Joseph B. Hellige

Information processing in the cerebral hemispheres: selective hemispheric activation and capacity limitations.

Several previous experiments have found that concurrently maintaining verbal information in memory influences visual laterality patterns (e.g., Hellige & Cox, 1976; Kinsbourne, 1975). The present article critically reviews existing experiments and reports five additional experiments designed to identify the mechanisms responsible for such effects. Experiment 1 demonstrates that laterality patterns are not influenced by a concurrent memory task that does not require verbal processing. (The verbal nature of the concurrent task was an important aspect of previous experiments.) Experiments 2 and 3 were designed to determined whether concurrent verbal memory primarily influences very early visuospatial processes or later processes such as those involved in visuospatial memory. In Experiment 2, observers indicated whether two simulteneously presented nonsense forms had the same shape. Observers held 0, 2, 4, or 6 words in memory during each shape judgment trial. Responses were faster when the forms were presented to the left visual field--right hemisphere (LVF-RH) than to the right visual field--left hemisphere (RVF-LH). This effect did not interact with memory set size. In Experiment 3, observers indicated whether either of two simultaneously presented forms was identical to a target form held in memory. Observers held 0, 2, or 6 words in memory on each trial. On same-as-target trials, responses were faster on LVF-RH trials than on RVF-LH trials in the no-word memory condition; this difference was reversed in the two-word and six-word conditions. The combined results of Experiments 2 and 3 suggest that concurrent verbal memory influences stages of processing beyond the initial registration of visuospatial information. Experiments 4 and 5 examined the influence of concurrent verbal memory on verbal laterality tasks. Observers indicated whether two simultaneously presented letters of different cases had the same name. In Experiment 4, different groups of observers held 0, 2, 4, or 6 words in memory on each letter-pair trial. In Experiment 5, memory set size was manipulated within subjects. On the same-pair trials of Experiment 4 and the first session of Experiment 5, responses in the no-memory condition were faster on RVF-LH trials than on LVF-RH trials; this difference was reversed in all of the work memory conditions. This shift is opposite to that found when the laterality task does not require verbal processing and further indicates that concurrent verbal memory influences processing stages beyond those that are common to the form-pair and letter-pair tasks. Neither directness-of-pathway nor attention-gradient laterality models can explain the entire pattern of results from the present experiments. Rather, the results suggest that the left hemisphere functions as a typical limited-capacity information processing system that can be influenced somewhat separately from the right hemisphere system.

Categorization versus distance: Hemispheric differences for processing spatial information

University of Southern California, Los Angeles, California It has been hypothesized that the brain computes two different kinds of spatial-relation representations: one used to assign a spatial relation to a category and the other used to specify metric distance with precision. The present visual half-field experiment offers support for this distinction by showing that the left and right cerebral hemispheres make more effective use of the categorization and metric distance representations, respectively. Furthermore, the inclusion of a bilateral stimulus presentation condition permits the computation of a reversed association that offers additional support for the distinction between two types of spatial-relation representation.

Effects of concurrent verbal memory on recognition of stimuli from the left and right visual fields

Two experiments examined the effect of concurrently holding 0, 2, 4, or 6 nouns in memory on the recognition of visual stimuli briefly presented to the left or right visual fields. When stimuli to be visually recognized were complex visuospatial forms it was found that a relatively easy memroy load of 2 or 4 nouns improved visual recognition accuracy on right visual field (left-hemisphere) trials relative to the no-memory condition; however, a more difficult memory load of 6 nouns decreased visual recognition accuracy to a level slightly below the no-memory condition. There were no effects of concurrent verbal memroy on visual form recognition on left visual field (right-hemisphere) trials. When the stimuli to be visually recognized were words it was found that a relatively easy memroy load of 2 or 4 nouns improved visual recognition accuracy and a more difficult load of 6 nouns decreased visual recognition accuracy on both left and right visual field trials. The complete pattern of results indicates that several factors including cerebral hemisphere specialization, stimulus codability, selective perceptual orientation, and selective cerebral hemisphere interference interact in systematic ways to produce overall visual laterality effects.

Role of input factors in visual-field asymmetries

This paper examines the implications of lateral tachistoscopic presentation of information for the processing efficiency of the intact cerebral hemispheres. Considering that the understanding of the processes underlying the particular competences of each hemisphere may require, as a preliminary step, the specification of the characteristics of the input on which the brain operates, anatomical, physiological, and psychophysical consequences of briefly stimulating the retinal periphery for the representation of information in the brain are outlined, with special reference to the spatial-frequency spectral composition of the stimuli. Retinal eccentricity and brief exposure duration converge to making the representation of information qualitatively different from the information that the brain normally operates on, which constrains the interpretation of findings with respect to the normal functions of the cerebral hemispheres. A review and discussion of empirical findings relevant to these issues suggest that manipulation of procedural variables may differentially affect the processing efficiency of the cerebral hemispheres and indicate that a given pattern of visual-field asymmetry may be overdetermined by a multitude of variables interacting in complex ways.

Changes in same-different laterality patterns as a function of practice and stimulus quality

Accuracy and reaction time (RT) of judgments about sameness vs. difference of (a) names of two letters and (b) shapes of two nonverbal forms were examined for stimuli presented to the center, left (LVF), and right (RVF) visual fields. For same-name letter pairs during Experiment I, responses were more accurate and faster for LVF than for RVF trials on an initial 90-trial block, but this difference was reversed by a third 90-trial block. The RVF advantage for RT was maintained over Trial Blocks 4 and 5, given during a second session, but had disappeared on Trial Blocks 6 through 9 as RT reached the same asymptotic level for both visual fields. No LVF-RVF differences were obtained at any level of practice for different-name letter pairs or for any of the form pairs. Experiment II replicated the shift from LVF toward RVF advantage that occurred over the first three trial blocks of Experiment I and demonstrated that such a shift does not occur when the letters are perceptually degraded. The results were discussed in terms of differences in cerebral hemisphere specialization for visuospatial vs. abstract stages of letter processing and changes with practice in the relative difficulty of these stages.

Hemispheric asymmetry in the processing of absolute versus relative spatial frequency

Observers indicated whether a stimulus presented to one visual field or the other consisted of two sine-wave gratings (the baseline stimulus) or those same two gratings with the addition of a 2 cycle per degree (cpd) component. When the absolute spatial frequencies of the baseline stimulus were low (0.5 and 1.0 cpd), there was a left visual field-right hemisphere (LVF-RH) advantage in reaction time (RT) to respond to the baseline stimulus which disappeared when the 2 cpd component was added (i.e., the stimulus consisted of 0.5, 1.0, and 2.0 cpd components). When the absolute spatial frequencies of the baseline stimulus were moderate to high (4.0 and 8.0 cpd), a right visual field-left hemisphere advantage in RT to respond to the baseline stimulus approached significance and shifted to a significant LVF-RH advantage when the 2 cpd component was added (i.e., the stimulus consisted of 2.0, 4.0, and 8.0 cpd components. That is, adding the same 2 cpd component caused opposite shifts in visual laterality depending on whether 2 cpd was a relatively high or relatively low frequency compared to the baseline.

Individual variation in hemispheric asymmetry: multitask study of effects related to handedness and sex.

: Functional hemispheric asymmetries were examined for right- or left-handed men and women. Tasks involved (a) auditory processing of verbal material, (b) processing of emotions shown on faces, (c) processing of visual categorical and coordinate spatial relations, and (d) visual processing of verbal material. Similar performance asymmetries were found for the right-handed and left-handed groups, but the average asymmetries tended to be smaller for the left-handed group. For the most part, measures of performance asymmetry obtained from the different tasks did not correlate with each other, suggesting that individual subjects cannot be simply characterized as strongly or weakly lateralized. However, ear differences obtained in Task 1 did correlate significantly with certain visual field differences obtained in Task 4, suggesting that both tasks are sensitive to hemispheric asymmetry in similar phonetic or language-related processes.

Right hemisphere superiority for initial stages of letter processing

Abstract When single letters, which could be perfectly recognized when presented alone, were embedded in an overlapping masking stimulus, observers recognized more letters from the left than from the right visual field. This left visual field-right hemisphere advantage persisted over short time intervals between the letter and the mask, regardless of which stimulus occurred first. Such results suggest that the right cerebral hemisphere is more efficient than the left at extracting relevant visual features of letters when the letters are perceptually degraded, even though letters are highly associated with language and, therefore, readily processed along verbal-analytic dimensions.

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.

Effects of concurrent hemisphere-specific activity on unimanual tapping rate

Abstract In Experiment 1, concurrently reading paragraphs reduced unimanual tapping rate more for the right than for the left index finger. This effect was larger when subjects were reading aloud rather than silently and larger for subjects who expected a test of paragraph content than for subjects who did not. In Experiment 2, the opposite hand difference was obtained when the concurrent activity involved solving a Block Design problem. Together, the experiments indicate that lateral differences in interference with manual performance depend on the differential involvement of the two cerebral hemispheres in the concurrent activity and that such effects are influenced by both motor and cognitive aspects of the concurrent task.