Frequency discrimination after bilateral section of the brachium of the inferior colliculus

Leveling up: a long-range olivary projection to the medial geniculate without collaterals to the central nucleus of the inferior colliculus in rats

The medial nucleus of the trapezoid body (MNTB) is one of the monaural cell groups situated within the superior olivary complex (SOC), a constellation of brainstem nuclei with numerous roles in hearing. Principal MNTB neurons are glycinergic and express the calcium-binding protein, calbindin (CB). The MNTB receives its main glutamatergic, excitatory input from the contralateral cochlear nucleus via the calyx of Held and converts this into glycinergic inhibition directed toward nuclei in the SOC and the ventral and intermediate nuclei of the lateral lemniscus (VNLL and INLL). Through this inhibition, the MNTB plays essential roles in localization of sound sources and encoding spectral and temporal features of sound. In rats, very few MNTB neurons project to the inferior colliculus. However, our recent study of SOC projections to the auditory thalamus revealed a substantial number of retrogradely labeled MNTB neurons. This observation led us to examine whether the rat MNTB provides a long-range projection to the medial geniculate body (MGB). We examined this possible projection using retrograde and anterograde tract tracing and immunohistochemistry for CB and the glycine receptor. Our results demonstrate a significant projection to the MGB from the ipsilateral MNTB that does not involve a collateral projection to the inferior colliculus.

The multi-channel cochlear implant: multi-disciplinary development of electrical stimulation of the cochlea and the resulting clinical benefit

This multi-disciplinary research showed sound could be coded by electrical stimulation of the cochlea and peripheral auditory nervous system. But the temporal coding of frequency as seen in the experimental animal, was inadequate for the important speech frequencies. The data indicated the limitation was due in particular to deterministic firing of neurons and failure to reproduce the normal fine temporo-spatial pattern of neural responses seen with sound. However, the data also showed the need for the place coding of frequency, and this meant multi-electrodes inserted into the cochlea. Nevertheless, before this was evaluated on people we undertook biological safety studies to determine the effects of surgical trauma and electrical stimuli, and how to prevent infection. Then our research demonstrated place of stimulation had timbre and was perceived as vowels. This led to our discovery in 1978 of the formant-extraction speech code that first enabled severely-profoundly deaf people to understand running speech. This result in people who had hearing before becoming severely deaf was an outcome not previously considered possible. In 1985 it was the first multi-channel implant to be approved by the US Food and Drug Administration (FDA). It was also the fore runner of our advanced formant and fixed filter strategies When these codes were used from 1985 for those born deaf or deafened early in life we discovered there was a critical period when brain plasticity would allow speech perception and language to be developed near- normally, and this required in particular the acquisition of place coding. In 1990 this led to the first cochlear implant to be approved by the FDA for use in children. Finally, we achieved binaural hearing in 1989 with bilateral cochlear implants, followed by bimodal speech processing in 1990 with a hearing aid in one ear and implant in the other. The above research has been developed industrially, with for example 250,000 people worldwide receiving the Cochlear device in 2013, and as of December 2012 the NIH estimated that approximately 324,200 people worldwide had received this and other implants (NIH Publication No. 11-4798). This article is part of a Special Issue entitled <Lasker Award>.

Subcollicular projections to the auditory thalamus and collateral projections to the inferior colliculus

Experiments in several species have identified direct projections to the medial geniculate nucleus (MG) from cells in subcollicular auditory nuclei. Moreover, many cochlear nucleus cells that project to the MG send collateral projections to the inferior colliculus (IC) (Schofield et al., 2014). We conducted three experiments to characterize projections to the MG from the superior olivary and the lateral lemniscal regions in guinea pigs. For experiment 1, we made large injections of retrograde tracer into the MG. Labeled cells were most numerous in the superior paraolivary nucleus, ventral nucleus of the trapezoid body, lateral superior olivary nucleus, ventral nucleus of the lateral lemniscus, ventrolateral tegmental nucleus, paralemniscal region and sagulum. Additional sources include other periolivary nuclei and the medial superior olivary nucleus. The projections are bilateral with an ipsilateral dominance (66%). For experiment 2, we injected tracer into individual MG subdivisions. The results show that the subcollicular projections terminate primarily in the medial MG, with the dorsal MG a secondary target. The variety of projecting nuclei suggest a range of functions, including monaural and binaural aspects of hearing. These direct projections could provide the thalamus with some of the earliest (i.e., fastest) information regarding acoustic stimuli. For experiment 3, we made large injections of different retrograde tracers into one MG and the homolateral IC to identify cells that project to both targets. Such cells were numerous and distributed across many of the nuclei listed above, mostly ipsilateral to the injections. The prominence of the collateral projections suggests that the same information is delivered to both the IC and the MG, or perhaps that a common signal is being delivered as a preparatory indicator or temporal reference point. The results are discussed from functional and evolutionary perspectives.

The multiple-channel cochlear implant: the interface between sound and the central nervous system for hearing, speech, and language in deaf people-a personal perspective

The multiple-channel cochlear implant is the first sensori-neural prosthesis to effectively and safely bring electronic technology into a direct physiological relation with the central nervous system and human consciousness, and to give speech perception to severely-profoundly deaf people and spoken language to children. Research showed that the place and temporal coding of sound frequencies could be partly replicated by multiple-channel stimulation of the auditory nerve. This required safety studies on how to prevent the effects to the cochlea of trauma, electrical stimuli, biomaterials and middle ear infection. The mechanical properties of an array and mode of stimulation for the place coding of speech frequencies were determined. A fully implantable receiver-stimulator was developed, as well as the procedures for the clinical assessment of deaf people, and the surgical placement of the device. The perception of electrically coded sounds was determined, and a speech processing strategy discovered that enabled late-deafened adults to comprehend running speech. The brain processing systems for patterns of electrical stimuli reproducing speech were elucidated. The research was developed industrially, and improvements in speech processing made through presenting additional speech frequencies by place coding. Finally, the importance of the multiple-channel cochlear implant for early deafened children was established.

Central acoustic tract in an echolocating bat: an extralemniscal auditory pathway to the thalamus

To determine the sources and targets of auditory pathways that bypass the inferior colliculus in the mustache bat, we injected WGA-HRP in the medial geniculate body and related auditory nuclei of the thalamus as well as in the lower brainstem. We used electrophysiological methods to verify that the injection electrode was in an area responsive to sound. The only thalamic injections that produced retrograde transport to cells in auditory nuclei caudal to the inferior colliculus were those that included the suprageniculate nucleus. These injections labeled a group of large multipolar cells lying between the ventral nucleus of the lateral lemniscus and the superior olivary complex. Neurons in this cell group have also been shown to project to the deep layers of the superior colliculus in the mustache bat. The pathway revealed by these studies is almost identical to the "central acoustic tract" in which fibers course medial to the lateral lemniscus and bypass the inferior colliculus to reach the deep superior colliculus and the suprageniculate nucleus.

Hearing loss in dogs after lesions of the brachium of the inferior colliculus and medial geniculate

Seven dogs were tested for their sensitivity to pure tones following lesions of the brachium of the inferior colliculus and medial geniculate body. Bilateral section of the brachium of the inferior colliculus consistently resulted in an average hearing loss of as much as 37 dB in the midrange of the animals' audiograms. Lesions of the medial geniculate appear to produce a similar hearing loss if the ventral division of the medial geniculate is completely destroyed.

Ontogenesis of learning: II. Variation in the rat's reflexive and learned responses to acoustic stimulation

The ontogenesis of the rat's reflexive and simple learned reactions to acoustic stimulation was investigated in five experiments. The pattern of results suggests that (a) reflexive reactions to tonal stimulation emerge earlier in ontogenesis than the capacity to learn about these stimuli, and (b) the age at which simple Pavlovian conditioning is first observed depends on the nature of the acoustic conditioned stimulus (CS). Specifically, these data suggest that pups are not capable of simple Pavlovian conditioning to a 2000-Hz tone (CS) paired with an oral infusion of sucrose (US) until they are 14-15 days old. The same acoustic stimulus will, however, evoke reflexive reactions (startle inspiration) in pups as young as 10-12 days old. Additionally, when a different auditory stimulus (an intermittent train of clicks) serves as the CS, there is evidence of conditioning in pups only 12-13 days old. It is suggested that these age-related dissociations in the emergence of reflexive and simple learned responses to sound reflect a caudal-to-rostral maturational sequence of the components of the ascending auditory system.

Evidence of sub-collicular auditory projections to the medial geniculate nucleus in the cat: an autoradiographic and horseradish peroxidase study

Connection of a posteromedial region of the ventral nucleus of the lateral lemniscus were examined in the cat using the autoradiographic tracing method. This sub-collicular region previously had been shown, using retrograde transport of horseradish peroxidase, to send axons to the superior colliculus. The autoradiographic findings revealed that many axons from the posteromedial region of the ventral nucleus of the lateral lemniscus that entered the superior colliculus continued into the midbrain reticular formation. Moreover, other axons traced rostral to the interior colliculus into the thalamus ended in the medial geniculate nucleus, bilaterally. Experiments in which horseradish peroxidase was placed in the medial geniculate nucleus retrogradely labeled the large neurons in the posteromedial region supporting the autoradiographic observations. Other sub-collicular regions also contained labeled cells in these cases, including the main body of the ventral nucleus of the lateral lemniscus and scattered cell groups around the superior olivary complex.

Startle habituation in rats after lesions in the brachium of the inferior colliculus

Bilateral interruption of the primary ascending auditory pathway at the level of the brachium of the inferior colliculus (BIC) did not affect short- or long-term habituation of the startle response provoked by auditory stimuli. Animals with BIC lesions and control animals exhibited comparable habituation following manipulations of stimulus intensity, inter-stimulus interval, and intensity of background noise, although animals with lesions in the BIC were more responsive than controls to auditory stimuli and to tactile stimuli. The integrity of the primary auditory pathway above the inferior colliculus is not necessary for short- or long-term habituation of the acoustic startle response.

Classical conditioning with auditory discrimination of the eye blink in decerebrate cats

Cats were subjected to complete lower brainstem transection, and were then tested for learning ability according to a classical conditioning paradigm. An auditory stimulus was systematically paired with a brief shock to the eyelid. Within a few weeks after the operation, the decerebrate cats could learn the conditioned response with a tone frequency discrimination and then a discrimination reversal. Our results support the notion that the brainstem reticular formation can support a conditioned response which isbehaviorally similar to that obtained in the intact animal.

Auditory pathways to the cortex in Tupaia glis

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.