Fig. 4.4 Averaged response latencies for ABR waves I–VI as a function of horizontal angle of incidence. Data from four subjects. Sizes of circles indicate number of data points represented. The sound transmission delay between the loudspeaker and the center of the listener’s head was 2 ms, indicated by the α on the axis. Filled circles: left ABR; empty circles: right ABR

The relative latencies of ABR peaks also change with horizontal angle. Latencies of peaks of waves I through VI are computed relative to the time when the short click pulses were supplied to the loudspeaker. The shortest latencies were seen for angles around ξ = 90◦ Fig. 4.4). Significant differences exist (p < 0.01) between averaged latencies at lateral locations and those in the median plane (in front ξ = 0◦ or in back, at ξ = 180◦). The differences are approximately 640 μs on average, which corresponds to the interaural time difference created in a typical human head by a sound incident at ξ = 90◦. It is likely that the relative latency at waves I–III may be reflected by the interaural time difference, particularly at wave III. No significant differences could be seen between the latencies of the left and right of waves I–IV responding to such a relative factor.

4.1.2Brainstem Response Correlates of Listening Level (LL) and Interaural Crosscorrelation Magnitude (IACC)

Neural auditory brainstem responses (ABRs) can be compared with interaural crosscorrelation functions (IACFs) derived from acoustic measurements made at the two ears of a dummy head. We found a number of parallels between acoustic crosscorrelation functions computed from sound pressure levels at the left and right ears and neural crosscorrelation functions computed from ABRs that reflect the successive activation of populations of neurons in the left and right auditory pathways.

A-weighted signals were presented and free-field sound pressure measurements were taken at the two ear entrances of a dummy head as a function of the horizontal angle of the sound source. Figure 4.5 depicts the signal power at the two

Fig. 4.5 Acoustic correlation magnitudes of sound signals arriving at the leftand right-ear entrances of a dummy head as a function of horizontal angle of incidence. Magnitudes are normalized by their respective values at ξ = 0◦ (directly in front). Φ: maximum interaural crosscorrelation, |Φlr(τ )|max, |τ | < 1 ms. (L): Φll(0) measured at the left ear. (R): Φrr(0) measured at the right ear

ears (R, L) for different angles (power is the zero-lag term of a signal’s autocorrelation function) and the maximum value of the interaural crosscorrelation function (ø), which are normalized only by the respective values at ξ = 0◦. Received signal power is greatest for the ear ipsilateral to the speaker R when it is situated at 90◦ azimuth, and least for the contralateral ear. These acoustic measures can be compared with the corresponding, neurally generated ABR potentials (Fig. 4.6). Here the neural correlate of the relative power of the received signals at the left and right ears is the average of the peak amplitudes of waves IV and V (left and right), normalized to those at the frontal incidence. Although differences in units and scaling confound direct comparison between the results in Figs. 4.5 and 4.6, there are nevertheless qualitative similarities between these acoustic and physiological responses.

Fig. 4.6 Averaged amplitudes of ABR waves IV and V as a function of horizontal angle of incidence. Data from four subjects. Symbol “l” is wave IVl; symbol “r” is wave IVr, and the symbol “V” indicates the averaged amplitudes of waves Vl and Vr. Amplitudes are normalized to their values at the frontal incidence

The relative behavior of wave IV (l) in Fig. 4.6 is similar to Φrr(0) in Fig. 4.5, which was measured at the right-ear entrance r. Also, the relative behavior of wave IVr is similar to Φll(0) at the left-ear entrance l. In fact, amplitudes of wave IV (left and right) are proportional to Φrr(0) and Φll(0), respectively, due to the interchange of signal flow. The behavior of wave V is similar to that of the maximum value, |Φlr(τ )|max, |τ | < 1 ms. Because correlations have the dimensions of the power of the sound signals, i.e., the square of ABR amplitude, the acoustic interaural correlation magnitude (IACC) defined by Equation (2.23) has a corresponding neural measure of binaural correlation P:

where AV is the amplitude of the wave V, which is a reflection of the “maximum” synchronized neural activity (≈ | lr (τ )|max)in the inputs to the inferior colliculus (see Fig. 4.3). And, AIV,r and AIV,l are amplitudes of wave IV from the right and left, respectively. The results obtained by Equation (4.1) are plotted in Fig. 4.7. It is clear that the behavior of the IACC and P are in good agreement (r = 0.92, p < 0.01).

Fig. 4.7 Values of the IACC measured due to Equation (2.23) together with (2.26) and values of P obtained by Equation (4.1). Data at ξ = 150◦ were not available, except for one indicating a P value far over unity (unusual)

4.1.3 Remarks

Here we summarize the neural correlates of binaural hearing that can be observed in the relative amplitudes and latencies of waves in the left and right auditory brainstem responses (ABRs).

The amplitudes of the ABR clearly differ according to the horizontal angle of the incidence of sound relative to the listener (Fig. 4.2). In particular, the amplitudes of waves IVl and IVr appear to be nearly proportional to the sound pressures at the right and left ear entrances, respectively, when the amplitude is normalized to that in either the front (ξ = 0◦) or back (ξ = 180◦).

Due to the successive reversals of left and right amplitudes of corresponding ABR peaks discussed above in Section 4.1.1, we have inferred that a significant

part of the neural signal must cross the midline in at least four places. The first interchange of the neural signal appears to occur between right and left cochlear nuclei; the second at the level of the superior olivary complex and the third at the level of nuclei of the lateral lemniscus as shown in Fig. 4.3. Thompson and Thompson (1988), using neuroanatomical tract-tracing methods in guinea pigs, found four separate pathways connecting the two cochleae with themselves and each other via afferent and efferent pathways that pass through brainstem nuclei. This relates to the first interchange at the entrance of the cochlear nucleus, in the three interchanges in the auditory pathway.

We found that the relative latencies of wave III in left and right ABRs correspond clearly to the interaural time difference. Moving up the pathway to stations above the neural binaural processors in the brainstem, the behavior of the magnitude of acoustic interaural crosscorrelation (IACC) is mirrored by “maximum” of neural activity for ABR wave V (associated with inputs to the inferior colliculus). The latency of wave V is around 8.5 ms latency relative to the time the sound signal is delivered to the loudspeakers, of which approximately 2 ms is the transit time of the sound from the loudspeakers (68±1 cm away) to the ear (Fig. 4.4).

At the level of the inferior colliculus, on the other hand, Hecox and Galambos (1974) found that the latency of wave V decreases with an increasing sensation level, as shown in Fig. 4.8. This implies binaural summation for the sound energy or the sound pressure level (SPL), whose acoustic correlate can be seen in both Φ11(0) and Φ11(0) corresponding to Equation (2.24).

Fig. 4.8 Latency of ABR wave V as a function of sensation level SL, expressed in dB re: sound detection threshold (Hecox and Galambos, 1974). This ABR latency may correspond to a binaural summation of sound energies from the left and right ears. (See also the longer and “stretched out” P1 and N1 single vertex response (SVR) latencies plotted as a function of SL in Fig. 4.14)

To summarize, the activity of the short-latency auditory brainstem responses (ABRs) that occur 0–10 ms after the sound signal has arrived at the eardrums likely reflects mechanisms in auditory binaural pathways that analyze interaural correlation patterns (e.g., the IACC).

Fig. 4.9 (a) Relationship between the IACC and N2-latency, p < 0.025 (Ando et al., 1987a). •, N2- latency of SVR over the left hemisphere; ◦, N2-latency of SVR over the right hemisphere; (____), regression of these. (b) Relationship between the IACC and ECD movement, p < 0.01 (Soeta, et al., 2004). •, ECD movement over the left hemisphere; (. . .. . .), regression; ◦, ECD movement over the right hemisphere; (____), regression

4.2 Slow Vertex Responses (SVRs)

The SVR is a longer-latency (10–500 ms) averaged evoked auditory potential that reflects patterns of synchronized activity in populations of cortical neurons in the temporal lobe. We present the results of our measurements of SVRs of human listeners in the following sections. In these experiments we sought to observe the cortical neural response correlates both of acoustic parameters associated with different temporal and spatial percepts and with their preferred values. Correlates of temporal factors of the sound field reflect that reverberation patterns, such as the initial time delay gap between the direct sound and the first reflection ( t1) and subsequent reverberation time (Tsub), were found in SVRs associated with the left hemisphere. In contrast, typical spatial factors of the sound field, the IACC, and the binaural listening level (LL) were found in SVRs associated with the right hemisphere.

4.2.1 SVR Correlates of First Reflection Time t1 Contrast

In experiments involving subjective preferences, we integrated the SVR for paired stimuli in a similar manner obtaining the scale value of subjective preference based on the paired-comparison method. First, subjective preferences were obtained by paired-comparison tests between different acoustic stimuli with different delay times of reflection. The preferred delay of single reflections observed in these tests

is typically in the 0–125 ms range. In order to compare slow vertex responses (SVR) with these subjective preferences, a reference stimulus was first presented, and then an adjustable test stimulus was presented. Many cortical neurons are more sensitive to changes in their inputs than those that stay the same. Typically, SVR magnitudes are greatest when stimulus patterns change abruptly, i.e., there is a pronounced contrast between paired stimuli in which spatial and/or temporal factors change substantially. The method of paired stimuli is therefore the most effective procedure because of this relativity of brain response.

Electrical responses were obtained from the left and right temporal regions (scalp locations T3 and T4, Fig. 4.24) according to the standard international 10–20 system for electrode placement (Jasper, 1958). Reference electrodes were located on the right and left earlobes and were connected together. Each subject had been asked to abstain from both smoking and drinking alcohol for 12 hours prior to the experiment. Pairs of stimuli were presented alternately 50 times through two loudspeakers which were located directly in front of the subject (68±1 cm away). This arrangement ensured that the magnitude of interaural crosscorrelation (IACC) could be kept at a constant value near unity for all stimuli. The stimulus-triggered SVR waveforms for each trial were averaged together, as is done with other auditory-evoked potentials (AEPs).

Examples of such averaged SVRs are shown in Fig. 4.10. The stacked plots show averaged SVR waveforms as a function of the delay time of a single sound reflection, i.e., a sound delayed and added to itself. The source signal was a 0.9 s fragment of a continuous speech, the Japanese word, “ZOKI-BAYASHI,” which means grove or thicket. The reference “direct” stimulus sound field was the source signal without

Fig. 4.10 Example of the averaged SVR recorded for a single subject. Dotted lines are the loci of P2-latency for the delay time of the reflection (Ando et al., 1987b). The upward direction indicates negativity. (a) Left hemisphere. (b) Right hemisphere