This review examined the hypothesis that mental imagery vividness can be used as a predictor of hallucinatory experience. Earlier studies provided supporting evidences to this hypothesis, showing hallucinating population has higher mental imagery vividness comparing to nonhallucianting population. However, as a result of varied operationalization and measurements of mental imagery, contradicting results abound, showing no significant difference of mental imagery vividness between halluciantors and nonhallucinators. No clear evidences can be used to determine whether the hypothesis is valid or not so far. On the other hand, development of neurological studies provided a new perspective for looking into the relationship between mental imagery and the experience of hallucination.
Keywords: mental imagery vividness, hallucination, schizophrenia
Mental Imagery Vividness as a Predictor of Hallucination: A Literature Review Mental imagery, as defined by Finke (1989), is an experience significantly resembling that of perceiving, but it occurs in the absence of an adequate physical stimulus. It exists in all of the seven sensory modalities, such as visual, auditory, and olfactory (Thomas, 1999). Mental imagery is believed to be in close relationship with some core psychological mechanisms such as perception and memory, and holding its unique role in contributing to cognitive performance (Kosslyn, 1994). For example, evidences suggested that visual imagery ability predicts visuospatial memory performance (Kail, 1997). Hallucination is an experience that largely resembles mental imagery, because of its perceptual nature as well as absence of appropriate stimuli (Sack, Van de Ven, Etschenberg, Schatz, & Linden, 2005). Nevertheless, the two distinguish from each other by the individual’s ability of voluntary control, as well as his/her ability to determine the source of the experience.
Specifically, mental imagery is generally regarded as being actively generated and can be intentionally controlled, while hallucination is most times beyond intention and control. The operator of mental imagery is usually aware of its internal source, while not necessarily so hallucinator. (Bentall, 1990). Hallucination, especially the form of auditory verbal hallucinations (AVH), is an important hallmark of schizophrenia (Wible, 2009). AVH is the perception of voices in the absence of sensory input. AVH has distinct clinical significance, as it affects about 70% of patients with schizophrenia (Sartorius et al., 1978; Silbersweig & Stern, 1996). Furthermore, there have been increasing evidences supporting that hallucination is not a rare case in non-clinical population, either.
It is now believed to exist on a spectrum from comparably innocuous forms in non-clinical population to a more pathological manifestation in schizophrenics (see review by Bentall, 1990). Because of its significant implication in psychopathology, continuous work has been going on to find out the risk factors and predictors for hallucination. The relationship between mental imagery and hallucination has interested researchers for long because of their shared features. Among all the characteristics of mental imagery that are potentially related with hallucination, vividness has been most substantively examined. This review summarized theories and empirical evidences for the relationship between mental imagery vividness and hallucination. Although still in its budding stage, relevant neurological evidences were also examined.
Vividness of mental imagery can be defined as the degree of perceptual detail experienced when having a mental image (Oertel et al., 2009). It has been associated with hallucination and schizophrenia for decades. In 1883, Galton suggested that increased vividness of mental imagery might be associated with hallucinatory experiences (as cited in Aleman, Bocker, & de Haan, 1999). Later in the 20th century, West (1962) and Horowitz (1975) proposed separate models suggesting that hallucinations were mental images derived from internal source, but mistakenly attributed to external source. More specifically, the model suggested that vividness of perception was typically higher than that of mental imagery. As vividness of mental imagery increased, it became more difficult for the individual to decide its source as internal or external, and eventually these images could grow into hallucinatory.
This model suggested that mental imagery and hallucination shared fundamental features and were possibly on the same continuum. Johnson and Raye (1981) supported the importance of imagery vividness, stating that memories from internal and external sources could usually be differentiated by comparing the amount of sensory, contextual and semantic attributes of events. Mintz and Alpert (1972) developed their model relevant to mental imagery vividness as well. They proposed that vividness of imagery was not the only factor contributing to hallucination. According to their argument, increased vividness of imagery, and impaired reality testing, referring to the ability to recognize the distortions in one’s own perception, are necessary but not sufficient prerequisites to generation of hallucination. Bentall (1990) summarized predecessors’ work and introduced the concept of “reality discrimination” – the idea that “hallucinators mistake their own internal, mental, or private events for external, publicly observable events”.
They stated that one type of information used in the reality discrimination process was the amount of sensory information present in conscious: rich sensory information implied a perception being experiences, while poor sensory information implied a mental imagery. Following this argument, individuals who have the propensity to construct images rich in sensory details are more likely to experience their internal imagery as a perception experience, and thus resulting in hallucination. Barrett (1993) provided empirical evidences in support of the above hypothesis, yet he also pointed out some questions of the model. He argued that it was unclear where the locus of the imagery effects was.
The imagery vividness discrepancy between individuals with and without hallucinations could be the result of either storage or retrieval difference of sensory information. Specifically, it could be that hallucinators were able to store more abundant sensory information than nonhallucinators; and it could also be that hallucinators were better at retrieving sensory information than nonhallucinators, with same storage capacity.
Despite of some questioning opinions, there are very limited direct theoretical dissents or alternative models. Holt (1972) is one of the very few that he argued that hallucination and mental imagery involved independent systems and should not correlate with each other. However, this is not saying that the hypotheses of mental imagery vividness and hallucination have not been subject to scrutinize. A large amount of researchers have attempted to verify or disprove these hypotheses through empirical evidences. Below is an incomplete summary of the empirical studies that are relevant.
Evidences in Support
Mintz and Alpert (1972) provided empirical evidences in support of their own hypothesis. Their study found that auditory hallucinating schizophrenics had a significantly higher vividness of auditory mental imagery comparing to non-hallucinating controls. They also identified an impaired ability to assess the accuracy of auditory perceptions in hallucinating schizophrenics. Similar results were presented by other studies. Barrett (1993) found that nonclinical subjects with hallucinations had higher vividness of mental imagery, and at the same time, had lower control of these images comparing to nonclinical subjects without hallucinations. Barrett’s (1993) study differed from that of Mintz and Alpert (1972) in the sense that he assessed mental imagery vividness on all seven sensory modalities, with one single factor emerged after factor analysis – general imagery vividness factor.
In another study by Bocker (2000), no group difference in perceptual acuity was identified between the schizophrenia group and the normal controls, suggesting perceptual degradation is not a reason for hallucination. For the hallucinating schizophrenia patients, the vividness for visual imagery was significantly lower than control, while that for auditory imagery was not. Although the decreased visual mental imagery contradicted with earlier studies, the relatively high auditory mental imagery ability suggested the possibility that auditory imagery for hallucinating patients was more percept-like and thus harder to differentiate, which is in line with the fact that most of the patients experienced hallucinations in auditory modality.
Evidences in Contradiction
Brett and Starker (1977) found no significant difference of auditory mental imagery vividness between hallucinating schizophrenics, nonhallucinating schizophrenics medical patients with no history of psychiatric problems. Starker and Jolin (1982) also found no significantly different vividness strength of auditory mental imagery between schizophrenics, possible schizophrenics, or nonschizophrenic psychiatrics, nor between schizophrenic subgroups of currently hallucinatory, previously hallucinatory, or nonhallucinatory. In fact, Starker and Jolin (1982) suggested that hallucinating schizophrenics might actually have less vivid auditory imagery than schizophrenics who had never hallucinated. Aleman et al.’s (1999) study revealed interesting results.
In their study, hallucinating subjects reported higher imagery vividness than nonhallucinating controls when subjective (self-report) scales were used to measure vividness. However, the pattern was reversed (i.e. hallucinating subjects demonstrated lower mental imagery vividness) when objective measure was used. Van de Ven and Merckelbach (2003) examined the mental imagery vividness and fantasy proneness in non-clinical population with and without hallucination experiences. They found that although subjects with hallucination scored higher on mental imagery vividness than subjects without such experiences, their imagery vividness was highly correlated with their fantasy proneness. Further analysis indicated that hallucination experience was better predicted by fantasy proneness, comparing to mental imagery vividness.
Sack and his colleagues (2005) incorporated in their study measures sensitive to cognitive capacity, in order to rule it out as a confounding variable. Their results showed that paranoid schizophrenics reported higher vividness of mental imagery in comparison to age- and sex-matched healthy controls, controlling for general intelligence and psychomotor speed of the subjects. More importantly, the higher imagery vividness of paranoid schizophrenics was not statistically dependent of the patients’ individual psychopathology, including the presence of hallucinations. The authors thus argued that mental imagery could be an independent trait marker of schizophrenia, and it operated on a separate system with hallucination for schizophrenics. Sack and colleagues (2005) also found that patient group performed worse on tasks that require involvement of mental imagery processes, which could indicate an impaired overall mental imagery ability.
However, this performance deficit could be attributed to general cognitive capacity deficit. In comparison, Doninger, Silipo, Rabinowics, Snodgrass and Javitt (2001) conducted a study with a task that was more purely targeted at mental imagery ability without time constraint. Their results also revealed worse performance from schizophrenia patients comparing to healthy controls. Results of these studied implied the possibility that group difference in vividness of mental imagery is not due to an enhancement of mental imagery abilities, but to altered information processing. Oertel and colleagues (2009) conducted a further study to address the question that whether vividness of mental imagery is a trait marker of the schizophrenia spectrum (i.e. including non-clinical population with schizophrenia-like traits – schizotypy) that is independent of presence of hallucination.
Subjects include schizphrenia patients, first-degree relatives of schizophrenics, as well as high- and low-schizotypy healthy controls. Results showed significantly higher mental imagery vividness across all modalities in schizophrenics, first-degree relatives, as well as high-schizotypy controls comparing to low-schizotypy controls, with cognitive abilities controlled. In replication of Sack et al.’s work, this study also revealed that the vividness of mental imagery and hallucinations were independent of each other. Results also indicated that first-degree relatives obtained highest score in vividness, and high-schizotypy controls’ score did not differ significantly from that of schizophrenia patients.
The results strengthened Sack et al.’s (2005) argument that vivid imagery is a trait marker of schizotypyrather, and could be related to the genetic liability to develop schizophrenia. However, longitudinal studies including premorbid data are required for further investigation. Bell and Halligan (2010) repeated Oertel et al.’s study on high- and low-schizotypy population with a larger sample size, yet with a different measure of schizotypy and a specific assessment of visual mental imagery. Their results revealed no significant difference on visual mental imagery vividness between the two groups.
Explanation for Results Variation
Although large variations exist across different studies, it is not justified to say whether the hypothesis of high mental imagery vividness predicts hallucination has been rejected or not. The main reason for failing to do so is that procedures, especially the measures of mental imagery, of these studies varies a lot, thus leaving considerable space for alternative explanations. Specifically, some of the measures were subjective scale (i.e. self-report), while some of the measures were objective (i.e. behavioral tasks). On the other hand, some of the measures assess mental imagery vividness of a single sensory modality, while some of them assess mental imagery vividness as a whole across all seven sensory modalities. S
ubjective Measure vs. Objective Measure of Mental Imagery
Kosslyn, Brunn, Cave and Wallach (1984) had presented an insignificant relation between performance on an objective task of visual imagery acuity and a self-report imagery vividness measure, indicating that subjective and objective measures of imagery vividness may not be necessarily measuring the same constructs and/or processes. Aleman et al. (1999) pointed out that objective and subjective mental imagery measures could yield contradictory results within the normal population. This distinction could be true for clinical population as well. For example, Mintz and Alpert (1972) utilized subjective measures (suggestion paradigms or self-report measures) and obtained the finding of an association between increased mental imagery and hallucinations, while Bocker et al. (2000) used a more objective mental imagery task, resulting in insignificant difference of mental imagery performance between patients and controls.
Aleman et al. (1999), who generated contradicting results of subjective and objective measures in one study, summarized the disadvantages of subjective and objective measures. For subjective measures, the process of introspection is involved, which is an ability varied across individuals, and can be especially impaired in hallucinating and/or schizophrenia populations. Also, the effect of social demand could influence a subject’s self-report, depending on how the concept of mental imagery vividness is interpreted. Subject’s idiosyncratic definition of imagery vividness also heavily impact his/her self-report. For objective measures, tasks are not real-life resembling, which can fail to evaluate one’s true capacity in daily life.
In addition, commonly used objective measures fail to purely target at the vividness of mental imagery alone, other processes such as conceptualization and reasoning are in effect as well. In addition, the contents of cued mental images could largely vary across individuals. Standardized scoring procedure fail to take into account these confounding processes and experiences, indicating that high imagery vividness does not guarantee a high score as it is designed to, and vice versa. General Mental Imagery vs. Single Sensory Modality Mental Imagery Barrett (1993) introduced the idea of general imagery vividness factor. If that is a valid factor, then measurement of general mental imagery vividness and measurement of single sensory modality imagery vividness could be more different than the mere part-whole relationship. Mintz and Alpert (1972) measured auditory imagery vividness only, and Barrett (1993) measured general mental imagery vividness.
These two studies both resulted in significant difference of imagery vividness between halluciantors and controls. However, Brett and Starker (1977) and Starker and Jolin (1982) both measured auditory only imagery vividness and resulted in no significant difference between hallucinators and controls. In addition, in replication of Oertel’s (2009) study, Bell and Halligan (2010) changed the general imagery vividness measure to a visual-specific mental imagery measure, and their results differed from that of Oertel et al.. Bell and Halligan (2010) proposed that vividness per se might not be modality specific, and this argument is in line with the modality-independent salience dysregulation theories of psychosis and the psychosis continuum (Murray, Lappin, & Di Forti., 2008).
Despite of the fact that neurological evidences for mental imagery and hallucination experiences did not start to bloom until late 20th century (Linden et al., 2010) and that there is no specific neurological evidence for mental imagery vividness’s relationship with hallucination, this session is included as it provides new perspectives on how experiences of mental imagery and hallucination overlap. Evidences from earlier studies suggested that imagery and hallucinations were associated with overlapping neural networks (Allen, Laroi, McGuire, & Aleman, 2008). McGuire et al. (1995) and Shergill et al. (2001) found that auditory mental imagery and auditory hallucination were involved with overlaps in fronto temporal language circuits. In terms of the main difference between hallucination and mental imagery – voluntary controllability, Linden et al. (2011) speculated that it was generated from the altered forward model of sensory productions.
Forward model (Wolpert, Ghahramani, & Jordan, 1995) stated that perceptual consequences were predicted upon known actions, and the prediction in turn influenced the actual experience of the self-generated action. The forward model could be in effect in speech production. According to it, self-generated speech is subject to prediction of resulted sensory experience. This hypothesis was partially supported by Linden et al.’s (2011) study. They found that the voice-selective area on the banks of superior temporal sulcus (STS) was consistently activated during auditory hallucination and voluntary auditory mental imagery.
This area had been regarded as reacting to external sensory stimulation only, but it was later speculated of having been monitoring the self-generated speech in mental imagery and hallucination. Linden et al. (2011) identified in their study that the monitoring and modulating functioning of relevant auditory regions were disturbed in nonclinical hallucinators, resulting in the likelihood of attributing internally generated speech to external sources. These budding evidences suggested that hallucination and mental imagery indeed share fundamental neural networks and do not operate on completely distinctive systems. However, detailed examinations of the activity of these neural networks in terms of different features of mental imagery and hallucination, as well as differed neural system contributing to their differences are still in need.
Mental imagery vividness has interested researchers for long because of their potential relationship with the experience of hallucination. However, empirical studies have resulted in contradicting results. Earlier studies proposed that increased vividness of mental imagery is a predictor of hallucination, and they have found that higher mental imagery vividness was related with hallucination experiences. Nevertheless, many of the later replications with various methodological modifications failed to reproduce any significant relationship.
Because of the variations of measurement of mental imagery vividness, it is hard to decide whether the hypothesis of the relationship between mental imagery vividness and hallucination can be rejected or not. The development of neuroscience based study methods provided a new perspective of testing the hypothesis. Current neurological studies have focused on neural activity comparison between general mental imagery process and hallucination experience. More specific studies targeting at features of mental imagery and hallucination are needed to provide a higher-level understanding of the relationship between the two experiences.
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