Comparison of hemispheric asymmetry in global and local information processing and interference in divided and selective attention using spatial frequency filters

Asymmetries in event-related potentials part 1: A systematic review of face processing studies

The human brain shows distinct lateralized activation patterns for a range of cognitive processes. One such function, which is thought to be lateralized to the right hemisphere (RH), is human face processing. Its importance for social communication and interaction has led to a plethora of studies investigating face processing in health and disease. Temporally highly resolved methods, like event-related potentials (ERPs), allow for a detailed characterization of different processing stages and their specific lateralization patterns. This systematic review aimed at disentangling some of the contradictory findings regarding the RH specialization in face processing focusing on ERP research in healthy participants. Two databases were searched for studies that investigated left and right electrodes while participants viewed (mostly neutral) facial stimuli. The included studies used a variety of different tasks, which ranged from passive viewing to memorizing faces. The final data selection highlights, that strongest lateralization to the RH was found for the N170, especially for right-handed young male participants. Left-handed, female, and older participants showed less consistent lateralization patterns. Other ERP components like the P1, P2, N2, P3, and the N400 were overall less clearly lateralized. The current review highlights that many of the assumed lateralization patterns are less clear than previously thought and that the variety of stimuli, tasks, and EEG setups used, might contribute to the ambiguous findings.

Contralateral Hemisphere Activation by Unilateral Hand Contraction: ReExamining Global and Local Attention

While previous studies have shown that left- (vs. right-) hand contractions can improve the performance of global (vs. local) attention, these results were inconsistent in certain behavioral studies in which the left cerebral hemisphere was found to be specialized for local attention, while the right hemisphere was not specialized for global attention. Thus, we reexamined whether different global (vs. local) attention performances follow left- (vs. right-) hand contractions due to right (vs. left) hemisphere activation. We recruited 100 right-handed healthy female, university student (aged 19-26 years) participants. We used the Navon task to present 80 letter-stimuli (40 in the global task; 40 in the local task) to either the participants' left or right visual fields. Half of the participants (randomly selected) were instructed to squeeze a dynamometer with the left hand, while the other half squeezed with the right hand, prior to completing the Navon task. In line with previous research, we observed a perceptual advantage of global (large letters) over local (small letters) stimuli. Participants who contracted the right hand showed an enhanced local attention performance (from left hemisphere activation), whereas those who contracted the left hand did not improve global attention performance (from right hemisphere activation). Thus, we supported prior behavioral research suggesting that hemispheric asymmetry for attention processing is evident for left but not right cortical hemisphere processing.

Perceptual asymmetry during free viewing of words and faces: The effect of context on recognition

There is ample evidence supporting the dissociation between the role of the left and right cerebral hemispheres in processing words and faces, respectively. Nevertheless, research has not yet studied the effect of perceptual asymmetry in memory context effect tasks using words and faces. Thus, the present study researches the advantages of presenting information in the right versus left hemispace and the effect of context on recognition when using faces compared to words presented in the right versus left hemispace. Participants (n=60) were assigned either to the group presented with pairs of words, or with pairs of faces. One stimulus in each pair was designated as the target (i.e., to be remembered) and the other served as context (i.e., to be ignored). Half of the targets were presented in the right hemispace, and half were presented in the left hemispace. As predicted, words were better recognized when presented in the right hemispace, while faces were better remembered when presented in the left hemispace. The most interesting finding is the influence of context on lateralized processing of words and pictures. That is, only when words or faces were presented in the left hemispace did contextual information affect target memory (though it yielded a different pattern of effect). Hence, the findings of the present study may be interpreted either as reflecting attentional bias to the left hemispace or structural differences between the hemispheres. Thus, cognitive processes and the content of the stimuli determine which hemisphere will be involved in processing contextual information.

Parallel processing of general and specific threat during early stages of perception

Differential processing of threat can consummate as early as 100 ms post-stimulus. Moreover, early perception not only differentiates threat from non-threat stimuli but also distinguishes among discrete threat subtypes (e.g. fear, disgust and anger). Combining spatial-frequency-filtered images of fear, disgust and neutral scenes with high-density event-related potentials and intracranial source estimation, we investigated the neural underpinnings of general and specific threat processing in early stages of perception. Conveyed in low spatial frequencies, fear and disgust images evoked convergent visual responses with similarly enhanced N1 potentials and dorsal visual (middle temporal gyrus) cortical activity (relative to neutral cues; peaking at 156 ms). Nevertheless, conveyed in high spatial frequencies, fear and disgust elicited divergent visual responses, with fear enhancing and disgust suppressing P1 potentials and ventral visual (occipital fusiform) cortical activity (peaking at 121 ms). Therefore, general and specific threat processing operates in parallel in early perception, with the ventral visual pathway engaged in specific processing of discrete threats and the dorsal visual pathway in general threat processing. Furthermore, selectively tuned to distinctive spatial-frequency channels and visual pathways, these parallel processes underpin dimensional and categorical threat characterization, promoting efficient threat response. These findings thus lend support to hybrid models of emotion.

Reciprocal organization of the cerebral hemispheres

The cerebral hemispheres are anatomically and neurophysiologically asymmetrical. The evolutionary basis for these differences remains uncertain. There are, however, highly consistent differences between the hemispheres, evident in reptiles, birds, and mammals, as well as in humans, in the nature of the attention each applies to the environment. This permits the simultaneous application of precisely focused, but narrow, attention, needed for grasping food or prey, with broad, open, and uncommitted attention, needed to watch out for predators and to interpret the intentions of conspecifics. These different modes of attention can account for a very wide range of repeated observations relating to hemisphere specialization, and suggest that hemisphere differences lie not in discrete functional domains as such, but distinct modes of functioning within any one domain. These modes of attention are mutually incompatible, and their application depends on inhibitory transmission in the corpus callosum. There is also an asymmetry of interaction between the hemispheres at the phenomenological level.

Callosal degradation in HIV-1 infection predicts hierarchical perception: a DTI study

HIV-1 infection affects white matter circuits linking frontal, parietal, and subcortical regions that subserve visuospatial attention processes. Normal perception requires the integration of details, preferentially processed in the left hemisphere, and the global composition of an object or scene, preferentially processed in the right hemisphere. We tested whether HIV-related callosal white matter degradation contributes to disruption of selective lateralized visuospatial and attention processes. A hierarchical letter target detection paradigm was devised, where large (global) letters were composed of small (local) letters. Participants were required to identify target letters among distractors presented at global, local, both or neither level. Attention was directed to one (global or local) or both levels. Participants were 21 HIV-1 infected and 19 healthy control men and women who also underwent Diffusion Tensor Imaging (DTI). HIV-1 participants showed impaired hierarchical perception owing to abnormally enhanced global facilitation effects but no impairment in attentional control on local-global feature selection. DTI metrics revealed poorer fiber integrity of the corpus callosum in HIV-1 than controls that was more pronounced in posterior than anterior regions. Analysis revealed a double dissociation of anterior and posterior callosal compromise in HIV-1 infection: compromise in anterior but not posterior callosal fiber integrity predicted response conflict elicited by global targets, whereas compromise in posterior but not anterior callosal fiber integrity predicted response facilitation elicited by global targets. We conclude that component processes of visuospatial perception are compromised in HIV-1 infection attributable, at least in part, to degraded callosal microstructural integrity relevant for local-global feature integration.

Local or global? Attentional selection of spatial frequencies binds shapes to hierarchical levels

Contrary to the traditional view that shapes and their hierarchical level (local or global) are a priori integrated in perception, recent evidence suggests that the identity of a shape and its level are encoded independently, implying the need for shape-level binding to account for normal perception. What is the binding mechanism in this case? Using hierarchically arranged letter shapes, we obtained evidence that the left hemisphere has a preference for binding shapes to the local level, whereas the right hemisphere has a preference for binding shapes to the global level. More important, binding is modulated by attentional selection of higher or lower spatial frequencies. Attention to higher spatial frequencies facilitated subsequent binding by the left hemisphere of elements to the local level, whereas attention to lower spatial frequencies facilitated subsequent binding by the right hemisphere of elements to the global level.

Schizophrenia patients show deficits in shifts of attention to different levels of global-local stimuli: evidence for magnocellular dysfunction

Abnormalities of attention and visual perception are well documented in schizophrenia. The global-local task is a measure of attention and perceptual organization that utilizes visual stimuli comprised of large letters (global level) made up of smaller letters (local level). Subjects identify target letters appearing at either the global or local level of the stimulus. In this study, we used a version of the global-local task specifically designed to examine lateralized hemispheric processing and attention shifting in 30 schizophrenia patients and 24 normal controls. Global-local stimuli were presented in couplets (consecutive pairs). Reaction time for the second target in a couplet was compared under conditions in which the target remained at the same level (global-global, local-local) and when the target changed levels (global-local, local-global). Level-specific priming (ie, global to global and local to local) and the local-to-global level shift were similar in both groups. Schizophrenia patients were significantly slower, however, shifting attention from the global to the local level. These results implicate an impairment in shifting attentional resources from predominantly right lateralized magnocellular/dorsal stream processing of global targets to predominantly left lateralized parvocellular/ventral stream processing of local targets. Local interference effects in global processing provide further support for impaired magnocellular processing in schizophrenia patients.

Global-local interference is related to callosal compromise in alcoholism: a behavior-DTI association study

BACKGROUND

Visuospatial ability is a multifactorial process commonly impaired in chronic alcoholism. Identification of which features of visuospatial processing are affected and which are spared in alcoholism, however, has not been clearly determined. We used a global-local paradigm to assess component processes of visuospatial ability and MR diffusion tensor imaging (DTI) to examine whether alcoholism-related microstructural degradation of the corpus callosum contributes to disruption of selective lateralized visuospatial and attention processes.

METHODS

A hierarchical letter paradigm was devised, where large global letters were composed of small local letters. The task required identification of target letters among distractors presented at global, local, both, or neither level. Attention was either selectively directed to global or local levels or divided between levels. Participants were 18 detoxified chronic alcoholics and 22 age-matched healthy controls. DTI provided quantitative assessment of the integrity of corpus callosal white matter microstructure.

RESULTS

Alcoholics generally had longer reaction times than controls but obtained similar accuracy scores. Both groups processed local targets faster than global targets and showed interference from targets at the unattended level. Alcoholics exhibited moderate compromise in selectively attending to the global level when the global stimuli were composed of local targets. Such local interference was less with longer abstinence. Callosal microstructural integrity compromise predicted degree of interference from stimulus incongruency in the alcoholic group. This relationship was not observed for lateral or third ventricular volumes, which are measures of nonspecific cortical volume deficits.

CONCLUSION

Global-local feature perception was generally spared in abstinent chronic alcoholics, but impairments were observed when directing attention to global features and when global and local information interfered at stimulus or response levels. Furthermore, the interference-callosal integrity relationship in alcoholics indicates that compromised visuospatial functions include those requiring bilateral integration of information.