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 complex—LOC) 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

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 ‘region of interest’-analyses from fMRI to MEG at a high temporal resolution

Processing of global vs. Local shape information in the human visual cortex

Coherent visual perception requires the integration of local elements into global shapes. However, the involvement of the various visual areas in the integration of local features into global shapes remains largely unknown. We used event-related fMRI to test for local and global shape processing in visual areas known to be involved in the processing of shapes. The stimuli consisted of images of houses or faces (global shapes) rendered by smaller images of stimuli from these categories (local shapes). We tested four conditions: a) global faces rendered by local faces; b) global faces rendered by local houses; c) global houses rendered by local faces and d) global houses rendered by local houses. Subjects were instructed to judge whether global and local shapes where from the same or different categories. We tested for responses in the Fusiform Face Area (FFA) known to respond selectively to faces (Kanwisher et al., 1997) and the Parahippocampal Place Area (PPA) known to be involved in the analysis of spatial layout (Epstein et al., 1998). Our results showed strong fMRI responses for global faces in the FFA and global houses in the PPA independent of the stimulus category at the local level. Lower category specific responses to the local shapes were observed when the global shapes were from a different category than the local shapes. These results suggest differential processing of global and local shape information in category selective areas. Further studies will test for the processing of global and local shape information at different stimulus scales

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 simultanagnosia

Global and Local Mechanisms of Shape Processing in the Human Visual Cortex

Coherent visual perception requires the integration of local elements into global shapes. However, the involvement of the various visual areas in the integration of local features into global shapes remains largely unknown.Event-related fMRI was used to test for local and global mechanisms of shape processing in higher visual object related areas. We tested for responses in the Fusiform Face Area (FFA) known to respond selectively to faces [1] and the Parahippocampal Place Area (PPA) known to be involved in the analysis of spatial layout [2]. The stimuli consisted of images of houses or faces (global shapes) rendered by smaller images of stimuli from these categories (local shapes). We tested four conditions: a) global faces rendered by local faces; b) global faces rendered by local houses; c) global houses rendered by local faces and d) global houses rendered by local houses. Subjects were instructed to judge whether global and local shapes where from the same or different categories. Our results showed strong fMRI responses for global faces in the FFA and global houses in the PPA independent of the stimulus category at the local level. Lower category specic responses to the local shapes were observed when the global shapes were from a different category than the local shapes. Further studies tested for fMRI responses at different stimulus scales and attentional shifts. Stronger responses to the local faces in the FFA and local houses in the PPA were observed compared to global faces and global houses. Our results suggest differential processing of global and local shape information in category selective areas. Furthermore, attention and spatial scale inuence the processing of local and global shape information

Perception of biological motion in visual agnosia

Over the past 25 years, visual processing has been discussed in the context of the dual stream hypothesis consisting of a ventral (“what”) and a dorsal (“where”) visual information processing pathway. Patients with brain damage of the ventral pathway typically present with signs of visual agnosia, the inability to identify and discriminate objects by visual exploration, but show normal perception of motion perception. A dissociation between the perception of biological motion and non-biological motion has been suggested: perception of biological motion might be impaired when “non-biological” motion perception is intact and vice versa. The impact of object recognition on the perception of biological motion remains unclear. We thus investigated this question in a patient with severe visual agnosia, who showed normal perception of non-biological motion. The data suggested that the patient's perception of biological motion remained largely intact. However, when tested with objects constructed of coherently moving dots (“Shape-from-Motion”), recognition was severely impaired. The results are discussed in the context of possible mechanisms of biological motion perception

Visual feature integration is a function of the temporo-parietal-junction

The integration of individual features is a fundamental process for the intact perception of a global scene. Disturbance of this process has been found in patients with Bálint's syndrome following bilateral damage of the temporo-parietal cortex. However, the cortical mechanisms underlying grouping processes remain largely unknown. In an event-related fMRI study, we investigated the neural correlates of spatial integration of visual features in healthy subjects. We compared the observed neural activity with the psychophysical performance and tested for attentional modulation of the fMRI responses. The stimuli consisted of images of circles or squares (global level) rendered by smaller images of stimuli from these categories (local level) resulting in four combinations: a) global circle, local circle; b) global circle, local square; c) global square, local circle and d) global square, local square. Furthermore, we parametrically degraded the objects at the global level by exchanging the smaller images of objects at the local level with each other resulting in the following conditions: 1) Intact Global Perception at 20%- Scrambled, 2) Reduced Global Perception at 40%- Scrambled, 3) Reduced Global Perception at 60%- Scrambled and 4) Disturbed Global Perception at 80%- Scrambled. Subjects were engaged in a 2AFC-task on the category at the global (Experiment 1) or local (Experiment 2) level. The fMRI responses of Experiment 1 revealed a network of cortical areas to be involved in the spatial integration of visual features including the precuneus (PC), anterior cingulate cortex (ACC) and the temporo-parieto-occipital junction (TPJ). Strong relation between neural activity and psychophysical performance was found for TPJ but not PC and ACC. In contrast, no significant differences across conditions were observed in the fMRI responses of Experiment 2. Our results suggest a critical involvement of the TPJ in the spatial integration of visual features