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Several structures associated with the auditory pathway have been examined by radioimmunoassay for their content of somatostatin. Of these, the medial geniculate body had the highest content, followed by the cochlear nucleus, inferior colliculus, auditory cortex and cochlea. Cochlear perilymph had no detectable somatostatin-like immunoreactivity.
The present experiments were designed to trace the central auditory pathways in an extant reptile, the New Worlkd lizard--Iguana iguana, utilizing anterograde axonal degeneration stained by the Fink-Heimer ('67) method and the retrograde axonal transport of horseradish peroxidase (LaVail and LaVail, '74). Beginning with the projections of the auditory portion of the VIIIth nerve, the ascending pathways were traced through successive relay nuclei to the telencephalon. The auditory portion of the VIIIth nerve projects to two nuclei in the dorsomedial medulla-nucleus angularis and nucleus magnocellularis medialis. These two nuclei together with a third cll group, nucleus magnocellularis lateralis (intercalated between nucleus angularis and nucleus magnocellularis medialis), have been referred to as the auditory tubercle in previous studies (cf. Miller, '75). The axonal degeneration following large lesions of the auditory tubercle and small lesions of nucleus angularis demonstrated the second order auditory pathways. Fibers leave nucleus angularis ventrally and travel to the ventral surface of the medulla where they cross the midline and ascend to the midbrain in pathways resembling the trapezoid body and the lateral lemniscus of mammals. Along these pathways, terminal arborizations of some fibers were seen in three lower brainstem nuclei while other fibers ascent to the midbrain and terminate in the central nucleus of the torus semicircularis. Experiments in which horseradish peroxidase injections were made in the torus semicircularis demonstrated that nucleus angularis is a primary source of second order auditory fibers to the midbrain and, in addition, that two of the lower brainstem targets of the auditory tubercle project to the torus semicircularis. These lower brainstem pathways were shown to be associated with the auditory system by electrophysiologically recording sound-evoked responses from clusters of cells in the torus semicircularis. Ascending fibers arising from the central nucleus of the torus semicircularis were followed rostrally where they entered the dorsal thalamus and terminated throughout nucleus medialis. Finally, a thalamotelencephalic auditory pathway was traced from nucleus medialis into the lateral forebrain bundle. Terminations of this pathway from nucleus medialis were seen in the medial dorsal ventricular ridge and in the striatum. It was concluded that the ascending auditory pathways of the iguana bear a remarkable resemblance to both the mammalian and avian auditory pathways from the level of the first order neurons in the VIIIth nerve to the level of the telencephalon. At the same time, there are important specializations of the auditory system in birds and mammals such as the development of particular lower brainstem nuclei. Nevertheless, a basic plan for the organization of the auditory system in terrestrial vertebrates can be recognized which invites comparisons with the vertebrate classes that remained in aquatic habitats...
The auditory system of the tree shrew, Tupaia glis, was investigated by identifying axonal degeneration after lesions of the lateral lemniscus, the inferior colliculus, the medial geniculate nucleus and the auditory cortex. The results show that the lateral lemniscus projects to the central nucleus of the inferior colliculus which in turn projects principally to the ventral division of the medial geniculate nucleus but to a lesser extent to the magnocellular division of the medial geniculate nucleus. The final step in the pathway to the cortex is achieved by a projection from the ventral division to the fourth layer of auditory koniocortex. There appear to be several auditory pathways parallel to this primary path. The lateral lemniscus projects to the dorsal division of the medial geniculate nucleus; the deeper layers of the superior colliculus project to the posterior nucleus; and both the dorsal division and the posterior nucleus project to the belt caudal to auditory koniocortex. The caudal division of the medial geniculate nucleus may constitute a relay in still another path from the pericentral division of the inferior colliculus. Finally, the magnocellular division also appears to be distinct insofar as its cortical projections are confined chiefly to the deeper layers. A comparison between the tree shrew and the cat reveals a similar organization in the two species. In the cat the starting point for understanding the organization of the several auditory pathways is the distinction between a core cortical zone which corresponds to konicortex and to AI and a peripheral belt. The core receives essential projections from the ventral division; the belt receives sustaining projections from the cell groups which surround the ventral division. It is reasonable to hypothesize that this difference between the core and the belt is characteristic of all mammals.
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Evoked responses originating from cochlea, brain stem, and cortex are clinically used for differential diagnosis of hearing losses. The diagnostic range and the reliability of the different ERA methods are discussed. In our clinic brain stem potentials recorded by a nonsurgical method have been used as a routine audiometric test in more than 900 cases. The procedure has proved to be easier than electrocochleography and gives nearly the same information about cochlear and middle ear function if there is no VIIIth nerve or lower brain stem damage. For topical diagnosis of the lower auditory pathway additional recording of ECoG is necessary. In the examination of a cortical deafness the recording of brain stem potentials yields the same result as the electrocochleography according to Aran (Fig. 2). In a case of an Apallic syndrome (Fig. 3) brain stem potentials are found only for high intensity clicks, and latencies are abnormally increased. The cochlear potentials simultaneously recorded from the promontory are quite normal. So damage of the brain stem is confirmed.
In order to evaluate their reliability for determing the hearing threshold, the cochlear microphonic potentials, the auditory nerve and brain stem neural evoked responses as well as the cortical evoked responses were compared with the behavioural hearing thresholds of the same subjects in the same session. The threshold for recording the cochlear microphonic potnetial was found to be appreciably higher than the behavioral threshold. The threshold for recording the auditory nerve and brain stem responses was within a few decibels of the behavioural threshold. The thresold of the cortical evoked response was several decibels higher. It is concluded that (1) the auditory nerve and brain stem neural evoked responses are the best indicators of hearing threshold; (2) the cortical evoked responses are usually comparable, and (3) all types of evoked responses are indispensable aids in the evaluation of hearing and the determination of site of lesion in the auditory system.
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Reports in the literature suggest that patients with otosclerosis, neural lesions such as acoustic neuroma, pontine lesions, and temporal lobe lesions may show abnormal sound localization ability. In the present experiment, 15 normal subjects and 22 patients with various auditory pathway lesions had sound localization testing using special apparatus. The results confirmed that patients with otosclerosis were unable to localize low frequency sounds. Those with temporal lobe lesions made judgments not significantly different from normal subjects. Patients with unilateral sensorineural deafness made errors localizing high frequency sounds. Those with acoustic neuromas made greater errors than those without, but the difference between these two groups just failed to reach statistical significance. The number of cases was however small, and further studies are in progress to attempt to define numerical criteria to assist clinical decisions with regard to the selection of patients for invasive investigations such as oil cisternogram.
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In 57 patients with operated expansive tumors in the postcranial fossa auditory evoked brain stem potentials were investigated. A group of 36 with neurological evidence for the brain stem compression showed significantly prolonged latencies (Fig. 1), whereas the evoked potentials were nearly normal in the preceding state of CSF circulatory disturbances (21 cases). For one patient with a neurinoma of the right trigeminal root the post-operative reduction of the pathological latency shift is demonstrated (Fig. 2). Latency evaluation of evoked brain stem potentials are used in our clinical routine for differential diagnosis of retrocochlear hearing damages.
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This study is carried out on single (not averaged) recordings combining the spontaneous activity preceding the stimulus onset and the EP recorded upon acoustical stimulation. These recordings, which we call EEG-EPograms, are measured simultaneously from different subdural structures, such as the auditory cortex, medial geniculate nucleus, inferior colliculus, reticular formation and the hippocampus of the cat brain during the slow wave sleep stage. Using a combined analysis procedure (C.A.P.), the relevant frequency components of spontaneous EEG and EPs, recorded simultaneously from these brain nuclei, are analyzed according to the consistent selectivity bands depicted by the determined amplitude-frequency characteristics for the SWS-stage. In parallel with the results which we obtained for the waking stage, these analyses provide also the following information: (1) there is an important congruency in the time courses of simultaneous response components in common frequency bands, especially in the alpha and beta frequency ranges; (2) there exist significant coupling and synchrony between the evoked amplitude enhancements in the simultaneously recorded single response components; (3) the inter-nuclei coherency in the brain's electrical activity is enormously increased upon stimulation;(4) the evoked response magnitude can be predicted, with reasonable accuracy, from the spontaneous activity preceding the stimulus. All these findings are discussed with reference to those obtained for the waking stage.
This study is carried out on single (not averaged) recordings combining the spontaneous activity preceding the stimulus onset and the EP recorded upon acoustical stimulation. These recordings, which we call EEG-EPograms, are measured simultaneously from different subdural brain structures, such as the auditory cortex, medial geniculate nucleus, inferior colliculus, reticular formation and the hippocampus of awake cats. Using a combined analysis procedure (C.A.P.), the relevant frequency components of spontaneous EEG and EPs, recorded simultaneously from these brain nuclei, are analyzed according to the consistent selectivity bands depicted by the determined amplitude-frequency characteristics. These analyses provide us the following information: (1) there is an important congruency in the time courses of simultaneous response components in common frequency bands, especially in the alpha and beta frequency ranges; (2) there exist significant coupling and synchrony between the evoked amplitude enhancements in the simultaneously recorded single response components; (3) the inter-nuclei coherency in the brain's electrical activity is enormously increased upon stimulation; (4) the evoked response magnitude can be predicted, with reasonable accuracy, from the spontaneous activity preceding the stimulation. The strong dependence of the response magnitude on the stimulus-preceding EEG is explained by means of a model network consisting of a population of relaxation oscillators, which can be brought to different states of synchrony and asynchrony. Some suggestions and comments are also made for investigators working toward theories of signal transmission in the brain.