Neurophysiology of the optokinetic system

This chapter provides a review of early studies into the neural substrate for optokinetic-vestibular responses. Properties and connections of retinal and brainstem neurons contributing to optokinetic responses in the afoveate rabbit are summarized. Electrophysiological and lesion studies provide support for confluence of optokinetic and vestibular signals in the vestibular nucleus to provide the brain's estimate of self-rotation. Evidence for optokinetic-vestibular symbiosis in humans comes from the observation that individuals who have lost vestibular function show no optokinetic after-nystagmus in darkness, following full-field stimulus motion. An anatomical scheme for brainstem elaboration of optokinetic responses is proposed and cerebellar contributions are reviewed.

Paediatric auditory brainstem implantation: The South Asian experience

INTRODUCTION

Paediatric Auditory Brainstem Implantation (ABI) is indicated for children with congenital cochlear aplasia, absent/hypoplastic vestibulocochlear nerve, for whom cochlear implantation is not possible. Knowledge of the anatomical landmarks and variants in anatomy of the brainstem is vital for ABI surgery.

METHOD

Study was done at Auditory implant centre in Madras ENT research foundation, which includes 24 children who had undergone ABI surgery and are being followed up for 1 year, post operatively. Aims were to study the anatomical variants and the outcomes of ABI implantation. To determine if different anatomical variants effect placement of ABI electrode. To assess the patient outcomes by Categories of auditory Performance (CAP) scores and Speech Intelligibility Ratings (SIR) scores.

RESULTS

All the candidates showed gradual improvement in audiological and verbal outcomes after the ABI. The mean CAP and SIR scores after 6 months of AVHT were 2.07 and 1.37 respectively. After 1 year of auditory verbal rehabilitation therapy CAP was 3.42 and SIR was 2.33. Flocculus of the cerebellum can be of different grades. Though, there was difficulty in insertion of the electrode in subjects with anatomical variants, the outcomes were comparable with other subjects.

CONCLUSION

ABI surgery involves frequent anatomical variations surrounding the lateral recess which makes the positioning of the auditory prosthesis difficult. Variants during the surgery can make the placement of ABI electrodes difficult, but promising results were seen all the implantees.

Computational Theory Underlying Acute Vestibulo-ocular Reflex Motor Learning with Cerebellar Long-Term Depression and Long-Term Potentiation

The vestibulo-ocular reflex (VOR) can be viewed as an adaptive control system that maintains compensatory eye movements during head motion. As the cerebellar flocculus is intimately involved in this adaptive motor control of the VOR, the VOR has been a popular model system for investigating cerebellar motor learning. Long-term depression (LTD) and long-term potentiation (LTP) at the parallel fiber-Purkinje cell synapses are considered to play major roles in cerebellar motor learning. A recent study using mutant mice demonstrated cerebellar motor learning with hampered LTD; the study concluded that the parallel fiber-Purkinje cell LTD is not essential. More recently, multiple forms of plasticity have been found in the cerebellum, and they are believed to contribute to cerebellar motor learning. However, it is still unclear how synaptic plasticity modifies the signal processing that underlies motor learning in the flocculus. A computational simulation suggested that the plasticity present in mossy fiber-granule cell synapses improves VOR-related sensory-motor information transferred into granule cells, whereas the plasticity in the molecular layer stores this information as a memory under guidance from climbing fiber teaching signals. Thus, motor learning and memory are thought to be induced mainly by LTD and LTP at parallel fiber-Purkinje cell synapses and by rebound potentiation at molecular interneuron-Purkinje cell synapses among the multiple forms of plasticity in the cerebellum. In this study, we focused on the LTD and LTP at parallel fiber-Purkinje cell synapses. Based on our simulation, we propose that acute VOR motor learning accomplishes by simultaneous enhancement of eye movement signals via LTP and suppression of vestibular signals via LTD to increase VOR gain (gain-up learning). To decrease VOR gain (gain-down learning), these two signals are modified in the opposite directions; namely, LTD suppresses eye movement signals, whereas LTP enhances vestibular signals.

Does cerebellar flocculus size affect subjective outcomes in pediatric auditory brainstem implantation

OBJECTIVES

The objectives of study was to 1) Describe relevant surgical anatomy in defining and accessing the lateral recess for placement of electrode, 2) Propose a working classification for grades of Flocculus; 3) To determine if different grades of cerebellar flocculus effects placement of ABI electrode and subjective outcomes in implantees.

METHODS

Our study was a prospective study, and comprised of cohort of 12 patients who underwent ABI surgery via retrosigmoid approach between 1 Jan 2012 to 31 Dec 2014. All children with congenital profound sensorineural hearing loss with either absent cochlea or cochlear nerve were included in the study. Relevant anatomy was noted. We also noted down the difficulty encountered during the placement of ABI electrode. Auditory perception and speech intelligibility was scored post operatively for 1 year.

RESULTS

Cerebellar flocculus was divided into 4 grades depending on the morphology of cerebellar flocculus. It was noted that Grade 3 & 4 flocculus (Group B) had difficult ABI electrode placement in comparison to Grade 1 & 2 flocculus (Group A). The subjective outcomes of Group A was better than Group B. However the p value was not statistically significant.

CONCLUSION

Cerebellar flocculus can be graded depending on morphology and size. Flocculus of higher grades can make the placement of ABI electrodes difficult and adversely effects the postoperative subjective outcomes.