Binaural interaction in auditory brain-stem responses: effects of masking

56

Electroencephalography and clinical Neurophysiology, 1985.62:56-64 Elsevier Scientific Publishers Ireland, Ltd.

BINAURAL INTERACTION IN AUDITORY B R A I N - S T E M R E S P O N S E S : E F F E C T S O F M A S K I N G t ROBERT A. DOBIE and MICHAEL J. WILSON

Veterans Administration Medical Center and University of Washington, Seattle, WA (U.S.A.) (Accepted for publication: June 7, 1984)

Binaural interaction (BI) in auditory brain-stem responses (ABRs) has been demonstrated in man (Dobie and Norton 1980; Levine 1981; Robinson and Rudge 1981) and experimental animals (Dobie and Berlin 1979; Gardi and Berlin 1981). Using a single electrode ensemble (e.g., vertex-nuchal region-forehead), ABR is recorded in response to right monaural, left monaural, and binaural clicks (or other stimuli). It is assumed that, if there were no BI, the binaural ABR would be nearly perfectly predicted by algebraic summation of the monaural ABR wave forms. Subtraction of the summed monaural ABR from the binaural ABR yields a wave form ('difference trace') which should contain only response components attributable to BI. A possible serious problem is acoustic crossover. If the monaural clicks are intense enough to stimulate both ears, the ' m o n a u r a l ' ABR will really be the algebraic sum of the ABR from the ipsilateral ear and a presumably smaller contribution from the contralateral ear. The binaural response would then not be expected to be well predicted by the sum of the monaural responses, and a derived difference trace would reveal responses attributable to the crossover in the monaural conditions, even in the absence of real BI. Evidence against acoustic crossover as the sole source of B I / A B R has been presented by studies showing that BI is present at levels which are probably too low to stimulate the contralateral ear (Dobie and Berlin 1979; Dobie and Norton 1980; Levine 1981). However, these studies do not rule out the possibility that acoustic crossover may be 1 Supported by the Research Service of the Veterans Administration and by an N I N C D S Teacher-Investigator Award (5KO7-NS-00432).

an important contaminant in B I / A B R at higher levels. An obvious solution would be always to use contralateral masking in the monaural conditions. However, Robinson and Rudge (1981) point out that this could introduce artifacts of another kind: even when both ipsilateral clicks and contralateral noise are well below crossover levels, the responses of binaurally innervated neurons to the clicks could be altered by the masking noise. This would be analogous to 'central masking' of ABR. Thus, the purpose of this study was to examine the effects of contralateral noise on ABR, and on B1/ABR, to determine whether contralateral masking should be used in recording, and subsequently deriving, B I / A B R .

Method and Material

Unfiltered rarefaction clicks were produced by presenting 0.1 msec pulses (20/sec) to matched earphones (TDH-49, in M X - 4 1 / A R cushions). Pulses were generated (Systron-Donner 100A), attenuated (Hewlett-Packard, 350C), and amplified (Dynaco Stereo 70) by parallel systems, one for each ear. Masking noise (20-20,000 Hz, GrasonStadler model 455C) was presented either ipsilaterally (through a mixing circuit) or contralaterally through the same transducers. Noise levels were measured with a sound level meter (B&K, 2203), a 1 in. microphone ( B & K , 4132), and a 6 ml coupler (B & K, 4152). Click levels in peak equivalent sound pressure level (maximum sound pressure re. 20 /~Pa) were obtained by measuring the baseline-topeak voltage of the transduced click, then using an oscillator to produce a 3 kHz sine wave transduced

0168-5597/85/$03.30 © 1985 Elsevier Scientific Publishers Ireland, Ltd.

BINAURAL INTERACTION by the same system. The output of the oscillator was adjusted to produce the same baseline-to-peak voltage, and the sound pressure level of the tone was read from the meter. To this level, 3 dB were added to convert from ordinary SPL, which is an RMS value, to peak equivalent SPL. The spectral features of this stimulus have been described (Dobie and Norton 1980). Silver cup electrodes (Grass, E5S) were applied with tape to vertex (positive), nuchal region (negative), and forehead (ground), after skin cleansing with alcohol; impendances were always less than 5000 I2 at 30 Hz. Surface potentials were amplified (105) and filtered (100-3000 Hz) by a Grass P5IIJ preamplifier, averaged (N = 2048, sampling interval = 40 /~sec, epoch = 10.24 msec) by a Nicolet 1170 signal averager employing artifact rejection, then stored on floppy disc by a microcomputer system (Digital Equipment, MINC-11/23). Subjects were paid volunteers, all with normal hearing (thresholds <10 dB HL, ANSI-1969), between the ages of 18 and 30 years. There were no pure-tone interaural threshold asymmetries exceeding 10 dB. In addition to 10 normal subjects, one unilaterally deaf subject participated in these studies. Testing was performed in a double-walled sound-proof room (IAC, 1204-A) with the subject reclining, and often sleeping. White noise intensity was as high as 88 dB SPL (linear scale), but was never presented for more than 2 min continuously at levels above 70 dB SPL, and total duration of exposure at levels above 70 dB SPL never exceeded 10 m i n / d a y . In addition, experimental conditions requiring high noise levels were always placed at the end of the recording session to avoid possible temporary threshold shift effects on lowintensity conditions. Behavioral thresholds for clicks (masked and unmasked) and noise were obtained using a modified method of limits (staircase method), using 1 dB steps. Levels of masking noises can be expressed in dB SPL, dB SL (re: a subject's threshold for the masker), or as effective masking level. For example, if a click at 40 dB SL is just masked by a noise at 70 dB SPL, the latter may be said to be equivalent to 40 dB effective masking level. Adding 10 dB to the noise level (to 80 dB SPL, or 50 dB

57 effective masking level) would yield a noise just capable of masking a 50 dB H L click. For this example, a 30 dB SPL noise would equal (by extrapolation) 0 dB effective masking level. Effective masking levels were determined for each subject using 40 dB SL clicks and varying ipsilateral noise levels: from the noise level required to just mask these clicks, 40 dB were subtracted to yield an estimate of 0 dB effective masking level. The selection of a noise level to be used for contralateral masking in the monaural conditions of the B I / A B R paradigm required the acquisition of pilot data on ipsilateral and contralateral masking of monaural ABR. Thus, this will be presented in Results.

Results

Table I shows the thresholds for detection of clicks and noise, and the effective masking levels, obtained for the 10 normal subjects. The behavioral threshold for our clicks was 5-10 dB higher than reported by some authors, but this may be due to the variability in methods of making peSPL measurements. For example, many authors match a transduced click to the peak-to-peak amplitude of a continuous tone (instead of baseline-to-peak) and do not add 3 dB to convert from RMS to peak intensity. This would lead to a ' p e S P L ' measurement as much as 9 dB less than obtained by our method (Burkard 1984). Interaural attenuation levels for noise were estimated in the 10 normal subjects by calculating the difference between the noise levels required to mask a very weak click (5 or 10 dB SL) under both ipsilateral and contralateral noise conditions. The mean attenuation was 65 dB (S.D. = 5.0 dB, range = 56-73 dB). Our single unilaterally deaf subject displayed interaural threshold differences consistent with these data: 61 dB for noise, 63 dB for clicks. Three normal subjects participated in a pilot study of masking of ABR. Fig. 1 shows a typical pattern when moderately intense clicks (105 dB peSPL, or about 70 dB SL) were presented with varying levels of ipsilateral noise. The first effects

R.A. DOBIE, M.J. WILSON

58 TABLE 1 Behavioral thresholds (dB).

Clicks (peSPL) Noise (SPE) Effective masking (SPL)

Mean

S.D.

36 14 26

2.0 2.5 1.25

seen were usually loss of early waves (especially II a n d IV), a n d increase of wave V latency. A t higher noise levels, wave V d i s a p p e a r e d altogether. N o t e that, because of the m i d l i n e electrode e n s e m b l e used in this study, wave I was never well seen. Fig. 2 shows the effects of ipsilateral noise on wave V latency, for varying levels of b o t h clicks a n d noise. In general, A B R wave V was o b l i t e r a t e d when the ratio of click level (dB p e S P L ) to noise level (dB SPL) was a b o u t 10 dB. F o r example, for a click at 85 dB peSPL, A B R s could be recorded with ipsilateral noise up to 63 dB SPL (latency = 7.8 msec), while the next higher noise level (73 dB SPL) o b l i t e r a t e d the A B R , as i n d i c a t e d by the arrow. However, there was a close c o r r e s p o n d e n c e between click SL a n d noise level expressed as dB effective m a s k i n g level. F o r example, A B R for 95

dB p e S P L clicks (59 dB SL) was c o m p l e t e l y e l i m i n a t e d by noise at 83 dB SPL (57 dB effective m a s k i n g level) - - the same intensity required to r e n d e r the clicks i n a u d i b l e (see Fig. 2). F o r lower noise levels, there was a range of 30 40 dB in which wave V latency p r o l o n g a t i o n was seen, prior to o b l i t e r a t i o n of the ABR. A second pilot g r o u p of 3 subjects had A B R r e c o r d e d with varying levels of contrahzteral noise. Figs. 3 and 4 show, for 1 subject, the a p p a r e n t lack of effect on A B R of noise levels up to 88 dB SPL. In each figure, the A B R o b t a i n e d under ' n o - n o i s e ' c o n d i t i o n s is shown as a solid trace in both the u p p e r and lower sets of traces, while A B R s obtained with a d d e d ipsilateral noise are superimp o s e d (low noise levels in the u p p e r half of each

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Fig. 1. Effects of ipsilateral white noise on click-evoked ABR. The uppermost trace (' - 12 dB') is essentially a no-noise condition in which an additional 35 dB of attenuation have been added in the noise channel, bringing it well below the threshold of audibility.

;3 13 2'3 3'3 4'3 5'3 6'3 7'3 IPSILATERAL WHITE NOISE INTENSITY (dB SPL)

83

Fig. 2. Effects of ipsilateral noise on latency of wave V in ABR. Parameter is click intensity level (55 ] 15 dB peSPL). Data are from 3 normal subjects and median values are shown. Arrowheads indicate no measurable response at higher noise levels.

BINAURAL

INTERACTION

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~.." Fig. 3. Effects of contra]ateral white noise on ABR (click level- 95 dB peSPL). Solid traces in both upper and lower p a r t s of the figure r e p r e s e n t u n m a s k e d A B R . Levels of c o n t r a l a t e r a l noise were as follows: in the u p p e r set of traces, 38 d B (light d a s h e d ) , 48 d B ( h e a v y d a s h e d ) , a n d 58 d B ( d o t t e d ) ; in the l o w e r set of traces, 68 d B (light d a s h e d ) , 78 d B ( h e a v y d a s h e d ) , a n d 88 dB (dotted).

Fig. 5. Effects of c o n t r a l a t e r a l w h i t e noise on l a t e n c y of w a v e V.

figure, higher noise levels in the lower half-figures). Results for all 3 subjects (Figs. 5 and 6) showed no increase in wave V latency for these conditions. From the earlier behavioral estimates of interaural attenuation for noise (about 65 dB; see Table I), this was not surprising. One would estimate that the noise level present at the ear receiving the clicks was 20-25 dB SPL (about 10 dB SL), not enough to observably affect ABR to moderately intense clicks. For the B I / A B R study, again involving all 10 normal subjects, a contralateral masking level of 73 dB SPL was chosen (SL = 59 dB, effective masking level = 47 dB). Assuming a mean interaural attenuation of 65 dB, this noise would have been present at the opposite ear at a level just below audibility, although allowing for intersubject variability, it may have been audible for some subjects. Responses were recorded, at different click levels, for 5 conditions: binaural; monaural right and monaural left without contralateral noise; and monaural right and monaural left with conSUBJEC TS • JG 0 Jd

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CLICK INTENSITY(dBpeSPL)

I contralateral

white noise on ABR (click

level = 75 dB p e S P L ) . N o i s e levels are as d e s c r i b e d for Fig. 3

Fig. 6. C h a n g e in w a v e V l a t e n c y w h e n c o n t r a l a t e r a l w h i t e n o i s e at 78 d B S P L w a s p r e s e n t e d , as a f u n c t i o n o f click intensity.

60

R.A. DOBIE, M.J. WILSON 7.5-

t r a l a t e r a l n o i s e (73 d B S P L ) , M o n a u r a l

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forms were generated for both the no-noise and the contralateral noise conditions, and these were

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Fig. 7. Wave V latency-intensity functions for 3 conditions: binaural clicks (B); sum of monaural responses, or predicted binaural (P); and sum of monaural responses obtained in the presence of contralateral masking at 73 dB SPL (PN). Means and standard errors of the mean are shown; N = 1 0 normal subjects.

s u b t r a c t e d f r o m t h e b i n a u r a l A B R to y i e l d difference traces estimating BI/ABR -- with and without the use of contralateral noise. Fig. 7 s h o w s w a v e V l a t e n c y - i n t e n s i t y f u n c t i o n s f o r 3 c o n d i t i o n s : b i n a u r a l (B); m o n a u r a l s u m ( P ='predicted binaural'); and monaural sum with contralateral noise (PN). This last condition simply consists of the algebraic sum of the ABRs r e c o r d e d f o r r i g h t c l i c k s w i t h left n o i s e a n d f o r left c l i c k s w i t h r i g h t n o i s e . T h e e x p e c t e d d e c r e a s e in w a v e V l a t e n c y f o r i n c r e a s i n g click i n t e n s i t y is o b v i o u s . T h e r e a l s o a p p e a r s to b e a v e r y s l i g h t l a t e n c y i n c r e a s e b y c o n d i t i o n , f r o m B to P to P N . A two-way analysis of variance was performed ( T a b l e II), a n d c o n f i r m s t h a t t h i s is so. W a v e V latency was clearly longer in the monaural sum c o n d i t i o n s (P, P N ) t h a n i n t h e b i n a u r a l c o n d i t i o n . The same effect has been noted previously (Dobie a n d N o r t o n 1980; K e l l y - B a l l w e b e r a n d D o b i e 1984). T h e r e w a s a less s u b s t a n t i a l , b u t b a r e l y significant (P = 0.032) effect of contralateral noise on wave V latency.

TABLE 11 Analysis of variance summary (probability values). Condition"

Intensity b

Interaction ~

< 0.001 0.002 0.001 0.032 N.S.

< < < < <

0.001 0.001 0.001 0.001 0.001

N.S.

N.S. N.S. N.S. N.S.

0.046 N.S. 0.008 N.S. N.S.

< < < <

0.001 0.001 0.001 0.001 0.018

0.034 N.S. 0.013 N.S. N.S.

Latency Wave V. ABR B vs. P vs. PN B vs. P B vs. PN P vs. PN N 1 (D vs. DN)

Amplitude Wave V-V', ABR B vs. P vs. PN B vs. P B vs. PN P vs. PN NI-P2 (D vs. DN)

a Binaural ABR (B), predicted binaural ABR with no masking (P), predicted binaural ABR with 73 dB SPL contralateral masking (PN): BI difference wave form with no masking (D), BI difference wave form with 73 dB SPL contralateral masking (DN). b Unfiltered clicks presented at intensity levels of 65-115 dB peSPL in 10 dB steps. " Interaction of condition and intensity variables.

BINAURAL INTERACTION

61 S:JG

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(dBpeSPL)

,\ 7.5-

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msI

Fig. 8. Difference traces representing binaural interaction in ABR, obtained by subt:acting summed monaural traces from binaurally evoked responses. The filled triangles indicate the latencies of wave V in the binaural response for each intensity level. The difference traces are characterized by a negative peak (N I) with latency just after wave V. The solid traces were obtained without masking; the dashed traces were obtained using contralateral white noise at 73 dB SPL during the monaural trials.

CLICK INTENSITY

(dBpeSPL)

Fig. 9: Latency-intensity functions for peak N] in the difference traces (see Fig. 8), for 2 conditions: without noise (D), and with the use of contralateral noise at 73 dB SPL during the monaural trials (DN).

Discussion

T h e absence of significant i n t e r a c t i o n effects ( b e t w e e n c o n d i t i o n s a n d intensity) m a k e s it unlikely that crossover accounts for this small l a t e n c y shift, since this w o u l d be expected to affect A B R for low-intensity clicks d i s p r o p o r t i o n a t e l y (see Fig. 2). B I / A B R wave forms a p p e a r e d to be unaffected b y the presence or absence of c o n t r a l a t e r a l noise. Fig. 8 shows a typical subject's response. All subjects showed replicable B I / A B R at click levels d o w n to at least 95 dB p e S P L ( a b o u t 59 dB SL), a n d most subjects h a d responses at 75 dB peSPL. Difference traces o b t a i n e d using m o n a u r a l responses r e c o r d e d with c o n t r a l a t e r a l noise were n o t o b v i o u s l y different from those r e c o r d e d w i t h o u t noise. Fig. 9 ( a n d T a b l e I) c o n f i r m s this for m e a s u r e m e n t s of p e a k N 1 latency a n d a m p l i t u d e .

As expected, a n d previously reported, ipsilateral noise p r e s e n t e d at increasing levels first delays, then obliterates, A B R ( K r a m e r and Teas 1982; T h ~ m m l e r et al. 1981). However, contralateral noise, at levels up to 88 dB SPL, h a d no gross effect on A B R . Based on these results, a n d on estimates of t r a n s c r a n i a l a t t e n u a t i o n of a b o u t 65 dB for our system, we used c o n t r a l a t e r a l m a s k ing at 73 dB SPL in the m a i n p o r t i o n of the study. Binaural i n t e r a c t i o n in A B R is present for click levels which are e x t r e m e l y unlikely to be c a p a b l e of s t i m u l a t i n g the c o n t r a l a t e r a l ear b y acoustic crosstalk. F u r t h e r , the use of c o n t r a l a t e r a l noise, at a level intense e n o u g h to m a s k a n y possible crossover stimulation, does not affect B I / A B R . Thus, even at high levels, B I / A B R is not simply an artifact due to acoustic crosstalk (ACT). Others (Levine 1981; Ainslie and Boston 1980)

62

have attributed part or all of the B I / A B R phenomenon to ACT, yet we were unable to demonstrate any effects of contralateral masking, even for click levels of 115 dB peSPL (79 dB SL). This may be in part due to differences in transducers. Levine (personal communication, 1982) noted that A C T with the Sennheiser H D 424 earphone used in his 1981 study was reduced by 5 dB through the addition of a chinstrap, but was still 10 dB poorer than that measured for TDH-39 earphones mounted in standard M X - 4 1 / A R cushions. Ainslie and Boston (1980) used TDH-39 phones, but these were mounted in mu-metal shields to reduce stimulus artifact (Boston and Ainslie 1979). A C T properties may have been quite different than for the same phones mounted in standard earphone cushions rather than the metal shielding cans. Ainslie and Boston (1980) demonstrated a large negative peak in their BI wave forms, with mean latency of 7.2 msec (compared to wave V latency of 5.8 msec in their data). We have never seen such a potential; the N 1 which we routinely identify has a latency about 0.5 msec later than wave V for a given intensity level (see Figs. 7, 8, and 9). When contralateral noise was added in the monaural trials, the large negative peak disappeared, prompting their conclusion that ACT accounted for 'most, if not all' of the observed interaction. However, inspection of their single BI trace obtained with masking (Ainslie and Boston 1980; Fig. 5, middle trace) reveals that the large negative peak attributed to ACT has been replaced by a smaller peak with an earlier latency, about 6 msec; this would be consistent with the type of BI we normally see. Precisely this effect was suggested by Levine (1981): at very high click levels, both the 'true' BI and a larger, later negative wave were present. This larger peak (Levine's ' 8 ' ) could be demonstrated in a unilaterally deaf subject to be due to ACT. Although contralateral masking had no effect on B I / A B R wave forms, there was a small but significant increase in wave V latency in the monaural sum wave forms when contralateral noise was used. This is contrary to Humes and Ochs (1982). However, these investigators used slightly less intense contralateral noise (40 dB effective masking level, SPL not specified). Since the con-

R.A. DOBIE, M.J. WILSON

tralateral masking noise we used was intense enough that it could have been present at audible levels in at least some ipsilateral ears, we cannot rule out a peripheral effect. However, the effect was not limited to low-intensity clicks, as demonstrated by the lack of interaction effect in the analysis of variance, and a very low level of noise would be unlikely to affect wave V latency for high-intensity clicks. The other possibility is a central masking effect, as postulated by Robinson and Rudge (1981). This interpretation is supported by the study of Rosenhamer and Holmqvist (1983) who showed, for slightly higher contralateral noise levels, wave V latency shifts in the absence of wave I shifts. It appears that contralateral masking is unnecessary in recording B I / A B R , at least for our equipment. Interaural attenuation may vary from laboratory to laboratory, due to differences in stimulus type and transducer. On the other hand, any central masking effect, if present, is small, and it probably does no harm to use contralateral masking in conditions of monaural stimulation. A masking level slightly lower than that used in this study (e.g., 30-40 dB effective masking level) would probably be more than adequate to assure that the contralateral ear does not contribute to monaural ABR. Interaural attenuation for clicks has been measured by several investigators using both unilaterally deaf (Humes and Ochs 1982; Levine 1981: Ozdamar and Stein 1981) and binaurally normal hearing subjects (Ainslie and Boston 1980; Levine 1981). Mean values have ranged from 70.75 dB (Humes and Ochs 1982) to 49 dB (Levine 1981). However, a figure of 60 dB for T D H earphones in supra-aural cushions would be a representative, conservative estimate based on our findings.

Summary Binaural interaction (BI) in auditory brain-stem responses (ABRs) can be shown by comparing a binaurally elicited ABR to the algebraic sum of monaurally elicited ABRs. Subtracting the summed monaural ABR from binaural ABR yields a wave form assumed to contain response components

BINAURAL INTERACTION a t t r i b u t a b l e to BI. It has been suggested that acoustic crosstalk accounts for some of the ' B I ' seen with this technique a n d that c o n t r a l a t e r a l m a s k i n g should be used d u r i n g m o n a u r a l stimulation to eliminate crosstalk. However, this practice might in itself c o n f o u n d the results, even if the m a s k i n g noise were not intense enough to affect the o p p o s i t e ear, b y ' c e n t r a l m a s k i n g ' of b r a i n - s t e m neural activity. W e s t u d i e d the effects of contralateral w i d e - b a n d m a s k i n g on B I / A B R in 10 n o r m a l a d u l t subjects. Clicks were p r e s e n t e d at levels from 55 to 115 dB peSPL, at 10 dB intervals. M a s k i n g was p r e s e n t e d at 73 dB S P L (47 dB effective m a s k i n g level); b a s e d on pilot studies of interaural a t t e n u a t i o n , this was a level e x p e c t e d to be at, or j u s t below, the threshold of a u d i b i l i t y in the c o n t r a l a t e r a l ear. B I / A B R wave forms were n o t n o t i c e a b l y affected b y the a d d i t i o n of contralateral noise in the m o n a u r a l trials. In a d d i t i o n , B I / A B R was seen (as previously r e p o r t e d ) at levels well below any p o s s i b i l i t y of crossover artifact. Thus, B I / A B R is not simply a t t r i b u t a b l e to crossover. C o n t r a l a t e r a l m a s k i n g is n o t necessary in r e c o r d i n g B I / A B R , within the limits of the stimuli a n d transducers used in this study.

Resume

Interaction binaurale clans les rOponses bvoqubes du tronc cbrbbral; effets du masquage L ' i n t e r a c t i o n b i n a u r a l e d a n s les r6ponses a u d i tives du tronc c6r6bral p e u t ~tre mise en 6vidence en c o m p a r a n t les r6ponses p r o d u i t e s b i n a u r a l e m e n t h la s o m m e alg6brique de celles p r o d u i t e s m o n a u r a l e m e n t . En s o u s t r a y a n t la s o m m e des r6ponses m o n a u r a l e s de la r6ponse b i n a u r a l e , on o b t i e n t une o n d e qui est consid6r6e c o m m e conten a n t les r6ponses c o r n p o s a n t e s dues h l'interaction. 11 a 6t6 sugg6r6 que l'interf6rence entre oreilles explique, au m o i n s en partie, l ' i n t e r a c t i o n mise en 6vidence avec cette technique et q u ' u n m a s q u a g e c o n t r a l a t 6 r a l devrait dtre utilis6 au cours de la s t i m u l a t i o n m o n a u r a l e p o u r 61iminer les effets d'interf6rence. T o u t e f o i s cette p r a t i q u e p o u r r a i t en elle-mEme affecter les r6sultats, mEme si le son de m a s q u a g e n'est p a s assez intense p o u r agir sur

63 l'oreille oppos6e, et ce p a r ' m a s q u a g e central' de l'activit6 nerveuse du tronc c6r6bral. N o u s avons 6tudi6 les effets d ' u n m a s q u a g e contralat6ral p a r large b a n d e sur l ' i n t e r a c t i o n b i n a u r a l e des r6ponses du tronc chez 10 sujets adultes n o r m a u x . Des clics o n t 6t6 pr6sent6s h des niveaux de 55 h 115 dB p e S P L , avec des intervalles de 10 dB. Le m a s q u a g e 6tait pr6sent6 g 73 dB SPL (47 dB de niveau efficace d e m a s q u a g e ) ; d ' a p r 6 s des 6tudes pr61iminaires d ' a t t 6 n u a t i o n inter-aurale, ce niveau 6tait consid6r6 c o m m e j u s t e au seuil ou en-dessous d u seuil d ' a u d i b i l i t 6 de l'oreille contralat6rale. Les c o m p o s a n t s de l ' i n t e r a c t i o n n ' o n t pas 6t6 n o t a b l e m e n t affect6s p a r l ' a d d i t i o n d u b r u i t en contralat6ral, lors des essais m o n a u r a u x . D e plus, l ' i n t e r a c t i o n a 6t6 vue (ce qui a d6jh 6t6 publi6) h des niveaux bien au-dessous d ' u n possible art6fact d'interf6rence. Ainsi, l'effet n'est pas u n i q u e m e n t a t t r i b u a b l e ~ l'interf6rence. Le m a s q u a g e contralat6ral n'est pas n6cessaire lors de son enregistrement; dans les limites des stimulus et appareils utilis6s d a n s cette 6tude.

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