75 research outputs found
The role of temporo-parietal junction (TPJ) in global Gestalt perception
Grouping processes enable the coherent perception of our environment. A number of brain areas has been suggested to be involved in the integration of elements into objects including early and higher visual areas along the ventral visual pathway as well as motion-processing areas of the dorsal visual pathway. However, integration not only is required for the cortical representation of individual objects, but is also essential for the perception of more complex visual scenes consisting of several different objects and/or shapes. The present fMRI experiments aimed to address such integration processes. We investigated the neural correlates underlying the global Gestalt perception of hierarchically organized stimuli that allowed parametrical degrading of the object at the global level. The comparison of intact versus disturbed perception of the global Gestalt revealed a network of cortical areas including the temporo-parietal junction (TPJ), anterior cingulate cortex and the precuneus. The TPJ location corresponds well with the areas known to be typically lesioned in stroke patients with simultanagnosia following bilateral brain damage. These patients typically show a deficit in identifying the global Gestalt of a visual scene. Further, we found the closest relation between behavioral performance and fMRI activation for the TPJ. Our data thus argue for a significant role of the TPJ in human global Gestalt perceptio
Saliency modulates global perception in simultanagnosia
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 viewâ is exceeded. A possible explanation for these observations might be a shift from global to local saliency in simultanagnosi
Enabling global processing in simultanagnosia by psychophysical biasing of visual pathways
A fundamental aspect of visual cognition is our disposition to see the âforest 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 âforest' but not the âtrees' (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 âforest before the trees
Consensus classification of posterior cortical atrophy
INTRODUCTION: A classification framework for posterior cortical atrophy (PCA) is proposed to improve the uniformity of definition of the syndrome in a variety of research settings. METHODS: Consensus statements about PCA were developed through a detailed literature review, the formation of an international multidisciplinary working party which convened on four occasions, and a Web-based quantitative survey regarding symptom frequency and the conceptualization of PCA. RESULTS: A three-level classification framework for PCA is described comprising both syndrome- and disease-level descriptions. Classification level 1 (PCA) defines the core clinical, cognitive, and neuroimaging features and exclusion criteria of the clinico-radiological syndrome. Classification level 2 (PCA-pure, PCA-plus) establishes whether, in addition to the core PCA syndrome, the core features of any other neurodegenerative syndromes are present. Classification level 3 (PCA attributable to AD [PCA-AD], Lewy body disease [PCA-LBD], corticobasal degeneration [PCA-CBD], prion disease [PCA-prion]) provides a more formal determination of the underlying cause of the PCA syndrome, based on available pathophysiological biomarker evidence. The issue of additional syndrome-level descriptors is discussed in relation to the challenges of defining stages of syndrome severity and characterizing phenotypic heterogeneity within the PCA spectrum. DISCUSSION: There was strong agreement regarding the definition of the core clinico-radiological syndrome, meaning that the current consensus statement should be regarded as a refinement, development, and extension of previous single-center PCA criteria rather than any wholesale alteration or redescription of the syndrome. The framework and terminology may facilitate the interpretation of research data across studies, be applicable across a broad range of research scenarios (e.g., behavioral interventions, pharmacological trials), and provide a foundation for future collaborative work
Percepção viso-motora de escolares com Transtorno do Déficit de Atenção com Hiperatividade
Global shape recognition is modulated by the spatial distance of local elements: Evidence from simultanagnosia
Simultanagnosia is a rare deficit that impairs individuals in perceiving several objects at the same time. It is usually observed following bilateral parieto-occipital brain damage. Despite the restrictions in perceiving the global aspect of a scene, processing of individual objects remains unaffected. The mechanisms underlying simultanagnosia are not well understood. Previous findings indicated that the integration of multiple objects into a holistic representation of the environment is not impossible per se, but might depend on the spatial relationship between individual objects. The present study examined the influence of inter-element distances between individual objects on the recognition of global shapes in two patients with simultanagnosia. We presented Navon hierarchical letter stimuli with different inter-element distances between letters at the Local Scale. Improved recognition at the Global Scale was observed in both patients by reducing the inter-element distance. Global shape recognition in simultanagnosia thus seems to be modulated by the spatial distance of local elements and does not appear to be an all-or-nothing phenomenon depending on spatial continuity. The findings seem to argue against a deficit in visual working memory capacity as the primary deficit in simultanagnosia. However, further research is necessary to investigate alternative interpretations
Spatiotemporal characteristics of form analysis in the human visual cortex revealed by rapid event-related fMRI adaptation.
The integration of local elements to coherent forms is at the core of understanding visual perception. Accumulating evidence suggests that both early retinotopic and higher occipitotemporal areas contribute to the integration of local elements to global forms. However, the spatiotemporal characteristics of form analysis in the human visual cortex remain largely unknown. The aim of this study was to investigate form analysis at different spatial (global vs. local structure) and temporal (different stimulus presentation rates) scales across stages of visual analysis (from V1 to the lateral occipital complexâLOC) in the human brain. We used closed contours rendered by Gabor elements and manipulated either the global contour structure or the orientation of the local Gabor elements. Our rapid event-related fMRI adaptation studies suggest that contour integration and form processing in early visual areas is transient and limited within the local neighborhood of their cells' receptive field. In contrast, higher visual areas appear to process the perceived global form in a more sustained manner. Finally, we demonstrate that these spatiotemporal properties of form processing in the visual cortex are modulated by attention. Attention to the global form maintains sustained processing in occipitotemporal areas, whereas attention to local elements enhances their integration in early visual areas. These findings provide novel neuroimaging evidence for form analysis at different spatiotemporal scales across human visual areas and validate the use of rapid event-related fMRI adaptation for investigating processing across stages of visual analysis in the human brain
Temporal and spatial properties of shape processing in the human visual cortex: combined fMRI and MEG adaptation
Recent studies have shown that global information about shapes is processed in both early ventral (i.e. V1, V2, Vp, V4) and higher occipitotemporal visual areas (i.e. Lateral Occipital Complex/LOC). However, the temporal and spatial properties of shape processing across visual areas in the human brain are largely unknown. The present thesis addressed this question in a combined fMRI and MEG study, that made use of the complimentary spatial and temporal resolution of the two techniques. An event-related adaptation paradigm was applied, in which lower neural responses are observed for two identical than two different consecutivelypresented stimuli. The stimuli were shapes, that consisted of collinear Gabor elements. To investigate the temporal properties of shape processing, the interstimulus interval between the two consecutively-presented stimuli was manipulated (ISI: 100 vs. 400 msec). The results showed adaptation for both the short and the long ISI in the LOC, but only for the short ISI in early visual areas. Further, the spatial properties (Local vs. Global) were tested by changes in the local orientation of the Gabor elements or different global changes. Strong fMRI adaptation effects to local changes were observed in early visual areas (V1, V2, VP and V4) and to a smaller extent also in LOC. In contrast, fMRI adaptation effects to global changes were found only for the LOC, but not the early visual areas. In summary, the findings indicated, that processing of shape information in early visual areas is transient and restricted to a local neighbourhood around the receptive fields of their cells resulting in an analysis at high spatiotemporal resolution in early visual areas. In contrast, a rather coarse spatiotemporal resolution is implemented in the processing of shape information in higher visual areas resulting in sustained analysis. Further, recurrent processing between early and higher visual areas via feedforward and feedback projections might play a critical role in local-to-global and global-to-local mechanisms in shape analysis. In addition, the experiments confirmed the role of event-related fMRI adaptation paradigms as a sensitive tool to study shape analysis at different spatial and temporal scales across visual areas in the human brain and finally indicated that combined fMRI and MEG studies allow the investigation at high temporal and spatial resolution
Zeitliche und rÀumliche Eigenschaften in der Objektverarbeitung im visuellen Cortex des Menschen : kombinierte fMRI und MEG-Adaptationsstudien
Recent studies have shown that global information about shapes is processed in both early ventral (i.e. V1, V2, Vp, V4) and higher occipitotemporal visual areas (i.e. Lateral Occipital Complex/LOC). However, the temporal and spatial properties of shape processing across visual areas in the human brain are largely unknown.
The present thesis addressed this question in a combined fMRI and MEG study, that made use of the complimentary spatial and temporal resolution of the two techniques. An event-related adaptation paradigm was applied, in which lower neural responses are observed for two identical than two different consecutivelypresented stimuli. The stimuli were shapes, that consisted of collinear Gabor elements. To investigate the temporal properties of shape processing, the interstimulus interval between the two consecutively-presented stimuli was manipulated (ISI: 100 vs. 400 msec). The results showed adaptation for both the short and the long ISI in the LOC, but only for the short ISI in early visual areas. Further, the spatial properties (Local vs. Global) were tested by changes in the local orientation of the Gabor elements or different global changes. Strong fMRI adaptation effects to local changes were observed in early visual areas (V1, V2, VP and V4) and to a smaller extent also in LOC. In contrast, fMRI adaptation effects to global changes were found only for the LOC, but not the early visual areas. In summary, the findings indicated, that processing of shape information in early visual areas is transient and restricted to a local neighbourhood around the receptive fields of their cells resulting in an analysis at high spatiotemporal resolution in early visual areas. In contrast, a rather coarse spatiotemporal resolution is implemented in the processing of shape information in higher visual areas resulting in sustained analysis. Further, recurrent processing between early and higher visual areas via feedforward and feedback projections might play a critical role in local-to-global and global-to-local mechanisms in shape analysis. In addition, the experiments confirmed the role of event-related fMRI adaptation paradigms as a sensitive tool to study shape analysis at different spatial and temporal scales across visual areas in the human brain and finally indicated that combined fMRI and MEG studies allow the investigation at high temporal and spatial resolution.Die Integration lokaler Elemente in eine globale Form wird generell als Voraussetzung fĂŒr die intakte und kohĂ€rente Wahrnehmung unserer Umwelt angenommen. KĂŒrzliche Studien konnten zeigen, dass globale Information sowohl in frĂŒhen (z. B. V1, V2, Vp, V4) als auch höheren (z. B. Lateral Occipital Complex/LOC) visuellen Arealen des ventralen Pfades verarbeitet wird, die sich im okzipito-temporalen Cortex befinden. Die zeitlichen und rĂ€umlichen Eigenschaften der Formverarbeitung sind jedoch weitestgehend unbekannt.
Die aktuelle Doktorarbeit hat sich mit dieser Fragestellung in kombinierten fMRT- und MEG- Studien beschĂ€ftigt. Hierbei wurden insbesondere die komplementĂ€ren Eigenschaften dieser Methoden hinsichtlich Zeit und Raum genutzt. Es wurde ein so genanntes „event-related“ Adaptationsparadigma verwendet, bei dem geringere neuronale Antworten fĂŒr die aufeinanderfolgende PrĂ€sentation identischer, im Vergleich zu verschiedener, Stimuli beoabchtet werden. Die Stimuli beinhalteten Formen, die aus kollinear angeordneten Gabor-Elementen bestanden. Um zeitliche Eigenschaften der Formverarbeitung zu untersuchen, wurde das Interstimulus-Interval (ISI) zwischen zwei aufeinanderfolgenden Stimuli manipuliert (ISI 100 vs. 400 msec). Die Ergebnisse zeigten Adaptation sowohl fĂŒr das kurze als auch das lange ISI im LOC, wĂ€hrend dies nur fĂŒr das kurze ISI in den frĂŒhen visuellen Arealen der Fall war. Weiterhin wurden rĂ€umliche Eigenschaften (Local vs. Global) durch VerĂ€nderungen in der lokalen Orietierung der Gabor-Elemente oder einer Variation der globalen Form untersucht. Starke fMRT-Adaptationseffekte auf lokale Ănderungen wurden in den frĂŒhen visuellen Arealen (V1, V2, VP und V4) beobachtet und zu einem geringeren AusmaĂ auch im LOC. Im Gegensatz hierzu zeigten sich fMRT-Adaptationseffekte auf globale Ănderungen nur im LOC, nicht jedoch in den frĂŒhen visuellen Arealen.
Zusammenfassend sind diese Befunde hinweisend fĂŒr eine transiente Verarbeitung von Forminformation in frĂŒhen visuellen Arealen des ventralen Pfades, die innerhalb eines lokalen Bereich um die rezeptiven Felder dieser Neurone stattfindet und auf eine Analyse von hoher rĂ€umlicher und zeitlicher Auflösung deutet. Im Gegensatz hierzu ist in der Verarbeitung von Forminformation in höheren visuellen Arealen eher eine grobe rĂ€umliche und zeitliche Auflösung implementiert, die sich in anhaltender Analyse Ă€ussert. Neben diesen Ergebnissen spielen möglicherweise rekurrente Verbindungen in Form von "feedforward"- und "feedback"-Projektionen zwischen frĂŒhen und höheren visuellen Arealen eine kritische Rolle fĂŒr lokal-globale als global-lokale Mechanismen der Formverarbeitung. ZusĂ€tzlich bestĂ€tigten die Ergebnisse die Bedeutung von "event-related" Adaptationsparadigmen als ein sensitives Instrument zur Untersuchung der Formverarbeitung innerhalb verschiedener Areale des humanen Gehirns und zeigten zuletzt, dass kombinierte fMRT-und MEG- Studien eine Untersuchung mit hoher rĂ€umlicher und zeitlicher Auflösung ermöglichen
Temporal properties of shape processing by event-related MEG adaptation
Object recognition is a fundamental mechanism of visual processing and requires the extraction of shape information. Early visual areas have been linked to the analysis of local image features, while higher visual areas of the ventral visual pathway rather mediate the perception and recognition of global shapes. Investigations of the spatiotemporal characteristics of shape analysis in the human visual cortex by rapid event-related fMRI adaptation in combination with a region of interest analysis suggested a transient manner of contour integration and shape processing in early visual areas compared to sustained processing in higher visual areas. fMRI adaptation (or repetition suppression) paradigms offer the possibility to enhance the restricted spatial resolution of conventional fMRI by focusing on decreased responses for repeated stimulus presentation. However, improving our understanding of complex neuronal mechanisms in the human brain requires the investigation not only at high spatial but also temporal resolution. A limitation of fMRI adaptation can be found in its poor temporal resolution which EEG- and MEG-techniques can overcome, though at a lower spatial resolution. The present study aimed to investigate temporal characteristics of shape processing in the human brain by adapting the principles of fMRI adaption in a MEG study. In parallel to an earlier fMRI study, the two stimuli of a trial were presented at varied interstimulus intervals. Additional analyses by means of a dipole analysis and co-registration of MEG and fMRI data were conducted. Adaptation was observed for the short as well as the longer interstimulus interval. Interestingly, the latency of the adaptation effects varied with the interstimulus interval. The findings support a late onset of adaption that possibly underlies global discrimination processes and recognition in higher areas of the ventral visual pathway. Further, the present results indicate a useful extension of adaptation paradigms and âregion of interestâ-analyses from fMRI to MEG at a high temporal resolution
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