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PloS One 2012While early and higher visual areas along the ventral visual pathway in the inferotemporal cortex are critical for the recognition of individual objects, the neural...
While early and higher visual areas along the ventral visual pathway in the inferotemporal cortex are critical for the recognition of individual objects, the neural representation of human perception of complex global visual scenes remains under debate. Stroke patients with a selective deficit in the perception of a complex global Gestalt with intact recognition of individual objects - a deficit termed simultanagnosia - greatly helped to study this question. Interestingly, simultanagnosia typically results from bilateral lesions of the temporo-parietal junction (TPJ). The present study aimed to verify the relevance of this area for human global Gestalt perception. We applied continuous theta-burst TMS either unilaterally (left or right) or bilateral simultaneously over TPJ. Healthy subjects were presented with hierarchically organized visual stimuli that allowed parametrical degrading of the object at the global level. Identification of the global Gestalt was significantly modulated only for the bilateral TPJ stimulation condition. Our results strengthen the view that global Gestalt perception in the human brain involves TPJ and is co-dependent on both hemispheres.
Topics: Adult; Analysis of Variance; Female; Gestalt Theory; Humans; Magnetic Resonance Imaging; Male; Parietal Lobe; Photic Stimulation; Reaction Time; Stroke; Temporal Lobe; Transcranial Magnetic Stimulation; Visual Pathways; Visual Perception
PubMed: 23110106
DOI: 10.1371/journal.pone.0047820 -
Brain : a Journal of Neurology May 2012A fundamental aspect of visual cognition is our disposition to see the 'forest before the trees'. However, damage to the posterior parietal cortex, a critical brain... (Randomized Controlled Trial)
Randomized Controlled Trial
A fundamental aspect of visual cognition is our disposition to see the 'forest before the trees'. However, damage to the posterior parietal cortex, a critical brain region along the dorsal visual pathway, can produce a neurological disorder called simultanagnosia, characterized by a debilitating inability to perceive the 'forest' but not the 'trees' (i.e. impaired global processing despite intact local processing). This impairment in perceiving the global shape persists even though the ventral visual pathway, the primary recognition pathway, is intact in these patients. Here, we enabled global processing in patients with simultanagnosia using a psychophysical technique, which allowed us to bias stimuli such that they are processed predominantly by the intact ventral visual pathway. Our findings reveal that the impairment in global processing that characterizes simultanagnosia stems from a disruption in the processing of low-spatial frequencies through the dorsal pathway. These findings advance our understanding of the relationship between visuospatial attention and perception and reveal the neural mechanism mediating the disposition to see the 'forest before the trees'.
Topics: Adult; Aged; Agnosia; Bias; Brain Injuries; Brain Mapping; Contrast Sensitivity; Female; Fluorodeoxyglucose F18; Humans; Male; Parietal Lobe; Photic Stimulation; Positron-Emission Tomography; Psychophysics; Psychophysiologic Disorders; Visual Pathways; Young Adult
PubMed: 22418740
DOI: 10.1093/brain/aws066 -
Experimental Brain Research Aug 2010Patients with parieto-occipital brain damage may show simultanagnosia, a selective impairment in the simultaneous perception and integration of multiple objects (global...
Patients with parieto-occipital brain damage may show simultanagnosia, a selective impairment in the simultaneous perception and integration of multiple objects (global perception) with normal recognition of individual objects. Recent findings in patients with simultanagnosia indicate improved global perception at smaller spatial distances between local elements of hierarchical organized complex visual arrays. Global perception thus does not appear to be an all-or-nothing phenomenon but can be modified by the spatial relationship between local elements. The present study aimed to define characteristics of a general principle that accounts for improved global perception of hierarchically organized complex visual arrays in patients with simultanagnosia with respect to the spatial properties of local elements. In detail, we investigated the role of the number and size of the local elements as well as their relationship with each other for the global perception. The findings indicate that global perception increases independently of the size of the global object and depends on the spatial relationship between the local elements and the global object. The results further argue against the possibility of a restriction in the attended or perceived area in simultanagnosia, in the sense that the integration of local elements into a global scene is impaired if a certain spatial "field of view" is exceeded. A possible explanation for these observations might be a shift from global to local saliency in simultanagnosia.
Topics: Aged; Agnosia; Attention; Brain Damage, Chronic; Brain Mapping; Case-Control Studies; Female; Humans; Magnetic Resonance Imaging; Male; Middle Aged; Parietal Lobe; Pattern Recognition, Visual; Photic Stimulation; Reading; Space Perception; Visual Cortex; Visual Pathways
PubMed: 20593278
DOI: 10.1007/s00221-010-2328-x -
Neuropsychologia May 2010Patients with simultanagnosia following bilateral parieto-temporo-occipital brain damage show a characteristic impairment of global gestalt perception, while their...
Patients with simultanagnosia following bilateral parieto-temporo-occipital brain damage show a characteristic impairment of global gestalt perception, while their perception of individual objects or elements remains intact. For instance, when shown 'hierarchical' stimuli comprising a larger global object (e.g. a large letter) made up from smaller components (e.g. multiple small letters), they typically report seeing one of the smaller components but not the global figure. Recent work on simultanagnosia revealed that global perception can be improved if local element spacing is reduced. However, it is still unclear whether the retinal separation or the physical (post-size-constancy) spatial separation is critical. Here, we presented various hierarchical global/local letter stimuli at different viewing distances and sizes to separate the impacts of retinal versus physical size. Our findings indicate a key role for visual angle in determining simultanagnosic perception. We observed that not only retinal spacing (in terms of visual angle) between local elements had a major impact on global perception in simultanagnosia, but also the physical size of the separation between local elements, provided that binocular cues to viewing distance were available. The results indicate both pre-size-constancy retinal influences and binocular-post-constancy influences upon conscious perception in simultanagnosia.
Topics: Aged; Agnosia; Brain Mapping; Discrimination Learning; Female; Fluorodeoxyglucose F18; Humans; Magnetic Resonance Imaging; Pattern Recognition, Visual; Photic Stimulation; Positron-Emission Tomography; Psychophysics; Retina; Size Perception; Visual Pathways
PubMed: 20170667
DOI: 10.1016/j.neuropsychologia.2010.02.013 -
Vision Research Jul 2009Simultanagnosia is a neuropsychological disorder characterized by a restriction of visuospatial attention. In addition, patients are able to identify local elements of a... (Comparative Study)
Comparative Study
Simultanagnosia is a neuropsychological disorder characterized by a restriction of visuospatial attention. In addition, patients are able to identify local elements of a scene, but not the global whole. This may be due to a failure to scan and assemble local elements into a global whole (i.e. connect-the-dots). We monitored the eye movements of a simultanagnosic patient while she identified local and global elements of hierarchical letters. Scanning each local element was not necessary, nor sufficient, for successful global level identification. Our results argue against a connect-the-dots strategy of global identification and suggest that residual global processing may be occurring.
Topics: Agnosia; Attention; Case-Control Studies; Eye Movements; Female; Humans; Magnetic Resonance Imaging; Middle Aged; Neuropsychological Tests; Pattern Recognition, Visual; Space Perception; Visual Pathways
PubMed: 19460397
DOI: 10.1016/j.visres.2009.05.002 -
The Clinical Neuropsychologist May 2009We present a 56 year-old, right-handed, congenitally deaf female who exhibited a partial Balint's syndrome accompanied by positive visual phenomena restricted to her...
We present a 56 year-old, right-handed, congenitally deaf female who exhibited a partial Balint's syndrome accompanied by positive visual phenomena restricted to her lower right visual quadrant (e.g., color band, transient unformed visual hallucinations). Balint's syndrome is characterized by a triad of visuo-ocular symptoms that typically occur following bilateral parieto-occipital lobe lesions. These symptoms include the inability to perceive simultaneous events in one's visual field (simultanagnosia), an inability to fixate and follow an object with one's eyes (optic apraxia), and an impairment of target pointing under visual guidance (optic ataxia). Our patient exhibited simultanagnosia, optic ataxia, left visual field neglect, and impairment of all complex visual-spatial tasks, yet demonstrated normal visual acuity, intact visual fields, and an otherwise normal neurocognitive profile. The patient's visuo-ocular symptoms were noticed while she was participating in rehabilitation for a small right pontine stroke. White matter changes involving both occipital lobes had been incidentally noted on the CT scan revealing the pontine infarction. As the patient relied on sign language and reading ability for communication, these visuo-perceptual limitations hindered her ability to interact with others and gave the appearance of aphasia. We discuss the technical challenges of assessing a patient with significant barriers to communication (e.g., the need for a non-standardized approach, a lack of normative data for such special populations), while pointing out the substantial contributions that can be made by going beyond the standard neuropsychological test batteries.
Topics: Aphasia; Ataxia; Cognition; Deafness; Female; Humans; Middle Aged; Neuropsychological Tests; Occipital Lobe; Stroke; Stroke Rehabilitation; Tomography, X-Ray Computed; Visual Acuity; Visual Fields; Visual Perception
PubMed: 18923965
DOI: 10.1080/13854040802448718 -
International Journal of... Apr 2009We report a psychophysiological study of "recognition without awareness" in patient 2354, who had severe but circumscribed atrophy in the occipitoparietal region...
We report a psychophysiological study of "recognition without awareness" in patient 2354, who had severe but circumscribed atrophy in the occipitoparietal region bilaterally (caused by visual-variant Alzheimer's disease, documented by structural and functional neuroimaging) and an accompanying Balint syndrome that prevented her from recognizing the emotional valence of many highly charged negative visual scenes (e.g., a burned body). Despite this lack of overt recognition, patient 2354 nonetheless generated large amplitude skin conductance responses to highly charged negative pictures, demonstrating the same kind of recognition without awareness that has been reported previously in patients with bilateral occipitotemporal dysfunction and prosopagnosia [e.g., Tranel, D., & Damasio, A. R. (1985). Knowledge without awareness: an autonomic index of facial recognition by prosopagnosics. Science, 228, 1453-1454.]. Our case complements both previous evidence of covert, nonconscious recognition in patients with prosopagnosia, and previous behavioral studies of patients with Balint syndrome that have shown evidence of "preattentive" visual processing. The findings add to the small but important set of empirical observations regarding nonconscious visual processing in neurological patients, and indicate that recognition without awareness can occur in the setting of dorsal visual stream dysfunction and Balint syndrome. The findings in our patient suggest that she has patent pathways from higher-order visual cortices to autonomic effectors in the amygdala or hypothalamus, even though the results of such information processing are not made available to conscious awareness.
Topics: Analysis of Variance; Awareness; Emotions; Female; Fluorodeoxyglucose F18; Galvanic Skin Response; Humans; Magnetic Resonance Imaging; Middle Aged; Neuropsychological Tests; Pattern Recognition, Visual; Photic Stimulation; Positron-Emission Tomography; Prosopagnosia; Recognition, Psychology; Vision, Ocular
PubMed: 18824046
DOI: 10.1016/j.ijpsycho.2008.02.012 -
Neuropsychologia 2008When confronted with two identical stimuli in a very brief period of time subjects often fail to report the second stimulus, a phenomenon termed "repetition blindness"....
When confronted with two identical stimuli in a very brief period of time subjects often fail to report the second stimulus, a phenomenon termed "repetition blindness". The "type-token" account attributes the phenomenon to a failure to individuate the exemplars. We report a subject, KE, who developed simultanagnosia (the inability to see more than one item in an array) as a consequence of bilateral parietal lobe infarctions. With presentation of two words, pictures or letters for an unlimited time, KE typically reported both stimuli on less than half of trials. Performance was significantly influenced by the semantic relationship between items in the array. He reported both items significantly more frequently if they were semantically related; in contrast, when presented either identical or visually different depictions of the same item, he reported both items on only 2-4% of trials. Performance was not influenced by the visual similarity between the stimuli; he reported visually dissimilar objects less frequently than visually similar but different objects. We suggest that KE's bilateral parietal lesions prevent the binding of preserved object representations to a representation computed by the dorsal visual system. More generally, these data are consistent with the claim that the posterior parietal cortex is crucial for individuating a stimulus by computing its unique spatio-temporal characteristics.
Topics: Agnosia; Attention; Cerebral Infarction; Form Perception; Functional Laterality; Humans; Male; Mental Recall; Middle Aged; Neuropsychological Tests; Parietal Lobe; Pattern Recognition, Visual; Photic Stimulation; Psycholinguistics; Reaction Time; Reading; Recognition, Psychology; Semantics; Space Perception; Verbal Behavior; Visual Perception
PubMed: 18514677
DOI: 10.1016/j.neuropsychologia.2007.08.024 -
Behavioural Neurology 2004Following bi-parietal lesions patient AT showed a severe inability to relocate her attention within a visual field which perimetry proved to be near-normal. An...
Following bi-parietal lesions patient AT showed a severe inability to relocate her attention within a visual field which perimetry proved to be near-normal. An experimental approach with tasks testing visuo-spatial attention demonstrated a shrinkage of A.T.'s attentional visual field. With her visual attention narrowed to a kind of functional tunnel vision, the patient exhibited simultanagnosia (Wolpert, 1924), a symptom previously described in 1909 by Balint under the label of Psychic paralysis of "Gaze". In striking contrast AT showed an efficient and effortless perception of complex natural scenes, which, according to recent work in normal subjects, necessitate few if any attentional resources.
Topics: Adult; Agnosia; Attention; Brain Damage, Chronic; Female; Humans; Occipital Lobe; Parietal Lobe; Space Perception; Stroke; Vision Disorders; Visual Fields; Visual Perception
PubMed: 15201489
DOI: 10.1155/2004/836830 -
Developmental Medicine and Child... May 2003A 10-year-old male was referred with difficulties at school. He had particular difficulty with reading long words, following the sequence of text down a page, writing...
A 10-year-old male was referred with difficulties at school. He had particular difficulty with reading long words, following the sequence of text down a page, writing words in the correct order, writing words in line, and copying from the blackboard. He had a history of infective endocarditis complicated by intracerebral haemorrhage at the age of three years. Detailed history taking revealed symptoms typical of 'dorsal stream' pathology, namely a deficit of 'vision for action'. This included a spatial disorder of attention (simultanagnosia), defective hand and foot movements under visual control (optic ataxia), and acquired oculomotor apraxia which are consistent with Balint's syndrome. Strategies were suggested for coping with the symptoms and one year later a distinct improvement in adapting to the disability was found.
Topics: Adaptation, Psychological; Agnosia; Apraxias; Ataxia; Cerebral Infarction; Child; Disabled Persons; Humans; Learning Disabilities; Magnetic Resonance Imaging; Male; Oculomotor Nerve Diseases; Parietal Lobe; Self Care; Syndrome
PubMed: 12729150
DOI: 10.1017/s0012162203000641