Biomedical subjects

E D Young

Publications and source records attributed to E D Young.

10 recordsLinked to original sources

PubMed1992

Neural organization and responses to complex stimuli in the dorsal cochlear nucleus.

The dorsal division of the cochlear nucleus (DCN) is the most complex of its subdivisions in terms of both anatomical organization and physiological response types. Hypotheses about the functional role of the DCN in hearing are as yet primitive, in part because the organizational complexity of the DCN has made development of a comprehensive and predictive model of its input-output processing difficult. The responses of DCN cells to complex stimuli, especially filtered noise, are interesting because they demonstrate properties that cannot be predicted, without further assumptions, from responses to narrow band stimuli, such as tones. In this paper, we discuss the functional organization of the DCN, i.e. the morphological organization of synaptic connections within the nucleus and the nature of synaptic interactions between its cells. We then discuss the responses of DCN principal cells to filtered noise stimuli that model the spectral sound localization cues produced by the pinna. These data imply that the DCN plays a role in interpreting sound localization cues; supporting evidence for such a role is discussed.

Acoustic Stimulation↗

PubMed1992

The parameter identification problem for the somatic shunt model.

The somatic shunt model, a generalized version of the Rall equivalent cylinder model, is used commonly to describe the passive electrotonic properties of neurons. Procedures for determining the parameters of the somatic shunt model that best describe a given neuron typically rely on the response of the cell to a small step of hyperpolarizing current injected by an intrasomatic recording electrode. In this study it is shown that the problem of estimating model parameters for the somatic shunt model using physiological data is ill-posed, in that very small errors in measured data can lead to large and unpredictable errors in parameter estimates. If the somatic shunt is assumed to be a real property of the intact neuron, the effects of these errors are not severe when predicting EPSP waveshapes resulting from synaptic input at a given location. However, if the somatic shunt is assumed to be a consequence of a leakage pathway around the recording electrode, and a correction for the shunt is applied, then the instability of the inverse problem can introduce large errors in estimates of EPSP waveshape as a function of synaptic location in the intact cell. Morphological constraints can be used to improve the accuracy of the inversion procedure in terms of both parameter estimates and predicted EPSP responses.

Mathematics↗

PubMed1992

Pinna-based spectral cues for sound localization in cat.

The directional dependence of the transfer function from free field plane waves to a point near the tympanic membrane (TM) was measured in anesthetized domestic cats. A probe tube microphone was placed approximately 3 mm from the TM from beneath the head in order to keep the pinna intact. Transfer functions were computed as the ratio of the spectrum of a click recorded near the TM to the spectrum of the click in freefield. We analyze the transfer functions in three frequency ranges: low frequencies (less than 5 kHz) where interaural level differences vary smoothly with azimuth; midfrequencies (5-18 kHz) where a prominent spectral notch is observed; and high frequencies (greater than 18 kHz) where the transfer functions vary greatly with source location. Because no two source directions produce the same transfer function, the spectrum of a broadband sound at the TM could serve as a sound localization cue for both elevation and azimuth. In particular, we show that source direction is uniquely determined, for source directions in front of the cat, from the frequencies of the midfrequency spectral notches in the two ears. The validity of the transfer functions as measures of the acoustic input to the auditory system is considered in terms of models of sound propagation in the ear canal.

Acoustic Stimulation↗

PubMed1992

Neural network models of sound localization based on directional filtering by the pinna.

Three-layer neural-network functions were developed to transform spectral representations of pinna-filtered stimuli at the input to a space-mapped representation of sound-source direction at the output. The inputs are modeled after transfer functions of the external ear of the cat; the output is modeled on the spatial sensitivity of superior colliculus neurons. Network solutions are obtained by backpropagation and by a method that enforces uniform task distribution in the hidden layer of the model. Solutions are characterized using bandlimited inputs to study the relative strength of potential sound localization cues in various frequency regions. This analysis suggests that the frequency region containing the first spectral notch (5-18 kHz) provides the best localization cues. Response properties of model neurons were studied using input patterns modeled after auditory nerve response profiles to pure tones at various frequencies and sound levels. The response properties of hidden layer model neurons resemble cochlear nucleus types III and IV and their composites. Neurons in both hidden and output layers show the properties of spectral notch detectors. Although neural networks have limitations as models of real neural systems, the results illustrate how they can provide insight into the computation of complex transformations in the nervous system.

Auditory Perception↗

PubMed1991

Organization of dorsal cochlear nucleus type IV unit response maps and their relationship to activation by bandlimited noise.

1. Response maps of 49 type IV neurons in cat dorsal cochlear nucleus (DCN) were studied by moving a tone in small steps along the frequency dimension and along the intensity dimension. Type IV responses are recorded from DCN principal cells. Data were collected from 38 units with best frequencies (BFs) from 2.16 to 50.3 kHz with the use of electrode penetrations along the long (strial) axis of the DCN; an additional 11 units from a previous study were analyzed. A stereotypical type IV response map is defined as consisting of two excitatory and two inhibitory regions. Type IV units from both the pyramidal cell layer (probably pyramidal cells) and the deep layer (probably giant cells) show the same types of response maps. 2. Two of the regions, one excitatory and one inhibitory, are seen in all type IV units. These regions are a low-threshold excitatory region at best frequency (BFER) and an inhibitory area at higher levels, usually centered below BF but extending upward in frequency to include BF (central inhibitory area, or CIA). The high resolution of the response maps in this paper allows us to show that type IV units fall into two groups on the basis of whether their CIAs are narrow with well-defined borders (35 units) or broad with poorly defined borders (14 units). 3. Two additional features of type IV response maps can be defined, most consistently in units with well-defined CIAs. These features are an excitatory region along the high-frequency edge of the CIA (upper excitatory region, UER) and an upper inhibitory sideband (UIS). The BFER and UER are continuous in many units, but in some cases their continuity is broken by the CIA. It seems likely that the BFER and UER represent a single excitatory input to type IV units and are revealed because the tuning curve of the stronger inhibitory inputs that produce the CIA has thresholds greater than and BFs lower than the excitatory inputs. 4. The CIA is probably produced by inhibitory inputs from DCN type II neurons. The bandwidths of type IV CIAs are about 1-3 times larger (at 40 dB above threshold) than the excitatory bandwidths of DCN type II units, suggesting a convergence of the equivalent in tuning of about two type II units onto each type IV unit. The BF of the CIA is below the excitatory BF of the type IV unit in most cases.(ABSTRACT TRUNCATED AT 400 WORDS)

Acoustic Stimulation↗

PubMed1990

Cross-correlation analysis of inhibitory interactions in dorsal cochlear nucleus.

1. Cross-correlation analysis was used to study the organization of inhibitory connections between type II or type III units and type IV principal cells in cat dorsal cochlear nucleus (DCN). Pairs of units were isolated using two microelectrodes so that information about the distance over which connections are made could be analyzed. Data were obtained from 51 pairs consisting of a type II and a type IV unit and from 22 pairs consisting of a type III and a type IV unit. The analyses in this paper concentrate on type II-type IV pairs. 2. Inhibitory troughs (ITs) are observed in the cross-correlograms of type II-type IV pairs (21/51 cases). An IT is a transient decrease in discharge probability in the postsynaptic (type IV) unit immediately after spikes in the presynaptic unit (type II). The average latency to the start of ITs is 0.73 ms, and the troughs are asymmetric with a faster leading phase. Small excitatory peaks accompany the ITs in type II units, but these are probably secondary effects associated with the IT. ITs are consistent with a monosynaptic, inhibitory connection between type II and type IV units. A variety of evidence suggests that type II responses are recorded from vertical cells, an interneuron in the deep layer of the DCN that may be glycinergic. 3. The cross-correlograms of type III-type IV pairs are more complex and variable than those of type II-type IV pairs--ITs are seen in 4/22 cases, and peaks of correlation that are symmetrically located around the origin (central mound or CM) are seen in 4/22 cases; two cases have both an IT and a CM. CMs result from shared sources of input. Whereas type II-type IV correlogram features change primarily in amplitude as stimulus conditions change, correlogram features in some type III-type IV pairs change qualitatively with stimulus conditions; correlograms are flat for some stimuli and show ITs or CMs or mixtures of the two for others. This variability suggests that the circuitry associated with type III-type IV pairs is more complex than a monosynaptic connection, and further analysis of type III-type IV pairs was not done. 4. The strength of inhibition for an IT is measured as the area under the IT (effectiveness) and as effectiveness divided by the postsynaptic discharge rate (association index).(ABSTRACT TRUNCATED AT 400 WORDS)

Animals↗

PubMed1979

Encoding of steady-state vowels in the auditory nerve: representation in terms of discharge rate.

Responses of large populations of auditory-nerve fibers to synthesized steady-state vowels were recorded in anesthetized cats. Driven discharge rate to vowels, normalized by dividing by saturation rate (estimated from the driven rate to CF tones 50 dB above threshold), was plotted versus fiber CF for a number of vowel levels. For the vowels /I/ and /e/, such rate profiles showed a peak in the region of the first formant and another in the region of the second and third formants, for sound levels below about 70 dB SPL. For /a/ at levels below about 40 dB SPL there are peaks in the region of the first and second formants. At higher levels these peaks disappear for all the vowels because of a combination of rate saturation and two-tone suppression. This must be qualified by saying that rate profiles plotted separately for units with spontaneous rates less than one spike per second may retain peaks at higher levels. Rate versus level functions for units with CFs above the first formant can saturate at rates less than the saturation rate to CF to-es or they can be nonmonotonic; these effects are most likely produced by the same mechanism as that involved in two-tone suppression.

Acoustic Stimulation↗

PubMed1979

Representation of steady-state vowels in the temporal aspects of the discharge patterns of populations of auditory-nerve fibers.

This paper is concerned with the representation of the spectra of synthesized steady-state vowels in the temporal aspects of the discharges of auditory-nerve fibers. The results are based on a study of the responses of large numbers of single auditory-nerve fibers in anesthetized cats. By presenting the same set of stimuli to all the fibers encountered in each cat, we can directly estimate the population response to those stimuli. Period histograms of the responses of each unit to the vowels were constructed. The temporal response of a fiber to each harmonic component of the stimulus is taken to be the amplitude of the corresponding component in the Fourier transform of the unit's period histogram. At low sound levels, the temporal response to each stimulus component is maximal among units with CFs near the frequency of the component (i.e., near its place). Responses to formant components are larger than responses to other stimulus components. As sound level is increased, the responses to the formants, particularly the first formant, increase near their places and spread to adjacent regions, particularly toward higher CFs. Responses to nonformant components, exept for harmonics and intermodulation products of the formants (2F1,2F2,F1 + F2, etc), are suppressed; at the highest sound levels used (approximately 80 dB SPL), temporal responses occur almost exclusively at the first two or three formants and their harmonics and intermodulation products. We describe a simple calculation which combines rate, place, and temporal information to provide a good representation of the vowels' spectra, including a clear indication of at least the first two formant frequencies. This representation is stable with changes in sound level at least up to 80 dB SPL; its stability is in sharp contrast to the behavior of the representation of the vowels' spectra in terms of discharge rate which degenerates at stimulus levels within the conversational range.

Acoustic Stimulation↗

PubMed1976

Responses to tones and noise of single cells in dorsal cochlear nucleus of unanesthetized cats.

1. Single-unit responses in the dorsal cochlear nucleus of unanesthetized, decerebrate cats have been divided into two categoreis. These have been differentiated on the basis of responses to best-frequency tones. Type IV units responded to best-frequency tones with excitation from threshold to about 20 or 30 dB above threshold; at higher levels, their response was inhibitory. In a few cases, the excitatory area near threshold was not seen and in a few others, the response became excitatory again at high levels. Type IV units could be divided into two groups based on the length of time that inhibition was maintained in response to long tones. Type IV units are not seen in anesthetized cats. 2. Type II/III units responded to best-frequency tones of all levels with excitation. Nonmonotonic rate versus level functions were seen in type II/III units, but they were of much less drastic character; the discharge rate of nonmonotonic type II/III units was still well above spontaneous rate for tones 50 dB above threshold. Type II/III units defined in this way were found to have, on the average, lower rates of spontaneous activity and higher thresholds than type IV units. 3. Type II/III units responded weakly to broad-band noise in comparison to auditory nerve fibers and many of them did not respond at all to noise. Type IV units, with best frequencies above 0.9 kHz, gave excitatory responses to noise. 4. The inhibitory response areas of type IV units could be divided into two areas: a central inhibitory area in the vicinity of best frequency where on- and off-discharges and afterdischarges were seen; and inhibitory side bands at higher and lower frequencies where simple inhibitory responses were seen. In four units, it was possible to show that the central inhibitory area was converted to an excitatory area after administration of an anesthetic dose of pentobarbital. 5. Most type II/III and type IV units could be excited or inhibited by stimuli in the contralateral ear. Broad-band noise was a more effective contralateral stimulus than tones at the ipsilateral best frequency. 6. On the basis of the properties of type II/III and type IV cells, it is suggested that type II/III responses are recorded from interneurons which provide a large share of the inhibitory imput to type IV cells.

Acoustic Stimulation↗

PubMedDate not supplied

Responses of squirrel monkey vestibular neurons to audio-frequency sound and head vibration.

A study was made of the response of peripheral vestibular neurons in the squirrel monkey to head vibration and air-borne sound in the frequency range from 50-4 00 Hz. Responses were measured in terms of the phase locking of discharge and changes in firing rate. The lowest phase-locking thresholds for vibration were -70 to -80 dB re 1 g, and median values in the most sensitive frequency range (200-400 Hz) were -20 to -40 dB re 1 g; the minimum and median thresholds for sound were 76 and 120-130 dB SPL, respectively. Rate-change thresholds were 10-30 dB above phase-locking thresholds. The squirrel monkey sacculus has no special sensitivity to vibration in comparison with the other vestibular end-organs; the median phase-locking threshold to sound of saccular neurons exceeded 100 dB SPL. Irregularly discharging neurons are more sensitive than regularly discharging units. Evidence is presented that the response to intense sound involves a hair-cell mechanism.

Acoustic Stimulation↗