OBJECTIVES:
Evidence for inhibitory difficulties in individuals with 'pure' (no
comorbid diagnoses) Tourette Syndrome (TS) is inconsistent, and a complete
neurological understanding of how tics are formed is lacking.
In
Experiments 1 and 2 we postulated that individuals with unmedicated
TS, free of unmedicated comorbid symptomatology, would not differ from
controls in cognitive inhibition or motor inhibition success at any
age. However, individuals with TS would differ from controls in terms
of motor inhibition failures once at an age where normal development
of motor inhibition is expected to have stabilized.
In
Experiments 3 and 4 we hypothesized that tics were the product of incidentally
learned associations between various motor movements, elicited due to
failures in motor inhibition.
METHODS:
In Experiments 1 and 2 the Stroop task and a tactile variant of the
Simon task were used to measure "cognitive inhibition" and
"motor inhibition" respectively. Both tasks measure the ability
of individuals to inhibit a prepotent response (semantic activation
of words in the Stroop, and ipsilateral motor responses in the Simon).
Moreover interference in both tasks during incongruent trials depends
upon the correspondence between the irrelevant stimulus attribute and
the response. Reaction times and errors were analyzed in TS and control
groups split into younger (7=9 years) and older (10-21) samples (n=40).
In
Experiments 3 and 4 half of the older participants in each group were
primed in an incidental motor association. A tactile stimulus, followed
by a response button press, was defined as the goal oriented action.
A RESET button press, allegedly to prompt the next trial, followed the
response button press and was therefore incidentally associated to it.
All older participants were later tested for problems inhibiting this
association by being told that pressing the RESET button was no longer
necessary. Inhibition problems were assessed in three different ways:
number of times the incidental motor association (i.e. RESET Button
Press) was engaged in, number of times the incidental motor association
was initiated (i.e RESET Button Initiation), and reported urge to engage
in the incidental motor association.
RESULTS:
In Experiments 1 and 2 the younger groups did not differ on any measures
of cognitive or motor inhibition success or failure. In the older groups,
individuals with TS committed significantly more errors than controls
on the Simon task.
In
Experiments 3 and 4 all participants exposed to the incidental association
procedure reported a greater urge to engage in the associated movement
than unexposed participants. Only the exposed individuals with TS showed
failures in inhibiting this incidental motor association.
CONCLUSIONS:
Experiment 1 suggested that cognitive inhibition difficulties are not
present in TS when comorbid conditions are controlled for. Experiment
2 suggested that motor inhibition failures are a feature of TS. Experiments
3 and 4 suggest that stereotypical movements may become associated with
numerous goal-directed behaviours. In the absence of appropriate motor
inhibition these associations strengthen over time. When inhibition
fails, these goal-directed behaviours elicit the stereotypical movements
at seemingly random intervals and are considered 'tics'. This model
gives direction to future diagnostic testing and treatment methods,
provides explanations for a considerable body of phenomenological evidence
and past research, and suggests many future areas of exploration.
A thesis
presented to the University of Waterloo
in fulfillment of the
thesis requirement for the degree of
Doctor of Philosophy
in
PsychologyWaterloo,
Ontario, Canada, 2001
©
Bruce Duncan McKinlay, 2001
