An examination of the right-hemisphere hypothesis of the lateralization of emotion

The Right-Hemisphere Hypothesis posits that emotional stimuli are perceived more efficiently by the right hemisphere than by the left hemisphere. The current research examines this hypothesis by examining hemispheric asymmetries for the conscious and unconscious perception of emotional stimuli. Negative, positive, and neutral words were presented for 17 ms to one visual field or the other. Conscious perception was measured by using a subjective report-of-awareness measure reported by participants on each trial. Unconscious perception was measured using an "exclusion task," a form of word-stem-completion task. Consistent with previous research, there was a right-hemisphere advantage for the conscious perception of negative information. As in previous studies, this advantage for conscious perception occurred at the expense of unconscious perception. Specifically, there was a right-hemisphere inferiority for the unconscious perception of negative information. Contrary to the predictions of the Right-Hemisphere Hypothesis, there were no hemispheric asymmetries for the perception of positive emotional information, thus suggesting that the Right-Hemisphere Hypothesis may not be applicable to all behavioral studies.     

Visual-field bias in the judgment of facial expression of emotion

The left and right hemispheres of the brain are differentially related to the processing of emotions. Although there is little doubt that the right hemisphere is relatively superior for processing negative emotions, controversy exists over the hemispheric role in the processing of positive emotions. Eighty right-handed normal male participants were examined for visual-field (left-right) differences in the perception of facial expressions of emotion. Facial composite (RR, LL) and hemifacial (R, L) sets depicting emotion expressions of happiness and sadness were prepared. Pairs of such photographs were presented bilaterally for 150 ms, and participants were asked to select the photographs that looked more expressive. A left visual-field superiority (a right-hemisphere function) was found for sad facial emotion. A hemispheric advantage in the perception of happy expression was not found.     

Visual-Field Bias in the Judgment of Facial Expression of Emotion

The left and right hemispheres of the brain are differentially related to the processing of emotions. Although there is little doubt that the right hemisphere is relatively superior for processing negative emotions, controversy exists over the hemispheric role in the processing of positive emotions. Eighty right-handed normal male participants were examined for visual-field (left-right) differences in the perception of facial expressions of emotion. Facial composite (RR, LL) and hemifacial (R, L) sets depicting emotion expressions of happiness and sadness were prepared. Pairs of such photographs were presented bilaterally for 150 ms, and participants were asked to select the photographs that looked more expressive. A left visual-field superiority (a right-hemisphere function) was found for sad facial emotion. A hemispheric advantage in the perception of happy expression was not found.     

The Contribution of Attention to the Right Visual Field Advantage for Word Recognition

Perceptual asymmetries have been explained by structural, attentional bias and attentional advantage models. Structural models focus on asymmetries in the physical access information has to the hemispheres, whereas attentional models focus on asymmetries in the operation of attentional processes. A series of experiments was conducted to assess the contribution of attentional mechanisms to the right visual field (RVF) advantage found for word recognition. Valid, invalid and neutral peripheral cues were presented at a variety of stimulus onset asynchronies to manipulate spatial attention. Results indicated a significant RVF advantage and cueing effect. The effect of the cue was stronger for the left visual field than the RVF. This interaction supports the attentional advantage model which suggests that the left hemisphere requires less attention to process words. The attentional asymmetry is interpreted in terms of the different word processing styles used by the left and right hemispheres. These results have ramifications for the methodology used in divided visual field research and the interpretation of this research.     

Visuospatial processing and the right-hemisphere interpreter

Popular views of hemispheric asymmetry hold that the left hemisphere is specialized for linguistic and cognitive processes and fine motor control, whereas the right is specialized for visuospatial processing. Although this dichotomy contains more than a grain of truth, it is an oversimplification. Experiments with split-brain patients have demonstrated that the left hemisphere retains relatively sophisticated visuospatial abilities, and that the asymmetries that favor the right hemisphere are subtler than those that favor the left. A consideration of the constructive nature of visual perception, and the organization of the visual system in the two hemispheres suggests that asymmetries are likely to arise relatively late in visual processing in areas that represent both sides of visual space. I present evidence in favor of the view that the right hemisphere can be considered more "visually intelligent" than the left, and postulate the existence of a "right-hemisphere interpreter" dedicated to constructing a representation of the visual world.     

Cerebral asymmetry in the perception of American sign language.

American Sign Language (ASL) offers a valuable opportunity for the study of cerebral asymmetries, since it incorporates both language structure and complex spatial relations: processing the former has generally been considered a left-hemisphere function, the latter, a right-hemisphere one. To study such asymmetries, congenitally deaf, native ASL users and normally-hearing English speakers unfamiliar with ASL were asked to identify four kinds of stimuli: signs from ASL, handshapes never used in ASL, Arabic digits, and random geometric forms. Stimuli were presented tachistoscopically to a visual hemifield and subjects manually responded as rapidly as possible to specified targets. Both deaf and hearing subjects showed left-visual-field (hence, presumably right-hemisphere) advantages to the signs and to the non-ASL hands. The hearing subjects, further, showed a left-hemisphere advantage to the Arabic numbers, while the deaf subjects showed no reliable visual-field differences to this material. We infer that the spatial processing required of the signs predominated over their language processing in determining the cerebral asymmetry of the deaf for these stimuli.     

刺激呈现时间对汉字语义加工脑偏侧化的影响

以往关于汉字字词识别脑功能偏侧化的研究发现了左半球优势、右半球优势或者大脑两半球均势三种不同的结果。该研究采用一侧化Stroop范式(刺激分别只呈现于左视野、中央视野或右视野中),通过系统地改变刺激呈现时间以期探讨刺激呈现时间是可以解释这些不一致结果的可能因素之一。结果显示:对于右利手被试,在刺激呈现时间为60 ms时右半球出现了较强的Stroop效应,在刺激呈现200 ms时左右半球的Stroop效应没有表现出差异,在刺激呈现时间较长时左半球表现出较强的Stroop效应。该结果提示,随着刺激呈现时间的延长,语义优势发生了从右半球到左半球的转换。     

A hemispheric asymmetry for the unconscious perception of emotion

Previous research has demonstrated that hemispheric asymmetries for conscious visual perception do not lead to asymmetries for unconscious visual perception. These studies utilized emotionally neutral items as stimuli. The current research utilized both emotionally negative and neutral stimuli to assess hemispheric differences for conscious and unconscious visual perception. Conscious perception was measured using a subjective measure of awareness reported by participants on each trial. Unconscious perception was measured by an "exclusion task," a form of word-stem-completion task. Consistent with predictions, negative stimuli were consciously perceived most often when presented to the right hemisphere. Negative stimuli presented to the right hemisphere showed no evidence of unconscious perception, suggesting that the hemispheric asymmetry for the conscious perception of negative information occurs at the expense of unconscious perception.     

Reevaluating split-fovea processing in word recognition: Hemispheric dominance, retinal location, and the word-nonword effect

Many studies have claimed that hemispheric projections are split precisely at the foveal midline and so hemispheric asymmetry affects word recognition right up to the point of fixation. To investigate this claim, four-letter words and nonwords were presented to the left or right of fixation, either close to fixation in foveal vision or farther from fixation in extrafoveal vision. Presentation accuracy was controlled using an eyetracker linked to a fixation-contingent display. Words presented foveally produced identical performance on each side of fixation, but words presented extrafoveally showed a clear left-hemisphere (LH) advantage. Nonwords produced no evidence of hemispheric asymmetry in any location. Foveal stimuli also produced an identical word-nonword effect on each side of fixation, whereas extrafoveal stimuli produced a word-nonword effect only for LH (not right-hemisphere) displays. These findings indicate that functional unilateral projections to contralateral hemispheres exist in extrafoveal locations but provide no evidence of a functional division in hemispheric processing at fixation.     

Are variations among right-handed individuals in perceptual asymmetries caused by characteristic arousal differences between hemispheres?

We propose that much of the variance among right-handed subjects in perceptual asymmetries on standard behavioral measures of laterality arises from individual differences in characteristic patterns of asymmetric hemispheric arousal. Dextrals with large right-visual-field (RVF) advantages on a tachistoscopic syllable-identification task (assumed to reflect characteristically higher left-hemisphere than right-hemisphere arousal) outperformed those having weak or no visual-field asymmetries (assumed to reflect characteristically higher right-hemisphere than left-hemisphere arousal). The two groups were equal, however, in asymmetries of error patterns that are thought to indicate linguistic or nonlinguistic encoding strategies. For both groups, relations between visual fields in the ability to discriminate the accuracy of performance followed the pattern of syllable identification itself, suggesting that linguistic and metalinguistic processes are based on the same laterally specialized functions. Subjects with strong RVF advantages had a pessimistic bias for rating performance, and those with weak or no asymmetries had an optimistic bias, particularly for the left visual field (LVF). This is concordant with evidence that the arousal level of the right hemisphere is closely related to affective mood. Finally, consistent with the arousal model, leftward asymmetries on a free-vision face-processing task became larger as RVF advantages on the syllable task diminished and as optimistic biases for the LVF, relative to the RVF, increased.     

Naming reaction times to tachistoscopically presented pictures: some evidence of right hemisphere encoding

The present study measured naming reaction times by normal subjects to unilaterally presented picture stimuli. Significant differences in picture-naming reaction time did not exist between left and right visual-field stimulations. The right hemisphere in the intact brain is capable of generating a verbal label for pictured stimuli. A psychological model suggests that a possible processing synergy between the two hemispheres may be tied to spatial processing and elementary linguistics.     

Hemifield asymmetries differentiate VSTM for single- and multiple-feature objects

Visual short-term memory (VSTM) is a capacity-limited system for maintaining visual information across brief durations. Limits in the amount of information held in memory reflect processing constraints in the intraparietal sulcus (IPS), a region of the frontoparietal network also involved in visual attention. During VSTM and visual attention, areas of IPS demonstrate hemispheric asymmetries. Whereas the left hemisphere represents information in only the right hemifield, the right hemisphere represents information across the visual field. In visual attention, hemispheric asymmetries are associated with differences in behavioral performance across the visual field. In order to assess the degree of hemifield asymmetries in VSTM, we measured memory performance across the visual field for both single- and two-feature objects. Consistent with theories of right-hemisphere dominance, there was a memory benefit for single-feature items in the left visual hemifield. However, when the number of features increased, the behavioral bias reversed, demonstrating a benefit for remembering two-feature objects in the right hemifield. On an individual basis, the cost of remembering an additional feature in the hemifields was correlated, suggesting that the shift in hemifield biases reflected a redistribution of resources across the visual field. Furthermore, we demonstrate that these results cannot be explained by differences in perceptual or decision-making load. Our results are consistent with a flexible resource model of VSTM in which attention and/or working memory demands result in representation of items in the right hemifield by both the left and right hemispheres.     

The symmetry of visual fields in chromatic discrimination

Both classical and recent reports suggest a right-hemisphere superiority for color discrimination. Testing highly-trained normal subjects and taking care to eliminate asymmetries from the testing situation, we found no significant differences between left and right hemifields or between upper and lower hemifields. This was the case for both of the cardinal axes of color space. In addition, there was no difference according to whether the discriminanda were delivered to the same or to different hemispheres, and we note that the same number of synapses may lie between the retina and the site of comparison whether or not the stimuli are delivered to the same hemisphere.     

Hemispheric asymmetries for global and local visual perception: effects of stimulus and task factors.

Although neurotogical and physiological studies indicate a right hemisphere superiority in global processing and a left hemisphere superiority in local processing of Navon-type hierarchical letters (D. Navon, 1977), most investigations of lateralized perception in healthy participants report neither asymmetry. In 6 experiments the authors examined the influence of attentional demands, stimulus properties, and mode of response on perceptual asymmetries for global and local perception. Consistent with their theoretical predictions, asymmetries were more robust on divided- than focused-attention tasks and in response to stimuli in which local and global levels were equally salient compared with those with greater global than local saliency. Contrary to their prediction, perceptual asymmetries were not influenced by the complexity of the motor response.     

Visual field asymmetries and allocation of attention in visual scenes

Single items such as objects, letters or words are often presented in the right or left visual field to examine hemispheric differences in cognitive processing. However, in everyday life, such items appear within a visual context or scene that affects how they are represented and selected for attention. Here we examine processing asymmetries for a visual target within a frame of other elements (scene). We are especially interested in whether the allocation of visual attention affects the asymmetries, and in whether attention-related asymmetries occur in scenes oriented out of alignment with the viewer. In Experiment 1, visual field asymmetries were affected by the validity of a spatial precue in an upright frame. In Experiment 2, the same pattern of asymmetries occurred within frames rotated 90 degrees on the screen. In Experiment 3, additional sources of the spatial asymmetries were explored. We conclude that several left/right processing asymmetries, including some associated with the deployment of spatial attention, can be organized within scenes, in the absence of differential direct access to the two hemispheres.     

Functional hemispheric asymmetries of global/local processing mirrored by the steady-state visual evoked potential

While hemispheric differences in global/local processing have been reported by various studies, it is still under dispute at which processing stage they occur. Primarily, it was assumed that these asymmetries originate from an early perceptual stage. Instead, the content-level binding theory (Hübner & Volberg, 2005) suggests that the hemispheres differ at a later stage at which the stimulus information is bound to its respective level. The present study tested this assumption by means of steady-state evoked potentials (SSVEPs). In particular, we presented hierarchical letters flickering at 12 Hz while participants categorised the letters at a pre- cued level (global or local). The information at the two levels could be congruent or incongruent with respect to the required response. Since content-binding is only necessary if there is a response conflict, asymmetric hemispheric processing should be observed only for incongruent stimuli. Indeed, our results show that the cue and congruent stimuli elicited equal SSVEP global/local effects in both hemispheres. In contrast, incongruent stimuli elicited lower SSVEP amplitudes for a local than for a global target level at left posterior electrodes, whereas a reversed pattern was seen at right hemispheric electrodes. These findings provide further evidence for a level-specific hemispheric advantage with respect to content-level binding. Moreover, the fact that the SSVEP is sensitive to these processes offers the possibility to separately track global and local processing by presenting both level contents with different frequencies.     

Hemisphere differences in conscious and unconscious word reading

Hemisphere differences in word reading were examined using explicit and implicit processing measures. In an inclusion task, which indexes both conscious (explicit) and unconscious (implicit) word reading processes, participants were briefly presented with a word in either the right or the left visual field and were asked to use this word to complete a three-letter word stem. In an exclusion task, which estimates unconscious word reading, participants completed the word stem with any word other than the prime word. Experiment 1 showed that words presented to either visual field were processed in very similar ways in both tasks, with the exception that words in the right visual field (left hemisphere) were more readily accessible for conscious report. Experiment 2 indicated that unconsciously processed words are shared between the hemispheres, as similar results were obtained when either the same or the opposite visual field received the word stem. Experiment 3 demonstrated that this sharing between hemispheres is cortically mediated by testing a split-brain patient. These results suggest that the left hemisphere advantage for word reading holds only for explicit measures; unconscious word reading is much more balanced between the hemispheres.     

Facilitation and disruption of lateralized syllable processing by unattended stimuli in the opposite visual field

The influence of lateralized unattended stimuli on the processing of attended stimuli in the opposite visual field can shed light on the nature of information that is transferred between hemispheres. On a cued bilateral task, participants tried to identify a syllable in the attended visual field, which elicits a left hemisphere (LH) advantage and different processing strategies by the two hemispheres. The same or a different syllable or a neutral stimulus appeared in the unattended field. Transmission of unattended syllable codes between hemispheres is symmetric, as revealed by equal interference for the two visual fields. The LH is more accurate than the RH in encoding unattended syllables, as indicated by facilitation in the left but not right visual field and a greater frequency of identifiable intrusions into the left than right field. However, asymmetric encoding strategies are different for attended and unattended syllables.