Complex sounds, including human speech, contain time-varying signals like f
requency modulation (FM) and amplitude modulation (AM) components. In spite
of various attempts to characterize their neuronal coding in the mammalian
auditory systems, a unified view of their responses has not been reached.
We compared FM and AM coding in terms of receptive space with reference to
the input-output relationship of the underlying neural circuits. Using extr
acellular recording, single unit responses to a novel stimulus (i.e. random
AM or FM tone) were obtained at the auditory midbrain of the anesthetized
rat. Responses could be classified into three general types, corresponding
to selective sensitivity to one of the following aspects of the modulation:
(a) steady state, (b) dynamic state, or (c) steady-and-dynamic states. Suc
h response typing was basically similar between FM and AM stimuli. Furtherm
ore, the receptive space of each unit could be characterized in a three-dim
ensional Cartesian co-ordinate system formed by three modulation parameters
: velocity, range and intensity. This representation applies to both FM and
AM responses. We concluded that the FM and AM codings are very similar at
the auditory midbrain and may likely involve similar neural mechanisms. (C)
2000 Elsevier Science Ireland Ltd. All rights reserved.