The Science: Digit & Letter Span
The concept of digit/letter span working memory tasks was created from short-term memory and is considered to be the cognitive analogy of workspace to be used while working on such tasks. Span memory for words involves the articulatory loop where items can be stored for 2 seconds. Therefore articulation speed is an important determiner of span memory and processing efficiency, as digit span is a measure of working memory (Brown, 1958; Peterson & Peterson, 1959). Two processes are involved in digit span: the identification of the items and the retention of order information. Individuals who are slow in identification have a shorter memory span.
In most research about short-term memory, the Wechsler Adult Intelligence Scale (WAIS) or Wechsler Intelligence Scale for Children (WISC) digit span battery was employed.
Verbal information presented visually and auditorily is not typically integrated into a unitary sequence for subsequent rehearsal (Whitehouse, 2002). There is a better short-term memory for verbal stimuli with auditory presentation than for visual stimuli (Vitulli & Mcneil, 1990). Bush and Geist (1991) reported that forward digit span has significant longer mean latency for the auditory channel. However, Selnes (1991) found that visual presentation in youngsters is significantly stronger than for auditory presentation of backward digit span (Powell & Hiatt, 1996).
Steele, Ball, and Runk (1997), reported no Mozart Effect on digit span. The researchers used the backward digit test because it was highly correlated with memory scores, incorporating spatial and temporal transformation. It was impossible to conclude whether listening to Mozart improved performance or listening to a progressive relaxation tape reduced performance.
Digit forward is not as sensitive to the early stages of dementia or brain damage as is digit backward. Digit/letter forward appears to involve primarily sequential processing, whereas digit backward appears to involve both planning ability and sequential processing (Crowder, 1993). Digit/letter backward demands exceptional attention and concentration (Searls, 1975). The ability to repeat digits backward is not only dependent on attention, concentration, general cognitive, and short-term memory functioning, but also requires verbal and visual (nonverbal visualization) mediation. Improvement demonstrated in recall of reverse digit span tasks also showed progress in areas of organization (Baddeley, 1990).
Right hemisphere dysfunction reduces backward digits, while left hemisphere dysfunction reduces forward digits performance (Steele et al., 1997). Even though digit repetition as a verbal function is predominantly subsumed by the left or dominant hemisphere, different neuropsychological functions play a role in digit repetition performance. Thus, digit repetition is not totally a left or dominant hemisphere function, and backward digit repetition is not a specific right or subdominant hemisphere function. Impairment of digits backward in patients with a right hemisphere lesion might be due to visual or spatial deficits.
In Black's (1986) study, the right hemisphere sample showed no significant impairment on forward and backward digit span, although performance was somehow inadequate on the backward digits. The left hemisphere sample demonstrated only a mild impairment on forward digits, but performance was significantly poor on backward digits. However, no evidence with brain-damaged patients exists to support that forward digit repetition is more highly correlated with verbal measures than backward digit repetition. Both hemispheres seem to have a role in digit repetition with different neuropsychological functions associated with forward and backward digit. (Searls, 1975). Interestingly, while Spafford (1989) suggested that speed and accuracy of coding contribute to reading speed and comprehension, Standing and Curtis (1989) found no correlation between speed of encoding and memory span.
