Two simple models are examined in order to explain the observation that a portion of the binaural-evoked response is less than the sum of monaural-evoked responses in human and animal subjects. The sum of monaural responses minus the binaural response is called the binaural difference (BD). Each model acts on binaural input signals and applies a single memoryless nonlinearity. One model (IE) applies a rectifying nonlinearity to the difference of input signals, while the other (EE) applies a compressive nonlinearity to the sum of input signals. These models are suggested by properties of inhibitory-excitatory (IE) and excitatory-excitatory (EE) neurons of the auditory brainstem. Parameters can be found that enable each model to produce a ratio of BD to summed monaural response which is invariant with input stimulus level. The IE model, but not the EE model, has a BD whose level is linearly related to input stimulus level.

The binaural difference (BD) is the sum of monaural acoustic brainstem responses (ABRs) minus the binaural ABR. Its construction implies operation of a nonlinear element with binaural input. The BD in five rats was studied using insert earphones that provided 60 dB of interaural isolation measured in the occluded external meatus. Scalp to chin BD consisted of a double-peaked complex with a following trough occurring during peaks IV and V of the sum of monaural ABRs. BD magnitude was defined as the maximum peak-to-trough waveform amplitude. As stimulus intensity increased, BD magnitude increased linearly with the magnitude of ABR peak I. A bilaterally symmetric stimulus generally yielded smaller BD magnitude than did stimuli with small bilateral asymmetry (8-dB interaural intensity difference, 64-μs interaural time difference). With larger asymmetry, BD magnitude decreased. The linear increase of BD magnitude with ABR peak I magnitude is consistent with operation of an element that applies a rectification nonlinearity to the difference of binaural signals during peaks IV and V of the ABR.

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B i o n g i n e e r i n g Program, Penn S t a t e U n i v e r s i t y , N o n -l i n e a r i n t e r a c t i o n i n t h e a c o u s t i c b r a i n s t e m r e s p o n s e c a n b e d e m o n s t r a t e d by s u b s t r a c t i n g t h e r e s p o n s e t o b i n a u r a l s t i m u l a t i o n from t h e sum o f r e s p o n s e s t o monaural s t i m u l a t i o n . The r e s u l t i n g b i n a u r a l d i f f e r e n c e waveform i s n o n -f l a t d u r i n g p e a k s I V a n d / o r V o f t h e a c o u s t i c b r a i n s t e m r e s p o n s e . The n o n -l i n e a r i t y p r o b a b l y a r i s e s a t some n e u r a l e l e m e n t s which l i e i n t h e a s c e n d i n g a u d i t o r y b r a i n s t e m a n d which r e c e i v e i n p u t s from b o t h e a r s . W e t e s t e d t h e growth o f t h e b i n a u r a l d i f f e r e n c e i n r a t s u n d e r a n i n c r e a s i n g s t i m u l u s l e v e l series a n d compared t h e r e s u l t s w i t h t h e r e s p o n s e s o f two h y p o t h e t i c a l models t h a t c a n a c c o u n t f o r t h e b i n a u r a l d i f f e r e n c e g e n e r a t i o n . Our d a t a a r e i n good agreement w i t h one o f t h e models, which assumes t h a t t h e b i n a u r a l d i f f e r e n c e i s g e n e r a t e d from c o n t r a l a t e r a l i n h i b i t i o n u n d e r b i n a u r a l s t i m u l a t i o n . I n t r o d u c t i o n The a c o u s t i c b r a i n s t e m r e s p o n s e (ABR) i s t h e s c a l p s u r f a c e evoked r e s p o n s e t h a t c a n b e r e c o r d e d i n t h e 1 0 msec p e r i o d f o l l o w i n g p r e s e n t a t i o n o f an a c o u s t i c s t i m u l u s a t t h e e a r . I t c o n s i s t s o f 4 t o 7 p e a k s , d e p e n d i n g on t h e s p e c i e s . I n t h e r a t t h e ABR commonly h a s 4 t o 5 p e a k s ( F i g . 1 ) . The i n t e r p e a k i n t e r v a l s a r e a p p r o x i m a t e l y 1 msec whereas t h e peak l a t e n c i e s may d i f f e r u n d e r d i f f e r e n t e x p e r i m e n t a l c o n d i t i o n s . ACOUSTIC BRAINSTEM RESPONSE 5-2 4 I . . . .V 0 MILLISECONDS 10 F i g . 1 : A c o u s t i c b r a i n s t e m r e s p o n s e from r a t . Peaks a r e i n d i c a t e d by L a t i n c h a r a c t e r s . The ABR may a r i s e a s a s e r i e s o f volume c o n d u c t e d p o s t s y n a p t i c p o t e n t i a l s o r from a c t i o n p o t e n t i a l s c a r r i e d a l o n g m y e l i n a t e d f i b e r s , o r b o t h . I n e i t h e r case s c a l p p o t e n t i a s o r i g i n a t e from t h e p r i m a r y a u d i t o r y pathway o f t h e b r a i n s t e m . Although t h e o r i g i n s o f e a c h i n d i v i d u a l peak h a v e n o t y e t been s p e c i f i e d i n d e t a i l , t h e r e i s i n c r e a s i n g l y more e v i d e n c e of t h e c o n t r i b u t i o n of f u n c t i o n a l l y s e p e r a t e s u b s t r a t e s y s t e m s t o t h e ABR waves. a s t h e d i f f e r e n c e between t h e sum o f r e s p o n s e s t o monaural s t i m u l a t i o n...