Temporal orientation and time estimation in chronic schizophrenics

Cerebellar transcranial magnetic stimulation in psychotic disorders: intermittent, continuous, and sham theta-burst stimulation on time perception and symptom severity

Background

The cerebellum contributes to the precise timing of non-motor and motor functions, and cerebellum abnormalities have been implicated in psychosis pathophysiology. In this study, we explored the effects of cerebellar theta burst stimulation (TBS), an efficient transcranial magnetic stimulation protocol, on temporal discrimination and self-reported mood and psychotic symptoms.

Methods

We conducted a case-crossover study in which patients with psychosis (schizophrenias, schizoaffective disorders, or bipolar disorders with psychotic features) were assigned to three sessions of TBS to the cerebellar vermis: one session each of intermittent (iTBS), continuous (cTBS), and sham TBS. Of 28 enrolled patients, 26 underwent at least one TBS session, and 20 completed all three. Before and immediately following TBS, participants rated their mood and psychotic symptoms and performed a time interval discrimination task (IDT). We hypothesized that cerebellar iTBS and cTBS would modulate these measures in opposing directions, with iTBS being adaptive and cTBS maladaptive.

Results

Reaction time (RT) in the IDT decreased significantly after iTBS vs. Sham (LS-mean difference = -73.3, p = 0.0001, Cohen's d = 1.62), after iTBS vs. cTBS (LS-mean difference = -137.6, p < 0.0001, d = 2.03), and after Sham vs. cTBS (LS-mean difference = -64.4, p < 0.0001, d = 1.33). We found no effect on IDT accuracy. We did not observe any effects on symptom severity after correcting for multiple comparisons.

Conclusion

We observed a frequency-dependent dissociation between the effects of iTBS vs. cTBS to the cerebellar midline on the reaction time of interval discrimination in patients with psychosis. iTBS showed improved (adaptive) while cTBS led to worsening (maladaptive) speed of response. These results demonstrate behavioral target engagement in a cognitive dimension of relevance to patients with psychosis and generate testable hypotheses about the potential therapeutic role of cerebellar iTBS in this clinical population.

Clinical Trial Registration

clinicaltrials.gov, identifier NCT02642029.

Temporal imprecision and phase instability in schizophrenia resting state EEG

Schizophrenia is characterized by temporal imprecision and irregularities on neuronal, psychological cognitive, and behavioral levels which are usually tested during task-related activity. This leaves open whether analogous temporal imprecision and irregularities can already be observed in the brain's spontaneous activity as measured during the resting state; this is the goal of our study. Building on recent task-related data, we, using EEG, aimed to investigate the temporal precision and regularity of phase coherence over time in healthy, schizophrenia, and bipolar disorder participants. To this end, we developed a novel methodology, nominal frequency phase stability (NFPS), that allows to measure stability over phase angles in selected frequencies. By applying sample entropy quantification to the time-series of the nominal frequency phase angle time series, we found increased irregularities in theta activity over a frontocentral electrode in schizophrenia but not in bipolar disorder. We therefore assume that temporal imprecision and irregularity already occur in the brain's spontaneous activity in schizophrenia.

Selective impairment of timing in a NMDA hypofunction animal model of psychosis

Schizophrenia is severe neuropsychiatric disease, which is commonly accompanied not only by positive or negative symptoms, but also by cognitive impairment. To study neuronal mechanisms underlying cognitive distortions and mechanisms underlying schizophrenia, animal pharmacological models of cognitive symptoms are commonly used. Between various cognitive impairments in schizophrenia patients, disturbed time perception has often been reported. Here, we examined temporal and spatial cognition in a modified Carousel maze task in the animal model of schizophrenia induced by non-competitive NMDA-receptor antagonists MK-801. Male Long-Evans rats (n = 18) first learned to avoid the aversive sector on a rotating arena in both dark and light intervals. We verified that during dark, rats used temporal cues, while during light they relied predominantly on spatial cues. We demonstrated that the timing strategy depends on the stable rotation speed of the arena and on the repositioning clues such as aversive stimuli. During testing (both in light and dark intervals), half of the rats received MK-801 and the control half received saline solution. We observed dose-dependent disruptions of both temporal and spatial cognition. Namely, both doses of MK-801 (0.1 and 0.12 mg/kg) significantly impaired timing strategy in the dark and increased locomotor activity. MK-801 dose 0.1 mg/kg, but not 0.12, also impaired spatial avoidance strategy in light. We found that the timing strategy is more sensitive to NMDA antagonist MK-801 than the spatial strategy. To conclude, a modified version of the Carousel maze is a useful and sensitive tool for detecting timing impairments in the MK-801 induced rodent model of schizophrenia.

Setting the beat of an internal clock: Effects of dexamphetamine on different interval ranges of temporal processing in healthy volunteers

Drug studies are powerful models to investigate the neuropharmacological mechanisms underlying temporal processing in humans. This study administered dexamphetamine to 24 healthy volunteers to investigate time perception at different time scales, along with contributions from working memory. Healthy volunteers were administered 0.45 mg/kg dexamphetamine or placebo in a double-blind, crossover, placebo-controlled design. Time perception was assessed using three experimental tasks: a time-discrimination task, which asked participants to determine whether a comparison interval (1200 ± 0, 50, 100, 150, 200 ms) was shorter or longer than a standard interval (1200 ms); a retrospective time estimation task, which required participants to verbally estimate time intervals (10, 30, 60, 90 and 120 s) retrospectively; and a prospective time-production task, where participants were required to prospectively monitor the passing of time (10, 30, 60, 90 and 120 s). Working memory was assessed with the backwards digit span. On the discrimination task, there was a change in the proportion of long-to-short responses and reaction times in the dexamphetamine condition (but no association with working memory), consistent with an increase in the speed of an internal pacemaker, and an overestimation of durations in the timing of shorter intervals. There was an interaction between dexamphetamine, working memory, and performance on the estimation and production tasks, whereby increasing digit span scores were associated with decreasing interval estimates and increased produced intervals in the placebo condition, but were associated with increased interval estimates and decreased produced intervals after dexamphetamine administration. These findings indicate that the dexamphetamine-induced increase in the speed of the internal pacemaker was modulated by the basal working memory capacity of each participant. These findings in healthy humans have important implications for the role of dopamine, and its contributions to timing deficits, in models of psychiatric disorders.

Positive symptoms and time perception in schizophrenia: A meta-analysis

Positive symptoms of schizophrenia may be related to distortions in time perception. To examine this issue, we conducted a meta-analysis to determine whether positive symptoms are associated with deficits in time processing performance. MEDLINE and Web of Science were searched from January 1980 through March 2017, and all related articles and their references were scrutinized to find relevant studies. Studies were selected if they included participants with a diagnosis of schizophrenia, and reported data from behavioral measures of interval timing (e.g. duration discrimination and temporal order judgement). The results indicated that positive symptoms of schizophrenia are related with overestimation of interval timing (i.e., acceleration of the “internal clock”), and suggest that time perception may be associated with psychosis.

Motor Synchronization in Patients With Schizophrenia: Preserved Time Representation With Abnormalities in Predictive Timing

Objective: Basic temporal dysfunctions have been described in patients with schizophrenia, which may impact their ability to connect and synchronize with the outer world. The present study was conducted with the aim to distinguish between interval timing and synchronization difficulties and more generally the spatial-temporal organization disturbances for voluntary actions. A new sensorimotor synchronization task was developed to test these abilities. Method: Twenty-four chronic schizophrenia patients matched with 27 controls performed a spatial-tapping task in which finger taps were to be produced in synchrony with a regular metronome to six visual targets presented around a virtual circle on a tactile screen. Isochronous (time intervals of 500 ms) and non-isochronous auditory sequences (alternated time intervals of 300/600 ms) were presented. The capacity to produce time intervals accurately versus the ability to synchronize own actions (tap) with external events (tone) were measured. Results: Patients with schizophrenia were able to produce the tapping patterns of both isochronous and non-isochronous auditory sequences as accurately as controls producing inter-response intervals close to the expected interval of 500 and 900 ms, respectively. However, the synchronization performances revealed significantly more positive asynchrony means (but similar variances) in the patient group than in the control group for both types of auditory sequences. Conclusion: The patterns of results suggest that patients with schizophrenia are able to perceive and produce both simple and complex sequences of time intervals but are impaired in the ability to synchronize their actions with external events. These findings suggest a specific deficit in predictive timing, which may be at the core of early symptoms previously described in schizophrenia.

Explicit Time Deficit in Schizophrenia: Systematic Review and Meta-Analysis Indicate It Is Primary and Not Domain Specific

Although timing deficits are a robust finding in schizophrenia (SZ), the notion of a genuine time perception disorder in SZ is still being debated because distortions in timing might depend on neuropsychological deficits that are characteristics of the illness. Here we used meta-analytic methods to summarize the evidence of timing deficits in SZ and moderator analyses to determine whether defective timing in SZ arises from nontemporal sources or from defective time perception. PubMed Services, PsycNET, and Scopus were searched through March 2015, and all references in articles were investigated to find other relevant studies. Studies were selected if they included subjects with a primary diagnosis of SZ compared to a healthy control (HC) group and if they reported behavioral measures of duration estimation (perceptual and motor explicit timing). Data from 24 studies published from 1956 to 2015, which comprised 747 SZ individuals and 808 HC, were included. Results indicate that SZ individuals are less accurate than HC in estimating time duration across a wide range of tasks. Subgroup analyses showed that the fundamental timing deficit in SZ is independent from the length of the to-be-timed duration (automatic and cognitively controlled timing) and from methods of stimuli estimation (perceptual and motor timing). Thus, time perception per se is disturbed in SZ (not just task-specific timing processes) and this perturbation is independent from more generalized cognitive impairments. Behavioral evidence of disturbed automatic timing should be more thoroughly investigated with the aim of defining it as a cognitive phenotype for more homogeneous diagnostic subgrouping.

Acute effects of THC on time perception in frequent and infrequent cannabis users

RATIONALE

Cannabinoids have been shown to alter time perception, but existing literature has several limitations. Few studies have included both time estimation and production tasks, few control for subvocal counting, most had small sample sizes, some did not record subjects' cannabis use, many tested only one dose, and used either oral or inhaled administration of Δ⁹-tetrahydrocannabinol (THC), leading to variable pharmacokinetics, and some used whole-plant cannabis containing cannabinoids other than THC. Our study attempted to address these limitations.

OBJECTIVES

This study aims to characterize the acute effects of THC and frequent cannabis use on seconds-range time perception. THC was hypothesized to produce transient, dose-related time overestimation and underproduction. Frequent cannabis smokers were hypothesized to show blunted responses to these alterations.

METHODS

IV THC was administered at doses from 0.015 to 0.05 mg/kg to 44 subjects who participated in several double-blind, randomized, counterbalanced, crossover, placebo-controlled studies. Visual time estimation and production tasks in the seconds range were presented to subjects three times on each test day.

RESULTS

All doses induced time overestimation and underproduction. Chronic cannabis use had no effect on baseline time perception. While infrequent/nonsmokers showed temporal overestimation at medium and high doses and temporal underproduction at all doses, frequent cannabis users showed no differences. THC effects on time perception were not dose related.

CONCLUSIONS

A psychoactive dose of THC increases internal clock speed as indicated by time overestimation and underproduction. This effect is not dose related and is blunted in chronic cannabis smokers who did not otherwise have altered baseline time perception.

The Concept of Time in Rehabilitation and Psychosocial Adaptation to Chronic Illness and Disability

The first part of this article focused on providing the reader with a general overview of the concept of time with special emphasis on understanding time’s role in the structure of personality theories and their associated therapeutic approaches, as well as linking the discussion to the understanding of time in the context of psychosocial adaptation to chronic illnesses and disabilities (CIDs). In the second part of this article, the author seeks to (a) briefly comment on the association among death, disability, and time; (b) discuss findings from the clinical and empirical literatures regarding time perception/orientation within the context of psychiatric disabilities; (c) review findings on the relationships between time perception/orientation and psychosocial adaptation to CIDs; and (d) highlight those treatment modalities that have been suggested for individuals whose disabilities have resulted in time distortions.

Timing as a window on cognition in schizophrenia

Distorted interval timing is a common feature of the cognitive impairment observed in patients with schizophrenia. The neural circuits which are required for interval timing and those thought to be compromised in schizophrenia overlap and include the cortico-striatal pathways. Here, we suggest that a focus on temporal information processing offers a window into understanding the cognitive deficits of schizophrenia and how deficits might contribute to a variety of symptoms. A disruption in the functioning of the cortico-striatal pathways may lead to cognitive deficits which in turn lead to impaired processing of temporal information. Disrupted temporal processing may also contribute to a variety of other symptoms associated with the disorder. Because interval timing is a cognitive/behavioral phenotype that can easily be assessed in animals it can be used as a sensitive screen for deficits in animal models. Using a recently developed transgenic mouse that models increased D2 receptor upregulation in the striatum similar to that observed in patients with schizophrenia we illustrate the utility of an interval timing approach in assessing cognitive impairment. We further discuss how variants of timing procedures can be used to assess attention and working memory performance as well as other necessary components of adaptive cognitive function.

Are impairments of time perception in schizophrenia a neglected phenomenon?

Based on clinical, phenomenological and neurobiological observations, psychiatrists often report a deficit in time estimation in patients with schizophrenia. Cognitive models of time estimation in healthy subjects have been proposed and developed for approximately 30 years. The current theory in the field of time perception, which is supported by a connectionist model, postulates that temporal judgement is based upon a pacemaker-counter device that depends mostly upon memory and attentional resources. The pacemaker emits pulses that are accumulated in a counter, and the number of pulses determines the perceived length of an interval. Patients with schizophrenia are known to display attentional and memory dysfunctions. Moreover, dopamine regulation mechanisms are involved in both the temporal perception processes and schizophrenia. Thus, it is still unclear if temporal impairments in schizophrenia are related to a specific disturbance in central temporal processes or are due to certain cognitive problems, such as attentional and memory dysfunctions, or biological abnormalities. The authors present a critical literature review on time perception in schizophrenia that covers topics from psychopathology to neuroscience. Temporal perception appears to play a key role in schizophrenia and to be partially neglected in the current literature. Future research is required to better ascertain the underlying mechanisms of time perception impairments in schizophrenia.

Modeling motivational deficits in mouse models of schizophrenia: behavior analysis as a guide for neuroscience

In recent years it has become possible to develop animal models of psychiatric disease in genetically modified mice. While great strides have been made in the development of genetic and neurobiological tools with which to model psychiatric disease, elucidation of neural and molecular mechanisms thought to underlie behavioral phenotypes has been hindered by an inadequate analysis of behavior. This is unfortunate given the fact that the experimental analysis of behavior has created powerful methods for isolating and describing the functional properties of behavioral mechanisms that are capable of providing deep understanding of behavioral phenotypes. A better understanding of the biological basis of normal behavior and its disturbance in psychiatric disease will require the application of these rigorous behavior analytic tools to animal models. In this review we provide an example of a merging of genetic and behavioral methods and illustrate its utility in the analysis of a mouse model of the motivational deficits in schizophrenia. The synergy between basic behavior analysis, neuroscience, and animal models of psychiatric disease has great potential for achieving a deeper understanding of behavior and its neurobiological mechanisms as well as for leading to improvements in diagnosis and treatment in clinical settings.

Ketamine perturbs perception of the flow of time in healthy volunteers

RATIONALE

Disturbances in the subjective experience of time have been observed both in schizophrenia and following acute administration of ketamine. However, effects of ketamine on more objective timing tasks have not yet been measured in humans, nor has it been established that timing effects are not merely secondary to a more general dysfunction in working memory (WM).

OBJECTIVE AND METHODS

In a double-blind placebo-controlled crossover study, we characterised the effects of ketamine (100 ng/ml blood plasma level) on performance of perceptual timing and colour discrimination tasks, which were matched for WM and attentional demands. To test the ubiquity of ketamine's effects on timing, we also examined two distinct measures of temporal predictability.

RESULTS

Ketamine significantly distorted the subjective experience of time as measured by the Clinician-Administered Dissociative States Scales. Critically, ketamine also impaired accuracy on the perceptual timing task while having no effect on performance of the colour perception task. Although ketamine did not impair the ability to use prelearned temporal (or spatial) cues to predict target onset (or location), it did slow reaction times at long delays following non-informative neutral cues, suggesting an impaired ability to use the unidirectional flow of time itself to make temporal predictions.

CONCLUSIONS

Ketamine induced selective impairments in timing, which could not be explained by more fundamental effects on the ability to hold information in WM. Rather our collected findings suggest that ketamine may disturb timing by selectively impairing the way in which information is temporally manipulated within WM.

Timing dysfunctions in schizophrenia as measured by a repetitive finger tapping task

Schizophrenia may be associated with a fundamental disturbance in the temporal coordination of information processing in the brain, leading to classic symptoms of schizophrenia such as thought disorder and disorganized and contextually inappropriate behavior. Although a variety of behavioral studies have provided strong evidence for perceptual timing deficits in schizophrenia, no study to date has directly examined overt temporal performance in schizophrenia using a task that differentially engages perceptual and motor-based timing processes. The present study aimed to isolate perceptual and motor-based temporal performance in individuals diagnosed with schizophrenia using a repetitive finger-tapping task that has previously been shown to differentially engage brain regions associated with perceptual and motor-related timing behavior. Thirty-two individuals with schizophrenia and 31 non-psychiatric control participants completed the repetitive finger-tapping task, which required participants to first tap in time with computer-generated tones separated by a fixed intertone interval (tone-paced tapping), after which the tones were discontinued and participants were required to continue tapping at the established pace (self-paced tapping). Participants with schizophrenia displayed significantly faster tapping rates for both tone- and self-paced portions of the task compared to the non-psychiatric group. Individuals diagnosed with schizophrenia also displayed greater tapping variability during both tone- and self-paced portions of the task. The application of a mathematical timing model further indicated that group differences were primarily attributable to increased timing--as opposed to task implementation--difficulties in the schizophrenia group, which is noteworthy given the broad range of impairments typically associated with the disorder. These findings support the contention that schizophrenia is associated with a broad range of timing difficulties, including those associated with time perception as well as time production.

Temporal processing dysfunction in schizophrenia

Schizophrenia may be associated with a fundamental disturbance in the temporal coordination of information processing in the brain, leading to classic symptoms of schizophrenia such as thought disorder and disorganized and contextually inappropriate behavior. Despite the growing interest and centrality of time-dependent conceptualizations of the pathophysiology of schizophrenia, there remains a paucity of research directly examining overt timing performance in the disorder. Accordingly, the present study investigated timing in schizophrenia using a well-established task of time perception. Twenty-three individuals with schizophrenia and 22 non-psychiatric control participants completed a temporal bisection task, which required participants to make temporal judgments about auditory and visually presented durations ranging from 300 to 600 ms. Both schizophrenia and control groups displayed greater visual compared to auditory timing variability, with no difference between groups in the visual modality. However, individuals with schizophrenia exhibited less temporal precision than controls in the perception of auditory durations. These findings correlated with parameter estimates obtained from a quantitative model of time estimation, and provide evidence of a fundamental deficit in temporal auditory precision in schizophrenia.

Transient overexpression of striatal D2 receptors impairs operant motivation and interval timing

The striatum receives prominent dopaminergic innervation that is integral to appetitive learning, performance, and motivation. Signaling through the dopamine D2 receptor is critical for all of these processes. For instance, drugs with high affinity for the D2 receptor potently alter timing of operant responses and modulate motivation. Recently, in an attempt to model a genetic abnormality encountered in schizophrenia, mice were generated that reversibly overexpress D2 receptors specifically in the striatum (Kellendonk et al., 2006). These mice have impairments in working memory and behavioral flexibility, components of the cognitive symptoms of schizophrenia, that are not rescued when D2 overexpression is reversed in the adult. Here we report that overexpression of striatal D2 receptors also profoundly affects operant performance, a potential index of negative symptoms. Mice overexpressing D2 exhibited impairments in the ability to time food rewards in an operant interval timing task and reduced motivation to lever press for food reward in both the operant timing task and a progressive ratio schedule of reinforcement. The motivational deficit, but not the timing deficit, was rescued in adult mice by reversing D2 overexpression with doxycycline. These results suggest that early D2 overexpression alters the organization of interval timing circuits and confirms that striatal D2 signaling in the adult regulates motivational process. Moreover, overexpression of D2 under pathological conditions such as schizophrenia and Parkinson's disease could give rise to motivational and timing deficits.

Interval-timing deficits in individuals at high risk for schizophrenia

A duration-bisection procedure was used to study the effects of signal modality and divided attention on duration classification in participants at high genetic risk for schizophrenia (HrSz), major affective disorder (HrAff), and normal controls (NC). Participants learned short and long target durations during training and classified probe durations during test. All groups classified visual signals as shorter than equivalent duration auditory signals. However, the difference between auditory and visual signal classification was significantly larger for the HrSz group than for the NC group. We posit a model in which there is a clock rate difference between auditory and visual signals due to an attentional effect at the level of a mode switch that gates pulses into an accumulator. This attentionally mediated clock rate difference was larger for the HrSz participants than for the NC participants, resulting in a larger auditory/visual difference for the HrSz group.

Eyeblink conditioning deficits indicate timing and cerebellar abnormalities in schizophrenia

Accumulating evidence indicates that individuals with schizophrenia manifest abnormalities in structures (cerebellum and basal ganglia) and neurotransmitter systems (dopamine) linked to internal-timing processes. A single-cue tone delay eyeblink conditioning paradigm comprised of 100 learning and 50 extinction trials was used to examine cerebellar timing circuits in 13 medicated patients with schizophrenia and 13 age- and sex-matched controls. Patients with schizophrenia showed impaired learning of the conditioned response compared to controls and also greater within-subject variability in the timing of their responses. These findings are consistent with models of schizophrenia in which timing deficits underlie information-processing abnormalities and clinical features of the disorder.

Temporal discrimination in schizophrenic and affective disorders: evidence for a dopamine-dependent internal clock

Performance in temporal discrimination of time intervals in the range of milliseconds was compared in 80 healthy subjects, 27 patients with schizophrenic disorders, 33 patients with major depression, 21 patients with dysthymic disorders. For schizophrenic patients as well as for patients with major depression, pronounced deficits in duration discrimination could be demonstrated as compared to the healthy control group (p less than .01). Patients with dysthymic disorders and schizophrenic patients differed significantly from the melancholic group (p less than .01 and p less than .05, respectively). The results are discussed on the basis of the assumption of an internal clock, implying that the clock rate is highest and therefore temporal resolution is best with healthy subjects. With psychiatric patients performance in temporal discrimination was impaired to a slowing down in clock rate and thus decreased temporal resolution. There is strong evidence that changes in clock rate depend on the effective level of dopamine. This leads to the conclusion that temporal discrimination thresholds may be seen as an indicator for deviations from the optimal level of dopaminergic activity in psychiatric patients. In addition, possible effects due to age and medication are discussed.

A longitudinal study of time estimation in psychotic disorders

5 healthy subjects and 9 patients with hospital diagnoses of schizophrenia were studied by means of repeated measurements of time estimation (production method) during a 1- to 2-yr period. The healthy subjects exhibited moderate variability in their time estimations and tended to over- or under-estimate somewhat. Both over- and under-estimation were also found among patients, but the patients were often more variable in their estimations and in some cases deviated more from the correct estimation than the healthy subjects. Two patients were decided under-estimators. They were both young, subchronic schizophrenics. Chronic schizophrenic patients tended to over-estimate time, but the amount of over-estimation had no noticeable correlation to the degree of clinical disturbance. Two patients gradually changed from under- to over-estimation during the observation period. One patient was evidently misdiagnosed. He suffered from a bipolar affective disorder, and his time estimations seemed to depend on his clinical state during the course of the disorder. The results are discussed in the context of earlier findings and physiological disturbances in chronic schizophrenia.

Estimation of time and the subclassification of schizophrenic disorders

Estimation of short time intervals by 60 healthy subjects, 50 patients with schizophrenic disorders, and 8 with schizotypal personality disorders, was investigated using the three different methods, adjusting a metronome, verbal estimation, and operative estimation (production). The schizophrenic patients tended to over-estimate time with all three methods. Overestimation was also found when longer intervals were studied. Patients with different types of schizophrenic disorders, classified according to DSM-III criteria, over-estimated time about the same; no significant differences were found. Different courses of schizophrenia were also studied. Patients in remission over-estimated time to the same extent as chronic patients; the subchronic patients probably over-estimated less. Schizotypal personality disorders did not seem to be associated with a tendency to over-estimate short time intervals. The results were discussed in the context of perceptual disturbances in schizophrenic disorders.

Time estimation by healthy subjects and schizophrenic patients: a methodological study

Three different methods were used to investigate the perception of short time intervals by 60 healthy subjects and a pilot group of 15 schizophrenic patients. The usefulness of metronome adjustment, verbal estimation, and operative estimation was evaluated. Estimation of longer time intervals was also studied. The influence of age, sex, oral temperature, and pulse rate was assessed. Earlier findings that schizophrenic subjects tend to overestimate short time intervals were supported by data from all three methods. Operative estimation--the subject's production of a requested time interval--could possibly best discriminate between the two groups. Estimation of longer time intervals did not differ significantly for the two groups.

Time Estimation among Schizophrenics

Studies of time estimation among schizophrenics have sometimes been difficult to integrate with one another because of differing methodologies and inconsistent definitions. The present study should increase clarity by employing several methods of time estimation within the same study and maintaining a consistent definition of overestimation and underestimation across tasks. 26 schizophrenic and 26 control subjects were given three types of time-estimation tasks. Longer Interval Estimation involved judging, at different points in the interview/testing session, how much time had passed. Verbal Estimation required subjects to judge the length of brief intervals signalled by the examiner. Operative Estimation required subjects to indicate when a specified number of seconds had passed. Schizophrenic subjects were significantly more inaccurate than controls in the Verbal and Operative Estimation tasks. Moreover, with overestimation defined consistently as judging more time to have passed than actually has, both Verbal and Operative Estimation results indicated schizophrenics were significantly more likely to overestimate. These results indicate support for the suggestion that schizophrenics have a disturbed sense of time, with real (clock) time experienced as passing more slowly than is actually the case. Long Interval Estimation produced quite different results, but it was felt that the retrospective and unfocused nature of the time judgments in this task made it a less valid indicator of ability to estimate.