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Experiment 1

To evaluate developmental changes in the delta, theta, and alpha responses, a total of 50 healthy children from 6 to 10 years of age and 10 healthy young adults from 20 to 30 years were studied. The children’s ages ranged between 6 and 11 years, and they were divided into 5 age groups consisting of 10 subjects (4–6 females) each. The children were free of neurological disturbances, without attentional, behavioral, or learning problems, and had normal and above IQ scores.

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Figure 2.5. Methods for evaluation of single-sweep phase-locking. (a) A flowchart of the procedure for single-sweep wave identification (SSWI). Extremes in the filtered single sweeps are identified, and corresponding single-sweeps are modified by using the values of +1 and –1 only. Modified single-sweeps are averaged so that a SSWI histogram is obtained. For measurements, the SSWI histogram is rectified (absolute values) and normalized according to the number of sweeps, pre-stimulus EEG, and frequency of the waves. (b) Phase-locking factor (PLF) is calculated after continuous wavelet transform with complex Morlet’s wavelet (seeAppendix) and presented in 3D on the left. Time courses of PLF for different frequency bands (layers) are shown on the right.

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28 Juliana Yordanova and Vasil Kolev

ERPs were elicited by auditory stimuli (intensity of 60 dB SPL, duration of 50 ms, rise/fall 10 ms, random inter-stimulus intervals 3.5–6.5 s) in two conditions: (1) Passive – tone bursts of 800 Hz frequency (N= 50) were presented, with participants instructed to relax silently; and (2) Oddball – a total of 75 high (1200 Hz) and 25 low (800 Hz) tones were delivered ran- domly, with the instruction given to the child to press a button as quickly and accurately as possible in response to the low tones. In both the passive and oddball conditions, the participants were instructed to keep their eyes closed.

Behavioral Data

Children’s response times (RTs) to targets decreased with increasing age (group means of 6-, 7-, 8-, 9-, and 10-year-old children: 716, 702, 675, 602, and 472 ms respectively), and were significantly slower than those of adults (mean 390 ms). Error rate tended to be higher in children than in adults. Time-Domain Averaged ERPs

Figure2.6illustrates time-domain ERPs from passive, target, and nontar- get stimuli and shows the components obtained by means of the classical averaging method. In the time domain, ERPs of both children and adults were characterized by N1, P2, N2, and P3 (P300) components. Additionally, the children displayed a fronto-central P1 component, a frontal late nega- tive wave N400–700, and a parietal late positive wave P400-700 identified as P3b. P400–700 occurred primarily in response to the targets and decreased in latency with increasing age in children (Yordanova et al.,1992), a finding that is commonly found in developmental ERP studies (Kurtzberg et al.,1984; Ladish & Polich,1989).

Frequency Domain Analysis

Figure2.7shows the AFCs of ERPs of six representative participants from each age group. It illustrates that the AFCs of children were different from those of adults with respect to the number of identifiable peaks. The auditory AFCs of adults were characterized by a major compound response cover- ing the range of the theta and alpha frequencies (4–12 Hz) and peaking at 6–9 Hz (Schrmann & Basar,1994; Yordanova & Kolev,1998a). A similar AFC pattern was observed for 10-year-old children. In younger children, distinct peaks were detected in the delta, theta, and alpha ranges. The number of separable peaks in the AFCs decreased with increasing age in children. This finding indicates that in the course of development, a specialization occurs in the resonant frequencies involved in auditory stimulus processing.

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Figure 2.6. Grand average passive, nontarget, and target auditory ERPs at three elec- trode locations (Fz, Cz, and Pz) from different age groups: 6-year-olds, 7-year-olds, 8-year-olds, 9-year-olds, 10-year-olds, and adults (AD). Each age group consists of 10 participants (from Yordanova & Kolev,1998b).

Time-Frequency Analysis

Time-frequency ERP components were analyzed for the midline frontal, central, and parietal electrode sites (Fz, Cz, Pz) by means of the methods described above. Single-sweep and averaged ERPs were digitally filtered in the respective frequency bands (delta, theta, and alpha).

Measurable parameters were

(1) power of the pre-stimulus EEG activity,

(2) amplitudes of averaged time-frequency ERP components calculated as the maximal peak-to-peak amplitude in a defined time window, and (3) single-sweep parameters – amplitude, phase-locking, and enhance-

ment relative to the pre-stimulus period. Amplitude of single-sweep responses was measured as the mean value of the maximal peak-to- peak amplitude in a defined time window. Phase-locking (between- sweep synchronization) was evaluated by means of the SSWI method

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30 Juliana Yordanova and Vasil Kolev

Figure 2.7. Amplitude-frequency charac- teristics for six representative participants at 6, 7, 8, 9, and 10 years of age, and an adult, calculated from the passive ERPs recorded at Cz. Along the x-axis – log(frequency), along the y-axis – 20log(|AFC|) (dB); (from Yordanova & Kolev,1996).

(seeAppendix). The enhancement relative to the prestimulus period was measured by calculating the so-called enhancement factor (EF; see Appendix). The EF reflects the change of the magnitude of the post-stimulus oscillations relative to the magnitude of the ongoing (pre-stimulus) EEG.

Experiment 2

In a second study, developmental gamma band responses (GBRs) were ana- lyzed. For this study, a total of 114 children and adolescents from 9 to 16 years of age were used. They were divided into 8 age groups of 9, 10, 11, 12, 13, 14, 15, and 16 years, each comprising 12 to 17 subjects. Details on group characteristics are reported in Yordanova et al. (2002).

All children and adolescents were healthy and reported no history of neu- rologic, somatic, or psychiatric problems, nor did they have any learning, emotional, or other problems.

In each of the two recording conditions described below, a total of 240 auditory stimuli were used. Two stimulus types were presented randomly

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to the left and right ear via headphones. The stimuli were low nontarget (1000 Hz, n= 144, p = 0.6) and high target (1500 Hz, n = 96, p = 0.4) tones with a duration of 120 ms, rise/fall of 10 ms, and intensity of 85 dB SPL. Inter-stimulus intervals varied randomly from 1150 to 1550 ms. Equal numbers of each stimulus type were presented to the left and right ears. In the first condition, participants were instructed to press a button in response to the high tones (targets) presented to the right, while in the sec- ond condition the attended targets were the high tones presented to the left. Thus, there were four stimulus types in each series: target-attended (n= 48), target-unattended (n= 48), nontarget-attended (n = 72), and nontarget- unattended (n= 72).

Apart from overall developmental effects, this task permitted exploration of whether and how the functional reactivity of GBRs changed with age. The following factors which are related to cognitive stimulus processing could also be examined: attended channel (attended vs. unattended) and stim- ulus type relevance (target vs. nontarget). In addition, it was possible to assess whether developmental GBRs depended on the side of stimulation (left vs. right), regardless of whether left or right stimuli were attended or unattended.

To describe developmental changes in the stability and functional reactiv- ity of auditory GBRs at specific scalp locations, these GBRs were analyzed in the time-frequency domain by means of the WT (see above section on Time- Frequency Decomposition of ERPs). The measurable parameters were (1) power of the spontaneous and pre-stimulus gamma band activity, (2) power of phase-locked GBRs, and (3) phase-locking of GBR (see above section on Phase-Locking between Single Sweeps). Age-related differences in the func- tional involvement of GBRs were assessed at separate scalp locations. The effects of attended channel and stimulus type relevance were examined for GBR power and phase-locking and compared among age groups.