The distorted body: The perception of the relative proportions of the body is preserved in Parkinson's disease.

Given humans' ubiquitous visual experience of their own body, one reasonable assumption is that one's perceptions of the lengths of their body parts should be accurate. However, recent research has shown that large systematic distortions of the length of body parts are present in healthy younger adults. These distortions appear to be linked to tactile sensitivity such that individuals overestimate the length of body parts of low tactile sensitivity to a greater extent than body parts of high tactile sensitivity. There are certain conditions featuring reduced tactile sensitivity, such as Parkinson's disease (PD) and healthy older ageing. However, the effect of these circumstances on individuals' perceptions of the lengths of their body parts remains unknown. In this study, participants visually estimated the length of their body parts using their hand as a metric. We show that despite the reductions in tactile sensitivity, and potential alterations in the cortical presentation of body parts that may occur in PD and healthy older ageing, individuals with mild-moderate PD and older adults of comparable age experience body size distortions comparable to healthy younger controls. These findings demonstrate that the ability to perceive the length of one's body parts is well preserved in mild-moderate PD.

How far can I reach? The perception of upper body action capabilities in Parkinson's disease.

Successful interaction within the environment is contingent upon one's ability to accurately perceive the extent over which they can successfully perform actions, known as action boundaries. Healthy young adults are accurate in estimating their action boundaries and can flexibly update them to accommodate stable changes in their action capabilities. However, there are conditions in which motor abilities are subject to variability over time such as in Parkinson's disease (PD). PD impairs the ability to perform actions and can lead to variability in perceptual-motor experience, but the effect on the perceptions of their action boundaries remains unknown. This study investigated the influence of altered perceptual-motor experience during PD, on the perceptions of action boundaries for reaching, grasping, and aperture passing. Thirty participants with mild-to-moderate idiopathic PD and 26 healthy older adults provided estimates of their reaching, grasping, and aperture-passing ability. Participants' estimates were compared with their actual capabilities. There was no evidence that individuals with PD's perceptions were less accurate than those of healthy controls. Furthermore, there was some evidence for more conservative estimates than seen in young healthy adults in reaching (both groups) and aperture passing (PD group). This suggests that the ability to judge action capabilities is preserved in mild to moderate PD.

The influence of perceptual-motor variability on the perception of action boundaries for reaching.

Successful interactions within the environment are contingent upon the perceiver's ability to perceive the maximum extent over which they can perform actions, commonly referred to as action boundaries. Individuals are extremely calibrated to their action boundaries, and the perceptual system can quickly and flexibly recalibrate to changes in the size of action boundaries in the event of physiological and/or environmental changes. However, because even the most basic motor activities are subject to variability over time, the information upon which action boundaries are based must also be subject to variability. In this set of studies, we examined the effect of random and systematic variability in reaching experience on the perception of action boundaries for reaching using virtual reality. Participants were asked to estimate their reachability following experience reaching with either a long virtual arm, short virtual arm, or a virtual arm that varied in size. Overall, we found that individuals tended toward liberal estimates of their reachability; however, individuals can be influenced to be slightly more conservative after a higher percentage of short reaches. Consequently, when anticipating our reaching capability in the event of perceptual motor variability, individuals employ a liberal approach as it would result in the highest number of successful attempts. (PsycInfo Database Record (c) 2020 APA, all rights reserved).