Stochastic properties of cat auditory nerve responses to electric and acoustic stimuli and application to intensity discrimination

Statistical properties of electrically stimulated (ES) and acoustically sti mulated (AS) auditory nerve fiber responses were assessed in undeafened and short-term deafened cats, and a detection theory approach was used to dete rmine fibers' abilities to signal intensity changes. ES responses differed from AS responses in several ways. Rate-level functions were an order of ma gnitude steeper, and discharge rate normally saturated at the stimulus puls e rate. Dynamic ranges were typically 1-4 dB for 200 pps signals, as compar ed with 15-30 dB for AS signals at CF, and they increased with pulse rate w ithout improving threshold or changing absolute rate-level function slopes. For both ES and AS responses, variability of spike counts elicited by repe ated trials increased with level in accord with Poisson-process predictions until the discharge rate: exceeded 20-40 spikes/s. AS variability continue d increasing monotonically at higher discharge rates, but more slowly. In c ontrast, maximum ES variability was usually attained at 100 spikes/s, and a t higher discharge rates variability reached a plateau that was either main tained or decreased slightly until discharge rate approached the stimulus p ulse rate. Variability then decreased to zero as each pulse elicited a spik e. Increasing pulse rate did not substantially affect variability for rates up to 800 pps; lather, higher pulse rates simply extended the plateau regi on. Spike count variability was unusually high for some ES fibers. This was traced to response nonstationarities that stemmed from two sources, namely level-dependent fluctuations in excitability that occurred at 1-3 s interv als and, for responses to high-rate, high-intensity signals, fatigue that a rose when fibers discharged at their maximum possible rates. intensity disc rimination performance was assessed using spike count as the decision varia ble in a simulated 2IFC task. Neurometric functions (percent correct versus intensity difference) were obtained at several levels of the standard (I), and the intensity difference (Delta I) necessary for 70% correct responses was estimated. AS Weber fractions (10 log Delta I/I) averaged +0.2 dB (Del ta I-dB = 3.1 dB) for 50 ms tones at CF. ES Weber fractions averaged -12.8 dB (Delta I-dB = 0.23 dB) for 50 ms. 200 pps signals, and performance was a pproximately constant between 100 and 1000 pps. Intensity discrimination by single cells in ES conditions paralleled human psychophysical performance for similar signals. High ES sensitivity to intensity changes arose primari ly from steeper rate-level functions and secondarily from reduced spike cou nt variability. (C) 2000 Acoustical Society of America. [S0001-4966(99)0791 2-6].