Ponemos SW por encima de nuestra capacidad auditiva, ¿por que no supertweeters ?

PET measurement and analysis

The sound presentation equipment was installed and calibrated in the PET laboratory of Kyoto University Hospital. Subjects lay supine, with their eyes naturally open, on the PET scanner bed in a quiet, dimly lit room. Their heads were fixed in individually molded helmet-shaped rests that were contoured to leave their ears undisturbed. The distance from the speakers to the subjects' ears was approximately 1.5 m. As in the EEG study, special attention was paid to the immediate environment to minimize the subjects' discomfort. Six of the subjects were studied using FRS, HCS, and baseline conditions, and the other six were studied using FRS, LCS, and baseline conditions. The order of the conditions was randomized across the subjects and a total of six scans was performed on each subject with intervals of 7 min. For each of the FRS, HCS, and LCS presentations, 30 mCi of ¹⁵O-labeled water was injected into the right cubital vein 80 s after the beginning of each session. The same procedure was carried out for the baseline condition after a minimum 1-min rest without any presentation other than the ambient background noise of the PET scanner room. Following the injection, the head was scanned for radioactivity with a multi-slice PET scanner (PCT3600W, Hitachi Medical Co., Tokyo, Japan) for 120 s. The scanner acquired 15 slices with a center-to-center distance of 7 mm and an axial resolution of 6.5 mm full-width at half-maximum (FWHM) at the center (Endo et al. 1991). The in-plane spatial resolution with stationary mode acquisition used in this protocol was 6.7 mm of FWHM, which was blurred to ∼10 mm in the reconstructed PET images. The field of view and pixel size were 256 mm and 2 × 2 mm, respectively. Prior to the emission measurements, transmission data were obtained using a⁶⁸Ge/⁶⁸Ga standard plate source for attenuation correction. Reconstructed images were obtained by summing up the activity throughout the 120-s period. No arterial blood sampling was performed; therefore the images collected were of tissue activity. Tissue activity recorded by this method is linearly related to rCBF (Fox et al. 1984; Fox and Mintun 1989).
The PET data were analyzed with statistical parametric mapping (SPM96 software, Wellcome Department of Cognitive Neurology, London, UK) implemented in MATLAB (Mathworks, Inc., Sherborn, MA). Statistical parametric maps are spatially extended statistical processes that are used to characterize regionally specific effects in imaging data (Friston et al. 1991, 1994,1995b; Worsley et al. 1992). The scans from each subject were realigned using the first image as the reference (Friston et al. 1995a). After realignment, the images were transformed into a standard anatomical space (Friston et al. 1995a; Talairach and Tournoux 1988). As a result, each scan was resampled into voxels that were 2 × 2 × 4 mm each in the x (right-left), y(anterior-posterior), and z (superior-inferior) directions. Each image was smoothed with an isotropic Gaussian kernel (FWHM = 15 mm) to account for the variation in normal gyral anatomy and to increase signal-to-noise ratio. The effect of global differences in rCBF between scans was removed by scaling the activity in each pixel proportional to the global activity so as to adjust the mean global activity of each scan to 50 ml/100g/min. To explore regions showing significant differences in rCBF among different conditions, the general linear model with contrasts was employed at each voxel (Friston et al. 1995b). Since the different conditions were run in different subjects, the contrasts of FRS versus HCS and HCS versus baseline were examined for six subjects, and those of FRS versus LCS and LCS versus baseline were examined for the other six subjects. The contrast of FRS versus baseline was examined for all 12 subjects, inclusive. The resulting set of voxel values for each contrast constituted a statistical parametric map of the t statistic. The t values were transformed into the unit normal distribution (Z score), which was independent of the degree of freedom of error, and were thresholded at 3.09. To account for multiple non-independent comparisons, the significance of the activation in each brain region detected was estimated by the use of distributional approximations from the theory of Gaussian fields in terms of spatial extent and/or peak height (Friston et al. 1994). An estimated P value of 0.05 was used as a final threshold for significance. The resulting set of Zscores for the significant brain regions was mapped onto a standard spatial grid (Talairach and Tournoux 1988).
In all of the subjects, EEGs were simultaneously recorded throughout the PET measurement, which lasted approximately 60 min, from 12 electrodes as in the EEG experiment. The EEGs obtained during the total 200-s sound presentation were subjected to power spectra analysis and, in particular, those during each 120-s PET scan were used for correlation analysis with the rCBF. The data of one subject were excluded because of an excessive amount of electrical noise in the EEG. We used ANOVA followed by Fisher's PLSD post hoc test to assess the statistical significance of the different conditions. In addition, we used SPM software to calculate a correlation map between rCBF and the occipital alpha-EEG, to examine the relationship between them. An estimated P value of 0.05 with correction for multiple comparisons was used as the final threshold for significance.
Psychological evaluation of sound quality

We also evaluated the subjective perception of sound quality. Since the subjective impression of sounds is closely related to the subjects' psychological condition, this evaluation was performed separately from the EEG and PET experiments. We used the same piece of gamelan music as was used for the EEG and PET experiments. First, a pair of FRS and HCS, each lasting 200 s, was presented. The order of the conditions was randomized across the subjects. After an intermission of 3 min, another pair of FRS and HCS was presented in reverse order. Therefore the stimuli were presented in an A-B-B-A fashion, in which FRS and HCS were assigned to A and B or B and A, respectively, in a randomly counterbalanced way across the subjects. Neither the subjects nor the experimenter knew what the sound conditions were, although they did know that the presentation was in an A-B-B-A fashion. The subjects filled out a questionnaire to rate the sound quality in terms of 10 elements, each expressed in a pair of contrasting Japanese words (e.g., soft vs. hard). Each element of each condition was graded on a scale of 5 to 1. The scores were statistically evaluated by the paired comparison method described byScheffé (1952). Note that the method used in the present study differs from that recommended by the CCIR (1978) and its modified version, which were widely used to determine the digital format of CDs around 1980 (e.g.,Muraoka et al. 1978; Plenge et al. 1979). In the previous studies, sound materials were never longer than 20 s and the interval between two successive sound materials was 2–3 s or less. Therefore if neuronal response to sound stimuli is characterized by delay and persistence for longer than 20 s, it is difficult to exclude the possibility that those studies might have introduced a subjective evaluation that might not precisely correspond to each sound condition.



RESULTS

EEG Experiment 1

Figure 2, A and B, shows the grand average BEAMs and occipital alpha-EEGs, respectively, for the 11 subjects, calculated over the entire period of the sound presentation. The alpha-EEGs were enhanced during FRS compared with those during the other conditions. This enhancement was especially predominant in the occipital and parietal regions (Fig. 2 A). ANOVA on the occipital alpha-EEG revealed a significant main effect of condition [F(2,63) = 3.74, P < 0.05]. The post hoc tests showed that the occipital alpha-EEG during FRS was significantly greater than that during HCS (P < 0.05) (Fig.2 B). There was a similar tendency when FRS was compared with the baseline (P = 0.10). Figure 2 C shows the averaged time course of the BEAMs calculated for each 30 s of the FRS and HCS conditions for all subjects, inclusive. The alpha-EEG showed a gradual increase during the first several tens of seconds of FRS; there was a gradual decrease at the beginning of the following HCS. Taking into account the delay and persistence of the enhancement of the alpha-EEG, statistical evaluation was also made of the data from the latter half of the recording session (from the 100-s to 200-s class mark). In this analysis, compared with the data obtained by analyzing the entire period of the sound presentation, ANOVA followed by post hoc tests revealed a more significant main effect of condition [F(2,63) = 4.43, P < 0.05] and a greater difference between FRS and HCS (P < 0.01).

EEG Experiment 2

The grand average BEAMs and occipital alpha-EEGs across all 17 subjects over the latter half of the session (from the 100-s to 200-s class mark) are shown in Fig. 3. The amount of eye movement did not differ for different conditions. The alpha-EEG showed significant enhancement in FRS compared with the other conditions (Fig. 3 A). This enhancement was predominant in the occipital and parietal regions. ANOVA on the occipital alpha-EEG revealed a significant main effect of condition [F(3,131) = 3.74,P < 0.05]. The post hoc tests showed that the occipital alpha-EEG in FRS was significantly greater than that in the other three conditions (Fig. 3 B). There was no significant difference among HCS, LCS, and baseline (P > 0.8 for all comparisons). A similar but weaker tendency was recognized when the data from the entire period of the sound presentation were subjected to the analysis (main effect of condition, P = 0.26; FRS vs. baseline, P = 0.05). This is reasonable because the time course of the grand average occipital alpha-EEG in this experiment showed, as in Experiment 1, a gradual increase over the first several tens of seconds of FRS (data not shown).



Fig. 3.
Normalized alpha-EEG potentials in each experimental condition (FRS, HCS, LCS, and baseline) during the latter half of the sound presentation in EEG Experiment 2. A: BEAMs averaged across all 17 subjects over the time period from the 100- to 200-s class marks. B: mean and standard error of the occipital alpha-EEG for all 17 subjects. FRS significantly enhanced the occipital alpha-EEG relative to the other conditions.



PET experiment

When the conditions with audible sounds (i.e., FRS or HCS) were compared with those without audible sounds (i.e., LCS or baseline), the bilateral temporal cortex, presumably the primary and secondary auditory cortex, always showed significantly increased rCBF as expected (Table 1; see also Fig. 5 C). More importantly, when FRS was compared with HCS, deep-lying structures in the brain were significantly more activated during the presentation of FRS than during that of HCS (Fig. 4and Table 1). The activated areas corresponded to the brain stem (Fig.4 B) and the lateral part of the left thalamus (Fig.4 C). The same areas also showed an increased rCBF when FRS was compared with either the baseline or LCS (Fig.5, A and B). This tendency was also recognizable in the comparison of FRS versus baseline with a lower threshold (Z > 1.64 with correction for multiple comparisons) (Fig. 5 C and Table 1). Conversely, when HCS was presented, these areas in fact showed a decreased rCBF compared with the baseline (Fig. 5, A and B). When LCS was compared with the baseline, no significant differential activation was observed anywhere in the brain and neither the left thalamus nor the brain stem showed changes in rCBF.

***** y como el artículo es mucho más extenso, para seguir leyéndolo y obtener las conclusiones, cito la fuente: http://jn.physiology.org/content/83/6/3548 *****