Abstract

During perceptually intensive tasks such as reading, there is a bottleneck in the information transfer between the large number of alphanumeric characters available and the acquiring of these characters. This is due mainly to the limited number of characters that one can report at a glance (also known as the “magic number 7 ± 2”) [ Psychol. Rev. 63, 81 ( 1956)]. To examine where in the perceptual pathway this bottleneck occurred, several investigators tested and compared performance with simultaneous and with sequential target presentations [ J. Exp. Psychol. 79, 1 ( 1969); J. Exp. Psychol. 93, 72 ( 1972); Percept. Psychophys. 14, 231 ( 1973)]. They found that performance was nearly equal in the two cases and concluded that the bottleneck must be due to the limitation of short-term memory. However, these studies were limited either by a long stimulus-onset asynchrony (SOA), or time interval between onsets of icon presentations, or by a lack of poststimulus masking. We report on experiments designed to overcome these limitations. We used shorter SOA’s than did previous investigators, and we removed persistence effects by poststimulus masking. Our stimuli were presented either sequentially or simultaneously. For the sequential presentation a numeral ranging from 0 to 9 was displayed at any one of eight positions 1.5 deg from a central fixation cross. The appearance of the next numeral in another part of the display coincided with the masking of the previous numeral. This was done for trials of one to four numerals and SOA’s of 16.7, 33.3, and 50.0 ms. For the simultaneous presentation, similar numbers of numerals and SOA’s were used, but the numerals were presented at the same time, all followed by masks. We found that, in general, performance declined with reduced SOA. Significantly, in contrast to previous findings, performance at all three SOA’s was better for simultaneous than for sequential presentation, especially for larger numbers of numerals. In addition, control experiments with either one or two sequential masks followed by two simultaneous numerals showed that forward masking and apparent motion cannot be the main factors responsible for the poorer sequential performance. Since the manner in which the numerals were presented (either simultaneous or sequential) influenced performance, the bottleneck is most likely due to the properties of the process for transferring information into short-term memory rather than to the limitation of short-term memory itself. In addition, we confirmed the previous finding that it is still possible to report four sequentially presented numerals with acceptable accuracy if temporal order is not required, and we extended the result to show that it can be done at SOA as brief as 16.7 ms/numeral. Further, we found that eccentricity had relatively little influence on performance.

© 1995 Optical Society of America

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