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juliana Yordanova, vasil kolev
The spontaneous EEG theta activity has
been found to decrease in absolute and relative
band power with progressing age in children
(Gasser, Verleger, Bacher, & Sroka, 1988; John
et al., 1980; Matousek & Petersen, 1973; Mat-
this, Scheffner, Benninger, Lipinski, & Stolzis,
1980). However, the information about theta
response development is insufficient although
the event-related theta activity may also vary
with age in children. First, the evoked potential
magnitude is known to be significantly corre-
lated with the power of the background EEG
but in many instances EEG and ERP ampli-
tudes have shown relatively independent be-
havior (e. g., Shagass, 1976). It is an open ques-
tion whether a decrease in theta activity during
stimulus processing accompanies the develop-
mental reduction of the spontaneous EEG the-
ta power. Secondly, increased theta activity in
human adults has been consistently associated
with higher cognitive processes such as memo-
ry, concept learning, attention, etc. (Ba⅛ar-Ero-
glu et al., 1992; Inouye, Shinosaki, Iyama, Mat-
sumoto, & Toi, 1994; Klimesch et al., 1994;
Klimesch, Doppelmayr, Russeger, & Pachin-
ger, 1996; Lang, Lang, Diekmann, & Kom-
huber, 1989; Mizuki, Takii, Nishijima, &
Inanaga, 1983). Given these correlations, it is
striking that although the spontaneous EEG
theta power decreases with age in children and
is relatively small in adults, the efficiency of
cognitive functioning improves in the course of
development (Piaget, 1969). This implies that
changes in theta system involvement during
event processing as reflected by the theta re-
sponse should occur with brain development.
Finally, theta responses of young (3-year-old)
children were demonstrated to be larger, de-
layed, and more variable than those of adults
(Ba§ar, 1982; Ba⅛ar-Eroglu, Kolev, Ritter,
Aksu, & Ba§ar, 1994; Kolev, Ba$ar-Eroglu,
Aksu, & Ba§ar, 1994). Altogether, these find-
ings suggest that the EEG theta response un-
dergoes specific developmental variations but
it is unknown how and which response charac-
teristics change as children mature. Therefore,
the present study aimed to assess age-depend-
ent alterations of EEG theta responses in 6 to
10-year-old children. Healthy young adults
were also studied to evaluate the mature theta
response.
As mentioned above, the EEG frequency re-
sponses are proposed to originate from the re-
organization of the spontaneous EEG. There-
fore, theta response characteristics reflecting
the stimulus-related changes in the ongoing
EEG should be regarded as relevant for anal-
ysis. Such characteristics are the response
phase-coupling to stimulus and amplitude en-
hancement or suppression in the post-stimulus
epoch (Kalcher & Pfurtscheller, 1995; Sayers,
Beagley, & Riha, 1979). In the averaged ERPs,
however, amplitude and phase-locking effects
are Confotmded and cannot be analyzed sepa-
rately (e. g., Ruchkin, 1988). To quantify theta
response phase-locking independently of am-
plitude effects, an original method for single-
sweep analysis was applied (Kolev &
Daskalova, 1990; Kolev & Yordanova, 1997;
Yordanova & Kolev, 1996a, in press). Thus, the
major questions addressed in this research
were how single theta response amplitude,
phase-locking, and enhancement relative to
prestimulus activity vary with age in children
and whether their developmental changes de-
pend on the background EEG theta activity?
Previous studies on adults have shown that
event-related theta activity differs between
processing conditions (Ba§ar-Eroglu et al.,
1992; Demiralp & Ba$ar, 1992; Klimesch et al.,
1994; Mecklinger, Kramer, & Strayer, 1992;
Yordanova & Kolev, in press). To examine
whether the age-related variations in the theta
response are restricted to a specific processing
condition, passive and task auditory stimuli
were used. Since the latency of the maximal
theta response in the averaged ERP has been
shown to be longer in younger children (Ba⅛ar-
Eroglu et al., 1994; Yordanova & Kolev, 1996b),
measurement and evaluation were made for
early and late post-stimulus epochs to enable
assessment of single theta response dynamics
over time after stimulus appearance. The rela-
tionship between single theta response param-
eters and the ongoing (prestimulus) theta band
power was also evaluated. Results of the alpha
frequency range in the same groups of children
are presented in a related paper (Yordanova &
Kolev, 1996a).
Methods
Subjects
A total of 50 healthy children from 6 to 10
years of age served as subjects, together with
ten adults from 20 to 30 years of age. As indi-