Visual search and its disorders:

Lateralized alpha oscillatory activity in the inferior parietal lobule to the right hemisphere during left-side visual stimulation

Understanding the neural mechanisms underlying spatial attention is crucial for unraveling the pathogenesis of unilateral spatial neglect (USN). However, the neural link between spatial attention and USN remains unclear. Thus, the neural mechanisms of spatial attention in the left and right hemispheres were compared. Twenty healthy volunteers participated in a hand mental rotation task in which they determined whether images depicted as left or right hands. The hand images were randomly displayed in the upper, lower, left, and right directions, centered on a fixation point. The laterality index for the alpha oscillatory activity was determined to assess the lateralization of neural activity during visual stimulation. Our results revealed a significant shift in alpha oscillatory neural activity in the inferior parietal lobule (IPL) towards the right hemisphere when visual stimulation occurred on the left side. In contrast, no significant oscillatory shift in the alpha band towards the left hemisphere was observed in the IPL when the visual stimulus was presented on the right side. These findings indicate that the spatial attention on the left side depends on oscillatory alpha activity in the right IPL, whereas that on the right side doesn't depend on either hemispheric alpha activity. These results provide valuable insights into the neural mechanisms of hemispatial neglect.

Virtual Reality and Eye-Tracking Assessment, and Treatment of Unilateral Spatial Neglect: Systematic Review and Future Prospects

Unilateral spatial neglect (USN) is a disorder characterized by the failure to report, respond to, or orient toward the contralateral side of space to a brain lesion. Current assessment methods often fail to discover milder forms, cannot differentiate between unilateral spatial neglect subtypes and lack ecological validity. There is also a need for treatment methods that target subtypes. Immersive virtual reality (VR) systems in combination with eye-tracking (ET) have the potential to overcome these shortcomings, by providing more naturalistic environments and tasks, with sensitive and detailed measures. This systematic review examines the state of the art of research on these technologies as applied in the assessment and treatment of USN. As we found no studies that combined immersive VR and ET, we reviewed these approaches individually. The review of VR included seven articles, the ET review twelve. The reviews revealed promising results. (1) All included studies found significant group-level differences for several USN measures. In addition, several studies found asymmetric behavior in VR and ET tasks for patients who did not show signs of USN in conventional tests. Particularly promising features were multitasking in complex VR environments and detailed eye-movement analysis. (2) No VR and only a few ET studies attempted to differentiate USN subtypes, although the technologies appeared appropriate. One ET study grouped USN participants using individual heatmaps, and another differentiated between subtypes on drawing tasks. Regarding (3) ecological validity, although no studies tested the prognostic validity of their assessment methods, VR and ET studies utilized naturalistic tasks and stimuli reflecting everyday situations. Technological characteristics, such as the field of view and refresh rate of the head-mounted displays, could be improved, though, to improve ecological validity. We found (4) no studies that utilized VR or ET technologies for USN treatment up until the search date of the 26th of February 2020. In conclusion, VR-ET-based systems show great potential for USN assessment. VR-ET holds great promise for treatment, for example, by monitoring behavior and adapting and tailoring to the individual person's needs and abilities. Future research should consider developing methods for individual subtypes and differential diagnostics to inform individual treatment programs.

Influence of Static Magnetic Field Stimulation on the Accuracy of Tachystoscopically Presented Line Bisection

Transcranial static magnetic stimulation (tSMS) has been known to reduce human cortical excitability. Here, we investigated whether tSMS would modulate visuo-spatial cognition in healthy humans. Subjects performed a visuo-spatial task requiring judgements about the symmetry of pre-bisected lines. Visual stimuli consisted of symmetrically or asymmetrically transected lines, tachystoscopically presented for 150 ms on a computer monitor. Task performance was examined before, immediately after, and 10 min after tSMS/sham stimulation of 20 min over the posterior parietal cortex (PPC: P4 from the international 10-20 system) or superior temporal gyrus (STG: C6). Nine out of 16 subjects misjudged pre-bisected lines by consistently underestimating the length of the right-side segment (judging lines to be exactly pre-bisected when the transector was located to the left of the midpoint, or judging the left-side segment to be longer when the transector was located at the midpoint). In these subjects showing a leftward bias, tSMS over the right STG reduced the magnitude of the leftward bias. This did not occur with tSMS over the right PPC or sham stimulation. In the remaining right-biased subjects, no intervention effect was observed with any stimulation. Our findings indicate that application of tSMS over the right STG modulates visuo-spatial cognition in healthy adults.

Unbalancing the Attentional Priority Map via Gaze-Contingent Displays Induces Neglect-Like Visual Exploration

Selective spatial attention is a crucial cognitive process that guides us to the behaviorally relevant objects in a complex visual world by using exploratory eye movements. The spatial location of objects, their (bottom-up) saliency and (top-down) relevance is assumed to be encoded in one “attentional priority map” in the brain, using different egocentric (eye-, head- and trunk-centered) spatial reference frames. In patients with hemispatial neglect, this map is supposed to be imbalanced, leading to a spatially biased exploration of the visual environment. As a proof of concept, we altered the visual saliency (and thereby attentional priority) of objects in a naturalistic scene along a left-right spatial gradient and investigated whether this can induce a bias in the exploratory eye movements of healthy humans (n = 28; all right-handed; mean age: 23 years, range 19–48). We developed a computerized mask, using high-end “gaze-contingent display (GCD)” technology, that immediately and continuously reduced the saliency of objects on the left—“left” with respect to the head (body-centered) and the current position on the retina (eye-centered). In both experimental conditions, task-free viewing and goal-driven visual search, this modification induced a mild but significant bias in visual exploration similar to hemispatial neglect. Accordingly, global eye movement parameters changed (reduced number and increased duration of fixations) and the spatial distribution of fixations indicated an attentional bias towards the right (rightward shift of first orienting, fixations favoring the scene’s outmost right over left). Our results support the concept of an attentional priority map in the brain as an interface between perception and behavior and as one pathophysiological ground of hemispatial neglect.

Clinical provision of compensatory visual training after neurological injury: example of a multisite outpatient program

AIM

Treatment efficacy is established via controlled research trials, but treatment in real-world clinical environments is typically highly variable and may differ from research protocols by necessity. Here, we examined provision of visual retraining for adults after neurological injury at an outpatient rehabilitation program in Calgary, Canada.

METHODS

Retrospective chart audits extracted demographic data, assessment outcomes, and details related to provision of training.

RESULTS

Treatment was provided to individuals with both visual field and visual-perceptual impairments due to neurological injury (mostly stroke). Tools and techniques of visual retraining at this program are discussed, the common denominator being repetitive practice of compensatory visual behaviors. Across this multisite program, there was significant variability in the number of treatment sessions, 13.00 (±10.21) sessions for those with visual-perceptual impairments and 14.41 (±9.63) sessions for those with field loss. Descriptive statistics and confidence intervals suggest improved outcomes on some measures for those with visual field and visual perceptual impairments.

CONCLUSIONS

Our data suggest that visual retraining is feasible in this clinical outpatient setting. Implications for rehabilitation This program of visual retraining was provided to individuals with visual impairment (e.g., hemianopia) and visual perceptual impairment (e.g., unilateral spatial neglect) as a result of neurological injury. In this outpatient program, visual rehabilitation was feasible and appeared to improve outcomes among a heterogeneous clinical population. Fundamental characteristics of visual compensatory training at this program included repetitive practice of adaptive scanning behaviors across multiple contexts to promote automaticity and generalization of skills.

Eye-Tracking Provides a Sensitive Measure of Exploration Deficits After Acute Right MCA Stroke

The eye-tracking study aimed at assessing spatial biases in visual exploration in patients after acute right MCA (middle cerebral artery) stroke. Patients affected by unilateral neglect show less functional recovery and experience severe difficulties in everyday life. Thus, accurate diagnosis is essential, and specific treatment is required. Early assessment is of high importance as rehabilitative interventions are more effective when applied soon after stroke. Previous research has shown that deficits may be overlooked when classical paper-and-pencil tasks are used for diagnosis. Conversely, eye-tracking allows direct monitoring of visual exploration patterns. We hypothesized that the analysis of eye-tracking provides more sensitive measures for spatial exploration deficits after right middle cerebral artery stroke. Twenty-two patients with right MCA stroke (median 5 days after stroke) and 28 healthy controls were included. Lesions were confirmed by MRI/CCT. Groups performed comparably in the Mini-Mental State Examination (patients and controls median 29) and in a screening of executive functions. Eleven patients scored at ceiling in neglect screening tasks, 11 showed minimal to severe signs of unilateral visual neglect. An overlap plot based on MRI and CCT imaging showed lesions in the temporo-parieto-frontal cortex, basal ganglia, and adjacent white matter tracts. Visual exploration was evaluated in two eye-tracking tasks, one assessing free visual exploration of photographs, the other visual search using symbols and letters. An index of fixation asymmetries proved to be a sensitive measure of spatial exploration deficits. Both patient groups showed a marked exploration bias to the right when looking at complex photographs. A single case analysis confirmed that also most of those patients who showed no neglect in screening tasks performed outside the range of controls in free exploration. The analysis of patients' scoring at ceiling in neglect screening tasks is of special interest, as possible deficits may be overlooked and thus remain untreated. Our findings are in line with other studies suggesting considerable limitations of laboratory screening procedures to fully appreciate the occurrence of neglect symptoms. Future investigations are needed to explore the predictive value of the eye-tracking index and its validity in everyday situations.

What Does It Take to Search Organized? The Cognitive Correlates of Search Organization During Cancellation After Stroke

OBJECTIVES

Stroke could lead to deficits in organization of visual search. Cancellation tests are frequently used in standard neuropsychological assessment and appear suitable to measure search organization. The current aim was to evaluate which cognitive functions are associated with cancellation organization measures after stroke.

METHODS

Stroke patients admitted to inpatient rehabilitation were included in this retrospective study. We performed exploratory factor analyses to explore cognitive domains. A digital shape cancellation test (SC) was administered, and measures of search organization (intersections rate and best r) were computed. The following cognitive functions were measured by neuropsychological testing: neglect (SC, line bisection; LB, Catherine Bergego Scale; CBS, and Balloons Test), visuospatial perception and construction (Rey Complex Figure Test, RCFT), psychomotor speed (Trail Making Test; TMT-A), executive functioning/working memory (TMT-B), spatial planning (Tower Test), rule learning (Brixton Test), short-term auditory memory (Digit Span Forward; DSF), and verbal working memory (Digit Span Backward; DSB).

RESULTS

In total, 439 stroke patients were included in our analyses. Four clusters were separated: "Executive functioning" (TMT-A, TMT-B, Brixton Test, and Tower Test), "Verbal memory" (DSF and DSB), "Search organization" (intersections rate and best r), and "Neglect" (CBS, RCFT copy, Balloons Test, SC, and LB).

CONCLUSIONS

Search organization during cancellation, as measured with intersections rate and best r, seems a distinct cognitive construct compared to existing cognitive domains that are tested during neuropsychological assessment. Administering cancellation tests and analyzing measures of search organization could provide useful additional insights into the visuospatial processes of stroke patients. (JINS, 2018, 24, 424-436).

Visual Search Performance in Patients with Vision Impairment: A Systematic Review

PURPOSE

Patients with visual impairment are constantly facing challenges to achieve an independent and productive life, which depends upon both a good visual discrimination and search capacities. Given that visual search is a critical skill for several daily tasks and could be used as an index of the overall visual function, we investigated the relationship between vision impairment and visual search performance.

METHODS

A comprehensive search was undertaken using electronic PubMed, EMBASE, LILACS, and Cochrane databases from January 1980 to December 2016, applying the following terms: "visual search", "visual search performance", "visual impairment", "visual exploration", "visual field", "hemianopia", "search time", "vision lost", "visual loss", and "low vision". Two hundred seventy six studies from 12,059 electronic database files were selected, and 40 of them were included in this review.

RESULTS

Studies included participants of all ages, both sexes, and the sample sizes ranged from 5 to 199 participants. Visual impairment was associated with worse visual search performance in several ophthalmologic conditions, which were either artificially induced, or related to specific eye and neurological diseases.

CONCLUSIONS

This systematic review details all the described circumstances interfering with visual search tasks, highlights the need for developing technical standards, and outlines patterns for diagnosis and therapy using visual search capabilities.

Visual search training in occupational therapy – an example of expert practice in community-based stroke rehabilitation

Introduction

Visual searching is an essential component of many everyday activities. Search training is practised as part of occupational therapy to improve performance skills both in people with hemianopia and those with spatial inattention post stroke. Evaluation of the effectiveness of such training first requires a systematic and detailed description of the intervention. To this end, this study describes the practice of a specialist occupational therapist.

Method

Single sessions of intervention delivered by the occupational therapist to five participants with visual search disorders post stroke were video recorded. The recordings were analysed for content using a framework approach.

Results

The occupational therapist educated participants about the impact of their visual impairment on everyday activities. She used instructions, spatial cueing, placement of objects and verbal feedback to train increased amplitudes of eye and head movements, to direct attention into the blind field or neglected space and to encourage systematic searching during occupations and activities. Activities were graded by manipulating the area for attention and complexity in the environment.

Conclusion

This investigation provides a detailed description of a specialist occupational therapist’s community-based intervention for improving visual search post stroke.

Target-to-Distractor Ratio Effects on Decision Time in the Orderly Array Shape Cancellation Task

The cancellation task is a paper-and-pencil test commonly used to assess attention or planning. This study investigated whether the decision time performance on the task was influenced by the number of targets and distractors. This study reduced the demand of planning and used an index of the decision time, an estimate of the time taken to decide whether to mark a stimulus. Forty healthy adults ( M age = 21.3 yr., SD = 1.5) performed five cancellation tasks. Four tasks were conducted with instructions to mark a detected target. The target-to-distractor ratio varied from 35/15,40/10,45/5, and 50/0, and one task with instructions to mark all stimuli (50/0), to measure the motor time to mark targets. One-way analysis of variance indicated statistically significant differences between conditions. There was a linear relationship between decision time and target-to-distractor ratio; the decision time increased as the proportion of distractors increased. The results suggested the decision time reflects the frequency of switches between responses to targets and distractors or attention modulation of processing to targets and distractors.

Animal foraging and the evolution of goal-directed cognition

Foraging- and feeding-related behaviors across eumetazoans share similar molecular mechanisms, suggesting the early evolution of an optimal foraging behavior called area-restricted search (ARS), involving mechanisms of dopamine and glutamate in the modulation of behavioral focus. Similar mechanisms in the vertebrate basal ganglia control motor behavior and cognition and reveal an evolutionary progression toward increasing internal connections between prefrontal cortex and striatum in moving from amphibian to primate. The basal ganglia in higher vertebrates show the ability to transfer dopaminergic activity from unconditioned stimuli to conditioned stimuli. The evolutionary role of dopamine in the modulation of goal-directed behavior and cognition is further supported by pathologies of human goal-directed cognition, which have motor and cognitive dysfunction and organize themselves, with respect to dopaminergic activity, along the gradient described by ARS, from perseverative to unfocused. The evidence strongly supports the evolution of goal-directed cognition out of mechanisms initially in control of spatial foraging but, through increasing cortical connections, eventually used to forage for information.

Impact of dynamic bottom-up features and top-down control on the visual exploration of moving real-world scenes in hemispatial neglect

Patients with hemispatial neglect are severely impaired in orienting their attention to contralesional hemispace. Although motion is one of the strongest attentional cues in humans, it is still unknown how neglect patients visually explore their moving real-world environment. We therefore recorded eye movements at bedside in 19 patients with hemispatial neglect following acute right hemisphere stroke, 14 right-brain damaged patients without neglect and 21 healthy control subjects. Videos of naturalistic real-world scenes were presented first in a free viewing condition together with static images, and subsequently in a visual search condition. We analyzed number and amplitude of saccades, fixation durations and horizontal fixation distributions. Novel computational tools allowed us to assess the impact of different scene features (static and dynamic contrast, colour, brightness) on patients' gaze. Independent of the different stimulus conditions, neglect patients showed decreased numbers of fixations in contralesional hemispace (ipsilesional fixation bias) and increased fixation durations in ipsilesional hemispace (disengagement deficit). However, in videos left-hemifield fixations of neglect patients landed on regions with particularly high dynamic contrast. Furthermore, dynamic scenes with few salient objects led to a significant reduction of the pathological ipsilesional fixation bias. In visual search, moving targets in the neglected hemifield were more frequently detected than stationary ones. The top-down influence (search instruction) could neither reduce the ipsilesional fixation bias nor the impact of bottom-up features. Our results provide evidence for a strong impact of dynamic bottom-up features on neglect patients' scanning behaviour. They support the neglect model of an attentional priority map in the brain being imbalanced towards ipsilesional hemispace, which can be counterbalanced by strong contralateral motion cues. Taking into account the lack of top-down control in neglect patients, bottom-up stimulation with moving real-world stimuli may be a promising candidate for future neglect rehabilitation schemes.

Gaze patterns predicting successful collision avoidance in patients with homonymous visual field defects

Aim of the present study was to identify efficient compensatory gaze patterns applied by patients with homonymous visual field defects (HVFDs) under virtual reality (VR) conditions in a dynamic collision avoidance task. Thirty patients with HVFDs due to vascular brain lesions and 30 normal subjects performed a collision avoidance task with moving objects at an intersection under two difficulty levels. Based on their performance (i.e. the number of collisions), patients were assigned to either an "adequate" (HVFD(A)) or "inadequate" (HVFD(I)) subgroup by the median split method. Eye and head tracking data were available for 14 patients and 19 normal subjects. Saccades, fixations, mean number of gaze shifts, scanpath length and the mean gaze eccentricity, were compared between HVFD(A), HVFD(I) patients and normal subjects. For both difficulty levels, the gaze patterns of HVFD(A) patients (N=5) compared to HVFD(I) patients (N=9) were characterized by longer saccadic amplitudes towards both the affected and the intact side, larger mean gaze eccentricity, more gaze shifts, longer scanpaths and more fixations on vehicles but fewer fixations on the intersection. Both patient groups displayed more fixations in the affected compared to the intact hemifield. Fixation number, fixation duration, scanpath length, and number of gaze shifts were similar between HVFD(A) patients and normal subjects. Patients with HVFDs who adapt successfully to their visual deficit, display distinct gaze patterns characterized by increased exploratory eye and head movements, particularly towards moving objects of interest on their blind side. In the context of a dynamic environment, efficient compensation in patients with HVFDs is possible by means of gaze scanning. This strategy allows continuous update of the moving objects' spatial location and selection of the task-relevant ones, which will be represented in visual working memory.

Does age matter? Age and rehabilitation of visual field disorders after brain injury

Homonymous visual field disorders (HVFD) are frequent and disabling consequences of acquired brain injury, particularly in older age. Their rehabilitation is therefore of great importance. Compensatory oculomotor therapy has been found to be effective in improving the associated functional impairments in reading and visual exploration. But older age is commonly considered to adversely affect practice-dependent functional plasticity and, thus, functional and rehabilitation outcome after acquired brain injury. The effect of age in the compensatory treatment of HVFD, however, has never been investigated hitherto. It remains unknown whether age determines not only patients' functional impairments but also the rehabilitation outcome and the required amount of treatment. We therefore present the first study to determine the effect of age in 38 patients with HVFD receiving compensatory oculomotor treatment for their reading and visual exploration impairments. We investigated whether older patients with HVFD (1) show more pronounced impairments and less spontaneous adaptation, (2) show lesser compensatory treatment-related improvement in reading and visual exploration, and (3) require a higher amount of treatment than younger patients. Our main finding is that older patients achieve the same treatment-induced improvements in reading and visual exploration with the same amount of treatment as younger patients; severity of functional impairment also did not differ between older and younger patients, at least in reading. Age does not seem to be a critical factor determining the functional and rehabilitation outcome in the compensatory treatment of HVFD. Older age per se is not necessarily associated with a decline in practice-dependent functional plasticity and adaptation. To the contrary, the effectiveness of compensatory treatment to reduce the functional impairments to a similar extent in younger and older patients with HVFD adds to the growing evidence for a life-long potential for adaptation to the adverse effects of brain injury.

The relationship between adult neuropsychological profiles and diabetic patients' glycemic control

The purpose of this study was to assess, in relation to metabolic control, the cognitive, depressive, and anxiety symptoms among 40 adult patients (age: 18-60 years) with either type 1 (n = 28) or type 2 (n = 12) diabetes mellitus (DM1, DM2). Nineteen healthy subjects matched for age, gender, and education served as the control group. For most cognitive domains, no significant performance differences were found between subjects from the diabetic groups and control subjects. However, diabetes patients demonstrated reduced information processing accuracy along with impaired visual and verbal working memory performance. In addition, psychopathology scores were significantly elevated but did not reach the clinical criteria for depression or anxiety. Overall, there were no significant differences between diabetic subgroups, and no significant correlation was found between cognitive performance, psychopathology scores, and HbA1c values for either subgroup. Thus, patients with DM1 or DM2 may show mild-to-moderate cognitive impairment as well as subtle psychopathological symptoms. While cognitive impairments may be understood in terms of diabetes-associated cognitive dysfunction, psychopathological symptoms may also result from unsuccessful coping with high task demands in everyday life activities. The outcome of the current study underscores the importance of early clinical neuropsychological standardized assessment as well as the diagnosis of cognitive and psychopathological symptoms in adult patients with diabetes.

Interhemispheric balance of overt attention: a theta burst stimulation study

Interhemispheric imbalance is discussed as a pathophysiological mechanism in visuospatial neglect. It is suggested that after a lesion of the right hemisphere the mutual transcallosal inhibition is impaired, resulting in an increased activity of the left hemisphere. We investigated the interhemispheric balance of attention in healthy subjects by using a free visual exploration task and by interfering with the neural activity of the posterior parietal cortex (PPC) of either hemisphere using an inhibitory transcranial magnetic stimulation routine with theta burst stimulation (TBS). Subjects explored colour photographs of real-life scenes presented on a computer screen under four conditions: (i) without TBS; (ii) after TBS over the right PPC; (iii) after TBS over the left PPC; and (iv) after TBS over the right PPC and, after the first half of the task, over the left PPC. Eye movements were measured, and distribution of mean cumulative fixation duration over screen halves was analyzed. TBS over the right PPC resulted in a significant rightward shift of mean cumulative fixation duration of approximately 30 min. The shift could be reversed when a subsequent train of TBS was applied over the left PPC. However, left PPC stimulation alone had no significant effect on visual exploration behaviour. The present study shows that the effect of TBS on the PPC depends on which hemisphere is stimulated and on the state of the contralateral homologue area. These findings are in accordance with the predictions of the interhemispheric rivalry model in neglect.

Differential organization of gamma-aminobutyric acid type A and glycine receptors in the somatic and dendritic compartments of rat abducens motoneurons

Premotor inhibitory neurons responsible for the decrease in the firing discharge during fast or slow eye movements selectively target the cell bodies and the dendrites of abducens motoneurons. Gamma-aminobutyric acid (GABA) and glycine, the main inhibitory synaptic neurotransmitters in the central nervous system, act via glycine and GABAA receptors, assembled from various types of subunits, which determine the kinetics of the currents mediated. Therefore, our hypothesis was that the expression of the inhibitory receptors on the somatic and the dendritic compartments, involved in different functions, may differ. In this study, we compared the subcellular patterns of expression of the main GABAA receptor subunits (GABAARalpha1, alpha2, alpha3, alpha5), glycine receptors (GlyRalpha1), and gephyrin in the somatic and dendritic compartments of rat abducens motoneurons, using double or triple immunocytochemical experiments with confocal microscopy. Significant differences exist in the patterns of organization and the synaptic expression of the GlyR and GABAAR subunits in the cell bodies and dendrites of abducens motoneurons. In the somata, only the GABAARalpha1 subunit was expressed, whereas both GABAARalpha1 and GABAARalpha3 were present in the dendrites. The GlyRalpha1 to GABAARalpha1 density ratio was reversed in the somatic and dendritic compartments (0.9 vs. 2.3). A quantitative electron microscopy study showed that the modes whereby gephyrin reaches its postsynaptic inhibitory synaptic target differ between the somata and the dendrites. Therefore, our results support the idea that a structure-function adaptation occurs at the single-neuron level.

NovaVision: vision restoration therapy

PURPOSE OF REVIEW

The aim of this article is to review the controversial findings for NovaVision's vision restoration therapy.

RECENT FINDINGS

It has been claimed that NovaVision's computerized therapy results in expansion of the visual field in optic nerve and occipital lesions, but the outcome has been challenged on the grounds of unsatisfactory perimetric control of central fixation and disputed mechanisms.

SUMMARY

In clinical practice NovaVision's therapy should not currently gain acceptance in view of unacceptable perimetric standards and equivocal results. Possible effects on a relative scotoma at the edge of a lesion have not been adequately explored. In the interim, research should also be focused on compensatory eye movement strategies.

Exploring the visual world: The neural substrate of spatial orienting

Inspecting the visual environment, humans typically direct their attention across space by means of voluntary saccadic eye movements. Neuroimaging studies in healthy subjects have identified the superior parietal cortex and intraparietal sulcus as important structures involved in visual search. However, in apparent contrast, spatial disturbance of free exploration typically is observed after damage of brain structures located far more ventrally. Lesion studies in such patients disclosed the inferior parietal lobule (IPL) and temporo-parietal junction (TPJ), the superior temporal gyrus (STG) and insula, as well as the inferior frontal gyrus (IFG) of the right hemisphere. Here we used functional magnetic resonance imaging to investigate the involvement of these areas in active visual exploration in the intact brain. We conducted a region of interest analysis comparing free visual exploration of a dense stimulus array with the execution of stepwise horizontal and vertical saccades. The comparison of BOLD responses revealed significant signal increases during exploration in TPJ, STG, and IFG. This result calls for a reappraisal of the previous thinking on the function of these areas in visual search processes. In agreement with lesion studies, the data suggest that these areas are part of the network involved in human spatial orienting and exploration. The IPL dorsally of TPJ seem to be of minor importance for free visual exploration as these areas appear to be equally involved in the execution of spatially predetermined saccades.

Visual search for item- and array-centered locations in patients with left middle cerebral artery stroke

In this study we systematically explored the impact of left hemisphere (LH) lesions on array-centered and item-centered spatial attention. We investigated 16 LH first ever stroke patients, focusing on strokes of the Middle Cerebral Artery (MCA), and 15 healthy control subjects with a parallel and serial search paradigm. None of the LH patients had a hemianopia or neglect. We systematically varied the item-centered (left- or right-side of a single item) and the array-centered position (left or right position in the search array of ten items) of critical features. Lesion sites were evaluated using MRIcro (Version 1.37; Rorden and Brett, 2000). The results show that patients had no specific problem with parallel search. In serial search patients showed a left to right gradient-like increase in response time for array-positions and they omitted more items if the critical feature was located on the right side of the items in the right half of the array. For low performing patients we found an overlapping lesion area around and anterior to the precentral sulcus (Brodmann's area 6 and 44), encompassing the frontal eye field. We conclude that LH MCA strokes may lead to search impairments in spatial attention, in particular in shifting to the right side of the visual field. Impaired rightward shifting moreover reduces the chance of detecting right-sided item features (but not left-sided). This suggests that spatial attention works with different reference frames, with spatial orientation being more basic than analyzing spatial aspects of objects.

Cerebellar infarction affects visual search

Recent studies have discussed the role of the cerebellum in not only motor but also cognitive functions, and in particular, fronto-executive operations. Similar to a previous study on hemineglect patients, we recorded eye movements during a visual search task to investigate patients with isolated infarction of the cerebellum compared with controls. Patients showed longer search durations, associated with mild saccadic dysmetria, longer single fixation durations and a higher number of repeated fixations of items. Systematic search strategies were preserved, but less frequent in patients. In conclusion, though basic mechanisms of visual search including spatial memory were not affected by cerebellar lesions, patients' search behaviour was slower and less efficient, indicating a mild deficit of visual attention and motor planning.