Visuo-motor adaptation with optical prisms that displace the visual scene (prism adaptation, PA) ... more Visuo-motor adaptation with optical prisms that displace the visual scene (prism adaptation, PA) has been widely used to study visuo-motor plasticity in healthy individuals and to decrease the lateralized bias of brain-damaged patients suffering from spatial neglect. Several factors may influence PA aftereffects, such as the degree of optical deviation (generally measured in dioptres of wedge prisms) or the direction of the prismatic shift (leftward vs. rightward). However, the mechanisms through which aftereffects of adaptation in healthy individuals and in neglect affect performance in tasks probing spatial cognition remain controversial. For example, some studies have reported positive effects of PA on auditory neglect, while other studies failed to obtain any changes of performance even in the visual modality. We here tested a new adaptation method in virtual reality to evaluate how sensory parameters influence PA aftereffects. Visual vs. auditory-verbal feedback of optical devi...
The medial temporal lobe (MTL) is crucial for memory encoding and recognition. The time course of... more The medial temporal lobe (MTL) is crucial for memory encoding and recognition. The time course of these processes is unknown. The present study juxtaposed encoding and recognition in a single paradigm. Twenty healthy subjects performed a continuous recognition task as brain activity was monitored with a high-density electroencephalography. The task presented New pictures thought to evoke encoding. The stimuli were then repeated up to 4 consecutive times to produce over-familiarity. These repeated stimuli served as “baseline” for comparison with the other stimuli. Stimuli later reappeared after 9–15 intervening items, presumably associated with new encoding and recognition. Encoding-related differences in evoked response potential amplitudes and in spatiotemporal analysis were observed at 145–300 ms, whereby source estimation indicated MTL and orbitofrontal activity from 145 to 205 ms. Recognition-related activity evoked by late repetitions occurred at 405–470 ms, implicating the MTL...
Several arguments suggest that mental rotation (MR) and motor planning may share embodied neural ... more Several arguments suggest that mental rotation (MR) and motor planning may share embodied neural mechanisms, but the overlap between cognitive processes recruited during MR of objects and body parts is not well established. We here used high-density EEG to examine the cognitive similarity between MR of non-manipulable objects (chairs) and bodily stimuli (hands). We selected chairs because they may appear in a recognizable left-right orientation and are not automatically associated with a manual action. Participants had identical response options for both types of stimuli, and they gave responses orally in order to prevent possible interference with motor imagery. MR of hands and chairs generated very similar behavioral responses, time-courses and neural sources of evoked-response potentials (ERPs). ERP segmentation analysis revealed distinct time windows during which differential effects of stimulus type and angular disparity were observed. An early period (90-160 ms) differentiated...
Illusory visual phenomena, such as palinopsia, polyopsia or allesthesia, are rare manifestations ... more Illusory visual phenomena, such as palinopsia, polyopsia or allesthesia, are rare manifestations of posterior cortical damage. Symptoms are characterized by illusory perceptions, ranging from isolated stationary objects to scenes and moving persons. Such illusions may appear while the original object is still in view, or become manifest with a delay and last for minutes, hours or even days. Some authors have suggested a disinhibited cortical response underlying visual illusions, but experimental studies supporting this hypothesis are lacking. Here, we examined a rare patient who after focal right parietal injury consistently reported a second stimulus on the left when briefly shown a target in his right hemifield. The patient perceived the illusory stimulus as less intense, and therefore concluded that it must have a different shape than the original stimulus. A masking experiment revealed that the frequency of the illusion was inversely related to the visibility of the original sti...
Lesion-symptom studies of spatial neglect and the attention deficits associated with this disorde... more Lesion-symptom studies of spatial neglect and the attention deficits associated with this disorder draw a complex picture of the brain areas involved in spatial awareness. Several cortical regions and fiber tracts have been identified as predictors of behavioral performance, a pattern reflecting the large degree of methodological variance and modest sample sizes of many studies. Here, we examined the anatomical predictors of deficits of spatial exploration, reading and line bisection in 134 unselected stroke patients with post-acute, right-hemispheric brain injury. In order to neutralize shortcomings of traditional lesion-symptom analyses we used several methodological approaches: voxel-based lesion-symptom mapping focusing on binary groups or continuous performance measures, region-of-interest analyses and a 'minimal-lesion' method, comparing patients with highly selective deficits to specific brain areas. All four approaches converged on the central role of the right temporo-parietal junction and frontoparietal connections conveyed through the superior longitudinal fasciculus for contralateral deployment of attention and detection of taskrelevant stimuli.
The neural mechanisms underlying the access to object knowledge from early representations of sha... more The neural mechanisms underlying the access to object knowledge from early representations of shape are little known. Functional imaging studies support the view that representations of visual properties are distributed across occipito-temporal cortex of both cerebral hemispheres. By contrast, brain lesion studies show that focal occipito-temporal damage may lead to object agnosiaa specific impairment of object recognition. How does distributed processing fit with functional specialization implied by the existence of stimulus-specific agnosias? Using fMRI we studied functional connectivity (FC) in a patient with object agnosia following left lateral occipital damage. Despite intact global and local processing of 2D and 3D object structure, the patient made consistent object identification errors. Seven experiments testing naming, visual matching or object priming showed that his errors mainly reflected the global shape similarity between objects. Compared to controls the patient exhibited strongly reduced FC between the damaged left and the intact right medial/lateral occipital cortex. In addition, controls showed stronger connectivity between the right occipital cortex and the left and right inferior and anterior temporal cortices. Interestingly, the patient also showed compensatory increases of FC between dorsal occipital and medial parietal cortex. These findings show that focal damage to the lateral occipital cortex may have global effects on representations of objects in bilateral occipito-temporal cortex, thus supporting the view that bilaterally distributed coding is necessary for the retrieval of associative knowledge from shape.
International journal of psychophysiology : official journal of the International Organization of Psychophysiology, 2018
Exaggerated attentional biases toward specific elements of the environment contribute to the main... more Exaggerated attentional biases toward specific elements of the environment contribute to the maintenance of several psychiatric conditions, such as biases to threatening faces in social anxiety. Although recent literature indicates that attentional bias modification may constitute an effective approach for psychiatric remediation, the underlying neurophysiological mechanisms remain unclear. We addressed this question by recording EEG in 24 healthy participants performing a modified dot-probe task in which pairs of neutral cues (colored shapes) were replaced by probe stimuli requiring a discrimination judgment. To induce an attentional bias toward or away from the cues, the probes were systematically presented either at the same or at the opposite position of a specific cue color. This paradigm enabled participants to spontaneously develop biases to initially unbiased, neutral cues, as measured by the response speed to the probe presented after the cues. Behavioral result indicated t...
Visuo-motor adaptation with optical prisms that displace the visual scene (prism adaptation, PA) ... more Visuo-motor adaptation with optical prisms that displace the visual scene (prism adaptation, PA) has been widely used to study visuo-motor plasticity in healthy individuals and to decrease the lateralized bias of brain-damaged patients suffering from spatial neglect. Several factors may influence PA aftereffects, such as the degree of optical deviation (generally measured in dioptres of wedge prisms) or the direction of the prismatic shift (leftward vs. rightward). However, the mechanisms through which aftereffects of adaptation in healthy individuals and in neglect affect performance in tasks probing spatial cognition remain controversial. For example, some studies have reported positive effects of PA on auditory neglect, while other studies failed to obtain any changes of performance even in the visual modality. We here tested a new adaptation method in virtual reality to evaluate how sensory parameters influence PA aftereffects. Visual vs. auditory-verbal feedback of optical devi...
The medial temporal lobe (MTL) is crucial for memory encoding and recognition. The time course of... more The medial temporal lobe (MTL) is crucial for memory encoding and recognition. The time course of these processes is unknown. The present study juxtaposed encoding and recognition in a single paradigm. Twenty healthy subjects performed a continuous recognition task as brain activity was monitored with a high-density electroencephalography. The task presented New pictures thought to evoke encoding. The stimuli were then repeated up to 4 consecutive times to produce over-familiarity. These repeated stimuli served as “baseline” for comparison with the other stimuli. Stimuli later reappeared after 9–15 intervening items, presumably associated with new encoding and recognition. Encoding-related differences in evoked response potential amplitudes and in spatiotemporal analysis were observed at 145–300 ms, whereby source estimation indicated MTL and orbitofrontal activity from 145 to 205 ms. Recognition-related activity evoked by late repetitions occurred at 405–470 ms, implicating the MTL...
Several arguments suggest that mental rotation (MR) and motor planning may share embodied neural ... more Several arguments suggest that mental rotation (MR) and motor planning may share embodied neural mechanisms, but the overlap between cognitive processes recruited during MR of objects and body parts is not well established. We here used high-density EEG to examine the cognitive similarity between MR of non-manipulable objects (chairs) and bodily stimuli (hands). We selected chairs because they may appear in a recognizable left-right orientation and are not automatically associated with a manual action. Participants had identical response options for both types of stimuli, and they gave responses orally in order to prevent possible interference with motor imagery. MR of hands and chairs generated very similar behavioral responses, time-courses and neural sources of evoked-response potentials (ERPs). ERP segmentation analysis revealed distinct time windows during which differential effects of stimulus type and angular disparity were observed. An early period (90-160 ms) differentiated...
Illusory visual phenomena, such as palinopsia, polyopsia or allesthesia, are rare manifestations ... more Illusory visual phenomena, such as palinopsia, polyopsia or allesthesia, are rare manifestations of posterior cortical damage. Symptoms are characterized by illusory perceptions, ranging from isolated stationary objects to scenes and moving persons. Such illusions may appear while the original object is still in view, or become manifest with a delay and last for minutes, hours or even days. Some authors have suggested a disinhibited cortical response underlying visual illusions, but experimental studies supporting this hypothesis are lacking. Here, we examined a rare patient who after focal right parietal injury consistently reported a second stimulus on the left when briefly shown a target in his right hemifield. The patient perceived the illusory stimulus as less intense, and therefore concluded that it must have a different shape than the original stimulus. A masking experiment revealed that the frequency of the illusion was inversely related to the visibility of the original sti...
Lesion-symptom studies of spatial neglect and the attention deficits associated with this disorde... more Lesion-symptom studies of spatial neglect and the attention deficits associated with this disorder draw a complex picture of the brain areas involved in spatial awareness. Several cortical regions and fiber tracts have been identified as predictors of behavioral performance, a pattern reflecting the large degree of methodological variance and modest sample sizes of many studies. Here, we examined the anatomical predictors of deficits of spatial exploration, reading and line bisection in 134 unselected stroke patients with post-acute, right-hemispheric brain injury. In order to neutralize shortcomings of traditional lesion-symptom analyses we used several methodological approaches: voxel-based lesion-symptom mapping focusing on binary groups or continuous performance measures, region-of-interest analyses and a 'minimal-lesion' method, comparing patients with highly selective deficits to specific brain areas. All four approaches converged on the central role of the right temporo-parietal junction and frontoparietal connections conveyed through the superior longitudinal fasciculus for contralateral deployment of attention and detection of taskrelevant stimuli.
The neural mechanisms underlying the access to object knowledge from early representations of sha... more The neural mechanisms underlying the access to object knowledge from early representations of shape are little known. Functional imaging studies support the view that representations of visual properties are distributed across occipito-temporal cortex of both cerebral hemispheres. By contrast, brain lesion studies show that focal occipito-temporal damage may lead to object agnosiaa specific impairment of object recognition. How does distributed processing fit with functional specialization implied by the existence of stimulus-specific agnosias? Using fMRI we studied functional connectivity (FC) in a patient with object agnosia following left lateral occipital damage. Despite intact global and local processing of 2D and 3D object structure, the patient made consistent object identification errors. Seven experiments testing naming, visual matching or object priming showed that his errors mainly reflected the global shape similarity between objects. Compared to controls the patient exhibited strongly reduced FC between the damaged left and the intact right medial/lateral occipital cortex. In addition, controls showed stronger connectivity between the right occipital cortex and the left and right inferior and anterior temporal cortices. Interestingly, the patient also showed compensatory increases of FC between dorsal occipital and medial parietal cortex. These findings show that focal damage to the lateral occipital cortex may have global effects on representations of objects in bilateral occipito-temporal cortex, thus supporting the view that bilaterally distributed coding is necessary for the retrieval of associative knowledge from shape.
International journal of psychophysiology : official journal of the International Organization of Psychophysiology, 2018
Exaggerated attentional biases toward specific elements of the environment contribute to the main... more Exaggerated attentional biases toward specific elements of the environment contribute to the maintenance of several psychiatric conditions, such as biases to threatening faces in social anxiety. Although recent literature indicates that attentional bias modification may constitute an effective approach for psychiatric remediation, the underlying neurophysiological mechanisms remain unclear. We addressed this question by recording EEG in 24 healthy participants performing a modified dot-probe task in which pairs of neutral cues (colored shapes) were replaced by probe stimuli requiring a discrimination judgment. To induce an attentional bias toward or away from the cues, the probes were systematically presented either at the same or at the opposite position of a specific cue color. This paradigm enabled participants to spontaneously develop biases to initially unbiased, neutral cues, as measured by the response speed to the probe presented after the cues. Behavioral result indicated t...
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Papers by Radek Ptak