From Perception to Execution in Parkinson’s Disease
Parkinson’s disease (PD) is a progressive neurodegenerative disorder that affects one’s ability to control voluntary movements. Typically, PD is considered a disease of motor output, without considering neuropsychological variables that might contribute to the impairment of movement. The goal of this chapter is to investigate issues of motor control, in addition to the observable motor symptoms found in PD from a perceptual and cognitive perspective. To this end, we divide movement into three components: perception, initiation, and execution.
Perception is required initially, and throughout movement, and is generally used to define characteristics of a movement goal. Thus, in the earliest stages of movement preparation, an error in perceptual judgment might contribute to an overall motor deficit by incorrectly identifying parameters of the movement. Within this chapter, we will review recent research that demonstrates perceptual impairments in PD, and evaluate the impact that this may have on overall motor control.
Deficits early in the movement continuum would likely have observable consequences in the selection of a motor response, and might contribute to an overall deficit in movement initiation. The variable most commonly used to reflect deficits in movement initiation and preparation is ‘reaction time’. Significant impairment on tests of simple reaction time have frequently been demonstrated in PD, and yet there remains some controversy as to whether this impairment may be interpreted as ‘pure’ slowness at the input stage, or whether it represents a delay in the output of a selected motor response to the effector. We will review the evidence establishing the dissociation between motor initiation and stimulus perception in PD.
One of the more interesting controversies in contemporary motor control is something that may be referred to as the ‘movement time paradox’. While it is well known that healthy adults use advance information to prepare a movement (i.e., improve their reaction time), the ability of patients with PD to utilize this advance information has been debated. Historically, research has suggested that PD impairs the ability to utilize advance information. More recent research in this population has suggested, however, that certain stimuli can be used to enhance an individual’s reaction time, but only at the expense of their speed of motor execution (typically measured as ‘movement time’). We discuss this neuropsychological phenomenon, in the context of what it might mean to our understanding of the neurological control of movement.