4.3 Electroencephalographic (EEG) Correlates of Subjective Preference

was observed in the right hemisphere (Soeta and Nakagawa, 2006). Analogous to our results for evoked electrical potentials, for spatial sensations the magnetic ECD dynamic range in the right hemisphere was much larger than in the left.

Going back to Fig. 4.14, let us look at the behavior of early latencies of P1 and N1. These were almost constant when the first reflection time and the IACC were changed. However, information related to SL or loudness is typically seen the N1 latency. This tendency agrees well with the results of Botte et al. (1975).

To summarize the neural response latency data from 40 to 170 ms latency, hemispheric dominance was found for SVR amplitude components, a difference that may reflect different functional specializations. Early latency differences corresponding to sensation level SL were found in the range of 120–170 ms. Finally, we found that the N2-latency components in the delay range between 200 and 310 ms correspond well with subjective preferences for sensation level (SL), time delay of first reflection ( t1), and indirectly, interaural correlation magnitude (IACC). Because the longest latency was always observed for the most preferred condition, one might speculate that the brain is most relaxed and less aroused at the preferred condition and that the slightly lower degree of neuronal excitability causes the longer observed response latencies to occur. For awake subjects, alpha rhythms observed in electroencephalography (EEG) and magnetoencephalography (MEG) usually have the longest periods of any waves that are present.

4.3Electroencephalographic (EEG) Correlates of Subjective Preference

Thus far, we have discussed the neural correlates of changes in sensation level SL, first reflection time t1 and interaural correlation magnitude IACC that can be seen in averaged long-latency (0–500 ms) auditory-evoked potentials (SVR) in response to short signals less than a second long. However, for a wide range of reverberation times (Tsub), no useful neural correlates of either their associated perceptual qualities or preferences could be obtained from examining slow vertex responses (SVRs). For these longer time windows one must look at ongoing neural responses to continuous sounds rather than brief, evoked auditory potentials. We therefore embarked on a series of experiments to find some distinctive feature in EEG signals that follows changes in the Tsub over long signal durations. In analyzing the data from these experiments we sought to find EEG correlates of subjective preferences in the duration that alpha rhythms persist, i.e., in the effective duration of alpha rhythms in the EEG signal.

4.3.1 EEG Correlates of First Reflection Time t1 Changes

In order to introduce the methods that we developed for analyzing EEG responses to sounds of extended duration, we shall first consider as preliminary study of the delay time of a single reflection to reconfirm the SVR results that were discussed in Section 4.2.3.

56 4 Electrical and Magnetic Responses in the Central Auditory System

In this experiment, music motif B (Arnold’s Sinfonietta, Opus 48, a 5 s segment of the third movement) was selected as the sound source (Table 2.1, Burd, 1969). The delay time of the first, single reflection t1 was alternatively adjusted to 35 ms (a preferred condition) and 245 ms (a condition of echo disturbance). The EEG from electrode positions T3 and T4 (Fig. 4.24) was recorded for about 140 s in response to ten delay-change pairs, and experiments were repeated over a total of 3 days. Eleven 22to 26-year-old subjects participated in the experiment. Each subject was asked to close his eyes when listening to the music during the recording of the EEG. Two loudspeakers were arranged in front of the subject. Thus, the IACC was kept at a constant value near unity. The sound pressure level was fixed at 70 dBA peak, in which the amplitude of the single reflection was the same as that of the direct sound, A0=A1=1. The leading edge of each sound signal was recorded as a time stamp so that the recorded EEG responses could be precisely synchronized to and correlated with the presented sound. EEG signals amplified, passed through a filter with a 5–40 Hz bandwidth and slope of 140 dB/octave, and then digitally sampled at 100 Hz or more.

In order to find brain activity patterns corresponding to subjective preference, we analyzed the effective durations τ e of the autocorrelation functions (ACF) of EEG and MEG signals in the α-wave range (8–13 Hz). The recorded signals were first passed through a 8–13 Hz digital band-pass filter, and effective durations (Fig. 2.2) were then computed from the ACFs of the filtered, α-band signals. This gives a measure of how long, on the average, a temporally coherent alpha rhythm persists in a given part of the brain.

What is the length of the autocorrelation analysis window that is needed to reliably detect perceptually relevant changes in EEG signals? First, on the consideration that the subjective preference judgment needs at least 2 s to develop a psychological present, the running integration interval (2T) was examined for periods between 1.0 and 4.0 s. A satisfactory duration 2T in the ACF analysis was found only from the left hemisphere for 2–3 s, but not from the right (Ando and Chen, 1996).

Table 4.3 indicates the results of an analysis of variance (ANOVA) for values of the effective duration τ e of EEG signals in the α-band obtained using analysis windows of 2T = 2.5 s. LR in the table is analysis of differences in hemispheric response (electrode location T3 vs. T4). Though individual subject differences were

significant (p < 0.01), the factor of first reflection time t1 (LR: p < 0.025) by itself is also significant. However, it is significant for an interference effect between factors t1 and LR (p < 0.01). Therefore, in order to analyze the data in more detail for each category, t1 and LR, we show the averaged value of τ e for the α-band with 11 subjects in Fig. 4.16. A clear tendency is apparent. Effective durations τ e at t1 = 35 ms are significantly longer than those at t1 = 245 ms (p < 0.01) only in the left hemisphere, not in the right. Ratios of τ e values in the α-range at t1 = 35 and 245 ms, for each subject, are shown in Fig. 4.17. Remarkably, all individual data indicate that the ratios in the left hemisphere at the preferred condition of 35 ms are much longer than those in the right hemisphere. A long effective duration τ e of α-rhythms may relate to the long N2 latency of SVR in the preferred condition that was discussed in Section 4.2.3.

Fig. 4.16 Averaged effective durations τ e of EEG alpha rhythms in response to a change of first reflection timet1 to either 35 ms (the preferred time) or 245 ms (a condition of echo disturbance). Data from 11 subjects recorded from scalp electrodes on either left (“left hemisphere”) or right side (“right hemisphere”)

Fig. 4.17 Ratios of effective durations τ e of EEG alpha rhythms in response to a change of first reflection timet1 for individual subjects A–K. Change in t1 was made either to the preferred value of 35 ms or to the unpreferred, echo disturbance value of 245 ms. Ratios are between the effective durations of the preferred condition divided by the durations of the unpreferred condition, i.e., (τ e value at 35 ms)/(τ e value at 245 ms)

Thus, the results reconfirm that, when the t1 is changed, the left hemisphere is highly activated (Fig. 4.11), and the effective durations τ e of α-rhythms in this hemisphere correspond well with subjective preference. The α-rhythm has the longest period in the EEG in the awake state and may be related to and associated with feelings of “pleasantness” and “comfort,” a condition that is universally preferred.

4.3.2 EEG Correlates of Reverberation Time Tsub Changes

Now, let us examine how effective durations τ e of α-rhythms change in response to reverberation times (Tsub) and their subjective preference values.

Ten student subjects participated in the experiment (Chen and Ando, 1996). The sound source used was music motif B, Arnold’s lively, rollicking allegro con brio movement from his Sinfonetta No. 1 (Table 2.1). Ten 25to 33-year-old subjects participated in the experiment. EEG signals from left and right hemispheres were recorded. Values of τ e of EEG signals in the α-band were also analyzed using 2T = 2.5 s moving windows.

First consider the averaged values of the effective duration τ e of α-activity, shown in Fig. 4.18. For this music, the preferred reverberation time Tsub is 1.2 s. Clearly, for the left hemisphere the values of τ e are much longer at the preferred condition Tsub = 1.2 s than those at either of the non-preferred conditions Tsub = 0.2 and 6.4 s. Thus, alpha rhythms persist longer in the left hemisphere for preferred reverberation time values. However, the contrary is true for the right hemisphere, where the effective durations of the non-preferred conditions Tsub = 0.2 and 6.4 s are longer than those for the preferred conditions.

Fig. 4.18 Averaged effective durations τ e of EEG alpha rhythms in response to a change of later reverberation time Tsub for two pairs of reverberation conditions (0.2 and 1.2 s) and (1.2 and 6.4 s). The preferred condition for late reverberation time is

1.2 s. Data from 10 subjects

The results of analysis of the variance are indicated in Table 4.4. Although there are large differences between individual subjects, a significant difference is achieved for Tsub in the pair of 0.2 and 1.2 s (p < 0.05), and interference effects are observed for factors subject and LR (p < 0.01), and LR and Tsub (p < 0.01). No such significant differences are achieved for the pair at 1.2 and 6.4 s, but there are interference effects between subject and LR and subject and Tsub. Thus, in order to discuss the matter in more detail, the ratio of values of τ e for the α-band is shown in Fig. 4.19 for each

Fig. 4.19 Ratios of effective durations τ e of EEG alpha rhythms in response to changes of later reverberation time Tsub for individual subjects a–j. The preferred value for reverberation time Tsub is 1.2 s. (a) (τ e value at 1.2 s)/(τ e value at 0.2 s). (b) (τ e value at 1.2 s)/(τ e value at 6.4 s)

subject. All of the individual data indicate the ratios in the left hemisphere are much longer than those in the right hemisphere at Tsub = 1.2 s in reference to Tsub = 0.2 s (Fig. 4.19a). However, this is not the case for Tsub = 1.2 s relative to Tsub = 6.4 s, indicating large individual differences (Fig. 4.19b).

In fact, these individual results correspond well to the scale values of individual subjective preference. Figure 4.20 shows the scale values of preference as a function of Tsub for each subject. The most preferred values for Tsub, which were different for each subject, average at about 1.2 s. The ratio of values for τ e of the α-wave at 1.2 and 6.4s is well correlated to the difference of the scale values of subjective preference for each individual, also reflecting large individual differences, as shown in Fig. 4.21 (r = 0.70, p < 0.01).