Keith Darrow

Robustness of cortical topography across fields, laminae, anesthetic states, and neurophysiological signal types.

Topographically organized maps of the sensory receptor epithelia are regarded as cornerstones of cortical organization as well as valuable readouts of diverse biological processes ranging from evolution to neural plasticity. However, maps are most often derived from multiunit activity recorded in the thalamic input layers of anesthetized animals using near-threshold stimuli. Less distinct topography has been described by studies that deviated from the formula above, which brings into question the generality of the principle. Here, we explicitly compared the strength of tonotopic organization at various depths within core and belt regions of the auditory cortex using electrophysiological measurements ranging from single units to delta-band local field potentials (LFP) in the awake and anesthetized mouse. Unit recordings in the middle cortical layers revealed a precise tonotopic organization in core, but not belt, regions of auditory cortex that was similarly robust in awake and anesthetized conditions. In core fields, tonotopy was degraded outside the middle layers or when LFP signals were substituted for unit activity, due to an increasing proportion of recording sites with irregular tuning for pure tones. However, restricting our analysis to clearly defined receptive fields revealed an equivalent tonotopic organization in all layers of the cortical column and for LFP activity ranging from gamma to theta bands. Thus, core fields represent a transition between topographically organized simple receptive field arrangements that extend throughout all layers of the cortical column and the emergence of nontonotopic representations outside the input layers that are further elaborated in the belt fields.

Planar multipolar cells in the cochlear nucleus project to medial olivocochlear neurons in mouse

Medial olivocochlear (MOC) neurons originate in the superior olivary complex and project to the cochlea, where they act to reduce the effects of noise masking and protect the cochlea from damage. MOC neurons respond to sound via a reflex pathway; however, in this pathway the cochlear nucleus cell type that provides input to MOC neurons is not known. We investigated whether multipolar cells of the ventral cochlear nucleus have projections to MOC neurons by labeling them with injections into the dorsal cochlear nucleus. The projections of one type of labeled multipolar cell, planar neurons, were traced into the ventral nucleus of the trapezoid body, where they were observed terminating on MOC neurons (labeled in some cases by a second cochlear injection of FluoroGold). These terminations formed what appear to be excitatory synapses, i.e., containing small, round vesicles and prominent postsynaptic densities. These data suggest that cochlear nucleus planar multipolar neurons drive the MOC neurons response to sound. J. Comp. Neurol. 520:1365–1375, 2012.

Commissural axons of the mouse cochlear nucleus.

The axons of commissural neurons that project from one cochlear nucleus to the other were studied after labeling with anterograde tracer. Injections were made into the dorsal subdivision of the cochlear nucleus in order to restrict labeling only to the group of commissural neurons that gave off collaterals to, or were located in, this subdivision. The number of labeled commissural axons in each injection was correlated with the number of labeled radiate multipolar neurons, suggesting radiate neurons as the predominant origin of the axons. The radiate commissural axons are thick and myelinated, and they exit the dorsal acoustic stria of the injected cochlear nucleus to cross the brainstem in the dorsal half, near the crossing position of the olivocochlear bundle. They enter the opposite cochlear nucleus via the dorsal and ventral acoustic stria and at its medial border. Reconstructions of single axons demonstrate that terminations are mostly in the core and typically within a single subdivision of the cochlear nucleus. Extents of termination range from narrow to broad along both the dorsoventral (i.e., tonotopic) and the rostrocaudal dimensions. In the electron microscope, labeled swellings form synapses that are symmetric (in that there is little postsynaptic density), a characteristic of inhibitory synapses. Our labeled axons do not appear to include excitatory commissural axons that end in edge regions of the nucleus. Radiate commissural axons could mediate the broadband inhibition observed in responses to contralateral sound, and they may balance input from the two ears with a quick time course. J. Comp. Neurol. 521:1683–1696, 2013.

Social Media Utilization in the Cochlear Implant Community

BACKGROUND More than 200,000 individuals worldwide have received a cochlear implant (CI). Social media Websites may provide a paramedical community for those who possess or are interested in a CI. The utilization patterns of social media by the CI community, however, have not been thoroughly investigated. PURPOSE The purpose of this study was to investigate participation of the CI community in social media Websites. RESEARCH DESIGN We conducted a systematic survey of online CI-related social media sources. Using standard search engines, the search terms cochlear implant, auditory implant, forum, and blog identified relevant social media platforms and Websites. Social media participation was quantified by indices of membership and posts. STUDY SAMPLE Social media sources included Facebook, Twitter, YouTube, blogs, and online forums. Each source was assigned one of six functional categories based on its description. INTERVENTION No intervention was performed. DATA COLLECTION AND ANALYSIS We conducted all online searches in February 2014. Total counts of each CI-related social media source were summed, and descriptive statistics were calculated. RESULTS More than 350 sources were identified, including 60 Facebook groups, 36 Facebook pages, 48 Twitter accounts, 121 YouTube videos, 13 forums, and 95 blogs. The most active online communities were Twitter accounts, which totaled 35,577 members, and Facebook groups, which totaled 17,971 members. CI users participated in Facebook groups primarily for general information/support (68%). Online forums were the next most active online communities by membership. The largest forum contained approximately 9,500 topics with roughly 127,000 posts. CI users primarily shared personal stories through blogs (92%), Twitter (71%), and YouTube (62%). CONCLUSIONS The CI community engages in the use of a wide range of online social media sources. The CI community uses social media for support, advocacy, rehabilitation information, research endeavors, and sharing of personal experiences. Future studies are needed to investigate how social media Websites may be harnessed to improve patient-provider relationships and potentially used to augment patient education.

Histologic Analysis of Chronos in an Optogenetic-Based Auditory Brainstem Implant Model

Objectives: Optogenetics affords the potential for improved spatial resolution compared with electric stimulation in future auditory neuroprostheses. No histologic studies have yet examined the virally-mediated gene transfer of Chronos, a new opsin, to the murine cochlear nucleus (CN). Herein, we aim to (1) identify CN regions and neurons receptive to gene transfer of Chronos and (2) describe the morphology of putative cell types that express Chronos. Methods: CBA/CaJ mice underwent CN-targeted injection of Chronos. The Chronos construct consisted of an adeno-associated viral vector (AA2/8), CAG promoter, and a fluorescent marker. Following a 4-week incubation period, mice were sacrificed and intravascularly fixed with paraformaldehyde, and brains were extracted, sucrose cryoprotected, and cryostatically sectioned. Sections of 35-µm thickness were co-labeled with neuron-specific markers microtubule associated protein-2 and anti-tubulin, beta III isoform and DAPI-fluoromounted. Sections of 60-µm thickness were DAPI-fluoromounted, and confocal microscopy revealed cellular morphologies. Results: Opsin-linked fluorescence demonstrates Chronos expression throughout the dorsal CN with contiguous extension routinely into the ventral CN and variably into the auditory nerve and inferior cerebellar peduncle. Chronos localizes to neuronal-specific and nonneuronal populations. Confocal microscopy suggests involvement of a wide array of CN cell types, including morphologies consistent with pyramidal cells and giant cells. Conclusions: Our histologic analyses confirm widespread infection of multiple neuronal populations throughout the CN. This work sets the stage for correlation with ongoing neurophysiology experiments. Future work with CN-specific promoters to target neuronal subpopulations may further improve clinical potential for an optogenetics-based auditory neuroprosthesis.