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W E Sullivan

Publications and source records attributed to W E Sullivan.

10 recordsLinked to original sources
PubMed1999

A morphological study of the cochlear nuclei of the pigeon (Columba livia).

The pigeon cochlear nucleus angularis (NA) and nucleus magnocellularis (NM) were analyzed with Golgi and Nissl techniques. NA was divided into a medial NAm and NA proper, which could be subdivided further into an intermediate NAi and lateral NAl. NAm contained a mostly homogeneous population of a unique multipolar cell type with very short dendrites and large somatic spines. NA proper contained four cell types: large, medium, and small multipolars, and medium bipolar. The medium multipolar cells were most common, and resembled the multipolar cells of the mammalian ventral cochlear nucleus. NM presented a homogenous appearance with a mediolateral gradient of cell size and shape. Medially located higher best frequency NM principal cells had round cell bodies with small somatic spines and few dendrites. By comparison, laterally located low best frequency NM stellate cells had more dendrites and spindle shaped cell bodies. The similarities between the cell types of NA and the cell types in the cochlear nuclei of other amniote vertebrates may be due to homology or convergent evolution.

Animals↗
PubMed1991

The military as a partner.

The Pentagon has authorized the military to contract with private medical groups to supply board certified and board eligible physicians and support personnel to military hospitals to administer specialty medical services. The arrangement, known as a partnership agreement, provides volume to the civilian medical groups and reduces military medical outlays for expensive services at civilian hospitals. This professional paper addresses the initiation, procurement, and execution of a partnership agreement.

Cost Control↗
PubMed1986

Neural map of interaural phase difference in the owl's brainstem.

Neurons of the barn owl's (Tyto alba) nucleus laminaris, the first site of binaural convergence, respond in a phase-locked fashion to a tone delivered to either ear. It may take longer to elicit phase-locked spikes from one ear than from the other. This disparity in delay differs from neuron to neuron and is independent of tonal frequency. In binaural stimulation, neurons respond best when sound in one ear leads that in the other by an amount equal to their delay disparities but opposite in sign. This condition causes simultaneous arrival of phase-locked spikes from the two sides. Laminaris neurons can thus be described as coincidence detectors. The phase of a tone-induced evoked potential, termed "neurophonic," varies systematically with position in nucleus laminaris. From dorsal to ventral within the nucleus, the phase delay of a contralaterally elicited potential decreases and that of its ipsilateral counterpart increases. Therefore, if the neurophonic delay is due to the delay of phase-locked spikes, an orderly representation of delay disparities is shown. Because they act as coincidence detectors, laminaris neurons should show selectivity for interaural phase difference based on their place in the nucleus. Thus, nucleus laminaris presumably measures and maps interaural phase differences by using the principles of delay lines and coincidence detection.

Action Potentials↗
PubMed1986

Processing of acoustic temporal patterns in barn owls and echolocating bats: similar mechanisms for the generation of neural place representations of auditory space.

Evidence for 'coincidence detection' or temporal summation mechanisms of temporal-pattern processing is presented for interaural-phase comparison in the barn owl's brainstem and echo delay sensitivity in the auditory cortex of the echolocating bat Myotis lucifugus. In each system, temporal tuning is correlated with the synchronous arrival of excitatory inputs from different acoustic events. Input coincidence is achieved by differences in neural transmission delay that counterbalance acoustic time disparities. Neural transmission delays are spatially organized, thus producing a topographic representation of acoustic time disparity.

Animals↗
PubMed1985

The owl's cochlear nuclei process different sound localization cues.

This paper discusses how the barn owl's brain stem auditory pathway is divided into two physiologically and anatomically segregated channels for separate processing of interaural phase and intensity cues for sound localization. The paper also points out the power of the "downstream" approach by which the emergence of a higher-order neuron's stimulus selectivity can be traced through lower-order stations.

Animals↗
PubMed1985

Classification of response patterns in cochlear nucleus of barn owl: correlation with functional response properties.

Response patterns of neurons in the cochlear nuclei of the barn owl (Tyto alba) were studied by obtaining poststimulus time histograms (PSTHs) and interspike interval histograms for the response to short tone bursts at the neuron's characteristic frequency. The observed response patterns can be classified according to the scheme developed for neurons of the mammalian cochlear nuclear complex (22). Neurons of the magnocellular cochlear nucleus (n. magnocellularis), which respond in a phase-locked manner to sinusoidal signals and do not show large increases in spike discharge rate with changes in stimulus intensity (26), have "primarylike" (PSTH) discharge patterns and broad interspike interval histograms. This indicates that magnocellular neurons have irregular firing patterns, with the timing of individual spikes being dependent on the phase of the stimulus waveform. Neurons of the angular cochlear nucleus (n. angularis), which show little or no phase-locking and large increases in spike rate with increasing intensity (26), had almost exclusively "transient chopper" discharge patterns. The interspike interval histograms of these angular units are sharp, indicating that their discharge is very regular. At the onset of the response where the chopper pattern is observed, both discharge regularity and rate-intensity sensitivity are at their maximum levels. Several "onset" units were isolated in the angular cochlear nucleus, but no "pauser" or "buildup" units were seen. Also, all of the units in the angular nucleus had monotonic rate-intensity functions. Thus no neural response patterns typical of mammalian dorsal cochlear nucleus units were observed. The relationship of response pattern type to neural function is discussed in relation to the acoustic cues used by the owl for two-dimensional sound localization. The primarylike, phase-locked discharge of magnocellular units is undoubtedly involved in the analysis of interaural differences in stimulus phase, which the owl uses for horizontal localization. There is strong evidence suggesting that the angular nucleus is involved in processing stimulus intensity information, which is important for determining sound elevation (due to asymmetries in vertical directionality of the owl's external ears). The predominant chopper patterns seen in the angular nucleus suggest that in the owl, this response type is correlated with stimulus intensity processing. Similarities in both anatomy and physiology suggest that the magnocellular nucleus is analogous to the spherical cell or bushy cell population of the anterior division of the mammalian anteroventral cochlear nucleus.(ABSTRACT TRUNCATED AT 400 WORDS)

Animals↗
PubMed1984

Segregation of stimulus phase and intensity coding in the cochlear nucleus of the barn owl.

The cochlear nucleus of the barn owl is composed of two anatomically distinct subnuclei, n. magnocellularis (the magnocellular nucleus) and n. angularis (the angular nucleus). In the magnocellular nucleus, neurons tend to respond at a particular phase of a stimulus sine wave. Phase locking was observed for frequencies up to 9.0 kHz. The intensity-spike count functions of magnocellular units are characterized by high rates of spontaneous activity, a narrow range of intensities over which spike counts changed from spontaneous to saturation levels, and a small increase in spike counts with intensity over that range. In the angular nucleus, neurons showed little or no tendency to respond at a certain sinusoidal phase, although some showed weak phase locking for frequencies below 3.5 kHz. Angular units typically had low spontaneous rates, large dynamic ranges, and large increases in spike counts with intensity, resulting in high saturation levels. The clear difference between the two nuclei in sensitivity to both phase and intensity and the reciprocity in response properties support the hypothesis that each nucleus is specialized to process one parameter (phase or intensity) and not the other.

Animals↗
PubMed1982

Neural representation of target distance in auditory cortex of the echolocating bat Myotis lucifugus.

1. Single- and multiunit recordings were obtained from neurons in the auditory cortex of the echolocating bat Myotis lucifugus, while trains of stimuli designed to simulate the bat's frequency-modulated (FM) orientation pulse and its returning echo were delivered. It was found that many neurons in the cortex responded selectively to pulse-echo pairs only if the time delay separating the artificial pulse and the echo was within a certain range. This response property is called "delay-dependent facilitation." Since echolating bats are known to utilize echo-delay information for the determination of target distance, it is postulated that these neurons are involved in the process of distance perception. 2. Two types of delay-sensitive neurons were characterized on the basis of their response patterns. P-type units had short maximum response delays, narrow delay response functions, and response latencies for pulse-echo pairs that were similar to their response latencies for single loud FM pulses. E-type units had longer maximum response delays, wide delay response functions, and pulse-echo pair response latencies that were time-locked to the echo. Another important difference between these two classes was that changes in the amplitude of the artificial echo caused systematic changes in the delay response of E-type units but not of P-type units. 3. The sharpness and stability of the delay response functions of P-type units suggested that they may encode target distance by responding at discrete echo delays. In contrast, delay tuning may not be an unambiguous determinant of echo delay in E-type units. Here, the most consistent and reliable response parameter for echo delay is the time at which the responses occurred. This suggested that echo delay could be encoded by the temporal pattern of responses in E-type units in relation to the responses evoked by the outgoing orientation cry. The different range of delay of delay sensitivity of P-type and E-type units indicates that these two mechanisms could be operating at different ranges of target distance. 4. P-type and E-type responses may not be due to different populations of neurons but to different response properties of the same population under different conditions. Evidence for this proposition was obtained by showing that in some recordings, decreases in the amplitude of the artificial pulse caused a switch in response from a long best delay, E-type response to a short best delay, P-type response. This suggested that the delay sensitivity of cortical neurons could be under the bat's control based on the intensity of its pulse emissions.

Animals↗
PubMed1982

Possible neural mechanisms of target distance coding in auditory system of the echolocating bat Myotis lucifugus.

1. In order to investigate the possible neural mechanisms underlying delay-dependent facilitation in the bat's auditory cortex (18), the responses to single FM pulses of varying amplitude were examined. Analysis of amplitude-spike count functions revealed three distinct types: monotonic, simple nonmonotonic, and complex nonmonotonic. The complex nonmonotonic function had two separate amplitude peaks, with a clear notch or worst amplitude between them. Other units had spike count functions that were mainly monotonic or nonmonotonic, but showed some evidence for a second response region. 2. Examination of response latency revealed another novel response property, which has been termed the paradoxical latency shift. Units with this response property responded at a shorter latency to sounds of low amplitude than to sounds of high amplitude. The paradoxical latency shift also appears to be related to the twin-peaked complex nonmonotonic response function. Units with the most prominent twin-peaked response functions also had the clearest latency shifts. In these units, the high-amplitude peak corresponded to the long-latency response and the low-amplitude peak to the short-latency-response. 3. These curious spike count and latency observations can be explained if they are considered in relation to the temporal and amplitude pattern of the acoustic input during echolocation. In echolocation, a loud orientation pulse is followed by a weaker echo. In delay-dependent facilitation, this pulse-echo sequence is followed by a neural response if the pulse-echo delay is appropriate. The simplest model for delay-dependent facilitation assumes that a synchronization of excitatory inputs from the pulse and echo is needed for facilitation. Since the weaker echo occurs after the pulse, it is closer in time to the postulated synchronization point. Therefore, in order for this model to work, the echo input must reach the summation place with less of a time lag than the pulse input. This is exactly what is seen with the paradoxical latency shift; the loud "pulse" response is delayed relative to the weak "echo" response.

Animals↗