As a general label for a range of symptoms associated with severe mental illness, “psychosis” is typically characterized as a “loss of contact with reality.” Although lacking a consistent operational definition, one of the most problematic aspects of the term, as traditionally employed, is its assumed categorical nature. In contrast to the traditional categorical approach to psychosis adopted in the Diagnostic and Statistical Manual of Mental Disorders1
(DSM-IV), there is growing interest in a more dimensional view, which argues that psychosis-like beliefs, perceptual distortions, and idiosyncrasies of thought and communication, considered the hallmark diagnostic criteria for psychosis, are distributed (albeit to varying degrees) throughout the general population. Such an approach considers florid psychosis as comprising the most extreme pole of the population spectrum.2–6
The view that psychotic manifestations may exist on a continuum, rather than as a discrete entity, however, is not new. In contrast to the more popular Kraepelinian view, Bleuler, and later others, argued throughout the twentieth century against a clear separation between sanity and madness.7–11
The development of psychometric measures that have attempted to capture the continuum of psychosis and psychosis-like experience has facilitated this noncategorical view. The focus for such scales, however, has varied, with some aiming to measure a general psychosis proneness, while other have focused on particular aspects of the psychosis continuum (such as delusional ideation or hallucination proneness) influenced by the symptom boundaries of clinical psychiatry.
One of the earliest attempts to capture a general concept of psychosis proneness was Eysenck's inclusion of the psychoticism dimension as an aspect of personality.12,13
Adopting a personality-theory standpoint, Eysenck aimed to capture psychosis proneness on a dimensional construct varying from normality (necessarily defined in culturally relative terms) and psychosis. This was subsequently developed into a multidimensional concept of schizotypy,14
based on a factor analysis of various psychosis-proneness scales,15
which has been developed into the “unusual experience,” “cognitive disorganization,” “introvertive anhedonia,” and “impulsive nonconformity” subscales of the Oxford and Liverpool Inventory of Feelings and Experiences (O-LIFE) schizotypy scale.16
In contrast to this approach, most other measures of psychosis proneness are grounded in clinical psychiatry and aimed at measuring attenuated or “soft” psychotic symptoms in the general population. Of particular relevance, understandably, are those measures that attempt to quantify aspects of the “positive symptoms” of psychosis, such as delusions and hallucinations.
The Magical Ideation Scale by Eckblad and Chapman17
covers a range of beliefs and experiences from first-rank symptoms of schizophrenia18
and ideas of reference to popular paranormal and conspiracy theory themes (eg, “The government refuses to tell us the truth about flying saucers”). The Peters et al. Delusions Inventory4,5
(PDI) is a measure of delusional ideation that inquires about beliefs, interpretations, and experiences, using items derived from the Present State Examination,19
an internationally recognized clinical measure, which is often used to detect and assess clinically defined psychotic symptoms. The PDI, however, is unique in that it not only measures the total number of beliefs or experiences endorsed but also the concurrent perceptions of distress, preoccupation, and conviction associated with the endorsed items.
Other measures have focused on perceptual and hallucinatory experiences associated with psychosis. The Perceptual Aberration Scale20
measures the level of body-image aberration, with items based on experiences of somatic distortions and hallucinations, as reported in the clinical literature on schizophrenia and associated diagnoses. Morrison, Wells, and Nothard created and revised the Launay-Slade Hallucinations Scale21,22
(RLSHS) to measure predisposition to hallucinations, in an attempt to capture some clinically recognizable hallucinatory phenomena (such as “hearing voices” and having nonveridical visual experiences), as well as any tendency to have vivid imagery and daydreams.
The Structured Interview for Assessing Perceptual Anomalies23
(SIAPA) is one alternative assessment method that does not rely on self-report. Although it aims to be comprehensive in its coverage of the “5 senses,” it is designed as an interview-based assessment of the frequency of sensory anomalies and, therefore, has the disadvantage of being time-consuming and requiring 1-to-1 assessment. It also is restricted in that, unlike some of the psychosis-inspired scales already mentioned, it does not assess hallucinatory phenomena directly but instead focuses on changes in sensory intensity, attention, and sensory flooding. It is clear that a measure is needed to assess the range of perceptual anomalies not covered by any single existing scale.
Furthermore, many of the psychometric measures of anomalous perceptual experience derive both their content and language from mainstream clinical psychiatry (which depends on frank and often chronic forms of mental illness), and it is apparent that they may lack face validity when trying to assess accurately the full range of perceptual anomalies in the general population.
These biases can make perceptual and cognitive distortions difficult to tease apart adequately. Several of the scales are not “pure” measures of perceptual anomaly (although deliberately so in many cases). For example, the Launay-Slade Hallucinations Scale, despite its name, conflates items concerning both perceptual experience (eg, “I hear the telephone ring and find that I am mistaken”) and delusional ideation (eg, “I fantasize about being someone else”) into a single measure.
There is also an implicit assumption in some scales that respondents are able to distinguish between experiences that stem from perceptions that exist out in the “real world” and those that may arise from distortions with the respondent's own cognitive processes—that is, those that are considered “not to be really there,” as illustrated by this item from the O-LIFE: “When in the dark, do you often see shapes and forms even though there's nothing there?”
Other measures rely on a related concept of strangeness or unusualness (for example, “When I look at things, they appear strange to me,” from the RLSHS) that presupposes a nonveridical perceptual experience will necessarily present as strange or anomalous. Both assumptions are potentially problematic “since virtually all waking perceptual experiences are veridical, a long personal history of validated perception would dictate accepting hallucinations as veridical.”24(p9)
Of course, it may be that perceptual anomalies are accompanied by insight into their unusual nature, but it is important that this is not the only criterion by which such anomalies are measured. In fact, there may be several indicators that a perceptual experience is not veridical for an individual, including those that may arise without a clear source, those that do not seem to be shared by other people in the vicinity, and those that that are accompanied by a sense of strangeness.
Another drawback of assessing perceptual anomalies by extrapolating exclusively from the context of clinical psychiatry is the overreliance on hallucinatory phenomena that occur in the visual and auditory modalities. For example, surveys of hallucinatory phenomena in the general population indicate that olfactory and gustatory hallucinations are particularly common,25
yet these modalities are rarely explored in psychometric measures of hallucination or psychosis proneness. Likewise, alterations in sensory intensity, rather than the experience of discrete perceptual phenomena, are not normally covered by existing scales. Another legacy of clinical psychiatry is the lack of coverage of perceptual anomalies associated with temporal lobe disturbance, despite the fact that temporal lobe disturbance has been linked to almost every “stage” on the psychosis continuum, from full-blown psychosis26,27
to paranormal beliefs and experiences,28
as well as to anomalous perceptual phenomena in nonclinical participants.29
Thus, there is a need for a comprehensive scale capable of measuring a range of sensory experience, covering both clinical and nonclinical populations.
Consequently, the purpose in designing the Cardiff Anomalous Perceptions Scale (CAPS) was to construct a valid and reliable psychometric measure of perceptual anomalies. Critically, it is not dependent on the clinical psychiatric context and considers subjective experiences from a range of different perspectives of insight awareness (including knowing that the percept is “not really there,” the percept seeming strange or unusual, or the percept being a nonshared sensory experience). Moreover, the CAPS includes items pertaining to distortions in perceptual intensity, to experiences in all appropriate sensory modalities, and to sensory experiences traditionally associated with temporal lobe disturbances. Following the usefulness of their inclusion in the PDI,4,5
we also included dimensional ratings to measure associated distress, intrusiveness, and frequency for each experience endorsed.