Disrupted pursuit compensation during self-motion perception in early Alzheimer's disease

Contrast Sensitivity, Visual Field, Color Vision, Motion Perception, and Cognitive Impairment: A Systematic Review

OBJECTIVES

To examine relationships between visual function (ie, contrast sensitivity, visual field, color vision, and motion perception) and cognitive impairment, including any definition of "cognitive impairment," mild cognitive impairment, or dementia.

DESIGN

Systematic review and meta-analyses.

SETTING AND PARTICIPANTS

Any settings; participants with (cases) or without (controls) cognitive impairment.

METHODS

We searched 4 databases (to January 2024) and included published studies that compared visual function between cases and controls. Standardized mean differences (SMD) with 95% CIs were calculated where data were available. Data were sufficient for meta-analyses when cases were people with dementia. The Joanna Briggs Institute checklists were used for quality assessment.

RESULTS

Fifty-one studies/69 reports were included. Cross-sectional evidence shows that people with dementia had worse contrast sensitivity function and color vision than controls: measured by contrast sensitivity (log units) on letter charts, SMD -1.22 (95% CI -1.98, -0.47), or at varied spatial frequencies, -0.92 (-1.28, -0.57); and by pseudoisochromatic plates, -1.04 (-1.59, -0.49); color arrangement, -1.30 (-2.31, -0.29); or matching tests, -0.51 (-0.78, -0.24). They also performed more poorly on tests of motion perception, -1.20 (-1.73, -0.67), and visual field: mean deviation, -0.87 (-1.29, -0.46), and pattern standard deviation, -0.69 (-1.24, -0.15). Results were similar when cases were limited to participants with clinically diagnosed Alzheimer disease. Sources of bias included lack of clarity on study populations or settings and definitions of cognitive impairment. The 2 included longitudinal studies with follow-ups of approximately 10 years were of good quality but reported inconsistent results.

CONCLUSIONS AND IMPLICATIONS

In the lack of longitudinal data, cross-sectional studies indicate that individuals with cognitive impairment have poorer visual function than those with normal cognition. Additional longitudinal data are needed to understand whether poor visual function precedes cognitive impairment and the most relevant aspects of visual function, dementia pathologies, and domains of cognition.

A corollary discharge mediates saccade-related inhibition of single units in mnemonic structures of the human brain

Despite the critical link between visual exploration and memory, little is known about how neuronal activity in the human mesial temporal lobe (MTL) is modulated by saccades. Here, we characterize saccade-associated neuronal modulations, unit-by-unit, and contrast them to image onset and to occipital lobe neurons. We reveal evidence for a corollary discharge (CD)-like modulatory signal that accompanies saccades, inhibiting/exciting a unique population of broad-/narrow-spiking units, respectively, before and during saccades and with directional selectivity. These findings comport well with the timing, directional nature, and inhibitory circuit implementation of a CD. Additionally, by linking neuronal activity to event-related potentials (ERPs), which are directionally modulated following saccades, we recontextualize the ERP associated with saccades as a proxy for both the strength of inhibition and saccade direction, providing a mechanistic underpinning for the more commonly recorded saccade-related ERP in the human brain.

Impact of Alzheimer's Disease in Ocular Motility and Visual Perception: A Narrative Review

PURPOSE

Alzheimer's disease (AD) is a neurodegenerative disease generating a progressive neuronal loss as well as cognitive deficiencies. This disease can be accompanied by ocular manifestations, including alterations in ocular motility and visual perception. The aim of the current review article was to collect all the information about these alterations and to analyze if there is scientific evidence supporting the potential use of these changes as biomarkers of the disease.

METHODS

A bibliographic search was performed using two different databases, Pubmed and Google Scholar, as well as a search of material in non-peer reviewed journals about Alzheimer's and Neurology. A total of 227 articles were found in the initial search, but only 76 were included considering their relevance according to the purpose of the reviewResults: This narrative review describes the findings obtained in this area to this date, confirming that deficiencies in saccades is the most common condition among AD patients. Furthermore, other visual alterations have also been reported in these patients, including a compromise of visual acuity and contrast sensitivity, fluctuations of colour vision, stereopsis impairment and visual field losses. Likewise, other complex visuo-spatial and visuo-perceptual impairments can be present. More studies are still needed to understand better what type of changes occurs in ocular alignment, binocularity, and fixation pattern in AD patients.

CONCLUSIONS

AD is associated to visual perception and ocular motility alterations. All the scientific information found in this review should be considered as a guide for designing future studies and to define adequate clinical protocols for the visual evaluation of patients with AD, considering the cognitive limitations that are normally present in this type of patients.