15.2 MEG Correlates of Preferences for Flickering Lights

Fig. 15.15 Scale preference values of flicker period T for different subjects. Different symbols represent preferences of different subjects. The thick black line shows averaged scale values of preference for all subjects tested. This reconfirmed the applicability of Equation (15.2)

to the period of a flickering light was analyzed (Soeta et al., 2002). The scale values of individual preference obtained by the PCT together with the averaged scale values of eight subjects are shown in Fig. 15.15. Effects of period on the scale values of preference were examined for all eight subjects by using one-way ANOVA. Results clearly indicated that the effects of period were significant (p < 0.01), and the most preferred period, [T]p, for each subject was estimated by fitting a suitable polynomial curve to a graph on which the scale values were plotted as shown in Fig. 15.16. The preferred period ranged from 0.6 to 2.0 s, and its averaged value was roughly 1.0 s.

Figure 15.17 shows an example of a recorded MEG alpha wave. We selected 16 channels for the ACF analysis that were located around the occipital area and analyzed each response to the single stimulus in the pair for each subject. The selected

Fig. 15.16 An example of obtaining the most-preferred period [T]p and less preferred periods of the flickering light. [T]0.5 ≈ 0.71, [T]1.0 ≈ 2.22, and [T]1.5 ≈ 0.48 (s). (Suffix number of [T] denotes the differences between scale values of preference)

Central area

Fig. 15.17 Examples of recorded MEG alpha rhythms. Response durations were 2.5 s

16 channels were divided into three areas as shown in this figure. Table 15.5 shows the results of the two-way ANOVA for τe, (0), and φ1 values of the alpha wave obtained for the eight subjects. Significant effects of preference were found on τe and φ1 values under all conditions tested. It is clear that the values of τe correlated with the value of φ1 (r = 0.75, p < 0.01). The values of τe and φ1 for the most preferred stimuli were larger than those for less preferred stimuli for all subjects, as shown in Figs. 15.18 and 15.19, respectively. There were no clear differences, however, in the value of (0) as shown in Fig. 15.20. Such a difference of averaged values of τe and φ1 from the left area was significantly larger than those from the central and right areas (p < 0.01). The preferred period of the flickering light clearly induces a much longer τe in the alpha wave than that of the less preferred ones. This

Fig. 15.18 Effective durations of averaged MEG alpha wave rhythms in response to flickering light for the different occipital sensor regions shown in Fig. 15.17. Error bars represent 95% confidence. The difference of the scale value of preference was (a) 1.5. (b) 1.0. (c) 0.5. ◦ : Higher preference; •: lower preference

tendency for longer τe for the preferred period of the flickering light rather than φ1 in the alpha wave was also found in a Section 15.1 on EEG. The fact that the brain repeats a similar rhythm under preferred conditions was reconfirmed.

The ratio of high to low preference in terms of the averaged value of τe obtained here was small in the range 1.01–1.06, but the difference is significant. This is much smaller than that derived in the study on EEG, which was 1.18–1.74. The EEG results from extracellular volume currents triggered mainly by the postsynaptic potential. MEG signals are thought to arise from the intracellular currents that flow from dendritic trees to cell bodies in large numbers (> 50,000) neurons. The MEG and EEG field distributions are mutually orthogonal. Only the current that has a component tangential to the surface of a spherically symmetric conductor produces an exterior magnetic field; the radial source is thus externally silent. Therefore, it one could think that the radial source might more directly reflect the neural activity patterns associated with preferred visual stimuli. Differential results from EEG and MEG for the analysis of human cognition have been discussed previously (Eulitz et al., 1997). Also, numerous studies have reported relationships between EEG and MEG coherence and mental processes (Rappelsberger and Petsche, 1988;

Fig. 15.19 Values of MEG alpha rhythm regularity φ1 in response to flickering light for the three occipital sensor regions shown in Fig. 15.17. Error bars represent 95% confidence. The difference of the scale value of preference was (a) 1.5. (b) 1.0. (c) 0.5. ◦ : Higher preference; •: lower preference

Hinrichs and Machleidt, 1992; Petsche, 1996). Those studies concentrated on the interchannel relations, for example, synchronization of alpha frequency rhythms.

Here, we found that the values of τe and φ1 from the left occipital area were significantly larger than those from the central and right occipital areas. Such a clear tendency was not found in the previous study on EEG mentioned above. The significant values τe and φ1 in the left hemisphere may reflect the specialization of the human brain, reconfirming specifically the left-hemisphere dominance for the temporal factors. Average alpha band signal amplitudes (0) from the right occipital area were significantly larger than those from the left occipital area. It is remarkable that the ratio of high to low preference in terms of τe is greater than that of the value of φ1. This indicates that the efffective duration τe of alpha waves reflects subjective preference better than does alpha rhythm regularity φ1.

These MEG experiments investigated human cortical responses corresponding to subjective preferences for flickering lights. The conclusions that we drew from these experiments are:

1.We reconfirmed that the most preferred flicker periods [T]p for individuals were 0.6 – 2 s, with an average value around 1 s.