Executive deficits in Parkinson's Disease in relation to depression, effort and motor symptoms

Parkinson's Disease (PD) is characterised by a progressive death of neurons in
nigrostriatal connections. Motor symptoms form the hallmark, however executive
dysfunctions are often present. Norman and Shallice proposed an influential
theory about executive functions. They suggest that the Supervisory Attentional
System (SAS) is important in new situations in which automatic behavior is not
sufficient. The SAS regulates the inhibition and activation of schemata.
Schemata are implicit memory structures containing 'behavior codes' that can be
activated in response to environmental stimuli. According to Brouwer factors
like motivation, planning and effort can also influence the SAS and a
dysfunction of one of these factors may cause executive dysfunctions.
Executive dysfunctions in PD may be caused by a dysfunction of three factors.
First, in a study of our researchgroup the new Cognitive Effort Test was used.
This test measures the amount of effort that is voluntarily invested. It
differs from other neuropsychological tests in which the amount of effort that
needs to be invested to succesfully complete the test is fixed. It was found
that PD patients allocated less effort than healthy, age-matched, controls. Are
executive dysfunctions in PD caused by an inability to allocate sufficient
effort?
Second, the SAS can also be influenced by an inability to perform motor
responses when confronted with a stimuli. According to the current models of
basal ganglia organization, the dopaminergic functioning of the putamen is
related to motor function. However, in a study of our researchgroup it was
found that the dopaminergic functioning of the putamen was related to executive
functioning in PD. Monchi et al. offer a possible explanation for this finding.
They concluded that the putamen was activated during actions that followed a
cognitive switch. The mental components of this cognitive switch were unrelated
to putamen activity. To what extent are motor symptoms of PD responsible for
decreased scores on executive functioning tests?
Third, depression might also influence executive functioning in PD. It was
previously reported that depressed PD patients performed significantly worse on
cognitive tests than non-depressed PD patients. However, Muslimovic et al.
corrected depression for the clinical characteristics of PD and found no
associations between depression and cognition. The previously reported
discrepancies between depressed and non-depressed PD patients may have been
caused by the depression rating scales that were used in these studies. These
scales measure the affective and somatic symptoms of depression. In a study of
our researchgroup it was found that the somatic symptoms of depression can also
be PD symptoms. Furthermore, the motor symptoms of PD might also influence the
performance on neuropsychological tests. To what extent is the executive
functioning of PD patients influenced by the affective symptoms of depression?
Are the motor symptoms of PD a third variable in the association between
cognition and depression?

This study will be focused on the executive functioning of PD patients and the
associations between effort, motor symptoms, depression and the executive
functioning of PD patients. By investigating to what extent executive
dysfunctions in PD are based on these factors the treatment of executive
dysfunctions in PD may be improved.
If the motor symptoms are responsible for decreased scores on executive
functioningtests, antiparkinsonian medication is justified. If the affective
symptoms of depression influence executive functioning anti-depressives might
bring comfort, while cognitive revalidation is applicable if executive
dysfunctions in PD are caused by an inability to allocate sufficient effort.

Executive functioning, depression and the amount of effort that is voluntarily
invested will be measured with (neuro)psychological tests in PD patients and
healthy controls. These groups will be matched for age, gender and level of
education.

The assessment of (neuro)psychological tests will require concentration.
Normally a break is sufficient to recover.
Since the level of performance will remain unknown to the participants no
(psychological) damage is expected as a consequence of these tests.