Student Perceptions of Superimposed Mixed Reality Anatomy: A Bridge Between the Virtual and Physical.

A prototype mixed reality model was developed in which three-dimensional holograms of musculoskeletal anatomy were superimposed over a physical skeleton model, conferring tactile sensation to the bony attachments of virtually observed muscles. Fifty-three second-year medical students piloted this innovative format and provided feedback on its effectiveness as a learning modality.

Altered brainstem auditory evoked potentials in a rat central sensitization model are similar to those in migraine.

Migraine symptoms often include auditory discomfort. Nitroglycerin (NTG)-triggered central sensitization (CS) provides a rodent model of migraine, but auditory brainstem pathways have not yet been studied in this example. Our objective was to examine brainstem auditory evoked potentials (BAEPs) in rat CS as a measure of possible auditory abnormalities. We used four subdermal electrodes to record horizontal (h) and vertical (v) dipole channel BAEPs before and after injection of NTG or saline. We measured the peak latencies (PLs), interpeak latencies (IPLs), and amplitudes for detectable waveforms evoked by 8, 16, or 32 kHz auditory stimulation. At 8 kHz stimulation, vertical channel positive PLs of waves 4, 5, and 6 (vP4, vP5, and vP6), and related IPLs from earlier negative or positive peaks (vN1-vP4, vN1-vP5, vN1-vP6; vP3-vP4, vP3-vP6) increased significantly 2h after NTG injection compared to the saline group. However, BAEP peak amplitudes at all frequencies, PLs and IPLs from the horizontal channel at all frequencies, and the vertical channel stimulated at 16 and 32 kHz showed no significant/consistent change. For the first time in the rat CS model, we show that BAEP PLs and IPLs ranging from putative bilateral medial superior olivary nuclei (P4) to the more rostral structures such as the medial geniculate body (P6) were prolonged 2h after NTG administration. These BAEP alterations could reflect changes in neurotransmitters and/or hypoperfusion in the midbrain. The similarity of our results with previous human studies further validates the rodent CS model for future migraine research.

Slc4a11 gene disruption in mice: cellular targets of sensorineuronal abnormalities.

NaBC1 (the SLC4A11 gene) belongs to the SLC4 family of sodium-coupled bicarbonate (carbonate) transporter proteins and functions as an electrogenic sodium borate cotransporter. Mutations in SLC4A11 cause either corneal abnormalities (corneal hereditary dystrophy type 2) or a combined auditory and visual impairment (Harboyan syndrome). The role of NaBC1 in sensory systems is poorly understood, given the difficulty of studying patients with NaBC1 mutations. We report our findings in Slc4a11(-/-) mice generated to investigate the role of NaBC1 in sensorineural systems. In wild-type mice, specific NaBC1 immunoreactivity was detected in fibrocytes of the spiral ligament, from the basal to the apical portion of the cochlea. NaBC1 immunoreactivity was present in the vestibular labyrinth, in stromal cells underneath the non-immunoreactive sensory epithelia of the macula utricle, sacule, and crista ampullaris, and the membranous vestibular labyrinth was collapsed. Both auditory brain response and vestibular evoked potential waveforms were significantly abnormal in Slc4a11(-/-) mice. In the cornea, NaBC1 was highly expressed in the endothelial cell layer with less staining in epithelial cells. However, unlike humans, the corneal phenotype was mild with a normal slit lamp evaluation. Corneal endothelial cells were morphologically normal; however, both the absolute height of the corneal basal epithelial cells and the relative basal epithelial cell/total corneal thickness were significantly increased in Slc4a11(-/-) mice. Our results demonstrate for the first time the importance of NaBC1 in the audio-vestibular system and provide support for the hypothesis that SLC4A11 should be considered a potential candidate gene in patients with isolated sensorineural vestibular hearing abnormalities.

Binaural unmasking of frequency-following responses in rat amygdala.

Survival in natural environments for small animals such as rats often depends on precise neural coding of life-threatening acoustic signals, and binaural unmasking of species-specific pain calls is especially critical. This study investigated how species-specific tail-pain chatter is represented in the rat amygdala, which receives afferents from both auditory thalamus and auditory association cortex, and whether the amygdaloid representation of the chatter can be binaurally unmasked. The results show that chatter with a fundamental frequency (F0) of 2.1 kHz was able to elicit salient phase-locked frequency-following responses (FFRs) in the lateral amygdala nucleus in anesthetized rats. FFRs to the F0 of binaurally presented chatter were sensitive to the interaural time difference (ITD), with the preference of ipsilateral-ear leading, as well as showing features of binaural inhibition. When interaurally correlated masking noises were added and ipsilateral chatter led contralateral chatter, introducing an ITD disparity between the chatter and masker significantly enhanced (unmasked) the FFRs. This binaural unmasking was further enhanced by chemically blocking excitatory glutamate receptors in the auditory association cortex. When the chatter was replaced by a harmonic tone complex with an F0 of 0.7 kHz, both the binaural-inhibition feature and the binaural unmasking were preserved only for the harmonic of 2.1 kHz but not the tone F0. These results suggest that both frequency-dependent ascending binaural modulations and cortical descending modulations of the precise auditory coding of the chatter in the amygdala are critical for processing life-threatening acoustic signals in noisy and even reverberant environments.

Auditory frequency-following responses in rat ipsilateral inferior colliculus.

Auditory frequency-following responses (FFRs) are sustained potentials based on phase-locked neural activity preserving low-frequency information. Some neurons in rat inferior colliculus are excited by stimuli at either ear. This study shows that FFRs in inferior colliculus can be elicited by presenting pure tone bursts with frequencies from 225 to 4025 Hz at the ipsilateral ear in anesthetized rats. Moreover, chemical block of glutamate transmissions in the contralateral inferior colliculus markedly reduced the ipsilaterally driven FFRs, which, however, were significantly enhanced by blocking the contralateral dorsal nucleus of the lateral lemniscus. Thus, FFRs in inferior colliculus to ipsilateral stimulation were facilitated by excitatory projections from the contralateral inferior colliculus but suppressed by inhibitory projections from the contralateral dorsal nucleus of the lateral lemniscus.

Ultrasonic evoked responses in rat cochlear nucleus.

Numerous studies have reported auditory brainstem responses evoked by stimuli within the "normal" hearing range of rats, with maximum sensitivity peaking around 16 kHz. Yet rats also emit and respond to sounds in the ultrasonic (US) frequency range (30-100 kHz). However, very few electrophysiological studies have recorded auditory brainstem responses using US stimuli, and none have exceeded 70 kHz. We report here short-latency (1-3 ms) evoked potentials recorded in rat cochlear nucleus (CN) to US stimuli ranging from 40 to 90 kHz. Robust responses were recorded in 33 of 36 CN recording sites to stimuli ranging from 40 to 60 kHz; and twenty-eight of these sites continued to yield well-defined responses out to 90 kHz. Latencies systematically increased and overall amplitudes decreased with increasing US frequency. Amplitudes differed significantly in the three CN subnuclei, being largest in posterior-ventral (PVCN) and smallest in anterior-ventral (AVCN). The fact that well-defined responses can be recorded to stimuli as high as 90 kHz significantly extends the recorded upper frequency range of neural activity in the brainstem auditory pathway of the rat. These evoked potential results agree with the well-documented behavioral repertoire of rats in the US frequency range.

Neural crest cell deficiency of c-myc causes skull and hearing defects.

The proto-oncogene c-myc has a central role in multiple processes important for embryonic development, including cell proliferation, growth, apoptosis, and differentiation. We have investigated the role of c-myc in neural crest by using Wnt1-Cre-mediated deletion of a conditional mutation of the c-myc gene. c-myc deficiency in neural crest resulted in viable adult mice that have defects in coat color, skull frontal bone, and middle ear ossicle development. Physiological hearing studies demonstrated a significant hearing deficit in the mutant mice. In this report, we focus on the craniofacial and hearing defects. To further examine neural crest cells affected by c-myc deficiency, we fate mapped Wnt1-Cre expressing neural crest cells using the ROSA26 Cre reporter transgene. The phenotype obtained demonstrates the critical role that c-myc has in neural crest during craniofacial development as well as in providing a model for examining human congenital skull defects and deafness.

Auditory evoked responses in the rat: transverse mastoid needle electrodes register before cochlear nucleus and do not reflect later inferior colliculus activity.

A previously described technique putatively differentiates short-latency auditory evoked potentials in peripheral and central neural pathways of the mouse and rat [Galbraith G, Waschek J, Armstrong B, Edmond J, Lopez I, Liu W, et al. Murine auditory brainstem evoked response: putative two-channel differentiation of peripheral and central neural pathways. J Neurosci Methods 2006;153:214-20]. This technique involves recording from orthogonally oriented subdermal needle electrode pairs, using fast sample rates (100k/s) to accurately measure differences in neural timing and waveform morphology. Electrodes oriented in a transverse plane (mastoid-to-mastoid) register an initial positive-going peak earlier than peaks recorded from electrodes oriented along the scalp midline (anterior and posterior to the interaural line). The absolute latency of the early mastoid component is consistent with an origin in the primary auditory nerve, while delayed midline latencies implicate activity in central neural pathways. We report here the results of recording simultaneously from transverse mastoid (M) needle electrodes and electrodes acutely implanted in cochlear nucleus (CN) and inferior colliculus (IC). The results show a highly consistent pattern in which the initial mastoid component leads CN by an average of 0.16 ms, suggesting an obligatory neural site of origin of the mastoid response that is distal to IC, namely the auditory nerve. Moreover, later IC components (beyond approximately 3.5 ms) are completely absent in mastoid recordings, indicating that the transverse mastoid recordings provide a relatively isolated measure of early auditory neural activity.

Murine auditory brainstem evoked response: putative two-channel differentiation of peripheral and central neural pathways.

Standard noninvasive recordings of the auditory brainstem evoked response (ABR) from a single pair of obliquely oriented electrodes (typically midline vertex referenced to mastoid) confound inherently distinct signals propagating over peripheral and central neural pathways differing in location and spatial orientation. We describe here a technique for recording short-latency auditory evoked potentials that putatively differentiates peripheral and central neural activity in the mouse and rat. The technique involves recording from two orthogonally oriented electrode pairs using fast sample rates (100 k/s) to accurately measure differences in neural timing and waveform morphology. Electrodes oriented in a transverse plane (mastoid-to-mastoid) register an initial positive-going ABR peak (P1T) earlier than a series of peaks recorded from electrodes oriented along the midline (anterior and posterior to the inter-aural line). The absolute P1T latency is consistent with an origin in the primary auditory nerve, while the delayed midline latencies implicate activity farther along central neural pathways. Differences between these latencies (midline minus transverse) provide new and precise measures of central conduction time (CCT), which in one case is as brief as 0.10 ms. Results in wild type (WT) and knockout (KO) mice, as well as rats, show significant differences in absolute latencies as well as CCT.

Time course of auditory impairment in mice lacking the electroneutral sodium bicarbonate cotransporter NBC3 (slc4a7).

Mice with a targeted disruption of the gene encoding the stilbene-insensitive electroneutral sodium bicarbonate cotransporter (NBC3; slc4a7) exhibit cochlear and retinal degeneration. To establish the progressive nature of sensory cells loss in slc4a7-/- deficient mice, we studied the morphology of cochleas of slc4a7-/- and slc4a7+/+ mice from postnatal day two (P2) to ninety (P90). Cell death was evaluated in slc4a7-/- cochleas using the TUNEL technique and caspase-3 immunoreactivity. The time course of NBC3 expression in the cochlea was assessed by immunohistochemistry using an antibody against NBC3. Between P2 and P8, slc4a7-/- mice cochlea exhibit normal morphology. There was a normal complement of inner and outer hair cells from the hook to the apical region. At P15, slc4a7-/- mice cochlea inner and outer hair cells were still present at the hook region, and vacuoles were seen underneath Hensen's cells. At P21, inner and outer hair cells were degenerated in this region. Between P30 and P90, there was a pronounced loss of hair cells and spiral ganglia neurons. Morphological analysis of the spiral ligament showed a progressive loss of type II and IV fibrocytes beginning at day 21. Transmission electron microscopy observations at P30 and P90 revealed that type II and IV fibrocytes showed shrinkage and vacuolization. In addition, hair cells were deteriorated with evidence of shrinkage and picnotic nuclei. TUNEL staining showed apoptotic cells at P8 in the organ of Corti at the basal region of the cochlea. At P15, caspase-3 immunoreactivity was present in supporting cells of the organ of Corti. NBC3 mild immunoreactivity was detected in the organ of Corti at P11. There was an increase in the expression of NBC3 in the spiral ligament between P17 and P19. From P21 to P90, NBC3 expression was confined to the spiral ligament and inner and outer sulcus cells. The vestibular sensory epithelia from slc4a7-/- mice were normal from P2 to P90. Damage of the sensory epithelia at the high frequency zone of the cochlea suggests that NBC3 may play an important physiological role in this region.

Correlated brain stem and cortical evoked responses to auditory tone change.

Numerous human studies have separately observed the effects of auditory stimuli at brain stem and cortical levels, but little research has focused on possible functional coupling between these diverse brain areas. The present study recorded the cortical C-process [5] evoked by a pitch change between two successive tones, as well as the brain stem frequency-following response (FFR) evoked by each tone. The results replicated expected C-process component waveforms, including a late, negative (N2) component. FFR spectral intensity differences between the two tones were significantly correlated with N2 amplitude. These results suggest that signal processing reflected in long-latency auditory evoked response components is not exclusively a cortical phenomenon, but also depends upon patterns of neural processing occurring in brain stem pathways.

Brain stem evoked response to forward and reversed speech in humans.

Speech stimuli played in reverse are perceived as unfamiliar and alien-sounding, even though phoneme duration and fundamental voicing frequency are preserved. Although language perception ultimately resides in the neocortex, the brain stem plays a vital role in processing auditory information, including speech. The present study measured brain stem frequency-following responses (FFR) evoked by forward and reverse speech stimuli recorded from electrodes oriented horizontally and vertically to measure signals with putative origins in auditory nerve and rostral brain stem, respectively. The vertical FFR showed increased amplitude due to forward speech. It is concluded that familiar phonological and prosodic properties of forward speech selectively activate central brain stem neurons.

Blindness and auditory impairment caused by loss of the sodium bicarbonate cotransporter NBC3.

Normal sensory transduction requires the efficient disposal of acid (H+) generated by neuronal and sensory receptor activity. Multiple highly sensitive transport mechanisms have evolved in prokaryotic and eukaryotic organisms to maintain acidity within strict limits. It is currently assumed that the multiplicity of these processes provides a biological robustness. Here we report that the visual and auditory systems have a specific requirement for H+ disposal mediated by the sodium bicarbonate cotransporter NBC3 (refs. 7,8). Mice lacking NBC3 develop blindness and auditory impairment because of degeneration of sensory receptors in the eye and inner ear as in Usher syndrome. Our results indicate that in certain sensory organs, in which the requirement to transduce specific environmental signals with speed, sensitivity and reliability is paramount, the choice of the H+ disposal mechanism used is limited.

Selective attention affects human brain stem frequency-following response.

Selective attention modifies long-latency cortical event-related potentials. Amplitudes are typically enhanced and/or latencies reduced when evoking stimuli are attended. However, there is controversy concerning the effects of selective attention on short-latency brain stem evoked potentials. The objective of the present study was to assess possible attention effects on the brain stem auditory frequency-following response (FFR) elicited by a periodic tone. Young adult subjects heard a repetitive auditory stimulus while detecting infrequent target stimuli in either an auditory or visual detection task. Five channels of high frequency electroencephalographic (EEG) activity were recorded along the scalp midline with the center electrode positioned at the vertex. The FFR was elicited by the repetitive tone during both tasks. There were significant individual differences in the electrode sites yielding maximum response amplitudes, but overall FFR amplitudes were significantly larger during the auditory attention task. These results suggest that selective attention in humans can modify signal processing in sensory (afferent) pathways at the level of the brain stem. This may reflect top-down perceptual preprocessing mediated by extensive descending (efferent) pathways that originate in the cortex. Overall, the FFR appears to be a robust indicator of early auditory neural processing and shows effects not seen in brain stem auditory evoked response studies employing transient (click) acoustic stimuli.