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INTRODUCTION
The construct of attention has been defined in many different ways in the cognitive and neuropsychological literatures. It is currently thought that attention is not a unitary process, but rather that it comprises multiple, dissociable processes dependent in part on the task or situation at hand during measurement, input modalities, stimulus features, behavioral relevance, and the active processes employed to search, shift, focus, and maintain attention (Luck & Vecera, 2002). Accordingly, current literature distinguishes between different types of attention (e.g., selective, focused, and sustained attention), as well as different types of deficits in attention (e.g., neglect, perseveration, distractibility), each testable with unique neuropsychological approaches and associated with distinct attentional models (e.g., Treisman & Gelade, 1980; Posner & Petersen, 1990; Desimone & Duncan, 1995). The most widely used neuropsychological tests of attention (e.g., the Continuous Performance Test, CPT) can distinguish deficits in speed of processing (both mental and sensory), reaction time (motor), and the interaction between processing speed and task complexity, but they are frequently inadequate for identifying specific clinical populations (e.g., Halperin et al., 1991). Given that clinical identification of patient populations is central to the neuropsychologist, tests of attention that offer alternative means of classifying attentional deficits may be valuable.
The d2 Test, a cancellation test involving simultaneous presentation of visually similar stimuli, has been proposed as a particularly useful measure of attention and concentration processes (Brickenkamp & Zillmer, 1998). The task is to cancel out all target characters (a "d" with a total of two dashes placed above and/or below), which are interspersed with nontarget characters (a "d" with more or less than two dashes, and "p" characters with any number of dashes), in 14 successive timed trials (Brickenkamp, 1962). The diagnostic utility and construct validity of the d2 Test have been well supported in European samples, yet this test remains relatively unknown in the U.S. (see Brickenkamp & Zillmer, 1998, for a review). The primary aims of the current study were to (1) examine construct validity and internal consistency of the d2 Test using a U.S. adult sample; (2) determine the psychometric characteristics of performance measures that have been proposed; and (3) derive process measures that may be useful for examining performance constancy and test-taking strategies.
Although the d2 Test was...