Johannes Rennig

The anatomy underlying acute versus chronic spatial neglect: a longitudinal study

Our aim was to examine how brain imaging in the initial phase of a stroke could predict both acute/subacute as well as chronic spatial neglect. We present the first voxel-wise longitudinal lesion-behaviour mapping study, examining acute/subacute as well as chronic performance in the same individuals. Acute brain imaging (acquired on average 6.2 days post-injury) was used to evaluate neglect symptoms at the initial (mean 12.4 days post-stroke) and the chronic (mean 491 days) phase of the stroke. Chronic neglect was found in about one-third of the patients with acute neglect. Analysis suggests that lesion of the superior and middle temporal gyri predict both acute/subacute as well as chronic neglect. At the subcortical level, the basal ganglia as well as the inferior occipitofrontal fasciculus/extreme capsule appear to play a significant role for both acute/subacute as well as chronic neglect. Beyond, the uncinate fasciculus was critically related to the emergence of chronic spatial neglect. We infer that individuals who experience spatial neglect in the initial phase of the stroke yet do not have injury to these cortical and subcortical structures are likely to recover, and thus have a favourable prognosis.

The temporo-parietal junction contributes to global gestalt perception—evidence from studies in chess experts

In a recent neuroimaging study the comparison of intact vs. disturbed perception of global gestalt indicated a significant role of the temporo-parietal junction (TPJ) in the intact perception of global gestalt (Huberle and Karnath, 2012). This location corresponded well with the areas known to be damaged or impaired in patients with simultanagnosia after stroke or due to neurodegenerative diseases. It was concluded that the TPJ plays an important role in the integration of individual items to a holistic percept. Thus, increased BOLD signals should be found in this region whenever a task calls for the integration of multiple visual items. Behavioral experiments in chess experts suggested that their superior skills in comparison to chess novices are partly based on fast holistic processing of chess positions with multiple pieces. We thus analyzed BOLD data from four fMRI studies that compared chess experts with chess novices during the presentation of complex chess-related visual stimuli (Bilalić et al., 2010, 2011a,b, 2012). Three regions of interests were defined by significant TPJ clusters in the abovementioned study of global gestalt perception (Huberle and Karnath, 2012) and BOLD signal amplitudes in these regions were compared between chess experts and novices. These cross-paradigm ROI analyses revealed higher signals at the TPJ in chess experts in comparison to novices during presentations of complex chess positions. This difference was consistent across the different tasks in five independent experiments. Our results confirm the assumption that the TPJ region identified in previous work on global gestalt perception plays an important role in the processing of complex visual stimulus configurations.

Fact learning in complex arithmetic—the role of the angular gyrus revisited

In recent theoretical considerations as well as in neuroimaging findings the left angular gyrus (AG) has been associated with the retrieval of arithmetic facts. This interpretation was corroborated by higher AG activity when processing trained as compared with untrained multiplication problems. However, so far neural correlates of processing trained versus untrained problems were only compared after training. We employed an established learning paradigm (i.e., extensive training of multiplication problems) but measured brain activation before and afte training to evaluate neural correlates of arithmetic fact acquisition more specifically. When comparing activation patterns for trained and untrained problems of the post‐training session, higher AG activation for trained problems was replicated. However, when activation for trained problems was compared to activation for the same problems in the pre‐training session, no signal change in the AG was observed. Instead, our results point toward a central role of hippocampal, para‐hippocampal, and retrosplenial structures in arithmetic fact retrieval. We suggest that the AG might not be associated with the actual retrieval of arithmetic facts, and outline an attentional account of the role of the AG in arithmetic fact retrieval that is compatible with recent attention to memory hypotheses. Hum Brain Mapp 37:3061–3079, 2016.

Investigating structure and function in the healthy human brain: validity of acute versus chronic lesion-symptom mapping.

Modern voxel-based lesion-symptom mapping (VLSM) analyses techniques provide powerful tools to examine the relationship between structure and function of the healthy human brain. However, there is still uncertainty on the type of and the appropriate time point of imaging and of behavioral testing for such analyses. Here we tested the validity of the three most common combinations of structural imaging data and behavioral scores used in VLSM analyses. Given the established knowledge about the neural substrate of the primary motor system in humans, we asked the mundane question of where the motor system is represented in the normal human brain, analyzing individual arm motor function of 60 unselected stroke patients. Only the combination of acute behavioral scores and acute structural imaging precisely identified the principal brain area for the emergence of hemiparesis after stroke, i.e., the corticospinal tract (CST). In contrast, VLSM analyses based on chronic behavior—in combination with either chronic or acute imaging—required the exclusion of patients who had recovered from an initial paresis to reveal valid anatomical results. Thus, if the primary research aim of a VLSM lesion analysis is to uncover the neural substrates of a certain function in the healthy human brain and if no longitudinal designs with repeated evaluations are planned, the combination of acute imaging and behavior represents the ideal dataset.

Involvement of the tpj area in processing of novel global forms

The neuropsychological syndrome “simultanagnosia” is characterized by the inability to integrate local elements into a global entity. This deficit in Gestalt perception is mainly apparent for novel global structures administered in clinical tests or unfamiliar visual scenes. Recognition of familiar complex objects or well-known visual scenes is often unaffected. Recent neuroimaging studies and reports from simultanagnosia patients suggest a crucial involvement of temporoparietal brain areas in processing of hierarchically organized visual material. In this study, we investigated the specific role of the TPJ in Gestalt perception. On the basis of perceptual characteristics known from simultanagnosia, we hypothesized that TPJ is dominantly involved in processing of novel object arrangements. To answer this question, we performed a learning study with hierarchical stimuli and tested behavioral and neuronal characteristics of Gestalt perception pre- and posttraining. The study included 16 psychophysical training sessions and two neuroimaging sessions. Participants improved their behavioral performance for trained global stimuli and showed limited transfer to untrained global material. We found significant training dependent neuronal signal modulations in anterior right hemispheric TPJ regions. These activation changes were specific to trained global stimuli, whereas no systematic neuronal response changes were observed for recognition of untrained global stimuli, local elements and regular objects that served as control stimuli. In line with perceptual characteristics in simultanagnosia, the results argue for an involvement of TPJ in processing of novel global structures. We discuss the signal modulations in the context of a more efficient or different neuronal strategy to process familiar global stimuli.

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.

Hemifield coding in ventral object-sensitive areas – Evidence from visual hemiagnosia

Electrophysiological monkey and human neuroimaging studies have reported a lateralization of signal processing in object perception. However, it is unclear whether these results point to a unique topographically organized signal processing in either hemisphere, or if these results represent a rather negligible spatial organization of otherwise redundant object perception systems in both hemispheres. We tested a group of 10 patients with lesions to ventral object processing regions and spared primary visual functions with lateral presentations of different categories of object stimuli. Object perception in the contralesional visual field was impaired while object perception on the ipsilesional hemifield was intact. These results demonstrate that the object perception system needs two intact ventral pathways for unimpaired object perception across the whole visual field; the loss of one system cannot be fully compensated by its contralateral homolog or spared parts of the lesioned ventral stream.

Preserved Expert Object Recognition in a Case of Visual Hemiagnosia

We examined a stroke patient (HWS) with a unilateral lesion of the right medial ventral visual stream, involving the right fusiform and parahippocampal gyri. In a number of object recognition tests with lateralized presentations of target stimuli, HWS showed significant symptoms of hemiagnosia with contralesional recognition deficits for everyday objects. We further explored the patients capacities of visual expertise that were acquired before the current perceptual impairment became effective. We confronted him with objects he was an expert for already before stroke onset and compared this performance with the recognition of familiar everyday objects. HWS was able to identify significantly more of the specific (“expert”) than of the everyday objects on the affected contralesional side. This observation of better expert object recognition in visual hemiagnosia allows for several interpretations. The results may be caused by enhanced information processing for expert objects in the ventral system in the affected or the intact hemisphere. Expert knowledge could trigger top–down mechanisms supporting object recognition despite of impaired basic functions of object processing. More importantly, the current work demonstrates that top–down mechanisms of visual expertise influence object recognition at an early stage, probably before visual object information propagates to modules of higher object recognition. Because HWS showed a lesion to the fusiform gyrus and spared capacities of expert object recognition, the current study emphasizes possible contributions of areas outside the ventral stream to visual expertise.

Magnitude processing of symbolic and non-symbolic proportions: an fMRI study

BackgroundRecent research indicates that processing proportion magnitude is associated with activation in the intraparietal sulcus. Thus, brain areas associated with the processing of numbers (i.e., absolute magnitude) were activated during processing symbolic fractions as well as non-symbolic proportions. Here, we investigated systematically the cognitive processing of symbolic (e.g., fractions and decimals) and non-symbolic proportions (e.g., dot patterns and pie charts) in a two-stage procedure. First, we investigated relative magnitude-related activations of proportion processing. Second, we evaluated whether symbolic and non-symbolic proportions share common neural substrates.MethodsWe conducted an fMRI study using magnitude comparison tasks with symbolic and non-symbolic proportions, respectively. As an indicator for magnitude-related processing of proportions, the distance effect was evaluated.ResultsA conjunction analysis indicated joint activation of specific occipito-parietal areas including right intraparietal sulcus (IPS) during proportion magnitude processing. More specifically, results indicate that the IPS, which is commonly associated with absolute magnitude processing, is involved in processing relative magnitude information as well, irrespective of symbolic or non-symbolic presentation format. However, we also found distinct activation patterns for the magnitude processing of the different presentation formats.ConclusionOur findings suggest that processing for the separate presentation formats is not only associated with magnitude manipulations in the IPS, but also increasing demands on executive functions and strategy use associated with frontal brain regions as well as visual attention and encoding in occipital regions. Thus, the magnitude processing of proportions may not exclusively reflect processing of number magnitude information but also rather domain-general processes.

Simultanagnosia does not affect processes of auditory Gestalt perception

ABSTRACT Simultanagnosia is a neuropsychological deficit of higher visual processes caused by temporo‐parietal brain damage. It is characterized by a specific failure of recognition of a global visual Gestalt, like a visual scene or complex objects, consisting of local elements. In this study we investigated to what extend this deficit should be understood as a deficit related to specifically the visual domain or whether it should be seen as defective Gestalt processing per se. To examine if simultanagnosia occurs across sensory domains, we designed several auditory experiments sharing typical characteristics of visual tasks that are known to be particularly demanding for patients suffering from simultanagnosia. We also included control tasks for auditory working memory deficits and for auditory extinction. We tested four simultanagnosia patients who suffered from severe symptoms in the visual domain. Two of them indeed showed significant impairments in recognition of simultaneously presented sounds. However, the same two patients also suffered from severe auditory working memory deficits and from symptoms comparable to auditory extinction, both sufficiently explaining the impairments in simultaneous auditory perception. We thus conclude that deficits in auditory Gestalt perception do not appear to be characteristic for simultanagnosia and that the human brain obviously uses independent mechanisms for visual and for auditory Gestalt perception. HIGHLIGHTSSimultanagnosia does not appear to occur across sensory domains.Visual simultanagnosia without comparable deficits in auditory Gestalt perception.Visual simultanagnosia thus does not appear to affect the auditory domain.The brain uses independent mechanisms for visual and auditory Gestalt perception.