Biomedical Engineering Reference
In-Depth Information
lar lesions due to surgery were studied. Subjects received 40 acoustic startle
stimuli daily for five successive days. Data were analyzed for response decre-
ment within the training session of one day (short-term habituation) and for a
decrease in the startle response across the five training days (long-term habitua-
tion). Long-term habituation of the blink component of the acoustic startle re-
sponse recorded at the orbicularis oculi muscles was significantly impaired in
patients with cerebellar lesions compared with control subjects, whereas short-
term habituation was preserved in both groups. Findings of involvement of the
human cerebellum in habituation of the startle response are further supported by
PET studies in healthy human subjects (72,57). These findings of impaired star-
tle habituation provide evidence that cerebellum is involved in non-associative
learning.
2.3. Tests for Mixed, Associative and Non-Associative Processes
2.3.1. Associative and Non-Associative Processes in Postural Reflexes
The eyeblink or the lower limb withdrawal reflex represents protective
avoidance reactions. An important driving force for an organism to establish
conditioned responses (CRs) preceding the unconditioned stimulus (US) is thus
to increase protection against the harmful US. This is in fact accomplished in
classically conditioned eyeblink experiments with an air puff as US but is in
contrast to the methodological-conceptual issues suggested by Gormezano and
Kehoe (25), who claimed that subjects should not profit from the CR. Postural
reflexes represent compensatory responses of an organism to unexpected exter-
nal perturbations of the body equilibrium. In contrast to nociceptive reflexes,
postural reflexes must be classified as protective and non-nociceptive and, thus,
must be based on a driving force different from avoidance. To analyze these
reflexes Nashner introduced a method using a dynamic platform that allows re-
producible translatory or rotational perturbations (e.g. (52)). Unexpected move-
ments of a tilting platform (toes-up) evokes a passive deviation of the body with
characteristic changes in the activation of corresponding leg and/or trunk mus-
cles. Following repetitive perturbations, the reflex patterns may alter minimally
only, whereas the amplitude of the responses may change. Different plastic
processes have been discussed to be involved. Functional habituation of postural
responses induced by "toes-up" rotations has been studied in detail by Hansen et
al. (26), who reported for the gastrocnemius muscle (GA) an initial reduction
due to habituation of a startle-like response and a subsequent more gradual de-
cay (26). "Habituation" or "adaptation" of automatic postural responses is a
functional mechanism that allows subjects to minimize energy expenditure (36).
Such decays or attenuations of responses are based presumably on non-
associative processes. Classical conditioning of postural reflexes, however, re-
