Donald C. Teas

Acoustically dependent latency shifts of BSER (wave V) in man

The latencies of wave V in Brain Stem Evoked Responses (BSER) elicited by a set of acoustic transients were measured. The stimuli were produced by delivering pulses to two filters, arranged in series. The filters were set so that the maximum acoustic energy in the transients, i.e., filtered clicks, occurred at 0.5, 1, 2, 4, or 8 kHz. The filtered clicks were presented via earphones at a rate of 30/s at 20, 40, or 60 dB HL to ten subjects with normal hearing. The latencies of wave V varied systematically with center frequency of the filtered clicks when they were each at the same HL. Stimuli presented at 40 dB HL produced the greatest opportunity for relating stimulus frequency to latency. The latencies for a smaller set of responses to stimuli presented at 10/s were the same as those for the principal data taken at 30/s. The changes in latency of wave V due to frequency are similar to those observed by other investigators in whole-nerve responses recorded in man.

Electrophysiological Studies on the Spatial Distribution of the Crossed Olivocochlear Bundle along the Guinea Pig Cochlea

The effectiveness of electrical stimulation of the crossed olivocochlear bundle (COCB) was measured at different locations along the cochlear partition. Differential electrodes were used to sample the cochlear microphonic (CM) at turns I, II, and III. A pipette inserted into scala media in turns I, II, and III was used to measure the slow negative potential associated with electrical stimulation of the COCB. The responses of auditory nerve fibers were sampled within the modiolus and included a wide range of best frequencies. The magnitude of the slow negative potential was maximum in turn I and decreased in turns II and III. The increment in CM, produced by COCB stimulation, depended not only upon the location along the cochlear partition from which it was recorded, but also upon acoustic frequency. For low‐frequency acoustic signals (below 700 Hz), the increment in CM was greatest at turn I and decreased toward the apex. At 5 kHz, COCB stimulation produced no detectable increment in CM at turn I. The reduction in discharges of auditory nerve fibers produced by COCB stimulation was greatest (16–20 dB in titration) when best frequency lay between 7 and 10 kHz. As best frequency decreased, the reduction in discharge rate with COCB stimulation decreased. Nerve fibers with very high best frequencies also showed minimal COCB effects. These data are discussed in relation to other recent information.

Effects of Electrical Current Applied to Cochlear Partition on Discharges in Individual Auditory‐Nerve Fibers. I. Prolonged Direct‐Current Polarization

This study is designed to reveal the effects of electrical polarization applied to the cochlear partition on discharges in individual auditory‐nerve fibers in guinea pigs. Direct current is introduced in the basal turn across the organ of Corti. Its effects on the impulse discharges of the primary auditory fibers are measured during systematic variation of electrical and acoustic parameters. Data, recorded on tape, are analyzed by computing latency and interval histograms. With external current flowing from scala vestibuli to scala tympani, the spontaneous activity and sound‐evoked responses increase in most of the auditory‐nerve fibers. Current in the opposite direction causes decrease of impulse discharges. These nerve impulses show adaptation during application of the current and an aftereffect following its offset. It appears that electrical polarization modifies the resting current through the hair cells and also the excitability of the initial segment of the afferent auditory fibers.

Effects of Electrical Current Applied to Cochlear Partition on Discharges in Individual Auditory‐Nerve Fibers. II. Interaction of Electrical Polarization and Acoustic Stimulation

This report describes the temporal patterns of discharges of single nerve fibers recorded by a microelectrode in the modiolus in response to short‐term dc (5‐sec) and ac stimulation delivered across scala media in the basal turn of the guinea pigs cochlea. These electrical stimuli produce increases or decreases in discharge rates, depending on the polarity of the current, but do not show adaptation effects when the current strength is moderate. Currents with scala vestibuli positive with respect to scala tympani (SV‐ST) are usually excitatory, while currents with ST‐SV produce reductions in discharge rate. The effect of a given polarity is found to depend on a units prestimulation spontaneous rate. When electrical stimulation is added to acoustic stimulation, the intensity function at best frequency is shifted laterally, with the direction of the shift depending on current polarity. When the effects of two sources of stimulation, acoustic and electric, are added, the maximum discharge rate seen for acoustic inputs is not altered; it only occurs at a lower strength of the acoustic signal. The addition of electrical stimulation to acoustic frequencies other than best frequency may show different effects.

Interaural attenuation versus frequency for guinea pig and chinchilla CM response.

Interaural attenuation was estimated for the guinea pig and the chinchilla by determining isopotential curves for the cochlear microphonic (CM) response produced by contralateral and by ipsilateral stimulation at frequencies from 300 Hz to 14.3 kHz. Intracochlear electrodes were used to record the CM from the basal turn. For ipsilateral stimulation, bulla‐sealed to bulla‐open sound pressure ratios showed effects similar to those reported by other investigators: 10–12‐dB loss for chinchilla and 20–22‐dB loss for the guinea pig for the bulla‐sealed condition. The variability of the sound pressure measures, considered across animals, was larger in chinchillas than in guinea pigs, and the variance was greatest at frequencies near those at which the external auditory canals and bulla cavities show resonant peaks. There appears to be additional low‐frequency resonances associated with contralateral stimulation for the closed bulla in the chinchilla, largest at 1.1 kHz for frequencies sampled in this report. The...

Responses of Inferior Collicular Units in the Chinchilla to Binaural Stimuli

Extracellularly recorded discharges of units in the inferior colliculus of the Flaxedilized chinchila will be described. The two ears were stimulated independently so that interaural intensity and phase could be manipulated. Of those units encountered whose activity could controlled by acoustic stimuli below about 2 kHz, a large majority were found to be differentially responsive to alterations of interaural phase and intensity. The time patterns (histograms) in response to binaural stimulation showed characteristic alterations of excitation and inhibition similar to those which have been reported for neurons in the cat. The effect of interaural phase changes could be maximized by appropriate choice of interaural intensity differences.

Tentative Classification for Auditory‐Nerve Fibers

The distribution of spontaneous discharge rates for auditory‐nerve fibers recorded from the guinea pig modiolus suggests that there is a greater number of fibers in the 10–30 discharges/sec range than in the cat. Some tentative groupings on the basis of discharge rates has been possible from an inspection of the cumulative frequency distribution. Three groups are suggested, with means of 10, 24, and 71 discharges/sec. The standard deviations are 2.8, 2.3 and 2.7 discharges/sec, respectively. The spontaneous discharges are reduced by COCB stimulation. The “high‐rate” group shows less reduction by COCB than the two “low‐rate” groups. For nerve fibers with best frequencies above 1 kHz, the effect of the COCB on spontaneous discharges varies as Δu varies with frequency. Below 1 kHz, there is more COCB effect on spontaneous discharges than would be expected from Δu.

Is Sensitivity (Audibility) Related to Frequency of the Acoustic Stimulus Showing Maximum Efferent Inhibition

Certain general features of the data described above (see the two preceding papers) are discussed. In particular, the variation of efferent inhibition as a function of best frequency is similar in the guinea pig to that shown by Wiederhold and Swift [J. Acoust. Soc. Amer. 41, 1585 (1967)] in the cat. Since, in the cat, the frequency region of greatest inhibition was also the region of greatest auditory sensitivity (MAF), were were prompted to compare audibility curves for the two species [Miller and Murray, J. Comp. Physiol. Psychol. 61, 227 (1966) and Miller, Watson, and Covell, Acta Otolaryngol., Suppl. 176 (1963)], as well as their Δu functions. The variation in magnitude of Δu suggests that the audibility curve for the guinea pig may actually be more sensitive in the 5–10 kHz region than previously thought. Our data at present suggest the hypothesis that the audibility curve for the guinea pig is similar in form to that for the cat but less sensitive. The effects of COCB stimulation on the tuning curv...

Electrical Stimulation of Guinea Pig Cochlea: Effects on Primary Fibers

The effects of dc and ac stimulation, introduced in the basal turn across the organ of Corti [re Tasaki and Fernandez, J. Neurophysiol. (1952)] on the discharges of 144 units recorded with pipet electrodes in the modiolus [Tasaki, J. Neurophysiol. (1954)] were measured during systematic variation of electrical and acoustic parameters. Data, recorded on tape, were analyzed by computing latency and interval histograms. Tasaki and Fernandez [ibid.] reported that V/T polarization (scala vestibuli positive re scala tympani) produced an increase of cochlear microphonic and action potential, while T/V showed a decrease. Our results show that the electric current always altered the discharge rate of primary fibers when their best frequency was greater than 3000 Hz, but the direction of the alteration varied between fibers and depended on the parameters of both acoustic and electric inputs. The rate of spontaneous discharges was also modified but the direction of the change in discharge rate was not always the sam...

Auditory‐Nerve Responses as a Function of Repetition Rate and Background Noise

Auditory nerve responses to 2‐ and 6‐kHz tone pips were analyzed for changes in magnitude and latency. The signals were presented at repetition rates from 2/sec to 160/sec with and without background noise present. The bandwidths of the noises and signals were identical, although unlike signal and noise were also paired. The responses to the 2‐kHz pip presented at 2/sec in weak 2‐kHz background noise showed an increase in magnitude as compared to the no‐noise condition. The responses to the 6‐kHz pip showed a similar increase in magnitude for increases in repetition rate up to 10/sec. However, at 10/sec, 6‐kHz background noise was maximally effective (over‐all repetition rates) in reducing response magnitude. Associated with response enhancement was a decrease in peak latency. Stronger noise and faster repetition rates increased the latency and decreased response magnitude. [This research was partially supported by PHS grant.]