Altered visual and haptic verticality perception in posterior cortical atrophy and Alzheimer's disease

Abstract

There is increasing theoretical and empirical support for the brain combining multisensory information to determine the direction of gravity and hence uprightness. A fundamental part of the process is the spatial transformation of sensory signals between reference frames: eye‐centred, head‐centred, body‐centred, etc. The question ‘Am I the right way up?’ posed by a patient with posterior cortical atrophy (PCA) suggests disturbances in upright perception, subsequently investigated in PCA and typical Alzheimer's disease (tAD) based on what looks or feels upright. Participants repeatedly aligned to vertical a rod presented either visually (visual‐vertical) or haptically (haptic‐vertical). Visual‐vertical involved orienting a projected rod presented without or with a visual orientation cue (circle, tilted square (±18°)). Haptic‐vertical involved orientating a grasped rod with eyes closed using a combination of side (left, right) and hand (unimanual, bimanual) configurations. Intraindividual uncertainty and bias defined verticality perception. Uncertainty was consistently greater in both patient groups than in control groups, and greater in PCA than tAD. Bias in the frontal plane was strongly directionally affected by visual cue tilt (visual‐vertical) and grip side (haptic‐vertical). A model was developed that assumed verticality information from multiple sources is combined in a statistically optimal way to produce observed uncertainties and biases. Model results suggest the mechanism that spatially transforms graviceptive information between body parts is disturbed in both patient groups. Despite visual dysfunction being typically considered the primary feature of PCA, disturbances were greater in PCA than tAD particularly for haptic‐vertical, and are considered in light of posterior parietal vulnerability. Key points The perception of upright requires accurate and precise estimates of orientation based on multiple noisy sensory signals. The question ‘Am I the right way up?’ posed by a patient with posterior cortical atrophy (PCA; purported ‘visual variant Alzheimer's’) suggests disturbances in the perception of upright. What looks or feels upright in PCA and typical Alzheimer's disease (tAD) was investigated by asking participants to repeatedly align to vertical a rod presented visually (visual‐vertical) or haptically (haptic‐vertical). PCA and tAD groups exhibited not only greater perceptual uncertainty than controls, but also exaggerated bias induced by tilted visual orientation cues (visual‐vertical) and grip side (haptic‐vertical). When modelled, these abnormalities, which were particularly evident in PCA haptic‐vertical performance, were compatible with disruption of a mechanism that spatially transforms verticality information between body parts. The findings suggest an important role of posterior parietal cortex in verticality perception, and have implications for understanding spatial disorientation in dementia.

Remote versus face-to-face neuropsychological testing for dementia research: a comparative study in people with Alzheimer’s disease, frontotemporal dementia and healthy older individuals

“…Healthy controls and participants with AD both performed significantly worse on the remote version of the VOSP object decision task, in which participants are presented with four silhouettes and asked to select the drawing of a real object; the three distractor silhouettes are based on nonsense shapes. The typical amnestic AD phenotype can include prominent visuospatial impairments24 25 and it is feasible that a reduction in stimulus quality may have stressed cortical apperceptive mechanisms still further, akin to a dynamic ‘stress test’ of degraded input processing 26–28. However, it is worth noting that there was no such discrepancy across the AD cohorts for other tasks involving visual administration (eg, WASI matrix).…”

Graded multidimensional clinical and radiological variation in patients with Alzheimer’s disease and posterior cortical atrophy

“…Principal component analyses, like neuropsychological investigations more generally, are constrained by the types and coverage of the assessments included. The current study was designed to evaluate the expected neuropsychological features of PCA and thus less typical symptoms, such as motor and non-visual perceptual features 56,57,63 , were omitted. When new types of assessment are included then additional dimensions can emerge if there is substantive variation of the patient group in this regard 64 65 .…”

“…In addition to the overlap in visual processing impairments, considerable overlap of tAD and PCA on the emergent cognitive dimensions reiterates the importance of non-visual impairments in PCA 6,56,57 . Fully characterising cognitive impairments in PCA is important because these symptoms could contribute to the misdiagnosis of PCA 7,21,58 .…”

Perception of verticality in the post-COVID-19 condition correlates to infection severity

“…[42][43][44][45] In addition, more severely ill patients undergoing hospitalization have a greater storm of pro-inflammatory cytokines in the acute phase, with an increased risk of developing a neurodegenerative condition. 46,47 Neurodegenerative processes can alter long-term verticality, 31 graviceptive information perception due to the reduced activity of somatosensory cortical areas. It can also be inferred that changes in verticality observed in this study may have been caused by changes in the peripheral nervous system.…”