FIGURE 6.

Phase locking of a single primary utricular neuron. (A,B) Time series of successive action potentials of the neuron to bone-conducted vibration (BCV) at 985 Hz. Panel (B) shows 142 action potentials superimposed and the onset of each successive action potential is shown by the arrow. The red trace shows the x channel of the 3D linear accelerometer. The neuron is locked to a narrow phase band of this stimulus. Panel (A) shows the circular histogram of the phases of the action potentials clustered around a mean of 129.1°, with angular deviation 28.7°. The test of circular uniformity (Rayleigh’s z), is highly significant showing the probability of a uniform phase distribution is <0.001. The neuron misses some cycles, but when it fires is locked to the stimulus waveform. Reproduced from Curthoys et al. (2019a) with permission of Elsevier. (C–F) Histograms of interspike intervals to show phase locking in the same utricular afferent neuron in guinea pig at two high frequencies of BCV (C,E) and air- conducted sound (ACS) stimuli (D,F). The bin width is 0.16 ms. The dots below each histogram show integral multiples of the period for the given stimulus frequency. The clustering around these integral multiples demonstrates phase locking at both frequencies. Reproduced from Grant and Curthoys (2017) with permission of Elsevier. (G–I) To show the differences in otolithic stimulation between the accelerometer and seismometer modes. (G) Show the gross schematic. (H) and (I) Show the otolithic macula in motion in relation to an inertial reference. In the low frequency accelerometer mode, not only is neuroepithelial layer (NEL) in motion, the otoconial layer (OL) s also in motion lagging behind the NEL due to its inertia, resulting in relative displacement between the two layers. This is also true for the otolith in the seismometer mode (I), where it is the NEL that is in motion with the OL remaining at rest or only slightly in motion, again due to its inertia (I).