DEVELOPMENTAL THETA RESPONSE
121
dren groups (F(l,54) > 6.94, P < .05 for each
contrast). For the passive ERPs, significant de-
creases were observed for the groups of 7- and
9-year-olds (F(l,54) > 9.7, P < .05).
The significant main effect of time window
was due to the overall higher early than late
theta responses. However, a significant age ×
time window interaction also was found. Figure
5a illustrates that only in adults and 10-year-old
children (for targets) were the early theta re-
sponses larger than the late ones. As indicated
by the significant electrode × time window in-
teraction (Table 2), the difference between ear-
ly and late theta responses was most pro-
nounced at Cz, with a simple time window ef-
fect at Cz yielding P < .001.
In sum, single theta response amplitudes de-
creased with age in children and were lowest
in adults. For each group, the early theta re-
sponses were maximal at the vertex. The differ-
ence between early and late response ampli-
tudes was significant only in adults.
- Phase-locking
The statistical results were again similar for the
passive, target, and nontarget ERPs (Table 2).
Figure 5b demonstrates that the phase-locking
was stronger in adults than in children from
each group (F(l,54) > 25.05, P < .005 for each
contrast). As indicated by the significant age ×
time window interaction (P < .001), the phase-
locking of the early theta responses increased
with age in children, whereas no difference be-
tween children groups was found for the late
theta responses. Also, the early theta responses
were more strongly phase-coupled than the
late ones (time window: P < .001) but the early
vs. late difference was significant only for the
adult subjects (Fig. 5b). As shown in Figure 6a,
the developmental increase in the phase-lock-
ing of the early theta responses was expressed
mainly at Cz and much less evident at Fz and
Pz. These effects were statistically reliable only
for the nontarget ERPs (age x time window ×
electrode: P < .05). In addition, the early but
not the late theta responses were best synchro-
nized at the vertex (electrode: P < .001; elec-
trode × time window: P < .001).
In total, the phase-locking of theta responses
was significantly stronger in adults than in chil-
dren. A developmental increase was observed
for the phase-locking of the early theta re-
sponses at the vertex location.
- Enhancement factors
Figure 5c illustrates that the EFs of adults were
significantly larger than those of children at
each age (age: P < .001; and for each between-
group contrast: F(l,54) > 32.3, P < .005), with
no statistical differences obtained among the
groups of children. The effect of the time win-
dow originated mainly from the significantly
greater early than late EFs in adults (age ×
time window: P < .001). Simple time window
effects tested for each age group showed that
in contrast to adults, 6-year-old children mani-
fested significantly stronger enhancement for
the late than for the early theta responses in
the task-related ERPs, with no reliable early vs.
late differences found for the rest of the chil-
dren groups, and for the passive ERPs (Fig. 5c).
In addition, unlike adults, all children had
greater EFs for the late than for the early theta
responses to task-related (target and nontar-
get) ERPs over the frontal brain area (age x
time window X electrode: P < .05 for the non-
target) - an effect illustrated in Figure 6b. For
adults the most pronounced differences be-
tween early and late EFs were found at the
mid-central location.
Altogether, the enhancement of theta re-
sponses relative to prestimulus theta activity
was substantially greater in adults than in chil-
dren. No increase in EFs was observed in chil-
dren from 6 to 10 years. In contrast to adults,
children tended to produce a stronger en-
hancement of the late theta responses, espe-
cially from 6 to 7 years of age, to task-related
stimuli and at the frontal site.
- Multiple regression analyses
The results obtained so far demonstrated the
presence of developmental variations in the
amplitude and phase-locking of the early theta
responses. However, the prestimulus theta
power also decreased with increasing age,
which may be responsible for the observed
age-related changes in these two single theta
response parameters. To determine to what ex-
tent age influenced theta response parameters
due to the developmental power reduction of
the prestimulus theta activity, step-wise multi-
ple regression analyses were performed for
children data from each stimulus type. The de-
pendent variables were single theta response
amplitude and phase-locking, and the predic-