Mw. Spitzer et Mn. Semple, RESPONSES OF INFERIOR COLLICULUS NEURONS TO TIME-VARYING INTERAURAL PHASE DISPARITY - EFFECTS OF SHIFTING THE LOCUS OF VIRTUAL MOTION, Journal of neurophysiology, 69(4), 1993, pp. 1245-1263
1. Motion of sound sources results in temporal variation of the binaur
al cues for sound localization. We evaluated the influence of virtual
motion on neural tuning to one of these cues, interaural phase dispari
ty (IPD). Responses to dichotic stimuli were recorded from single unit
s in the inferior colliculus of the anesthetized cat and gerbil (Merio
nes unguiculatus). Static IPDs were generated by presenting dichotic t
one pairs with a constant phase offset maintained for the duration of
the stimulus. Time-varying IPDs were generated by simultaneously prese
nting a pure tone to one ear and a phase-modulated tone to the other e
ar. Sets of time-varying stimuli consisted of modulations through part
ially overlapping ranges of IPD, corresponding to movement of a sound
source through partially overlapping arcs in the horizontal plane. 2.
In agreement with previous results, neuronal discharge was typically a
peaked function of static IPD resulting from both binaural facilitati
on at favorable IPDs and binaural suppression at unfavorable IPDs. Res
ponses to time-varying IPD stimuli appeared to be shaped by the same f
acilitative and inhibitory mechanisms that underlie static IPD tuning.
Modulation toward the peak of binaural facilitation increased the pro
bability of discharge, and modulation toward the peak of binaural supp
ression decreased the probability of discharge. However, it was also c
lear that IPD tuning could be significantly altered by the temporal co
ntext of the stimulus. For the vast majority of units in response to m
odulation through partially overlapping ranges of IPD the discharge ra
te profiles were generally nonoverlapping. This shift in IPD tuning in
duced by the virtual motion reflects the fact that the binaural intera
ction associated with a given IPD depends on the recent history of sti
mulation. In addition, modulation in opposite directions through the s
ame range of IPDs often elicited asymmetric responses. These nonlinear
ities imply that most inferior colliculus neurons do not unambiguously
encode a specific IPD, but instead may encode small changes of IPD oc
curring virtually anywhere within their receptive fields. In a few cas
es modulation through overlapping ranges of IPD elicited contiguous re
sponse profiles, indicating that for these units responses were determ
ined entirely by instantaneous IPD. 3. The nonlinearity of responses t
o time-varying IPD stimuli could not be attributed to monaural entrain
ment to the phase-modulated signals, did not depend on the phase modul
ation waveform, and occurred irrespective of which ear received the ph
ase-modulated signal. Responses were similar in cats and gerbils, sugg
esting that the underlying mechanisms are common to binaural processin
g in diverse mammalian species. 4. The consistent shifts in IPD tuning
displayed by most neurons in our sample suggests that sensitivity to
dynamic spatial cues is a general property of neurons in the inferior
colliculus. A measure was developed to quantify the magnitude of the s
hifts in IPD tuning across a large sample of units. This index of moti
on sensitivity was distributed continuously across the sample of units
and was independent of frequency tuning, preferred static IPD, and sh
arpness of tuning to static IPD. 5. The initial neural encoding of IPD
is believed to occur through a process of coincidence detection or cr
oss-correlation at the superior olivary complex. The present finding t
hat IPD tuning in the inferior colliculus is so dependent on the dynam
ic context suggests that the output of the original cross-correlator m
ust be modified in the ascending auditory pathway. These data reveal t
hat within the inferior colliculus the neural representation of IPD, a
nd consequently sound location, is influenced by movement of a sound s
ource.