Mood and spatial memory: emotion and right hemisphere contribution to spatial cognition.

Depressed persons show an impairment of spatial cognition that may reflect the influence of affective arousal on right hemisphere cognition. We examined normal university students to determine whether individual differences in mood and arousal levels would be related to performance on a spatial memory task. Right-hemisphere specialization for this spatial memory task was confirmed by a left field advantage for the targets and this field asymmetry was enhanced as task difficulty was increased. Event-related brain potentials (ERPs), assessed with a 64-channel sensor array, showed a processing negativity contralateral to the target in the P300 interval (300-500 ms after the target appeared). This effect increased as task difficulty was increased. A stronger posterior negativity for good (rather than bad) targets may suggest that attention was allocated toward the good locations. A suggestion of right hemisphere sensitivity to mood in this normal sample was a tendency for the subjects high in Negative Arousal not to show the normal right hemisphere (left field) superiority for the spatial memory task. Interestingly, a medial frontal lobe negativity was elicited in the ERPs by the bad targets, perhaps paralleling the error-related negativity observed in other paradigms. This medial frontal negativity was also seen in response to the feedback stimulus for the bad targets. Motivation may be important to this frontal effect: It was enhanced for subjects describing themselves as high in either positive or negative affective arousal during the task.     

ERP evidence for hemispheric asymmetries in abstract but not exemplar‐specific repetition priming

Implicit memory retrieval is thought to be exemplar‐specific in the right hemisphere (RH) but abstract in the left hemisphere (LH). Yet, conflicting behavioral priming results illustrate that the level at which asymmetries take effect is difficult to pinpoint. In the present divided visual field experiment, we tried to address this issue by analyzing ERPs in addition to behavioral measures. Participants made a natural/artificial decision on lateralized visual objects that were either new, identical repetitions, or different exemplars of studied items. Hemispheric asymmetries did not emerge in either behavioral or late positive complex (LPC) priming effects, but did affect the process of implicit memory retrieval proper as indexed by an early frontal negativity (N350/(F)N400). Whereas exemplar‐specific N350/(F)N400 priming effects emerged irrespective of presentation side, abstract implicit memory retrieval of different exemplars was contingent on right visual field presentation and the ensuing initial stimulus processing by the LH.     

Hemispheric Asymmetry for Word Perception: Behavioral and ERP Evidence

Reaction times (RTs) to words or patterns presented in the left or right visual fields and averaged Event-Related Potentials (ERPs) evoked by the same stimuli presented foveally were measured separately. In both experiments, an oddball paradigm known to elicit the P3 component was used. Twenty-four subjects (12 males) were tested in each experiment. In the electrophysiological study EEG was measured differentially from five different scalp sites, each referred to linked ear lobes. There were two frontal and two parietal lateralized electrodes, and a fifth which was located at the vertex (C_z). The RTs to words were significantly delayed relative to the RTs to patterns. Furthermore, the responses were significantly shorter in the right visual field when the target was a word, yet only slightly shorter in the left visual field when the target was a nonsense pattern. The same pattern of results was found with the evoked potentials: A positive wave evoked by the target stimulus was measured at vertex with mean latency of 511.3 msec in the verbal task, and 464.4 msec in the nonverbal task. At the lateralized locations, a significant Task by Hemisphere interaction was found. When the target was a word the base to peak P3 amplitudes were higher over the left than over the right hemisphere. A reverse trend was found with nonverbal targets. The base to peak P3 amplitudes were slightly (but not significantly) higher over the right than over the left hemisphere.     

Contralateral and ipsilateral motor activation in visual simple reaction time: a test of the hemispheric coactivation model

Motor potentials contralateral versus ipsilateral to the responding hand were examined in a visual simple reaction time (RT) experiment in order to test the hemispheric coactivation model of Miller (Cogn Psychol 49:118-154, 2004). Visual stimuli were presented on the left side of fixation, on the right side, or on both sides, and in the RT task participants had to respond as quickly as possible to the onset of any stimulus. The same stimulus displays were also presented in a counting task, for which participants had merely to count the stimuli. Hemisphere-specific movement-related potentials contralateral and ipsilateral to the responding hand were isolated by subtracting count-task ERPs from RT-task ERPs. Consistent with the hemispheric coactivation model, there was evidence of movement-related ipsilateral positivity as well as contralateral negativity, suggesting that the motor areas of both hemispheres contribute to response initiation in simple RT. The distinction between contralateral and ipsilateral motor activation appears useful in clarifying the roles of the two hemispheres in response initiation.     

The characteristics of the nogo-N140 component in somatosensory go/nogo tasks

Nogo-related brain potentials may not be dependent on sensory modalities but reflect common neural activities specific to the inhibitory process. Recent studies reported that nogo potentials were elicited by not only visual and auditory but also somatosensory stimulation. However, the characteristics of this nogo potential evoked by somatosensory stimulation have been unclear because of the small number of reports. In the present study, therefore, to determine the characteristics of this potential, the effects of stimulus site and response hand were investigated. Electrical stimulation was delivered to the second and fifth digit of one hand, and the subjects had to respond to a go stimulus by pushing a button with the thumb contralateral to the stimulated side as quickly as possible. The amplitudes of the nogo-N140 component (N140 evoked by the nogo stimuli), which is very similar to the nogo-N2 components following visual and auditory stimulation, were unrelated to the stimulated digits, the second and fifth digit of the left and right hand. However, differences between go and nogo ERPs were significantly larger in the hemisphere contralateral to the response hand than the ipsilateral hemisphere. This result was inconsistent with visual and auditory go/nogo studies showing a right-hemisphere dominance or bilateral activities in nogo trials. Therefore, nogo-N140 should be considered to reflect the inhibitory process especially in the hemisphere contralateral to the response hand and the sensory modality dependency of nogo potentials.     

The processing of sound duration after left hemisphere stroke: Event-related potential and behavioral evidence

The ability of left-hemisphere stroke patients ( n = 8) and healthy control subjects ( n = 8) to process sounds preattentively and attentively was studied by recording auditory event-related potentials (ERPs) and behavioral responses. For the right-ear stimulation, the mismatch negativity (MMN) was significantly smaller in the patients than control subjects over both hemispheres. For the left-ear stimuli, the MMN was significantly smaller in the patient group than in the control group over the left hemisphere, whereas no group differences were obtained over the right hemisphere. In addition, the N1 amplitude was reduced bilaterally for the right-ear stimulation (with the reduction being larger over the left hemisphere), whereas no significant effects on the N1 amplitude were found for the left-ear stimulation. Behaviorally, the patients detected significantly fewer deviant tones than did the control subjects irrespective of the stimulated ear. The present results thus suggest that the long-latency ERPs can be used to probe such auditory processing deficits that are difficult to define with behavioral measures. Especially by recording MMN to monaural stimuli, the discrimination accuracy can be separately determined for the left and right temporal lobes.     

Sensory and motor attentional modulation during the manual gap effect in humans: a high-density ERP study

The present study investigated reaction time (RT) and event-related potential (ERP) differences between gap and step conditions using visual stimulation and manual responses. RTs during the gap condition were facilitated with respect to those of the step condition. The ERPs, which were obtained from electrodes placed at 58 scalp sites, showed differences when the gap and step conditions were compared for the following components: an early positive component centred at the vertex, an enhanced P1 component, a frontal negativity, a negative lateralized motor potential, and an increased P3. All these results suggest that the facilitation induced by the gap is mediated by a modulation of the neural circuits involved in sensory, motor, and cognitive functions.     

Event-related potentials suggest early interaction between syntax and semantics during on-line sentence comprehension

Event-related potentials (ERPs) were used to investigate interaction between syntactic parsing and semantic integration processes during a visual sentence comprehension task. The linguistic stimuli were Finnish five-word sentences containing morphosyntactic and/or semantic violations. Single morphosyntactic violations evoked left anterior negativity (LAN) and P600 components. Single semantic violations elicited a robust N400 effect over the left hemisphere. A later and weaker N400-like response was also observed in the right hemisphere, left-right hemispheric latency difference being 40 ms. Combined morphosyntactic and semantic violations elicited a P600 component and a negative ERP component within the latency range of the LAN and N400 components. Further analysis of these ERP effects provided evidence for early processual interaction between syntax and semantics during on-line sentence comprehension. The hemispheric distribution of the LAN and N400 components was taken to suggest lateralization of initial morphosyntactic parsing and semantic integration processes to the left hemisphere. In contrast, the later syntax-related P600 component was observed as being more pronounced over the posterior areas of the right hemisphere.     

Bilateral Electrodermal and Electrocortical Activity in Anticipation of Sensorimotor Tasks

The present study investigated the relationship between lateralized sensorimotor activation and lateralized elevation of electrodermal activity. Within a two-stimulus reaction time paradigm skin conductance responses (SCRs) were recorded from both hands, while subjects awaited and performed unilateral sensorimotor tasks. A tactile warning stimulus was applied 6 s prior to a tactile imperative stimulus either to the same or to the contralateral hand; the imperative stimulus was associated with a choice reaction time task. Task-dependent lateral asymmetry with larger skin conductance responses contralateral to the involved hemisphere indicated contralateral facilitatory processes rather than inhibition. This asymmetry disappeared with repeated stimulation (across trials). Only right-hand (left hemispheric) tasks proved to be sensitive to habituation. The asymmetry, recorded in slow brain potentials (SP_s), assured that lateralized cortical processing was induced through the present experimental arrangement. The brain potentials specified the time course of lateralized processing, exhibiting larger late negative shifts at the precentral areas contralateral to the hand involved in the sensorimotor task and larger early negativity and a more pronounced positive evoked potential (late positive complex, LPC) contralateral to the hand stimulated by the tactile warning stimulus.     

Hemispheric control of unilateral and bilateral movements of proximal and distal parts of the arm as inferred from simple reaction time to lateralized light stimuli in man

Summary In 12 normal, right-handed male subjects simple reaction time of key-pressing and leverpulling responses to light flashes presented in the right or left visual fields was faster with the arm ipsilateral to the visual stimulus, provided the responses were made unilaterally. Key-pressing responses involved a movement of a single finger, whereas lever-pulling responses involved an integrated movement of the proximal parts of the arm. The mean difference in reaction time between reactions ipsilateral to the stimulus and reactions contralateral to the stimulus was about 2 ms for both key-pressing and lever-pulling responses. When key-pressing and lever-pulling responses to a single lateralized light stimulus were made bilaterally, the advantage in favor of the ipsilateral responses was still present, although it was mainly limited to the right hand, on key-pressing, whereas it was completely absent on lever pulling.The difference between ipsilateral and contralateral reactions on unilateral responding, whether proximal or distal, is attributed to the consistent initiation of the response by the contralateral hemisphere. Given that a lateralized visual stimulus is projected to the opposite hemisphere, ipsilateral responses can be integrated within a single hemisphere, whereas contralateral responses are integrated interhemispherically and therefore require additional time. The reduction or lack of the ipsilateral advantage on bilateral responding is attributed to the engagement of a bilaterally distributed motor control that is preferentially directed to the proximal musculature.     

Hemispheric lateralization of somatosensory processing

Processing of both painful and nonpainful somatosensory information is generally thought to be subserved by brain regions predominantly contralateral to the stimulated body region. However, lesions to right, but not left, posterior parietal cortex have been reported to produce a unilateral tactile neglect syndrome, suggesting that components of somatosensory information are preferentially processed in the right half of the brain. To better characterize right hemispheric lateralization of somatosensory processing, H(2)(15)O positron emission tomography (PET) of cerebral blood flow was used to map brain activation produced by contact thermal stimulation of both the left and right arms of right-handed subjects. To allow direct assessment of the lateralization of activation, left- and right-sided stimuli were delivered during separate PET scans. Both innocuous (35 degrees C) and painful (49 degrees C) stimuli were employed to determine whether lateralized processing occurred in a manner related to perceived pain intensity. Subjects were also scanned during a nonstimulated rest condition to characterize activation that was not related to perceived pain intensity. Pain intensity-dependent and -independent changes in activation were identified in separate multiple regression analyses. Regardless of the side of stimulation, pain intensity--dependent activation was localized to contralateral regions of the primary somatosensory cortex, secondary somatosensory cortex, insular cortex, and bilateral regions of the cerebellum, putamen, thalamus, anterior cingulate cortex, and frontal operculum. No hemispheric lateralization of pain intensity-dependent processing was detected. In sharp contrast, portions of the thalamus, inferior parietal cortex (BA 40), dorsolateral prefrontal cortex (BA 9/46), and dorsal frontal cortex (BA 6) exhibited right lateralized activation during both innocuous and painful stimulation, regardless of the side of stimulation. Thus components of information arising from the body surface are processed, in part, by right lateralized systems analogous to those that process auditory and visual spatial information arising from extrapersonal space. Such right lateralized processing can account for the left somatosensory neglect arising from injury to brain regions within the right cerebral hemisphere.     

Modulation of Brain Activities by Hierarchical Processing: A High-Density ERP Study

The present study investigated how attention to global or local levels of hierarchical patterns modulates brain activities by recording high-density event-related brain potentials (ERPs) evoked by hierarchical stimuli. 120-channel recordings of ERPs were obtained from subjects while they detected targets at global or local levels of hierarchical stimuli displayed in the left or the right visual field. We found that attention to local stimulus features enhanced posterior PI and N2 components, with the N2 enhancement showing a left hemisphere predominance regardless of stimulus positions. Difference was also seen in the distribution of the frontal P2. Reaction times were slowed when global and local levels of stimuli were incompatible, and an interference effect was observed on anterior N2 amplitudes and latencies. Three-dimensional current distributions showed common sources over the posterior cortex between 80-230 ms and a contralateral frontal source between 300-400 ms for global and local conditions. However, an additional ipsilateral frontal focus between 230-350 ms was found specially for local processing. The results corroborate the findings of previous ERP studies, and suggest that the frontal lobe is particularly important for the selective processing of local parts of a global structure.     

TMS of the right angular gyrus modulates priming of pop-out in visual search: combined TMS-ERP evidence

During priming of pop-out, performance at discriminating a pop-out feature target in visual search is affected by whether the target on the previous trial was defined by the same feature as on the upcoming trial. Recent studies suggest that priming of pop-out relies on attentional processes. With the use of simultaneous, combined transcranial magnetic stimulation and event-related potential recording (TMS-ERP), we tested for any critical role of the right angular gyrus (rANG) and left and right frontal eye fields (FEFs)-key attentional sites-in modulating both performance and the ERPs evoked by such visual events. Intertrial TMS trains were applied while participants discriminated the orientation of a color pop-out element in a visual search array. rANG TMS disrupted priming of pop-out, reducing reaction time costs on switch trials and speeding responses when the color of the pop-out target switched. rANG TMS caused a negativity in the ERP elicited in response to the visual stimulus array, starting 210 ms after stimulus onset. Both behavioral and ERP effects were apparent only after rANG TMS, on switch trials, and when the target in the visual search array was presented in the left visual field, with no effects after left or right FEF TMS. These results provide evidence for an attentional reorienting mechanism, which originates in the rANG and is modulated by the implicit memory of the previous trial. The rANG plays a causal role on switch trials during priming of pop-out by interacting with visual processing, particularly in the ipsilateral hemisphere representing the contralateral hemifield.     

Mismatch negativity in dichotic listening: Evidence for interhemispheric differences and multiple generators

The characteristics of mismatch negativity (MMN) elicited by dichotic stimulation were examined using frequency-deviant stimuli presented to the right, to the left, or to both sides. The experiment was run twice, once using earphones and once using loudspeakers in free field. With both modes of stimulation, deviants presented in the left, right, or both ears, or tones that were switched between ears, elicited comparable MMNs, with a peak latency of about 180 ms. With earphones, the amplitude of the MMN was bigger at the frontal-lateral right hemisphere sites than at the homologous left-hemisphere sites for all deviance conditions. Scalp current density analysis revealed that deviance in the right side elicited bilaterally equivalent frontal current sinks and a trend towards stronger contralateral current sources at the mastoid sites. In contrast, left side deviance elicited frontal sinks and temporal current sources stronger over the right hemiscalp. These results are compatible with the multiple-generator model of MMN. The attention-related role of the MMN is discussed, suggesting comparable attention mechanisms for vision and audition.     

Intermodal spatial attention differs between vision and audition: an event-related potential analysis

Subjects were required to attend to a combination of stimulus modality (vision or audition) and location (left or right). Intermodal attention was measured by comparing event-related potentials (ERPs) to visual and auditory stimuli when the modality was relevant or irrelevant, while intramodal (spatial) attention was measured by comparing ERPs to visual and auditory stimuli presented at relevant and irrelevant spatial locations. Intramodal spatial attention was expressed differently in visual and auditory ERPs. When vision was relevant, spatial attention showed a contralateral enhancement of posterior N1 and P2 components and enhancement of parietal P3. When audition was relevant, spatial attention showed a biphasic fronto-central negativity, starting after around 100 ms. The same effects were also present in ERPs to stimuli that were presented in the irrelevant modality. Thus, spatial attention was not completely modality specific. Intermodal attention effects were also expressed differently in vision and audition. Taken together, the obtained ERP patterns of the present study show that stimulus attributes such as modality and location are processed differently in vision and audition.     

Right‐lateralized alpha desynchronization during regularity discrimination: Hemispheric specialization or directed spatial attention?

When actively classifying abstract patterns according to their regularity, alpha desynchronization (ERD) becomes right lateralized over posterior brain areas. This could reflect temporary enhancement of contralateral visual inputs and specifically a shift of attention to the left, or right hemisphere specialization for regularity discrimination. This study tested these competing hypotheses. Twenty‐four participants discriminated between dot patterns containing a reflection or a translation. The direction of the transformation, which matched one half onto the other half, was either vertical or horizontal. The strategy of shifting attention to one side of the patterns would not produce lateralized ERD in the horizontal condition. However, right‐lateralized ERD was found in all conditions, regardless of orientation. We conclude that right hemisphere networks that incorporate the early posterior regions are specialized for regularity discrimination.     

The Effect of Lateral Visual Fixation and the Direction of Eye Movements on Heartbeat Discrimination

This study was undertaken to determine whether the asymmetrical activation of the two cerebral hemispheres affects the accuracy of heartbeat perception. Hemispheric preference--the tendency to activate one hemisphere rather than the other--was assessed by the directionality of conjugate lateral eye movements. Actual differential hemispheric activation was achieved by contralateral visual fixation. The results of 44 right-handed male subjects showed that right hemisphere preferent subjects ("left-movers") performed better on a heartbeat discrimination task than left hemisphere preferent subjects ("right-movers"). The direction of lateral visual fixation also influenced heartbeat discrimination: subjects fixating to the left were more accurate than those fixating to the right.     

Spatial attention freezes during the attention blink

A variant of the rapid serial visual presentation paradigm was used to display sequentially two lateral sequences of stimuli, one to the left and one to the right of fixation, embedding two pairs of target stimuli, T1 and T2. T1 was composed of a pair of alphanumeric characters, and subjects had either to ignore T1 or to encode T1 for a delayed response. T2 was a lateral square of a prespecified color. The square had a small gap in one side, and the task for this stimulus was to report which side had the gap. When subjects were required to ignore T1, the T2-locked ERP produced a clear N2pc, that is, a greater negativity at electrode sites contralateral to the position occupied by T2. This N2pc was followed by a sustained posterior contralateral negativity (SPCN). When subjects were required to monitor T1 in addition to T2, both the N2pc and the SPCN components amplitude depended on the difficulty of the task associated with T1. If T1 was composed of digits that had to be encoded for a delayed same/different judgment, both the N2pc and the SPCN components were entirely suppressed. Although attenuated, such components were present when T1 was composed of a pair of symbols that subjects could disregard. The results suggest that a set of mechanisms subserving the allocation of attention in the spatial domain, resulting in the N2pc, suffer significant interference from concurrent cognitive operations required to encode information into visual short-term memory.     

Event-related potentials before saccades and antisaccades and their relation to reaction time

In the present study, reaction time (RT) was measured in 12 healthy subjects in a saccade and antisaccade task while recording electroencephalographic activity (EEG) from 62 electrodes on the scalp. Event-related potentials averaged both on target appearance and on saccade onset were larger in amplitude (increased negativity) for the antisaccade task compared to the saccade task. The relation of RT variability to EEG amplitude was studied by averaging stimulus-aligned and movement-aligned individual trials for each subject into four RT quartile groups. The analysis showed a relation of EEG amplitude to RT for both saccades and antisaccades. More specifically, the ERP negativity at 100–120 ms after stimulus onset in the saccade task and at 160–200 ms after stimulus onset in the antisaccade task for stimulus-aligned ERPs decreased monotonically with increasing RT as would be expected if this signal would be related to the eye movement preparation processes. This was much more pronounced and wide spread for the antisaccades than for visually triggered saccades. The peak negativity before movement onset for movement-aligned ERPs also covaried with RT suggesting no relation of this activity to movement preparation processes. This study then confirmed that only a particular ERP signal variation was related to the saccadic eye movement preparatory processes while other components of the ERP have no specific relation to the movement preparation. This particular signal was more prominent for antisaccades compared to visually triggered saccades supporting previous evidence for the cortical involvement in the preparation of these voluntary eye movements. In conclusion, this study validates the use of ERPs in the study of the planning and execution of saccadic eye movements.     

Hemispheric differences in auditory oddball responses during monaural versus binaural stimulation

Hemispheric lateralization of early event-related potentials (ERPs; e.g. N1) is largely based on anatomy of the afferent pathway; lateralization of later auditory ERPs (P2/N2, P250, P3b) is less clear. Using 257-channel EEG, the present study examined hemispheric laterality of auditory ERPs by comparing binaural and monaural versions of an auditory oddball task. N1 showed a contralateral bias over auditory cortex in both hemispheres as a function of ear of stimulation, although right hemisphere sources were activated regardless of which ear received input. Beginning around N1 and continuing through the time of P3b, right hemisphere temporal–parietal and frontal areas were more activated than their left hemisphere counterparts for stimulus evaluation/comparison and target detection. P250 and P3b component amplitudes, topographies, and source estimations were significantly influenced by ear of stimulation, with right hemisphere activity being stronger. This was particularly true for anterior temporal and inferior frontal sources which were more strongly associated with the later, more cognitive components (P250, P3b). Results are consistent with theories of a right hemisphere network that is prominently involved in sustained attention, stimulus evaluation, target detection, and working memory/context updating.