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Juliana Yordanova, vasil kolev
to brain mechanisms that reach maturity at de-
velopmental stages later than 10 years.
The phase-locking in children did not de-
pend on the power of the prestimulus EEG
theta activity Instead, subject age was the de-
terminant of the developmental increase in
phase-synchronization. This result demon-
strates that theta response phase-locking re-
flects processes specifically activated after
stimulus presentation. It is to be noted that the
developmental increase in phase-locking was
found only for the early theta responses and at
the central site where the early theta responses
of adults were mostly expressed (Fig,6a). Al-
though the phase-locking of late responses was
also significantly stronger in adults than in chil-
dren, no increase of late theta response phase-
locking was observed for children groups.
These differential time courses support the as-
sumption that the early and late responses may
have different functional roles.
In addition, major developmental differenc-
es were revealed for the time Structoe of all
single-sweep parameters: Adults manifested
pronounced differences between the early and
late theta responses, but in children younger
than 9-10 years the magnitude and phase-lock-
ing of the early theta responses were similar to
those of the late responses. Furthermore,
younger (6-7-year-old) children had a greater
enhancement for the late compared to the ear-
ly theta responses (Fig.5c) and all children
groups tended to enhance the frontal late theta
responses more than the early ones (Fig.5b).
Given the previous results of adults outlined
above, it cannot be ruled out that the relation
between early and late theta responses varies
as a function of the specific task conditions.
Whether the reverse pattern in children re-
flects a response delay, a different way of in-
volvement of processes functionally specific to
the early and late responses in adults, or a qual-
itatively different mode of organization of the
event-related theta activity, is an open ques-
tion. The precise functional significance of the
event-related theta activity in both adults and
children requires further investigation, espe-
cially with respect to the meaning of single-
sweep parameters. Nevertheless, the present
findings suggest that single theta response pa-
rameters and their timing may provide a sensi-
tive indicator of stimulus- and task-related in-
formation processing in children.
Altogether, the single-sweep analysis results
demonstrate that the theta response system is
not completely developed at the age of 10 years.
The age at which the adult values of single theta
response parameters are reached is not clear
from the results but the processes related to
theta response system functioning obviously
reach maturation at later stages of develop-
ment. It should be emphasized that at the age
of 10 years the frontal lobes, unlike other brain
structures, are reported not to have reached
functional maturity with respect to their ana-
tomical structure and input-output connections
(Rothenberger, 1990). Miller (1991) has raised
the hypothesis that the EEG theta rhythm re-
flects the fronto-hippocampal interplay during
context processing. The present findings of the
incomplete development of theta responses at
10 years of age as well as of the delayed en-
hancement of frontal theta responses in chil-
dren support the notion of the association be-
tween the theta response and frontal lobe pro-
cesses. In this regard, since frontal lobe
functioning has been assigned a major role for
the occurrence and ∞urse of psychiatric disor-
ders in children (Rothenberger, 1990), event-
related theta activity may appear informative
as a supplementary tool for such clinical studies
(e. g., Rothenberger, 1995; Yordanova, Dumais-
Huber, Rothenberger, & Woemer, 1997). It
should also be noted that the developmental
changes of single-sweep parameters, although
occurring at different ages, were rather step-
wise than gradual, which points to their possible
relation with stages of cognitive development
(Piaget, 1969) but the precise correlations of
single theta response parameters with cognitive
stage is still to be investigated.
Theoretical implications
The developmental time courses were differ-
ent for theta response amplitude, phase-lock-
ing, and enhancement. These differential de-
velopmental courses might reflect sequential
effects in the maturation of a common under-
lying mechanism. Alternatively, they may indi-
cate that specific mechanisms might be in-
volved in the maturation of the processes re-
lated to each of the three single-sweep
parameters.
The strong phase-locking of theta responses
in adults shows that repeatable and stable
waveforms are produced after stimulation. In