Cochlear Delay Research Articles

Effect of decreased and lost high frequency hearing on latency of acoustically evoked brain stem reaction

The diagnosis of retrocochlear damage is supported by an increased latency difference between peak I and peak V. In case of high frequency hearing loss peak I is often hard to determine, and peak V latency may be shifted not only by neural delay, but also by missing basal hair cells. The amount of cochlear delay can be estimated by a procedure presented here. High frequency decay was simulated by steep high-pass noise masking. Peak V latency turned out to be established by the highest unaffected frequency components of the click stimulus. Thus, in case of a high frequency gap (with normalization towards higher frequencies) latency may be almost normal. In case of prolonged latency the amount ascribable to the cochlea may be rather precisely be estimated. If the whole lag is explained this way, unnecessary further diagnostics can be avoided.

Transmission delay of phase-locked cells in the medial geniculate body

Over 4000 single unit recordingswere obtained from the medial geniculate body (MGB) of nitrous oxide anaesthetized cats. Out of 1600 cells sensitive to tone bursts below 4 kHz, 10% were responding in a sustained manner. From these, 121 were tested for phase-locked responses. The general characteristics of these units have been described in a previous report. The central tendency of the discharges distribution within the period or mean phase angle was studied for many frequencies in 24 phase-locked units. For each of them, the mean phase angle shifts linearly with the frequencies. The slope of these phase versus frequency lines is an accurate measure of the transmission delay from the cochlea to the MGB. This delaly is function of the unit's characteristics frequency and shows that the time spread introduced by the cochlea between the high and low frequency components of an acoustic signal is preserved up to the MGB. Substracting the cochlear delay, the neural delay from the eight nerve to the MGB was found to be 6.4 ms for neurons having a CF above 300 Hz; it was greater by 3 ms for cells with a CF below that frequency.

Transient response of the basilar membrane measured in squirrel monkeys using the Mössbauer effect.

Measurements of the transient response of the basilar membrane were conducted using the Mössbauer effect on 33 squirrel monkeys using an experimental preparation identical to that of Rhode (1971). The stimuli were acoustic clicks 150 μsec in duration repeated 100 000–400 000 times. The amplitude of the click was varied and the responses of the malleus and of the basilar membrane at a point in the basal turn were measured. The basilar membrane’s click response is oscillatory, with a period near that of the characteristic frequency. The first few response peaks behave almost linearly with stimulus intensity, while the later peaks exhibit a pronounced nonlinearity. This behavior is shown to be consistent with the nonlinearity reported using steady-state measurement methods (Rhode, 1971). The transient response observed in some of the preparations was very lightly damped; however, a wide range in the damping of the responses was found in the different animals. A progressive increase in the rate of decay of the transient response with the duration of the experiment was observed. Any damage to the cochlea also resulted in increased damping. The measured cochlear delay time of 300–390 μsec agrees with that derived from phase curves of the transfer ratio measured with steady-state methods. Subject Classification: [43]65.26, [43]65.20.