Lindsay Aitkin

Neuronal mechanisms of hearing

Session I. Cochlear mechanisms A.- The Responses of Hair Cells to Low Frequency Tones and Their Relationship to the Extracellular Receptor Potentials and Sound Pressure Level in the Guinea Pig Cochlea.- Some Comparative Aspects of a Cochlear Mechanism.- The Parasympathetic Innervation of the Inner Ear and the Problem of Cochlear Efferents: Enzyme and Autoradiographic Studies.- Adaptation and Dynamic Response Occurring at Hair Cell - Afferent Fiber Synapse.- Phase Versus Frequency Plots from Caiman Primary Auditory Fibres: Is There a Travelling Wave?.- Session II. Cochlear Mechanisms B.- The Dynamics of pO2-Changes in the Perilymphatic Perfusate of the Guinea Pig Cochlea Depending on the Metabolism.- Analytical Studies on Biochemistry and Physiology of Perilymph (Guinea Pig).- Distribution of Microphonic Potentials in the Four Turns of the Guinea Pig Cochlea.- Session III. Coding in the Auditory Nerve and Cochlear Nucleus.- The Dynamic Range Problem: Place and Time Coding at the Level of Cochlear Nerve and Nucleus.- Coding of Complex Sounds in the Auditory Nervous System.- On Predicting the Response of Auditory Nerve Fibers to Complex Tones.- Effects of Masking Noise on the Representation of Vowel Spectra in the Auditory Nerve.- Neuronal Circuits in the Dorsal Cochlear Nucleus.- The Internal Organization of the Dorsal Cochlear Nucleus.- Session IV. Central Auditory Mechanisms A.- Functional Organization of the Inferior Colliculus.- Integration and Segregation of Input to the Cat Inferior Colliculus.- Some Facets of the Organization of the Principal Division of the Cat Medial Geniculate Body.- Functional Organization of the Medial Geniculate Body Studied by Simultaneous Recordings of Single Unit Pairs.- Possibilities of Recording Multiunit Activity in the Auditory Pathway.- Poststimulatory Effects in the Medial Geniculate Body of Guinea Pigs.- How Biosonar Information Is Represented in the Bat Cerebral Cortex.- Three-Dimensional Study of Evoked Field Potentials in the Auditory Cortex of the Cat.- Differential Diagnosis of Hearing Disorders-Clinical Findings Contributing to Information Processing in the Auditory Pathway.- Session V. Central Auditory Mechanisms B.- Binaural Interaction in the Cat Inferior Colliculus: Comparison of the Physiological Data with a Computer Simulated Model.- Coding Properties of the Different Nuclei of the Cats Medial Geniculate Body.- Interaural Delay Sensitive Units in the MGB of the Cat.- Temporal Information in the Medial Geniculate Body.- Some Investigation of Acoustical Evoked Potentials from Peripheral and Central Structures of the Auditory Pathway in Rabbits.- Session VI. Auditory Localization.- Anatomical-Behavioral Analyses of Hindbrain Sound Localization Mechanisms.- Effects of Unilateral Ablation of Anteroventral Cochlear Nucleus on Localization of Sound in Space.- Binaural Interaction Models and Mechanisms.- Psychophysical and Neurophysiological Data on the Sound Source Perception.- Session VII. Neural Coding of Speech and Complex Stimuli.- Information Processing in Neuronal Populations of the Human Brain during Learning of Verbal Signals.- A Comparison of the Responses Evoked by Artificial Stimuli and Vocalizations in the Inferior Colliculus of Squirrel Monkeys.- Acoustic Properties of Central Auditory Pathway Neurons during Phonation in the Squirrel Monkey.- Selectivity of Auditory Neurons for Vowels and Consonants in the Forebrain of the Mynah Bird.- Some Aspects of Functional Organization of the Auditory Neostriatum (Field L) in the Guinea Fowl.- 14-C-Deoxyglucose Labeling of the Auditory Neostriatum in Young and Adult Guinea Fowl.- Integration of Voco-Auditory Centers in Song Birds.- Response Properties and Spike Waveforms of Single Units in the Torus Semicircularis of the Grassfrog (Rana Temporaria) as Related to Recording Site.- Coding of Amplitude-Modulated Sounds in the Midbrain Auditory Region of the Frog.- Session VIII. Deprivation and Developmental Studies.- Effects of Early Auditory Stimulation on Cortical Centers.- Effects of Acoustic Deprivation on Morphological Parameters of Development of Auditory Neurons in Rat.- Behavioral and Anatomical Studies of Central Auditory Development.- Input-Dependent 2-Deoxy-D-Glucose Uptake in the Central Auditory System of Rana Temporaria.- Plastic Changes in the Inferior Colliculus Following Cochlear Destruction.- Developmental Changes of Auditory Evoked Responses in Normal and Kanamycin Treated Rats.- Session IX. Auditory Prostheses, Physiological Background.- Physiological Background of Hearing Prostheses.- Electrical Stimulation of the Human Cochlea - Psychophysical and Speech Studies.- Preliminary Speech Perception Results through a Cochlear Prosthesis.- Tactile Aid for the Deaf Search of a Code Allowing Somesthetic Processing of Acoustic Messages.- Participants.

Acoustic input to the lateral pontine nuclei

Axon and terminal degeneration were studied in the cat dorsolateral pontine nucleus (DLPN) after lesion of the inferior colliculus. In separate experiments the acoustic responses of 111 units of the lateral pontine nuclei were studied in cats anesthetized with chloralose-urethane. Lesions of all three nuclei of the inferior colliculus (central, pericentral and external) lead to a very similar pattern of terminal degeneration in a discrete region of ipsilateral DLPN. This is suggestive of a highly convergent projection in which topography may be blurred. Most units responded to binaural stimulation, and the most common binaural response consisted of excitatory inputs from each ear which were facilitated at some binaural intensity levels and occluded at others. Discharge rates changed as a result of alterations in the number of spikes evoked at the onset of the stimulus, and sustained discharges were rarely encountered. Units were broadly and irregularly tuned; binaural inhibition was very uncommon. Unit response characteristics suggested that, while the projection from the inferior colliculus was highly convergent, only a subclass of inferior colliculus neurons may be involved. However, the acoustic properties of lateral pontine units were strikingly similar to those of the cerebellar vermis, a region to which DLPN is known to project.

Is the inferior colliculus and obligatory relay in the cat auditory system

The status of the inferior colliculus of the cat as an obligatory relay in the ascending auditory pathway was examined by attempting to infiltrate totally the fibres of the brachium of the inferior colliculus on one side with horseradish peroxidase. Following a transport time of 24 h, alternate sections from thalamus to caudal brainstem were reacted with a sensitive histochemical method to reveal tracer reaction product. Results for three cats revealed that the inferior colliculus is an obligatory relay for the overwhelming majority of axons comprising the lateral lemniscus and originating in the cochlear nucleus and superior olive.

Spatial receptive-fields in the cat inferior colliculus

Auditory spatial receptive fields of 122 single units in the inferior colliculus of 8 anesthetised cats were studied with free-field acoustic stimuli presented in the frontal hemisphere. The best frequency and best frequency threshold were determined for each unit with the speaker located in a position where the unit responded strongly. The intensity was then raised to 10 dB above threshold at the best frequency and the boundaries of the spatial receptive field were determined. For sounds of low intensity, receptive field size appeared to be a continuum with respect to best frequency. Units of high best frequency had small circumscribed fields located in the contralateral frontal hemifield. The boundaries of receptive fields for units of progressively lower best frequency expanded in all directions. Thus for intermediate frequencies, fields typically filled the contralateral hemifield whereas for low frequencies, units could be activated by stimulation from any position tested. At higher intensities, the boundaries of the receptive fields of units expanded. Circumscribed receptive field centres lay on a line corresponding to the acoustical axis of the contralateral pinna. For these units with small receptive fields, the free-field response to low intensity sounds appeared to be attributable more to the directional properties of the contralateral pinna than to significant binaural interaction.

The responses of neurons in subdivisions of the inferior colliculus of cats to tonal, noise and vocal stimuli.

The aim of this study was to gain information from anesthetized cats about the differential coding properties of neurons in the three major subdivisions of the inferior colliculus: the central (CNIC) and external (EN) nuclei and dorsal cortex (DC). Stimuli were presented in the free field from a speaker facing the contralateral pinna. For each unit, the characteristic frequency (CF, where threshold was lowest) was determined, and impulse rates to CF tone bursts, noise bursts and four feline vocal stimuli were measured as a function of increasing sound pressure level (rate/level functions). Peristimulus-time histograms were computed for responses to all stimuli. Sustained firing patterns to CF stimuli were observed for 81% of units in CNIC, for 50% of units in EN and 27% of units in DC. Sustained discharges were evoked by noise in 78–100% of units in all regions, and by at least one vocal stimulus in 86% of units in CNIC, 82% in EN and 55% in DC. In the CNIC, non-monotonic rate/level functions to CF stimuli were more common (41%) than either monotonie or plateau functions, whereas the reverse was the case with noise and vocal stimuli. Non-monotonic functions were uncommon to any stimulus in EN and DC (21–24%). Vocal stimuli were more effective in terms of higher firing rates than noise or CF stimuli in 27% of units in CNIC, 82% in EN and 72% in DC. There were no units that responded exclusively to one vocal stimulus, but a high proportion of units in EN responded strongly to broad band stimuli, and some of these showed clear preferences for one vocal stimulus over others.

Responses of neurones in the rabbit inferior colliculus. I. Frequency-specificity and topographic arrangement

Abstract The topographic arrangement and frequency sensitivity of units were studied in the inferior colliculus of urethane anaesthetized rabbits. Units in the central nucleus of the inferior colliculus were organized tonotopically, with neurones of low best frequency located dorsally and high best frequency ventrally in this structure. Units in the external nucleus of the inferior colliculus tended to be broadly tuned but the frequently observed habituative behaviour of these units often made the determination of tuning curves difficult in this region. In the central nucleus, tuning curves of low frequency elements were broader than those of high frequency units. Auditory activity was also observed in a small sample of units in the intermediate and deep layers of the superior colliculus. These results were discussed in relation to the neuronal architecture of the inferior colliculus and to the diversity of the sources of afferent endings in this structure. Units in the external nucleus would appear to have a different role to those in the central nucleus in the neural processes of hearing.

Physiology of pathway from dorsal cochlear nucleus to inferior colliculus revealed by electrical and auditory stimulation

SummaryThe dorsal cochlear nucleus (DCN) projects axons to the contralateral central nucleus of the inferior colliculus (ICC) via the dorsal acoustic stria (DAS). In the anaesthetised cat, when brief electrical stimuli are applied to the caudal surface of DCN, single unit and field activity is evoked preferentially in the ventro-lateral region of ICC. Most single ICC units judged by electrical stimulation to have received a direct input from DCN are excited by contralateral tonal stimulation and inhibited or uninfluenced by ipsilateral tones. The sharp non-monotonic intensity functions of most of these units are similar to those of units in the dorsal cochlear nucleus, but ipsilateral inhibition is likely to be provided by a source other than DCN. Thus, although it is suggested that axons of DAS terminate preferentially in the ventro-lateral aspect of the contralateral ICC, it is likely that at least some neurones in this latter region receive additional input from another source — possibly the lateral superior olivary nucleus.

Activity dependent plasticity of postsynaptic density structure in the ventral cochlear nucleus of the rat

Young adult male rats were anaesthetised with urethane and exposed to either 24 h of silence or 24 h of repetitive 77 dB tones in a sound-proofed anechoic chamber. The influence of these two conditions on the ultrastructure of the synaptic appositions made by auditory afferents in the anterior ventral cochlear nucleus (end bulbs of Held) was compared. Both the cross-sectional area and the mean thickness of the postsynaptic density (PSD) in the rats exposed to tones were significantly reduced when compared with rats maintained in silence. Similarly, the degree of curvature of the apposition was significantly reduced. The results of these experiments provide further evidence that the postsynaptic density material is a plastic structure significantly influenced by the amount of activity in the presynaptic element.

Neonatal sensorineural hearing loss affects synaptic density in the auditory midbrain.

WE examined the effect of neonatal sensorineural hearing loss on synaptic density in the central nucleus of the inferior colliculus (ICC) of adult cats to evaluate the role of auditory experience in synaptogenesis. Three groups of animals were used: bilaterally deafened, uni-laterally deafened and normal hearing controls. Synaptic density in bilaterally deafened animals was significantly lower than in normal hearing animals. By contrast, there was no significant difference in synaptic density between normal hearing animals and unilaterally deaf animals. These results demonstrate, for the first time, that a sensorineural hearing loss during development can affect synaptogenesis in the auditory midbrain.

Auditory cortex in the marsupial possum Trichosurus vulpecula

A microelectrode mapping survey was made of the auditory cortex of the acallosal marsupial possum Trichosurus vulpecula. Single unit and unit cluster responses, as well as evoked potentials, were obtained from 64 tangential, cortical penetrations. Pontamine sky-blue marking of successful tracks aided in later reconstruction of tracks and identification of recording locations. Only one auditory field was identified. The 98 units sampled in this field covered a wide range of best frequencies from 330 Hz to 39 kHz with the most sensitive units (thresholds of best frequency of less than 10 dB SPL) being in the 17-19 kHz range. An orderly representation of cochlear place was found in the cortex with high frequencies located dorsally and low frequencies more ventrally. This orientation of frequencies is unlike that seen in other mammals.