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.     

Hemispace and information control by the two cerebral hemispheres: which interaction?

Two experiments employing subjects with different experience in tactile discrimination (blind and seeing subjects) were carried out to investigate the effect of the space location of stimuli on the information processing activity of the two cerebral hemispheres. An angle discrimination task that yields a right hemisphere superiority was used. In Experiment 1, seeing subjects showed a general superiority of the left hand (right hemisphere) which was more pronounced in the left hemispace with respect to the central and the right hemispace performance. In Experiment 2, blind subjects showed a significant superiority of the left hand in the central and in the left hemispace and no difference between the two hands in the right hemispace. In both experiments hemispace differences were due only to the modification of the left hand (right hemisphere) performance. These results suggest that the hemispace control by the contralateral hemisphere interacts only with the activity of the hemisphere dominant in the information processing.     

Right hemispheric dominance in gaze-triggered reflexive shift of attention in humans

Recent findings suggest a right hemispheric dominance in gaze-triggered shifts of attention. The aim of this study was to clarify the dominant hemisphere in the gaze processing that mediates attentional shift. A target localization task, with preceding non-predicative gaze cues presented to each visual field, was undertaken by 44 healthy subjects, measuring reaction time (RT). A face identification task was also given to determine hemispheric dominance in face processing for each subject. RT differences between valid and invalid cues were larger when presented in the left rather than the right visual field. This held true regardless of individual hemispheric dominance in face processing. Together, these results indicate right hemispheric dominance in gaze-triggered reflexive shifts of attention in normal healthy subjects.     

Hemisphere differences in the effects of cuing in visual recognition tasks.

In Experiment 1 uncued recognition of single letters presented in left or right visual fields showed no hemispheric asymmetry, but cuing by alternatives produced a left-hemisphere advantage. Uncued recognition of words was better in the right visual field (left hemisphere), and this advantage was unchanged by cuing by alternatives or cuing by class. In Experiment 2 a mixed series of words, digits, and dots was presented. Uncued trials showed no asymmetry, but when a precue indicated which type or stimulus would appear next, a left-hemisphere advantage for words was evident. Cuing also produced a nonsignificant shift toward a left-hemisphere advantage for digits and a right-hemisphere advantage for dots. The asymmetrical effects of cuing can be explained by Kinsbourne's attentional model of lateralization, which suggests that cuing may selectively activate one hemisphere, and so bias attention toward the contralateral visual field. Repetition effects within and between visual fields were analyzed but no asymmetries were found.     

Apparent shift in visual field preference after unilateral stroke

Patients with either a left- or a right-hemisphere stroke lesion scored higher in tasks of word-picture matching and of nonverbal shape matching when information was presented tachistoscopically (120 msec) to the visual field (VF) projecting to their undamaged hemisphere. Left-hemisphere stroke patients (n = 13) were dissociated from right-hemisphere stroke patients (n = 15) by low word recognition from memory and by low right VF but nearly normal left VF accuracy in word-picture matching or shape matching; the former appeared to rely upon processing of word meaning by the right hemisphere. In contrast, right-stroke patients had higher right than left VF scores in both tasks, and their discrimination of nonverbal shapes via the right VF was not different from that of controls (n = 15). Preferred processing by the VF projecting to the undamaged hemisphere appeared as a shift in perceptual asymmetry but may indicate, in support of a "direct access" model, that each hemisphere responds more or less efficiently to word and to nonverbal shape discriminations.     

Does each hemisphere monitor the ongoing process in the contralateral one?

The present study was conducted to examine hemispheric division of labor in the initial processing and error monitoring in tasks for which hemispheric specialization exists. We used lexical decision as a left hemisphere task and bargraph judgment as a right hemisphere task, and manipulated cognitive load. Participants had to respond to one of two stimuli presented to both visual fields and were instructed to correct their errors. To achieve enough correctable errors, participants were encouraged to respond quickly by using a bonus system. The results showed the classical asymmetry for initial responses in both tasks and reversed asymmetry for corrections in the bargraph task at both load conditions, and in the lexical decision task at the high load condition. The results suggest that each hemisphere monitors the ongoing process in the contralateral one and that the dissociation of initial process and its monitoring grows with load of task.     

Rhythm length and hemispheric asymmetry

Conflicting evidence has appeared in the literature concerning hemispheric asymmetry in the perception of rhythm. The present study investigated the effects of rhythm length on relative cerebral dominance. Twenty-four subjects were presented with sequences of one to four time intervals bounded by light flashes. The subjects' task was to determine if two such sequences were the same or different. The first rhythm was presented in both visual fields and the second only to one visual field. Reaction times and number of errors were recorded. It was found that increasing rhythm length resulted in a shift in cerebral dominance from left to right hemisphere. An interpretation of these findings was suggested in terms of the preferred mode of processing of each hemisphere, analytic vs. holistic.     

语义距离半球效应的ERP研究

以通过视觉通路呈现的汉语双字词为实验材料,操纵启动词与目标词的语义距离。在词汇决定实验中记录ERP,发现：（1）除N300外,P240亦对语义距离敏感,它可能是一种与语义加工有关的新的ERP成分;（2）P240在头皮中央偏左的部位较早出现,但在头皮右侧波幅较大;（3）N300和P240都不存在语义距离的半球效应,该结果没有证实右脑更专擅远距离语义加工的假设,表明使用汉语与使用拼音文字的脑机制存在差异。     

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.     

Selective attention and cerebral dominance in perceiving and responding to speech messages

The effect of attention on cerebral dominance and the asymmetry between left and right ears was investigated using a selective listening task. Right handed subjects were presented with simultaneous dichotic speech messages; they shadowed one message in either the right or left ear and at the same time tapped with either the right or the left hand when they heard a specified target word in either message. The ear asymmetry was shown only when subjects' attention was focused on some other aspect of the task: they tapped to more targets in the right ear, but only when these came in the non-shadowed message; they made more shadowing errors with the left ear message, but chiefly for non-target words. The verbal response of shadowing showed the right ear dominance more clearly than the manual response of tapping. Tapping with the left hand interfered more with shadowing than tapping with the right hand, but there was little correlation between the degree of hand and of ear asymmetry over individual subjects. The results support the idea that the right ear dominance is primarily a quantitative difference in the distribution of attention to left and right ear inputs reaching the left hemisphere speech areas. This affects both the efficiency of speech perception and the degree of response competition between simultaneous verbal and manual responses.     

Right hemisphere participation in reading

This study examined whether the right hemisphere's contribution to lexical semantic processing is greatest when it is "disinhibited." Skilled reading may require the controlled modulation of interhemispheric interaction: the left hemisphere (or some other control mechanism) may regulate the subprocesses of reading by selectively "inhibiting" and "disinhibiting" right hemisphere function. Right-handed undergraduates concurrently performed two tasks: a lateralized semantic or rhyme task and a verbal memory task. It was hypothesized that right hemisphere reading processes would be disinhibited when the left hemisphere was "occupied" with the memory task. This hypothesis was supported for a subgroup of subjects who showed evidence of inhibition of right hemisphere function (i.e., left hemisphere dominance for lexical processing) when the lateralized semantic task was performed alone. Across subjects, there was a strong correlation between the degree of left hemisphere dominance in the single-task semantic conditions and the degree of disinhibition of right hemisphere function in the dual-task semantic condition.     

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.     

Gender differences in the recognition of laterally presented affective nouns

Evidence suggests that the cerebral hemispheres are differentially specialised for the processing of positively and negatively valenced affective material. The present study examined the consequences of this for the availability in each hemisphere of lexical entries related to positive, neutral, and negative affect. A threshold recognition task was used to assess the identification of positive, neutral, and negative emotionally toned nouns projected to the right or left visual field. A strong lateral asymmetry in favour of right visual field recognition of nouns was observed. Whereas women showed no evidence of changes in lateral asymmetry as a function of affect, lateral asymmetry in men was significantly greater for positive nouns. This finding points to a difference in the representation of affective material in the left and right lexicons of the male brain, and provides support for the notion that gender may be one of the keys to reconciling some of the contradictory findings that characterise this area of research.     

A functional near-infrared spectroscopy study of sustained attention to local and global target features

Despite a long history of vigilance research, the role of global and local feature discrimination in vigilance tasks has been relatively neglected. In this experiment participants performed a sustained attention task requiring either global or local shape stimuli discrimination. Reaction time to local feature discriminations was characterized by a quadratic trend over time-on-task with performance levels returning to initial levels late in the task. This trend did not occur in the global shape discrimination task. Functional near-infrared spectroscopy (fNIRS) was utilized in this study as an index of cerebral activation. In both tasks there was increased right hemisphere relative to left hemisphere oxygenation and right hemisphere oxygenation increased with time-on-task. Left hemisphere oxygenation, however, decreased slightly in the global task, but increased significantly in the local task as task duration increased. Indeed, total oxygenation, averaging both right and left, increased more with time-on-task in the local discrimination task. Both the performance and physiological results of this study indicate increased utilization of bilateral cerebral resources with time-on-task in the local, but not the global discrimination vigil.     

Order of feature recognition in familiar and unfamiliar faces

Three experiments measured order of processing for single faces presented to the left or right visual field (VF) using a same-different matching task. In contrast to earlier studies, the stimuli in the present experiments were carefully matched for overall similarity prior to the actual experiments. Experiments 1 and 2 showed that a significant top-to-bottom order of processing occurred for line drawings of unfamiliar faces but not for line drawings of familiar faces. Experiment 3 found evidence supporting top-to-bottom processing for unfamiliar photographic face stimuli. The photographic stimuli in Experiment 3 were matched more quickly when presented in the left VF (right hemisphere); however, this VF asymmetry was not related to previously reported differences in order of processing. It is suggested that under some conditions faces presented to the right hemisphere may be processed more like familiar faces than faces presented to the left hemisphere; however, this difference is not critical for the left VF (right hemisphere) superiority often found in face recognition tasks.     

Hemispheric control of spatial attention

According to the activation-orienting hypothesis the distribution of attention in space is biased in the direction contralateral to the more activated hemisphere. The present investigation tested this proposal and evaluated the nature of hemispheric differences in orienting control. Activation imbalance was produced by a unilateral visual stimulus. The distribution of attention was measured using a modified line bisection task in which subjects judged the location of an intersect on a tachistoscopically presented horizontal line. The first three experiments suggest that (a) attention is biased in the direction contralateral to the stimulated hemisphere, and (b) the biases do not depend on the task relevance or hemispatial position of the stimulus producing the activation imbalance. The final three experiments suggest that when orienting conflict is introduced the rightward bias becomes more robust than the leftward bias. The findings are consistent with the activation-orienting hypothesis. Each hemisphere generates a contralateral attentional bias and the rightward bias of the left hemisphere is stronger. The relevance of these findings to understanding unilateral neglect resulting from parietal damage is discussed.     

Effects of perceptual quality on the processing of human faces presented to the left and right cerebral hemispheres

Three experiments examined the effects of stimulus duration, retinal eccentricity, and visual noise on the processing of human faces presented to the left visual field/right hemisphere (LVF-RH) and right visual field/left hemisphere (RVF-LH). In Experiment 1 observers identified which of 10 similar male faces was presented on a screen. The single face was presented for 10, 55, or 100 ms at 1 degree, 4 degrees, or 9 degrees of visual angle to the left or right of fixation. Decreasing stimulus duration and increasing retinal eccentricity lowered face recognition. The effect of duration was the same for LVF-RH and RVF-LH trials, but the detrimental effect of increasing retinal eccentricity was larger on LVF-RH trials than on RVF-LH trials. In Experiment 2 observers indicated whether a single face from this same set was a member of a memorized set of five positive faces. The probe face on each trial was presented alone or embedded in visual noise. Visual noise increased the error rate more on LVF-RH trials than on RVF-LH trials. This effect was replicated in Experiment 3, which also required observers to make a much easier discrimination between male and female faces. In the male/female task visual noise tended to impair performance more on RVF-LH trials than on LVF-RH trials, opposite the effect for the male/male task. These results are discussed in terms of hemispheric asymmetry for global versus local features of faces, the level of feature analysis demanded by a task, and the level of feature analysis most disrupted by perceptual degradation.     

Hemispheric asymmetry in the efficiency of attentional networks

Despite the fact that hemispheric asymmetry of attention has been widely studied, a clear picture of this complex phenomenon is still lacking. The aim of the present study was to provide an efficient and reliable measurement of potential hemispheric asymmetries of three attentional networks, i.e. alerting, orienting and executive attention. Participants (N=125) were tested with the Lateralized Attention Network Test (LANT) that allowed us to investigate the efficiency of the networks in both visual fields (VF). We found a LVF advantage when a target occurred in an unattended location, which seems to reflect right hemisphere superiority in control of the reorienting of attention. Furthermore, a LVF advantage in conflict resolution was observed, which may indicate hemispheric asymmetry of the executive network. No VF effect for alerting was found. The results, consistent with the common notion of general right hemisphere dominance for attention, provide a more detailed account of hemispheric asymmetries of the attentional networks than previous studies using the LANT task.     

Visual field asymmetries for motion processing in deaf and hearing signers

Recently, we reported a strong right visual field/left hemisphere advantage for motion processing in deaf signers and a slight reverse asymmetry in hearing nonsigners (Bosworth & Dobkins, 1999). This visual field asymmetry in deaf signers may be due to auditory deprivation or to experience with a visual-manual language, American Sign Language (ASL). In order to separate these two possible sources, in this study we added a third group, hearing native signers, who have normal hearing and have learned ASL from their deaf parents. As in our previous study, subjects performed a direction-of-motion discrimination task at different locations across the visual field. In addition to investigating differences in left vs right visual field asymmetries across subject groups, we also asked whether performance differences exist for superior vs inferior visual fields and peripheral vs central visual fields. Replicating our previous study, a robust right visual field advantage was observed in deaf signers, but not in hearing nonsigners. Like deaf signers, hearing signers also exhibited a strong right visual field advantage, suggesting that this effect is related to experience with sign language. These results suggest that perceptual processes required for the acquisition and comprehension of language (motion processing in the case of ASL) are recruited by the left, language-dominant, hemisphere. Deaf subjects also exhibited an inferior visual field advantage that was significantly larger than that observed in either hearing group. In addition, there was a trend for deaf subjects to perform relatively better on peripheral than on central stimuli, while both hearing groups showed the reverse pattern. Because deaf signers differed from hearing signers and nonsigners along these domains, the inferior and peripheral visual field advantages observed in deaf subjects is presumably related to auditory deprivation. Finally, these visual field asymmetries were not modulated by attention for any subject group, suggesting they are a result of sensory, and not attentional, factors.     

The influence of emotional faces on the spatial allocation of attention

Background objectives: Studies suggest that the right hemisphere is dominant for emotional facial recognition. In addition, whereas some studies suggest the right hemisphere mediates the processing of all emotions (dominance hypothesis), other studies suggest that the left hemisphere mediates positive emotions the right mediates negative emotions (valence hypothesis). Since each hemisphere primarily attends to contralateral space, the goals of this study was to learn if emotional faces would induce a leftward deviation of attention and if the valence of facial emotional stimuli can influence the normal viewer’s spatial direction of attention. Methods: Seventeen normal right handed participants were asked to bisect horizontal lines that had all combinations of sad, happy or neutral faces at ends of these lines. During this task the subjects were never requested to look at these faces and there were no task demands that depended on viewing these faces. Results: Presentation of emotional faces induced a greater leftward deviation compared to neutral faces, independent of where (spatial position) these faces were presented. However, faces portraying negative emotions tended to induce a greater leftward bias than positive emotions. Conclusions: Independent of location, the presence of emotional faces influenced the spatial allocation of attention, such that normal subjects shift the direction of their attention toward left hemispace and this attentional shift appears to be greater for negative (sad) than positive faces (happy).