Effects of Bilateral Hearing Aid Use on Balance in Experienced Adult Hearing Aid Users

PURPOSE

The purpose of this study was to evaluate the balance of experienced adult hearing aid users with and without their hearing aids via computerized posturography.

METHOD

Computerized posturography was accomplished by employing the Sensory Organization Test (SOT) on the NeuroCom Balance Master (Natus Medical Incorporated). The SOT assessed each participant's balance and the strategy used to maintain balance in 6 progressively challenging conditions. Twenty-two adults using bilateral at-the-ear hearing aids participated in the study. All participants completed all SOT protocols with and without their hearing aids.

RESULTS

No statistically significant differences in participants' balance were identified regardless of the presence or absence of their hearing aids during the SOT.

CONCLUSIONS

These results failed to support previous research, which indicated that amplification of auditory input could benefit balance in individuals with hearing and balance disorders. Further research utilizing randomized controlled trials is needed to resolve the disparity between the current results and those of previous studies.

Inactivation of Salmonella on tainted foods: using blue light to disinfect cucumbers and processed meat products

Foodborne illness resulting from infectious organisms occurring in vegetables and processed meat is a serious health concern in the United States. Improved and cost-effective techniques for disinfection are needed. Visible light in the blue range (405 nm) was administered to processed meat that had been inoculated with Escherichia coli. One application of light energy at doses of 10, 30, 60, and 100 J/cm² was applied, in vitro. In the case of vegetables contaminated with Salmonella (cucumbers), 464 nm light was used at 6, 12, and 18 J/cm². In both cases, after 20 hours of incubation, colony-forming units were counted and compared to controls to determine whether the light energy inhibited growth of E. coli or Salmonella. E. coli - 405 nm light at doses of 30, 60, and 100 J/cm² were all effective inhibitors of the organism. Kill rates of 75.61 - 96.34% were achieved. Salmonella - 464 nm light at doses of 6, 12, and 18 J/cm² produced significant inactivation of the organism. Kill rates of 80.23-100% were obtained. Blue light, delivered in the wavelength/dose combinations used in this study is an effective inhibitor of both E. coli and Salmonella on actual foodstuffs. Blue light should be considered as a potentially effective tool in the effort to protect humans from foodborne illnesses.

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.

Distribution of GABAergic cells in the inferior colliculus that project to the thalamus

A GABAergic component has been identified in the projection from the inferior colliculus (IC) to the medial geniculate body (MG) in cats and rats. We sought to determine if this GABAergic pathway exists in guinea pig, a species widely used in auditory research. The guinea pig IC contains GABAergic cells, but their relative abundance in the IC and their relative contributions to tectothalamic projections are unknown. We identified GABAergic cells with immunochemistry for glutamic acid decarboxylase (GAD) and determined that ~21% of IC neurons are GABAergic. We then combined retrograde tracing with GAD immunohistochemistry to identify the GABAergic tectothalamic projection. Large injections of Fast Blue, red fluorescent beads or FluoroGold were deposited to include all subdivisions of the MG. The results demonstrate a GABAergic pathway from each IC subdivision to the ipsilateral MG. GABAergic cells constitute ~22% of this ipsilateral pathway. In addition, each subdivision of the IC had a GABAergic projection to the contralateral MG. Measured by number of tectothalamic cells, the contralateral projection is about 10% of the size of the ipsilateral projection. GABAergic cells constitute about 20% of the contralateral projection. In summary, the results demonstrate a tectothalamic projection in guinea pigs that originates in part from GABAergic cells that project ipsilaterally or contralaterally to the MG. The results show similarities to both rats and cats, and carry implications for the role of GABAergic tectothalamic projections vis-à-vis the presence (in cats) or near absence (in rats and guinea pigs) of GABAergic interneurons in the MG.

Projections from the dorsal and ventral cochlear nuclei to the medial geniculate body

Direct projections from the cochlear nucleus (CN) to the medial geniculate body (MG) mediate a high-speed transfer of acoustic information to the auditory thalamus. Anderson etal. (2006) used anterograde tracers to label the projection from the dorsal CN (DCN) to the MG in guinea pigs. We examined this pathway with retrograde tracers. The results confirm a pathway from the DCN, originating primarily from the deep layers. Labeled cells included a few giant cells and a larger number of small cells of unknown type. Many more labeled cells were present in the ventral CN (VCN). These cells, identifiable as multipolar (stellate) or small cells, were found throughout much of the VCN. Most of the labeled cells were located contralateral to the injection site. The CN to MG pathway bypasses the inferior colliculus (IC), where most ascending auditory information is processed. Anderson etal. (2006) hypothesized that CN-MG axons are collaterals of axons that reach the IC. We tested this hypothesis by injecting different fluorescent tracers into the MG and IC and examining the CN for double-labeled cells. After injections on the same side of the brain, double-labeled cells were found in the contralateral VCN and DCN. Most double-labeled cells were in the VCN, where they accounted for up to 37% of the cells labeled by the MG injection. We conclude that projections from the CN to the MG originate from the VCN and, less so, from the DCN. A significant proportion of the cells send a collateral projection to the IC. Presumably, the collateral projections send the same information to both the MG and the IC. The results suggest that T-stellate cells of the VCN are a major source of direct projections to the auditory thalamus.

Multiple origins of cholinergic innervation of the cochlear nucleus

Acetylcholine (Ach) affects a variety of cell types in the cochlear nucleus (CN) and is likely to play a role in numerous functions. Previous work in rats suggested that the acetylcholine arises from cells in the superior olivary complex, including cells that have axonal branches that innervate both the CN and the cochlea (i.e. olivocochlear cells) as well as cells that innervate only the CN. We combined retrograde tracing with immunohistochemistry for choline acetyltransferase to identify the source of ACh in the CN of guinea pigs. The results confirm a projection from cholinergic cells in the superior olivary complex to the CN. In addition, we identified a substantial number of cholinergic cells in the pedunculopontine tegmental nucleus (PPT) and the laterodorsal tegmental nucleus (LDT) that project to the CN. On average, the PPT and LDT together contained about 26% of the cholinergic cells that project to CN, whereas the superior olivary complex contained about 74%. A small number of additional cholinergic cells were located in other areas, including the parabrachial nuclei.The results highlight a substantial cholinergic projection from the pontomesencephalic tegmentum (PPT and LDT) in addition to a larger projection from the superior olivary complex. These different sources of cholinergic projections to the CN are likely to serve different functions. Projections from the superior olivary complex are likely to serve a feedback role, and may be closely tied to olivocochlear functions. Projections from the pontomesencephalic tegmentum may play a role in such things as arousal and sensory gating. Projections from each of these areas, and perhaps even the smaller sources of cholinergic inputs, may be important in conditions such as tinnitus as well as in normal acoustic processing.

Cholinergic cells in the tegmentum send branching projections to the inferior colliculus and the medial geniculate body

The pontomesencephalic tegmentum (PMT) provides cholinergic input to the inferior colliculus (IC) and the medial geniculate body (MG). PMT cells are often characterized as projecting to more than one target. The purpose of this study was to determine whether individual PMT cholinergic cells, (1) innervate the auditory pathways bilaterally via collateral projections to left and right auditory thalamus; or, (2) innervate multiple levels of the auditory pathways via collateral projections to the auditory thalamus and inferior colliculus. We used multiple retrograde tracers to identify individual PMT cells that project to more than one target. We combined the retrograde tracer studies with immunohistochemistry for choline acetyltransferase to determine whether the projecting cells were cholinergic. We found that individual PMT cells send branching axonal projections to two or more auditory targets in the midbrain and thalamus. The collateral projection pattern that we observed most frequently was to the ipsilateral IC and ipsilateral MG. Cells projecting to both MGs were somewhat less common, followed by cells projecting to the contralateral IC and ipsilateral MG. Both cholinergic and non-cholinergic cells contribute to each of these projection patterns. Less often, we found cells that project to one IC and both MGs; there was no evidence for non-cholinergic cells in this projection pattern. It is likely that collateral projections from PMT cells could have coordinated effects bilaterally and at multiple levels of the ascending auditory pathways.

Cholinergic cells of the pontomesencephalic tegmentum: connections with auditory structures from cochlear nucleus to cortex

Acetylcholine (ACh) is a neuromodulator that is likely to play a role in plasticity as well as other phenomena at many sites in the auditory system. The auditory cortex receives cholinergic innervation from the basal forebrain, whereas the cochlea receives cholinergic innervation from the superior olivary complex. Much of the remainder of the auditory pathways receives innervation from the pedunculopontine and laterodorsal tegmental nuclei, two nuclei referred to collectively as the pontomesencephalic tegmentum (PMT). The PMT provides the major source of ACh to the auditory thalamus and the midbrain, and is a substantial source (in addition to the superior olivary complex) of ACh in the cochlear nucleus. Individual cholinergic cells in the PMT often have axon branches that innervate multiple auditory nuclei, including nuclei on both sides of the brain as well as nuclei at multiple levels of the auditory system. The auditory cortex has direct axonal projections to the PMT cells, including cholinergic cells that project to the inferior colliculus or cochlear nucleus. The divergent projections of PMT cholinergic cells suggest widespread effects on the auditory pathways. These effects are likely to include plasticity as well as novelty detection, sensory gating, reward behavior, arousal and attention. Descending projections from the forebrain, including the auditory cortex, are likely to provide a high level of cognitive input to these cholinergic effects. Dysfunction associated with the cholinergic system may play a role in disorders such as tinnitus and schizophrenia.

Cholinergic and non-cholinergic projections from the pedunculopontine and laterodorsal tegmental nuclei to the medial geniculate body in Guinea pigs

The midbrain tegmentum is the source of cholinergic innervation of the thalamus and has been associated with arousal and control of the sleep/wake cycle. In general, the innervation arises bilaterally from the pedunculopontine tegmental nucleus (PPT) and the laterodorsal tegmental nucleus (LDT). While this pattern has been observed for many thalamic nuclei, a projection from the LDT to the medial geniculate body (MG) has been questioned in some species. We combined retrograde tracing with immunohistochemistry for choline acetyltransferase (ChAT) to identify cholinergic projections from the brainstem to the MG in guinea pigs. Double-labeled cells (retrograde and immunoreactive for ChAT) were found in both the PPT (74%) and the LDT (26%). In both nuclei, double-labeled cells were more numerous on the ipsilateral side. About half of the retrogradely labeled cells were immunonegative, suggesting they are non-cholinergic. The distribution of these immunonegative cells was similar to that of the immunopositive ones: more were in the PPT than the LDT and more were on the ipsilateral than the contralateral side. The results indicate that both the PPT and the LDT project to the MG, and suggest that both cholinergic and non-cholinergic cells contribute substantially to these projections.

Projections from auditory cortex to cholinergic cells in the midbrain tegmentum of guinea pigs

Anterograde and retrograde tracing techniques were used to characterize projections from the auditory cortex to the pedunculopontine and laterodorsal tegmental nuclei (PPT and LDT, respectively) in the midbrain tegmentum in guinea pigs. For anterograde tracing, tetramethylrhodamine dextran (FluoroRuby) was injected at several sites within auditory cortex. After sufficient time for transport, the brain was processed for immunohistochemistry with anti-choline acetyltransferase to reveal presumptive cholinergic cells. Anterogradely labeled axons were observed ipsilaterally and, in smaller numbers, contralaterally, in both the pedunculopontine and laterodorsal tegmental nuclei. In all four nuclei, tracer-labeled boutons appeared to contact immunolabeled (i.e., cholinergic) cells. The contacts occurred on cell bodies and dendrites. The results were similar following injections that spread across multiple auditory cortical areas or injections that were within primary auditory cortex. In order to confirm the anterograde results, in a second series of experiments, retrograde tracers were deposited in the pedunculopontine tegmental nucleus. These injections labeled layer V pyramidal cells in the auditory cortex. The results suggest an excitatory projection from primary auditory cortex bilaterally to cholinergic cells in the midbrain tegmentum. Such a pathway could allow auditory cortex to activate brainstem cholinergic circuits, possibly including the cholinergic pathways associated with arousal and gating of acoustic stimuli.

Sources of cholinergic input to the inferior colliculus

We combined retrograde tracing with immunohistochemistry for choline acetyltransferase to identify the source of cholinergic input to the inferior colliculus (IC) in guinea pigs. Injection of a retrograde tracer into one IC labeled cells in many brainstem nuclei. Retrogradely-labeled cells that were also immunoreactive for choline acetyltransferase were identified in two nuclei in the midbrain tegmentum: the pedunculopontine tegmental nucleus (PPT) and the laterodorsal tegmental nucleus (LDT). More PPT and LDT cells project ipsilaterally than contralaterally to the IC and, on both sides, there are more projecting cells in the PPT than in the LDT. Double-labeled cells were not found in any other brainstem nucleus. A common feature of cholinergic cells in PPT and LDT is collateral projections to multiple targets. We placed different retrograde tracers into each IC to identify cells in PPT and LDT that project to both ICs. In both PPT and LDT, a substantial proportion (up to 57%) of the immunoreactive cells that contained tracer from the contralateral IC also contained tracer from the ipsilateral IC. We conclude that acetylcholine in the IC originates from the midbrain tegmental cholinergic nuclei: PPT and LDT. These nuclei are known to participate in arousal, the sleep/wake cycle and prepulse inhibition of acoustic startle. It is likely that the cholinergic input to the IC is directly associated with these functions.

Distribution of cholinergic cells in guinea pig brainstem

We used an antibody to choline acetyltransferase (ChAT) to label cholinergic cells in guinea pig brainstem. ChAT-immunoreactive (IR) cells comprise several prominent groups, including the pedunculopontine tegmental nucleus, laterodorsal tegmental nucleus, and parabigeminal nucleus, as well as the cranial nerve somatic motor and parasympathetic nuclei. Additional concentrations are present in the parabrachial nuclei and superior colliculus. Among auditory nuclei, the majority of ChAT-IR cells are in the superior olive, particularly in and around the lateral superior olive, the ventral nucleus of the trapezoid body and the superior paraolivary nucleus. A discrete group of ChAT-IR cells is located in the sagulum, and additional cells are scattered in the nucleus of the brachium of the inferior colliculus. A group of ChAT-IR cells lies dorsal to the dorsal nucleus of the lateral lemniscus. A few ChAT-IR cells are found in the cochlear nucleus and the ventral nucleus of the lateral lemniscus. The distribution of cholinergic cells in guinea pigs is largely similar to that of other species; differences occur mainly in cell groups that have few ChAT-IR cells. The results provide a basis for further studies to characterize the connections of these cholinergic groups.

On the use of retrograde tracers for identification of axon collaterals with multiple fluorescent retrograde tracers

A common method for identifying collateral projections is to inject different retrograde tracers into two targets and examine labeled cells for the presence of both tracers. Double-labeled cells are considered to have collateral projections to the two injection sites. This method is widely considered to underestimate the extent of collaterals. To test the efficiency of double-labeling, we mixed equal volumes of two tracers, injected them into one site in a guinea-pig brain, and counted the resulting labeled cells. Ideally, the tracers would have precisely overlapping injection sites and all labeled cells would contain both tracers. We tested several combinations of tracers: 1) Fast Blue and fluorescein dextran; 2) fluorescein dextran and FluoroGold; 3) fluorescein dextran and FluoroRuby; 4) FluoroGold and green beads; 5) FluoroGold and red beads; 6) FluoroRuby and green beads; and, 7) green beads and red beads. For each combination, a mixture was injected into the left inferior colliculus. After 1 week to allow for transport, labeled cells were counted in the right inferior colliculus and the left temporal cortex. For each mixture, the results were similar for the two areas. The percentage of cells that were double-labeled varied from 0% to 100%, depending on tracer combination. The highest efficiencies (>96%) were observed with red beads and green beads or with FluoroRuby and fluorescein dextran. The limited efficiency of other mixtures could be accounted for only in part by incomplete overlap of the two tracers at the injection site. The results indicate that the specific combination of tracers used to search for collateral projections can greatly affect the findings.