Coding space within but not between objects: evidence from Balint's syndrome

Beyond visual integration: sensitivity of the temporal-parietal junction for objects, places, and faces

One important role of the TPJ is the contribution to perception of the global gist in hierarchically organized stimuli where individual elements create a global visual percept. However, the link between clinical findings in simultanagnosia and neuroimaging in healthy subjects is missing for real-world global stimuli, like visual scenes. It is well-known that hierarchical, global stimuli activate TPJ regions and that simultanagnosia patients show deficits during the recognition of hierarchical stimuli and real-world visual scenes. However, the role of the TPJ in real-world scene processing is entirely unexplored. In the present study, we first localized TPJ regions significantly responding to the global gist of hierarchical stimuli and then investigated the responses to visual scenes, as well as single objects and faces as control stimuli. All three stimulus classes evoked significantly positive univariate responses in the previously localized TPJ regions. In a multivariate analysis, we were able to demonstrate that voxel patterns of the TPJ were classified significantly above chance level for all three stimulus classes. These results demonstrate a significant involvement of the TPJ in processing of complex visual stimuli that is not restricted to visual scenes and that the TPJ is sensitive to different classes of visual stimuli with a specific signature of neuronal activations.

Visuospatial representation in patients with mild cognitive impairment: Implication for rehabilitation

Behavioral and neurophysiological experiments have demonstrated that distinct and common cognitive processes and associated neural substrates maintain allocentric and egocentric spatial representations. This review aimed to provide evidence from previous behavioral and neurophysiological studies on collating cognitive processes and associated neural substrates and linking them to the state of visuospatial representations in patients with mild cognitive impairment (MCI). Even though MCI patients showed impaired visuospatial attentional processing and working memory, previous neuropsychological experiments in MCI largely emphasized memory impairment and lacked substantiating evidence of whether memory impairment could be associated with how patients with MCI encode objects in space. The present review suggests that impaired memory capacity is linked to impaired allocentric representation in MCI patients. This review indicates that further research is needed to examine how the decline in visuospatial attentional resources during allocentric coding of space could be linked to working memory impairment.

Temporo-parietal brain regions are involved in higher order object perception

Lesions to posterior temporo-parietal brain regions are associated with deficits in perception of global, hierarchical shapes, but also with impairments in the processing of objects presented under demanding viewing conditions. Evidence from neuroimaging studies and lesion patterns observed in patients with simultanagnosia and agnosia for object orientation suggest similar brain regions to be involved in perception of global shapes and processing of objects in atypical ('non-canonical') orientation. In a localizer experiment, we identified individual temporo-parietal brain areas involved in global shape perception and found significantly higher BOLD signals during the processing of non-canonical compared to canonical objects. In a multivariate approach, we demonstrated that posterior temporo-parietal brain areas show distinct voxel patterns for non-canonical and canonical objects and that voxel patterns of global shapes are more similar to those of objects in non-canonical compared to canonical viewing conditions. These results suggest that temporo-parietal brain areas are not only involved in global shape perception but might serve a more general mechanism of complex object perception. Our results challenge a strict attribution of object processing to the ventral visual stream by suggesting specific dorsal contributions in more demanding viewing conditions.

Cortical Hemodynamic Response Associated with Spatial Coding: A Near-Infrared Spectroscopy Study

Allocentric and egocentric are two types of spatial coding. Previous studies reported the dorsal attention network's involvement in both types. To eliminate possible paradigm-specific confounds in the results, this study employed fine-grained cue-to-target paradigm to dissociate allocentric (aSC) and egocentric (eSC) spatial coding. Twenty-two participants completed a custom visuospatial task, and changes in the concentration of oxygenated hemoglobin (O₂-Hb) were recorded using functional near-infrared spectroscopy (fNIRS). The least absolute shrinkage and selection operator-regularized principal component (LASSO-RPC) algorithm was used to identify cortical sites that predicted the aSC and eSC conditions' reaction times. Significant changes in O₂-Hb concentration in the right inferior parietal lobule (IPL) and post-central gyrus regions were common in both aSC and eSC. Results of inter-channel correlations further substantiate cortical activities in both conditions were predominantly over the right parieto-frontal areas. Together with right superior frontal gyrus areas be the reaction time neural correlates, the results suggest top-down attention and response-mapping processes are common to both spatial coding types. Changes unique to aSC were in clusters over the right intraparietal sulcus, right temporo-parietal junction, and left IPL. With the left pre-central gyrus region, be the reaction time neural correlate, aSC is likely to involve more orienting attention, updating of spatial information, and object-based response selection and inhibition than eSC. Future studies will use other visuospatial task designs for testing the robustness of the findings on spatial coding processes.

The one-is-more illusion: Sets of discrete objects appear less extended than equivalent continuous entities in both space and time

We distinguish between discrete objects and continuous entities in categorization and language, but might we actually see such stimuli differently? Here we report the one-is-more illusion, wherein 'objecthood' changes what we perceive in an unexpected way. Across many variations and tasks, observers perceived a single continuous object (e.g. a rectangle) as longer than an equated set of multiple discrete objects (e.g. two shorter rectangles separated by a gap). This illusion is phenomenologically compelling, exceptionally reliable, and it extends beyond space, to time: a single continuous tone is perceived to last longer than an equated set of multiple discrete tones. Previous work has emphasized the importance of objecthood for processes such as attention and visual working memory, but these results typically require careful analyses of subtle effects. In contrast, we provide striking demonstrations of how perceived objecthood changes the perception of other properties in a way that you can readily see (and hear!) with your own eyes (and ears!).

Simultanagnosia does not affect processes of auditory Gestalt perception

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.

A world unglued: simultanagnosia as a spatial restriction of attention

Simultanagnosia is a disorder of visual attention that leaves a patient's world unglued: scenes and objects are perceived in a piecemeal manner. It is generally agreed that simultanagnosia is related to an impairment of attention, but it is unclear whether this impairment is object- or space-based in nature. We first consider the findings that support a concept of simultanagnosia as deficit of object-based attention. We then examine the evidence suggesting that simultanagnosia results from damage to a space-based attentional system, and in particular a model of simultanagnosia as a narrowed spatial window of attention. We ask whether seemingly object-based deficits can be explained by space-based mechanisms, and consider the evidence that object processing influences spatial deficits in this condition. Finally, we discuss limitations of a space-based attentional explanation.

Implicit location encoding via stored representations of familiar objects: neuropsychological evidence

We report data on the visual localisation ability of a patient with Balint's syndrome, GK. We show that, with relatively long exposures of displays, GK is better able to judge the spatial relations between separate objects (a "between-object judgement") than the spatial relations between a part and a whole object (a "within-object judgement") (Experiments 1-3). This dissociation occurred even when the same stimulus was used for both judgements, and the task instructions biased GK to parse the stimulus as either a single or as two separate objects (Experiments 2 and 6). However, when he could use a stored representation to make a within-object judgement, then performance was better than on a comparable spatial judgement of the relations between two separate objects (Experiments 4-7). The data demonstrate that stored representations of objects can support the spatial coding of parts to perceptual wholes. In the absence of stored representations, part-whole relations must be explicitly coded by attention, a process that is impaired in this patient.

Neuropsychological evidence for a competitive bias against contracting stimuli

Two experiments examined extinction to stimuli presented either with contracting or expanding motion. Experiment 1 used solid shapes which either increased or decreased in size rapidly, consistent with looming motion. Experiment 2 employed random dots so that stimulus size was not confounded with type of motion. In both experiments extinction was modulated by the type of motion presented, with extinction most evident when a contracting object was in the weaker visual field. In addition, in Experiment 2 there was evidence for grouping modulating extinction, when there were looming stimuli in both fields. The results suggest that looming motion is a powerful determinant of stimulus salience in selective attention.

Attention, competition, and the parietal lobes: insights from Balint's syndrome

Simultanagnosia (resulting from occipito-parietal damage) is a profound visual deficit, which impairs the ability to perceive multiple items in a visual display, while preserving the ability to recognise single objects. Here we demonstrate in a patient presenting with Balint's syndrome that this deficit may result from an extreme form of competition between objects which makes it difficult for attention to be disengaged from an object once it has been selected.

What is Meant by Impaired Configural Processing in Acquired Prosopagnosia?

Apperceptive prosopagnosia is supposedly characterised by impaired configural processing, which could refer to either perception of spatial structure or holistic mechanisms. Ten prosopagnosic patients were tested with (i) dot patterns, to determine if manipulations of complexity, size, orientation, or the regularity of global structure generated effects consistent with the holistic hypothesis; and (ii) hierarchical letters, to probe for a global ‘whole-object’ processing deficit. With dot patterns (experiment 1) patients were impaired even for simple two-dot stimuli, but did better with more complex patterns, when size or orientation varied, or with a regular global structure. In experiment 2, they demonstrated normal latency effects of global-level processing. Apperceptive prosopagnosia, including that from lesions encompassing the right fusiform gyrus, is thus associated with a ‘configural deficit’ that impairs perception of spatial structure, not just for faces but also for non-facial patterns. While it cannot be concluded that holistic processing is entirely normal in these subjects, their performance shows significant modulation by whole-object structure, indicating that some whole-object processing is occurring in these patients.

Vision and touch: multiple or multisensory representations of objects?

The relationship between visually and haptically derived representations of objects is an important question in multisensory processing and, increasingly, in mental representation. We review evidence for the format and properties of these representations, and address possible theoretical models. We explore the relevance of visual imagery processes and highlight areas for further research, including the neglected question of asymmetric performance in the visuo-haptic cross-modal memory paradigm. We conclude that the weight of evidence suggests the existence of a multisensory representation, spatial in format, and flexibly accessible by both bottom-up and top-down inputs, although efficient comparison between modality-specific representations cannot entirely be ruled out.

The “Special Effect” of Case Mixing on Word Identification: Neuropsychological and Transcranial Magnetic Stimulation Studies Dissociating Case Mixing from Contrast Reduction

We present neuropsychological and transcranial magnetic stimulation (TMS) evidence with normal readers, that the effects of case mixing and contrast reduction on word identification are qualitatively different. Lesions and TMS applied to the right parietal lobe selectively disrupted the identification of mixed relative to single-case stimuli. Bilateral lesions and TMS applied to the occipital cortex selectively disrupted the identification of low-contrast words. These data suggest that different visual distortions (case mixing, contrast reduction) exert different effects on reading, modulated by contrasting brain regions. Case mixing is a “special” distortion and involves the recruitment of processes that are functionally distinct, and dependent on different regions in the brain, from those required to deal with contrast reduction.

Attending to space within and between objects: Implications from a patient with Balint's syndrome

Neuropsychological conditions such as Balint's syndrome have shown that perceptual organization of parts into a perceptual unit can be dissociated from the ability to localize objects relative to each other. Neural mechanisms that code the spatial structure within individual objects or words may seem to be intact, while between-object structure is compromised. Here we investigate the nature of within-object spatial processing in a patient with Balint's syndrome (RM). We suggest that within-object spatial structure can be determined (a) directly by explicit spatial processing of between-part relations, mediated by the same dorsal pathway as between-object spatial relations; or (b) indirectly by the discrimination of object identities, which may involve implicit processing of between-part relations and which is probably mediated by the ventral system. When this route is ruled out, by testing discrimination of differences in part location that do not change the identity of the object, we find no evidence of explicit within-object spatial coding in a patient without functioning parietal lobes.

Restricted ocular exploration does not seem to explain simultanagnosia

One major function of parietal cortex is to direct our attention towards salient stimuli. The present data suggest that it also plays an important role in visual gestalt perception. Patients with simultanagnosia following lesions in this area are not able to extract the meaning of a visual scene whereas being perfectly able to recognise individual objects of this scene. We tested two patients with simultanagnosia with hierarchical Navon figures combined with eye movements recordings. The patients' performance allowed us to compare directly the scan paths in trials in which the global letter shape was recognised with trials in which the global letter shape was not recognised. We did not find any obvious differences in the eye movement pattern related to the two perceptual situations. The two patients did not show a significant problem in shifting their eyes (and thus possibly also their attentional focus) to all aspects of the complex visual stimulus when attempting to bind together the different elements of spatially distributed information. The results demonstrate that restricted ocular exploration cannot be the reason for the patients' inability to recognise the global shape of stimuli. Our data rather suggest a role of parietal cortex in visual gestalt perception that is beyond its role of directing attention towards relevant objects.

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.

Reflexive shifts of covert attention operate in an egocentric coordinate frame

Covert shifts of attention have been shown to improve detection and discrimination thresholds for a range of visual stimuli. Although there is some evidence to suggest that the allocation of attention to a particular region of interest occurs in a retinotopic frame of reference, the importance of an allocentric, or object-based, framework has gained widespread empirical support. The current experiment investigates the nature of the spatial representation in which covert shifts of attention occur in response to a reflexive prime. Primes and targets were presented in four conditions designed to vary systematically the validity of the spatial relationship between the prime and target in egocentric or allocentric coordinate frameworks. A significant advantage, in terms of reaction time and correct identification, was found for targets located in positions previously primed in an egocentric (but not allocentric) framework whereas there was no advantage for locations primed in an allocentric (but not egocentric) framework. These results suggest that the allocation of covert spatial attention within an egocentric framework may be more important than previously thought.