Topographic representation of periodicities in the forebrain of the mynah bird: one map for pitch and rhythm?

Coding of amplitude modulated acoustic stimuli was studied within isofrequency planes of the tonotopically organized field L, the avian analogue of the mammalian auditory cortex. The synchronization of unit responses to envelopes of sinusoidally amplitude modulated tones (AM) and repetitive noise bursts (RN) were determined as a function of envelope frequency (EF). From 249 synchronizing units, 66% were tuned to a best envelope frequency (BEF) using a synchronization criterion. BEFs varied from 0.3 to 380 Hz and showed an orderly representation within isofrequency planes orthogonal to the lamination of field L. The majority (68%) of these units had BEFs below 20 Hz down to 0.3 Hz. Thus, they were tuned to rhythms typical for animal communication sounds, speech, and music. Thirty-two % had BEFs between 20 Hz and 380 Hz and covered at least 4 of 5 octaves of the range of periodicity pitch sensation.