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M N Semple

Publications and source records attributed to M N Semple.

27 records · Page 2Linked to original sources
PubMed1985

Single-unit responses in the inferior colliculus: effects of neonatal unilateral cochlear ablation.

Monaural responses of single units isolated in the inferior colliculus of adult gerbils that have developed postnatally with one cochlea were compared with monaural responses recorded in animals that have developed with both cochleas. One cochlea of 2-day-old gerbils was ablated, and at approximately 6 mo of age, excitatory responses to stimulation of the nonoperated ear were recorded in the ipsilateral inferior colliculus. These responses were compared quantitatively with responses evoked by ipsilateral and contralateral monaural stimulation in normal gerbils. Responses to ipsilateral stimulation in adult gerbils subjected at 2 days of age to ablation of the contralateral cochlea are significantly different from ipsilateral responses in nonoperated gerbils. In several respects they are very similar to contralateral responses in nonoperated gerbils. (Differences between monaural contralateral and ipsilateral responses in control animals are documented in the companion paper, Ref. 24.) These conclusions are based on analyses of response threshold, peak discharge rate, response pattern, and minimum response latency. The mean dynamic range of ipsilateral rate/intensity functions obtained in neonatally ablated gerbils is significantly larger than the mean ipsilateral and contralateral dynamic ranges in control animals. Analyses of threshold tuning curves indicate that the frequency tuning of units in the inferior colliculus of neonatally ablated animals does not differ significantly from the tuning of units in control animals in response to either ipsilateral or contralateral stimulation. These data reveal that in normal gerbils responses of single units in the inferior colliculus to stimulation of the ipsilateral ear result in part from interactions during postnatal development between pathways that convey information from the contralateral ear. The results are discussed in terms of the known anatomic consequences of a neonatal cochlear ablation and the competition for available synaptic space in the development of the retinotectal system.

Acoustic Stimulation↗
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↗
PubMed1983

Some acoustic properties of neurones in the ferret inferior colliculus.

Single neurones in the central nucleus of the ferret inferior colliculus (ICC) were studied using extracellular recording. Responses to pure tone stimuli were analyzed to assess the frequency organization of the nucleus, the sensitivity and tuning properties of neurones and the effects of binaural sound presentation. Excitatory tuning curves had the characteristic shape found for neurones in the auditory systems of other species. Many ferret neurones were inhibited by stimulus frequencies on either side of the range producing excitatory responses. Sharpness of excitatory tuning was found to be comparable with that reported for the cat. Neurones having best frequencies in the range 4-15 kHz showed the greatest sensitivity. All electrode penetrations revealed a dorsal-to-ventral progression of neurones of increasing best frequency. Some neurones were classified according to the predominant type of input from each ear. Over 80% of these units were binaurally influenced. Nearly all cells received an excitatory input from the contralateral ear and about one-third of those units were excited by ipsilateral stimulation. The remainder showed either a simple inhibitory ipsilateral input or ipsilateral inhibition that was manifested as a suppression of the excitatory, contralateral response. These experiments demonstrate that the ferret may be a satisfactory alternative to other carnivores for studies of the auditory system.

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↗
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↗
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↗