Three experiments were conducted to isolate the effects of retinal locus and string position in tachistoscopic letter recognition. Retinal locus proved to be an important variable even when its range' was restricted to less than a degree from the center of the fovea. Performance was maximal at the center of the fovea, dropping off rapidly to about 1.5 deg from the center. From that distance on, the decline in performance was quite gradual. String position was also an important factor. Retinal locus and string position interacted in such a way that the end positions were less affected by retinal locus than the middle positions. It was also found that processing order, as distinct from report order, was a significant component of the string position effect.

When unrelated letter strings are presented tachistoscopically, the end letters are reported more often than their neighbors; and when spaces are inserted into strings, performance on certain adjacent letters is superior to performance on those letters when no spaces were present. An experiment was conducted to determine the nature of those spacing effects. Letter strings were presented at a variety of retinal locations, and spaces were inserted into different positions in the instructed left-right processing order. The space effect was unrelated to processing order, but it was dependent on retinal location. To account for the various asymmetries, it was necessary to postulate that letters tend to interact with adjacent letters and that the interaction was not spatially symmetric. Furthermore, it was found that spaces had greater effects on "right-hand" letters than on symmetric letters. It was therefore concluded that letters interact at the feature level.When a string of letters is presented tachistoscopically in the right visual field (RVF), the probability of identifying one of the letters as a function of serial position is usually U-shaped (see, e.g., Estes & Wolford, 1971). Variables which contribute to the correct identification of a letter include retinal location, report order, and the number and position of surrounding letters. The space following the last letter is of particular interest as none of the other three variables predict an upturn at the end of a string in the RVF. Compared to the penultimate letter, the last letter is both further from the center of the fovea and later in the instructed processing and report orders. It is possible that the last letter is aided by the absence of contour interaction on one of its sides.There is evidence in the literature that the presence of a space in a string can have a marked effect on performance. Shaw (1969) introduced spaces into the middle of letter strings and found that a space which followed a letter in the instructed processing order (postspace) increased performance on that letter. A prespace, however, had little or no effect on the identification of that letter. A question of some importance in the understanding of visual information processing is whether that space effect is sensory in nature (due to lessened contour interaction) or cognitive (based on considerations of processing and memory). Shaw asserted that the asymmetry .The project reported herein was performed pursuant to a grant from the U.S. Office of Education, Department of Health, Education and Welfare. The opinions expressed herein, however, do not necessarily reflect the position or policy of the U.S. Office of Education, and no official endorsement by the U.S. Office of Education should be inferred. This article was written while the senior author was visiting the Center for Human Information Processing at the University of California, San Diego, The center (under partial support from NIMH Grant MH-15828) was generous in its assistance. implied a cognitive effect because of its...

An experiment was conducted to determine whether acoustic short-term memory is the factor which limits performance in the tachistoscopic full-report paradigm. Many Es have demonstrated the existence of phonemic encoding in short-term memory. The confusion errors from a tachistoscopic full-report task were, therefore, analyzed for the presence of acoustic confusions. Absolutely no evidence for acoustic confusions was found; visual confusions, however, were abundant. It was concluded that acoustic short-term memory is not the limiting factor in the full-report paradigm.

Two experiments were conducted to study the number biases of subjects in situations not involving the usual psychophysical stimuli. In Exp. I subjects were asked to generate numbers (whithin boundary conditions) they thought other people would produce under the same conditions. In Exp. II only a single lower boundary (e.g., t, i0 or 100) was employed and subjects generated a set of numbers larger than the boundary. Results suggested that definite number biases exist. ~ultiples of 1, 10, t00 and to a lesser extent 5, 50 and 500 dominate and are appropriate to the log cycle. That is, multiples of i occur most often in the cycle 1--10, multiples of t0 in the cycle 10--t00, etc. The implications of these results are noted for several psychophysical theories. * We thank Charles Lewis of the University of Illinois for advice on statistical problems.

No abstract