Hemispheric competence and perceptual confusability

What do lateralized displays tell us about visual word perception? A cautionary indication from the word-letter effect

A common assumption underlying laterality research is that visual field asymmetries in lateralized word perception indicate the hemispheric specialisation of processes generally available for the perception of words, including words viewed in a more typical setting (i.e. in the central visual field). We tested the validity of this assumption using a phenomenon (the word-letter effect) frequently reported for displays viewed in the central visual field, where letters in words are perceived more accurately than the same letters in isolation. Words and isolated letters were presented in the left visual field (LVF), right visual field (RVF) and central visual field (CVF), the Reicher-Wheeler task was used to suppress influences of guesswork, and an eye-tracker ensured central fixation. In line with previous findings, lateralized displays revealed a RVF-LVF advantage for words (but not isolated letters) and CVF displays revealed an advantage for words over isolated letters (the word-letter effect). However, RVF and LVF displays both produced an advantage for isolated letters over words (a letter-word effect), indicating that processing subserving the advantage for words when participants viewed stimuli in the central visual field was unavailable for lateralized displays. Implications of these findings for studies of lateralized word perception are discussed.

Hemifield differences in perceived spatial frequency

Measurements of the perceived spatial frequency of stationary sinewave gratings were made with the gratings presented at the same eccentricity in the left, right, upper, and lower visual hemifields. Ten subjects performed the task binocularly with spatial frequencies of 1, 2, and 4 cycles deg-1. Two of these subjects also performed the task monocularly at 2 cycles deg-1. In the majority of cases, the spatial frequency of stimuli presented in the left and lower visual hemifields was overestimated relative to stimuli presented in the right and upper visual hemifields. The results were similar for all spatial frequencies tested, and the direction of the asymmetry was the same whether viewing was with the left eye, right eye or binocular, suggesting that the differences in perceived spatial frequency are not retinal in origin.

Spatial frequencies and the cerebral hemispheres: contrast sensitivity, visible persistence, and letter classification

The hypothesis that the two cerebral hemispheres are specialized for processing different visual spatial frequencies was investigated in three experiments. No differences between the left and right visual fields were found for: (1) contrast-sensitivity functions measured binocularly with vertical gratings ranging from 0.5 to 12 cycles per degree (cpd); (2) visible persistence durations for 1- and 10-cpd gratings measured with a stimulus alternation method; and (3) accuracy (d') and reaction times to correctly identify digitally filtered letters as targets (L or H) or nontargets (T or F). One significant difference, however, was found: In Experiment 3, a higher decision criterion (beta) was used when filtered letters were identified in the right visual field than when they were identified in the left. The letters were filtered with annular, 1-octave band-pass filters with center spatial frequencies of 1, 2, 4, 8, and 16 cpd. Combining four center frequencies with three letter sizes (0.5 degrees, 1 degree, and 2 degrees high) made some stimuli equivalent in distal spatial frequency (cycles per object) and some equivalent in proximal spatial frequency (cycles per degree). The effective stimulus in the third experiment seemed to be proximal spatial frequency (cycles per degree) not distal (cycles per object). We conclude that each cerebral hemisphere processes visual spatial frequency information with equal accuracy but that different decision rules are used.

Cerebral asymmetries and interhemispheric processes

Those who attempt to characterize the functions of the cerebral hemispheres tend, broadly speaking, to do so either in terms of structural specializations or of different information-processing modes. Often little attention is paid to the possible importance, in determining the outcome of experiments in this field, of interhemispheric processes. An experiment is described which concurrently studies hemispheric response differences and interhemispheric processes. Sets of dot patterns are learned, each made up of an original and three distortions of the original. The degree of distortion is systematically changed and is quantified in terms of information theory. Subjects then examine pairs of patterns and decide whether they belong to the 'same' family, i.e. an original and one of its distortions, or 'different', i.e. a pattern previously learned and a completely new one. Manual response times are recorded for 'same' and 'different' responses and functions plotted of response latencies against degree of pattern distortion. The pairs of patterns are presented under three different conditions. Either both patterns in one visual field (unilateral condition), one pattern in each visual field (bilateral condition) or the patterns one above the other straddling the vertical meridian (central condition). Response latencies are shortest for the bilateral condition, next shortest for the unilateral condition and slowest for the central condition. Models which may account for these results are examined and one crucially involving changed information transmission rates with increasing distortion of patterns to be compared is found to provide the most parsimonious explanation of the results from all three experimental conditions.

Role of task factors in visual field asymmetries

Any tachistoscopic study of cerebral hemisphere asymmetry imposes a variety of task demands on the participants, ranging from demands imposed by specific viewing conditions to demands imposed by response output requirements. The present article discusses the role of several task factors that influence processing after the initial reception of the stimulus input, suggests a theoretical rationale for some of the effects of these task factors, and considers implications for future studies of visual laterality. The task factors discussed include both those that are relevant for minimizing artifacts that have nothing to do with hemispheric asymmetry and those that are relevant for interpreting hemispheric asymmetry in terms of specific perceptual and cognitive processes.