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



DEVELOPMENTAL THETA RESPONSE

Table 1 Distribution according to age of the subjects studied. Each group contained 10 subjects.

Children

6 years

7 years

8 years

9 years

10 years

Adults

20-30 years

mean age (years)

6.50

7.60

8.50

9.30

10.60

24.10

(months)

77.90

91.20

102.10

111.50

127.10

289.20

SD (months)

4.65

3.25

4.68

3.03

4.43

44.40

115


cated in Table 1, the ages of the children ranged
between 72 and 132 months, and were divided
into 5 age groups consisting of 10 subjects (4-6
females) each. Children were obtained from
local schools in Sofia and adult subjects were
volunteers, primarily students from the Medi-
cal University in Sofia. Children were selected
from a large sample after a neurological
screening and were free of neurological dis-
turbances. Interviews with teachers and par-
ents of the children revealed no signs of atten-
tional, behavioral, or learning problems. The
children were of similar socio-economic status
(professional - executive) as assessed by par-
ents’ education and occupation, and had nor-
mal or higher IQ scores as indicated by Ra-
ven’s and verbal tests. Subjects were right-
handed (Annett, 1985), without any history of
neurologic, psychiatric, or hearing problems,
and were paid for their participation in the ex-
periment.

Stimuli and procedure

The subjects were assessed in a dimly lit, elec-
trically shielded room and were monitored by
means of a closed-circuit TV and intercom sys-
tem. Children were given enough time before
the recording sessions to become acquainted
with the environment.

The auditory stimuli were generated by a
PC, filtered, amplified, and reproduced by a
loudspeaker in a free-sound field. All stimuli
were presented with intensity of 60 dB SPL and
duration of 50 ms (rise∕fall 10 ms). The stimuli
were delivered with random interstimulus in-
tervals (3.5-6.5 s) in two different task situa-
tions:

1) Tone bursts of 800 Hz frequency (N = 50)
were presented in a passive listening condi-
tion, with subjects instructed to relax silent-
ly. Before the recording session, they were
told that the stimuli would be delivered for
testing the technical equipment and would
be of no relevance for them.

2) Auditory target and nontarget stimuli were
presented in an oddball task. During the
task condition, 100 high and low frequency
tones (1200 Hz and 800 Hz) were delivered
randomly, with probability
P = 0.75 for the _
high tones, and
P = 0.25 for the low tones.
Subjects were required to press a button
with their dominant hand as quickly and ac-
curately as possible in response to the low
tones. In both the passive and oddball con-
ditions the subjects kept their eyes closed.

Data collection and processing

Electrodes

The EEG was recorded with Ag-AgCl disc
electrodes placed on midline frontal, central
and parietal sites (Fz, Cz, and Pz), with linked
mastoids as a reference. The ground electrode
was positioned on the forehead. The electrooc-
ulogram (EOG) was recorded bipolarly with
electrodes placed below and at the outer can-
thus of the left eye. Electrode impedance did
not exceed 10 kθhms.

EEG recording and data storage

EEG was amplified with cutoff frequencies of
0.5 and 70 Hz by means of a Nihon Kohden
electroencephalograph (model EEG-4314F).
Stop-band filtering (band limits 48-52 Hz) was
used for eliminating line frequency interfer-
ence. The amplified EEG analog signals were
digitized with a sampling frequency of 250 Hz
(12 bit analog-to-digital converter) and stored
for offline processing with epoch length of
1024 ms pre- and 1024 ms post-stimulus. Reac-
tion times (RTs) were recorded automatically.

Artifact rejection

The stored raw single sweeps were inspected
visually offline to eliminate EEG segments
contaminated with blink, muscular, or any oth-
er type of artifact activity, with any EEG or
EOG trial exceeding ± 50 μV for adults and ±
90 μV for children excluded from further anal-
ysis. Thus, the number of artifact-free sweeps



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