Elsevier

Schizophrenia Research

Volume 193, March 2018, Pages 91-97
Schizophrenia Research

Schizotypy is associated with reduced mnemonic precision in visual working memory

https://doi.org/10.1016/j.schres.2017.07.046Get rights and content

Abstract

Background

Working memory (WM) maintains a limited amount of information over a short period of time at the service of other ongoing mental activities. Deficits in this function are often observed in schizophrenia spectrum disorders. The present study examined whether self-report schizotypy was associated with the qualitative, quantitative, or both aspects of visual WM and whether these impairments could be accounted for by sensory memory deficits and/or depressed mood in a group of non-clinical, medication-naïve participants.

Method

Visual WM and sensory memory were assessed in 164 medication-naïve college students using delayed and immediate color estimation tasks, respectively. Self-report measures of schizotypy and depressed mood were also collected.

Results

Individuals with more schizotypal features retained less precise representations in visual WM, without a significant reduction in the number of retained WM representations (i.e., capacity). In contrast, there was no significant correlation between sensory memory precision and schizotypy, suggesting that schizotypy-related imprecision in visual WM was unlikely a result of imprecise sensory memory. Furthermore, opposite patterns of WM deficits were observed for depressed mood in that it was negatively associated with WM capacity, but not with WM precision.

Conclusion

Together, the present findings demonstrated dissociable WM deficits in schizotypy and depressed mood, providing strong evidence for unstable mental representations in schizotypy and reduced cognitive resource in depressed mood.

Introduction

Working memory (WM) maintains a limited amount of information over a short period of time at the service of other ongoing mental activities (Baddeley, 2012, Cowan, 2001). Impairments in this function are often observed in schizophrenia spectrum disorders, including schizophrenia patients (Park and Gooding, 2014), their first-degree relatives (R. Zhang et al., 2016), and individuals with schizotypal features (Park et al., 1995, Park and McTigue, 1997, Rosell et al., 2014; but see Kane et al., 2016, Matsui et al., 2007, Xie et al., 2014). The present study aims to examine whether self-report schizotypy is associated with the qualitative, quantitative, or both aspects of visual WM in a group of non-clinical, medication-naïve participants.

Schizotypy is often referred to as a multidimensional personality organization of subclinical schizophrenia-like traits, ranging from ordinary dissociative, imaginative states to more extreme states associated with schizophrenia pathology (Kwapil and Barrantes-Vidal, 2015, Meehl, 1962, Meehl, 1990). It may be a manifestation of inherited genetic diathesis and adverse environmental experiences (Meehl, 1962, Meehl, 1990). Schizotypal features, such as affective flattening and social disinterest (i.e., negative schizotypy) as well as odd beliefs and unusual perceptual experiences (i.e., positive schizotypy), can be captured by self-report checklists or psychometric inventories (J. P. Chapman et al., 1995; L. J. Chapman et al., 1978, Chapman et al., 1976, Chapman et al., 1980, Eckblad and Chapman, 1983, Raine, 1991, Winterstein et al., 2011b). The current study focuses on this subclinical stage of schizophrenia spectrum disorders for three specific reasons. First, although individuals with schizotypal features have increased risks of developing full psychosis (Cannon, 2015), they seldom take anti-psychotic medication. This allows examination of memory impairments potentially associated with schizophrenia-related symptoms without confounds of antipsychotic medication. Second, intermediate schizophrenia-spectrum phenotypes, such as schizotypy, may originate from shared genetic roots with schizophrenia (Nicolson et al., 2003, Torgersen, 1985). Consequently, individuals with schizotypal features often demonstrate neuroanatomical (Modinos et al., 2009) and neuropsychological (Chan et al., 2016b) impairments that largely overlap with those in schizophrenia patients, including deficits in WM (Hazlett et al., 2014, Park et al., 1995, Park and McTigue, 1997, Roitman et al., 2000). Third, assessment of WM impairments in a subclinical stage of schizophrenia spectrum disorders could potentially facilitate early diagnosis and intervention of the disease by using cognitive impairments as target features to capture schizophrenia-related deficits (e.g., Germine and Hooker, 2010).

Impairments in visual WM have been reported in schizotypy (e.g., Farmer et al., 2000, Mitropoulou et al., 2005, Roitman et al., 2000). However, effects on the compound measures of visual WM performance, such as accuracy, could have been driven by a reduction in the quantitative (i.e., how many stimuli are remembered, capacity), qualitative (i.e., how precise is the retained memory), or both aspects of retained WM representations (Xie and Zhang, 2017a, Xie and Zhang, 2017b, Xie and Zhang, 2016; W. Zhang and Luck, 2011, Zhang and Luck, 2008). Recent studies assessing WM impairments associated with schizotypy have focused on the quantitative aspect of WM (i.e., capacity) and yielded mixed findings (e.g., Kane et al., 2016, Matsui et al., 2007, Xie et al., 2014). It is thus possible that visual WM impairments associated with schizotypal features could reflect a reduction in the precision, instead of the number, of retained WM representations (e.g., Bates, 2005).

To independently and simultaneously measure the quantitative and qualitative aspects of visual WM, the current study used an established delayed estimation paradigm and analytical approach (Gold et al., 2010; W. Zhang and Luck, 2008). In this task, participants memorized distinctive colors of briefly presented colored squares on a computer screen and then, after a 1-second blank delay, estimated the color of a randomly picked square from memory by matching it to a color on a continuous color wheel. Participants' performance can be fitted with Zhang and Luck's (2008) mixture model, yielding independent estimates of the likelihood of correct recall and mnemonic precision. Specifically, the likelihood of correct recall (multiplied by the total number of to-be-remembered colors yielding the number of remembered colors) is inversely related to the proportion of non-guessing responses and memory precision is the inverse of performance variability (W. Zhang and Luck, 2011, Zhang and Luck, 2009, Zhang and Luck, 2008).

To isolate the deficits in visual WM from potential deficits in perceptual and/or sensory processing (Coleman et al., 2012), a sensory memory task was also included (also see Gold et al., 2010). In this task, to-be-remembered colored squares were briefly presented (100 ms), and observers estimated the color of a randomly picked square immediately following the offset of the memory array. This task thus captures sensory memory representations available for up to a few hundred milliseconds after the offset of briefly presented visual stimuli (Sperling, 1960). These sensory memory representations are fundamentally different from WM representations in several ways. For instance, they are fragile and easily disrupted by masking procedures (Vogel et al., 2006). In addition, sensory memory has an exceedingly large capacity in contrast to the limited storage capacity in WM (Sperling, 1960). Furthermore, a recent study suggests that observers' performance in sensory memory tasks may be conjointly supported by two internal representations, namely a fine-grained representation and a coarse-grained representation (see Cappiello and Zhang, 2016 for details). Between these two, the fine-grained representation is more likely to be transferred into WM (Cappiello and Zhang, 2016). In the current study, if individuals with more schizotypal features also demonstrate more deficits in sensory memory (especially the fine-grained trace), then the observed deficits in WM performance could potentially result from impairments in perceptual and/or sensory processing instead of WM (Tek et al., 2002).

Furthermore, given the large co-occurrence of schizotypy (especially positive schizotypy) and mood-related symptoms in the general population (Lewandowski et al., 2006), it is important to examine whether the pattern of compromised visual WM representations can be accounted for by mood-related symptoms, such as depressed mood. Specifically, a growing literature suggests reduced WM capacity as one of the critical cognitive impairments associated with depressed mood (Adams et al., 2001, Arnett et al., 1999, Hubbard et al., 2015). There may even be a causal relationship between these two because interventions that alleviate depressed mood can also improve WM capacity (e.g., Klein and Boals, 2001). The current study thus assessed and used depressed mood as a control variable when examining the relationship between schizotypy and WM to reveal whether depressed mood could account for WM deficits associated with schizotypy, if WM deficits in schizotypy were more related to capacity instead of precision.

In summary, the current study used an individual-differences approach to assess whether self-report schizotypy and depressed mood were associated with different impairments in visual WM.

Section snippets

Participants

One-hundred-and-sixty-four typical college students (19.30 ± 1.16 [Mean ± std.] years old, 85 female) voluntarily participated this study for course credits at University of California, Riverside. All participants reported normal (or corrected-to-normal) visual acuity and normal color vision. No participant reported use of antipsychotic medication. Written informed consent was given by all participants at the beginning of the study. The Institutional Review Board at University of California,

Correlation analysis

Summary statistics of the measures (see Table 1 and Supplementary Table 1) were more or less comparable as those in previous studies (Cappiello and Zhang, 2016, Winterstein et al., 2011b; W. Zhang and Luck, 2008). Of primary interest, individuals with more schizotypal features tended to have worse WM precision: increases in WM SD (and hence decreases in WM precision) were significantly associated with increases in the total score of Wisconsin Schizotypal Scales (r = 0.24 [0.09, 0.38], p = 0.0019).

Discussion

The current study characterizes several key features regarding visual WM impairments in schizotypy. Specifically, individuals with either more positive or negative schizotypal features tend to have less precise representations retained in visual WM, without a reduction in WM capacity. In contrast, depressed mood, although positively correlated with schizotypy (positive schizotypy), is primarily associated with reduced WM capacity, but not WM precision. Furthermore, the reduced WM precision in

Conclusion

In conclusion, by demonstrating dissociable WM deficits in schizotypy and depressed mood, the present study has provided strong evidence for unstable mental representations in schizotypy, in comparison to reduced overall cognitive resources in depressed mood. Our findings add to the growing literature suggesting that WM impairments can be used as an endophenotypic marker of schizophrenia spectrum disorders (e.g., Park and Gooding, 2014).

Author contributions

W. Xie and W. Zhang developed the study concept. W. Xie, M. Cappiello, and H. Park performed data analysis. P. Deldin and R. C. K. Chan commented the first draft of the manuscript critically. All authors contributed to manuscript preparation.

Declaration of conflicting interests

The authors declared that they had no conflicts of interest with respect to their authorship or the publication of this article.

Role of the funding source

This study is made possible by a start-up fund from University of California, Riverside to Weiwei Zhang.

Acknowledgement

We thank Jonathan Caplan, Miram Rizk, Debora Bina Handojo, Hanna Boparai, Aaron Koay, Adrian Pardo, and Jessica Tarango for their assistance with data collection.

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