Introduction

The presence of psychotic features such as hallucinations, delusions or disorganized thinking is common in a wide range of mental disorders [1].

The prevalence of psychotic experiences in the population is difficult to assess given a range of methodological issues. However, numerous studies in recent years have emerged suggesting that psychotic experiences are more common in the general population than it was thought [2,3,4].

Some individuals without psychiatric history may manifest an attenuated form of hallucinations, of which only a minority will develop a psychotic or other mental disorder over time [5]. Such individuals may be relatives of patients with schizophrenia or individuals with psychometric schizotypy, or participants with physiological, neurological and cognitive characteristics similar to hallucinations found in patients with schizophrenia [6].

Psychotic experiences including auditory verbal hallucinations (AVH) may be phenomenologically and temporally continuous across different levels of clinical severity, ranging from subclinical psychotic experiences in the general population to full-blown psychotic disorder [7,8].

Contemporary models of psychosis suggest that an increase in mesolimbic dopaminergic neurotransmission occurs during the prodromal phase of psychosis, not corresponding to normal learning mechanisms [9]. Dysregulation of dopamine transmission, associated with alterations in top-down processing, causes neutral or irrelevant stimuli—associated with both external and internal representations—to be interpreted incorrectly. The tendency to assign altered meaning or emotional value to a neutral or irrelevant stimulus (aberrant salience) drives the individual to develop a cognitive scheme that alters its ability to process the experience and the surrounding environment appropriately [10,11].

Evidence suggests that alterations in salience attribution mediate the continuum of experiences of subclinical and attenuated psychoses to the sustained expression of psychotic disorder [4].

In order to explore the aberrant salience hypothesis in adults with psychotic disorders, their siblings and general population, a tool was developed to induce speech illusions, the "White Noise Task" (WN). It was first introduced in a study in 2011 [12]. Top-down processing was analyzed by detecting individuals who experienced speech illusions caused by this task and its association with variables of vulnerability to psychosis. The study found that the tendency to detect speech illusions was more frequent in the group of patients with psychotic disorder followed by the group at elevated familial risk. It also revealed that speech illusions were associated in healthy controls with high levels of positive schizotypy.

An attempt at replication was published by Catalan and colleagues, showing replication of the finding that patients had higher rates of WN than controls [13]. However, no association was found between WN and schizotypy in the control group.

In another study, speech illusions were examined in relation to psychotic phenomena in large general population sample of pre-adolescents, using an abbreviated version of the WN task. In this sample, speech illusions were associated with hallucinatory experiences [14].

Here, we present a novel analysis of the expanded non-clinical sample of the adult population presented in the earlier study by Catalan and colleagues [13]. Thus, the original sample was n = 150; the sample in the current analysis was n = 185. The aim was to determine the rate of speech illusions with the WN task and to analyse the association between speech illusions and schizotypal traits. Given the possibility that any association between WN and indicators of psychosis risk may be mediated by known risk cognitive factors, in particular cognitive ability, analyses were adjusted for IQ.

Method

Results

Sample

Controls were 55.14% males and more than half were single (58.38%). Mean age of controls was 31.81 years old (SD = 11.56). The majority was from middle social class (76.76%) and had had full-time education (38.92%). The majority (92.97%) did not live alone, living either with their parents or with their own family (wife/husband and children). The majority (77.84%) had an occupation in the form of either a job (52.43%) or studies (25.41%). Mean WAIS-IQ was 109.01 (SD = 14.93) (Table 1).

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Table 1. Socio-demographic and cognition variables https://figshare.com/s/a9b7606c8ef68f9253fd.

https://doi.org/10.1371/journal.pone.0192373.t001

Clinical variables were as follows: mean positive schizotypy score was 0.26 (SD = 0.28) and mean negative schizotypy 0.11 (SD = 0.23); CAPE positive dimension 0.22 (0.14) and negative dimension 0.50 (SD = 0.30). 12.97% of controls perceived at least two instances of any positive, negative or neutral voice in white noise (Table 2).

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Table 2. SIS-R and CAPE scores in healthy participants https://figshare.com/s/0b71b7caee297f3f2050.

https://doi.org/10.1371/journal.pone.0192373.t002

Schizotypy and speech illusions

Any speech illusion was associated with positive schizotypy (OR: 4.139, 95% CI: 1.074–15.938; p = 0.039) but not with negative schizotypy (OR: 1.151, 95% CI: 0.183–7.244; p = 0.881). However, the association of positive schizotypy with speech illusions disappeared after adjustment for age, sex and WAIS-III (OR: 2.577, 95% CI: 0.620–10.700; p = 0.192). Speech illusions were not associated with the CAPE positive (OR: 7.221, 95% CI: 0.471–110.497; p = 0.155) and negative scales (OR: 1.250, 95% CI: 0.294–5.315; p = 0.762) (Table 3).

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Table 3. Associations between white noise speech illusion and SIS-R and CAPE https://figshare.com/s/09c048d70d0c4ef3ed0c.

https://doi.org/10.1371/journal.pone.0192373.t003

Discussion

The degree to which WN reflects vulnerability for expression of psychosis in healthy participants, possible reflecting alterations in processing top-down or aberrant salience in healthy population, remains uncertain. No associations were apparent with self-reported measures of psychotic experiences. While WN was associated with interview-based measures of positive schizotypy, this appeared to be mediated to a large extent by other variables including cognitive ability.

In accordance with the current findings, a recent research [19] speculates that neurocognitive mechanism underlying perceptual abnormalities might differ between psychotic patients and the non clinical population, based on the findings showing no association between white noise speech illusion and subtle psychosis expression in a large general population adolescent and young adult twin cohort (n = 704).

The strengths of our study were, first, the use of both the SIS-R and CAPE scales, designed to measure the prevalence of positive experiences in the general population. Second, we included adequate control for cognitive ability. Lastly, the use of a representative sample in terms of age and education, reducing the risk of bias. A potential weakness is that an even larger sample may be required to detect the small association that may remain after adjustment for confounders. Similarly, sensitivity of the analyses may be enhanced if preferentially young people are included, given the high prevalence of psychotic experience around adolescents. This latter factor may explain the finding of a positive association between WN and hallucinatory experiences in an earlier study [14].

In conclusion, although there was an apparent association between the tendency to detect speech illusions in random noise and interview-based positive schizotypal traits, the association appeared to be mediated to a large extent by cognitive ability. Research in larger samples and/or uniformly young people may shed more light on this issue.

Acknowledgments

We would like to warmly thank all the volunteers who participated to the study.