Abstract
Many studies have demonstrated that vestibular sensory input is important for spatial learning and memory. However, it has been unclear what contributions the different parts of the vestibular system – the semi-circular canals and otoliths – make to these processes. The advent of mutant otolith-deficient mice has made it possible to isolate the relative contributions of the otoliths, the utricle and saccule. A number of studies have now indicated that the loss of otolithic function impairs normal spatial memory and also impairs the normal function of head direction cells in the thalamus and place cells in the hippocampus. Epidemiological studies have also provided evidence that spatial memory impairment with aging, may be linked to saccular function. The otoliths may be important in spatial cognition because of their evolutionary age as a sensory detector of orientation and the fact that velocity storage is important to the way that the brain encodes its place in space.
Highlights
The otolith organs in the vestibular inner ear, which comprise the utricle and saccule, represent the most ancient part of the vestibular system in evolutionary terms
Any role that the otoliths have in these processes is especially important since: aging is associated with reduced otolith function (e.g., Agrawal et al, 2012; Zu Eulenburg et al, 2017); impaired vestibular function and otolithic function, has been recently associated with an increased risk of cognitive impairment and dementia (Aranda-Moreno and Jáuregui-Renaud, 2016; Harun et al, 2016; Wei et al, 2017, 2019; Xie et al, 2017; Kamil et al, 2018, in press; see Agrawal et al, 2019, in press, for a review); and otolithic lesions can occur in humans independently of lesions of the semi-circular canals (e.g., Manzari et al, 2014)
galvanic vestibular stimulation (GVS) studies will not be discussed here. These studies suggest that the otolithic part of the peripheral vestibular system, the utricle and saccule, make some contribution to spatial learning and memory in tests such as the radial arm maze, Y maze and homing tasks, but not the Barnes task or non-spatial memory tests such as the object recognition test or elevated plus maze task
Summary
The otolith organs in the vestibular inner ear, which comprise the utricle and saccule, represent the most ancient part of the vestibular system in evolutionary terms. The maculae of the utricle and saccule are oriented at right angles to one another, and in the usual supine position of the head for humans, the saccule responds to changes in acceleration by gravity as the head is tilted relative to gravitational vertical (see Figure 1). In both the utricle and saccule, the hair cells, which are oriented in different directions, are activated by the inertial force exerted upon them by otoconia (calcium carbonate crystals) which sit above, on an epithelial layer (see Lim and Brichta, 2016 for a review; Figure 1).
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