Evan M. Relkin

A reexamination of forward masking in the auditory nerve

Forward masking, as measured behaviorally, is defined as an increase in a signals detection threshold resulting from a preceding masker. Previously, forward masking in the auditory nerve has been measured as a reduction in the neural response to a signal when preceded by a masker. However, detection threshold depends on both the magnitude of the response to the signal and the variance of the response. Thus changes in detectability cannot be inferred from response reduction alone. Relkin and Pelli (1987) have described a two-interval forced-choice procedure that may be used to measure the threshold for the detection of a probe signal in recordings of spike counts in single auditory neurons. These methods have been used to study the forward masking of characteristic frequency probe tones by characteristic frequency maskers as masker intensity was varied. Although the masker does reduce the detectability of the probe tone, it was found that the threshold shifts are much less than those observed behaviorally, particularly for intense maskers. In part, the small threshold shifts can be attributed to the reduction in response variance following the masker, which is the result of the adaptation of spontaneous activity. These results imply that behavioral forward masking must result from suboptimal processing of spike counts from auditory neurons at a location central to the auditory nerve.

Displacement of the Malleus in Neonatal Golden Hamsters

A capacitive probe was used to measure displacements of the umbo of the malleus in neonatal golden hamsters for discrete frequencies from 0.6 to 35.0 kHz. Displacement, extrapolated to a sound pressure of 100 dB SPL, plotted as a function of frequency demonstrated low-pass characteristics with a cutoff frequency near 9.0 kHz. The amplitude of displacement increased with age to a plateau at all frequencies. Analysis of the data below the cutoff frequency indicated that low frequency displacements were dominated by a compliance which increased with age. It was also found that high frequency responses showed evidence of increased mass limitation in younger as compared with older subjects. Comparisons between the inverse of velocity and evoked response threshold curves indicated that the middle ear plays an important role in determining thresholds throughout development.

The development of middle‐ear admittance in the hamster

A high-frequency admittance meter was developed and used to study the maturation of physiological function in the middle ear of neonatal golden hamsters (Mesocricetus auratus). The middle-ear input admittance in the frequency range of 0.8-1.8 kHz was measured in animals ranging in age from 6 to 69 days postpartum. Admittance magnitude was found to increase steadily with age, beginning on day 16, to asymptotic values at each test frequency. There were no obvious differences in admittance growth rates within the range of frequencies tested. However, an analysis of the slopes of the admittance ma;nitude frequency response curves revealed an increase from 4.6 dB/octave for animals 25 days old or younger, to 6.3 dB/octave for all older animals. This difference between younger and older subjects indicates that the development of the middle ear in the golden hamster is more complex than a simple increase in pure compliance.

Changes in middle-ear input admittance during postnatal auditory development in chicks

Admittance at the tympanic membrane was measured in one ear of 73 chicks in 8 age groups between 1 and 150 days post-hatch. Admittance at 7 frequencies (0.66-1.8 kHz) was calculated from measurements of the magnitude and phase angle of sound pressure in front of the tympanic membrane. The results show that admittance magnitude at all frequencies increased exponentially with age during the first 70 days of life. The admittance angle also changed systematically with age. These developmental events may be related to changes in the compliance and resistance of the tympanic membrane.

Threshold and suprathreshold temporal integration effects in the crossed and uncrossed human acoustic stapedius reflex

Threshold and suprathreshold temporal integration (TI) effects were studied in the crossed and uncrossed human acoustic stapedius reflex. Changes in reflex threshold were compared at four stimulus durations and for six threshold response criteria; suprathreshold effects were similarly compared at four stimulus durations and at five stimulus sound-pressure levels. Our results showed that reflex thresholds were significantly lower for the uncrossed condition and for longer duration stimuli. Both effects were dependent on threshold response criteria. Threshold TI measurements in both crossed and uncrossed conditions were equivalent at low criterion levels (0%-4%, p greater than 0.05) but were significantly larger in the crossed condition at higher criterion levels (5%-6%, p less than 0.05). Prominent suprathreshold effects also occurred. The main effect was characterized by significantly larger reflex magnitudes in the uncrossed condition. Duration dependent slope differences in magnitude intensity functions (MIFs) were also observed and were characterized by steeper slopes for longer duration stimuli. Saturation effects and/or intensity-dependent slope decelerations in MIFs were predominant in the uncrossed condition and for stimulus durations exceeding 20 ms. These data provide further quantitative evidence for asymmetric threshold and suprathreshold response properties of the crossed and uncrossed human acoustic stapedius reflex and demonstrate the dependence of these effects on stimuli of different durations.

Neural contributions to the perstimulus compound action potential: Implications for measuring the growth of the auditory nerve spike count as a function of stimulus intensity

The perstimulus compound action potential (PCAP), unlike the more familiar compound action potential (CAP), can be recorded in response to asynchronous as well as synchronous auditory nerve activity. When all neurons contribute equally to the PCAP, the area under the PCAP (the PCAP area) is proportional to the number of action potentials fired by auditory nerve neurons (the auditory nerve spike count). The auditory nerve spike count is one proposed code for stimulus intensity, and our goal is to use the PCAP to test this hypothesis. In this study, two independent tests were developed to measure the contributions of neurons to the PCAP as a function of their characteristic frequency (CF). The test results were verified using a model of the auditory periphery designed to calculate the auditory nerve spike count as a function of pure tone intensity and frequency. In nearly all experiments, neurons having CFs that span contiguous three or four octave bands contribute equally to the PCAP. For pure tones that stimulate only those neurons contributing equally to the PCAP, the PCAP area grows over intensity ranges frequently exceeding 80 dB, and in one case equaling 108 dB. These results demonstrate that the auditory nerve spike count, at least for pure tones, is capable of encoding changes in stimulus intensity over the entire dynamic range of the auditory system.

Short‐term poststimulatory response characteristics of the human acoustic stapedius reflex: Monotic and dichotic stimulation

Two experiments were performed to study short-term poststimulatory response characteristics of the human acoustic stapedius reflex in the time and intensity domains. In experiment 1, monotic magnitude-intensity functions (MIFs) were obtained for a 20-ms test stimulus preceded by a conditioning stimulus varying in duration (20, 50, 100, 500 ms) and level (-10, 0, +10 dB re: stapedius-reflex threshold) and temporally separated from the test stimulus by various interstimulus intervals (ISIs) (0, 20, 50, 100, 500 ms). Experiment 2 was similar in design except that conditioner and test stimuli were presented dichotically and fewer ISIs were used. Both experiments demonstrated that a prior conditioning stimulus produced significant increases in test-stimulus response magnitude. These poststimulatory effects were characterized by complex interactions among stimulus variables (conditioner duration, conditioner level, and interstimulus interval) with similar interactions occurring for both monotic and dichotic stimuli. A simple superposition effect of the responses to the conditioner and test stimulus does not account for the effect of prior stimulation since responses often exceeded the sum of the responses to the conditioner and the test stimulus alone.

Forward masking of the compound action potential: Thresholds for the detection of the N1 peak

A two-interval forced-choice method was developed that provides a rapid and objective computerized measurement of the threshold for detection of the N1 peak of the compound action potential (CAP) recorded in response to a probe tone. The CAP was recorded at the round window of anesthetized chinchillas and several gerbils. An adaptive threshold-tracking procedure was verified by comparing measured thresholds to those obtained from neurometric functions, which plot the proportion of correct detections of the probe as a function of probe intensity. The adaptive procedure was applied in a forward masking paradigm to study the growth of masking of the CAP as a function of masker intensity. Results indicate that growth of masking of the CAP more closely corresponds to that observed psychophysically, than does forward masking observed in the response of a single neuron. Implications for neural encoding mechanisms are discussed.

Some temporal effects on the sensitivity of the CAP to changes in sound intensity and frequency

The sensitivity of the compound action potential (CAP) of the auditory nerve to changes in sound intensity and frequency was obtained in anesthetized Mongolian gerbils. Detection of the change in N1 peak magnitude caused by a probe stimulus was determined using a two‐interval tracking procedure [E. M. Relkin and R. L. Smith, J. Acoust. Soc. Am. Suppl. 1 83, S98 (1988)], a technique that depends on both the N1 magnitude and its variability. The probe was added to a background sound consisting of a train of short tone bursts, and the time delay from background onset to probe onset was varied. Adaptation, i.e., the number of prior tone bursts, reduced the magnitude and variability of the CAP in response to successive background tone bursts, but did not appear to directly affect detection of the probe tone. However, detection was found to be a sensitive function of the latency between the onset of the probe and the corresponding background tone burst, particularly for probes that differed in frequency from th...

Relative contributions of two neural population responses to intensity coding

Two neural population responses have been proposed to encode the 120‐dB intensity range: the spread of excitation along the entire basilar membrane and neurons of different thresholds within a narrow region of the basilar membrane. This study tests relative contributions of the two population codes by measuring forward‐masked intensity discrimination of a broadband noise (14 kHz) and pure tones from 125 to 8000 Hz in octave steps. The hypothesis is that the broadband noise and low‐frequency tones rely more on the spread of the excitation code, whereas high‐frequency tones rely more on the high‐threshold neuron code. To obtain appropriate forward masker levels, three normal‐hearing listeners balanced loudness of the broadband noise and tones to a 100‐ms, 90‐dB SPL, 1000‐Hz standard. Intensity discrimination was measured for a 25‐ms probe which had the same spectral characteristic as the masker and occurred after a 100‐ms delay from the masker’s offset. The discrimination data showed that the midlevel hump ...