M.Sc Thesis

M.Sc StudentMoont Ruth
SubjectEvaluation of Cognitive Capabilities in Early Parkinson's
Disease Patients
DepartmentDepartment of Medicine
Supervisor ASSOCIATE PROF. Shraga Hocherman


The last twenty years (1980's onwards) have revolutionized research into cognitive deficits in Parkinson's disease (PD) patients.  It is now established that deterioration of cognitive functioning is a specific and important feature of PD, separate from the motor difficulties, and may even precede the appearance of motor symptoms.  Accordingly, quantitative and objective measurement of cognitive functioning is vital to evaluate treatment effectiveness.  Currently, such testing is not common practice in neurology clinics.  Our goal was to increase the understanding of the cognitive pathology in PD, to quantify it by use of a simple methodology, and to generate a more comprehensive clinical description of PD patients' pathology by using a newly devised testing battery. 

We tested a group of early to moderate PD patients on 4 instrumental tests, assessing 4 frontal executive functions that are known to be impaired in PD: 1) a visuo-motor test (VMT) measuring visuo-motor coordination, 2) a visuo-motor-attention test (VMAT) measuring the ability to recruit attentional resources in order to cope with distraction, 3) a conditioned choice reaction time test (CCRTT) measuring set-formation and set-shifting abilities, and 4) a Wisconsin Card Sorting Test for comparison of the above test results with a standard test for frontal lobe cognitive functions.  A group of elderly age-matched controls and a group of young controls were also tested. 

Our results showed a visuospatial deficit in PD patients, as measured by difficulties in controlling movement speed and movement direction on both tracking and tracing tasks.  On the VMAT, the same performance measures on which the patients showed a tracking deficit were also sensitive to attentional load.  The PD patients and elderly controls were similarly susceptible to a moderate increase in attention load.  Since the performance of the patients was already poor at baseline, the cost (i.e. decreased scoring) of having to deal with a high increase in attentional load was greater in the controls than in the patients.  This suggests a saturation effect, whereby increasing the attentional load further does not reduce the patients' performance significantly since their performance has already reached its lowest limits.  Comparison with groups of young and elderly control subjects showed that the decline in the executive functions tested was disease rather than age related.  The results were independent of memory, cultural background and intellectual status.

The WCST results showed a significant impairment in rule learning, set shifting and mental flexibility in the PD patients.  Conversely, CCRT testing showed that both PD patients and the elderly controls had similar performance accuracy.  However, these preserved set formation and set shifting capabilities were blunted by the PD patients' significantly slower visual and auditory choice reaction times.  Reaction time showed a cumulative affect with respect to age and disease.  When tested on learnt associations between visual cues and auditory stimuli, patients were as fast as controls.  These results show that PD patients are able to use learnt associations in order to shorten their auditory reaction time down to normal.  We suggest that the temporal bottleneck in PD is in the sensory evaluation, perceptual-motor linkage and/or motor programming, and not in the triggering and execution of the actual planned movement.  In the final test condition the learned visual-auditory associations were reversed unexpectedly in 25% of the trials.  Both patients and controls became slower in the regular trials and were further slowed in the reversed trials.  Marked differences between the patients and controls were found in the regular and reversed trials.  The controls appeared to better suppress responses, when not required, than the patients, but this difference did not reach significance.

We conclude that early PD patients exhibit deterioration in visuospatial functioning and possess limited attention resources.  This supports many studies in the last 20 years of a deficit in frontal executive functions in PD.  The conflicting results between the WCST and CCRT results suggest that there is a difference between executive demands that involve a thought process (such as in WCST) and executive functions which underlay fast, reasoning-independent responses (such as in the CCRT).  We discuss our results with reference to the underlying pathology in the cortex-basal ganglia-cortical circuits.