A COMMON NEURAL CODE FOR FREQUENCY-MODULATED AND AMPLITUDE-MODULATED SOUNDS

MOST naturally occurring sounds are modulated in amplitude or frequenc g; important examples include animal vocalizations and species-specifi c communication signals in mammals, insects, reptiles, birds and amphi bians(1-9). Deciphering the information from amplitude-modulated (AM) sounds is a well-understood process, requiring a phase locking of prim ary auditory afferents to the modulation envelopes(10-12). The mechani sm for decoding frequency modulation (FM) is not as clear because the FM envelope is flat (Fig. 1). One biological solution is to monitor am plitude fluctuations in frequency-tuned cochlear filters as the instan taneous frequency of the FM sweeps through the passband of these filte rs, This view postulates an FM-to-AM transduction whereby a change in frequency is transmitted as a change in amplitude(13,14). This is an a ppealing idea because, if such transduction occurs early in the audito ry pathway, it provides a neurally economical solution to how the audi tory system encodes these important sounds. Here we illustrate that an FM and AM sound must be transformed into a common neural code in the brain stem. Observers can accurately determine if the phase of an FM p resented to one ear is leading or lagging, by only a fraction of a mil lisecond, the phase of an AM presented to the other ear. A single intr acranial image is perceived, the spatial position of which is a functi on of this phase difference.