Peripheral reaching in Alzheimer's disease and mild cognitive impairment

Abstract

Recent evidence has implicated areas within the posterior parietal cortex (PPC) as among the first to show pathophysiological changes in Alzheimer's disease (AD). Focal brain damage to the PPC can cause optic ataxia, a specific deficit in reaching to peripheral targets. The present study describes a novel investigation of peripheral reaching ability in AD and mild cognitive impairment (MCI), to assess whether this deficit is common among these patient groups. Individuals with a diagnosis of mild-to-moderate AD, or MCI, and healthy older adult controls were required to reach to targets presented in central vision or in peripheral vision using two reaching tasks; one in the lateral plane and another presented in radial depth. Pre-registered case-control comparisons identified 1/10 MCI and 3/17 AD patients with significant peripheral reaching deficits at the individual level, but group-level comparisons did not find significantly higher peripheral reaching error in either AD or MCI by comparison to controls. Exploratory analyses showed significantly increased reach duration in both AD and MCI groups relative to controls, accounted for by an extended Deceleration Time of the reach movement. These findings suggest that peripheral reaching deficits like those observed in optic ataxia are not a common feature of AD. However, we show that cognitive decline is associated with a generalised slowing of movement which may indicate a visuomotor deficit in reach planning or online guidance.

Keywords: Alzheimer's disease; Cognitive impairment; Optic ataxia; Reaching; Visuomotor.

Fig. 1

Lateral reaching task. (A) Stimuli were displayed on a tablet laptop in the fronto-parallel plane. The experimenter sat directly opposite the participant to monitor eye movements. Target locations, on radial spokes at 28, 33 & 38° are shown during (B) left-hand free reaching and (C) right-hand peripheral reaching. All possible target positions are shown in the figure, but only one was presented per trial.

Fig. 2

Radial reaching task. (A) Set-up for UOE with stimuli displayed in radial plane 500 mm in-front of participant. Eye movements were monitored via a live feed from webcam placed at centre of display. (B) Target locations were 100, 200, 300 and 400 mm to the left and right of fixation (webcam). All possible target locations are shown in the figure, but only one was presented per trial.

Fig. 3

Peripheral reaching error for the lateral reaching task. (A) Lateral Peripheral Misreaching Index (PMI) for each participant, for non-dominant (ND) and dominant (D) sides. (B) PMI averaged across side for each participant. Diamonds show significant deficits in case–control comparisons. Crosses show mean Peripheral Misreaching Index within groups and side (A) and within groups across side (B). (C) Peripheral reaching endpoint (mm) along the x and y-axes for each group relative to target position, collapsed across three target locations per eccentricity (empty circles), for both right and left sided targets. Target position along the x-axis is plotted from the centre of the screen, 150 mm to the left or right of fixation. Error bars represent 95% confidence intervals for Reach Endpoint along the x-axis. Note the scale differences between the x and y-axes.

Fig. 4

Exploratory results for the lateral reaching task, showing differences in (A) Lateral Reaching Error, (B) Reaction Time and (C) Movement Time between patient groups, across viewing conditions and target eccentricities. Error bars show 95% between-subject confidence intervals.

Fig. 5

Peripheral reaching error for the radial reaching task. (A) Radial Peripheral Misreaching Index (PMI) for each participant for non-dominant (ND) and dominant (D) sides. (B) PMI averaged across side for each participant. Diamonds show significant deficits in case–control. Crosses show mean Peripheral Misreaching Index within groups and side (A) and within groups across side (B). (C) Peripheral reaching endpoint (mm) along the x and y-axes for each group relative to target position at each eccentricity (empty circles) for both right and left sided targets. Target position is plotted from the centre of the screen. Error bars represent 95% confidence intervals for Reach Endpoint along the x-axis. Note the scale differences between the x and y-axes.

Fig. 6

Exploratory results for the radial reaching task, showing differences in (A) Absolute Error, (B) Reaction Time, (C) Movement Time, (D) Peak Speed, (E) Acceleration Time and (F) Deceleration Time between patient groups, across viewing conditions and target eccentricities. Error bars show 95% between-subject confidence intervals.