Luria (1980) posited the existence of three "blocks" or "functional units" in the brain: Block 1 concerns arousal or attention; Block 2 deals with successive and simultaneous coding functions, and pertains directly to the sequential-simultaneous dichotomy just discussed; and Block 3 involves higher-level planning processes (Luria, 1980; Naglieri, 1999; Naglieri & Das, 1988, 1997). Digit Symbol-Coding, apart from its sequential component, is for Rapaport a measure of concentration and is often a member of the dis-tractibility/working memory grouping; therefore, it may be thought of as a measure of arousal, a Block 1 function that "maintains a proper state of arousal or cortical tone...[which] is also important for effective performance because too much or too little interferes with proper processing of information" (Naglieri & Das, 1988, p. 36). In addition, Picture Arrangement has long been considered a measure of planning ability, of anticipation of consequences, and, by inference, of Luria's third functional unit.
From this perspective, an individual's performance on Digit Symbol-Coding (and perhaps Digit Span, a measure of attention according to
Rapaport) may reflect arousal or orientation toward the tasks in general; scores on Arithmetic, Letter-Number Sequencing, and Digit Span may denote successive or sequential processing; success or failure on the Spatial triad may indicate simultaneous processing; and Picture Arrangement and Symbol Search performance may denote planning, the ability to develop strategies, to generate hypotheses, and generally to program, regulate, and verify activity (Naglieri & Das, 1988, 1997).
From Naglieri's (1999; Naglieri and Das, 1988, 1997) research, one might treat Picture Arrangement, Symbol Search, and Digit Symbol-Coding as measures of the Block 3, or planning, ability. Naglieri and Das (1997) interpret as planning ability a scale on the Cognitive Assessment System (CAS) that includes several highly speeded tasks, each of which resembles traditional measures of processing speed. Naglieri (1999) has provided some construct validity evidence for the CAS Planning scale, including research findings that support the need to generate problem-solving strategies to perform well on the CAS measures of planning ability. From this vantage point, it seems reasonable to hypothesize a Luria-based Block 3 "planning ability" interpretation of high or low scores on the Processing Speed Index, especially for those individuals whose score on Picture Arrangement is consistent with the PSI (whether high or low). Whereas the clerical, psychomotor Digit Symbol-Coding subtest is questionable as a measure of Luria's Block 3, both Picture Arrangement and Symbol Search seem to embrace the essence of Naglieri and Das's (1987) description of Block 3 characteristics: "planning entails the aptitude for asking new questions, solving problems, and self-monitoring, which...may represent one of the most complex forms of human behavior" (p. 355).
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