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L M Aitkin

Publications and source records attributed to L M Aitkin.

At least 37 records · Page 2Linked to original sources
PubMed1984

Properties of spatial receptive fields in the central nucleus of the cat inferior colliculus. I. Responses to tones of low intensity.

Single neurones in the central nucleus of the inferior colliculus (ICC) of barbiturate-anesthetized cats were examined using free-field, pure-tone stimuli of low intensity at the neurones' best frequency. Receptive field size was inversely correlated with best frequency. Almost all neurones were maximally excited by stimulus positions in the hemifield contralateral to the recording electrode, irrespective of their best frequency. Simultaneous cochlear microphonic recording revealed that the neurones' best excitatory area was also the spatial region associated with maximum amplification by the contralateral outer ear. This amplification resulted in extremely low (less than -20 dB SPL in some neurones) best frequency thresholds. Response patterns were found not to vary markedly with speaker position. The results suggest that most ICC neurones are more sensitive to stimulation of the contralateral ear than to stimulation of the ipsilateral ear.

Animals↗
PubMed1984

Properties of spatial receptive fields in the central nucleus of the cat inferior colliculus. II. Stimulus intensity effects.

Single units in the central nucleus of the inferior colliculus (ICC) of barbiturate-anaesthetized cats were studied using pure-tone, best-frequency stimulation presented in the free field. At low stimulus intensities almost all neurones responded most strongly to stimuli positioned along the acoustical axis of the pinna contralateral to the recording electrode and there was little or no response to stimuli positioned in the ipsilateral hemifield. Four major classes of spatial response were distinguished when tones of moderate to high intensity were used. The simplest response (24% of the sample) to increasing intensity consisted of a monotonic increase in discharge level at all effective speaker positions and an expansion of the area of space from which a stimulus influenced the response (receptive field). A second class (21%) of units had a nonmonotonic increase in discharge level and an expanding receptive field with increasing intensity. Neither of these classes showed evidence of influence from the ipsilateral ear. The third class (26%) developed, at higher intensities, a second excitatory response region in the ipsilateral hemifield. The fourth class (20%) had receptive fields with fixed medial borders, irrespective of intensity. The third and fourth classes of units were thought to be binaurally influenced and to be sensitive to interaural phase and intensity differences, respectively.

Animals↗
PubMed1984

Responses of neurons in inferior colliculus to variations in sound-source azimuth.

This study aimed to classify the responses of single units in the auditory midbrain to acoustic stimuli presented in the free field in order to characterize those units likely to have a role in sound localization in the horizontal plane. The responses of 131 single units in the inferior colliculus of the cat and the brush-tailed possum were studied using tone and noise-burst stimuli presented from a speaker capable of movement at any point along a plane 10 degrees above the horizontal plane. Speaker positions along this plane are referred to as speaker azimuths; those on the same side as the recorded inferior colliculus as ipsilateral, and on the opposite side as contralateral, azimuths. For each unit, spike counts were measured as a function of azimuth either at the best frequency (BF) or using noise bursts. These functions are referred to as azimuth functions and were usually measured for at least two intensities, between 10 and 70 dB above threshold. The recording sites of most units were identified histologically with the aid of microlesions and were related to the major subdivisions of the inferior colliculus: the central nucleus (ICC), the lateral part of the external nucleus (ICX), and the rostroventral process (R-ICX). Two units were located in the pericentral nucleus and two in the dorsal nucleus of the lateral lemniscus. Two major classes of neuron were identified: omnidirectional and directionally sensitive. Omnidirectional units exhibited azimuth functions that were either flat or that declined gradually at progressively ipsilateral azimuths. For the latter units, discharge rates at all points monotonically increased with stimulus intensity. There was no indication, for either type of omnidirectional unit, of significant binaural interaction. A good correlation was found between the summed proportions of excitatory-excitatory (EE) and monaural (EO) units observed in dichotic studies (46-55%) and the proportion of omnidirectional units in the present study (47%). A subgroup of directionally sensitive units (36% of the total) displayed azimuth functions for which the azimuthal position of the discharge border or peak firing azimuth remained essentially unaltered over a range of stimulus intensities. These azimuth-selective units are likely to have a role in the detection of the location of stimuli in the horizontal plane and appear to include units that would be considered excitatory-inhibitory (EI) or delay sensitive in dichotic studies. The azimuths over which directionally sensitive units showed their marked directional effects were influenced by the position of the contralateral pinna.(ABSTRACT TRUNCATED AT 400 WORDS)

Animals↗
PubMed1983

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.

Acoustic Stimulation↗
PubMed1983

Connections of the auditory cortex of the brush-tailed possum, Trichosurus vulpecula.

The afferent connections of the auditory cortex of the acallosal marsupial Trichosurus vulpecula have been studied using the retrograde transport of horseradish peroxidase. The enzyme was applied by electrophoresis from a micropipette previously used to define physiologically the organization of the auditory cortex. Three major (lateral, dorsomedial and ventromedial) sectors of the medial geniculate contribute axons to the ipsilateral auditory cortex; the connections from at least the lateral sector are topographically arranged. Axons reach the auditory field from the contralateral cortex by way of the anterior commissure and fasciculus aberrans.

Afferent Pathways↗
PubMed1983

Ascending projections to the medial geniculate body of the cat: evidence for multiple, parallel auditory pathways through thalamus.

The sources of ascending input to the medial geniculate body (MGB) of the cat were studied using the retrograde transport of horseradish peroxidase (HRP). HRP injections were made iontophoretically through micropipettes which were also used to record physiological properties at the injection sites. This technique produced small injections which appeared to be restricted to single subnuclei. The tectothalamic projection of the auditory system was found to consist of at least four distinct and separate pathways. The ventral division of the MGB receives a topographical projection from the central nucleus of the inferior colliculus (ICC) which preserves tonotopicity and provides short latency, sharply frequency-tuned responses. The medial part of the ICC projects to the deep dorsal nucleus, which contains only units tuned to high frequencies. The major inputs to the caudodorsal nucleus (DC) stem from nucleus sagulum and the pericentral nucleus of the inferior colliculus (ICP). Units in DC and the ventrolateral nucleus, which also receive input from ICP, have very broad tuning properties and late, habituating responses. Injections of HRP into the medial division (MGM) produced labeled cells scattered throughout the external nucleus of the inferior colliculus and the ventral part of ICC. This widespread input is reflected in the wide range of auditory responses found in MGM. Auditory responses in the suprageniculate nucleus were poorly defined and many units did not respond to tonal stimuli; following HRP injections no filled cells were found in the inferior colliculus, but labeled cells were found in the deeper layers of the superior colliculus and in the interstitial nucleus of the brachium of the inferior colliculus. Together with recent findings on the auditory thalamocortical projection, these results provide evidence for multiple parallel auditory pathways through the thalamus.

Afferent Pathways↗
PubMed1982

Directionality of sound pressure transformation at the cat's pinna.

The directionality of the cat's pinna was studied by using the amplitude of the cochlear microphonic (CM) as a quantitative indicator of tympanic sound pressure level (SPL). It was found that tympanic SPL varied with the location of a free field stimulator in anechoic space. For high (tonal) frequencies, there was a circumscribed optimal area for tympanic SPL in the frontal ipsilateral sound field, in confirmation of previous findings with other techniques that the pinna has an acoustical axis. The directionality of the pinna, determined from the solid angle enclosed by the 5 dB isointensity-decrement line with respect to the optimal position, increased with frequency. For low tonal frequencies, no circumscribed optimal area in the frontal sound field could be distinguished, and tympanic SPL fell by only 10-12 dB for displacements of 90 degrees into the contralateral sound field. Excision of the pinna abolished the circumscribed optimal areas for tympanic SPL and revealed the pinna produces up to 28 dB amplification of acoustic signals delivered 'on-axis'.

Animals↗
PubMed1982

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.

Animals↗
PubMed1982

Some aspects of the auditory pathway and audition in the European mole, Talpa europaea.

The structure of the lower auditory pathway and certain responses to sounds were studied in the European mole (Talpa europaea). While the animal possesses a structurally normal cochlear nuclear complex and auditory midbrain, the medial superior olive, although small, is prominent and the lateral superior olive is lacking. This characteristic has been observed in other species with essentially low frequency hearing. Behavioural and electrophysiological responses to sound were evoked between 0.1 and 15 kHz, and moles were apparently at least sensitive to tones of 5-8 kHz as they were to low tones. In general, evidence is presented for the mole having essentially normal mammalian auditory characteristics.

Acoustic Stimulation↗
PubMed1981

The representation of the auditory and somatosensory systems in the external nucleus of the cat inferior colliculus.

Parallel single unit and retrograde tracing experiments were carried out in the anesthetized cat to elucidate the representation of the auditory and somatosensory systems in the external nucleus of the inferior colliculus (ICX). Units responding to tonal stimuli were more commonly encountered in ICX and the adjacent intercollicular area (ICA) than were units with identified tactile receptive fields. Concomitantly, a larger number of retrogradely labeled cells were identified in midbrain auditory structures, following injections of horseradish peroxidase (HRP) into CRX, than were found in the combined dorsal column nuclei. Microelectrode recording revealed that the entire accessible body surface was represented in ICX, and receptive fields of individual units were usually large and mostly located on the contralateral side. HRP-labeled cells were scattered throughout the contralateral cuneate, gracile, and spinal trigeminal nuclei. tations of tone pips. Responses to complex sound were commonly observed. Binaural stimuli influenced the firing of the majority of auditory units. Labeled auditory neurons following ICX injections were found mainly in the inferior colliculus of both sides. Spread of tracer into the central and pericentral nuclei was associated with labeling of many neurons in hind brain auditory structures. Only vague suggestions of somatotopy or tonotopy were observed in the electrophysiological experiments; similarly, no topographical relationship between HRP injection site and locus of retrograde label in a given projecting nucleus could be discerned. Speculations were made, in the light of the dual convergent sensory representation in ICX, about the role of this structure in acoustico-motor mechanisms.

Afferent Pathways↗
PubMed1980

The efferent projections of the central nucleus and the pericentral nucleus of the inferior colliculus in the cat.

The efferent projections of the central nucleus of the inferior colliculus (ICC) and the pericentral nucleus of the inferior colliculus (ICP) were examined by placing restricted injections of anterograde tracers at electrophysiologically defined loci in the inferior colliculus (IC) of the cat. It was found that single loci in the ICC projected bilaterally onto the ventral division of the medial geniculate body (MGB) in the form of caudorostrally oriented sheets of terminals. The ICC loci also projected bilaterally onto the MGB in the form of caudorostrally oriented columns of terminals; these columns had their caudal aspects located in the medial and their rostral aspects located in the deep dorsal nucleus of the dorsal division. The caudal aspect of the sheets of terminals in the ventral division was folded and passed through both pars lateralis and pars ovoidea of the ventral division. Every component of the ICC-to-MGB projection was cochleotopically ordered. Periodic discontinuities of two types were noted in the projections of ICC onto pars lateralis (VI) of the ventral division. The one type of periodic discontinuity sometimes approximated bands oriented caudorostrally in the caudal aspect of VI. The second type of discontinuity was of very thin parallel columns of more intense labeling oriented rough dorsoventrally and oblique or normal to the first type of discontinuity. Injections in the ICP produced autoradiographic labeling in the caudal dorsal nucleus (Dc) of the dorsal division of the MGB. Thus the ICC-to-MGB and ICP-to-MGB projections are segregated. The efferent connections of the IC with other brainstem auditory structures were noted.

Animals↗
PubMed1980

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

The 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.

Acoustic Stimulation↗
PubMed1979

Representation of sound frequency and laterality by units in central nucleus of cat inferior colliculus.

1. The discharges of 632 units were isolated extracellularly during 42 penetrations of the central nucleus of the inferior colliculus (ICC) in 21 adult cats lightly anesthetized with pentobarbital and ketamine. Microelectrode penetrations were directed from caudal to rostral through ICC, parallel to the Horsley-Clarke (H-C) horizontal and sagittal planes. 2. The threshold best frequency (BF) and binaural response properties were examined for each unit, with the aim of elucidating the organization of these discharge characteristics within ICC. 3. Binaural unit classes consisted of monaural (contralateral) (EO), binaurally phase-sensitive (delay), contralateral excitatory/ipsilateral inhibitory (EI), binaurally excitatory (EE), and other more heterogeneous interaction patterns (other). 4. Detailed histological reconstruction of electrode tracks allowed the recording site for each unit to be related to the three dimensions of ICC. This structure was divided into three lateromedial and three rostrocaudal blocks such that each block contained a similar number of units, enabling meaningful statistical comparisons. Low (3.2 kHz greater than BF) and high (3.2 kHz less than BF) best-frequency classes provided a correlate of dorsoventral location. 5. The arrangements of BFs within ICC were found to be compatible with a model of this structure in which units having similar BFs are organized into layers lying in the H-C horizontal plane medially and gradually tilting in both a ventrolateral and ventrorostral direction. Low frequencies are concentrated dorsally and laterally; high frequencies, ventrally and medially. A rostrocaudal BF difference arises only in lateral aspects of the ICC, where lower frequencies are encountered rostrally. 6. Binaural response classes were distributed differentially throughout ICC. Thus, EO units were concentrated caudally, ventrally, and laterally, while delay units were in greatest numbers rostrally, dorsally, and laterally--almost totally segregated from EO and EI units. The latter populations overlapped ventrally and laterally, but EI units were in greatest density rostrally. The EE class occurred throughout the nucleus, but was most common medially. 7. It is suggested that the differential distributions of binaural responses reflect a partial segregation of the afferents, arising in the superior olive and cochlear nucleus, which terminate in ICC. The central nucleus of the inferior colliculus thus may be composed of several functionally segregated subregions contained within a common tonotopic organization.

Animals↗
PubMed1978

Some features of the spatial organization of the central nucleus of the inferior colliculus of the cat.

A series of neuroanatomical and neurophysiological experiments have been conducted within the central nucleus of the inferior colliculus (ICC) of the cat in order to determine some features of the spatial organization of the nucleus. Results from these experiments have demonstrated: (1) the origins of the auditory brain stem afferents to ICC and the topography of those projections in relation to the cochleotopic organization of ICC; (2) the segregation of at least some of those brain stem projections within ICC; and (3) ICC neurons with similar response properties (response properties which are often similar or identical to those of neurons in brain stem auditory nuclei) are often clustered within the nucleus. These results provide evidence that the laminated division of the ICC probably consists of anatomically, physiologically, and functionally distinct subdivisions and that some aspects of auditory sensation may be encoded or represented separately (i.e., in spatially distinct "regions") within the nucleus.

Animals↗
PubMed1978

The auditory midbrain of a marsupial: the brush-tailed possum (Trichosurus vulpecula).

A microelectrode survey was made of the midbrain auditory nuclei of the brushtailed possum (Trichosurus vulpecula), a common Australian marsupial. Information was sought on the tuning characteristics of individual neurones, tonotopic organization and mechanisms of sound localization. It was felt that such information would be of use in future studies of the development and evolution of mammalian hearing. Twelve possums were anaesthetized with ketamine and chloralose-urethane, and recordings were made of extracellular unit discharges in the inferior colliculus during monaural and binaural tonal stimulation. The inferior colliculus of the possum consists of a central nucleus - a darkly stained, densely packed group of cells - flanked laterally by an external nucleus with a lower density of paler cells. Tonotopic organization was demonstrated by discretelytuned elements in the central nucleus, but was not observed in the external nucleus. In the latter region broad and irregular tuning was commonly seen. Most units in both divisions were influenced by binaural stimuli, with patterns of binaural interaction similar to those observed in the cat inferior colliculus. Cells influenced by changes in the interaural time and intensity difference were commonly observed, but only a subclass of these were suited in sensitivity for sound localization. In general, the midbrain auditory system of the possum was similar in unit discharge characteristics and organization to those of the eutherian mammals commonly studied.

Acoustic Stimulation↗
PubMed1978

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.

Acoustic Stimulation↗