Contralesional Stimuli Research Articles

Alexander's law during high-acceleration head rotations in humans

Alexander's law states that the amplitude of the spontaneous nystagmus grows with increasing gaze in the direction of the fast phase. Using the search-coil method we employed head impulses at various eye-in-orbit azimuth angles to test (i) whether the normal vestibulo-ocular reflex (VOR) in the behaviorally relevant high-frequency range has intrinsic properties that could account for Alexander's law and (ii) whether such properties can also be shown in patients with unilateral vestibulopathy. We showed that the gain of the VOR remained unaffected by eye-in-orbit position in contols and in patients, both on ipsilesional and contralesional stimuli. These findings suggest that eye-in-orbit position does not directly modulate the activity in VOR pathways, neither during unbalanced but reciprocal (in controls), nor during unbalanced and nonreciprocal natural vestibular stimulation (in patients).

Neurobiologische Grundlagen des Neglects: Implikationen für neue Therapieansätze

Neglect is characterised by a clinically relevant impairment of spatial perception and awareness observed after focal brain damage. The neglect-specific deficit in responding to contralesional stimuli in various sensory modalities is most commonly explained by impairments of spatial attention, spatial working memory, impaired spatial reference frames and lateralised movement initiation. Many neglect interventions have been developed in the last decades to alleviate symptoms of neglect in stroke patients. However, hardly any of these methods has been proven to significantly ameliorate the patient's deficits in everyday activities. Thus, the identification of predictors for recovery of function and of the success of rehabilitative measures provides a major challenge for future research on neglect.

Distinguishing non-spatial from spatial biases in visual selection: Neuropsychological evidence

Recently, it has been shown that patients with visual extinction can show enhanced awareness of contralesional stimuli that match the contents of working memory [Soto, D., & Humphreys, G.W. (2006). Proceedings of the National Academy of Sciences, USA, 103, 4789–4792]. Here we investigate whether these effects extend to cases where the working memory cue is verbal rather than visual (Study 1), and we show that non-spatial cues can overcome problems in spatial disengagement in patients, and this can affect the first eye movement made in search (Study 2). We discuss the implications of the data for understanding the relations between the cueing of attention from visual and conceptual information held in working memory and the cueing of spatial attention, and for understanding the relations between non-spatial and spatial biases in selection.

Asynchronous stimulus presentation in visual extinction: A psychophysical study

Patients showing visual extinction as a consequence of a unilateral brain lesion can correctly detect a single stimulus in either hemifield but fail to detect the contralesional stimulus (or at least process the stimulus less efficiently) when it is presented simultaneously with an ipsilesional stimulus. In an attempt to uncover the nature of the underlying deficit, some studies have manipulated the temporal characteristics of stimulus presentation. Contra- and ipsilesional stimuli with different stimulus onset asynchronies are typically used. In the present study, visual extinction was investigated in a group of left neglect patients (N=10) using a psychophysical paradigm with different stimulus onset asynchronies of target and distractor stimuli presented in different hemifields. Contrast thresholds for a target grating were determined with the target either in isolation or in the presence of an irrelevant distractor grating. When target and distractor gratings were presented simultaneously, neglect patients showed a significant extinction effect, i.e., a significant interference from the right hemifield distractor with left hemifield contrast sensitivity. When the right hemifield distractor preceded the left hemifield target stimulus by 250 ms, two different patterns of results were observed in the neglect patients. Five patients showed a significant improvement compared to the simultaneous presentation condition, five other patients showed a significant increase of the extinction effect. The results suggest that different underlying mechanisms, maybe due to different lesion locations, can cause extinction in neglect patients.

Repetitive long-term prism adaptation permanently improves the detection of contralesional visual stimuli in a patient with chronic neglect

The aim of the current study was to investigate long-term effects in spatial awareness after daily exposure to prism adaptation during three months in a patient with hemispatial neglect. Results showed improvement in the detection of stimuli in the contralesional visual field, as measured with perimetry, in the contralesional visual field up to 24 months after ending prism adaptation. These perimetrical results suggest that compensatory eye movements are an unlikely candidate for an underlying mechanism.

Horizontal and vertical dimensions of visual extinction: a theta burst stimulation study

After a lesion of the posterior parietal cortex (PPC), the perception of a contra-lesional stimulus in presence of a simultaneous, ipsilesional stimulus may be impaired, a phenomenon referred to as visual extinction. In the present study, visual extinction was transiently induced in healthy subjects by interfering with the function of the right PPC by means of continuous theta burst stimulation (TBS). We investigated to which extent the horizontal and vertical position of visual stimuli influenced the extinction rate. A single TBS train over the right PPC induced a significant increase of left visual extinctions of at least 30 min. Left visual extinction rate was higher when the left sided visual stimulus was presented at a more eccentric position on the horizontal axis (irrespective of right sided visual stimulus position) and in the lower part of the visual field. The results are discussed within the framework of current explanatory models and of putative inter- and intrahemispheric mechanisms directing visuospatial attention.

Parallel interhemispheric processing in hemineglect: Relation to visual field defects

Parallel interhemispheric processing is required to explore our visual environment and to integrate visual information from both hemifields simultaneously. Damage to the right temporo-parietal cortex can disrupt such parallel processes and result in neglect and visual extinction of stimuli in the left contralesional visual space. Neglected or extinguished stimuli can still be processed, yet without reaching the patient's awareness. Such unconscious processing has been attributed to structurally intact primary visual areas in neglect. To study whether unconscious parallel processing depends on visual functional integrity, we compared the performance of neglect patients with visual field defects (VFDs) (n=11) and hemianopic patients with partial or complete blindness of one visual hemifield (n=11) on redundant targets effects (RTE). The RTE manifests as faster reaction times to redundant paired (two stimuli, one in each hemifield) than single stimulation (in one hemifield). We found RTEs, i.e., unconscious processing, in neglect patients but not in hemianopic patients. Furthermore, neglect patients showed large crossed-uncrossed differences (CUDs), i.e., faster response times to ipsi- than contralesional hemifield stimulation, reflecting a difference in processing speed for single stimuli in the two hemispheres that were correlated with VFDs and visual extinction. The finding that extinction, but not RTE, was correlated with the CUD suggests that under competitive bilateral stimulus conditions the delayed contralesional visual field input may not be detected by the intact left hemisphere, which presumably mediates the task given the impairment of the right hemisphere. By contrast, unconscious parallel processing of contralesional stimuli (RTE) occurred even when contralesional visual field input is lacking (VFD) or delayed (CUD) and is possibly mediated via subcortical visual pathways.

Effects of Psychostimulants on Alertness and Spatial Bias in Healthy Participants

Converging evidence from neuropsychological and neuroimaging studies suggests that the ability to maintain an alert, ready-to-respond state is mediated by a network of right-hemisphere frontal and parietal cortical areas. This right lateralization may help to explain why visuospatial hemineglect, a cluster of deficits in detecting and responding to contralesional stimuli, is more common and persistent after right-hemisphere lesions. Indeed, it has been hypothesized that this asymmetry reflects a direct, functional link between alertness and spatial attention. In the present study, we investigated whether a pharmacologically induced increase in alertness would influence lateral bias in healthy people. Eighteen healthy participants were each given placebo or the psychostimulant drugs methylphenidate 40 mg or modafinil 400 mg on separate days and completed an hour-long version of the spatially sensitive landmark task. For those participants who demonstrated the expected alerting effect of modafinil, there was a significant Condition by Time interaction, consistent with the effects of the drug resisting time-on-task rightward drifts in spatial bias in the placebo condition. In contrast, no effect of methylphenidate on spatial bias was observed. These results suggest that spatial bias may be modulated by a psychostimulant-induced increase in alertness, supporting the hypothesis of a direct, functional link between right-hemisphere systems controlling alertness and visuospatial attention.

Preattentive surface and contour grouping in Kanizsa figures: Evidence from parietal extinction

Visual extinction commonly occurs after unilateral, parietal brain damage and manifests in a failure to identify contralesional stimuli when presented simultaneously with other, ipsilesional stimuli – but full awareness for single stimulus presentations. However, extinction can be substantially reduced when preattentive grouping operations link fragmentary items across hemifields into a coherent object. For instance, one study demonstrated preserved access to bilateral stimulus segments when these could be grouped to form a Kanizsa square [Mattingley, J. B., Davis, G., & Driver, J. (1997). Preattentive filling-in of visual surfaces in parietal extinction. Science, 275, 671–674]. Here, we investigated the relative contributions of distinct object attributes to the spared access in Kanizsa figure completion in extinction, by systematically varying the degree to which bilateral surface filling-in and contour interpolations group disparate items. We demonstrate that surface information can substantially reduce extinction, whereas contour completions showed comparably smaller influences. In summary, such graded influences of object attributes support recurrent models of grouping, first, linking fragmentary parts into coherent surfaces and, second, interpolating the precise boundaries.

Prism Adaptation Moves Neglect-related Perseveration to Contralesional Space

Hemi-neglect is a disorder characterized by a disregard of contralesional stimuli. Some neglect patients also show an "exaggerated attention" for the ipsilesional field, reflected in perseverative responses, such as repetitive fixations in the ipsilesional field, ipsilesional revisitings on standard cancellation, or exaggerations in drawings on the ipsilesional side. It is still unclear whether neglect and perseveration are due to a single underlying mechanism. In the present study, we will examine the effect of 4-day-in-a-row prism adaptation on neglect and perseveration severity in a patient with severe perseverations. Additionally, we will examine whether the position of omissions and perseverations on the Star Cancellation will change during the intervention. Our patient showed a decrease in neglect severity and an increase in perseveration severity, suggesting that perseveration and neglect are dissociated phenomena. Interestingly, 4-day-in-a-row prism adaptation gradually moved the predominant position of perseverative responses from right to left as neglect decreased with treatment. Perseverative responses do not necessarily occur in the ipsilesional sector of space as is generally assumed. Instead, the position of the revisits may be determined by the severity of the neglect, and may shift when the focus of attention moves more contralesionally with recovery.

Sustained interactions between perception and action in visual extinction and neglect: Evidence from sequential pointing

Interactions between perception and action were examined by assessing the effects of action programming on extinction and neglect. In an extension of prior work, effects of sequential motor programming were assessed under conditions in which attention was first directed to an ipsilesional stimulus. Despite pointing and reporting a stimulus on the ipsilesional side first, programming a second action to the contralesional side reduced the spatial deficit on report, improving the report of contralesional stimuli (relative to when the patient just pointed to the ipsilesional side) while decreasing the report of ipsilesional items. The data suggest that perception and action interact through motor feedback to early visual coding, helping a patient overcome a lack of visual awareness to contralesional stimuli. This is effective even when attention has to be disengaged from the ipsilesional side suggesting that motor programming decreases ipsilesional capture and exerts a sustained influence on perception.

When they see, they see it almost right: Normal subjective experience of detected stimuli in spatial neglect

Unilateral spatial neglect (USN) patients show reduced contrast sensitivity on their contralesional side and often miss their non-salient stimuli. What their subjective experience is when successfully reporting a stimulus remains unclear. Here, we report that despite large contrast sensitivity differences between the sides, the relative attenuation in perceived contrast measured in a contrast-matching task was small. This was true even at threshold levels where the patients missed up to 40% of the contralesional target patches, in contrast to a 100% detection rate on their ipsilesional side. When the misses were counted as zero perceived contrast events, the attenuation in perceived contrast was less than half of the sensitivity loss. When the misses were ignored, there was almost no attenuation in perceived contrast, implying that whenever the patients detected a target, they perceived it with the correct contrast. These findings suggest that contrast sensitivity reduction in USN is not due to attenuation occurring at a peripheral low-level processing stage. More likely it reflects a high-threshold added at a higher level of processing, which prevents sensory events from reaching conscious awareness. Hence, patients may often miss contralesional stimuli but see them in full contrast once they clear the high-level hurdle.

Impaired visual processing of contralesional stimuli in neglect patients: a visual-evoked potential study

Transient visual-evoked potentials (VEPs) were recorded in 11 patients with right brain damage and spatial neglect. High-resolution EEG was recorded using focal stimuli located in the four visual quadrants. VEPs to left stimuli, i.e. located in the neglected side, were compared to VEPs to right stimuli. Results showed that bottom-up processing of a visual stimulus located in the neglected hemifield was intact up to approximately 130 ms from stimulus onset. Hemispheric differences were not significant for either C1 or P1 components representing the activity of striate and extrastriate areas, respectively. In contrast, visual processing in more dorsal areas adjacent to the superior parietal lobe was changed from normal. We failed to record the N1a component for left visual field stimuli expected in the 130-160 ms time range. Furthermore, the N1p (140-180 ms) and P2 (180-220) components were delayed and/or reduced in amplitude for stimuli located on the neglected side. The source of the N1a was previously localized in the intraparietal sulcus in the dorsal occipital cortex; N1p may represent a reactivation of area V3A and P2 reactivation of occipital visual areas including V1 due to top-down feedbacks. Six patients with left brain damage (LBD) and no neglect and 21 healthy subjects were also tested in the same experimental conditions used for patients with neglect. In LBD patients, all components evoked by contralesional stimuli were comparable to ipsilesional components. Overall, data allow localizing in time and space the processing deficit specific for patients with neglect. The first takes place around 130 ms in the bottom-up processing at the level of the anatomically intact dorsal parietal areas; the second is located at the level of the reactivation of the striate and extrastriate areas via feedback connections from higher visual areas. The two functional impairments were limited to left-field stimuli.

A non-spatial bias favouring fixated stimuli revealed in patients with spatial neglect

The cardinal feature of spatial neglect is severely impaired exploration of the contralesional space, a failure resulting in unawareness of many contralesional stimuli. This deficit is exacerbated by a reflexive attentional bias toward ipsilesional items. Here we show that, in addition to these spatially lateralized failures, neglect patients also exhibit a severe bias favouring stimuli presented at fixation. We tested neglect patients and matched healthy and right-hemisphere damaged patients without neglect in a task requiring saccade execution to targets in the left or right hemifield. Targets were presented alone or simultaneously with a distracter that appeared in the same hemifield, in the opposite hemifield, or at fixation. We found two fundamental biases in saccade initiation of neglect patients: irrelevant distracters presented in the preserved hemifield tended to capture gaze reflexively, resulting in a large number of saccades erroneously directed toward the distracter. Additionally, distracters presented at fixation severely disrupted saccade initiation irrespective of saccade direction, leading to disproportionately increased latencies of left and right saccades. This latency increase was specific to oculomotor responses of neglect patients and was not observed when a manual response was required. These results show that, in addition to their failure to inhibit reflexive glances toward ipsilesional items neglect patients exhibit a strong oculomotor bias favouring fixated stimuli. We conclude that impaired initiation of saccades in any direction contributes to the deficits of spatial exploration that characterize spatial neglect.

Seeing Fearful Body Language Overcomes Attentional Deficits in Patients with Neglect

Survival depends to some extent on the ability to detect salient signals and prepare an appropriate response even when attention is engaged elsewhere. Fearful body language is a salient signal of imminent danger, easily observable from a distance and indicating to the observer which adaptive action to prepare for. Here we investigated for the first time whether fearful body language modulates the spatial distribution of attention and enhances visual awareness in neurological patients with severe attentional disorders. Patients with visual extinction and hemispatial neglect following right parietal injury have a rightward attentional bias accompanied by loss of awareness for contralesional left stimuli, especially when competing stimuli appear to the right. Three such patients were tested with pictures of fearful, happy, and neutral bodily expressions briefly presented either unilaterally in the left or right visual field, or to both fields simultaneously. On bilateral trials, unattended and task-irrelevant fearful bodily expressions modulated attentional selection and visual awareness. Fearful bodily expressions presented in the contralesional unattended visual field simultaneously with neutral bodies in the ipsilesional field were detected more often than left-side neutral or happy bodies. This demonstrates that despite pathological inattention and parietal damage, emotion and action-related information in fearful body language may be extracted automatically, biasing attentional selection and visual awareness. Our findings open new perspectives on the role of bodily expressions in attentional selection and suggest that a neural network in intact fronto-limbic and visual areas may still mediate reorienting of attention and preparation for action upon perceiving fear in others.

Temporal dynamics of visuo-tactile extinction within and between hemispaces.

The ability to detect left-sided stimuli (visual or tactile) was studied in visual and tactile extinction (RBD) patients and in healthy (C) subjects. Stimuli were single or double; double stimuli were always cross-modal and could be released across the 2 hemispaces or within the same hemispace. Moreover, the stimuli could be either simultaneous or separated by 1 of 3 different asynchronies (105, 505, 905 ms). C subjects were perfect in all conditions. RBD patients omitted the contralesional tactile stimulus in bilateral trials (across space, classical extinction). They also omitted the tactile stimulus in unilateral left trials (within hemispace). Both effects showed the same temporal modulation with lower extinction rates at longer stimulus onset asynchronies. Results suggest that attentional processing of a visual stimulus inhibits processing of a tactile one, even if both are delivered in the same contralesional hemispace.

A selective effect of parietal damage on letter identification in mixed case words

We investigated the reading of cAsE mIxInG and contrast reduction on word reading in patients with unilateral parietal lesions and attentional deficits. We show that, compared with control participants, the patients produce selective increases in lateralised errors when reading mixed case relative to same case words. However, there were not reliable increases in lateralised errors when words were degraded by low contrast. The patients also showed some increases in contralesional errors at a task aimed at feature processing in words (a gap detection task), but these effects were not increased for mixed case stimuli and errors were reduced relative to the word reading task. The results are consistent with mixed case words stressing attention-demanding letter identification, drawing-out an impairment in the patients in attending to contralesional stimuli. On the other hand, effects of contrast reduction are accommodated without necessarily recruiting attentional processes mediated by the posterior parietal lobe.

Action-perception dissociation; preserved reactive grip force despite tactile extinction due to cortical stroke

Sensory extinction following stroke, manifests as a bias in spatial attention towards ipsilesional spatial locations, which arises when stimuli from other locations compete for pathologically limited attentional resources. In the tactile domain, extinction results in a failure to verbally report tactile stimuli applied to a contralesional body part when they are timed to coincide with ipsilesional tactile contact. While it is typical for research in this area to focus on the verbal report of sensory stimuli as a measure of conscious awareness, work in visual extinction has shown that when contralesional stimuli fail to reach conscious awareness, they may still contribute to the control of actions. We describe the case of a woman with tactile extinction who failed to verbally report contralesional tactile input associated with perturbations to bimanual grasp. Despite this, the same stimulus was sufficient to drive reflexive grip force responses.

When supine is better than upright: Evidence from postural effects in extinction patients

There is no clear consensus on the specific effects of gravitational inputs on the manifestations of neglect and extinction. While some studies provided evidence that neglect patients improved their performance in supine position, other studies supported instead the notion that the manipulation of body posture has no effect on neglect and extinction. To investigate the effects of the postural manipulation of the entire body in space on visual extinction, a total of 24 right- and left-brain-damaged patients (with or without signs of visual extinction) and 8 healthy control subjects were tested in a visual detection task in two different gravitational conditions: upright and supine. Two dependent variables were considered: accuracy and speed of response. The results were clear-cut. Changing body posture had only a mild effect on the performance of healthy controls and non-extinction patients, but it clearly modulated the performance of extinction patients. In fact, extinction patients markedly improved their performance on contralesional stimuli in supine position in terms of both accuracy and speed of response. This suggests that the reduction of gravitational inputs obtained by placing patients in supine position results in a significant amelioration of visual extinction.

A dissociation between visual and auditory hemi-inattention: Evidence from temporal order judgements

Patients with right hemisphere brain lesions often suffer from deficits in spatial attention that can be manifested in different sensory modalities. It has recently been claimed that a relationship (i.e., association) could exist between symptoms of hemi-inattention in different modalities, based on correlations between the results of visual and auditory clinical tests of neglect or extinction. However, it should be noted that the visual and auditory tasks varied greatly both in response type and level of sensitivity. Here, we have examined cross-modal associations in spatial attention deficits using a temporal order judgment task (TOJ) in which patients were required to identify which of two visual or auditory objects had appeared first. When compared to age and education matched control participants, the patients needed, on average, the contralesional stimulus to lead the ipsilesional stimulus to achieve the point of subjective simultaneity (PSS). No association between the degree of visual and auditory hemi-inattention was observed amongst the patients, suggesting that there is a certain degree of independence between the mechanisms subserving spatial attention across sensory modalities.