M. Albert

Temporal and spatial coding of periodicity information in the inferior colliculus of awake chinchilla (Chinchilla laniger)

Amplitude modulation responses and onset latencies of multi-unit recordings and evoked potentials were investigated in the central nucleus of inferior colliculus (ICC) in the awake chinchilla. Nine hundred and one recording sites with best frequencies between 60 and 30 kHz showed either phasic (18%), tonic (25%), or phasic-tonic (57%) responses. Of 554 sites tested for responses to modulation frequencies 73% were responsive and 57% showed clear preference for a narrow range of modulation frequencies. Well defined bandpass characteristics were found for 32% of rate modulation transfer functions (rate-MTFs) and 36% of synchronization MTFs (sync-MTFs). The highest best modulation frequency (BMF) of a bandpass rate-MTF was 600 Hz. Neurons with phasic responses to best-frequency tones showed strong phase coupling to modulation frequencies and were dominated by bandpass rate-MTFs and sync-MTFs. Most neurons with tonic responses showed bandpass tuning only for sync-MTFs. Both BMFs and onset latencies changed systematically across frequency-band laminae of the ICC. Low BMFs and long latencies were located medially and high BMFs and short latencies laterally. Latency distributions obtained with evoked potentials to clicks showed a similar gradient to the multi-unit data. These findings are in line with previous findings in different animals including humans and support the hypothesis that temporal processing results in a topographic arrangement orthogonal to the spectral processing axis, thus forming a second neural axis of the auditory system.

Tonotopy and Periodotopy in the Auditory Midbrain of Cat and Guinea Fowl

A laminar functional organization has been demonstrated in the auditory midbrain of mammals and birds. In the midbrain of cat a fine structure of the tonotopic organization could be demonstrated, covering a frequency range of 1/6 octave in each of the functional laminae. Neurons in the midbrain of various animals have been found to be selective for certain modulation frequencies (BMF) relevant for the perception of periodicity pitch. An orderly topographical distribution of BMF could be demonstrated in the laminae of the ICC of the cat, approximately orthogonal to the tonotopic fine-structure within the laminae. An orderly spatial representation of BMF could be demonstrated also in the midbrain of Guinea fowl.