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 view” 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 ‘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

Zeitliche und räumliche 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 für die intakte und kohärente Wahrnehmung unserer Umwelt angenommen. Kürzliche Studien konnten zeigen, dass globale Information sowohl in frühen (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 räumlichen Eigenschaften der Formverarbeitung sind jedoch weitestgehend unbekannt. Die aktuelle Doktorarbeit hat sich mit dieser Fragestellung in kombinierten fMRT- und MEG- Studien beschäftigt. Hierbei wurden insbesondere die komplementären Eigenschaften dieser Methoden hinsichtlich Zeit und Raum genutzt. Es wurde ein so genanntes „event-related“ Adaptationsparadigma verwendet, bei dem geringere neuronale Antworten für die aufeinanderfolgende Präsentation 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 für das kurze als auch das lange ISI im LOC, während dies nur für das kurze ISI in den frühen visuellen Arealen der Fall war. Weiterhin wurden räumliche Eigenschaften (Local vs. Global) durch Veränderungen in der lokalen Orietierung der Gabor-Elemente oder einer Variation der globalen Form untersucht. Starke fMRT-Adaptationseffekte auf lokale Änderungen wurden in den frühen visuellen Arealen (V1, V2, VP und V4) beobachtet und zu einem geringeren Ausmaß auch im LOC. Im Gegensatz hierzu zeigten sich fMRT-Adaptationseffekte auf globale Änderungen nur im LOC, nicht jedoch in den frühen visuellen Arealen. Zusammenfassend sind diese Befunde hinweisend für eine transiente Verarbeitung von Forminformation in frühen visuellen Arealen des ventralen Pfades, die innerhalb eines lokalen Bereich um die rezeptiven Felder dieser Neurone stattfindet und auf eine Analyse von hoher räumlicher und zeitlicher Auflösung deutet. Im Gegensatz hierzu ist in der Verarbeitung von Forminformation in höheren visuellen Arealen eher eine grobe räumliche und zeitliche Auflösung implementiert, die sich in anhaltender Analyse äussert. Neben diesen Ergebnissen spielen möglicherweise rekurrente Verbindungen in Form von "feedforward"- und "feedback"-Projektionen zwischen frühen und höheren visuellen Arealen eine kritische Rolle für lokal-globale als global-lokale Mechanismen der Formverarbeitung. Zusätzlich bestätigten 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 räumlicher und zeitlicher Auflösung ermöglichen

Altered time judgements highlight common mechanisms of time and space perception

Space, numbers and time share similar processing mechanisms mediated by parietal cortex. In parallel to the spatial representation of numbers along a horizontal line, temporal information is mapped on a horizontal axis with short intervals (and the past) represented to the left of long intervals (and the future). Little is known about the representation of time in the presence of visuo-spatial deficits. We here report two experiments on the comparative judgment of time. Experiment 1 required patients with left-sided neglect to indicate which of two consecutively presented silent intervals was longer. Their judgments were better if the first interval was longer and they judged the first interval longer on trials in which the two intervals were equally long. These results were not present in right-hemispheric damaged patients without neglect and healthy controls. They are in line with a previously reported finding in a single patient with neglect, but not readily compatible with findings of neglect patients' comparative length judgments. In Experiment 2, healthy participants' performance on an identical task improved for trials with a first-longer interval after caloric vestibular stimulation (CVS) of the right ear with warm water

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 perception

Retinal versus physical stimulus size as determinants of visual perception in simultanagnosia

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

The role of size constancy for the integration of local elements into a global shape

Visual perception depends on the visual context and is likely to be influenced by size constancy, which predicts a size and distance invariant perception of objects. However, size constancy can also result in optical illusions that allow the manipulation of the perceived size. We thus asked whether the integration of local elements into a global object can be influenced by manipulations of the visual context and size constancy? A set of stimuli was applied in healthy individuals that took advantage of the “Kanizsa” illusion, in which three circles with open wedges oriented toward a center point are placed to form an illusionary perception of a triangle. In addition, a 3D-perspective view was implemented in which the global target (“Kanizsa” triangle) was placed in combination with several distractor circles either in a close or a distant position. Subjects were engaged in a global recognition task on the location of the “Kanizsa” triangle. Global recognition of “Kanizsa” triangles improved with a decreasing length of the illusory contour. Interestingly, recognition of “Kanizsa” triangles decreased when they were perceived as if they were located further away. We conclude that the integration of local elements into a global object is dependent on the visual context and dominated by size constancy