Phase-locked response to low-frequency tones in single auditory nerve fibers of the squirrel monkey

The human frequency-following response: its behavior during continuous tone and tone burst stimulation

The frequency following response (FFR) in humans has been elicited by continuous tones as well as tone bursts. The responses were observable over a range of frequencies extending from 70 c/sec to greater than 1.5 kc/sec. The threshold for the continuous response has a mean of about 40 dB SL and is perhaps several dB lower for the burst response. The response threshold as a function of sound pressure level (SPL) increases rapidly at frequencies below 125 c/sec and above 1 kc/sec. These thresholds are particularly sharp at the lower frequencies. Narrow-band noise that just produces subjective tone masking also masks the FFR. The latency of the FFR is about 6 msec when the tone burst intensity is 30 dB over FFR threshold. Comparison with click response latencies indicates that the onset of the FFR corresponds with early waves IV and V.

Neurons in the cochlear nucleus investigated with tone and noise stimuli

1. Responses of cochlear nucleus neurons to stationary and amplitude modulated noise stimulation are investigated and compared with responses to tonal stimuli. 2. Cross-correlation functions, computed from responses to stationary noise stimulation, showing a clear oscillation could be most easily obtained from low CF fibres presumed to be auditory nerve fibres and low CF cochlear nucleus neurons showing only activation response and a primarylike temporal pattern of response to tone bursts. This reflects good quality of phase locking in these neurons. 3. The CCF reflects strongly the frequency selectivity of the neuron as revealed in its response area but not the temporal pattern of response to tone bursts. 4. Responses to noise bursts are correlated with the responses to tone bursts of many different frequencies in both their sign (i.e. activation or suppression) and their temporal pattern. 5. The concept of two independently operating mechanisms, one depending on the fine time structure of the stimulus (the carrier) and the other on its amplitude, and determining respectively the fine time structure of the response pattern and its magnitude, is introduced. Experimental data are presented which lend support to the adequacy of the description in the majority of cases and reveal its shortcomings in others.

Statistical analysis and interpretation of the initial response of cochlear nucleus neurons to tone bursts

1. Subject of investigation is the initial response of cochlear nucleus neurons and units presumed to be auditory nerve fibres to CF tone burst stimulation. 2. The initial response is characterized by computing the distribution of the latency of the first spike and of the duration of the first interval in the ensemble of responses to a large number of stimuli. 3. In many of the neurons the properties of both distributions appear to be related. The presumed auditory nerve fibres and spontaneously active cochlear nucleus neurons showing only activation responses to tonal stimuli (A type) exhibit irregularity in both response onset and intervals. Minimum latency and minimum first intervals are short. On the other hand, spontaneously active neurons with both activation and suppression in the response area (AS type) and silent neurons showing only activation (A(S) type) often show a more precisely timed onset of response and narrow interval distributions. In many neurons this leads to oscillations in the PSTH (chopping). In these neurons minimum latency and minimum first interval have higher values. The longer minimum latency cannot be attribute-d to longer pure time delays in these neurons. 4. The results are interpreted as speaking in favour of temporal integration as an important mechanism in many of the AS and A(S) neurons, particularly those in the DCN. The firing patterns of A neurons are thought to indicate virtual absence of this mechanism. 5. Using pure time delay estimates derived from cross-correlation functions, computed from the responses to stationary noise, an attempt is made to estimate the integration time in the cochlear and in the cochlear nucleus neurons.

Organization of the cat trapezoid body and the discharge characteristics of its fibers

The laminae of the midline cat trapezoid body (TB) are delimited and named on the basis of their characteristic fiber diameters. Proceeding ventrally from the dorsal boundary of the TB are found the medium, mixed, large, and small diameter populations (the MDP, XDP, LDP, and SDP respectively). Specific cell types of the cochlear nuclear complex give rise to the fibers of the TB. Medium diameter fibers originate from spherical cells while globular cells are the source of the large diameter fibers. Bilateral cochlear nuclei ablations demonstrate that, withe the possible exception of a few thin fibers, all midline TB fibers are second order. The discharge characteristics of single units recorded near the midline in the MDP, XDP, and LDP are presented. The MDP displays a dorso-ventral progression of best frequencies, from low to high. Very high best frequencies (greater than 12 kHz) do not seem to be represented in the MDP but are found in the XDP and LDP. Except for latent periods, the properties of units in the MDP are the same as those of spherical cells and eight nerve fibers for all parameters where comparisons are available. The excitatory response characteristics of units in the XDP and LDP are similar to those of the MDP. They differ from MDP fibers in having tuning curves with inhibitory sidebands as well as a lower mean spontaneous rate of discharge. The results confirm previous observations on the spherical cells and define the discharge characteristics of the globular cells.

Human frequency-following responses to monaural and binaural stimuli

Frequency-following responses, with latencies circa 6 msec, were recorded from five normal-hearing human subjects to brief 500 c/sec tone bursts presented monaurally. The frequency-following responses appear as peaks occurring at 2 msec intervals superimposed on a slow wave (pedestal-like) component. Comparisons were made between the frequency-following responses evoked by binaural and monaural stimuli. The results show that the binaural responses may be interpreted as the sum of two monaural responses. It is concluded, therefore, that there are two independent populations of neurons, each capable of generating a frequency-following response is not a microphonic-like response but rather that the individual waves in the frequency-following response are evoked by the collective activity of phase-locked single units. Finally, on the basis of the distinctness of the individual waves in the frequency-following response, it is concluded that the neural generators of the response must be spatially compact.

Neural encoding of sound sensation evoked by electrical stimulation of the acoustic nerve

A series of psychoacoustic experiments was conducted in subjects implanted with a permanent intracochlear bipolar electrode. These experiments were designed to reveal the nature of the sensation evoked by direct sinusoidal electrical stimulation of the acoustic nerve. A series of single unit experiments in the inferior colliculus of cats was then conducted, using intracochlear stimulus electrodes identical to those implanted in human subjects in all respects except size, and using identical stimuli. These physiological experiments were designed to reveal how sounds evoked by intracochlear electrical stimulation in humans are generated and encoded in the auditory nervous system. Among the results were the following: 1) The sensation arises from direct electrical stimulation of the acoustic nerve. It is not “electrophonic” hearing arising from electro-mechanical excitation of hair cells. 2) While sounds are heard with electrical stimulation at frequencies from below 25 to above 10,000 Hz, the useful range of discriminative hearing is limited to frequencies below 400–600 Hz. 3) There is no “place” coding of electrical stimuli of different frequency. Tonal sensations generated by electrical stimulation must be encoded by the time order of discharge of auditory neurons. 4) The periods of sinusoidal electrical stimuli are encoded in discharges of inferior colliculus neurons at frequencies up to 400–600 Hz. 5) Both psychoacoustic and physiological evidence indicates that the low tone sensations evoked by electrical stimulation are akin to the sensations of “periodicity pitch” generated in the normal cochlea. 6) Most cochlear hair cells are lost with intracochlear implantation with this electrode. Most ganglion cells survive implantation. Implications of these experiments for further development of an acoustic prosthesis are discussed.

Coding of sounds in lower levels of the auditory system

The great number of investigations and advanced developments in neurophysiology and psychoacoustics during recent years have extensively increased our knowledge about the frequency analysis of simple sounds in the peripheral auditory system.

New methods have facilitated quantitative measurements of the amplitude of the submicroscopic vibration of a narrow segment of the basilar membrane in anaesthetized animals at physiological sound intensities. The results of these studies have quantitatively confirmed the results of past studies by showing that the basilar membrane has a selectivity with regard to tone frequency. In addition to this, the recent studies have increased our knowledge about the finer details of vibration of the basilar membrane. At the lowest levels used in the recent investigations, i.e. about 70 dB SPL, the selectivity in the 7 kHz region of the basilar membrane was found to be greater than expected on the basis of extrapolation of older data. Moreover, the high frequency slope of the tuning curves of the basilar membrane was found to be particularly steep. The results of these recent studies, furthermore, showed that the basilar membrane vibrates in a non-linear way at intensities within the physiological range. This non-linearity results in a broadening of the selectivity curves of a narrow segment of the basilar membrane when the sound intensity is increased.

Little is known as to how the motion of the basilar membrane is transformed to excitation of the cochlear sensory cells, i.e. the haircells. The excitation may be related to displacement, spatial differentiation or other transformations of the basilar membrane motion. Recording from the interior of mammalian haircells has so far been unsuccessful, and the neural excitatory process within the haircells in the cochlea is as yet practically unknown. Studies of the haircells in the lateral line organ of fish have provided fundamental knowledge about their excitation; since they in many respects resemble those in the mammalian cochlea, the results very probably can be applied to the excitatory process in the mammalian cochlea.

Responses of inferior collicular neurones of bats to tone bursts with different rise times

1. A study was made of the responses of single neurones in the inferior colliculus of bats to tone bursts with different rise times (or with different rates of amplitude increase) in order to determine whether the neurones were specialized for analysis of amplitude-modulated sound, especially the rising phase in amplitude.2. The response patterns of neurones which showed phasic on-responses usually did not change with rise time, although the response patterns of some neurones changed from phasic on-responses to inhibitory responses.3. The thresholds of responses to tone bursts increased when the rise time was lengthened. The amount of increase greatly differed from neurone to neurone. For the excitation of neurones which showed a large increase in threshold, the stimulus amplitude should quickly increase in amplitude. For tone bursts with a short rise time, some neurones showed an upper-threshold above which the sounds failed to excite them. The upper-threshold usually disappeared when the rise time was lengthened. For the excitation of neurones which showed an upper-threshold, the rate of amplitude increase and its extent were very important parameters.4. Lengthening the rise time of a tone burst from 0.5 to 98 msec caused various types of change in the excitatory area. In some cases, there was a diminution of the area and, in others, there was an expansion of the area. Neurones showing these changes may be considered to be specialized for responding to tone bursts with either a rapid or slow increase in amplitude.5. Changes with rise time occurred not only in the excitatory area, but also in the inhibitory area. The change in the inhibitory area of a given neurone, however, was not necessarily the same as that in the excitatory area.6. The latency of response usually changed as a function of stimulus amplitude and rise time. In 26% of the neurones studied, however, the latency was relatively constant regardless of stimulus amplitude and rise time. Some of the latency-constant neurones appeared to have properties suited for echo-ranging.7. The changes in excitatory area with rise time appear to involve neural inhibition and/or accommodation. Possible neural mechanisms for the changes in threshold, upper-threshold and response pattern and the properties of latency-constant neurones are discussed.

A neuron model with spatially distributed synaptic input

I have assembled a neuron model simulating contiguous patches of nerve cell membrane. With this model I have examined the functional significance of different spatial and temporal distributions of synaptic inputs. The model consists of two terminal electronic analogue circuits with inputs controlled by a LINC computer. One terminal represents the inside of a membrane patch, the other represents the outside. Two circuit designs are used: one simulates spike-generating regions of the neuron, the other simulates subthreshold activity in inexcitable regions. To simulate a neuron, patches are assembled in various spatial arrangements by suitable connection to the "intracellular" nodes. Thus the relation of neuron geometry to aspects of spatiotemporal summation of synaptic inputs can be investigated readily. Performance of the model is assessed by comparison with results from microelectrode studies in the cochlear nucleus of the cat. In particular, the peristimulus time (PST) histogram and averaged membrane potential are used for quantitative comparison. The model suggests that the geometry of the neuron's receptive surface can account for a wide variety of physiologically observed behavior, particularly in response to dynamic stimuli.

Echo-location and evoked potentials of bats after ablation of inferior colliculus

1. Echo-location and evoked potentials of blinded Yuma bats (Myotis yumanensis) were studied before and after ablation of the inferior colliculus (I.C.). A task of obstacle-avoidance was given to the bats: hits and misses of strands in the flight path were counted. Orientation sounds emitted by the bats during flight were recorded.2. Bilateral ablation of the dorso-medial region of I.C. including the internuclear cortex and commissure had no effect on obstacle-avoidance performance. The bats avoided even strands of 0.2 mm diameter with orientation sounds.3. Bilateral ablation of the dorsal half of I.C. including the external nucleus (lateral cortex) also had no effect on echo-location.4. Bilateral ablation of the ventral half of I.C. caused severe deficiency in ability to avoid obstacles. The main nucleus appeared to be very important for echo-location. When bilateral ablation including the main nucleus was moderate, the bats failed to avoid strands of less than 0.5 mm diameter in spite of detecting them, but avoided large obstacles such as 3.7 mm strands. With severe bilateral ablation including the main nucleus, the bats did not avoid even the 3.7 mm strands in spite of frequent emission of orientation sounds, but often avoided crashing into the wall.5. Severe unilateral ablation of I.C. including the main nucleus and a part of the lateral lemniscus had no effect on ability to avoid obstacles. Since sound localization by such bats are not explained by Van Bergeijk's model based on Békésy's, a modification of Van Bergeijk's model has to be considered.6. Of the positive evoked potentials recorded with an active electrode placed at the dorsal surface of I.C., the slow component with a 7-9 msec peak latency reflected activity of inferior collicular neurones, while the fast component (N(4)) with a 3 msec peak latency represented activity of ascending lateral lemniscal fibres.

A model of the peripheral auditory system responding to low-frequency tones

A model of the peripheral auditory system responding to low-frequency tone stimulation is given. The model is of the type previously introduced by Weiss (1966). It includes three interconnected parts: a linear model of the ear's mechanical system, a model of the cochlear transducer, and a stochastic model of an auditory nerve fiber. The output of the model accurately mimics many characteristics of the output of some auditory nerve neurons responding to sinusoidal stimuli but is unable to successfully match all reported aspects of data obtained from other of these neurons. Characteristics of the model neurons are discussed.