From the article I posted:

Article Text

The seminal work by Goldscheider (1898) distinguished four aspects of muscle sense: (1) passive motion sense, (2) active motion sense, (3) position sense, and (4) the appreciation of heaviness and resistance.[1] The last sense has also been called dynamesthesia, or sense of tension.[2] The perception of heaviness was first studied experimentally by the German psychologist Gustav Fechner in 1889, who investigated the human ability to detect and to discriminate between different weights.[3] Most of the subsequent work in this field addressed the question, which psychophysical mechanisms underlie the correct discrimination of two different weights. It has been shown that visual and haptic information is relevant for correct estimation of weights and might be altered by size-weight illusions.[4-6] Anesthesia to skin and joints causes changes in the perceived heaviness of weights, indicating that the sensation relies on an intact peripheral nervous systems.[7][8]

Although Parkinson's disease (PD) is predominately characterized by motor deficits, increasing evidence shows that processing of sensory signals might be altered in patients with PD. With respect to proprioception, we and others have shown that PD patients exhibit deficits in the conscious awareness of limb position and passive limb movements (kinesthesia) that can be partially reversed by deep brain stimulation of the subthalamic nucleus.[9-14] It is also known that proprioceptors next to cutaneous mechanoreceptors mediate the conscious perception of heaviness and tension. Thus, it seems plausible that such perception is affected in PD patients, although the psychophysical thresholds for load detection in PD are not known. Consequently, the purpose of the present study was to examine whether mild to moderately impaired PD patients show elevated detection thresholds for a gravitational load and to correlate possible perceptual deficits with markers of disease severity and medication.


Do Cerebro-Basal-Ganglia Loops Play a Role in Perception of Heaviness?

The significant group difference in weight perception thresholds between PD patients and control subjects indicates that intact cerebro-basal-ganglia loops are essential for an intact gravito-inertial sense that Goldscheider called the sensation of heaviness.

The critical role of this functional loop in proprioceptive processing has been documented by several psychophysical studies investigating kinesthesia in PD and focal dystonia patients.[9-15]

Taken together, all these reports indicate that the cerebro-basal-ganglia loops are more involved in proprioceptive signal processing than previously known. Recent anatomical and neurophysiological data corroborate these behavioral findings.

First, neuronal activity elicited by passive movements and somatosensory evoked potentials can be recorded within the subthalamic nucleus and pallidal globe.[16-18]

Second, the basal ganglia receive inputs not only from motor cortical areas but also from numerous other cortical areas, including the sensory cortex.[19] Third, with respect to patients, who have a dysfunction of the cerebro-basal ganglia loop, it is known that they exhibit an altered distribution proprioceptive-evoked cortical potentials in PD and in Huntington's disease.[20]

The functional significance of these proprioceptive deficits is still not fully understood. However, the reported kinesthetic deficits likely contribute to the endpoint errors in goal-directed movements of PD patients,[21] whereas the consistent hypometria of pointing movements against gravity becomes explainable on the basis of our findings that the thresholds for gravitational loads are altered.

In fact, PD patients have been demonstrated to exhibit disturbances in lifting an object in a precision grip between the thumb and the index finger.[22]

Could Other Factors Account for Deficits in PD Patients?

PD is a neurodegenerative disease that does not solely affect dopaminergic pathways but also cholinergic and serotoninergic circuits. Thus, one needs to consider other factors that possibly contributed to an impaired weight perception.

First, peripheral neuropathy or sensory tract involvement are known to affect weight perception.[23]

However, clinical examination did not reveal any clinical signs of sensory impairments in our PD patient sample. In addition, peripheral neuropathy is an uncommon feature of idiopathic Parkinson's disease and, if present, is due to nerve compression or rare side-effects of medication in advanced stages rather than to the disease itself.

Second, the dopaminergic medication could have induced a proprioceptive deficit since one study reported a suppressed position sense after administration of L-dopa and dopamine agonists.[24]

However, in our study, individual thresholds of PD patients did not correlate significantly with their L-dopa equivalent dose but increased with disease severity, indicating that the disease itself and not the medication was causative.

Third, attention, decision-making, or other cognitive deficits of PD patients might have led to the observed perceptual deficits. However, our neuropsychological results speak against such assumption. The MMSE and Tower of Hanoi scores were not different between our PD patients and controls.

As a caveat, we cannot completely rule out that subtle cognitive deficits have contributed to the perceptual deficits in our group of PD patients, because we did not administer a detailed neuropsychological test battery.

Lastly, fatigue might have influenced the results in PD patients but not in controls. However, both in controls and in PD patients, the performance recorded at the end of the experiment did not significantly differ from that at the beginning of the experiment. Therefore, it is unlikely that fatigue was a key factor in our experiment.

In summary, our results demonstrate that the perception of heaviness or weight is already affected in the early stages of PD. The findings of the current study underline growing evidence that proprioceptive and possibly haptic dysfunction is a common feature of PD and confirm the importance of the cerebro-basal-ganglia loops for proprioception.