Visual and Spatial Short-term Memory in Integrative Agnosia

Visuospatial memory in apraxia: Exploring quantitative drawing metrics to assess the representation of local and global information

Neuropsychological evidence suggests that visuospatial memory is subserved by two separable processing systems, with dorsal underpinnings for global form and ventral underpinnings for the integration of part elements. Previous drawing studies have explored the effects of Gestalt organisation upon memory for hierarchical stimuli, and we here present an exploratory study of an apraxic dorsal stream patient's (MH) performance. We presented MH with a stimulus set (previously reported by Riddoch et al., Cognitive Neuropsychology, 20(7), 641-671, 2003) and devised a novel quantitative scoring system to obtain a finer grain of insight into performance. Stimuli possessed either good or poor Gestalt qualities and were reproduced in a copy condition and two visual memory conditions (with unlimited viewing before the model was removed, or with 3 s viewing). MH's copying performance was impaired in comparison to younger adult and age-matched older adult controls, with a variety of errors at the local level but relatively few at the global level. However, his performance in the visual memory conditions revealed impairments at the global level. For all participants, drawing errors were modulated by the Gestalt qualities of the stimuli, with accuracy at the global and local levels being lesser for poor global stimuli in all conditions. These data extend previous observations of this patient, and support theories that posit interaction between dorsal and ventral streams in the representation of hierarchical stimuli. We discuss the implications of these findings for our understanding of visuospatial memory in neurological patients, and also evaluate the application of quantitative metrics to the interpretation of drawings.

Altered large-scale organization of shape processing in visual agnosia

Recent findings suggest that both dorsal and ventral visual pathways process shape information. Nevertheless, a lesion to the ventral pathway alone can result in visual agnosia, an impairment in shape perception. Here, we explored the neural basis of shape processing in a patient with visual agnosia following a circumscribed right hemisphere ventral lesion and evaluated longitudinal changes in the neural profile of shape representations. The results revealed a reduction of shape sensitivity slopes along the patient's right ventral pathway and a similar reduction in the contralesional left ventral pathway. Remarkably, posterior parts of the dorsal pathway bilaterally also evinced a reduction in shape sensitivity. These findings were similar over a two-year interval, revealing that a focal cortical lesion can lead to persistent large-scale alterations of the two visual pathways. These alterations are consistent with the view that a distributed network of regions contributes to shape perception.

Three-Dimensional Representations of Objects in Dorsal Cortex are Dissociable from Those in Ventral Cortex

An established conceptualization of visual cortical function is one in which ventral regions mediate object perception while dorsal regions support spatial information processing and visually guided action. This division has been contested by evidence showing that dorsal regions are also engaged in the representation of object shape, even when actions are not required. The critical question is whether these dorsal, object-based representations are dissociable from ventral representations, and whether they play a functional role in object recognition. We examined the neural and behavioral profile of patients with impairments in object recognition following ventral cortex damage. In a functional magnetic resonanace imaging experiment, the blood oxygen level-dependent response in the ventral, but not dorsal, cortex of the patients evinced less sensitivity to object 3D structure compared with that of healthy controls. Consistently, in psychophysics experiments, the patients exhibited significant impairments in object perception, but still revealed residual sensitivity to object-based structural information. Together, these findings suggest that, although in the intact system there is considerable crosstalk between dorsal and ventral cortices, object representations in dorsal cortex can be computed independently from those in ventral cortex. While dorsal representations alone are unable to support normal object perception, they can, nevertheless, support a coarse description of object structural information.

Working memory impairment in people with Williams syndrome: effects of delay, task and stimuli

Williams syndrome (WS) is a neurodevelopmental disorder associated with impaired visuospatial representations subserved by the dorsal stream and relatively strong object recognition abilities subserved by the ventral stream. There is conflicting evidence on whether this uneven pattern in WS extends to working memory (WM). The present studies provide a new perspective, testing WM for a single stimulus using a delayed recognition paradigm in individuals with WS and typically developing children matched for mental age (MA matches). In three experiments, participants judged whether a second stimulus 'matched' an initial sample, either in location or identity. We first examined memory for faces, houses and locations using a 5s delay (Experiment 1) and a 2s delay (Experiment 2). We then tested memory for human faces, houses, cat faces, and shoes with a 2s delay using a new set of stimuli that were better controlled for expression, hairline and orientation (Experiment 3). With the 5s delay (Experiment 1), the WS group was impaired overall compared to MA matches. While participants with WS tended to perform more poorly than MA matches with the 2s delay, they also exhibited an uneven profile compared to MA matches. Face recognition was relatively preserved in WS with friendly faces (Experiment 2) but not when the faces had a neutral expression and were less natural looking (Experiment 3). Experiment 3 indicated that memory for object identity was relatively stronger than memory for location in WS. These findings reveal an overall WM impairment in WS that can be overcome under some conditions. Abnormalities in the parietal lobe/dorsal stream in WS may damage not only the representation of spatial location but may also impact WM for visual stimuli more generally.

Impairments in part-whole representations of objects in two cases of integrative visual agnosia

How complex multipart visual objects are represented perceptually remains a subject of ongoing investigation. One source of evidence that has been used to shed light on this issue comes from the study of individuals who fail to integrate disparate parts of visual objects. This study reports a series of experiments that examine the ability of two such patients with this form of agnosia (integrative agnosia; IA), S.M. and C.R., to discriminate and categorize exemplars of a rich set of novel objects, "Fribbles", whose visual similarity (number of shared parts) and category membership (shared overall shape) can be manipulated. Both patients performed increasingly poorly as the number of parts required for differentiating one Fribble from another increased. Both patients were also impaired at determining when two Fribbles belonged in the same category, a process that relies on abstracting spatial relations between parts. C.R., the less impaired of the two, but not S.M., eventually learned to categorize the Fribbles but required substantially more training than normal perceivers. S.M.'s failure is not attributable to a problem in learning to use a label for identification nor is it obviously attributable to a visual memory deficit. Rather, the findings indicate that, although the patients may be able to represent a small number of parts independently, in order to represent multipart images, the parts need to be integrated or chunked into a coherent whole. It is this integrative process that is impaired in IA and appears to play a critical role in the normal object recognition of complex images.

Too Many Trees to See the Forest: Performance, Event-related Potential, and Functional Magnetic Resonance Imaging Manifestations of Integrative Congenital Prosopagnosia

Neuropsychological, event-related potential (ERP), and functional magnetic resonance imaging (fMRI) methods were combined to provide a comprehensive description of performance and neurobiological profiles for K.W., a case of congenital prosopagnosia. We demonstrate that K.W.'s visual perception is characterized by almost unprecedented inability to identify faces, a large bias toward local features, and an extreme deficit in global/configural processing that is not confined to faces. This pattern could be appropriately labeled congenital integrative prosopagnosia, and accounts for some, albeit not all, cases of face recognition impairments without identifiable brain lesions. Absence of face selectivity is evident in both biological markers of face processing, fMRI (the fusiform face area [FFA]), and ERPs (N170). Nevertheless, these two neural signatures probably manifest different perceptual mechanisms. Whereas the N170 is triggered by the occurrence of physiognomic stimuli in the visual field, the deficient face-selective fMRI activation in the caudal brain correlates with the severity of global processing deficits. This correlation suggests that the FFA might be associated with global/configural computation, a crucial part of face identification.

Exploring the role of motion in prosopagnosia: recognizing, learning and matching faces

HJA has been completely unable to recognize faces since suffering a stroke some 22 years ago. Previous research has shown that he is poor at judging expressions from static photographs of faces, but performs relatively normally at these judgements when presented with moving point-light patterns (Humphreys et al., 1993). Recent research with non-prosopagnosic participants has suggested a beneficial role for facial motion when recognizing familiar faces and learning new faces. Three experiments are reported that investigate the role of face motion for HJA when recognizing (Experiment 1), learning (Experiment 2) and matching faces (Experiment 3). The results indicate that HJA is unable to use face motion to explicitly recognize faces and is no better at learning names for moving faces than static ones. However, HJA is significantly better at matching moving faces for identity, an opposite pattern to that found with age-matched and undergraduate control participants. We suggest that HJA is not impaired at processing motion information but remains unable to use motion as a cue to identity.