Wednesday, June 17, 2009

More on schizophrenia and the mental clock

This is a follow-up post to a prior post re: a study (published in Brain and Cognition) linking mental timing and schizophrenia. I've now read the article closer and would like to share a little more of the content from the authors (prior post only included the abstract)

In the introduction, the authors state that the research literature suggests that:
Schizophrenia may be associated with a fundamental disturbance in the temporal coordination of information processing in the brain, leading to dysfunctions in the timing of perceptual, cognitive, and motor processes (Bressler, 2003; Paulus & Braff, 2003; Phillips & Silverstein, 2003; Tononi & Edelman, 2000). These impairments of neural timing have also been associated with ‘‘disturbances of consciousness,” and may give rise to the expression of clinical symptoms associated with both positive (e.g., hallucinations, delusions) and negative (e.g., psychomotor poverty, poverty of speech) subtype classifications of the disorder (Andreasen, 1999; Hyde, Ziegler, & Weinberger, 1993; McGlashan & Hoffman, 2000).
Support for these conceptualizations is emerging with evidence that brain structures and neurotransmitter systems – such as dopamine, glutamate, and serotonin – that are directly linked to neural timing processes are also impaired in schizophrenia (Andreasen, 1999; Andreasen et al., 1998; Buhusi & Meck, 2007; Cheng, Ali, & Meck, 2007; Rao, Mayer, & Harrington, 2001; Rao et al., 1997; Volz et al., 2001). Despite the growing interest and centrality of these time-dependent conceptualizations of the pathophysiology of schizophrenia, there remains a paucity of research directly examining overt timing performance in the disorder. In addition, the majority of studies that have examined timing behavior in schizophrenia have employed temporal durations in the range of several seconds (Densen, 1977; Johnson & Petzel, 1971; Tysk, 1983a, 1983b, 1990; Volz et al., 2001; Wahl & Sieg, 1980), requiring higher cognitive processes beyond initial sensory registration for temporal encoding (Fraisse, 1984; Michon, 1985; Rammsayer & Lima, 1991). Thus, the aim of the present study was to delineate deficits of temporal perception from more generalized cognitive impairments in schizophrenia by assessing duration estimates in both the millisecond and seconds range using a well-established task of time perception.
The primary purpose of the study was to move beyond prior studies that primarily focused on time estimation in the range of several secondsto time at the level of milliseconds. Also, the theoretical mental clock model used in this study was the classic pacemaker-accumulator model. One task frequently used in mental timing research is the temporal bisection task. The authors provide a nice description of this task:

a temporal bisection task was used to assess the timing of brief auditory durations (i.e., 300–600 ms) in individuals with schizophrenia and non-psychiatric control participants. The temporal bisection procedure required participants to first encode short and long anchor durations to which intermediate durations were subsequently compared and classified as most similar to either the short or the long anchor. The bisection point therefore refers to the duration at which short and long classifications are made with equal probability. In addition to brief durations in the millisecond range, participants in the present study were also required to estimate auditory durations in a second task employing intervals in the range of several seconds (i.e., 3–6 s).
Temporal bisection is unique to other tasks of time estimation with respect to its predictions and the inferences that can be drawn from temporal performance. In a simple timing task involving the estimation of a presented duration, differences in clock rate are reflected in performance differences. Specifically, a faster ‘‘internal clock” would result in the accumulation of a larger number of clock ‘‘pulses” during the timed interval, such that the summation of pulses would correspond to a longer duration than actually presented. A similar outcome would be observed for a slowed clock, with underestimations resulting from the accumulation of fewer pulses than would be needed to represent the presented duration
In the summary, the authors concluded that the timing deficits associated with schizophrenia are due primarily to perceptual or "clock" processes, and not other processes such as attention. Also implicated was the reference memory component of the pacemaker accumulator model.

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