Characteristics of reliable tone-evoked oscillations in the rat thalamo-cortical auditory system

Tone-evoked oscillations were studied from simultaneous recordings collecte d in the auditory cortex, auditory thalamus and auditory sector of the reti cular nucleus in urethane anesthetized rats. These oscillations were precis ely time-locked to tone onset and were easily observed on peristimulus time histograms (PSTHs). Visual inspection of PSTHs and rasters led us to disti nguish between 'reliable' oscillations (which exhibited oscillatory pattern s in more than 50% of the trials) and 'labile' oscillations (which exhibite d oscillations in less than 50% of the trials). Systematic quantification o f oscillations based on several indices derived from power spectra confirme d this distinction. 'Reliable' stimulus-locked oscillations were observed i n 51/184 (28%) of the recordings from auditory cortex, 9/55 (17%) of the re cordings from auditory thalamus and 11/26 (42%) of the recordings from the auditory sector of the reticular nucleus. The frequency range of these osci llations was the same in the three structures (5-14 Hz). Within the same an imal, when one electrode exhibited oscillations, there was a high probabili ty of detecting similar oscillations from electrodes located in the same st ructure, but not from electrodes located in the other structures. These osc illations were observed for pure tone frequency (or for clicks) whatever th e tone duration (1 s, 100 ms, 10 ms). The inter-tone interval (ITI) was fou nd to be the critical factor controlling the occurrence of these oscillatio ns: they were present for ITIs of 2 s or longer, but were absent for ITIs o f 1 s or less. In contrast, the occurrence of the oscillations was a funct: ion neither of the strength of the 'on' evoked response nor of the animal's temperature. However, lowering the animal's temperature from 37-38 degrees C to 35-36 degrees C systematically led to a decrease in the frequency and an increase in the duration of the tone-evoked oscillations. These results suggest that, even in well defined conditions (temperature, EEG, ITI, leve l of anesthesia), the oscillations triggered by presentation of the same st imulus can be stable or unstable. This temporal instability of stimulus-evo ked oscillations has to be taken into account before stating percentages of oscillations in a given brain structure. They also suggest that some gener al factors such as the animals temperature or the inter-stimulus interval c an considerably affect their characteristics and/or their occurrence. (C) 2 000 Elsevier Science B.V. All rights reserved.