Visual selection and action in Balint's syndrome

We examined the relations between attention and action in a patient with Balint's syndrome following bilateral damage involving the parietal lobes. The first two experiments used prolonged stimulus exposures and showed that unlike normals, the patient GK made independent actions to bilateral stimuli, even when explicitly instructed to make coordinated reaches. In contrast, bimanual actions to a single stimulus were coordinated in time. Experiments 3 and 4 employed reduced stimulus exposures to produce visual extinction. We found comparable extinction effects in detection and action, along with improved bimanual movements when the stimuli were grouped by collinearity and surface contrast, plus better coordination with bilateral stimuli proximity. The results are discussed in relation to the common role of visual selection processes in perception and action.     

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.     

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.     

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.     

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 use of visual feedback is independent of visual awareness: evidence from visual extinction

Milner and Goodale (The visual brain in action, Oxford Press, 1995) made a distinction between vision for perception and vision for action. In contrast to perception, many action tasks have strict temporal constraints, which can only be met if the visual information is relayed directly to the motor system without first passing through a conscious decision making process. Milner and Goodale therefore predict that visual stimuli do not have to reach visual awareness in order to guide rapid motor responses. Online visual feedback provides a good example of visual information that is used under tight temporal constraints to guide rapid motor responses. Online visual feedback provides information about the position of the moving limb. This information can be used to improve the accuracy of our movements. If vision for action operates independently of visual awareness, visual feedback should be beneficial even if the subject is unaware of this information. We tested this prediction in a patient (V.E.) with left-sided visual extinction, a condition in which a visual stimulus typically fails to reach awareness if a second stimulus is presented simultaneously at a more rightward location. V.E. was asked to point towards a central target with his left hand. In some trials a light-emitting diode (LED) provided brief visual feedback from the moving hand. However, in the majority of trials, V.E. was unaware of this LED, due to his extinction. His performance was nevertheless significantly better when visual feedback was present, regardless of whether or not the information was available for verbal report. We conclude that visual awareness is not essential for the effective use of online visual feedback. The study was carried out at: Cognitive Neuroscience Research Unit (CNRU), Wolfson Research Institute, University of Durham, UK     

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.     

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.     

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 role of the corpus callosum in the coupling of bimanual isometric force pulses

Two split-brain patients, a patient with callosal agenesis, and 6 age-matched control participants were tested on a bimanual force production task. The participants produced isometric responses with their index fingers, attempting to match the target force specified by a visual stimulus. On unimanual trials, the stimuli were presented in either the left or right visual field and the response was made with the ipsilateral hand. On bimanual trials, two stimuli were presented, one on each side, and the target forces could be either identical or different. Bimanual responses of the control subjects showed strong evidence of coupling. Forces produced by one hand were influenced by the forces produced by the other hand with positive correlations observed for all target force combinations. These assimilation effects and correlations were greatly attenuated in the acallosal group, with similar results observed for the split-brain patients and participant with callosal agenesis. Furthermore, the processes involved in selecting and planning the two responses occurred independently in the acallosal group; in contrast to the controls, the three acallosal participants exhibited no differences in reaction times or accuracy between bimanual trials in which the two target forces were the same or different. We also found a striking temporal desynchronization of the responses in the split-brain patients, indicating that in this context, temporal coupling is impaired after callosotomy. These results are congruent with the hypothesis that interference related to response selection and planning of bimanual force pulses arises from callosal interactions.     

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.     

Encoding of visual information by LGN bursts.

Encoding of visual information by LGN bursts. Thalamic relay cells respond to visual stimuli either in burst mode, as a result of activation of a low-threshold Ca2+ conductance, or in tonic mode, when this conductance is inactive. We investigated the role of these two response modes for the encoding of the time course of dynamic visual stimuli, based on extracellular recordings of 35 relay cells from the lateral geniculate nucleus of anesthetized cats. We presented a spatially optimized visual stimulus whose contrast fluctuated randomly in time with frequencies of up to 32 Hz. We estimated the visual information in the neural responses using a linear stimulus reconstruction method. Both burst and tonic spikes carried information about stimulus contrast, exceeding one bit per action potential for the highest variance stimuli. The "meaning" of an action potential, i.e., the optimal estimate of the stimulus at times preceding a spike, was similar for burst and tonic spikes. In within-trial comparisons, tonic spikes carried about twice as much information per action potential as bursts, but bursts as unitary events encoded about three times more information per event than tonic spikes. The coding efficiency of a neuron for a particular stimulus is defined as the fraction of the neural coding capacity that carries stimulus information. Based on a lower bound estimate of coding efficiency, bursts had approximately 1.5-fold higher efficiency than tonic spikes, or 3-fold if bursts were considered unitary events. Our main conclusion is that both bursts and tonic spikes encode stimulus information efficiently, which rules out the hypothesis that bursts are nonvisual responses.     

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.     

Context modulation of memory for fear extinction in humans

Distinct memories are formed during fear conditioning and subsequent extinction. In animals, the expression of the latter is gated by the context. The recall of extinction memory after a long delay, and the contextual modulation thereof, has not been directly tested in humans. Mentally healthy volunteers underwent a 2-day fear conditioning and extinction protocol that examined the recall of extinction memory and its relationship to context. Conditioned stimuli were paired with an aversive electric shock in one visual context and extinguished in a different context. Extinction recall and renewal were examined 24 h after training. We found that skin conductance responses were small when the conditioned stimulus was presented in the extinction context, but responses were renewed when the conditioned stimulus was presented in the conditioning context. This finding demonstrates context dependency of extinction recall in humans.     

Action‐associated modulation of visual event‐related potentials evoked by abstract and ecological stimuli

This study investigated the influence of action‐associated predictive processes on visual ERPs. In two experiments, we sought evidence for sensory attenuation (SA) indexed by ERP amplitude reductions for self‐induced stimuli when compared to passive viewing of the same images. We assessed if SA is (a) present for both ecological and abstract stimuli (pictures depicting hands or checkerboards), (b) modulated by the degree of stimulus predictability (certain or uncertain action‐effect contingencies), and (c) sensitive to laterality of hand movements (dominant or subdominant hand actions). We found reduced occipital responses in the early 77–90 ms time interval (C1 component), irrespective of stimulus type, predictability, or the laterality of hand movements. However, the subsequent P1 component was increased (rather than reduced) for all action‐associated stimuli. In addition, this P1 effect was influenced by the degree of stimulus predictability for ecological stimuli only. Finally, the posterior N1 component was not modulated by self‐initiated actions. Overall, our findings indicate that movement‐related predictive processes attenuate early visual responses. Moreover, we propose that amplitude modulations in the P1 time range reflect the interaction between expectation‐based SA and attention‐associated amplitude enhancements. These results can have implications for assessing the influence of action‐associated predictions on visual processing in psychiatric disorders characterized by aberrant sensory predictions and alterations in hemispheric asymmetry, such as schizophrenia.     

Interhemispheric interactions and redundancy gain: tests of an interhemispheric inhibition hypothesis

In simple reaction time (RT) tasks, responses are faster when stimuli are presented to both the left and right visual hemifields than when a stimulus is presented to a single hemifield. Paradoxically, this redundancy gain with bilateral stimuli is enhanced in split-brain individuals relative to normals. This article reports three experiments testing an account of that enhancement in which normals' responses to bilateral stimuli are slowed by interhemispheric inhibition. In simple RT tasks, normal participants responded bimanually to left, right, or bilateral visual stimuli. In choice RT tasks, they responded to each stimulus with one hand, responding bimanually only when both stimuli were presented. Measurements of response forcefulness (Experiment 1) and electroencephalographic activity (Experiments 2 and 3) showed no evidence of the correlation patterns predicted by the hypothesis of interhemispheric inhibition. The results suggest that such inhibition is unlikely to be the explanation for enhanced redundancy gain in split-brain individuals.     

Conceptual unifying constraints override sensorimotor interference during anticipatory control of bimanual actions

Traditional approaches to research on bimanual coordination focus on sensorimotor interference, motor programming, and effects of perception and feedback guidance; surprisingly, little is known about high-level conceptual constraints that might unify separate movements into coordinated actions. We investigated two possible forms of high-level unifying representations on anticipatory control (i.e., reaction time: RT) in two-limb (bimanual) movements. Specifically, we adapted a paradigmatic bimanual task involving reaching to targets by adding two novel manipulations. One involved a visual-perceptual manipulation in which target-objects were presented either separately (i.e., two circles) or as a unified object (i.e., two circles connected by a bar). The other involved variants on language representation to elicit separate action plans (i.e., separate instructional commands joined by ‘and’) or unified action plans (i.e., a single verb applying to both hands). Typical forms of sensorimotor interference were virtually abolished when these unifying constraints were available. These findings provide strong support for the theoretical account that unifying conceptual representations are primary forms of bimanual constraint. Findings further suggest that the organization and content of the language used to form action representations can strongly influence anticipatory planning of bimanual actions.     

Sensory versus motor loci for integration of multiple motion signals in smooth pursuit eye movements and human motion perception

We have investigated how visual motion signals are integrated for smooth pursuit eye movements by measuring the initiation of pursuit in monkeys for pairs of moving stimuli of the same or differing luminance. The initiation of pursuit for pairs of stimuli of the same luminance could be accounted for as a vector average of the responses to the two stimuli singly. When stimuli comprised two superimposed patches of moving dot textures, the brighter stimulus suppressed the inputs from the dimmer stimulus, so that the initiation of pursuit became winner-take-all when the luminance ratio of the two stimuli was 8 or greater. The dominance of the brighter stimulus could be not attributed to either the latency difference or the ratio of the eye accelerations for the bright and dim stimuli presented singly. When stimuli comprised either spot targets or two patches of dots moving across separate locations in the visual field, the brighter stimulus had a much weaker suppressive influence; the initiation of pursuit could be accounted for by nearly equal vector averaging of the responses to the two stimuli singly. The suppressive effects of the brighter stimulus also appeared in human perceptual judgments, but again only for superimposed stimuli. We conclude that one locus of the interaction of two moving visual stimuli is shared by perception and action and resides in local inhibitory connections in the visual cortex. A second locus resides deeper in sensory-motor processing and may be more closely related to action selection than to stimulus selection.