Task Switching and Attention Deficit Hyperactivity Disorder

Journal of Abnormal Child Psychology,  June, 2000  by Nicholas J. Cepeda,  Manuel L. Cepeda,  Arthur F. Kramer

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Nicholas J. Cepeda [1]

Manuel L. Cepeda [2]

Arthur F. Kramer [1]

The main goal of the present set of studies was to examine the efficiency of executive control processes and, more specifically, the control processes involved in task set inhibition and preparation to perform a new task in attention deficit hyperactivity disorder (ADHD) and non-ADHD children. This was accomplished by having ADHD children, both on and off medication, and non-ADHD children perform the task-switching paradigm, which involves the performance of two simple tasks. In nonswitch trials, an individual task is performed repeatedly for a number of trials. In switch trials, subjects must rapidly and accurately switch from one task to the other, either in a predictable or unpredictable sequence. Switch costs are calculated by subtracting performance on the nonswitch trials from performance on the switch trials. These costs are assumed to reflect the executive control processes required for the coordination of multiple tasks. ADHD children showed substantially larger switch costs than non-ADHD children. H owever, when on medication, the ADHD children's switch performance was equivalent to control children. In addition, medication was observed to improve the ADHD children's ability to inhibit inappropriate responses. These data are discussed in terms of models of ADHD and cognition.

KEY WORDS: Executive control; attention; inhibition; ADHD; child development; task switching.

The main goal of the present study was to examine differences in performance in the task switching paradigm among attention deficit hyperactivity disorder (ADHD) children on and off medication and age and IQ matched controls. This paradigm was employed in an effort to decompose and examine, in a fine-grained manner, a number of aspects of information processing that have been hypothesized to be less efficient in ADHD children than in non-ADHD children (Barkley, 1989, 1997; Douglas, 1983; Schachar et al., 1993; Seidman et al., 1995). More specifically, the task-switching paradigm has enabled us to examine executive control processes (Norman & Shallice, 1986; Meyer et al., 1997; Shallice, 1994) required to prepare to perform a task as well as those control processes involved in the inhibition of previously activated processing algorithms.

Two different classes of models have been proposed to account for the behavioral deficits observed in ADHD children. One class of models has argued that the behavioral deficits are the result of deficient inhibitory processing in ADHD (Barkley, 1997; Pennington & Ozonoff, 1996; Schachar et al., 1993, 1995; Quay, 1988, 1996). The most comprehensive of such theories has been proposed by Barkley (1997). This model suggests that faulty inhibition is the core deficit in ADHD and this deficit is responsible for secondary deficits observed in other neuropsychological functions, including working memory, self-regulation of affect and motivation, internalization of speech, and behavioral analysis and synthesis. The alternative class of models suggests that behavioral deficits observed in ADHID are the result of either deficient resource allocation policies (Sergeant, 1995a,b) or reduced arousal (Zentall, 1985). We contrast these two classes of models in the present study by examining the performance of ADHD children (on and off medication) with control children in situations that enable the dissociation of changes in resources and arousal with changes in the need for inhibitory processes.

Another goal of the present study is to more finely delineate the nature of the inhibitory deficit associated with ADHD. It has become increasingly clear that inhibition is not a unitary construct, but instead varies along a number of dimensions (Neill & Valdes, 1996), and that different varieties of inhibition can be dissociated in certain populations, such as older adults (Kramer et al., 1994). In the present study, we employ a relatively well-studied paradigm from Cognitive Psychology called the task-switching paradigm to enable us to decompose the nature of inhibitory and executive control deficits observed in ADHD and, in addition, to examine the efficacy of pharmacologic intervention as a means to reduce inhibitory deficits in ADHD children.

EXECUTIVE CONTROL AND INHIBITION IN THE TASK-SWITCHING PARADIGM

The task-switching paradigm involves the performance of two simple tasks, such as deciding whether a letter is a vowel or a consonant or deciding whether a number is odd or even. In one condition (i.e., the non-switch baseline or repetition condition), the same task is repeated a number of times. In the second condition (i.e., the switch or alternation condition), subjects switch from one task to the other. The time required to complete the executive control processes necessary to switch from one task to another, such as the selection from long-term memory and configuration in working memory of the appropriate processing algorithms and the inhibition of previously used processing algorithms, is inferred from the increased response time (RT) observed when a task switch occurs, compared to the RT for the same task performed separately or in a run of trials of the same task (i.e., switch cost RT = switch trial RT - nonswitch trial RT).