Time perception: modality and duration effects in attention-deficit/hyperactivity disorder

Journal of Abnormal Child Psychology, Oct, 2005 by Maggie E. Toplak, Rosemary Tannock

Attention-deficit/hyperactivity disorder (ADHD) is a neurodevelopmental disorder that is typically first diagnosed in childhood, but symptoms often persist into adolescence (DSM-IV-TR; APA, 2000). Current working models of ADHD suggest that it is best characterized as a disorder of executive function (Barkley, 1997a,b; 1998). Certainly, individuals with ADHD have demonstrated poorer performance on tasks involving inhibitory control (Schachar, Mota, Logan, Tannock, & Klim, 2000) and working memory, set shifting, and planning (Pennington & Ozonoff, 1996) relative to normal controls (e.g., reviewed by Barkley, 1997a, b; Pennington & Ozonoff, 1996; Tannock, 2003). However, these differences are not always observed consistently and are not unique to ADHD, as such deficits are often observed in other clinical samples (Pennington & Ozonoff, 1996; Sergeant, Geurts, & Oosterlaan, 2002).

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Cognitive studies of ADHD have implicated time perception, which refers to the perceived length of a time interval, as a potentially important cognitive deficit in ADHD (Barkley, 1997b, 1998; Barkley, Edwards, Laneri, Fletcher, & Metevia, 2001a; Barkley, Koplowitz, Anderson, & McMurray, 1997; Barkley, Murphy, & Bush, 2001b; Kerns, McInerney, & Wilde, 2001; Rubia et al., 1999a, 2001; Rubia, Noorloos, Smith, Gunning, & Sergeant, 2003; Rubia, Taylor, Taylor & Sergeant, 1999b; Smith, Taylor, Warner Rogers, Newman, & Rubia, 2002; Sonuga-Barke, Saxton, & Hall, 1998; Toplak, Rucklidge, Hetherington, John, & Tannock, 2003; West et al., 2000). The contention that time perception (also described as time estimation in this literature) is impaired in individuals with ADHD has emerged from the idea that time perception is an executive function (Barkley, 1997b). However, cognitive models of time perception implicate mechanisms other than frontal executive functions, such as cerebellar timing mechanisms (Ivry & Fiez, 2000). The conceptualization and measurement of time perception becomes crucial, as there are very different implications of understanding time perception as another measure of executive function versus understanding time perception as an index of an internal timing mechanism driven by cerebellar processes. From the latter perspective, evidence of time perception impairments in ADHD would indicate the need to broaden the current conceptualization of ADHD as a disorder of executive function to include possible cerebellar or perceptual deficits.

Recent advances in the neuroscience of ADHD highlight the plausibility of time perception deficits in ADHD. This is because of findings that children and adolescents with ADHD have smaller cerebellar volumes than comparison controls (Castellanos et al., 2001). Converging evidence from the neuroscience literature indicates that the cerebellum may be involved in time perception (Ivry & Fiez, 2000; Ivry & Spencer, 2004). Both of these literatures are in their infancy, as it is still not clear what types of time perception are reliably impaired in ADHD, and what the role of the cerebellum has in timing functions (Ivry & Fiez, 2000). Clinically, there is also good reason to examine the construct of time perception further, as individuals with ADHD have been reported to have a poor sense of time. Parents will often report that their adolescents blurt out answers before it is their turn to respond, and they cannot seem to wait for their turn, suggesting that even brief spans of time "are like forever" to these adolescents. The purpose of the present research was to further examine time perception systematically in a sample of adolescents with and without ADHD.

Time Perception in ADHD

The focus of the present study was on the perception of time intervals rather than on production or reproduction. One important reason for selecting perception is because time perception tasks minimize the motor demands of timing performance, unlike interval production and reproduction tasks. The impact of motor demands is a critical variable in the study of ADHD, as motor difficulties also characterize individuals with ADHD (Carte, Nigg, & Hinshaw, 1996; Denckla & Rudel, 1978; Riordan et al., 1999). Moreover, duration discrimination is reportedly impaired in patients with cerebellar lesions relative to normal controls (Ivry & Keele, 1989). Also, neuroimaging studies of healthy controls have implicated cerebellar involvement in duration discrimination (Smith, Taylor, Lidzba, & Rubia, 2003).

Duration discrimination is the typical method used to measure time perception, and involves the comparison of two brief intervals similar in duration (such as, 500 ms vs, 600 ms) to determine which is the longest or shortest. This task has been administered in two ways in the ADHD literature, using either discrete trials (Smith et al., 2002) or psychophysical methods (Toplak et al., 2003). Accuracy is the typical dependent measure when discrete trials are used, while duration threshold is the dependent measure when psychophysical methods are used. In psychophysical methods, an adaptive procedure is used to determine a participants' duration threshold. The duration threshold represents the duration of the comparison interval (i.e., the duration threshold) at which the participant can reliably distinguish the comparison from the target interval with 80% accuracy. Psychophysical methods of time perception afford more precise measurement than discrete trials.

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Time perception: modality and duration effects in attention-deficit/hyperactivity disorder

Journal of Abnormal Child Psychology, Oct, 2005 by Maggie E. Toplak, Rosemary Tannock

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