Shifts Of Visuospatial Attention Research Articles

On the interaction between implicit statistical learning and the alternation advantage: Evidence from manual and oculomotor serial reaction time tasks.

In this study, we examine how implicit statistical learning (ISL) interacts with the cognitive bias of the alternation advantage in serial reaction time (SRT) tasks. Our aim was to disentangle perceptual from motor aspects of learning, as well as to shed light on the cognitive sources of this alternation effect. We developed a manual (Study 1) and an oculomotor (Study 2) two-choice SRT task, with visual stimuli following the regularities of two binary artificial grammars (Fibonacci and its modification Skip). While these grammars share some deterministic transitional regularities, they differ in their probabilistic transitional regularities and distributional properties. The pattern of manual RTs in Study 1 provide evidence for ISL, showing that subjects learned the deterministic and probabilistic transitions in the two grammars. We also found a bias toward alternation (vs. repetition) in correspondence to non-deterministic points, regardless of their statistical properties in the grammars. Study 2 provides further evidence for both ISL and the alternation advantage, in terms of shorter manual RTs and higher accuracy rates of anticipatory eye movements. Saccadic responses preceding stimulus onset allow us to argue for the perceptual nature of ISL: participants detected regularities in the string by forming S-S associations based on the sequence of the perceived stimuli. Moreover, we propose that shifts in visuospatial attention preceding oculomotor programming play a role in the occurrence of the alternation advantage, and that such an effect is driven by the spatial location of the stimulus. These findings are also discussed with respect to the presence of two (possibly interacting) parsing strategies: statistical generalizations on the string vs. local hierarchical reconstruction.

Fearful faces straight ahead or in the periphery: Early neuronal responses independently of trait anxiety.

For humans, it is vitally important to rapidly detect potentially threatening stimuli such as fearful faces in the periphery. Trait anxiety has been related to biased responses to threatening faces, which might be more pronounced for peripheral stimuli. We examined the impact of spatial location and trait anxiety on event-related potentials to fearful faces (ERPs) in a large sample (N = 80). Using a face-unrelated oddball detection task and online eye-tracking, we ensured central fixation but sustained attention to the location of fearful and neutral faces at central or peripheral locations (12° left or right). Manipulation checks showed high task performance and successful shifts of visuospatial attention indexed by prestimulus alpha lateralization of the EEG spectra. Concerning ERPs, we observed increased P1 amplitudes for fearful compared to neutral faces, independent of the spatial location. In contrast, the N170 to fearful faces was only potentiated for centrally presented faces. Finally, fearful and neutral faces did not differ during the EPN window at any spatial location. Trait anxiety was unrelated to fearful-neutral ERP differences for each examined location. Our findings show that differential P1 responses are less constrained by the stimulus location, possibly due to coarse low-level stimulus information processing. However, the relative N170 increase for fearful faces depends on the central stimulus location. Furthermore, findings support the view that differential processing at the stage of the EPN depends on attentional conditions. Finally, our results question the hypothesis that trait anxiety differences correlate with mandatorily altered processing of threatening stimuli. (PsycInfo Database Record (c) 2022 APA, all rights reserved).

Two Oscillatory Correlates of Attention Control in the Alpha-Band with Distinct Consequences on Perceptual Gain and Metacognition.

Behavioral consequences and neural underpinnings of visuospatial attention have long been investigated. Classical studies using the Posner paradigm have found that visual perception systematically benefits from the use of a spatially informative cue pointing to the to-be-attended spatial location, compared with a noninformative cue. Lateralized α amplitude modulation during visuospatial attention shifts has been suggested to account for such perceptual gain. However, recent studies on spontaneous fluctuations of prestimulus α amplitude have challenged this notion. These studies showed that spontaneous fluctuations of prestimulus α amplitude were associated with the subjective appreciation of stimulus occurrence, while objective accuracy was instead best predicted by the frequency of α oscillations, with faster prestimulus α frequency accounting for better perceptual performance. Here, in male and female humans, by using an informative cue in anticipation of lateralized stimulus presentation, we found that the predictive cue not only modulates preparatory α amplitude but also α frequency in a retinotopic manner. Behaviorally, the cue significantly impacted subjective performance measures (metacognitive abilities [meta-d']) and objective performance gain (d'). Importantly, α amplitude directly accounted for confidence levels, with ipsilateral synchronization and contralateral desynchronization coding for high-confidence responses. Crucially, the contralateral α amplitude selectively predicted interindividual differences in metacognitive abilities (meta-d'), thus anticipating decision strategy and not perceptual sensitivity, probably via excitability modulations. Instead, higher perceptual accuracy both within and across participants (d') was associated with faster contralateral α frequency, likely by implementing higher sampling at the attended location. These findings provide critical new insights into the neural mechanisms of attention control and its perceptual consequences.SIGNIFICANCE STATEMENT Prior knowledge serves the anticipation of sensory input to reduce sensory ambiguity. The growing interest in the neural mechanisms governing the integration of sensory input into our internal representations has highlighted a pivotal role of brain oscillations. Here we show that distinct but interacting oscillatory mechanisms are engaged during attentional deployment: one relying on α amplitude modulations and reflecting internal decision processes, associated with subjective perceptual experience and metacognitive abilities; the other relying on α frequency modulations and enabling mechanistic sampling of the sensory input at the attended location to influence objective performance. These insights are crucial for understanding how we reduce sensory ambiguity to maximize the efficiency of our conscious experience, but also in interpreting the mechanisms of atypical perceptual experiences.

Cross-modal interactions at the audiovisual cocktail-party revealed by behavior, ERPs, and neural oscillations

Theories of attention argue that objects are the units of attentional selection. In real-word environments such objects can contain visual and auditory features. To understand how mechanisms of selective attention operate in multisensory environments, in this pre-registered study, we created an audiovisual cocktail-party situation, in which two speakers (left and right of fixation) simultaneously articulated brief numerals. In three separate blocks, informative auditory speech was presented (a) alone or paired with (b) congruent or (c) uninformative visual speech. In all blocks, subjects localized a pre-defined numeral. While audiovisual-congruent and uninformative speech improved response times and speed of information uptake according to diffusion modeling, an ERP analysis revealed that this did not coincide with enhanced attentional engagement. Yet, consistent with object-based attentional selection, the deployment of auditory spatial attention (N2ac) was accompanied by visuo-spatial attentional orienting (N2pc) irrespective of the informational content of visual speech. Notably, an N2pc component was absent in the auditory-only condition, demonstrating that a sound-induced shift of visuo-spatial attention relies on the availability of audio-visual features evolving coherently in time. Additional exploratory analyses revealed cross-modal interactions in working memory and modulations of cognitive control.

Early neural potentiation to centrally and peripherally presented fear-conditioned faces.

For humans, it is vitally important to rapidly detect and process threatening signals regardless of whether stimuli occur at fixation or in the periphery. However, it is unknown whether eccentricity affects early neuronal electrophysiological responses to fear-conditioned stimuli. We examined early event-related potentials (ERPs) of the electroencephalogram (EEG) to fear-conditioned faces to address this question. Participants (N=80) were presented with faces, either paired with an aversive (CS+) or neutral sound (CS-), at central or peripheral positions. We ensured constant central fixation using online eye-tracking but directed attention to either centrally or peripherally presented faces. Manipulation checks showed successful fear-conditioning (i.e., on average lower ratings in valence and higher ratings in arousal and perceived threat) and successful shifts of visuospatial attention indexed by high task performance and pre-stimulus alpha lateralization of the EEG spectra. We observed a generally increased P1 to fear-conditioned faces regardless of presentation location. An N170 difference between fear-conditioned and neutral stimuli was found but was restricted to the central location and depended on the effectivity of fear-conditioning. A similar effect was observed for the early posterior negativity (EPN). Trait anxiety was not related to differential ERP responses to CS+ versus CS- faces for any ERP component. These findings suggest that the P1 indexes early responses to centrally and peripherally presented fear-conditioned faces. Subsequent stages are modulated by the spatial location of the stimuli. This suggests different stages of neural processing of fear-conditioned faces depending on their spatial location. Finally, our results question the hypothesis that trait anxiety in healthy participants is related to altered visual processing of fear-conditioned faces.

Effects of attentional shifts along the vertical axis on number processing: An eye-tracking study with optokinetic stimulation

Previous studies suggest that associations between numbers and space are mediated by shifts of visuospatial attention along the horizontal axis. In this study, we investigated the effect of vertical shifts of overt attention, induced by optokinetic stimulation (OKS) and monitored through eye-tracking, in two tasks requiring explicit (number comparison) or implicit (parity judgment) processing of number magnitude. Participants were exposed to black-and-white stripes (OKS) that moved vertically (upward or downward) or remained static (control condition). During the OKS, participants were asked to verbally classify auditory one-digit numbers as larger/smaller than 5 (comparison task; Exp. 1) or as odd/even (parity task; Exp. 2). OKS modulated response times in both experiments. In Exp.1, upward attentional displacement decreased the Magnitude effect (slower responses for large numbers) and increased the Distance effect (slower responses for numbers close to the reference). In Exp.2, we observed a complex interaction between parity, magnitude, and OKS, indicating that downward attentional displacement slowed down responses for large odd numbers. Moreover, eye tracking analyses revealed an influence of number processing on eye movements both in Exp. 1, with eye gaze shifting downwards during the processing of small numbers as compared to large ones; and in Exp. 2, with leftward shifts after large even numbers (6,8) and rightward shifts after large odd numbers (7,9). These results provide evidence of bidirectional links between number and space and extend them to the vertical dimension. Moreover, they document the influence of visuo-spatial attention on processing of numerical magnitude, numerical distance, and parity. Together, our findings are in line with grounded and embodied accounts of numerical cognition.

Paired cognitive flexibility task with symptom factors improves detection of sports-related concussion in high school and collegiate athletes

Determining the sensitivity and specificity of short neurocognitive assessments to objectively detect concussion will help clinicians more confidently integrate such tools in clinical management decisions. This study quantified the sensitivity and specificity of a computerized cognitive flexibility task isolating shifts of visuospatial attention in combination with clinical symptoms acutely (< 72 h) following concussion. A total of 100 athletes (53 concussed; 47 non-injured control; 42% female) completed computerized neurocognitive testing and clinical symptom reports (Sport Concussion Assessment Tool 3rd edition: SCAT3). Separate discriminant function analyses were performed for individual, combination, and stepwise inclusion of neurocognitive and clinical symptomology assessments. Findings revealed the combination of neurocognitive outcomes (i.e., mean reaction time, response accuracy, and response accuracy cost) with clinical symptom factor scores exhibited the greatest sensitivity (95.7%) and specificity (88.7%) as well as the highest positive predictive value (95.9%) and negative predictive value (88%) relative to other approaches. Further, a stepwise approach predicting concussion status using the discriminant functions improved detection of concussion (98.2% sensitivity, 95.7% specificity, 96.4% positive predictive value, and 97.8% negative predictive value) when clinical symptom factors failed to indicate the presence of a concussion. Incorporating a cognitive flexibility task involving shifts of visuospatial attention combined with clinical symptom factor scores may improve clinical decision-making as this approach exceeds the sensitivity and specificity of widely popular neurocognitive test batteries and takes less than 10 min to administer.

Gaze-based and attention-based rehearsal in spatial working memory.

How do we maintain information about spatial configurations in mind? Many working memory (WM) models assume that rehearsal processes are used to counteract forgetting in WM. Here, we investigated the contributions of gaze-based and attention-based rehearsal for protecting spatial representations from time-based forgetting. Participants memorized 6 locations selected from a grid of 30 scattered dots. Memory was tested after 1.5 or 4.5 s, and this interval was either blank or the grid remained onscreen (which is assumed to provide rehearsal support). In 2 experiments, we monitored eye movements during the retention phase, or asked participants to fixate the screen center. In 3 subsequent experiments, we tested spatial WM under dual-task conditions inhibiting shifts of visuospatial attention or central attention to the memoranda. Memory was better and more resistant to time-based forgetting in the grid than blank condition. Recording of fixations showed more frequent and efficient gaze-based rehearsal in the presence of the grid. Fixations toward distractor locations occurred at a similar frequency in the blank and grid conditions, and it did not predict incorrect recalls. Inhibition of eye-movements or shifts of visuospatial attention impaired memory overall, but it did not change the grid benefit nor the rate of time-based forgetting. In contrast, distracting central attention increased time-based forgetting regardless of grid presence. These results indicate that (a) the grid benefit is only partially explained by rehearsal; (b) gaze-errors (i.e., distractor fixations) do not lead to more forgetting; and (c) the maintenance of spatial representations over time depends on central processing. (PsycInfo Database Record (c) 2020 APA, all rights reserved).

Left parietal tACS at alpha frequency induces a shift of visuospatial attention.

BackgroundVoluntary shifts of visuospatial attention are associated with a lateralization of parieto-occipital alpha power (7-13Hz), i.e. higher power in the hemisphere ipsilateral and lower power contralateral to the locus of attention. Recent noninvasive neuromodulation studies demonstrated that alpha power can be experimentally increased using transcranial alternating current stimulation (tACS).Objective/HypothesisWe hypothesized that tACS at alpha frequency over the left parietal cortex induces shifts of attention to the left hemifield. However, spatial attention shifts not only occur voluntarily (endogenous/ top-down), but also stimulus-driven (exogenous/ bottom-up). To study the task-specificity of the potential effects of tACS on attentional processes, we administered three conceptually different spatial attention tasks.Methods36 healthy volunteers were recruited from an academic environment. In two separate sessions, we applied either high-density tACS at 10Hz, or sham tACS, for 35–40 minutes to their left parietal cortex. We systematically compared performance on endogenous attention, exogenous attention, and stimulus detection tasks.ResultsIn the endogenous attention task, a greater leftward bias in reaction times was induced during left parietal 10Hz tACS as compared to sham. There were no stimulation effects in either the exogenous attention or the stimulus detection task.ConclusionThe study demonstrates that high-density tACS at 10Hz can be used to modulate visuospatial attention performance. The tACS effect is task-specific, indicating that not all forms of attention are equally susceptible to the stimulation.

Implicit location probability learning does not induce baseline shifts of visuospatial attention.

We tested whether implicit learning causes shifts of spatial attention in advance of or in response to stimulus onset. Participants completed randomly interspersed trials of letter search, which involved reporting the orientation of a T among Ls, and scene search, which involved identifying which of four scenes was from a target category (e.g., forest). In Experiment 1, an initial phase more often contained target letters in one screen quadrant, while the target scenes appeared equally often in all quadrants. Participants persistently prioritized letter targets in the more probable region, but the implicitly learned preference did not affect the unbiased scene task. In Experiment 2, the spatial probabilities of the scene and letter tasks reversed. Participants unaware of the probability manipulation acquired only a spatial bias to scene targets in the more probable region, with no effect on letter search. Instead of recruiting baseline shifts of spatial attention prior to stimulus onset, implicit learning of target probability yields task-dependent shifts of spatial attention following stimulus onset. Such shifts may involve attentional behaviors unique to certain task contexts.

Preliminary evidence for differential trajectories of recovery for cognitive flexibility following sports-related concussion.

A critical barrier to the understanding of disruptions to cognitive flexibility following sports-related concussion is the use of assessments that conflate shifts of visuospatial attention and contextual rules. Because these dissociable forms of cognitive flexibility are subserved by distinct neural networks, the utility of a cognitive flexibility assessment following concussion may be reduced, depending upon the extent to which the task requires shifting visuospatial attention relative to shifting contextual rules. Accordingly, the current investigation examined the extent to which these aspects of cognitive flexibility exhibit differential trajectories of recovery following a sports-related concussion. Twenty-two athletes with sports-related concussions were assessed on a cognitive flexibility task with 2 switch conditions (i.e., perceptual-based and contextual rule-based) within 72 hr of injury, after return to play, and within 1 month following return to play. Thirty-three healthy control athletes were tested at the same intervals. Findings revealed that concussed athletes demonstrated protracted disruptions in task performance on a visuospatial attention-based cognitive flexibility task relative to healthy controls, whereas disruptions in task performance on a contextual rule-based cognitive flexibility task resolved after the acute phase of injury. These findings suggest that dissociable forms of cognitive flexibility exhibit differential trajectories of recovery. Therefore, evaluations detecting sports-related concussion disruptions in cognitive flexibility may be reduced depending on the extent to which they rely on contextual rule-based decisions. Test batteries focusing on visuospatial attention-based demands may be useful additions to aid in the objective assessment and follow-up management of athletes following the acute phase of injury. (PsycINFO Database Record

Grasping remaps the distribution of visuospatial attention and enhances competing action activation.

We examined how action goals influence the distribution of visuospatial attention near the body (Experiment 1) and how the temporal relationship between distractors and targets modifies shifts in visuospatial attention (Experiment 2). Targets were light emitting diodes (LEDs) in the left and right hemispace of a visual display. Following left or right target illumination, participants reached to point-to or grasp target object in blocked trials. Coincident with target onset, a distractor LED illuminated in the same or opposite hemispace between the initiation point and target, or no distractor appeared. In Experiment 1, during grasping there was a larger temporal interference effect (slower reach initiation) than with pointing. When grasping versus pointing, participants deviated more towards same-side distractors and away from opposite-side distractors. In Experiment 2, distractors onset 200ms prior to (-200-ms), coincident with (0 ms), or 200ms following (+200 ms) the target. For both reach types, -200-ms distractors had greater onset temporal interference than 0 ms and +200-ms distractors. For grasping, +200 ms distractors had larger temporal interference than 0 ms distractors. For -200-ms, reach trajectories deviated more towards opposite-side distractors and away from same-side distractors, the reverse of the pattern for 0 ms and +200-ms distractors.

Functional connectivity between prefrontal and parietal cortex drives visuo-spatial attention shifts

It is well established that the frontal eye-fields (FEF) in the dorsal attention network (DAN) guide top-down selective attention. In addition, converging evidence implies a causal role for the FEF in attention shifting, which is also known to recruit the ventral attention network (VAN) and fronto-striatal regions. To investigate the causal influence of the FEF as (part of) a central hub between these networks, we applied thetaburst transcranial magnetic stimulation (TBS) off-line, combined with functional magnetic resonance (fMRI) during a cued visuo-spatial attention shifting paradigm.We found that TBS over the right FEF impaired performance on a visual discrimination task in both hemifields following attention shifts, while only left hemifield performance was affected when participants were cued to maintain the focus of attention. These effects recovered ca. 20min post stimulation. Furthermore, particularly following attention shifts, TBS suppressed the neural signal in bilateral FEF, right inferior and superior parietal lobule (IPL/SPL) and bilateral supramarginal gyri (SMG). Immediately post stimulation, functional connectivity was impaired between right FEF and right SMG as well as right putamen. Importantly, the extent of decreased connectivity between right FEF and right SMG correlated with behavioural impairment following attention shifts.The main finding of this study demonstrates that influences from right FEF on SMG in the ventral attention network causally underly attention shifts, presumably by enabling disengagement from the current focus of attention.

Handedness and Graspability Modify Shifts of Visuospatial Attention to Near-Hand Objects.

We examined how factors related to the internal representation of the hands (handedness and grasping affordances) influence the distribution of visuospatial attention near the body. Left and right handed participants completed a covert visual cueing task, discriminating between two target shapes. In Experiment 1, participants responded with either their dominant or non-dominant hand. In Experiment 2, the non-responding hand was positioned below one of two target placeholders, aligned with the shoulder. In Experiment 3 the near-monitor hand was positioned under the placeholder in the opposite region of hemispace, crossed over the body midline. For Experiments 2 & 3, in blocked trials the palmar and back-of hand surfaces were directed towards the target placeholder such that targets appeared towards either the graspable or non-graspable space of the hand respectively. In Experiment 2, both left and right handers displayed larger accuracy cueing effects for targets near versus distant from the graspable space of the right hand. Right handers also displayed larger response time cueing effects for objects near the graspable versus non-graspable region of their dominant hand but not for their non-dominant hands. These effects were not evident for left-handers. In Experiment 3, for right handers, accuracy biases for near hand targets were still evident when the hand was crossed over the body midline, and reflected hand proximity but not functional orientation biases. These findings suggest that biased visuospatial attention enhances object identity discrimination near hands and that these effects are particularly enhanced for right-handers.

Response control networks are selectively modulated by attention to rare events and memory load regardless of the need for inhibition

Recent evidence has sparked debate about the neural bases of response selection and inhibition. In the current study, we employed two reactive inhibition tasks, the Go/Nogo (GnG) and Simon tasks, to examine questions central to these debates. First, we investigated whether a fronto-cortical-striatal system was sensitive to the need for inhibition per se or the presentation of infrequent stimuli, by manipulating the proportion of trials that do not require inhibition (Go/Compatible trials) relative to trials that require inhibition (Nogo/Incompatible trials). A cortico-subcortical network composed of insula, putamen, and thalamus showed greater activation on salient and infrequent events, regardless of the need for inhibition. Thus, consistent with recent findings, key parts of the fronto-cortical-striatal system are engaged by salient events and do not appear to play a selective role in response inhibition. Second, we examined how the fronto-cortical-striatal system is modulated by working memory demands by varying the number of stimulus-response (SR) mappings. Right inferior parietal lobule showed decreasing activation as the number of SR mappings increased, suggesting that a form of associative memory – rather than working memory – might underlie performance in these tasks. A broad motor planning and control network showed similar trends that were also modulated by the number of motor responses required in each task. Finally, bilateral lingual gyri were more robustly engaged in the Simon task, consistent with the role of this area in shifts of visuo-spatial attention. The current study sheds light on how the fronto-cortical-striatal network is selectively engaged in reactive control tasks and how control is modulated by manipulations of attention and memory load.

The neural basis of dyslexia may originate in primary auditory cortex.

This scientific commentary refers to ‘Neuroanatomical precursors of dyslexia identified from pre-reading through to age 11’ by Clark et al. (doi:10.1093/brain/awu229). Longitudinal research studies that follow at-risk samples before a disorder is evident, ideally from infancy, are the gold standard in the field of neurodevelopmental disorders (Goswami, 2014). They are also the rarest. In this issue of Brain , Clark and colleagues report a longitudinal structural neuroimaging study of children at high versus low risk of dyslexia that began before reading instruction commenced. The structural scans were taken the year before reading was taught (at age 6), a year after reading tuition began (at age 8), and after dyslexia had been diagnosed (at age 11). Clark et al. ’s study thereby reveals the neurodevelopmental trajectory of the dyslexic brain, the only sure means of disentangling cause from effect in the aetiology of a neurodevelopmental disorder. Longitudinal studies are crucial because the acquisition of reading is one of the most complex cognitive feats that the brain achieves. As young children learn to recode print to sound, the brain undergoes intensive and highly specific experience-dependent learning, often on a daily basis. These learning experiences selectively train aspects of sensory processing and attention. For example, oculomotor control and small shifts of visuospatial attention may be practised for hours every day. Learning to read literally changes the brain. Therefore, the identification of pre-reading weaknesses in neural structures and processes is vital to understanding causation in developmental dyslexia. Clark and colleagues followed Norwegian children from preschool until sixth grade, identifying participants at high risk of dyslexia on the basis of family risk. They also followed a matched sample of children at low risk for dyslexia. The children’s neurocognitive skills were assessed yearly. Norwegian is a relatively transparent orthography, and reading instruction commences at …

Microsaccade-related brain potentials signal the focus of visuospatial attention

Covert shifts of visuospatial attention are traditionally assumed to occur in the absence of oculomotor behavior. In contrast, recent behavioral studies have linked attentional cueing effects to the occurrence of microsaccades, small eye movements executed involuntarily during attempted fixation. Here we used a new type of electrophysiological marker to explore the attention–microsaccade relationship, the visual brain activity evoked by the microsaccade itself. By shifting the retinal image, microsaccades frequently elicit neural responses throughout the visual pathway, scalp-recordable in the human EEG as a microsaccade-related potential (mSRP). Although mSRPs contain similar signal components (P1/N1) as traditional visually-evoked potentials (VEPs), it is unknown whether they are also influenced by cognition. Based on established findings that VEPs are amplified for visual inputs at currently attended locations, we expected a selective gain-modulation also for mSRPs. Eye movements and EEG were coregistered in a classic spatial cueing task with an endogenous cue. Replicating behavioral findings, the direction of early microsaccades 200–400ms after cue onset was biased towards the cued side. However, for microsaccades throughout the cue-target interval, mSRPs were systematically enhanced at occipital scalp sites contralateral to the cued hemifield. This attention effect resembled that in a control condition with VEPs and did not interact with the direction of the underlying microsaccade, suggesting that mSRPs reflect the focus of sustained visuospatial attention, which remains fixed at the cued location, despite microsaccades. Microsaccades are not merely an artifact source in the EEG; instead, they are followed by cognitively modulated brain potentials that can serve as non-intrusive electrophysiological probes of attention.

Shifts of visuospatial attention do not cause the spatial distortions of the Roelofs effect

When a visible frame is offset left or right of an observer's objective midline, subjective midline is pulled toward the frame's center, resulting in an illusion of perceived space known as the Roelofs effect. However, a large frame is not necessary to generate the effect-even a small peripheral stimulus is sufficient, raising the possibility that the effect would be brought about by any stimulus that draws attention away from the midline. To assess the relationship between attention and distortions of perceived space, we adopted a paradigm that included a spatial cue that attracted the participant's attention, and an occasional probe whose location was to be reported. If shifts of attention cause the Roelofs effect, the probe's perceived location should vary with the locus of attention. Exogenous attentional cues caused a Roelofs-like effect, but these cues created an asymmetry in the visual display that may have driven the effect directly. In contrast, there was no mislocation after endogenous cues that contained no asymmetry in the visual display. A final experiment used color-contingent attentional cues to eliminate the confound between cue location and asymmetry in the visual display, and provided a clear demonstration that the Roelofs effect is caused by an asymmetric visual display, independent of any shift of attention.

Difference in Visual Processing Assessed by Eye Vergence Movements

Orienting visual attention is closely linked to the oculomotor system. For example, a shift of attention is usually followed by a saccadic eye movement and can be revealed by micro saccades. Recently we reported a novel role of another type of eye movement, namely eye vergence, in orienting visual attention. Shifts in visuospatial attention are characterized by the response modulation to a selected target. However, unlike (micro-) saccades, eye vergence movements do not carry spatial information (except for depth) and are thus not specific to a particular visual location. To further understand the role of eye vergence in visual attention, we tested subjects with different perceptual styles. Perceptual style refers to the characteristic way individuals perceive environmental stimuli, and is characterized by a spatial difference (local vs. global) in perceptual processing. We tested field independent (local; FI) and field dependent (global; FD) observers in a cue/no-cue task and a matching task. We found that FI observers responded faster and had stronger modulation in eye vergence in both tasks than FD subjects. The results may suggest that eye vergence modulation may relate to the trade-off between the size of spatial region covered by attention and the processing efficiency of sensory information. Alternatively, vergence modulation may have a role in the switch in cortical state to prepare the visual system for new incoming sensory information. In conclusion, vergence eye movements may be added to the growing list of functions of fixational eye movements in visual perception. However, further studies are needed to elucidate its role.

P 10. Acute state-dependent changes in corticomotor threshold in the absence of overt motor activity

The cortical motor threshold (CMT) is often used to individually adjust the intensity of transcranial magnetic stimulation (TMS). It reflects the integrated excitability of the corticomotor projection, including the excitability at the spinal level. The CMT reflects axonal and synaptic excitability as CMT can be modified by drugs that voltage-gated sodium channel blockers or non-NMDA glutamatergic drugs. The CMT is not a static measure but is subject to state-dependent fluctuations. To test this hypothesis we used an adaptive stair-case procedure to track acute changes in CMT associated with motor imagery (MI) or shifts in visuospatial attention (VA). Eight healthy subjects were asked to imagine a continuous thumb-index movement with the left or right hand (MI-left and MI-right), to shift their visuospatial attention to the left or right FDI (VA-left and VA-right) or to fixate a cross in the central visual field. Using a fast (<15 pulses) threshold-hunting procedure (Borckardt, 2006), we assessed task-dependent CMT changes in the left primary motor hand area (M1) during each task. CMT was defined as the stimulus intensity, at which a single TMS pulse elicited a MEP of 50 μV in the relaxed right first dorsal interosseus muscle (FDI) with a probability of 0.5. The CMT measurements during the VA and MI task were normalized to the control condition. All participants underwent two experimental sessions on separate days, in which the TMS pulse either induced a posterior-to-anterior (PA) or anterior-to-posterior (AP) current in M1. The number of pulses needed to determine rMT did not significantly differ across conditions (average: 13.5). Using the normalized CMT values as dependent variable, we computed a repeated measures ANOVA with the factors Current (AP/PA), Task (MI/VA) and Site (Left/Right). We found a significant Task-by-Site interaction ( p = 0.004). Post hoc testing confirmed that CMT during MI-right was significantly lower than during MI-left or during VA-right. This was the case for AP and PA current direction. The resting CMT is subject to rapid state-dependent changes in the absence of overt motor activity. We show that these acute CMT changes can be captured with a threshold-hunting procedure. The “CMT tracking” approach might be particularly useful to assess the temporal dynamics of corticomotor excitability changes induced by interventional brain stimulation protocols.