Lateralized human cortical activity for shifting visuospatial attention and initiating saccades

Refixation behavior in naturalistic viewing: Methods, mechanisms, and neural correlates

When freely viewing a scene, the eyes often return to previously visited locations. By tracking eye movements and coregistering eye movements and EEG, such refixations are shown to have multiple roles: repairing insufficient encoding from precursor fixations, supporting ongoing viewing by resampling relevant locations prioritized by precursor fixations, and aiding the construction of memory representations. All these functions of refixation behavior are understood to be underpinned by three oculomotor and cognitive systems and their associated brain structures. First, immediate saccade planning prior to refixations involves attentional selection of candidate locations to revisit. This process is likely supported by the dorsal attentional network. Second, visual working memory, involved in maintaining task-related information, is likely supported by the visual cortex. Third, higher-order relevance of scene locations, which depends on general knowledge and understanding of scene meaning, is likely supported by the hippocampal memory system. Working together, these structures bring about viewing behavior that balances exploring previously unvisited areas of a scene with exploiting visited areas through refixations.

Psychophysical dual-task setups do not measure pre-saccadic attention but saccade-related strengthening of sensory representations

Visual attention and saccadic eye movements are linked in a tight, yet flexible fashion. In humans, this link is typically studied with dual‐task setups. Participants are instructed to execute a saccade to some target location, while a discrimination target is flashed on a screen before the saccade can be made. Participants are also instructed to report a specific feature of this discrimination target at the trial end. Discrimination performance is usually better if the discrimination target occurred at the same location as the saccade target compared to when it occurred at a different location, which is explained by the mandatory shift of attention to the saccade target location before saccade onset. This pre‐saccadic shift of attention presumably enhances the perception of the discrimination target if it occurred at the same, but not if it occurred at a different location. It is, however, known that a dual‐task setup can alter the primary process under investigation. Here, we directly compared pre‐saccadic attention in single‐task versus dual‐task setups using concurrent electroencephalography (EEG) and eye‐tracking. Our results corroborate the idea of a pre‐saccadic shift of attention. They, however, question that this shift leads to the same‐position discrimination advantage. The relation of saccade and discrimination target position affected the EEG signal only after saccade onset. Our results, thus, favor an alternative explanation based on the role of saccades for the consolidation of sensory and short‐term memory. We conclude that studies with dual‐task setups arrived at a valid conclusion despite not measuring exactly what they intended to measure.

In humans, the relation between visual attention and saccadic eye movements is usually studied with psychophysical dual‐task setups. Here, we employ concurrent EEG and eye‐tracking to directly compare dual‐task to single‐task setups and conclude in line with previous research that attention precedes saccades. However, our results suggest that dual‐task setups do not measure what they are supposed to measure, that is, the pre‐saccadic shift of attention.

Neural correlates of task-related refixation behavior

Eye movement research has shown that attention shifts from the currently fixated location to the next before a saccade is executed. We investigated whether the cost of the attention shift depends on higher-order processing at the time of fixation, in particular on visual working memory load differences between fixations and refixations on task-relevant items. The attention shift is reflected in EEG activity in the saccade-related potential (SRP). In a free viewing task involving visual search and memorization of multiple targets amongst distractors, we compared the SRP in first fixations versus refixations on targets and distractors. The task-relevance of targets implies that more information will be loaded in memory (e.g. both identity and location) than for distractors (e.g. location only). First fixations will involve greater memory load than refixations, since first fixations involve loading of new items, while refixations involve rehearsal of previously visited items. The SRP in the interval preceding the saccade away from a target or distractor revealed that saccade preparation is affected by task-relevance and refixation behavior. For task-relevant items only, we found longer fixation duration and higher SRP amplitudes for first fixations than for refixations over the occipital region and the opposite effect over the frontal region. Our findings provide first neurophysiological evidence that working memory loading of task-relevant information at fixation affects saccade planning.

Eye Movements and Fixation-Related Potentials in Reading: A Review

The present review is addressed to researchers in the field of reading and psycholinguistics who are both familiar with and new to co-registration research of eye movements (EMs) and fixation related-potentials (FRPs) in reading. At the outset, we consider a conundrum relating to timing discrepancies between EM and event related potential (ERP) effects. We then consider the extent to which the co-registration approach might allow us to overcome this and thereby discriminate between formal theoretical and computational accounts of reading. We then describe three phases of co-registration research before evaluating the existing body of such research in reading. The current, ongoing phase of co-registration research is presented in comprehensive tables which provide a detailed summary of the existing findings. The thorough appraisal of the published studies allows us to engage with issues such as the reliability of FRP components as correlates of cognitive processing in reading and the advantages of analysing both data streams (i.e., EMs and FRPs) simultaneously relative to each alone, as well as the current, and limited, understanding of the relationship between EM and FRP measures. Finally, we consider future directions and in particular the potential of analytical methods involving deconvolution and the potential of measurement of brain oscillatory activity.

Dizziness in patients with cognitive impairment

Accumulating evidences show that the vestibular system contributes to cognitive function, including visuospatial ability, memory, and attention. Conversely, cognitive processes appear to affect the vestibular system. Based on the assumption that cognitive impairment correlates to increased perception of dizziness, we recruited 308 adults with cognitive decline from neurodegenerative disorders and administered neuropsychological tests and the Dizziness Handicap Inventory. Global cognitive measures did not correlate with increased dizziness, whereas attentional and visuospatial cognitive ability was correlated with scores of the Dizziness Handicap Inventory. Furthermore, patients with both cognitive impairment and postural instability experienced notably worse dizziness than those without postural instability, suggesting that postural instability is an important determinant of dizziness.

Neural correlates of goal-directed enhancement and suppression of visual stimuli in the absence of conscious perception

An observer's current goals can influence the processing of visual stimuli. Such influences can work to enhance goal-relevant stimuli and suppress goal-irrelevant stimuli. Here, we combined behavioral testing and electroencephalography (EEG) to examine whether such enhancement and suppression effects arise even when the stimuli are masked from awareness. We used a feature-based spatial cueing paradigm, in which participants searched four-item arrays for a target in a specific color. Immediately before the target array, a nonpredictive cue display was presented in which a cue matched or mismatched the searched-for target color, and appeared either at the target location (spatially valid) or another location (spatially invalid). Cue displays were masked using continuous flash suppression. The EEG data revealed that target-colored cues produced robust N2pc and N_T responses-both signatures of spatial orienting-and distractor-colored cues produced a robust P_D-a signature of suppression. Critically, the cueing effects occurred for both conscious and unconscious cues. The N2pc and N_T were larger in the aware versus unaware cue condition, but the P_D was roughly equivalent in magnitude across the two conditions. Our findings suggest that top-down control settings for task-relevant features elicit selective enhancement and suppression even in the absence of conscious perception. We conclude that conscious perception modulates selective enhancement of visual features, but suppression of those features is largely independent of awareness.

Saccadic suppression measured by steady-state visual evoked potentials

Visual sensitivity is severely impaired during the execution of saccadic eye movements. This phenomenon has been extensively characterized in human psychophysics and nonhuman primate single-neuron studies, but a physiological characterization in humans is less established. Here, we used a method based on steady-state visually evoked potential (SSVEP), an oscillatory brain response to periodic visual stimulation, to examine how saccades affect visual sensitivity. Observers made horizontal saccades back and forth, while horizontal black-and-white gratings flickered at 5-30 Hz in the background. We analyzed EEG epochs with a length of 0.3 s either centered at saccade onset (saccade epochs) or centered at fixations half a second before the saccade (fixation epochs). Compared with fixation epochs, saccade epochs showed a broadband power increase, which most likely resulted from saccade-related EEG activity. The execution of saccades, however, led to an average reduction of 57% in the SSVEP amplitude at the stimulation frequency. This result provides additional evidence for an active saccadic suppression in the early visual cortex in humans. Compared with previous functional MRI and EEG studies, an advantage of this approach lies in its capability to trace the temporal dynamics of neural activity throughout the time course of a saccade. In contrast to previous electrophysiological studies in nonhuman primates, we did not find any evidence for postsaccadic enhancement, even though simulation results show that our method would have been able to detect it. We conclude that SSVEP is a useful technique to investigate the neural correlates of visual perception during saccadic eye movements in humans. NEW & NOTEWORTHY We make fast ballistic saccadic eye movements a few times every second. At the time of saccades, visual sensitivity is severely impaired. The present study uses steady-state visually evoked potentials to reveal a neural correlate of the fine temporal dynamics of these modulations at the time of saccades in humans. We observed a strong reduction (57%) of visually driven neural activity associated with saccades but did not find any evidence for postsaccadic enhancement.

Refixation control in free viewing: a specialized mechanism divulged by eye-movement-related brain activity

In free viewing, the eyes return to previously visited locations rather frequently, even though the attentional and memory-related processes controlling eye-movement show a strong antirefixation bias. To overcome this bias, a special refixation triggering mechanism may have to be recruited. We probed the neural evidence for such a mechanism by combining eye tracking with EEG recording. A distinctive signal associated with refixation planning was observed in the EEG during the presaccadic interval: the presaccadic potential was reduced in amplitude before a refixation compared with normal fixations. The result offers direct evidence for a special refixation mechanism that operates in the saccade planning stage of eye movement control. Once the eyes have landed on the revisited location, acquisition of visual information proceeds indistinguishably from ordinary fixations. NEW & NOTEWORTHY A substantial proportion of eye fixations in human natural viewing behavior are revisits of recently visited locations, i.e., refixations. Our recently developed methods enabled us to study refixations in a free viewing visual search task, using combined eye movement and EEG recording. We identified in the EEG a distinctive refixation-related signal, signifying a control mechanism specific to refixations as opposed to ordinary eye fixations.

Presaccadic EEG activity predicts visual saliency in free-viewing contour integration

While viewing a scene, the eyes are attracted to salient stimuli. We set out to identify the brain signals controlling this process. In a contour integration task, in which participants searched for a collinear contour in a field of randomly oriented Gabor elements, a previously established model was applied to calculate a visual saliency value for each fixation location. We studied brain activity related to the modeled saliency values, using coregistered eye tracking and EEG. To disentangle EEG signals reflecting salience in free viewing from overlapping EEG responses to sequential eye movements, we adopted generalized additive mixed modeling (GAMM) to single epochs of saccade-related EEG. We found that, when saliency at the next fixation location was high, amplitude of the presaccadic EEG activity was low. Since presaccadic activity reflects covert attention to the saccade target, our results indicate that larger attentional effort is needed for selecting less salient saccade targets than more salient ones. This effect was prominent in contour-present conditions (half of the trials), but ambiguous in the contour-absent condition. Presaccadic EEG activity may thus be indicative of bottom-up factors in saccade guidance. The results underscore the utility of GAMM for EEG-eye movement coregistration research.

RLS patients show better nocturnal performance in the Simon task due to diminished visuo-motor priming

OBJECTIVE

The restless legs syndrome (RLS) is characterized by sensory-motor symptoms which usually occur predominantly at rest in the evening and at night. It is assumed that this circadian rhythm is caused by low dopamine levels in the evening. Yet, it has never been investigated whether RLS patients show diurnal variations in cognitive functions modulated by dopamine and what neurophysiological and functional neuroanatomical processes underlie such modulations.

METHODS

We used a Simon task combined with EEG and source localization to investigate whether top-down response selection and/or automatic visuo-motor priming are subject to diurnal changes in RLS patients, as compared to matched healthy controls.

RESULTS

We found that RLS patients showed better task performance due to reduced visuo-motor priming in the evening, as reflected by smaller early lateralized readiness potential (e-LRP) amplitudes and decreased activation of the superior parietal cortex and premotor cortex. Top-down response selection and early attentional processing were unaffected by RLS.

CONCLUSIONS

Counterintuitively, RLS patients show enhanced task performance in the evening, i.e. when experiencing dopaminergic deficiency. Yet, this may be explained by deficits in visuo-motor priming that lead to reduced false response tendencies.

SIGNIFICANCE

This study reveals a counterintuitive circadian variation of cognitive functions in RLS patients.

Concealed Around-the-Ear EEG Captures Cognitive Processing in a Visual Simon Task

In theory, miniaturized systems such as the around-the-ear electrode arrays (cEEGrids) enable mobile monitoring of the electroencephalogram (EEG) in a variety of real life situations without interfering with the natural setting. However, the research benefit of such cEEGrid recordings critically depends on their validity. To investigate whether visual and motor processing are reflected in the cEEGrid-EEG, a direct comparison of EEG that was concurrently recorded with the cEEGrids and with a high-density cap setup was conducted. Thirteen participants performed a classic Simon task in which letters were presented laterally and a lateralized choice response was executed. N1, P1 and P300 event-related potential (ERP) waveforms were extracted from cEEGrid-EEG: they were found to be strongly correlated with corresponding waveforms extracted from cap-EEG but with lower signal strength and lower signal-to-noise-ratio (SNR). Event-related lateralizations (ERLs) recorded at posterior scalp sites were well reflected in middle cEEGrid pairs. Moreover, the effect size of the Simon correspondence effect on the extracted ERLs was similar between the two systems. However, lateralizations at central cap sites were less well reflected in the cEEGrid-EEG indicating a difficulty in capturing motor response preparation and execution. These results show that well-described visual and cognitive ERPs and ERLs can be measured using the cEEGrids, while motor-related cortical potentials are not well captured. This study further demonstrates the potential and possible limitations of unobtrusive cEEGrid-EEG recordings.

Early visual processing allows for selective behavior, shifts of attention, and conscious visual experience in spite of masking

Object-substitution masking (OSM) occurs when a briefly displayed target in a search array is surrounded by a mask, which remains onscreen after the target has disappeared. It has been suggested that OSM results from a specific interference with reentrant visual processing, while the initial feedforward processing is left intact. Here, we tested the prediction that the fastest saccadic responses towards a masked target, supposedly triggered before the onset of reentrant processing, are not impaired by OSM. Indeed, saccades faster than 350ms "escaped" the influence of the mask. Notably, participants' judgements of subjective awareness indicated that stimulus processing during this early stage is not entirely devoid of conscious awareness. Furthermore, the N2pc event-related potential component indicated shifts of spatial attention towards the masked targets on trials with correct fast saccades, suggesting that both target detection and spatial attention can be based on the computations accomplished during the initial feedforward sweep.

In vivo functional neurochemistry of human cortical cholinergic function during visuospatial attention

Cortical acetylcholine is involved in key cognitive processes such as visuospatial attention. Dysfunction in the cholinergic system has been described in a number of neuropsychiatric disorders. Levels of brain acetylcholine can be pharmacologically manipulated, but it is not possible to directly measure it in vivo in humans. However, key parts of its biochemical cascade in neural tissue, such as choline, can be measured using magnetic resonance spectroscopy (MRS). There is evidence that levels of choline may be an indirect but proportional measure of acetylcholine availability in brain tissue. In this study, we measured relative choline levels in the parietal cortex using functional (event-related) MRS (fMRS) during performance of a visuospatial attention task, with a modelling approach verified using simulated data. We describe a task-driven interaction effect on choline concentration, specifically driven by contralateral attention shifts. Our results suggest that choline MRS has the potential to serve as a proxy of brain acetylcholine function in humans.

Disrupted Saccade Control in Chronic Cerebral Injury: Upper Motor Neuron-Like Disinhibition in the Ocular Motor System

Saccades rapidly direct the line of sight to targets of interest to make use of the high acuity foveal region of the retina. These fast eye movements are instrumental for scanning visual scenes, foveating targets, and, ultimately, serve to guide manual motor control, including eye-hand coordination. Cerebral injury has long been known to impair ocular motor control. Recently, it has been suggested that alterations in control may be useful as a marker for recovery. We measured eye movement control in a saccade task in subjects with chronic middle cerebral artery stroke with both cortical and substantial basal ganglia involvement and in healthy controls. Saccade latency distributions were bimodal, with an early peak at 60 ms (anticipatory saccades) and a later peak at 250 ms (regular saccades). Although the latencies corresponding to these peaks were the same in the two groups, there were clear differences in the size of the peaks. Classifying saccade latencies relative to the saccade "go signal" into anticipatory (latencies up to 80 ms), "early" (latencies between 80 and 160 ms), and "regular" types (latencies longer than 160 ms), stroke subjects displayed a disproportionate number of anticipatory saccades, whereas control subjects produced the majority of their saccades in the regular range. We suggest that this increase in the number of anticipatory saccade events may result from a disinhibition phenomenon that manifests as an impairment in the endogenous control of ocular motor events (saccades) and interleaved fixations. These preliminary findings may help shed light on the ocular motor deficits of neurodegenerative conditions, results that may be subclinical to an examiner, but clinically significant secondary to their functional implications.

Dissociations Among Attention, Perception, and Awareness During Object-Substitution Masking

When a visual target object is surrounded by four dots that onset at the same time as the target but remain visible after the target terminates, the four dots dramatically impair target discrimination performance. This phenomenon is called object-substitution masking, reflecting the hypothesis that both the target and the four dots are identified, but the representation of the four dots replaces the representation of the target object before the target can be reported. The present study used the event-related potential technique to demonstrate that a target masked in this manner is identified by the visual system and triggers a shift of attention. However, by the time attention is shifted to the target, only the mask remains visible, leading to impaired behavioral detection performance. These findings support the object-substitution hypothesis and provide new evidence that perception, attention, and awareness can be dissociated.

Bias for the Left Visual Field in Rapid Serial Visual Presentation: Effects of Additional Salient Cues Suggest a Critical Role of Attention

Everyday experience suggests that people are equally aware of stimuli in both hemifields. However, when two streams of stimuli are rapidly presented left and right, the second target (T2) is better identified in the left hemifield than in the right hemifield. This left visual field (LVF) advantage may result from differences between hemifields in attracting attention. Therefore, we introduced a visual cue shortly before T2 onset to draw attention to one stream. Thus, to identify T2, attention was correctly positioned with valid cues but had to be redirected to the other stream with invalid ones. If the LVF advantage is caused by differences between hemifields in attracting attention, invalid cues should increase, and valid cues should reduce the LVF advantage as compared with neutral cues. This prediction was confirmed. ERP analysis revealed that cues evoked an early posterior negativity, confirming that attention was attracted by the cue. This negativity was earlier with cues in the LVF, which suggests that responses to salient events are faster in the right hemisphere than in the left hemisphere. Valid cues speeded up, and invalid cues delayed T2-evoked N2pc; in addition, valid cues enlarged T2-evoked P3. After N2pc, right-side T2 evoked more sustained contralateral negativity than left T2, least long-lasting after valid cues. Difficulties in identifying invalidly cued right T2 were reflected in prematurely ending P3 waveforms. Overall, these data provide evidence that the LVF advantage is because of different abilities of the hemispheres in shifting attention to relevant events in their contralateral hemifield.

Visual encoding and fixation target selection in free viewing: presaccadic brain potentials

In scrutinizing a scene, the eyes alternate between fixations and saccades. During a fixation, two component processes can be distinguished: visual encoding and selection of the next fixation target. We aimed to distinguish the neural correlates of these processes in the electrical brain activity prior to a saccade onset. Participants viewed color photographs of natural scenes, in preparation for a change detection task. Then, for each participant and each scene we computed an image heat map, with temperature representing the duration and density of fixations. The temperature difference between the start and end points of saccades was taken as a measure of the expected task-relevance of the information concentrated in specific regions of a scene. Visual encoding was evaluated according to whether subsequent change was correctly detected. Saccades with larger temperature difference were more likely to be followed by correct detection than ones with smaller temperature differences. The amplitude of presaccadic activity over anterior brain areas was larger for correct detection than for detection failure. This difference was observed for short "scrutinizing" but not for long "explorative" saccades, suggesting that presaccadic activity reflects top-down saccade guidance. Thus, successful encoding requires local scanning of scene regions which are expected to be task-relevant. Next, we evaluated fixation target selection. Saccades "moving up" in temperature were preceded by presaccadic activity of higher amplitude than those "moving down". This finding suggests that presaccadic activity reflects attention deployed to the following fixation location. Our findings illustrate how presaccadic activity can elucidate concurrent brain processes related to the immediate goal of planning the next saccade and the larger-scale goal of constructing a robust representation of the visual scene.

Perceptual grouping and visual enumeration

We used lateralized Event-Related Potential (ERP) measures - the N2pc and CDA/SPCN components - to assess the role of grouping by target similarity during enumeration. Participants saw a variable number (0, 1, 2 or 3) of same- or differently-colored targets presented among homogeneous distracters, and performed an enumeration task. Results showed that the N2pc, but not the CDA, was larger for multiple targets of identical color relative to targets of different colors. The findings are interpreted in terms of the effects of grouping on early versus late stages of multiple object processing. Within this framework, they reveal that grouping has an effect on early individuation mechanisms, while later processing mechanisms are less prone to such an influence.

Time-course of hemispheric preference for processing contralateral relevant shapes: P1pc, N1pc, N2pc, N3pc

A most sensitive and specific electrophysiological indicator of selective processing of visual stimuli is the N2pc component. N2pc is a negative EEG potential peaking 250 ms after stimulus onset, recorded from posterior sites contralateral to relevant stimuli. Additional deflections preceding or following N2pc have been obtained in previous studies, possibly produced by specific stimulus features or specific prime-target sequences. To clarify the entire time-course of the contralateral- ipsilateral (C-I) difference recorded from the scalp above visual cortex in response to left-right pairs of targets and distracters, C-I differences were here compared between two types of stimuli and between stimuli that were or were not preceded by masked neutral primes. The C-I difference waveform consisted of several peaks, termed here P1pc (60-100 ms after target onset), N1pc (120-160 ms), N2pc (220-280 ms), and N3pc (360-400 ms). Being markedly enhanced when stimuli were preceded by the neutral primes, P1pc may indicate a response to stimulus change. Also, when stimuli were primed, N2pc reached its peak earlier, thereby tending to merge with N1pc. N3pc seemed to increase when target discrimination was difficult. N1pc, N2pc, and N3pc appear as three periods of one process. N3pc probably corresponds to L400 or SPCN as described in other studies. These observations suggest that the neurophysiological basis of stimulus-driven focusing of attention on target stimuli is a process that lasts for hundreds of milliseconds, with the relevant hemisphere being activated in an oscillating manner as long as required by the task.

Electrophysiological recordings in humans reveal reduced location-specific attentional-shift activity prior to recentering saccades

Being able to effectively explore the visual world is of fundamental importance, and it has been suggested that the straight-ahead gaze position within the egocentric reference frame ("primary position") might play a special role in this context. In the present study we employed human electroencephalography (EEG) to examine neural activity related to the spatial guidance of saccadic eye movements. Moreover, we sought to investigate whether such activity would be modulated by the spatial relation of saccade direction to the primary gaze position (recentering saccades). Participants executed endogenously cued saccades between five equidistant locations along the horizontal meridian. This design allowed for the comparison of isoamplitude saccades from the same starting position that were oriented either toward the primary position (centripetal) or further away from it (centrifugal). By back-averaging time-locked to the saccade onset on each trial, we identified a parietally distributed, negative-polarity EEG deflection contralateral to the direction of the upcoming saccade. Importantly, this contralateral presaccadic negativity, which appeared to reflect the location-specific attentional guidance of the eye movement, was attenuated for recentering saccades relative to isoamplitude centrifugal saccades. This differential electrophysiological signature was paralleled by faster saccadic reaction times and was substantially more apparent when time-locking the data to the onset of the saccade rather than to the onset of the cue, suggesting a tight temporal association with saccade initiation. The diminished level of this presaccadic component for recentering saccades may reflect the preferential coding of the straight-ahead gaze position, in which both the eye-centered and head-centered reference frames are perfectly aligned and from which the visual world can be effectively explored.

Cooperative and opposing effects of strategic and involuntary attention

To assess whether working memory contents can effectively bias visual selection even when they do not represent the current target in the attention task, we recorded the ERP activity from participants performing both a memory task and, in the retention period, a visual search task. In this task, a distracter matching the memory content could be presented on the same side (congruent trials) or on the opposite side (incongruent trials) relative to the target location (Experiment 1 and Experiment 2). On some trials, only the matching distracter (but no target) was presented (catch trials, Experiment 2). Results showed that the N2pc component was modulated by the presence and location of a matching distracter. We interpret these results as evidence that the involuntary control exerted by the irrelevant memory contents coexists with the strategic mechanism related to the search target, influencing attention selection with roughly equal power. In Experiment 3, we found that the modulation of the N2pc is not strictly related to the active maintenance of the memory-target features but can also be elicited by repetition priming. Overall, these findings suggest that, together with the physical properties of the stimuli presented in the visual field, irrelevant memory contents represent a powerful class of factors that lead to involuntary attentional control.

Manual response preparation disrupts spatial attention: an electrophysiological investigation of links between action and attention

Previous behavioural and neuroscience studies have shown that the systems involved in the control of attention and action are functionally and anatomically linked. We used behavioural and event-related brain potential measures to investigate whether such links are mandatory or merely optional. Cues presented at the start of each trial instructed participants to shift attention to the left or right side and to simultaneously prepare to a finger movement with their left or right hand. In different trials, cues were followed by a central Go signal, requiring execution of the prepared manual response (motor task), or by a peripheral visual stimulus, which required a target-non-target discrimination only when presented on the cued side (attention task). Lateralised ERP components indicative of covert attention shifts were found when attention and action were directed to the same side (same side condition), but not when attention and action were directed to opposite sides (opposite sides condition). Likewise, effects of spatial attention on the processing of peripheral visual stimuli were present only when attention and action were directed to the same side, but not in the opposite sides condition. These results demonstrate that preparing a manual response on one side severely disrupts the attentional selection of visual stimuli on the other side, and suggest that it is not possible to simultaneously direct attention and action to different locations in space. They support the hypothesis that the control of spatial attention and action are implemented by shared brain circuits, and are therefore linked in a mandatory fashion.

On Why Left Events are the Right Ones: Neural Mechanisms Underlying the Left-hemifield Advantage in Rapid Serial Visual Presentation

When simultaneous series of stimuli are rapidly presented left and right, containing two target stimuli T1 and T2, T2 is much better identified when presented in the left than in the right hemifield. Here, this effect was replicated, even when shifts of gaze were controlled, and was only partially compensated when T1 side provided the cue where to expect T2. Electrophysiological measurement revealed earlier latencies of T1- and T2-evoked N2pc peaks at the right than at the left visual cortex, and larger right-hemisphere T2-evoked N2pc amplitudes when T2 closely followed T1. These findings suggest that the right hemisphere was better able to single out the targets in time. Further, sustained contralateral slow shifts remained active after T1 for longer time at the right than at the left visual cortex, and developed more consistently at the right visual cortex when expecting T2 on the contralateral side. These findings might reflect better capacity of right-hemisphere visual working memory. These findings about the neurophysiological underpinnings of the large right-hemisphere advantage in this complex visual task might help elucidating the mechanisms responsible for the severe disturbance of hemineglect following damage to the right hemisphere.

Links between eye movement preparation and the attentional processing of tactile events: an event-related brain potential study

OBJECTIVE

We investigated whether the covert preparation of saccadic eye movements results in spatially specific modulations of somatosensory processing.

METHODS

ERPs were recorded in a spatial cueing experiment where auditory cues preceded tactile stimuli delivered to the left or right hand. In the Saccade task, cues signalled that an eye movement towards the left or right hand had to be prepared. In the Covert Attention task, cues signalled the direction of a covert shift of tactile attention.

RESULTS

A lateralized component previously observed during cued shifts of spatial attention (ADAN) was elicited in the cue-target interval in both tasks. The somatosensory N140 component was enhanced for tactile stimuli presented to the hand on the cued side. This modulation was present not just in the Covert Attention task, but also in the Saccade task. Longer-latency effects of spatial cueing were only present in the Covert Attention task.

CONCLUSIONS

Covert shifts of attention and saccade preparation have similar effects on early stages of tactile processing, suggesting that both are mediated by overlapping control processes.

SIGNIFICANCE

These findings support the premotor theory of attention by demonstrating that the programming of eye movements has spatially selective effects on somatosensory processing.

Eye movement preparation causes spatially-specific modulation of auditory processing: new evidence from event-related brain potentials

To investigate whether saccade preparation can modulate processing of auditory stimuli in a spatially-specific fashion, ERPs were recorded for a Saccade task, in which the direction of a prepared saccade was cued, prior to an imperative auditory stimulus indicating whether to execute or withhold that saccade. For comparison, we also ran a conventional Covert Attention task, where the same cue now indicated the direction for a covert endogenous attentional shift prior to an auditory target-nontarget discrimination. Lateralised components previously observed during cued shifts of attention (ADAN, LDAP) did not differ significantly across tasks, indicating commonalities between auditory spatial attention and oculomotor control. Moreover, in both tasks, spatially-specific modulation of auditory processing was subsequently found, with enhanced negativity for lateral auditory nontarget stimuli at cued versus uncued locations. This modulation started earlier and was more pronounced for the Covert Attention task, but was also reliably present in the Saccade task, demonstrating that the effects of covert saccade preparation on auditory processing can be similar to effects of endogenous covert attentional orienting, albeit smaller. These findings provide new evidence for similarities but also some differences between oculomotor preparation and shifts of endogenous spatial attention. They also show that saccade preparation can affect not just vision, but also sensory processing of auditory events.

Disentangling neural processing of masked and masking stimulus by means of event-related contralateral - ipsilateral differences of EEG potentials

In spite of the excellent temporal resolution of event-related EEG potentials (ERPs), the overlapping potentials evoked by masked and masking stimuli are hard to disentangle. However, when both masked and masking stimuli consist of pairs of relevant and irrelevant stimuli, one left and one right from fixation, with the side of the relevant element varying between pairs, effects of masked and masking stimuli can be distinguished by means of the contralateral preponderance of the potentials evoked by the relevant elements, because the relevant elements may independently change sides in masked and masking stimuli. Based on a reanalysis of data from which only selected contralateral-ipsilateral effects had been previously published, the present contribution will provide a more complete picture of the ERP effects in a masked-priming task. Indeed, effects evoked by masked primes and masking targets heavily overlapped in conventional ERPs and could be disentangled to a certain degree by contralateral-ipsilateral differences. Their major component, the N2pc, is interpreted as indicating preferential processing of stimuli matching the target template, which process can neither be identified with conscious perception nor with shifts of spatial attention. The measurements showed that the triggering of response preparation by the masked stimuli did not depend on their discriminability, and their priming effects on the processing of the following target stimuli were qualitatively different for stimulus identification and for response preparation. These results provide another piece of evidence for the independence of motor-related and perception-related effects of masked stimuli.

When does the brain inform the eyes whether and where to move? An EEG study in humans

The current study addressed when in the course of stimulus processing, and in what brain areas, activity occurs that supports the interpretation of cues that signal the appropriateness of different and competing behaviors. Twelve subjects completed interleaved no-go-, pro-, and antitrials, whereas 64-channel electroencephalography was recorded. Principle component and distributed source analyses were used to evaluate the spatial distribution and time course of cortical activity supporting cue evaluation and response selection. By 158 ms poststimulus, visual cortex activity was lower for no-go trials than it was for both pro- and antitrials, consistent with an early sensory filter on the no-go cue. Prefrontal cortex (PFC) activity at 158 ms was highest during antitrials, consistent with this brain region's putative involvement in executive control. At 204 ms poststimulus, however, PFC activity was the same for pro- and antitrials, consistent with an ostensible role in response selection. PFC activity at 204 ms also was robustly inversely correlated (r = -0.75) with visual cortex activity on antitrials, perhaps indicating top-down modulation of early sensory processing that would decrease the probability of an error response. These data highlight how a distributed neural architecture supports the evaluation of stimuli and response choices.

ERP correlates of shared control mechanisms involved in saccade preparation and in covert attention

We investigated whether attention shifts and eye movement preparation are mediated by shared control mechanisms, as claimed by the premotor theory of attention. ERPs were recorded in three tasks where directional cues presented at the beginning of each trial instructed participants to direct their attention to the cued side without eye movements (Covert task), to prepare an eye movement in the cued direction without attention shifts (Saccade task) or both (Combined task). A peripheral visual Go/Nogo stimulus that was presented 800 ms after cue onset signalled whether responses had to be executed or withheld. Lateralised ERP components triggered during the cue-target interval, which are assumed to reflect preparatory control mechanisms that mediate attentional orienting, were very similar across tasks. They were also present in the Saccade task, which was designed to discourage any concomitant covert attention shifts. These results support the hypothesis that saccade preparation and attentional orienting are implemented by common control structures. There were however systematic differences in the impact of eye movement programming and covert attention on ERPs triggered in response to visual stimuli at cued versus uncued locations. It is concluded that, although the preparatory processes underlying saccade programming and covert attentional orienting may be based on common mechanisms, they nevertheless differ in their spatially specific effects on visual information processing.

Effects of repeated amphetamine administration on antisaccade in schizophrenia spectrum personality

Repeated amphetamine administration is used to examine the responsivity of cerebral dopaminergic systems. Schizophrenia spectrum personality (SSP) provides a unique opportunity to study the pathophysiology of schizophrenia because of shared neurobiology without the confounding factors of acute psychosis and psychotropic exposure. Previously we noted that on repeated amphetamine administration, dyskinesia and SSP symptoms were less likely to worsen in SSP than in healthy volunteers. In the current study, we report the effects of repeated amphetamine on antisaccade task performance. Eleven SSP and seven healthy subjects were given placebo once and amphetamine (30 mg) twice, in randomized double-blind fashion at least 1 week apart. Antisaccade eye measurements (error rate and latency) were recorded over 30 trials in each direction. Analysis of error rate showed no significant main effects of the drug. There was a significant group by field by drug interaction effect on the antisaccade latency. The SSP group showed a significant reduction in antisaccade latency for right field targets whereas no significant effects were noted in healthy control subjects. Findings from this preliminary study suggest SSP may be more receptive to the beneficial effects of repeated amphetamine on cognition than healthy controls.

Spatiotemporal overlap between brain activation related to saccade preparation and attentional orienting

Recent brain imaging studies provided evidence that the brain areas involved with attentional orienting and the preparation of saccades largely overlap, which may indicate that focusing attention at a specific location can be considered as an unexecuted saccade towards that location (i.e. the premotor theory of attention). Alternatively, it may be proposed that attentional orienting is simply relevant for preparing saccades, but the two processes may also be completely unrelated. In two experiments, we examined temporal activation of brain areas by measuring the electroencephalogram. Central cues indicated the likely side (left or right) at which a to-be-attended target would occur, or to which a saccade had to be prepared. Cue direction-related activity was determined, time-locked to cue onset. In addition, in our second experiment, delayed saccades had to be carried out, which allows to focus on processes strongly related to saccade execution. In nearly all tasks, an early directing attention negativity (EDAN), an anterior directing attention negativity (ADAN), and a late directing attention positivity (LDAP) were observed, time-locked to cue onset. Source analyses supported the view that this activity probably originates from areas within the ventral intraparietal sulcus (vIPS) and the frontal eye fields (FEF). The saccade-locked analysis also indicated that the FEF plays an important role in triggering saccades, but the role of vIPS appears to be minimal. The latter finding disfavors the premotor theory of attention, as it suggests that the relation between attention and action is less direct.

Cortico-cortical networks and cortico-subcortical loops for the higher control of eye movements

There are multiple distinct regions, or eye fields, in the cerebral cortex that contribute directly to the initiation and control of voluntary eye movements. We concentrate on six of these: the frontal eye field, parietal eye field, supplementary eye field, middle superior temporal area, prefrontal eye field, and area 7 m (precuneus in humans). In each of these regions: (1) there is neural activity closely related to eye movements; (2) electrical microstimulation produces or modifies eye movements; (3) surgical lesions or chemical inactivation impairs eye movements; (4) there are direct neural projections to major structures in the brainstem oculomotor system; and (5) increased activity is observed during eye movement tasks in functional magnetic resonance imaging or positron emission tomography experiments in humans. Each of these eye fields is reciprocally connected with the other eye fields, and each receives visual information directly from visual association cortex. Each eye field has distinct subregions that are concerned with either saccadic or pursuit eye movements. The saccadic subregions are preferentially interconnected with other saccade subregions and the pursuit subregions are preferentially interconnected with other pursuit subregions. Current evidence strongly supports the proposal that there are parallel cortico-cortical networks that control purposeful saccadic and pursuit eye movements, and that the activity in those networks is modulated by feedback information, via the thalamus, from the superior colliculus, basal ganglia, and cerebellum.

Electroencephalography/magnetoencephalography study of cortical activities preceding prosaccades and antisaccades

The temporal and spatial characteristics of brain activity preceding prosaccades and antisaccades were investigated using source reconstructions of 64-channel electroencephalography and 148-channel magnetoencephalography data. Stimulus-locked data showed early cuneus activity was stronger during antisaccades, and later occipital gyrus activity was stronger preceding prosaccades, which suggests a top-down influence on early visual processing. Response-locked data showed that supplementary eye field, prefrontal cortex, and medial frontal eye field activity was greater for antisaccades than for prosaccades prior to saccade generation. Lateral frontal eye field activity appeared to be inhibited prior to antisaccade response generation. The spatial and temporal resolution of combined electroencephalography/magnetoencephalography data allows the evaluation of specific cortical activities preceding saccades and for demonstration of how activities differ as a function of response contingencies.

Oculomotor inhibition in children with and without attention-deficit hyperactivity disorder (ADHD)

The aim of the present study was to distinguish between a general deficit in oculomotor control and a deficit restricted to inhibitory functions in children with attention deficit hyperactivity disorder (ADHD). In addition, we were interested in differentiating between a general inhibition deficit and deficient subfunctions of inhibition. We used a prosaccade task to measure general oculomotor abilities in 22 children with ADHD and in age- and gender-matched healthy controls. A fixation, an antisaccade and a countermanding saccade task were used to measure specific aspects of oculomotor inhibition. Two major results were obtained: First, our prosaccade task suggests similar saccadic response preparation and saccadic accuracy in the ADHD compared to the control children. Secondly, the fixation and the countermanding saccade task indicate deficits on measures of oculomotor inhibition in the ADHD group. While patients were specifically impaired in stopping an already initiated response or in suppressing exploratory saccades in a novel situation, inhibition of a prepotent response was not deficient. Our data thus indicate an underlying impairment in cognitive inhibition in ADHD that has been associated with prefrontal lobe functions. More specifically, as the anterior cingulate gyrus has been associated with the countermanding saccade task and group differences were most pronounced in this paradigm our data are in line with imaging data stressing the importance of this cortical structure in the pathophysiology of ADHD.

The Posterior Contralateral Negativity as a Temporal Indicator of Visuo-Spatial Processing

Abstract. The temporal segmentation of the processing stream between stimulus and response is a declared aim of many EEG-studies on cognitive processing. This goal, however, can only be reached when measures of high temporal sensitivity are available that are unequivocally assigned to a particular cognitive process. The present study evaluates the peak latency of the posterior contralateral negativity (PCN) as a temporal measure of visual spatial processing. In three experiments, we show systematic variability of the PCN latency with task demands. In visual search, PCN latency varied in the same direction as response times with the number of distractors. These effects, however, were smaller than the effects observed on response times, indicating that the process underlying the PCN might contribute to but did not determine response time effects. In contrast, in attentional cueing tasks and a stimulus localization task, where stimulus detection was the primarily addressed process, PCN latency varied to the same amount as manual response times. Thus, it is suggested that the latency of the PCN provides a reliable and valid temporal measure of target localization.

Comparison of early cortical networks in efficient and inefficient visual search: an event-related potential study

Functional magnetic resonance imaging studies have indicated that efficient feature search (FS) and inefficient conjunction search (CS) activate partially distinct frontoparietal cortical networks. However, it remains a matter of debate whether the differences in these networks reflect differences in the early processing during FS and CS. In addition, the relationship between the differences in the networks and spatial shifts of attention also remains unknown. We examined these issues by applying a spatio-temporal analysis method to high-resolution visual event-related potentials (ERPs) and investigated how spatio-temporal activation patterns differ for FS and CS tasks. Within the first 450 msec after stimulus onset, scalp potential distributions (ERP maps) revealed 7 different electric field configurations for each search task. Configuration changes occurred simultaneously in the two tasks, suggesting that contributing processes were not significantly delayed in one task compared to the other. Despite this high spatial and temporal correlation, two ERP maps (120-190 and 250-300 msec) differed between the FS and CS. Lateralized distributions were observed only in the ERP map at 250-300 msec for the FS. This distribution corresponds to that previously described as the N2pc component (a negativity in the time range of the N2 complex over posterior electrodes of the hemisphere contralateral to the target hemifield), which has been associated with the focusing of attention onto potential target items in the search display. Thus, our results indicate that the cortical networks involved in feature and conjunction searching partially differ as early as 120 msec after stimulus onset and that the differences between the networks employed during the early stages of FS and CS are not necessarily caused by spatial attention shifts.

How the Self Controls Its “Automatic Pilot” when Processing Subliminal Information

Human performance may be primed by information not consciously available. Can such priming become so overwhelming that observers cannot help but act accordingly? In the present study, well-visible stimuli were preceded by whole series of unidentifiable stimuli. These series had strong, additive priming effects on behavior. However, their effect depended on the frequency with which they provided information conflicting to the visible main stimuli. Thus, effects of subliminal priming are under observers' strategic control, with the criterion presumably set as a function of the openly observable error frequency. Electrical brain potentials show that this criterion acts simultaneously at the level of visual discrimination of the primes and at motor activation evoked by the primes, thereby shielding observers from unwanted information.

Visual search strategies are indexed by event-related lateralizations of the EEG

Two processes have been proposed for picking out information from a visual scene. A parallel process that detects salient features and a following serial process for higher order vision. However, this separation is still under dispute. The current study investigated whether event-related lateralizations (ERLs) of the electroencephalogram are a useful tool to examine these two processes. In a visual search for a colour- or form singleton or a conjunction target, reaction time (RT), P3 amplitudes and ERLs served as dependent variables. RT replicated earlier results for colour and conjunction targets. P3 amplitudes decreased and ERL latencies increased for these conditions. However, form singletons showed RT-, P3- and ERL-results comparable to conjunction targets. ERL-results differed in some conditions from RTs. The results suggest that target attributes alone cannot dissociate between different search strategies but showed that efficient as well as less efficient processes can be utilised for the same targets proposing that processing demands are determined by the inter-relation of target and the distractors.

EEG cortical potentials preceding vergence and combined saccade-vergence eye movements

Previous event-related potential (ERP) studies of eye movements were restricted to saccades, the rapid conjugate movements used to change direction of fixation. Our study explored ERP activity for vergence eye movements used for the adjustment of the angle of the visual axes to the distance of the object. These movements are essential when exploring the natural 3D world. Subjects made the following visually guided movements to LED targets: pure convergence or divergence along the median plane, pure lateral saccades within the same distance, and combined movements (lateral and divergent or convergent). ERPs were recorded from 14 surface electrodes and were analyzed for 200 ms prior to the onset of the eye movement. The highest activation was observed prior to vergence. Activation was diffused and peaked at 50 ms prior to the onset of vergence. Combined vergence and sacccadic movements revealed a composite pattern of ERP activity from both saccades and pure vergence. These findings suggest a cortical substrate for vergence eye movements in humans that is engaged in the natural exploration of visual space.

Traces left on visual selective attention by stimuli that are not consciously identified

Briefly presented information, even if unidentifiable, may speed or delay required responses to following events. It has been assumed that this priming of the motor system may occur without affecting attention to priming and following stimuli. In contrast to this notion, the present study reports that such unidentified stimuli have effects on a physiological indicator of the attentional system. A lateral posterior electroencephalogram component was evoked by laterally presented relevant shapes, reflecting shifts of attention to those shapes. This component was absent, however, when the relevant shape was preceded by a similar shape at the same location, even if this shape was completely masked by metacontrast. The attentional shift evidently became unnecessary in this situation. Thus, unidentifiable information may leave some trace for attention-controlled selection of the following event.

Influence of Time Pressure in a Simple Response Task, a Choice-by-Location Task, and the Simon Task

Abstract The influence of strategy was examined for a simple response task, a choice-by-location task, and the Simon task by varying time pressure. Besides reaction time (RT) and accuracy, we measured response force and derived two measures from the event-related EEG potential to form an index for attentional orienting (posterior contralateral negativity: PCN) and the start of motor activation (the lateralized readiness potential: LRP). For the choice-by-location task and the Simon task, effects of time pressure were found on the response-locked LRP, but not on the onset of the PCN and the stimulus-locked LRP. Thus, strategy influences processing after the start of motor activation in choice tasks. A small effect of time pressure was found on the peak latency of the PCN in the Simon task, which suggests that time pressure may affect attentional orienting. In the simple response task, time pressure reduced the amplitude of the PCN. This finding suggests that strategy affects attentional orienting to stimuli when these stimuli are not highly relevant. Finally, the effect of time pressure on RT was much larger in the simple response task than in the other tasks, which may be ascribed to the possibility of preparing the required response in the simple response task.

Posterior and Anterior Contribution of Hand-Movement Preparation to Late CNV

Abstract The late part of the Contingent Negative Variation (CNV) is assumed to be a composite potential, reflecting both movement preparation and several other processes. To assess the contribution of hand-motor preparation to overall CNV, three S1-S2 experiments were performed. Replicating earlier results that have been interpreted as demonstrating hand-motor preparation, experiment 1 showed that CNV gets larger centro-parietally under speed instruction. Experiments 2 and 3 compared preparation for hand responses (key-press) to preparation for ocular responses (saccades) varying the effector system either between blocks (exp. 2) or between trials (exp. 3) and also comparing these preparation situations to no preparation (exp. 3). Hand-motor preparation was reflected in CNV getting larger fronto-centrally, with this topography being significantly different from the effect in experiment 1. Thus, two different kinds of motor preparation appear to be reflected by CNV. One kind may consist of assembling and maintaining the stimulus-response links appropriate to the expected S2 patterns, the other is for activating the hand-motor area. These two motor contributions to CNV might reflect the two aspects of the parieto-frontal motor system.