Frederic G. Worden

Frequency-following (microphonic-like) neural responses evoked by sound☆

Abstract 1. 1. An acoustically evoked response is described which reproduces the frequency and wave form of the stimulus. This frequency-following response (FFR) is recordable from gross electrodes in the central auditory pathway. 2. 2. The FFR differs from an acoustic stimulus of graded onset, and the cochlear microphonic response (CM), in that it has a sharp onset, a latency appropriate to the locus from which it is recorded, an amplitude burst at the onset and a decrement of amplitude over time. 3. 3. The frequency range of FFR increases with stimulus intensity. At 80 dB sound pressure level the range is approximately 500–5000 c/sec. For recordings from the cochlear nucleus these frequency limits are not influenced by Nembutal anesthesia. 4. 4. The wave form and amplitude of FFR vary with stimulus frequency and with laterality of stimulus input. 5. 5. In contrast to the auditory evoked potential, which can be recorded widely in the brain, FFR is recordable only within, or close to, the auditory pathway. We have observed it only at, and below the level of the inferior colliculus. 6. 6. FFR has implications for the neurophysiology of hearing which are different from those of the auditory evoked potential. Some of these are discussed.

Auditory potentials during acoustic habituation: Cochlear nucleus, cerebellum and auditory cortex

Abstract 1. 1. Amplitude changes of auditory EPs and of cortical background activity were studied during acoustic habituation in 5 cats with chronically implanted electrodes. Click stimuli were delivered to the unrestrained animals through earphones in order to hold acoustic input constant. 2. 2. Data were collected in both non-alerted (sleeping) and alerted conditions after 2, 4 and 6 h of click repetition. 3. 3. At the cochlear nucleus, no consistent amplitude change in EPs was observed. There was no amplitude difference between the non-alerted and alerted conditions. 4. 4. At the auditory cortex, a consistent and progressive loss of amplitude of EPs over time occurred for the alerted but not for the non-alerted condition. 5. 5. At the auditory cortex, the amplitude of EPs was larger in sleep than in the alerted condition. 6. 6. At the auditory cortex, the amplitude of EPs covaried with the amplitude of background activity across the non-alerted and alerted conditions, and also within each condition. It is suggested that amplitude of cortical EPs is a function of the state of cortical synchrony, regardless of how this state of synchrony is produced. 7. 7. Large auditory EPs were recorded from the flocculus and adjacent cerebellum. Despite great variability, these showed a significant loss of amplitude during acoustic habituation for the alerted, but not for the non-alerted condition. In addition, they were larger in the sleep than in the alerted condition.

AMPLITUDE CHANGES OF AUDITORY POTENTIALS EVOKED AT COCHLEAR NUCLEUS DURING ACOUSTIC HABITUATION.

Abstract 1. 1. Evoked auditory potentials at the cochlear nucleus were recorded from sixteen chronically implanted electrodes in nine cats during acoustic habituation at each of two repetition rates — 1/sec and 1/10 sec. Seven animals were given 6 h habituation runs and two were given 5 day runs. 2. 2. Three simultaneous recordings were obtained from each bipolar electrode. Two of these were monopolar tracings, comparing each tip of the electrode to ground, and the third was a bipolar recording between the two electrode tips. 3. 3. Reduction of evoked CN potentials during acoustic habituation was not confirmed. Statistically significant changes in amplitude occurred but these were small and inconsistent in direction. Increases as well as decreases were observed. 4. 4. Amplitude changes were inconsistent between right and left CN as well as between two adjacent points within one CN. 5. 5. No correlation was found between electrode placement and direction of amplitude changes over time. Even from one electrode tip, amplitude changes on one run could be reversed on a second run. 6. 6. No consistent effect on peak to peak amplitudes was observed as a function of rate of stimulus presentation or amount of movement of the animal. 7. 7. The cumulative mean amplitude of potentials in the alert samples was larger than in the sleep samples. It is suggested that the acoustic effects secondary to position differences between the alerted and the sleeping samples would tend to contribute to the amplitude differences observed. 8. 8. The implications of these results for CN function during acoustic habituation are discussed.

Some Effects of Room Acoustics on Evoked Auditory Potentials

Auditory potentials were recorded from bipolar electrodes chronically implanted in the cochlear nuclei of four cats. In a training box modified to reduce echoes these animals were exposed to clicks and tone pulses presented from an overhead speaker. Slight changes in the position of the animal in the resulting sound field produced marked changes in the potentials evoked from the cochlear nucleus. These phenomena were observed in the unanesthetized, unrestrained subjects as well as in those under Nembutal anesthesia. It is suggested that these acoustic effects complicate the analysis and interpretation of potentials evoked from the cochlear nucleus under conditions of habituation, shifts in attention, and learning.

Auditory Frequency-Following Response: Neural or Artifact?

An electrical response which reproduces the waveform and frequency of the sound stimulus can be recorded from the central neural pathway for audition. Controversy has existed for some years over whether this frequency-following response (FFR) is neural or an artifact such as remote pickup of the cochlear microphonic or cross talk in the recording system. Two experiments resolve this issue by demonstrating that the frequency-following response depends upon functionally intact neural pathways. The frequency-following response, as well as auditory evoked potentials, is abolished by section of the eighth nerve; it is reversibly abolished by cooling of the cochlear nucleus.

Sound evoked frequency-following responses in the central auditory pathway.

An acoustically evoked response is described which reproduces the frequency and waveform of the stimulus. This frequency following response (FFR.) is recordable from gross electrodes in the central auditory pathway.

Receptor and neural responses in auditory masking of low frequency tones

Abstract Responses from cochlea and cochlear nucleus were recorded in cats through gross electrodes, using stimulus conditions under which masking effects were demonstrated with human subjects. With successive intensity increments of the noise masker relative to the tone stimulus, the neural “frequency-following response” (FFR) showed a significantly greater diminution in amplitude than did the cochlear microphonic. Results suggested masking is mediated neurally in the cochlear nucleus and is separable from interfernce effects known to occur at the cochlea. In order to explore further the neural mechanisms involved, experiments were performed to study the activity of single cells in the cochlear nucleus under the same stimulus conditions. Cells that fired in phase-locked fashion to the tone frequency showed progressive desynchronization with increasing intensity of the noise masker. These results support the hypothesis that the noise pre-empts the activities of units which would otherwise be part of the phase-locked neural population contributing to the grossly recorded FFR envelope and suggest at least one neural mechanism involved in the masking of low frequency tones.

Variability of evoke auditory potentials and acoustic input control

Abstract 1. 1. Samples of auditory EPs were recorded under two conditions of stimulus delivery: overhead speaker and earphones. For each condition the cats position in the cage differed between samples but was constant within samples. 2. 2. Variation of EP amplitude and waveform between-samples is markedly reduced with earphones as compared to overhead speaker. 3. 3. A gradient of acoustic influence on EPs was observed with the largest influence at CN and the least at cortex. 4. 4. The non-acoustic variability of EP amplitude (earphone condition) showed no relationships to level of the auditory system in absolute value, but, proportional to mean amplitude, variability at AC is greater than at CN or IC.

Some factors modulating neural activities in peripheral auditory centers.

Summary Some central and peripheral factors modulating sound-evoked neural activities, as measured by the frequency following response (FFR), were investigated in peripheral levels of the central auditory pathway in cat. It was shown that middle ear muscle activity modulates FFR, and that these modulations closely parallel those observed in the cochlear microphonic. In the absence of ear muscle activity (tenotomy, Flaxedil) we were unable to demonstrate any modulation of sound-evoked responses of the cochlea (microphonic) or of the central auditory relays (cochlear nucleas, trapezoid body, and medial accessory olivary nucleus). This suggests that, if central efferent influences act upon afferent activities at these levels of the auditory pathway, they exert their effects in forms not evident in the neural FFR. Some implications of these results are discussed.

Electrophysiological Analog of the Interaural Time‐Intensity Trade

Variations in the amplitude and polarity of evoked potentials with differences in intensity and arrival time of clicks delivered to the two ears were recorded from the superior‐olivary nucleus in cats with chronically implanted electrodes. An analog of the “time‐intensity” trade was seen in the cancellation of evoked potentials when time and intensity differences were opposed.