One of the many opposite symptoms of autism as opposed to psychosis that was apparent from the beginning was that a classic symptom of schizophrenia is hearing voices, whereas a common complaint about autistics is that they seem to be deaf, and many autistics report difficulty hearing what someone is saying in a noisy ambience.

Now two different studies, kindly brought to my attention by Bernard Crespi, not only confirm this feature of the diametric model of mental illness, but also go some considerable way towards explaining it.

Not only psychotics, but a minority of the general population also experience auditory hallucinations frequently and without distress. As a recent study by Ben Alderson-Day and colleagues points out, "non-clinical voice-hearing (NCVH) is featurally similar to auditory verbal hallucinations described in psychosis, but usually more controllable and positive in content."

We examine for the first time the cortical processing of ambiguous speech in people without psychosis who regularly hear voices. Twelve non-clinical voice-hearers and 17 matched controls completed a functional magnetic resonance imaging scan while passively listening to degraded speech (‘sine-wave’ speech), that was either potentially intelligible or unintelligible. Voice-hearers reported recognizing the presence of speech in the stimuli before controls, and before being explicitly informed of its intelligibility. Across both groups, intelligible sine-wave speech engaged a typical left-lateralized speech processing network. Notably, however, voice-hearers showed stronger intelligibility responses than controls in the dorsal anterior cingulate cortex and in the superior frontal gyrus [below].

10.1093/brain/awx206
Experimental design and behavioural results. Participants were scanned in functional MRI while (A) listening to intelligible SWS, unintelligible SWS, or noise-vocoded, unintelligible target sounds; (B) listening and rest trials were presented in a pseudo-random order across two 20-min runs, divided by a ‘reveal’ period including training to understand SWS stimuli; (C) each trial lasted 8.4 s, including jitter, a 2 s stimulus and 3.4 s of volume acquisition; (D) NCVH participants recognized speech being present earlier than control participants during Run 1 (left), and this correlated with voice-hearing during the previous week (PSYRATS – Physical Characteristics subscale). PSYRATS = Psychotic Symptoms Rating Scale.
Source: Brain 2017 doi:10.1093/brain/awx206

This suggests an enhanced involvement of attention and sensorimotor processes, selectively when speech was potentially intelligible. Altogether, these behavioural and neural findings indicate that people with hallucinatory experiences show distinct responses to meaningful auditory stimuli. A greater weighting towards prior knowledge and expectation might cause non-veridical auditory sensations in these individuals, but it might also spontaneously facilitate perceptual processing where such knowledge is required.

You could call this over-hearing in the sense that these voice-hearers were doing something we all do, but in a more intense manner. According to the researchers: “Such individuals do not appear to be differentially affected by explicit modulations of expectation—instead, people in this group report being able to spontaneously extract speech from degraded auditory signals (and report doing so earlier than matched controls).” They add that “This suggests that the fundamental mechanisms underlying hallucination involve—and may develop from—ordinary perceptual processes, illustrating the continuity of mundane and unusual experience. It has implications not only for ‘continuum’ views of experiences usually associated with psychosis (…) but also for the normalization, interpretation, and public understanding of a seriously misunderstood phenomenon."

Finally, the authors might have gone on to add that their findings fit one “continuum” view in particular: the diametric model, and especially one of its most distinctive features: the claim that symptoms of mental illness are not alien intrusions into the mind, but pathological elaborations or deficits of normal function. Indeed, given that language is a key factor in mentalism, our evolved capacity to relate to other people in mental terms such as knowledge, belief, feeling, intention, and so on, you could say that in its full clinical manifestation auditory hallucination was a case of hyper-mentalism: the key factor in psychosis according to the diametric model. Indeed, back in 2010 I published a post making exactly this point in relation to an early study of voice hallucination whose findings were completely in line with the new one. 

In that post I alluded to the prediction of the diametric model that autistics ought to be the exact opposite, and in another study also brought to my attention by Bernard Crespi,

Two groups of subjects participated in the study: 11 high-functioning normal-hearing adults and adolescents with confirmed diagnoses of autism or Asperger’s syndrome (HFA/AS); and 9 normal-hearing age/IQ matched adult and adolescent control subjects, with no previously reported speech-in-noise perception problems. All had normal hearing thresholds (<20 dB HL) across the audiometric frequencies (.25 to 8 kHz) and middle ear function within normal limits, and were paid for their services.

The authors conclude that

the results of the current study suggest that the problems commonly reported by autistic individuals of understanding speech when there is background noise are real and quantifiable. They may be due, at least in part, to abnormal peripheral processing, specifically, to a reduced ability to exploit information about the target speech present during the spectral and temporal dips in the background.

If voice hallucination could be described as “over-hearing”, then the deficit revealed here could certainly be called “under-hearing” understood as something quite different from being deaf. Taken together, the terms nicely illustrate the great strength of he diametric model: its aptness for describing both normality and pathology and its ability to account for symptoms at both ends of the mentalistic continuum.

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