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Steady-state auditory evoked potentials (SSAEPs) in alert adults are most detectable at stimulus or modulation rates of about 40 Hz. Sedation reduces the detectability of 40-Hz SSAEPs and increases it for higher rate SSAEPs. This study examined whether rates higher than 40 Hz would be preferable for detecting responses to low-intensity tones in sedated adults. Fourteen normal adults listened to 640-Hz tones at modulation rates (and toneburst rates) of 20-160 Hz, in 10-Hz steps, at levels of 38 and 58 dB peak equivalent sound-pressure level (peSPL) (20 and 40 dB normal hearing level (nHL) for amplitude-modulated (AM) tones), both alert and sedated (1-2 g chloral hydrate). Sedation reduced both signal (SSAEP) power and noise power at all rates, but noise power reduction was greater for higher rates. Detectability in the alert condition was always greatest at 40 Hz. Under sedation, a second detectability peak was present at 90 Hz for 58-dB peSPL tones, approximately equal to that seen at 40 Hz. At 38 dB peSPL (sedated), peak detectability moved from 40 to 50 Hz. These results suggest that presentation/modulation rates around 40 Hz may be optimal for SSAEP detectability at low levels in adults, whether alert or sedated.

Objective: Objective threshold estimation is possible using statistics such as magnitude-squared coherence (MSC) and responses such as the 40-Hz auditory evoked potential (AEP). The purpose of this paper is to compare three general methods of threshold estimation using MSC., Design: Using 500-Hz amplitude-modulated tones and 40-Hz AEPs from 10 paid-volunteer young adults with normal hearing, we compared three MSC-based threshold estimation methods--extrapolation, intensity series, and adaptive., Results: The extrapolation method yielded low thresholds but unacceptable variability. The intensity series method was insensitive and time-consuming. Two adaptive methods displayed good performance in terms of precision and sensitivity. The first employed an MSC criterion with a 13.5% false positive rate (per trial), but achieved overall false positive rates under 5% after 5 to 7 runs, requiring 30 to 40 minutes test time (a run is a single set of ascending or descending trials, terminated by a reversal). The second, which performed better overall, included only a single run, but collected more data at intensities near threshold than at higher intensities; test time could be further reduced (from 22 to 18 minutes) by using a stopping rule based on initial MSC and noise power estimates at each test intensity. If speed is paramount, a simple single-run method achieved threshold estimates in about 9 minutes, starting at 40 dB nHL, with acceptable precision but less sensitivity., Conclusions: A single-run adaptive method, with or without collection of additional data near threshold, yielded the best results, in terms of statistical performance and data collection time.

The relative performance of phase coherence (PC) and magnitude-squared coherence (MSC) for detection of steady-state evoked potentials was studied using 40 Hz auditory evoked potentials (AEPs) in 10 normal human subjects. In addition, simulation experiments were carried out to determine the effects of signal amplitude and phase variability on detection performance. All simulations showed MSC performance to be better than PC performance, with further improvements when MSC was supplemented with weighted averaging. However, human 40 Hz AEP data showed essentially identical detection performance for PC and MSC, with or without weighted averaging. These data support a "phase aggregation" model (at least near threshold) over the more usual model in which an AEP signal is added to a stationary noise. Human data collected under "no-stimulus" conditions agree well with theoretical distributions for both PC and MSC. For equal test time, long analysis periods (with less averaging) yielded equal performance to short analysis periods (with more averaging), for both PC and MSC.

A method is proposed for allocation of hearing handicap between noise and aging in individual cases when noise exposure level and duration are known or can be estimated. A recently published international standard (ISO-1999, 1990) provides statistical models for hearing threshold changes associated with aging and noise exposure. When an individual's hearing threshold level exceeds the sum of the median levels expected given that individual's age, gender, exposure level, and exposure duration, the appropriate allocation depends on the correlation between age-related and noise-induced changes. However, the differences in allocation between assumptions of perfect and absent correlation are small. Only very small errors result from calculating the allocation based on median expectations for noise and aging. In most cases, age-related changes exceed noise-induced changes for the 0.5, 1, 2, and 3 kHz pure-tone average; for men age 65, this is true for all exposure levels below 100 dBA.

Binaural interaction (BI) in auditory evoked potentials was defined as any deviation from the predictions of a model which assumes two independent populations of neurons whose outputs are, in the far field, simply additive. Monaural responses are added to yield the model's prediction of binaurally evoked response; this trace is then subtracted from the actual binaural response to yield a difference trace which is considered to represent BI. In a previous study, strong BI was seen in guinea pig BSEP, maximal at about 4 msec latency; the interaction was greatest for simultaneous (delta t = 0) stimuli of equal intensitsy (delta I = 0). Sixteen normal young adults had BSEP and middle-latency auditory evoked potentials (middle AEP) recorded in response to binaural and monaural clicks. Although BI (in the 6 msec latency region) was observed in the difference trace for the majority of subjects' BSEP, there were no significant differences between binaural and predicted-binaural responses in the group means for individual peak amplitudes and latencies. In middle AEP, the amplitude of peak Pa was usually much smaller in binaurally evoked responses than in the predicted-binaural response. Four subjects had middle AEP which were felt to be predominantly myogenic on the basis of short latency and large amplitude; these responses demonstrated a reverse pattern of interaction, with binaural responses much larger than the monaural sums.

Short-latency auditory responses were derived by cross correlation of pseudorandom white noise with averaged scalp potentials in guinea pigs. The cross-correlation functions were characterized by distinct cochlear microphonic and neural components, as distinguished by susceptibility to hypothermia and masking noise. This technique detects only linear, frequency-following responses of the auditory system, and demonstrated neural frequency following up to 3-4 kHz; thresholds were about 30-40 dB spectrum level. While conventional auditory brain stem responses reflect onset neural activity and are most responsive to high-frequency stimuli, cross-correlation responses reflect frequency-following activity, primarily to low frequencies, and thus may represent a complementary method of electrophysiologic assessment of the auditory system. Data are very rapidly acquired, and estimation of responses of limited areas of the cochlea may be possible by off-line digital filtering of cross-correlation functions obtained with broadband noise stimuli.

An overview of some recent developments in the study of central auditory processes is presented. We describe findings from single cell physiologic studies that contribute to our understanding of central auditory function, the development of behavioral techniques permitting precise evaluation of hearing in animals, and the power and potential of integrative neurophysiologic-behavioiral investigations in defining and analyzing central neural mechanisms that underlie normal perception and imperception. The influence of changes in intermodality attention on evoked activity of 25 cells of the auditory cortex is described and compared to effects of other attentional changes. A small but consistent increase in excitatory cell evoked discharge rate and a reduction in initial latency of response are shown to be correlated with a shift from a visual to an auditory task. In general, behavioral states ranging from sleep-waking to attention to a specific acoustic cue are shown to influence responsiveness of the auditory system and variability of response. Procedures are described for effective control of attentional factors and reduction of electrophysiologic variability. The contributions of these data and procedures to studies of the encoding of complex auditory signals and studies of neural mechanisms underlying such disorders as noise induced hearing loss are discussed.