Biomedical subjects

Radek Ptak

Publications and source records attributed to Radek Ptak.

11 recordsLinked to original sources

PubMed2006

Temporal dynamics of attentional control settings in patients with spatial neglect.

Patients with spatial neglect show disproportionately slow reactions to a contralesional stimulus presented shortly after an ipsilesional cue. We examined whether this attentional bias reflects purely automatic capture of attention by the cue or whether it is contingent on the similarity between cue and target. Patients with spatial neglect reacted to letters presented in the left or right visual field. These target letters were pre-cued by the same letter (similar cue) or a different letter (dissimilar cue) presented 100 or 1000 ms prior to target onset in the same or the opposite visual field. At the short interval, similar and dissimilar ipsilesional cues captured attention comparably and strongly slowed reactions to contralesional targets. In contrast, while similar ipsilesional cues still captured attention at the long interval dissimilar cues ceased to affect performance. In contrast, the different cueing conditions induced only small and insignificant differences in reaction times to ipsilesional targets. These findings suggest that attention of neglect patients is initially captured by all ipsilesional cues in a reflexive, stimulus-driven fashion, but that prolonged attentional capture may only be observed when cues share a property with the target.

Aged↗

PubMed2006

Decision-making in amnesia: do advantageous decisions require conscious knowledge of previous behavioural choices?

Previous work has reported that in the Iowa gambling task (IGT) advantageous decisions may be taken before the advantageous strategy is known [Bechara, A., Damasio, H., Tranel, D., & Damasio, A. R. (1997). Deciding advantageously before knowing the advantageous strategy. Science, 275, 1293-1295]. In order to test whether explicit memory is essential for the acquisition of a behavioural preference for advantageous choices, we measured behavioural performance and skin conductance responses (SCRs) in five patients with dense amnesia following damage to the basal forebrain and orbitofrontal cortex, six amnesic patients with damage to the medial temporal lobe or the diencephalon, and eight control subjects performing the IGT. Across 100 trials healthy participants acquired a preference for advantageous choices and generated large SCRs to high levels of punishment. In addition, their anticipatory SCRs to disadvantageous choices were larger than to advantageous choices. However, this dissociation occurred much later than the behavioural preference for advantageous alternatives. In contrast, though exhibiting discriminatory autonomic SCRs to different levels of punishment, 9 of 11 amnesic patients performed at chance and did not show differential anticipatory SCRs to advantageous and disadvantageous choices. Further, the magnitude of anticipatory SCRs did not correlate with behavioural performance. These results suggest that the acquisition of a behavioural preference--be it for advantageous or disadvantageous choices--depends on the memory of previous reinforcements encountered in the task, a capacity requiring intact explicit memory.

Adult↗

PubMed2005

Reflexive orienting in spatial neglect is biased towards behaviourally salient stimuli.

Patients with spatial neglect are impaired when detecting contralesional targets presented shortly after an ipsilesional cue. This "disengagement" deficit is believed to reflect reflexive orienting towards ipsilesional stimuli that is independent of behavioural goals. Here, we show that the extent of this spatial bias depends on the behavioural salience of ipsilesional stimuli. Healthy participants, brain-injured patients without neglect and neglect patients reacted to ipsilesional and contralesional visual targets. Prior to target presentation, a visual cue similar or dissimilar to the target was presented at target position or opposite the target. Although participants did not react to the similar cue, it had high behavioural salience since it shared features with the target stimulus. Neglect patients showed dramatically increased reaction times to contralesional targets, but only when these followed behaviourally relevant ipsilesional cues. No decrease of performance was observed with irrelevant cues. This performance pattern was not due to perceptual similarity, since the same effect was found when cue and target were semantically related but differed perceptually. Importantly, semantically related cues ceased to attract attention when they were defined as behaviourally irrelevant. These results show that neglect patients only orient attention reflexively towards ipsilesional stimuli with high behavioural salience.

Adult↗

PubMed2005

Dynamic modulation of visual detection by auditory cues in spatial neglect.

One of the most constant findings of studies about selective attention is that detection of visual stimuli is enhanced when a visual cue is presented at the position of the upcoming target. In healthy participants, comparable benefits were reported when the cue was presented in a different modality than the target. The aim of this study was to examine spatial and temporal dynamics of visual attention following auditory cues in patients with spatial neglect. Twelve healthy subjects and five patients with left-sided neglect were asked to react to a small vertical line presented randomly at one of four positions. The target appeared 150 or 1000 ms after an auditory cue that was either static (continuous 380 Hz tone presented to the left or right ear) or dynamic (380 Hz tone moving from the left to the right ear or vice versa). The reaction time pattern of healthy participants was unaffected by the different tones. In contrast, reaction times of neglect patients were significantly faster to left targets following a dynamic tone moving from right to left in comparison to a tone moving from left to right. Interestingly, static unilateral tones modulated visual attention of neglect patients to a lesser degree than dynamic tones. The modulation of visual attention by dynamic auditory cues was of short duration and disappeared after 1000 ms. These results demonstrate a fast automatic shift of spatial attention in the direction of a moving tone, suggesting strong dynamic links between visual and auditory attention in patients with a severe spatial deficit.

Acoustic Stimulation↗

PubMed2005

The inferior temporal lobe mediates distracter-resistant visual search of patients with spatial neglect.

Although impaired visual search is a core deficit of patients with spatial neglect, current evidence is not conclusive about the mechanisms underlying this failure. We present evidence from 14 neglect patients searching for a target defined by two perceptual features that visual search is mediated by mechanisms of attentional competition. Participants were tested in three search conditions with constant target and distracter positions: Distracters did not share any feature with the target; distracters shared one feature with the target; two distracters shared one feature and one distracter shared the other feature with the target (mixed condition). Whereas search performance of healthy participants was comparable across conditions, neglect patients had a significant contralesional slowing in the mixed condition compared with the other two conditions. A detailed lesion analysis revealed that involvement of the parietal lobe did not predict the degree of distractibility in visual search. In contrast, neglect patients with high distractibility had more frequent damage to the inferior temporal lobe, suggesting a preliminary role of this region for competitive attentional processes involved in visual search of spatial neglect patients.

Adult↗

PubMed2004

Defective spatial imagery with pure Gerstmann's syndrome.

Gerstmann's syndrome comprises finger agnosia, peripheral agraphia, anarithmetia, and right-left confusion. We here report a single-case study of an 85-year-old ambidextrous man who exhibited pure Gerstmann's syndrome (i.e., without aphasia) 10 weeks after a stroke involving the angular gyrus in the left parietal lobe. We hypothesize that, in this case, the main cognitive denominator of Gerstmann's tetrad was a severe dysfunction in mental rotation and translation. This report provides further evidence for the spatial nature of Gerstmann's syndrome.

Aged↗

PubMed2004

The space of senses: impaired crossmodal interactions in a patient with Balint syndrome after bilateral parietal damage.

Balint syndrome after bilateral parietal damage involves a severe disturbance of space representation including impaired oculomotor behaviour, optic ataxia, and simultanagnosia. Binding of object features into a unique spatial representation can also be impaired. We report a patient with bilateral parietal lesions and Balint syndrome, showing severe spatial deficits in several visual tasks predominantly affecting the left hemispace. In particular, we tested whether a loss of spatial representation would affect crossmodal interactions between simultaneous visual and tactile events occurring at the same versus different locations. A tactile discrimination task, where spatially congruent or incongruent visual cues were delivered near the patient's hands, was used. Following stimulation of the left hand in the left side of space, we observed visuo-tactile interactions that were not modulated by spatially congruent conditions. In contrast, performance following stimulation of the right hand in the right side of space was affected in a spatially selective manner--facilitated for congruent stimuli and slowed for incongruent stimuli. To dissociate effects on somatotopic and spatiotopic coordinates, we crossed the patient's hands during unimodal tactile discriminations. Tactile performance of the left hand improved when it was positioned in the right hemispace, whereas placing the right hand in left space produced no significant changes, suggesting that left-sided tactile inputs are coded with respect to a combination of limb- and trunk-centred coordinates. These data converge with recent findings in animals and healthy humans to indicate a critical role of the posterior parietal cortex in multimodal spatial integration, and in the fusion of different coordinates into a unified representation of space.

Analysis of Variance↗

PubMed2004

Disorganised memory after right dorsolateral prefrontal damage.

Neurophysiological and functional brain imaging studies suggest the importance of dorsolateral area 9/46 for modality-independent working memory. However, the behavioural manifestations following isolated damage to this area have not been clearly outlined. Here, we describe the dramatic derangement of pragmatic memory after circumscribed lesion of the right area 9/46. A 52 year-old woman had suffered a traumatic brain injury 13 years ago and sought help for what she thought was a severe memory disorder. She regularly missed appointments and had gradually lost all her friends. She had tried different strategies to compensate for her memory problem, such as writing notes on her wrist or using electronic organizers. Intelligence, attention, verbal long-term memory, and executive functions were normal. Furthermore, experimental exploration demonstrated intact time estimation, attribution of memories to ongoing reality, and anticipation of outcomes. However, she failed to rehearse and manipulate information in working memory (n-back task), confirming the findings of functional imaging studies that the mid-dorsolateral prefrontal cortex is involved in mental manipulation of information. This case exemplifies that an isolated lesion of the right area 9/46 may induce severe failure to schedule actions and memory retrieval, a disorder leading to severely disorganised behaviour and disability.

Brain Injuries↗

PubMed2003

Decoupling of autonomic and cognitive emotional reactions after cerebellar stroke.

Emotional blunting can be found after cerebellar lesions. However, the mechanism of such a modification is not clear. We present a patient with emotional flattening and increased risk taking after left cerebellar infarct who had an impaired autonomic reactivity to negative as compared with positive reinforcement. This impairment was demonstrated by the patient's undifferentiated skin conductance responses to negative and positive reinforcement, whereas controls produced larger skin conductance responses after negative feedback. The cooccurrence of emotional flattening and undifferentiated autonomic reactions to positively and negatively valenced stimuli strengthens the role of the cerebellum in the modulation of the autonomic responses.

Affect↗

PubMed2003

[Different stages of neurologic rehabilitation after acute brain damage].

Neurorehabilitation following acute brain damage, including stroke or head injury, aims to improve the patient's recovery and avoid further complications. It requires detailed functional assessment, in order to define specific objectives. These goals can only be achieved through an intense and targeted therapy practiced by a multidisciplinary team.

Acute Disease↗

PubMed2002

Expectation-based attentional modulation of visual extinction in spatial neglect.

Visual extinction, the failure of patients with unilateral focal brain damage to report the contralesional of two simultaneously presented stimuli, may be modulated by characteristics of the display such as similarity, collinearity, or connectedness. Since these factors affect the perceptual configuration of stimuli, the modulation of extinction is believed to reflect low-level perceptual grouping. In the present study, patient AG did not show any modulation of contralesional detection when the ipsilesional and contralesional stimulus grouped by colour, by form, or both (Experiment 1). In contrast, identification of the contralesional stimulus was facilitated when the stimuli grouped (Experiment 2), suggesting a modulation of extinction by specific task demands. Experiment 3 used a cueing procedure to demonstrate modulation of extinction by expectation biases. Prior to stimulus presentation, AG was cued to attend to a particular feature (e.g. colour). After stimulus exposure he was prompted to identify the expected feature on valid trials and the unexpected feature on invalid trials. AG showed a significant validity effect for contralesional stimuli i.e. he identified the expected feature (e.g. colour) significantly better than the unexpected feature (e.g. form). These results suggest that competition for selection between visual stimuli may not only be influenced by perceptual characteristics of the display, but also by high-level factors such as the response criterion or expectation biases.

Attention↗