Developmental changes in the theta response system: a single sweep analysis



DEVELOPMENTAL THETA RESPONSE

123


reports on adults have found that enhanced
(synchronized) event-related theta activity ac-
companies short-term memory activation
(Klimesch et al., 1994; Mecklinger et al., 1992)
and increased focused attention (Ba⅞ar-Eroglu
et al., 1992). These memory- and attention-re-
lated effects were observed for the post-stimu-
lus epochs later than 250-300 ms. In addition,
the late (300-600 ms) fronto-parietal theta re-
sponses to target oddball tones have been re-
ported to be more enhanced and more strongly
phase-locked than the late responses to passive
stimuli, whereas the early theta components
did not differ between target and passive stim-
ulus processing (Yordanova & Kolev, in press).
Stimulus type effect was not tested in the pre-
sent study because only the developmental
changes in theta activity were focused on.
However, the reports mentioned above suggest
that the early and late responses of adults as
observed here may relate to different process-
ing mechanisms.

Differences in theta responses Ofchildren
and adults

The organization of the auditory theta re-
sponse was specific for children. This was evi-
denced by the significant differences between
the single-sweep parameters of 6-10-year-old
children and adults: (1) Single theta responses
of children were larger but not enhanced
against prestimulus theta activity, and also less
synchronized than those of adults; (2) In adults,
the early theta oscillations showed higher re-
sponsiveness than the late ones, whereas in
children the late responses were either more
enhanced than the early ones or no reliable dif-
ferences were observed between the early and
late theta activity. It is noteworthy that the age-
related variations were similar for the three
stimulus types. These findings generally indi-
cate that during auditory stimulus processing
the theta response system in adults operates in
a manner different from that in 6-10-year-old
children. Since the passive and task-related
stimuli produced similar age-related differenc-
es, it may be further suggested that the specific
organization of the theta response in children
reflects developmental variations of basic stim-
ulus-processing mechanisms common for the
different processing conditions. Such a propos-
al is supported by the observation that single
theta response parameters do not correlate
with response speed.

Theta response system development

Single theta responses in children not only dif-
fered from those in adults, but also changed
with advancing age from 6 to 10 years: A de-
crease in single-response amplitudes occurred
at 10 years for the task stimuli and at 7 and 9
years for the passive stimuli, with the values of
the mature theta response not achieved even
by oldest (10-year-old) children. The develop-
mental alterations in amplitude are not likely
to result from differences in cranial parameters
such as thickness of the scalp or head circum-
ference (Gasser et al., 1988; Polich, Ladish, &
Burns, 1990) - a conclusion supported also by
the different time courses of single theta am-
plitudes for the passive and task stimuli. As re-
vealed by the multiple regression analysis re-
sults, these age-related amplitude effects re-
sulted exclusively from the developmental
decrease in the power of the ongoing EEG the-
ta activity. Furthermore, the early theta re-
sponses displayed a maximum at Cz as did the
prestimulus theta power. The observation of
the strong relationship between the pre- and
post-stimulus theta amplitudes may be regard-
ed as supporting the concept of the diffuse and
distributed theta system in the brain generat-
ing both the spontaneous and stimulus-related
4-7 Hz activity (Ba§ar, 1992). Hence, the devel-
opmental reduction of EEG theta activity may
indicate a decrease in the number and/or inten-
sity of the neuronal elements that determine
operative theta states of the brain.

The present results demonstrate, however,
that single theta responses of children are large
but do not change relative to the prestimulus
theta activity after stimulation. Also, the ability
to enhance the magnitude of the evoked theta
component does not improve from 6 to 10
years.

In parallel to the decrease in amplitude, an
increase in the phase-locking of early theta re-
sponses took place at about 9 years of age.
However, for all three types of ERPs adults had
remarkably stronger phase-locking than any of
the children groups. Hence, it may be concluded
that the capability to produce stable (congru-
ent) theta responses to auditory stimulation im-
proves with development but appears related



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