Reaction times and perceptual adjustments are sensitive to the illusory distortion of space

The Oppel-Kundt illusion (OKI) consists of the perception of a filled space as larger than an empty space of the same size. Here, we used a modified version of that illusion composed of a gradient of vertical lines whose spacing decreased progressively from one side to the other: space is expected to be perceived as larger where the lines are more compressed. We tested the hypothesis that a horizontal stimulus presented in a space perceived as larger will produce faster RTs by asking forty-four healthy subjects to respond as quickly as possible to lateralized stimuli (horizontal bars, vertical bars and circles) presented on different backgrounds (control condition: evenly spaced vertical lines or an empty space; illusory conditions: vertical lines progressively compressed to the right or the left). Subjects’ RTs were reliably faster for horizontal stimuli presented on the space perceived as larger than on the space perceived as smaller. To verify that this effect was actually due to a size illusion, the same subjects were asked to adjust the size of the stimuli presented on the same backgrounds as to make them equal to a reference stimulus. For horizontal stimuli, subjects produced adjustments in accordance with the predicted effect of the illusion. Together, these data show that the OKI produces a distortion of space that extends to stimuli placed in front of it and that RTs are influenced by the perceived and not the physical size of the stimuli. Implications for neural bases of illusions and for spatial neglect are discussed.     

Exploring the syndrome of spatial unilateral neglect through an illusion of length

Both a neuropsychological syndrome (unilateral spatial neglect) and a visual illusion of length (the Brentano version of the Müller-Lyer illusion) bring about a misjudgement of the subjective centre of a horizontal line, with a unilateral shift. In experiment 1 we investigated, in patients with left unilateral neglect, illusory effects of horizontal length, with the aim of exploring the functional and neural basis of horizontal space perception, and the role of visual processing in shaping the patients' bisection performance. Fourteen right-brain-damaged patients with left spatial unilateral neglect, seven with and seven without left visual half-field deficits (assessed by confrontation, perimetry, and visual event-related potentials), entered this study. Two conditions of manual line bisection were assessed: setting the mid-point of a horizontal line, and of the shaft of the Brentano-Müller-Lyer illusion, with either a left- or a right-sided expansion. Both groups of patients set the subjective midpoint to the right of the objective centre of the line, consistent with the presence of left neglect. Patients with neglect and left hemianopia showed no illusory effects and a greater bisection error. The effects of the illusion, by contrast, were fully present in neglect patients without hemianopia, in both illusory conditions, adding to, or subtracting from, the rightward bisection bias. Anatomoclinical correlations revealed an association of damage to the occipital regions with the lack of illusory effects. Conversely, more anterior damage, sparing these regions, did not disrupt the illusion, revealing a dissociation between visual and spatial processing of extension. These findings suggest that processing of the Müller-Lyer illusion of length is likely to occur in the occipital cortex, at a retinotopic level of representation. In neglect patients with left homonymous hemianopia the visual deficit adds to the spatial bias, yielding a greater error in line bisection, but not in other visual exploratory tasks, such as cancellation, where the contribution of retinotopic frames is likely to be comparatively minor. Experiment 2 showed preserved illusory effects in patients with homonymous visual field defects without spatial unilateral neglect, suggesting that preserved spatial processing may compensate for unilateral visual field defects.     

Neglect’s perspective on the Ponzo illusion

Visual illusions have been used to explore implicit perception in neglect. Previous studies have highlighted differences between length and surface illusion perception in neglect, but much less is known about depth illusion perception. In the Ponzo illusion (a classic depth illusion), two converging oblique lines modulate the perceived length of two horizontal lines. In the current study, we presented modified versions of the Ponzo illusion in which only one of the converging oblique lines was presented (alternatively the right or the left one). This manipulation allowed us to explore (1) how acute patients with neglect process depth illusions, and (2) whether awareness of both converging lines is necessary for the full effect of the illusion. To examine these questions, we had participants (i.e. healthy controls, patients with neglect and right brain-damaged patients) to make a perceptual judgment regarding the perceived length of the upper versus lower horizontal line within the Ponzo frame in four conditions: (1) the classic Ponzo illusion, (2) a “modified left” Ponzo illusion with a single oblique line on the left, (3) a “modified right” Ponzo illusion with a single oblique line on the right and (4) a control condition with parallel lines. The results indicated that all participants perceived the canonical Ponzo illusion and the modified right illusion. Critically, patients with neglect did not perceive the modified left illusion. In addition, for neglect patients, there was no difference in the strength of the perceived illusion when comparing the canonical illusion with the modified right illusion. Importantly, single case analysis revealed a high degree of variability in the neglect group that seemed to be linked with the amount of damage to occipital areas. Overall our results indicate that: (1) the classic Ponzo illusion might be perceived in neglect patients based solely on perception of the right side of the stimulus configuration, and (2) differences between types of illusions (i.e. depth vs. length), and variability between patients suggest that caution is needed when utilizing these kinds of illusions to assess implicit processing in neglect.     

Cognitive neuroscience of hemispatial neglect

Introduction. Patients with neglect often fail to be aware of stimuli located on their contralesional side - the side of space opposite their brain damage. Some patients show impairments in extrapersonal space; others within personal (body) space; and some in both. Although the neglect syndrome may be associated with a distorted perception of extrapersonal or personal space, recent investigations suggest that attentional, memory and motor deficits may be important additional components. Conclusion. Better understanding of these cognitive impairments holds the key to offering effective treatment for neglect, as well as providing important insights into normal brain function.     

Underestimation of contralateral space in neglect: a deficit in the “where” task

Space perception was investigated in right brain damaged patients with ( N=13) and without neglect ( N=5; control group). Patients were requested to localise a target tachistoscopically flashed at various eccentricities along the horizontal meridian. All patients had an intact visual field and spared ability to manually point to a target. To segregate magno- and parvo-pathway activity, stimuli modulated in either luminance or chromatic contrast were used. Patients were required to verbally report the stimulus position (verbal task) or to manually point to the stimulus (pointing task). Neglect patients reported the stimuli in the left visual field closer to the centre than they actually were. In the verbal test, underestimation was about 7 deg at the most eccentric position tested (20 deg), and decreased linearly for smaller eccentricities. The effect was similar but less marked in the pointing task. No difference was found for stimuli with luminance or chromatic contrast. Space underestimation was confined to the contralesional space; no evidence of misperception was detected in the ipsilesional hemifield. The present findings are consistent with the view that contralesional space representation is compressed in neglect patients.     

Cognitive neuroscience of hemispatial neglect

INTRODUCTION: Patients with neglect often fail to be aware of stimuli located on their contralesional side--the side of space opposite their brain damage. Some patients show impairments in extrapersonal space; others within personal (body) space; and some in both. Although the neglect syndrome may be associated with a distorted perception of extrapersonal or personal space, recent investigations suggest that attentional, memory and motor deficits may be important additional components. CONCLUSION: Better understanding of these cognitive impairments holds the key to offering effective treatment for neglect, as well as providing important insights into normal brain function.     

Interactions between perception and action programming: evidence from visual extinction and optic ataxia

We report a series of 7 experiments examining the interaction between visual perception and action programming, contrasting 2 neuropsychological cases: a case of visual extinction and a case with extinction and optic ataxia. The patients had to make pointing responses to left and right locations, whilst identifying briefly presented shapes. Different patterns of performance emerged with the two cases. The patient with "pure" extinction (i.e., extinction without optic ataxia) showed dramatic effects of action programming on perceptual report. Programming an action to the ipsilesional side increased extinction (on 2-item trials) and tended to induce neglect (on 1-item trials); this was ameliorated when the action was programmed to the contralesional side. Separable effects of using the contralesional hand and pointing to the contralesional side were apparent. In contrast, the optic ataxic patient showed few effects of congruency between the visual stimulus and the action, but extinction when an action was programmed. This effect was particularly marked when actions had to be made to peripheral locations, suggesting that it reflected reduced resources to stimuli. These effects all occurred using stimulus exposures that were completed well before actions were effected. The data demonstrate interactions between action programming and visual perception. Programming an action to the affected side with the contralesional limb reduces "pure" extinction because attention is coupled to the end point of the action. However, in a patient with deficient visuo-motor coupling (optic ataxia), programming an action can increase a spatial deficit by recruiting resources away from perceptual processing. The implications for models of perception and action are discussed.     

Perception of horizontal distances in patients with spatial neglect

Two different models have argued that neglect of contralateral stimuli following brain damage might be associated with either a compressed or an anisometric neural representation of space along the earth-horizontal axis. We tested these models by determining neglect patients’ perception of spatial distances in the horizontal plane. We found no evidence for any compression or expansion or for anisometry along the earth-horizontal axis. The findings argue against a distortion of subjective space along the horizontal axis in patients with neglect, which could account for their failure to orient towards and to explore the contralesional parts of space.     

Examining the crossmodal consequences of viewing the Müller-Lyer illusion

For many years, the Müller-Lyer illusion was studied as a purely visual illusion, but like many other optical illusions, the evidence now shows that it also occurs when stimuli are presented tactually. In the present study, we investigated whether the visual perception of the illusion would have any crossmodal consequences for haptic perception. The wings-in and wings-out parts of the Müller-Lyer illusion were placed end-to-end, sharing a central fin. This Brentano version of the illusion was presented visually on a screen in front of the participants, who had to compare the felt length of two sticks placed on the back of the screen, one behind either part of the illusion. Our results show that the presentation of the visual illusion modified the felt lengths of the sticks presented directly behind the illusion. In particular, the stick presented on the side of space perceived visually as being shorter (behind the wings-in part of the display) was perceived as longer, and vice versa for the stick mounted behind the space perceived visually as longer (behind the wings-out part of the display). These results highlight the crossmodal consequences of the visual perception of the Müller-Lyer illusion for the haptic perception of line length.     

忽视症多感觉通道缺陷研究的新进展

忽视症可表现为多感觉通道缺陷。该文着重综述了与视觉忽视并存的听觉忽视。视觉忽视的病人常表现出对病损对侧声音的定位缺陷，尤其是当病损同侧存在竞争性声音刺激时，视觉和听觉缺陷的严蘑程度有硅著的相关性。神经影像学研究结果表明，忽视症病人受损的脑区可能与多感觉通道的空间表征有关。     

Ocular exploration of space as a function of neck proprioceptive and vestibular input — observations in normal subjects and patients with spatial neglect after parietal lesions

We recently argued that the specific compensation of spatial neglect by manipulating neck proprioceptive and vestibular input is due to a central correction of the disturbed neural transformation process converting the afferent input coordinates from the peripheral sensory organs into a central representation of egocentric space. Both types of stimulation were proposed to induce a reorientation of the deviated or distorted egocentric spatial reference frame. The aim of the present study was to observe this process of reorientation under a condition in which no visual stimulus can attract the subject's attention and thus influence exploration behaviour from outside. We recorded eye movements of normal subjects and of three patients with spatial neglect after right parietal lesions while searching for a non-existent target in complete darkness. It was assumed that the area of the outer space that subjects spontaneously explore under this condition is a direct function of the subject's representation of egocentric space. Ocular space exploration was biased and confined almost entirely to the right side of the midsagittal plane in patients with neglect. This spatial distribution of exploratory eye movements changed remarkably with left-sided neck muscle vibration as well as with left-sided vestibular stimulation using ice water calorics. The spatial area of exploration was significantly enlarged to the contralesional side and the exploration maximum shifted in the same direction. Whereas with both types of stimulation space exploration of patients with neglect was similar to that of normal subjects when not being stimulated, neck proprioceptive and vestibular stimulation in normal subjects induced a quasi neglect-like exploration pattern, i.e. a bias to one side of the objective midsagittal plane. If ocular space exploration was, however, related to the subjectively perceived position of the midsagittal plane in space, eye movements were symmetrically distributed and carried out to both sides of subjective straight ahead in all experimental conditions, in normal subjects as well as in patients with neglect. The present results support the above hypothesis and indicate that neck proprioceptive as well as vestibular input directly contribute to the computation of the subject's central representation of egocentric space used for localizing body orientation and for guiding motor behaviour in space.     

The effects of competition and motor reprogramming on visuomotor selection in unilateral neglect

Patients with unilateral neglect following right hemisphere damage may have difficulty in moving towards contralesional targets. To test the hypothesis that this impairment arises from competing motor programs triggered by irrelevant ipsilesional stimuli, we examined 16 right hemisphere patients, eight with left visual neglect and eight without, in addition to eight healthy control subjects. In experiment 1 subjects performed sequences of movements using their right hand to targets on the contralesional or ipsilesional side of the responding limb. The locations of successive targets in each sequence were either predictable or unpredictable. In separate blocks of trials, targets appeared either alone or with a simultaneous distractor located at the immediately preceding target location. Neglect patients were significantly slower to execute movements to contralesional targets, but only for unpredictable movements and in the presence of a concurrent ipsilesional distractor. In contrast, healthy controls and right hemisphere patients without neglect showed no directional asymmetries of movement execution. In experiment 2 subjects were required to interrupt a predictable, reciprocating sequence of leftward and rightward movements in order to move to an occasional, unpredictable target that occurred either in the direction opposite to that expected, or in the same direction but twice the extent. Neglect patients were significantly slower in reprogramming the direction and extent of movements towards contralesional versus ipsilesional targets, and they also made significantly more errors when executing such movements. Right hemisphere patients without neglect showed a similar bias in reprogramming direction (but not extent) for contralesional targets, whereas healthy controls showed no directional asymmetry in either condition. On the basis of these findings we propose that neglect involves a competitive bias in favour of motor programs for actions directed towards ipsilesional versus contralesional events. We suggest that programming errors and increased latencies for contralesional movements arise because the damaged right hemisphere can no longer effectively inhibit the release of inappropriate motor programs towards ipsilesional events. Received: 1 October 1996 / Accepted: 21 October 1997     

Interactions between perception and action programming: Evidence from visual extinction and optic ataxia

We report a series of 7 experiments examining the interaction between visual perception and action programming, contrasting 2 neuropsychological cases: a case of visual extinction and a case with extinction and optic ataxia. The patients had to make pointing responses to left and right locations, whilst identifying briefly presented shapes. Different patterns of performance emerged with the two cases. The patient with “pure” extinction (i.e., extinction without optic ataxia) showed dramatic effects of action programming on perceptual report. Programming an action to the ipsilesional side increased extinction (on 2-item trials) and tended to induce neglect (on 1-item trials); this was ameliorated when the action was programmed to the contralesional side. Separable effects of using the contralesional hand and pointing to the contralesional side were apparent. In contrast, the optic ataxic patient showed few effects of congruency between the visual stimulus and the action, but extinction when an action was programmed. This effect was particularly marked when actions had to be made to peripheral locations, suggesting that it reflected reduced resources to stimuli. These effects all occurred using stimulus exposures that were completed well before actions were effected. The data demonstrate interactions between action programming and visual perception. Programming an action to the affected side with the contralesional limb reduces “pure” extinction because attention is coupled to the end point of the action. However, in a patient with deficient visuo-motor coupling (optic ataxia), programming an action can increase a spatial deficit by recruiting resources away from perceptual processing. The implications for models of perception and action are discussed.     

Bimanual coordination and perceptual grouping in a patient with motor neglect

Motor neglect refers to the underutilisation of a limb contralateral to a brain lesion in the absence of primary motor and sensory deficits. The related problem of motor extinction refers to a contralesional motor deficit that worsens or only becomes apparent when bilateral actions are required. We present a single case (MM) of a patient with motor neglect who also demonstrates a form of motor extinction that is influenced by visual grouping between stimuli. The comparisons of unimanual and bimanual reach to grasp movements towards one or two objects in Experiment 1 showed that MM made relatively normal unimanual contralesional movements but impaired contralesional movements under bimanual action conditions. Experiment 2 demonstrated that motor extinction was improved by asking MM to make bimanual movements towards a single object. In Experiment 3, the effects of object coding on bimanual movement were replicated across conditions that varied the distance between end points for the movements. MM did not show overt visual extinction. We suggest that MM demonstrates a late-acting attentional bias that is expressed in terms of competitive motor activity. Normally, the contralesional limb "loses" the competition for action, but this can be modulated by visual grouping between targets.     

Anti-extinction Following Unilateral Parietal Damage

Following unilateral damage to the parietal cortex, extinction of sensory events on the side of space opposite the lesion frequently occurs: A contralesional event is detected when it occurs alone, but missed when it is accompanied by an ipsilesional event. We describe a patientwith left visuospatial neglect subsequent to a right parietal infarct, who shows the opposite of this typical extinction pattern: On brief visual presentations, he consistently shows better detection and identification of contralesional targets when they appear simultaneously with an ipsilesional target, compared to when the contralesional target appears alone. This benefitfor contralesional targets on simultaneous presentation is found most clearly when the same task is performed with both contra- and ipsilesional items. These findings first provide a very clear demonstration of neglect in the absence of extinction. Second, our results demonstrate that visuospatial neglect can be mitigated by non-perceptual task variables.     

Extinction is not a natural consequence of unilateral spatial neglect: evidence from contrast detection experiments

To investigate whether the expression of visual extinction is dependent upon the contralesional low saliency existing in neglect, we tested stroke patients with neglect and extinction, as well as normal controls, on detection of a peripheral Gabor patch, while a competing patch was presented simultaneously on the other side. To compensate for uneven saliency we set the contrast level relative to the detection threshold on each side. Patients showed contralesional extinction even for stimuli set at threshold level. They differed from controls in their sensitivity to changes in relative contrast between sides, showing stronger tendency for extinction and requiring much higher contrast increments in the target patch in order to eliminate extinction. The differences between normal and pathological extinction, shown despite compensation for contralesional perceptual attenuation due to neglect, suggest an additional extinction-specific deficit related to an abnormal interplay between the bilaterally presented stimuli. These findings have important theoretical implications concerning the relationship between neglect and extinction. They demonstrate that the hypothetical ‘attentional gradient’, taken to explain reduced saliency of stimuli in the neglected side, cannot fully account for the phenomenon of extinction.     

The interactive contribution of neck muscle proprioception and vestibular stimulation to subjective “straight ahead” orientation in man

Seventeen normal subjects were asked to direct a laser point to the position they felt to lie exactly straight ahead of their body. Subjects were seated in complete darkness in an approximately spherical cabin in an upright position with the orientation of the trunk and head aligned. For both the horizontal and vertical plane, straight ahead judgements were closely scattered around the objective straight ahead body position. Posterior neck muscle vibration as well as caloric vestibular stimulation with ice water led to (1) an apparent motion and horizontal displacement of a stationary visual target to the side opposite to stimulation and (2) a horizontal deviation of subjective straight ahead perception toward the side of stimulation. Only those subjects who experienced an illusion of target motion also showed a deviation of their subjective body orientation. No systematic effect of a displacement of subjective body orientation in the vertical plane was detected. When vestibular stimulation and neck muscle vibration were combined their effects were additive, i.e. the horizontal deviation of subjective body orientation observed when either type of stimulation was applied in isolation, was linearly combined either by summation or by cancellation. The present results clearly support the assumption that afferent visual, vestibular and proprioceptive input converge to the neural generation of an egocentric, body-centred coordinate system that allows us to determine our body position with respect to visual space.     

Simulating and testing visual exploration in spatial neglect based on a new model for cortical coordinate transformation

Most studies of object and space perception have focused on neural representations of either object-centered or egocentric coordinate systems. But daily life requires interactions of both kinds of coordinates. We have recently proposed an 'integrated space-object (ISO-) map' combining both coordinate systems in one representation. Based on a lesioned version of this model, here we present results from visual search simulations demonstrating that the model accounts for contralesional neglect during space-centered and object-centered exploration tasks. Interestingly, the model simulations also predicted an amelioration of neglect symptoms during exploration with more ipsilesional object positions. By measuring the eye movements of neglect patients during different exploratory tasks, we confirmed all model predictions. These results corroborate the view that the brain might combine coordinates for object and space perception in an integrated coordinate system as suggested by the ISO-map model.     

Visual illusion in virtual world alters women’s target-directed walking

In this study we investigated whether a visual illusion located in far space alters a persons open-loop, target-directed walking path in the same manner as it alters the perception of the targets position. Through the use of immersive VR the subject was able to walk physically to the location of a target embedded in a scene that was manipulated to create a visual illusion, known as the induced Roelofs effect. This illusion has been shown to alter the perception of a targets position. The experiment consisted of two tasks: a perception task and an action task. In the perception task, subjects viewed the scene for 1 s, it disappeared, and they were to report the targets location verbally. The results showed that the visual illusion altered the reported positions in all but one subject. In the action task, subjects viewed the scene for 1 s, it disappeared, and the subjects were asked to walk to the targets location. The results showed that the illusion significantly altered the walking paths of most of the women and less than half of the men. A significant gender effect was observed; womens walking paths deviated, on average, by 7.1° and mens, by only 2.0°. These results indicate that action tasks in far space are susceptible to the effects of visual illusions, unlike the action tasks in near space that reportedly have been resistant to them. Furthermore, the significant gender effect suggests that men and women either have different strategies and/or employ different mechanisms when executing a visually guided task in far space.Supported by the NIH, National Eye Institute grant EY07839 to KAT     

Bimanual coordination and perceptual grouping in a patient with motor neglect

Motor neglect refers to the underutilisation of a limb contralateral to a brain lesion in the absence of primary motor and sensory deficits. The related problem of motor extinction refers to a contralesional motor deficit that worsens or only becomes apparent when bilateral actions are required. We present a single case (MM) of a patient with motor neglect who also demonstrates a form of motor extinction that is influenced by visual grouping between stimuli. The comparisons of unimanual and bimanual reach to grasp movements towards one or two objects in Experiment 1 showed that MM made relatively normal unimanual contralesional movements but impaired contralesional movements under bimanual action conditions. Experiment 2 demonstrated that motor extinction was improved by asking MM to make bimanual movements towards a single object. In Experiment 3, the effects of object coding on bimanual movement were replicated across conditions that varied the distance between end points for the movements. MM did not show overt visual extinction. We suggest that MM demonstrates a late-acting attentional bias that is expressed in terms of competitive motor activity. Normally, the contralesional limb “loses” the competition for action, but this can be modulated by visual grouping between targets.