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.     

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.     

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.     

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.     

Sensory and response contributions to visual awareness in extinction

Brain-damaged patients may extinguish contralesional stimuli when ipsilesional stimuli are presented simultaneously. Most theories of extinction postulate that stimuli compete for pathologically limited attentional resources with a bias to process ipsilesional over contralesional stimuli. Implicit in this view is the idea that responses follow the outcome of an earlier competition between inputs. In the current study of two patients, we used signal detection analyses to test the hypothesis that response criteria and response modalities also contribute to visual awareness. We found that identification was more sensitive than detection in uncovering deficits of contralesional awareness. Extinction was worse with bilateral stimuli when the ipsilesional stimulus was identical or similar to the target than when it was dissimilar. This diminished awareness was more likely to reflect a shift towards more conservative responses rather than diminished discrimination of contralesional stimuli. By contrast, one patient was better able to discriminate contralesional stimuli when using his contralesional limb to indicate awareness of targets than when using his ipsilesional limb. These data indicate that the nature of stimuli can modulate response criteria and the motor response can affect the sensory discriminability. Sensory discrimination and response output are not organized in a simple serial manner. Rather, input and output parameters interact in complicated ways to produce visual awareness. Visual awareness itself appears to be the outcome of two bottlenecks in processing, one having to do with sensory processing that may be covert and the other having to do with decision making, which by definition is overt. Finally, we advocate the use of signal detection analyses in studies of extinction, a method that has been surprisingly neglected in this line of research.     

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.     

Impaired orienting of attention in left unilateral neglect: a componential analysis

Twenty-six patients suffering from damage to the right side of the brain, 19 of whom exhibited signs of left neglect, as well as 32 matched controls, ran 3 spatial cuing tasks. Patients were also tested with 2 cancellation tests, a line-bisection test, the copy of a complex drawing, and a visual extinction procedure. Results first showed correlations between extinction and cancellation tests performance on one hand, and between line bisection and copy on the other hand. Second, results demonstrated that an engagement deficit toward contralesional targets appeared to be the most striking feature of neglect, and the engagement score was correlated with the cancellation score and extinction. Most patients with neglect also presented a deficit in disengagement, a deficit of inhibition of return, and probably a deficit of alertness. Deficits in engagement and in disengagement, as well as poor scores in cancellation tests, seemed to be related with posterior cortical and subcortical lesions. Most important, even if an endogenous deficit (frequently related with a thalamic lesion) could aggravate the neglect behavior, neglect syndrome was mainly explained by a deficit of exogenous attention.     

Visual extinction of similar and dissimilar stimuli: Evidence for level-dependent attentional competition

Repetition blindness (RB) is the failure to report a visual stimulus presented shortly after a first occurrence of the same stimulus (Kanwisher, 1987). A similar phenomenon is that visual extinction, the failure to identify a contralesional stimulus presented simultaneously with an ipsilesional stimulus, increases with increasing similarity between the contralesional and ipsilesional stimulus (Baylis, Driver, & Rafal, 1993). We report a patient who, after a right parietal stroke, presented increased extinction for letters in repeated (e.g., A + A) than in unrepeated (e.g., T + U) displays. Increased extinction due to RB was observed in all experimental conditions probing item identification and varied between 5.4% and 40.6% across conditions. RB was unaffected by temporal modulation of the display, but was significantly reduced when stimuli grouped by a surrounding contour. Identification of contralesional repeated and unrepeated letters could be enhanced by auditory cues presented prior to the visual display. These results suggest that perceptual processing of extinguished stimuli that are similar to the stimulus presented on the preserved side is relatively unimpaired, but that the patient fails to ascribe to the stimulus a separate identity, supporting the distinction between type recognition and token individuation (Kanwisher, 1987). The extinction patterns for similar and dissimilar stimuli indicate that competition for attentional selection does not only occur at low (perceptual) levels, but also at higher processing levels, suggesting the presence of attentional competition on different levels of analysis.     

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     

Visual extinction of similar and dissimilar stimuli: Evidence for level-dependent attentional competition

Repetition blindness (RB) is the failure to report a visual stimulus presented shortly after a first occurrence of the same stimulus (Kanwisher, 1987). A similar phenomenon is that visual extinction, the failure to identify a contralesional stimulus presented simultaneously with an ipsilesional stimulus, increases with increasing similarity between the contralesional and ipsilesional stimulus (Baylis, Driver, & Rafal, 1993). We report a patient who, after a right parietal stroke, presented increased extinction for letters in repeated (e.g., A?+?A) than in unrepeated (e.g., T?+?U) displays. Increased extinction due to RB was observed in all experimental conditions probing item identification and varied between 5.4% and 40.6% across conditions. RB was unaffected by temporal modulation of the display, but was significantly reduced when stimuli grouped by a surrounding contour. Identification of contralesional repeated and unrepeated letters could be enhanced by auditory cues presented prior to the visual display. These results suggest that perceptual processing of extinguished stimuli that are similar to the stimulus presented on the preserved side is relatively unimpaired, but that the patient fails to ascribe to the stimulus a separate identity, supporting the distinction between type recognition and token individuation (Kanwisher, 1987). The extinction patterns for similar and dissimilar stimuli indicate that competition for attentional selection does not only occur at low (perceptual) levels, but also at higher processing levels, suggesting the presence of attentional competition on different levels of analysis.     

Evidence for a unimodal somatosensory attention system

Extinction is generally viewed as a disorder of selective attention for spatial locations. Recent physiologic, behavioral and lesion studies view spatial locations as a complex construct in which multiple modalities and motor systems are integrated. Accordingly, cross-modal and sensory-motor conditions often modify extinction. In a patient with tactile extinction, we tested the hypothesis that attentional deficits can also be confined to a specific sensory modality. Using objectively and subjectively balanced tactile stimuli and signal detection analysis, we found that our patient's contralesional tactile discrimination was not modulated by proprioceptive or visual input or by movement. By contrast, increasing the salience of the contralesional tactile stimuli did improve her contralesional tactile discrimination, consistent with our hypothesis that she suffered from a modality-specific attentional deficit. Additionally, she did not have any evidence of visual extinction, again bolstering our claim that her extinction was confined to touch. These data suggest that in addition to polymodal and sensory-motor attentional systems, spatial attention also operates on specific sensations. We also advocate the use of signal detection analysis, a method that has been surprisingly neglected in extinction research.     

Transient binding by time: Neuropsychological evidence from anti-extinction

Anti-extinction occurs when there is poor report of a single stimulus presented on the contralesional side of space, but better report of the same item when it occurs concurrently with a stimulus on the ipsilesional side (Goodrich & Ward, 1997). We report a series of experiments that examine the factors that lead to anti-extinction in a patient GK, who has bilateral parietal lesions but more impaired identification of left-side stimuli. We show a pattern of anti-extinction when stimuli are briefly presented, which is followed by an extinction effect when stimuli are left for longer in the visual field. In Experiments 1 and 2 we present evidence that the anti-extinction effects are determined by stimuli onsetting together, and it is not apparent when stimuli are defined by offsets. In Experiments 3 and 4 we report that performance is not strongly affected by whether the same or different tasks are performed on the ipsi- and contralesional stimuli, and the anti-extinction effect also survives trials where eye movements are made to right-side stimuli. Experiment 5 provides evidence that anti-extinction is due to temporal grouping between stimuli, rather than to increased arousal or cueing attention to the contralesional side. Experiment 6 demonstrates that anti-extinction dissociates from GK's conscious perception of when contra- and ipsilesional stimuli occur together. We interpret the data as indicating that there is unconscious and transient temporal binding in vision.     

Transient binding by time: Neuropsychological evidence from anti-extinction

Anti-extinction occurs when there is poor report of a single stimulus presented on the contralesional side of space, but better report of the same item when it occurs concurrently with a stimulus on the ipsilesional side (Goodrich & Ward, 1997). We report a series of experiments that examine the factors that lead to anti-extinction in a patient GK, who has bilateral parietal lesions but more impaired identification of left-side stimuli. We show a pattern of anti-extinction when stimuli are briefly presented, which is followed by an extinction effect when stimuli are left for longer in the visual field. In Experiments 1 and 2 we present evidence that the anti-extinction effects are determined by stimuli onsetting together, and it is not apparent when stimuli are defined by offsets. In Experiments 3 and 4 we report that performance is not strongly affected by whether the same or different tasks are performed on the ipsi- and contralesional stimuli, and the anti-extinction effect also survives trials where eye movements are made to right-side stimuli. Experiment 5 provides evidence that anti-extinction is due to temporal grouping between stimuli, rather than to increased arousal or cueing attention to the contralesional side. Experiment 6 demonstrates that anti-extinction dissociates from GK's conscious perception of when contra- and ipsilesional stimuli occur together. We interpret the data as indicating that there is unconscious and transient temporal binding in vision.     

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.     

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.     

Impaired texture segregation but spared contour integration following damage to right posterior parietal cortex

We examined the relations between texture segregation and contour integration in patients with deficits in spatial attention leading to left or right hemisphere extinction. Patients and control participants were presented with texture and contour stimuli consisting of oriented elements. We induced regularity in the stimuli by manipulating the element orientations resulting in an implicit texture border or explicit contour. Participants had to discriminate curved from straight shapes without making eye movements, while the stimulus presentation time was varied using a QUEST procedure. The results showed that only patients with right hemisphere extinction had a spatial bias, needing a longer presentation time to determine the shape of the border or contour on the contralesional side, especially for borders defined by texture. These results indicate that texture segregation is modulated by attention-related brain areas in the right posterior parietal cortex.     

Effectiveness of sensory stimulation on tactile extinction

 Eleven brain-damaged patients with extinction were asked to report double tactile stimuli before, during, and after optokinetic stimulation and transcutaneous electrical stimulation of the posterior neck region. The goal of the study was to test whether tactile extinction is sensitive to these experimental manipulations in order to better understand the nature of the disorder. Both of these sensory stimulations are known to be effective in modulating only higher-order (cognitive) disorders of spatial coding, such as visual hemineglect, deficit of position sense, hemianesthesia, etc. When applied to the side contralateral to the cerebral lesion, both optokinetic and transcutaneous electrical stimulation significantly affected patients’ performances, increasing the amount of detections of contralesional double stimuli. A tendency towards worse performance was observed when sensory stimulation was applied to the ipsilesional side. The reported effectiveness in reducing tactile extinction suggests that the deficit can not be fully ascribed to a peripheral sensory disorder and that it reflects damage to a higher-order cognitive function involved in contralesional space representation or in the deployment of attention to that side of space. The nature of the close relationship between extinction and hemineglect is also discussed from the point of view of extinction as a deficit of space coding. Received: 19 September 1998 / Accepted: 19 February 1999     

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.     

The contribution of stimulus-driven and goal-driven mechanisms to feature-based selection in patients with spatial attention deficits

When people search a display for a target defined by a unique feature, fast saccades are predominantly stimulus-driven whereas slower saccades are primarily goal-driven. Here we use this dissociative pattern to assess whether feature-based selection in patients with lateralized spatial attention deficits is impaired in stimulus-driven processing, goal-driven processing, or both. A group of patients suffering from extinction or neglect after parietal damage, and a group of healthy, age-matched controls, were instructed to make a saccade to a uniquely oriented target line which was presented simultaneously with a differently oriented distractor line. We systematically varied the salience of the target and distractor by changing the orientation of background elements, and used a time-based model to extract stimulus-driven (salience) and goal-driven (target set) components of selection. The results show that the patients exhibited reduced stimulus-driven processing only in the contralesional hemifield, while goal-driven processing was reduced across both hemifields.