Long-term frequency tuning of local field potentials in the auditory cortex of the waking guinea pig

The goal of our study was to determine the extent of changes in frequency t iming in the auditory cortex over weeks. The subjects were awake adult male guinea pigs (n = 8) bearing electrodes chronically implanted in layers IV- VI of primary auditory cortex. Tuning was determined by presenting sequence s of pure tone bursts (similar to0.97-41.97 kHz, -20 to 80 dB, 100-ms tone duration, 5-ms rise-fall, 800-ms intertone intervals, 1.5-s intersequence i nterval) either in 0.5-octave steps (n = 5, 14 probes) or 0.25-octave steps (n = 3, 9 probes) delivered to the ear contralateral to recording sites. T iming curves were determined for local field potentials (LFPs), which were tuned to frequency (negative potential, latency to peak 15-20 ms), repeated ly for up to 27 days (0.5 octave) or 12 days (0.25 octave). Characteristic frequency (CF), best frequency at 10 and 30 dB above absolute threshold (BF 10, BF30), threshold (TH), and bandwidth (10 dB above threshold; BW) were m easured. Absolute amplitude often decreased across weeks, necessitating nor malization of amplitude. However, there were no significant trends in tunin g over days for CF, BF10, or BF30 for either the half- or the quarter-octav e group. Both groups exhibited random daily variations in frequency tuning, the quarter-octave group revealing larger variations averaging 0.228, 0.21 1, and 0.250 octave for CF, BF10, and BF30, respectively. Therefore, freque ncy tuning in waking animals does not exhibit directional drift over very l ong periods of time. However, daily tuning variations on the order of 0.20- 0.25 octave indicate that the peaks of tuning curves (CF, BF) represent a p referred frequency range rather than a fixed frequency.