The Inferior Colliculus

Inferior colliculus
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Inferior Colliculus

Do you have frequent and recurring nosebleeds? IC neurons are involved in sound localisation in the horizontal and vertical plains.

Their activity is mediated by the descending fibers from the cortex and the thalamus medial geniculate body. The tonotopic organisation seen in the SOC continues in the higher nuclei of the auditory system. The ascending projections from the SOC pass along the lateral lemniscus C and arrive at the external nucleus of the inferior colliculus A and B. The fibers that send low frequency information in red and marked with number 3 project along the lateral side of the lemniscus ipsilaterally and reach the external nucleus of the IC.

Only a few studies have been designed to investigate the responses of neurons in the IC to vocal stimuli Aitkin et al. The aim of the study by Aitkin et al. Four types of feline vocalizations kitten calls, screech, adult meow and low-meow were used in this study, in addition to tone bursts and noise bursts. In general, the average thresholds to CF stimuli were lower in the IC than those to noise and vocal stimuli.

As to the selectivity of responses, there appeared to be a continuum of response preferences, ranging from a complete lack of response to any of the complex stimuli used, to apparently unselective responding to all presented stimuli.

Inferior colliculus

There were no units that responded exclusively to one vocal stimulus, but a high proportion of units in the ECIC responded strongly to broadband stimuli, and some of these showed a clear preference for one vocal stimulus over another. Interestingly, the reactivity of neurons to pure tones and to noise bursts was different from that observed in response to animal calls.

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What functional meaning may the fact that vocal stimuli were more effective than pure tone stimuli in the ECIC have? It is possible that the efferent projections of the call-preferring neurons in the IC make synaptic contacts at the premotor areas necessary for the initiation of motor responses to certain vocalizations. In the study by Aitkin et al.

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A dominant observation arising from this study was that, throughout the IC, particularly in the external nucleus, neurons which were clearly tuned to the tone-burst stimuli responded vigorously to the bandpass stimuli noise and vocalizations. Neither the rate-level functions nor the peristimulus histograms suggested that strong inhibitory interactions existed between CF and off-CF frequencies for the majority of the units. For some units with non-mono-tonic rate-level functions to CF stimuli, monotonic functions were demonstrated using noise and vocal stimuli Fig.

This observation means that lateral inhibition is not evoked in the IC units by off-CF components. The widespread effectiveness of vocal stimuli on cellular responses in the IC may also relate to the amplitude modulation AM and frequency modulation FM of components within each call.

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The central nucleus is the largest division of the inferior colliculus. Neurons in the external nucleus EN and dorsal cortex DC tend to respond to the stimulus onset and have lost their frequency selectivity. This page was last edited on 30 May , at Clear Turn Off Turn On. The short answer is no. This indicates that the inferior colliculus is metabolically more active than many other parts of the brain.

The majority of neurons recorded in the IC by Rees and Moller responded effectively to modulation rates up to Hz. In cats, Langner and Schreiner reported even higher rates; over half of their sample responded to AM stimuli at frequencies up to Hz, with some units still sensitive at 1,Hz. For further review of the subcortical neural coding mechanisms for auditory temporal processing, see, for example, Frisina The most extensive study to date of the responses of the IC to vocalizations was performed in guinea pigs Syka et al. Guinea pigs represent a species where voluminous knowledge about the structure and function of the inner ear has been accumulated over the twentieth century, with less information being available about the structure and function of the central part of the auditory pathway.

Guinea pigs produce expressed vocalization signals in different behavioral situations. Their vocalization repertoire contains at least 11 distinct calls Berryman, , fundamentally different in their spectrotemporal features Syka et al. The four most recognized guinea pig calls are purr, whistle, chutter and chirp Fig. Purr consists of a series of regular low-frequency acoustical pulses with a fundamental frequency around Hz; whistle is a long-lasting frequency and amplitude modulated sound consisting of many harmonics over a wide frequency range; chutter is a sequence of irregular noise bursts; and chirp is an isolated brief acoustic impulse with a harmonic structure.

The selectivity of neurons in the IC of the guinea pig to individual vocalizations was found to be relatively low. The responsiveness to individual types of vocalizations also corresponded with the characteristic frequency of the recorded neurons, i.

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The low-frequency call, the purr, produced excitation in only a small number of high-frequency neurons; similarly, whistle, with its strong representation of high sound frequencies, elicited a response in a few low-frequency neurons. Since the s, one of the frequently used criteria for recognizing a selective response of a neuron to an animal call call detector was a difference between the response to a temporally reversed call and the response to a proper call. Such a procedure changes the temporal features of a sound, but preserves its spectral characteristics. The possibility that the weaker response to temporally manipulated sounds is attributable to their lack of behavioral relevancy is supported by the study of Wang and Kadia This study showed that cortical neurons in the cat responded similarly to marmoset natural and time-reversed communication sounds, but that marmoset cortical units preferred the naturally patterned call over the time-reversed call.

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Further details were studied by Gehr et al. Interestingly, the sustained part of the response to the time-reversed meows was slightly larger than that for the forward meows. There is a possibility that the preference for natural whistle could be based on the preference for rising amplitude versus falling amplitude, or for rising frequency over falling frequency of the whistle versus temporally reversed whistle.

This idea is supported by studies of neuronal responses to amplitude or frequency modulated sounds.

Inferior Colliculus

Similarly, Neuert et al. Harvey Babkoff, in Contributions to Sensory Physiology , The role of the inferior colliculus in the localization of sound sources has been established in physiological and ablation studies involving a variety of species. In all of the studies, units have been identified that are sensitive to interaural time and interaural intensity asymmetries Benevento and Coleman, ; Starr and Don, ; Flammino and Clopton, Two types of mechanisms have been proposed for the analysis of neural activity at the inferior colliculus.

One model specifies units with characteristic delays, i. In this model there is invariance in unit activity over frequency and intensity for the adequate stimulus. Each cell is thus capable of encoding a specific location in space depending upon its characteristic delay.

An array of cells thus can produce a neural mapping of auditory space. Altman reports that the majority of binaurally respondent neurons at the inferior colliculus have characteristic delays and characteristic interaural asymmetries to which they are maximally sensitive. Altman also points to the change in neural coding from the superior olive to the inferior colliculus. Another model Hall, specifies neuronal populations rather than individual units, and the relative firing rate of the two bilaterally symmetrical populations as the neural substrate for localization.

These asymmetries in neural activity are compared in ratio form, and the laterality and magnitude of the ratio are posited to represent spatial locus. The data reported by Flammino and Clopton for the albino rat favor a model emphasizing overall activity of a neural population.

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It appears that when a sound arrives at one ear sooner, the cells at the contralateral nucleus receive relatively more excitatory than inhibitory input in direct relation to the extent of the lead. When the sound reaches the ipsilateral ear first, the inhibitory effect dominates and overall activity at that locus is attenuated. Thus, interaural time asymmetry seems primarily to affect the ratio of excitatory to inhibitory inputs to inferior colliculus. When the sound from the contralateral side of the inferior colliculus is more intense than the sound at the ipsilateral side, this, in turn, produces a higher firing probability ; Benevento and Coleman, ; Flammino and Clopton, The opposite is true for a more intense sound at the ipsilateral ear.

This results in a decrease in firing rate. Also, the greater amount of excitation reaching the inferior colliculus from the contralateral ear, the greater the synchrony of firing within the colliculus and the smaller the interneuron variance in timing Flammino and Clopton, Knudsen and Konishi reported the results of a study of auditory space mapping in the owl's midbrain auditory area nucleus mesencephalicus lateralis dorsalis , the avian homolog of the inferior colliculus , using free-field conditions.

They report that the units in this region were sensitive to limited areas of space and were largely independent of the nature or intensity of the sound stimuli. These data thus support the concept of neural mapping of auditory space. Konishi, in Encyclopedia of Neuroscience , The lateral shell of the inferior colliculus LS is the site of convergence between the time and intensity pathways.

The LS receives direct inputs from nucleus angularis and LLDp and indirect inputs from nucleus laminaris and LLDa via the core of the inferior colliculus. LS neurons respond to combinations of ITD and ILD within a narrow range of frequencies, indicating that the convergence of the two pathways occurs initially in each frequency band. ICx neurons do not respond to ITD or ILD alone but require specific combinations of the two cues, and unfavorable combinations may induce inhibition. The auditory midbrain, or inferior colliculus , represents the first major convergence of both binaural and monaural neural information in the auditory system.