Asymmetries in perception of tachistoscopically presented horizontal and vertical random-letter strings

Two studies examined perception of briefly presented (100-msec.) strings of letters. In Study One, 20 subjects were presented horizontal 5-letter strings in the left, central, and right visual fields. These were compared with 5-letter vertical strings presented centrally in the lower, central, and upper visual fields. Similar within-string patterns were found for all presentations. Between strings there was a typical right over left visual-field advantage in accuracy of report for horizontal strings. There was no equivalent lower over upper visual-field advantage for vertical strings. In Study Two, 24 subjects were presented vertical strings in the right and left visual fields, vertical strings in the upper and lower visual fields, and horizontal strings in the right and left visual fields. A post-stimulus cueing technique for single letters was used. Between-strings, the same right over left visual-field advantage for horizontal strings was noted but not for vertical strings. Between strings no advantage for lower over upper visual fields was found. An interaction for within-string patterns and visual field was found for vertical strings presented in the upper and lower visual fields. These results are explained in terms of an interaction between scanning and masking effects depending upon orientation and visual field.