Left and right visual field superiority for letter classification

The effect of letter string length and report condition on letter recognition accuracy

PURPOSE

Letter sequence recognition accuracy has been postulated to be limited primarily by low-level visual factors. The influence of high level factors such as visual memory (load and decay) has been largely overlooked. This study provides insight into the role of these factors by investigating the interaction between letter sequence recognition accuracy, letter string length and report condition.

METHODS

Letter sequence recognition accuracy for trigrams and pentagrams were measured in 10 adult subjects for two report conditions. In the complete report condition subjects reported all 3 or all 5 letters comprising trigrams and pentagrams, respectively. In the partial report condition, subjects reported only a single letter in the trigram or pentagram. Letters were presented for 100ms and rendered in high contrast, using black lowercase Courier font that subtended 0.4° at the fixation distance of 0.57m.

RESULTS

Letter sequence recognition accuracy was consistently higher for trigrams compared to pentagrams especially for letter positions away from fixation. While partial report increased recognition accuracy in both string length conditions, the effect was larger for pentagrams, and most evident for the final letter positions within trigrams and pentagrams. The effect of partial report on recognition accuracy for the final letter positions increased as eccentricity increased away from fixation, and was independent of the inner/outer position of a letter.

CONCLUSIONS

Higher-level visual memory functions (memory load and decay) play a role in letter sequence recognition accuracy. There is also suggestion of additional delays imposed on memory encoding by crowded letter elements.

Sex differences in human brain asymmetry: a critical survey

Dual functional brain asymmetry refers to the notion that in most individuals the left cerebral hemisphere is specialized for language functions, whereas the right cerebral hemisphere is more important than the left for the perception, construction, and recall of stimuli that are difficult to verbalize. In the last twenty years there have been scattered reports of sex differences in degree of hemispheric specialization. This review provides a critical framework within which two related topics are discussed: Do meaningful sex differences in verbal or spatial cerebral lateralization exist? and, if so, Is the brain of one sex more symmetrically organized than the other? Data gathered on right-handed adults are examined from clinical studies of patients with unilateral brain lesions; from dichotic listening, tachistoscopic, and sensorimotor studies of functional asymmetries in non-brain-damaged subjects; from anatomical and electrophysiological investigations, as well as from the developmental literature. Retrospective and descriptive findings predominate over prospective and experimental methodologies. Nevertheless, there is an impressive accummulation of evidence suggesting that the male brain may be more asymmetrically organized than the female brain, both for verbal and nonverbal functions. These trends are rarely found in childhood but are often significant in the mature organism.

Hemispheric specialization in schizophrenics with perceptual aberration

Higher rates of left-handedness and atypical lateralization in schizophrenics paired with findings of morphological abnormalities in cerebral asymmetry suggest that the normal patterns of hemisphere specialization for processing verbal and spatial information may be anomalous in schizophrenics. The small number of studies that have addressed this question have produced inconsistent findings and varied with subtype diagnosis, gender, type of task employed, task difficulty, and control of handedness. Conflicting research findings also may be due to confounding from the heterogeneity of the schizophrenic construct and variability in clinical symptoms across patients. The present study was designed to control for factors that may have confounded earlier studies. Because the study used perceptual measures, the relationship between symptoms of perceptual aberration and hemisphere advantages was examined using Chapman et al.'s (1978) Perceptual Aberration Scale (PAS). Fifteen male schizophrenic patients and 14 male controls were administered tachistoscopic letter and facial recognition go/no-go reaction time tasks. Left hemisphere advantages were found for both controls and schizophrenics on the letter task. Right hemisphere advantages were found for controls on the facial task but not schizophrenics. Instead, a strong negative correlation was found between schizophrenics' PAS scores and hemisphere advantages (r = -0.685, p < 0.007). Further analysis identified a subgroup of schizophrenics with perceptual aberration who exhibited reversed left hemisphere advantages that increased as the PAS scores increased. Additional research is needed to determine whether this subgroup of schizophrenics constitutes a meaningful subtype with a distinct disease process that disrupts the development of normal cerebral lateralization. The findings provide further evidence for the importance of examining relationships between schizophrenics' performance on cognitive measures and their symptom patterns.

Outpatient treatment of dyslexia through stimulation of the cerebral hemispheres

Although a number of experimental investigations into the effects of hemisphere stimulation on the reading performance of individuals with dyslexia are currently available, only a few studies have addressed the effects of treatment in the setting of an outpatient clinic. The present study reports on the reading results after a treatment that was based on the balance model and incorporated notions from cognitive psychological origin in 80 children with severe dyslexia who were referred to the outpatient clinic of the Paedological Institute in Amsterdam. Treatment was individually tailored, depending on the type of dyslexia, the phase of the learning-to-read process, and the intermediate results of treatment. Effects on reading performance, measured after preclinical (home-training), clinical, and postclinical intervention periods, were analyzed through multiple time-series and multilevel analyses. Treatment with flash cards, exercising automatic letter-sound conversions, appeared to have a robust and slight effect in the preclinical and clinical phases, respectively, whereas hemisphere stimulation produced robust effects in both the clinical and the postclinical period. The results are discussed in terms of theoretical models, experimental findings of other investigations, intellectual and scholastic characteristics of the subjects, and such treatment factors as compliance (see Note).

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.

Hemispheric patterns in visual search

A visual search paradigm was employed to examine hemispheric serial and parallel processing. Stimulus arrays containing 4, 9, or 16 elements were tachistoscopically presented to the right visual field-left hemisphere (RVF-LH) or left visual field-right hemisphere (LVF-RH). Subjects judged whether all of the elements within an array were physically the same (all X's) or whether one (O) was different from the rest. Left hemisphere presentations were processed more quickly and accurately than LVF-RH presentations for all stimulus conditions. As the number of array elements increased, more errors and longer response times were obtained for different stimulus items whereas fewer errors and somewhat shorter response times were obtained for same stimulus items. These and previous results suggest that the left hemisphere obtains an advantage for visual search because of that hemisphere's superiority for fine-grained feature analysis rather than because of a fundamental hemispheric serial/parallel processing dichotomy.

Can group differences in hemispheric asymmetry be inferred from behavioral laterality indices?

A large and often contradictory literature purports to demonstrate different patterns--or at least different degrees--of hemispheric specialization across various groups of people. Schizophrenics, dyslexics, stutterers, musicians, Orientals, Jews, and many other groups have been alleged to display idiosyncratic laterality patterns. An examination of this literature reveals three important problems. First the groups concerned are rarely homogeneous. This makes it difficult to know which group characteristics, if any, are responsible for the observed differences. Second, most behavioral laterality indices are of low reliability making group differences highly unstable. Third, the validity of many behavioral laterality indices has not been substantiated. Because of these problems, it is concluded that caution should be exercised in using and interpreting laterality measures to make between-group comparisons. For now at least, group differences in laterality cannot be inferred.

Lateral differences for person perception: effects of task on visual processing of handwriting

Sergent and Bindra suggested that identification of faces using few different stimuli engages mainly the left hemisphere and, conversely, that discrimination of faces using more stimuli engages mainly the right hemisphere. The present study examined whether this suggestion holds for the perception of handwriting which, like faces, authorizes the perception of "persons". Normal adults were presented stimuli consisting of a word written in various hands, laterally displayed for 180 msec. Exp. 1 (identification) was conducted with 8 subjects, who were asked to identify (push-button) four different stimuli by means of an associated first name. Exp. 2 (discrimination) was conducted with 16 subjects who were asked to make same/different judgments between a central stimulus and a lateral one under three experimental conditions, normal, mirror-reversed, and inverted presentations of both members of the pair. The results suggest that the direction of asymmetry depends more on the amount of stimuli than on the task and that familiarity with the stimuli is an important variable.

Hemispheric competence and perceptual confusability

Hemispheric competence in performing easy and difficult letter classification was examined in an exact replication of a previous experiment by Jonides (1979). The present experiment failed to confirm Jonides's finding of right visual field advantage in conditions of perceptual confusability and left visual field superiority when the stimuli were easily discriminable. The results showed a trend in the opposite direction, but no significant interaction. This divergence is discussed with respect to existing evidence and methodological procedures.

The effects of task, practice and sequencing upon the lateralization of semantic decisions

We report three reaction time experiments in which first names were laterally presented and subjects made discriminatory manual or vocal responses (male/female), or named the stimuli, or judged whether name pairs were of same or opposite sex. The experiments addressed the issue of whether the strength and direction of laterality effects changed with practice and/or stimulus repetition. Performance facilitation with a repeated stimulus was greater when the same rather than the opposite hemisphere received the repeated stimulus, indicating that the effects of stimulus repetition are partly hemisphere-specific. Task practice was found to be a far more important determinant of changing asymmetries than stimulus repetition; however such practice may decrease as well as increase lateral asymmetries. These effects did not extend to the situation where a task was preceded by another of quite a different nature--name before face processing, and vice versa. We conclude that sequencing and practice effects are important but complex determinants of the direction and magnitude of lateral asymmetries, indicating that lateral asymmetries are labile rather than fixed and reflect relative rather than absolute specialization in interhemispheric processing capacity.

Individual variation in directional bias in visual perception

Directional biases in visual perception were examined for individual differences in sixty-five subjects on two tasks. One task required judgments of the onset asynchrony of pairs of dots presented at random, either one dot in each visual field, or both in the left visual field (LVF), or the right visual field (RVF). The second task required the recall of four letter strings presented randomly in either visual field. Dot-asynchrony judgments were influenced by two main biases: first, an outward from the centre bias in both visual fields, and second, a lateral bias which was significantly from left to right (L-R) in the total sample. A substantial minority of subjects were biased to judge the dots as occurring in right to left (R-L) order. Accuracy of letter report decreased fairly consistently from L-R in the RVF but varied in the LVF. Some subjects showed a L-R report gradient, some a R-L gradient, and some a U-shaped recall pattern. Significant correlations between measures of L-R and R-L biases on the two tasks show that the biases have some stable foundation. The findings suggest that there are directional biases affecting visual perception which are due neither to learned reading habits, nor to cerebral specialization of function.

Feature similarity and laterality effects in visual masking

Observers attempted to recognize single letters presented to the left or right visual field when preceded or followed by a masking stimulus. When the masking stimulus contained features identical to those of the target letter, there was a left visual field (right hemisphere) advantage for target recognition. When the target and mask contained extremely different features, the opposite visual field advantage was obtained. Implications are discussed for models of hemispheric asymmetry and for interpreting visual laterality studies that use masks to reduce the overall level of performance.

Dissociation between normal hemispheres in delayed recognition of verbal and spatial cues of the same visual pattern

Is each hemisphere capable of disentangling a specific kind of information from a complex 3-target set of stimuli in an expectancy-free delayed recognition experiment on normals? A tachistoscopical study set out to answer this question. It provided evidence for a double dissociation between the hemisphere of input of complex stimuli (comprising both spatial and verbal information) and the kind of information to be retrieved.

Hemispheric memory for random forms revisited

Hemispheric short-term memory was studied by projecting complex random forms that varied in their verbal association strength to the left and right hemispheres. Male and female subjects responded same or different to a centrally presented memory test stimulus which occurred 0, 5, 10, or 20 sec after the target item. Reaction time for both response judgments was significantly shorter for right-hemisphere presentations over all memory intervals. For both performance measures, response judgment interacted with the length of memory interval and the verbal association value of the stimulus items. No hemispheric differences were observed as a function of subject sex. Hemispheric memory for complex forms appears primarily dependent upon task processing demands rather than stimulus factors when response time is used to measure recognition under tightly controlled conditions.

Hemispheric asymmetry, early visual processes, and serial memory comparison

Observers indicated whether a single probe letter presented to the left visual field/right hemisphere (LVF-RH) or to the right visual field/left hemisphere (RVF-LH) matched one of two, three, four, or five set letters in both name and case. For positive trials during the initial experimental session, the slope of the linear memory set size reaction time function was increased by perceptually degrading the probe letter on LVF-RH trials, but not on RVF-LH trials. In addition, perceptual degradation of the probe letter increased the intercept of the memory set size function more on RVF-LH trials than on LVF-RH trials. During subsequent experimental sessions, the same pattern of intercept results was obtained but perceptual degradation of the probe no longer changed the slope for either LVF-RH or RVF-LH trials. However, the slopes were uniformly lower on RVF-LH trials than on LVF-RH trials. The major results are consistent with hypothesized right-hemisphere efficiency for early processing stages and left-hemisphere efficiency for serial processing of alphanumeric information. The results further illustrate the importance of separating stages of processing in studies of cerebral laterality and indicate that the relative difficulty of the various stages can be a determinant of laterality results.

Laterality effects in visual information processing: hemispheric specialisation or the orienting of attention?

The notion that visual laterality patterns may be attributable to attentional allocation rather than hemispheric specialisation was examined in three experiments. In Experiment I, high verbal ability subjects were found to be less lateralised on a letter name match task than low verbals. In Experiment II, stimulus probability was shown to affect laterality patterns for name but not for physical matches. Again, low verbals were affected more than highs. Experiment III produced results identical to those of Experiment II although, in the latter experiment, visual fields were defined vertically rather than horizontally from the midline. Together, these results support the following generalisations: (1) visual asymmetries have their locus in a post-perceptual information processing stage; (2) visual asymmetries may be altered by manipulating stimulus probability; (3) verbal ability differences in laterality may not reflect neuroanatomical differences but merely cognitive capacity and (4) it may be unnecessary to invoke differential hemispheric specialisation in order to account for visual lateral asymmetries.