The acoustic reflex pattern studied by the averaging technique

The effect of auditory stimulation on the tensor tympani in patients following stapedectomy

CONCLUSION

We believe that a tensor tympani reflex, in response to loud sound, is present in a minority of people, although its functional significance with regard to sound transmission is questionable. The absence of startle response in our stapedectomized subjects makes us question whether the tensor component of the startle response is large enough to be identified by manometry.

OBJECTIVES

This study was organized to examine reflex activity to stimulation by loud sound or by startle in the tensor tympani. Although many previous studies have been carried out, results have been contradictory, and methodological flaws have rendered the interpretations questionable.

PATIENTS AND METHODS

Stapedectomized patients were invited to take part in the study. Thirteen patients underwent testing by extratympanic manometry, using a standard tympanometer. Responses were observed during repeated stimuli with loud sound at 500, 1000, 2000 and 4000 Hz to the ipsilateral and contralateral ears, and with an air jet against the closed eye. A control group was also studied for the startle test.

RESULTS

Three clear ipsilateral (23%) and two clear contralateral (14%) responses to auditory stimuli were seen in the 13 patients. We found responses to startle stimuli in none of our study group and eight (42%) of control ears.

The effect of contralateral broad-band noise on acoustic distortion products from the human ear

In the first part of the study, the effect of ipsilateral stimulus level on contralateral suppression of the 2f2-f2 distortion product by broad-band noise at 60 dB SPL RMS is examined. The levels of the primary stimuli were manipulated independently, giving f1 and f2 growth curves at four different f2 frequencies for four subjects. These typically bell-shaped, growth curves are shifted vertically to lower distortion levels and, in some cases, horizontally to higher primary stimulus levels. These results can be interpreted as an attenuation of both the primary stimuli and the distortion product and could be caused by simple acoustic attenuation produced by middle ear muscle activity, efferent activity or a combination of the two. In the second part of the study, the same contralateral stimulus was used while measuring both 3f1-2f2 and 2f1-f2 distortion products from the ipsilateral ear. The frequency separation of the primary tones was varied. This produced an approximately bandpass shape with the level peaking when the distortion frequency was approximately half an octave below f2, as previously described (Brown and Gaskill, 1990). This shape is thought to be linked with frequency selectivity in the cochlea. Contralateral broadband noise did not affect the tuning or the centre frequency of the bandpass shape or the mean group delay. It did reduce the size of the distortion peak and, in particular, it affected the peak-to-trough height of the 'fine structure' in the amplitude. Vector analysis revealed that the fine structure was due to a signal with substantial delay (probably from the distortion product 'place') which was summed with a larger, less delayed component (probably directly from the f2 'place'). The greater effect of contralateral stimulation on the more delayed component may reflect differences in efferent effect with complex (stimulus place), rather than simple (distortion product place) stimuli.

Voluntary contraction of middle ear muscles: effects on input impedance, energy reflectance and spontaneous otoacoustic emissions

Two types of measurements were performed on a subject able to voluntarily contract her middle ear muscles (MEM). First, wideband measurements (0-11 kHz) of middle ear input impedance and energy reflectance were obtained when the subject was relaxed and when she contracted her MEM. The changes in impedance observed with voluntary MEM contraction were similar to those reported in the literature for acoustically-elicited MEM contractions. The energy reflectance increased for frequencies below about 4 kHz. Second, the effects of voluntary MEM contraction on the frequencies and levels of spontaneous otoacoustic emissions (SOAEs) were measured and compared to effects evoked by contralateral acoustic stimulation. Effects on SOAEs appear to be a more sensitive indicator of MEM activity than changes in impedance, and the effects due to voluntary MEM contraction were qualitatively similar to those evoked by contralateral acoustic stimulation. These results suggest that in subjects with normally-functioning middle ears, only some effects on otoacoustic emissions caused by contralateral stimuli whose levels are below the contralateral acoustic reflex threshold can be unequivocally attributed to the action of cochlear efferents. The temporal aspects of SOAE frequency shifts caused by voluntary contraction of MEM show that voluntary contraction fatigues rapidly over a time period of tens of seconds.

Event-related perturbations in an electrophysiological measure of auditory sensitivity: effects of probability, intensity and repeated sessions

It is often held that novel or salient stimuli are followed by a brief period of orienting or alerting during which sensory processes are facilitated. Evidence for such a period of facilitation was sought in a paradigm in which evoked responses to weak auditory probe stimuli were examined when given in the presence of salient foreground stimuli, which were varied in probability and intensity, and which were given in two replicate sessions. The background probe stimuli consisted of a continuous train of auditory pip stimuli delivered at a rate of 40 pips per second. Under such conditions of repetitive stimulation a steady-state rhythm (SSR), which is believed to reflect summated early and middle latency evoked responses, is established in the EEG at a corresponding frequency of 40 Hz. The 40 Hz SSR was extracted using a digital averaging and filtering technique and examined continuously for changes in amplitude and latency. The rhythm showed a brief episode during which the latencies of response were decreased. The reduction in latency was greatest at 186 ms after the foreground stimulus, at which time the latencies of individual peaks in the rhythm were reduced by about 3.5 ms. The magnitude of the latency reduction response was larger for intense and for rare stimuli, and showed long-term decrement during the second session. Event-related potential and heart rate responses to the foreground stimulus were also affected by probability, intensity and session, but not in the same pattern. It was hypothesized that the latency shift in the 40 Hz SSR reflects a brief period of sensitization during alerting or orienting responses.

Concomitant changes in the acoustic impedance and the cochlear microphonic potentials during twitch contractions of the middle ear muscles in cats

The effect of twitch contractions of the middle ear muscles in cats on sound transmission through the middle ear (as measured by the cochlear microphonic potential of the inner ear) was compared with the simultaneous changes in the acoustic input impedance of the middle ear at the same frequency. It was found that decreases in impedance were related to an increase in the amplitude of the cochlear microphonics and vice versa. This may imply that decreases in impedance measured during the initial phase of the acoustic reflex in man are true decreases and are not due to transient decoupling of the ossicular chain at any point.