Convergence Along the Visual Hierarchy Is Altered in Posterior Cortical Atrophy

The Role of Population Receptive Field Sizes in Higher-Order Visual Dysfunction

PURPOSE OF REVIEW

Population receptive field (pRF) modeling is an fMRI technique used to retinotopically map visual cortex, with pRF size characterizing the degree of spatial integration. In clinical populations, most pRF mapping research has focused on damage to visual system inputs. Herein, we highlight recent work using pRF modeling to study high-level visual dysfunctions.

RECENT FINDINGS

Larger pRF sizes, indicating coarser spatial processing, were observed in homonymous visual field deficits, aging, and autism spectrum disorder. Smaller pRF sizes, indicating finer processing, were observed in Alzheimer's disease and schizophrenia. In posterior cortical atrophy, a unique pattern was found in which pRF size changes depended on eccentricity. Changes to pRF properties were observed in clinical populations, even in high-order impairments, explaining visual behavior. These pRF changes likely stem from altered interactions between brain regions. Furthermore, some studies suggested that pRF sizes change as part of cortical reorganization, and they can point towards future prognosis.

Global Processing Deficit in Amnestic Mild Cognitive Impairment

Background

Visuo-perceptual and visuo-attentional disorders, such as global processing deficit and simultanagnosia, are not routinely investigated in prodromal forms of typical Alzheimer's disease, as amnestic mild cognitive impairment (MCI).

Objective

This study evaluated global processing abilities through Navon's classical paradigm in individuals with amnestic MCI and investigated the related visuo-perceptual and attentional components involved in simultanagnosia.

Methods

Sixteen consecutive patients with amnestic MCI (6 single-domain, 10 multiple-domain) and 16 matched controls were requested to identify global and local elements of hierarchical Navon letters, and to name large and small solid letters.

Results

While correctly identifying solid letters, patients with multiple-domain amnestic MCI were less accurate in processing the global level of hierarchical stimuli compared to controls. Single-case analyses suggested that global processing may also be impaired in single-domain amnestic MCI. In addition, patients with pathological performance in the Navon task showed perceptual and/or visual focal attention deficits.

Conclusions

Early dysfunction of holistic processing can be detected in amnestic MCI. Visuo-perceptual and/or visual focal attention mechanisms, which have been shown to be damaged in Posterior Cortical Atrophy patients with simultanagnosia, may be impaired in individuals with amnestic MCI. Investigation and identification of global processing deficits in MCI could contribute to early diagnosis and longitudinal monitoring of the disease.

Abnormal network connections to early visual cortex in posterior cortical atrophy

INTRODUCTION

Posterior Cortical Atrophy (PCA), a visual variant of Alzheimer's disease, initially manifests with higher-order visual disorders and parieto/temporo-occipital atrophy. Recent studies have shown remote functional impairment in both distant brain networks and along the calcarine sulcus (V1). Functional alteration in the calcarine differs along its length, reflecting center to periphery visual space differences. Herein, we aim to connect between these two sets of findings by looking at the retinotopic patterns of functional connectivity between large-scale brain networks and V1, comparing patients with normally sighted subjects.

METHODS

Resting state functional magnetic resonance imaging (fMRI) and T1 anatomical scans were obtained from 11 PCA patients and 17 age-matched healthy volunteers. Default mode network (DMN) and fronto parietal network (FPN) were defined and differences between the networks in patients and healthy controls were evaluated at the whole brain level, specifically their connectivity to V1.

RESULTS

Connectivity patterns within the DMN and the FPN were similar between the groups, although differences were found in regions within and beyond the networks. Focusing on V1, in the control group we identified the expected pattern of a distributed connectivity along eccentricity, with foveal regions showing stronger connectivity to the FPN and peripheral regions showing stronger connectivity to the DMN. However, in PCA patients we could not identify a clear difference in connectivity along the eccentricities.

CONCLUSION

Lost specialization of function along the calcarine in PCA patients may have further implications on large-scale networks or vice versa. This impairment, distant from the core pathology, might explain patients' visual disabilities.

The clinical approach to the identification of higher-order visual dysfunction in neurodegenerative disease

PURPOSE OF REVIEW

This review is intended to assist the reader in gaining the knowledge and skills necessary for the recognition and assessment of higher-order visual dysfunction due to neurodegenerative diseases including Alzheimer's disease, dementia with Lewy bodies, Parkinson's dementia, corticobasal degeneration, Creutzfeldt-Jakob disease, and the posterior cortical atrophy syndrome. Clinical problem-solving and pattern recognition must be developed and practiced to accurately diagnosis disturbances of higher-order visual function, and knowledge of higher-order visual brain regions and their visual syndromes forms the foundation for deciphering symptoms presented by patients and/or their care partners. Tests of higher-order visual dysfunction must be assembled by the clinician and assessment can take time and effort. The use of screening tests, follow-up visits, and formal neuropsychological referrals are critical components for accurate diagnosis and these principles are reviewed here.

RECENT FINDINGS

A recent survey of neuro-ophthalmologists revealed that over half of the respondents report that 5-10% of their new patient referrals carry a diagnosis of neurodegenerative disease and many patients were referred for visual symptoms of unknown cause. Despite over a century of discovery related to higher-order visual functions of the human brain, translation of discovery to the clinical assessment of patients has been slow or absent. As with the approach to translational medicine in general, to see meaningful progress, an interdisciplinary approach is indispensable. The first step involves the application of discoveries from the field visual neuroscience by clinicians from the fields of ophthalmology, neurology, and neuropsychology, and from the disciplines of neuro-ophthalmology and behavioral neurology. The unmet need for recognition, assessment, and management of higher-order visual dysfunction in neurodegeneration is evident and clinicians can contribute to closing the gap by using the approach and the tools outlined in the review.