Abstract
Visual event-related potentials were recorded from a group of 10 normal subjects while they judged the proximity of two letters of the alphabet. Subjects viewed singly the letters A, D, G, L, N, T, W and Z and indicated by button press whether the letter displayed occured before or after the comparison letter M. Reaction times to close letters (L and N) were longer than ordinally more distant letters (A, D, G, T, W, Z). A late parietally positive potential of approximately 475 ms covaried in latency amplitude with these judgments. Late potentials were delayed in latency and reduced in amplitude to close letter (L and N) judgments compared to the other letters. The results suggest that mental processes, such as alphabetic distance judgments, may be usefully studied by examining their associated event-related potentials.
Highlights
When individuals are asked to judge which of a pair of digits is larger, their decision time is a function of the numerical size or distance between the numbers (Moyer and Dumais, 1978; Moyer and Landauer, 1967)
An earlier positive peak in the average potential waveform, P2 (150-230 ms), did not reflect distance effects either in terms of latency or amplitude, and suggests that it was only after P2 that effects of letter comparison emerged. While both Reaction times (RTs) and late potential latency appeared to covary with ordinal letter position (Figs. 1A and 2A), the relatively low correlations obtained between these measures indicated that RT and latency were not strongly coupled to each other
Numerous reports in the literature have already demonstrated that increasing perceptual demands increase late potential latency and reduce late potential amplitude
Summary
When individuals are asked to judge which of a pair of digits is larger, their decision time is a function of the numerical size or distance between the numbers (Moyer and Dumais, 1978; Moyer and Landauer, 1967). Reaction times (RTs), for instance, are longer when the difference between the number pair is small as when, for example, subjects are asked to judge which is larger 7 or 8 (difference is equal to one) compared to the shorter RTs when the difference between the pair of numbers is greater, for example, 1 or 9 (difference is equal to eight). The distance effect has been extended to include other types of ordered magnitude estimations including judgments of animal sizes (Moyer, 1973) and common everyday objects (Paivio, 1975). In these instances, the distance effect presumably was working through some relative magnitude estimate assigned by the subject at the time of the judgment, or a memorycoded representation based on size. It is of interest to note that ordered distance judgments are resistant to repetition, and distance can persist for thousands of test trials (see, for example, the experiments reported by Banks and White, 1982)
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