Effects of visual-field and matching instruction on event-related potentials and reaction time

Vertical letter pairs were presented randomly in the left and right visual hemifields in a physical identity match and name identity match condition. The reaction times showed a right visual field superiority for name matches, and a left visual field superiority for physical matches. Event-related potentials to letter pairs showed a sequence of three waves: a negative wave (N2, around 270 msec), a positive wave (P3, around 500 msec), and a broad positive slow wave (SW, around 600-700 msec), respectively. P3 and SW amplitudes were consistently larger at the left hemisphere than at the right hemisphere, regardless of the field of stimulation. At both hemispheres, N2 waves were always larger to stimuli presented in the visual field contralateral to a hemisphere than stimuli presented in the visual field ipsilateral to a hemisphere. The positive waves (P3, SW) showed the opposite pattern: smaller amplitudes to stimuli that were presented contralaterally than stimuli that were presented ipsilaterally to a given hemisphere. These results were attributed to a shift in sustained negativity on the directly stimulated hemisphere, relative to the indirectly stimulated hemisphere, reflecting either sensory at attentional processes in the posterior cerebral hemispheres.     

Cerebral asymmetry in visual attention

The traditional view of cerebral lateralization of various cognitive functions has been challenged by results from recent experimental and clinical studies. Evidence has been gathered to suggest that a seemingly unitized cognitive function can be further broken down into various processing subcomponents which are distributed across the two hemispheres. For instance, according to such a more complex conceptualization of cerebral lateralization, language is seen not as a unitary ability, but rather as a collection of syntactic, semantic, and prosodic components, with each lateralized in particular manners. In much the same way, the present study attempts to examine the cerebral lateralization patterns of the seemingly unitary visual perception process. In a visual half-field experiment, 20 normal subjects were asked to make same/different judgments to laterally presented arrays of stimuli of the same type as previously studied by Treisman and her colleagues in experiments attempting to separate the preattentive and attentive stages of visual perception. Hemispheric differences were obtained only in tasks requiring attentive processing (i.e., Treisman's glueing). Results indicate a local attentional strategy for the left hemisphere and a global attentional strategy for the right hemisphere.     

Split-brain reveals separate but equal self-recognition in the two cerebral hemispheres

To assess the ability of the disconnected cerebral hemispheres to recognize images of the self, a split-brain patient (an individual who underwent complete cerebral commissurotomy to relieve intractable epilepsy) was tested using morphed self-face images presented to one visual hemifield (projecting to one hemisphere) at a time while making "self/other" judgments. The performance of the right and left hemispheres of this patient as assessed by a signal detection method was not significantly different, though a measure of bias did reveal hemispheric differences. The right and left hemispheres of this patient independently and equally possessed the ability to self-recognize, but only the right hemisphere could successfully recognize familiar others. This supports a modular concept of self-recognition and other-recognition, separately present in each cerebral hemisphere.     

The cortical representation of centrally presented words: A magnetic stimulation study

The right and left visual fields each project to the contralateral cerebral hemispheres. The current study aimed to investigate the extent of the functional overlap of the two hemifields along the vertical meridian. We applied repetitive transcranial magnetic stimulation (rTMS) over the left and right occipital cortex to investigate whether the foveal representation of words is bilaterally represented or is split between the two hemispheres. Employing a lateralized lexical decision task, we first showed a double dissociation between the stimulated cortical site and performance; right visual field (RVF) but not left visual field (LVF) performance was impaired when the left visual cortex was stimulated, and LVF but not RVF performance was impairred when the right visual cortex was stimulated. Unilateral stimulation also significantly impaired lexical decision latencies to centrally presented words. These findings support the suggestion that foveal representation of words is split. We discuss future strategies for the use of TMS in further tests of the split representation account.     

Spatial and Selective Attention in the Cerebral Hemispheres in Depression, Mania, and Schizophrenia

Two tachistoscopic tests examining distinct aspects of attention were administered to normal subjects and patients with depression, mania, and schizophrenia. The first examined spatial attentional bias using happy-sad chimeric faces, known to elicit a perceptual bias to the left side of space in normal right-handers provided the right cerebral hemisphere is intact. The second used a lateralized version of the Stroop task, a traditional test of selective attention. Normals showed the expected leftward perceptual bias but showed equivalent susceptibility to the Stroop effect in both visual fields. As previously demonstrated with chimeric faces viewed in free vision, depression and mania were associated with weak and strong biases respectively with schizophrenics showing no bias to either side of space. The relationship between perceptual bias, as assessed by reaction time and absolute performance and the Stroop effect, showed differences according to diagnosis. This may be interpreted as evidence for the dissociability of attentional processes as well as lateralized differences in the pattern of cerebral activation in affective disorders and schizophrenia. The independence of performance variables on these tests in the schizophrenic group points to severe neuropsychological dysfunction.     

Interlateral asymmetry in the time course of the effect of a peripheral prime stimulus

Evidence exists that both right and left hemisphere attentional mechanisms are mobilized when attention is directed to the right visual hemifield and only right hemisphere attentional mechanisms are mobilized when attention is directed to the left visual hemifield. This arrangement might lead to a rightward bias of automatic attention. The hypothesis was investigated by testing male volunteers, wherein a "location discrimination" reaction time task (Experiments 1 and 3) and a "location and shape discrimination" reaction time task (Experiments 2 and 4) were used. Unilateral (Experiments 1 and 2) and unilateral or bilateral (Experiments 3 and 4) peripheral visual prime stimuli were used to control attention. Reaction time to a small visual target stimulus in the same location or in the horizontally opposite location was evaluated. Stimulus onset asynchronies (SOAs) were 34, 50, 67, 83 and 100 ms. An important prime stimulus attentional effect was observed as early as 50 ms in the four experiments. In Experiments 2, 3 and 4, this effect was larger when the prime stimulus occurred in the right hemifield than when it occurred in the left hemifield for SOA 100 ms. In Experiment 4, when the prime stimulus occurred simultaneously in both hemifields, reaction time was faster for the right hemifield and for SOA 100 ms. These results indicate that automatic attention tends to favor the right side of space, particularly when identification of the target stimulus shape is required.     

Splitting attention across the two visual fields in visual short-term memory

Humans have the ability to attentionally select the most relevant visual information from their extrapersonal world and to retain it in a temporary buffer, known as visual short-term memory (VSTM). Research suggests that at least two non-contiguous items can be selected simultaneously when they are distributed across the two visual hemifields. In two experiments, we show that attention can also be split between the left and right sides of internal representations held in VSTM. Participants were asked to remember several colors, while cues presented during the delay instructed them to orient their attention to a subset of memorized colors. Experiment 1 revealed that orienting attention to one or two colors strengthened equally participants' memory for those colors, but only when they were from separate hemifields. Experiment 2 showed that in the absence of attentional cues the distribution of the items in the visual field per se had no effect on memory. These findings strongly suggest the existence of independent attentional resources in the two hemifields for selecting and/or consolidating information in VSTM.     

A simple technique for lateralizing visual input that allows prolonged viewing

A simplified technique is described for obtaining lateralization of visual input with prolonged viewing. This technique is based on the presence of constant normal lateral limits for horizontal rotation of the eyes with reference to the head. With head movement prevented by use of a standard bite bar, and the eyes rotated to the left and held at their lateral limit, the temporal half of the visual field of the left eye may be used for lateralized input to the right hemisphere or vice versa for input to the left hemisphere. Any form of visual stimuli or visually monitored task can be used if confined within one of the extreme temporal hemifields. In comparison to previous methods, this technique is technically simple, inexpensive, without significant risk or discomfort to the subject, readily applicable to normal and various brain-lesioned subjects, and permits prolonged in-depth viewing. An alternative version of this technique uses a stabilized spectacle frame fitted with adjustable central occluders set to allow vision through only one or both of the extreme temporal hemifields.     

Modes of word recognition in the left and right cerebral hemispheres

Four experiments are reported examining the effects of word length on recognition performance in the left and right visual hemifields (LVF, RVF). In Experiments 1 and 2 length affected lexical decision latencies to words presented in the LVF but not to words presented in the RVF. This result was found for both concrete and abstract nouns. Changing from a normal horizontal format to the use of unconventionally "stepped" words, however, produced length effects for words in both visual hemifields (Experiment 3). The Length x VHF interaction was found once again in Experiment 4 where subjects classified words as either concrete or abstract. A model proposing two modes of visual processing of letter strings is presented to account for these findings. Mode A operates independent of string length and is seen only in left hemisphere analysis of familiar words. Mode B is length dependent: it is the only mode possessed by the right hemisphere but is displayed by the left hemisphere to nonwords and to words in abnormal formats.     

Semantic and associative priming in the cerebral hemispheres: some words do, some words don't ... sometimes, some places

This study investigated spreading activation for words presented to the left and right hemispheres using an automatic semantic priming paradigm. Three types of semantic relations were used: similar-only (Deer-Pony), associated-only (Bee-Honey), and similar + associated (Doctor-Nurse). Priming of lexical decisions was symmetrical over visual fields for all semantic relations when prime words were centrally presented. However, when primes and targets were lateralized to the same visual field, similar-only priming was greater in the LVF than in the RVF, no priming was obtained for associated-only words, and priming was equivalent over visual fields for similar + associated words. Similar results were found using a naming task. These findings suggest that it is important to lateralize both prime and target information to assess hemisphere-specific spreading activation processes. Further, while spreading activation occurs in either hemisphere for the most highly related words (those related by category membership and association), our findings suggest that automatic access to semantic category relatedness occurs primarily in the right cerebral hemisphere. These results imply a unique role for the right hemisphere in the processing of word meanings. We relate our results to our previous proposal (Burgess & Simpson, 1988a; Chiarello, 1988c) that there is rapid selection of one meaning and suppression of other candidates in the left hemisphere, while activation spreads more diffusely in the right hemisphere. We also outline a new proposal that activation spreads in a different manner for associated words than for words related by semantic similarity.     

Left hemispheric interference with nonverbal performance in aphasics: comparison with data from split-brain studies

Two nonverbal tasks (Nebes' Figural Unification Test and Arc-Circle Matching Task) were given to 10 aphasic patients and 10 normal controls. Observations in split-brain patients have shown that the right hemisphere performs almost normally on these tasks while the left hemisphere fails completely. Left and right hand performances of aphasics and controls were compared with those of split-brain patients as reported in the literature. In the Figural Unification Test the aphasics' left hand performance was significantly poorer than that of split-brain patients. This is interpreted as an effect of left hemispheric interference with right hemispheric nonverbal performance. The lack of a similar impairment of left hand performance in the Arc-Circle Matching Task suggests that this interference is only provoked by tasks which, due to the possible verbalization of stimulus material, tend to stimulate left hemispheric activity.     

Name and face matching in one or two visual fields: a test of models of hemispheric specialization

In two experiments a name and a face (each male or female) were simultaneously flashed to either the same or opposite visual fields (left or right), for matching congruent (same sex) or incongruent (opposite sex), to test the predictions of various models of hemispheric specialization. While overall best performance occurred with a face in the left visual field (LVF) and a name in the right visual field (RVF), and worst with the opposite configuration, the general pattern of results was incompatible with either a direct access model or an activational/attentional account. The results were, however, most compatible with the predictions of a semispecialized hemispheres account, whereby cerebral asymmetries are seen as relative rather than absolute, either hemisphere being capable of processing either kind of material (verbal or visuospatial), but to different levels of efficiency. However, despite the fact that the stimulus materials had previously been shown to produce stable and consistent lateral asymmetries in the predicted directions when presented in isolation, in the composite, integrative matching task the position of the name seemed to be the major determinant of the resultant asymmetries. It would seem therefore that when such stimuli are to be cross matched, either left hemisphere (language) processes somehow dominate right hemisphere (visuospatial) processing (though not in the way that would be predicted by a simple activational/attentional account) or the left hemisphere's greater capacity predominates.     

Cognitive consequences of asymmetrical visual distraction

The authors explored whether manipulating the location of distraction in the participants' visual field influences the degree of competition between visual and other cognitive processes. If a cognitive task is lateralized to a particular hemisphere, visual distraction directed toward that same hemisphere should impair performance on that task more than should visual distraction directed toward the other hemisphere. Consistent with this hypothesis, the authors found in Experiments 1 and 2 that participants better recalled words of high imageability in a verbal memory task when the examiner was in the participant's left visual field (right hemisphere) than when the examiner was in the participant's right visual field (left hemisphere). In Experiment 3, the authors found that this effect reversed for performance on a right-hemisphere spatial task.     

The contribution of the right cerebral hemisphere to the recovery from aphasia: a single longitudinal case study

We examined the role of the right cerebral hemisphere in the recovery from aphasia of HJ, a 50-year-old right-handed and unilingual man who suffered from severe aphasia caused by an extensive left hemisphere (LH) lesion. He was followed-up over 10 months at 4-month intervals, with a lateralized lexical decision task (LDT), an attentional task, and a language battery. Testing started when HJ was 2 months poststroke. In the LDT, words were presented to central vision or lateralized to the left or right visual hemifield. At each test period, we examined the effect of the degree of imageability (high vs. low), and the grammatical class (noun vs. verb) of the targets on HJ's response times and error rates, with left visual field, right visual field, and central vision presentations. The results of the experiment showed that the pattern obtained with the LDT could not be accounted for by fluctuations in attention. There was an interaction of grammatical class with degree of imageability with left visual field displays only. The right hemisphere (RH) was faster with high-imageability words than with low-imageability words, regardless of their grammatical class. There was also an overall RH advantage on response times at 2 and 6 months after onset. This RH predominance coincided with a major recovery of language comprehension and the observation of semantic paralexias, while no major change in language expression was observed at that point. Ten months after onset, the pattern of lateralization changed, and response times for the LDT with either presentation site were equivalent. This LH improvement coincided with some recovery of language expression at the single-word level. The results of this study suggest that, in cases of severe aphasia caused by extensive LH lesions, the RH may play an important role in the recovery process. Furthermore, these results show that the contribution of the two cerebral hemispheres to recovery may vary overtime and affect specific aspects of language.     

Multitask investigation of individual differences in hemispheric asymmetry

Right-handed subjects (N = 120) participated in four different laterality tasks designed to measure aspects of cerebral hemisphere asymmetry: identification of dichotically presented consonant-vowel syllables (CVs), examination of the effects of concurrent repetition of CVs and concurrent anagram solution on finger-tapping by the right and left hands, lateralized identification of CVs presented tachistoscopically to the left and right visual fields, and left/right biases on a free-vision face task involving judgments of emotion. Ear differences in the dichotic listening task were related to the pattern of lateralized interference in the dual-task finger-tapping paradigm. There were no other significant relations between pairs of tasks, but when the present results are considered in the light of other recent experiments, there appears to be a relation between lateral bias on the free-vision face task and visual field differences in tachistoscopic identification. The pattern of results has implications for hypothesized individual differences among right-handers in cerebral dominance for verbal processes, input pathway dominance, and asymmetric arousal of the two cerebral hemispheres.     

An investigation of bilateral asymmetries in electrodermal activity

Research in cerebral laterality supports the idea that functional differences between the left and right hemispheres exist with respect to cognitive style and perceptual ability. Related research, which has examined autonomic nervous system (ANS) correlates of cerebral laterality, suggests that a component of the ANS, electrodermal activity (EDA), is also lateralized. Some findings in the literature report the occurrence of bilateral asymmetries in phasic and tonic EDA as a function of a left or right hemisphere preference for information processing. This experiment used normal male subjects who were either left or right movers in a test of Conjugate Lateral Eye Movement (CLEM). This selection procedure served to maximize a subject’s preference for either a right or left hemisphere mode of information processing. Bilateral EDA was recorded continuously while subjects performed a visual recognition task using word (left hemisphere) and shape (right hemisphere) stimuli. The data do not support the contention that performance on a procedure chosen to selectively activate a given hemisphere elicits asymmetric tonic and/or phasic EDA. The results show no significant difference in the frequency of elicited skin conductance responses under either of the experimental conditions. Bilateral tonic EDA rose continuously over time and did not vary in either hand as a function of task. The present results fail to offer support for either of the hypotheses which argue for I) increased contralateralexcitation with selective hemispheric activation, or 2) increased contralateralinhibition of the EDR with hemispheric arousal.     

Remembering 1500 pictures: the right hemisphere remembers better than the left

We hypothesized that the right hemisphere would be superior to the left hemisphere in remembering having seen a specific picture before, given its superiority in perceptually encoding specific aspects of visual form. A large set of pictures (N=1500) of animals, human faces, artifacts, landscapes, and art paintings were shown for 2s in central vision, or tachistoscopically (for 100ms) in each half visual field, to normal participants who were then tested 1-6 days later for their recognition. Images that were presented initially to the right hemisphere were better recognized than those presented to the left hemisphere. These results, obtained with participants with intact brains, large number of stimuli, and long retention delays, are consistent with previously described hemispheric differences in the memory of split-brain patients.     

Selective attention, inhibition for repeated events and hemispheric specialization

When two visual events appear consecutively in the same spatial location, our response to the second event is slower than to the first. This inhibition for repeated events may reflect a bias toward sampling novel locations, a bias useful for exploring visual space. It has been shown that the left hemisphere is more specialized in selective attentional processes than the right one. The aim of the present experiment was to test if this hemispheric specialization for selective attention may also affect the inhibition for repeated events. For this purpose, we asked 11 normal subjects to perform an identity-based discrimination task in which the target to be detected could appear alone or surrounded by flanking letters, in the left or in the right visual field. Results show that inhibition for repeated events is present only when selective attention is required and when the task is performed in the right specialized visual field.     

Right hemisphere specialization for color detection

Three experiments were carried out to investigate hemispheric asymmetry in color processing among normal participants. In Experiment 1, it was shown that the reaction times (RTs) of the dominant and non-dominant hands assessed using a visual target presented at the central visual field, were not significantly different. In Experiment 2, RTs of ipsilateral hands to lateralized chromatic stimuli revealed that the processing time was 17 ms shorter in the right hemisphere (RH) than that in the left hemisphere among the right-handed participants, whereas no significant difference was found among the left-handed participants. On the other hand, RTs to lateralized achromatic stimuli showed no such asymmetry among both the right- and left-handed participants (Experiment 3). These findings strongly suggest RH superiority for detection of color among right-handed individuals.