Y) hypothesis extends recent neuroscientific theories of schizophrenia, and its potential treatment applications.