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Juliana Yordanova, vasil kolev
tor variables were log-transformed prestimu-
lus theta power and children’s age in months.
The results obtained were similar for the three
stimulus types. At each electrode site, single
theta response amplitude was entirely predict-
ed by the prestimulus theta power, since the
age variable was removed from the equations
(R2total > .46, F(l,47) > 39.6, P < .001). In con-
trast, the stability of phase-locking at Cz de-
pended only on the age factor and was not pre-
dicted by variations of the prestimulus theta
power (R2total > .17, F(l,47) > 3.02, P < .005).
No significant correlations were found be-
tween children’s reaction times and single-
sweep parameters of theta responses to odd-
ball targets.
ИТіте Window0-300 ms □Time Window 300-600 ms
PHASE-LOCKING
(b) ENHANCEMENT FACTOR
2.2 -∣ Fz Cz Pz
Discussion
The present study assessed the event-related
theta activity in children and adults at the level
of single-sweep analysis. It was hypothesized
that the EEG theta responses would differ be-
tween children and adults as well as among chil-
dren groups, which can be evinced by differenc-
es in single-sweep parameters. This hypothesis
was confirmed and significant age-related vari-
ations were found for single theta response am-
plitude, phase-locking, and enhancement rela-
tive to prestimulus activity. Furthermore, these
variations were specific for each parameter. A
major finding was that although single-sweep
amplitudes decreased, the phase-locking in-
creased with age. As described in the results, the
averaged filtered ERPs did not manifest signif-
icant differences between children and adults.
This was because the smaller single-sweep am-
plitudes of adults were accompanied by a pro-
nounced phase-locking, and the opposite was
true for children. Thus, since additional infor-
mation was obtained that was obscured in the
averaged waveforms, the developmental chang-
es in the EEG theta response could be revealed
and evaluated precisely at the level of single-
sweep analysis.
Single-sweep theta responses in adults
In adults, the theta component of averaged au-
ditory ERPs has been described previously
(Ba§ar-Eroglu et al., 1992; Demiralp & Ba§ar,
1992). In the present study, by using single-
sweep analysis the following new characteris-
AGE GROUPS
* P< .05, ** P< .01, *** P< .001
Figure 6 Effects of time window on theta responses to
nontarget stimuli from each electrode and age group: (a)
number of phase-locked single theta waves, (b) enhance-
ment factor. The age groups are designated in the same
manner as in Figure 2.
tics of the event-related theta activity were
found: (1) Auditory theta responses display
strong phase-locking and amplitude enhance-
ment in the post-stimulus epoch, which verifies
the stimulus-related reorganization of the on-
going theta activity in adults; (2) The early the-
ta responses (0-300 ms) are higher in magni-
tude and enhancement, and stronger in phase-
locking than the late ones (300-600 ms), with
the differences between early and late compo-
nents being most pronounced at the mid-cen-
tral location. These observations were true for
passive, target, and nontarget ERPs, which
shows that the early and late theta responses
may reflect mechanisms activated during both
passive and task stimulus processing. Previous