Recordings from CVN neurons in labyrinth-intact animals
The spatial and dynamic properties of the responses of 85 CVN neurons to vertical tilts were fully characterized in three vestibular-intact animals. illustrates averaged responses of one cell to rotations in vertical planes. shows a subset of the traces used to determine the response vector orientation for the unit, which was calculated to be near nose down pitch based on the responses to CW and CCW wobble stimuli. The response vector orientation was confirmed by considering the relative gains of responses to tilts in single fixed planes. For example, pitch rotations elicited robust responses (signal-to-noise ratio of 1.3; gain of 12.4 spikes/sec/°), whereas roll tilts elicited little modulation of the neuron's firing (signal-to-noise ratio of 0.3; gain of 1.9 spikes/sec/°), indicating that the response vector orientation was much closer to the pitch plane than the roll plane. illustrates the responses of the unit to pitch rotations performed at frequencies of 0.05-1 Hz. The response phases led nose down pitch by > 90° at all frequencies, and the gain of the response elicited by 1 Hz rotations was seven times larger than the gain of the response to 0.1 Hz tilts.
contains Bode plots that illustrate the dynamic properties of CVN neuronal responses to rotations in fixed planes near the response vector orientation. Both response gain and phase are plotted with respect to stimulus position, such that a response whose phase leads stimulus position by 90° is synchronous with stimulus velocity. By considering differences in the phases of stimuli and responses across frequencies, all units could be subdivided into four types. Neurons were classified as having ‘graviceptive’ properties if the response phase was within 40° of stimulus position at 0.1 Hz, and either remained near or lagged stimulus position as the stimulus frequency increased (see ). ‘Phase advancing’ units exhibited a response phase that was within 40° of stimulus position at 0.1 Hz or lower frequencies, but that advanced >50° as the stimulus frequency was increased to 0.5-1 Hz (see ). ‘Phase lagging’ cells had response phases within 30° of stimulus velocity at 0.1 Hz or lower frequencies, but which lagged this value by >60° as the stimulus frequency was increased to 0.5-1 Hz (see ). Neurons whose response phases lagged stimulus velocity by no more than 30° across the frequency range of 0.1-1 Hz were described as having ‘velocity’ responses (see ). In total, 19/85 of the CVN neurons examined (22%) were classified as having graviceptive responses, 5 or 6% had phase advancing responses, 14 or 16% had phase lagging responses, and 47 or 55% had velocity responses.
Figure 2 Bode plots illustrating the dynamic properties of CVN neuronal responses to rotations at multiple frequencies. Response gain and phase are plotted relative to stimulus position. Thin gray lines designate responses of individual neurons, whereas thick (more ...)
indicates the relative response gain per stimulus decade (usually over the frequency range of 0.1-1 Hz, calculated by dividing the gain of the response to the high frequency stimulus by the gain of the response to the low frequency stimulus) for the 4 types of CVN neurons. On average, the relative gain per stimulus decade for graviceptive neurons was only moderate: a 3.2±0.3-fold increase. However, the relative gain per stimulus decade for phase-lagging and velocity neurons was much larger: 9.9±1.6-fold and 9.0±0.8-fold increases, respectively. A nonparametric one-way ANOVA (Kruskal-Wallis test) combined with Dunn's multiple comparison post-test revealed that the relative gain per decade for graviceptive neurons was significantly smaller than for phase lagging or velocity cells (p<0.001). The modest response gain increases with advancing stimulus frequency exhibited by graviceptive neurons is consistent with these cells signaling body position in space.
Figure 3 Relative gain per stimulus decade (usually over the range of 0.1-1 Hz) of different types of CVN neurons. Symbols indicate for each neuron the ratio of the gain of the response to the high frequency stimulus to the gain of the response to the low frequency (more ...)
The response vector orientations for the different types of CVN neurons are shown in , whereas summarizes whether the vector orientations were closest to the roll plane, the pitch plane, or the plane of one of the vertical semicircular canals. further indicates whether the response vector orientations were directed ipsilaterally or contralaterally with respect to the side of the brain from which recordings were made. When all neurons were considered together, the response vector orientations were typically closest to the plane of one of the vertical semicircular canals (50 units, 59%) or the roll plane (20/85 units, 24%), although 15 cells (18%) had response vector orientations closest to the pitch plane. Furthermore, 61% (52/85 cells) of the response vector orientations were directed towards the ipsilateral side.
Figure 4 Polar plots showing the response vector orientations and gains for CVN neurons, determined using wobble stimuli that were usually delivered at 0.5 Hz. Response vector orientations for different neuronal types (A,graviceptive; B,phase leading; C,phase (more ...)
Table 1 Number of CVN units of different types with response vector orientations closest to roll, pitch, or vertical canal planes. Neurons with response vector orientations directed ipsilateral or contralateral to the side where recordings were performed are (more ...)
Neurons with graviceptive responses were particularly likely to exhibit response vector orientations that were closest to the vertical canal planes: 13/19 or 68% of the neurons had such a characteristic. Similarly, the response vector orientations of 30/47 or 64% of the velocity neurons were closer to the vertical canal planes than the roll or pitch planes. Moreover, the response vector orientations of many of the velocity neurons (22) were almost directly aligned (within 10°) with the plane of a vertical semicircular canal. In contrast, half (7/14) of the phase lagging neurons had response vector orientations closest to the roll plane.
Some vestibular nucleus neurons, which we refer to as spatiotemporal convergence (STC) units, respond as though they receive vestibular inputs from receptors with differing spatial and frequency components (such as graviceptive and velocity responses with different spatial orientations) (Baker et al. 1984
; Kasper et al. 1988
; Schor and Angelaki 1992
). The response vector orientations for such cells vary as a function of tilt frequency; furthermore, the gains of responses to CW and CCW wobble rotations are usually significantly different (Kasper et al. 1988
; Schor and Angelaki 1992
). To determine if STC neurons are present in the CVN, the ratio of the gain of the responses to CW and CCW wobble stimulation was determined for the highest frequency rotations employed for a unit (usually 0.5 Hz), where the STC response is expected to be most evident. For the large majority of cells (79/85), the ratios were < 2:1 (i.e., no less than 0.5 and no larger than 2.0); the largest ratio was 2.6:1. In addition, wobble stimuli were delivered at two or more frequencies between 0.05 and 0.5 Hz for 23 neurons. These values changed no more than 23° for any cell, and typically the variability in response vector orientation was much smaller: the median was 8°. Thus, it appears that few CVN neurons exhibit robust STC behavior.
The locations of the CVN neurons that responded to vertical rotations are illustrated in ; the locations are plotted on a horizontal section through the caudal vestibular nuclei. Approximately 34% of the neurons were located in the medial vestibular nucleus, whereas 76% were situated in the inferior vestibular nucleus. Neurons with specific response characteristics did not appear to be clustered in a particular region of the CVN.
Figure 5 Locations where activity was recorded from CVN neurons, plotted on a horizontal section through the caudal half of the vestibular nucleus complex. A: recording locations in vestibular-intact animals; each unit type is designated by a different symbol. (more ...)
Recordings from CVN neurons in animals lacking labyrinthine inputs
Recordings were made from CVN neurons in 3 animals following a bilateral vestibular neurectomy; the recording locations are shown in . In one case (animal 1), the recordings were initiated two weeks following the elimination of vestibular inputs and continued for 2 months. In the other two cats (animals 2 and 3), data collection started the day after the vestibular neurectomies were performed and continued for one month. Every spontaneously-active neuron that was encountered and could be held for extended periods was tested for responses to wobble rotations up to 15° in amplitude delivered at 0.05-0.1 Hz. In animal 2, 6/79 neurons responded to 15° wobble rotations, but the activity of only 1/35 neurons in animal 3 and 0/54 neurons in animal 1 was modulated by 15° wobble stimuli. The signal-to-noise ratios were typically very low for responses of neurons whose activity was not modulated by 15° wobble rotations: the average values were 0.24±0.02 in animal 1, 0.22±0.1 in animal 2, and 0.21±0.03 in animal 3.
The response characteristics of the 7 neurons whose activity was consistently modulated by vertical rotations in animals lacking vestibular inputs are shown in . shows examples of the responses of one neuron to 15° pitch rotations; the recordings were performed three days following the bilateral vestibular neurectomy. Bode plots indicating the response dynamics of all of the units are provided in . All of the neurons could be classified as graviceptive, as their response phases were near stimulus position at low stimulus frequencies, and either remained near or lagged position as stimulus frequency increased. In addition, the response gains were relatively consistent across stimulus frequencies; on average, the response gain only increased 1.4±0.2-fold per stimulus decade. The response vector orientations for the units are shown in . Six of the neurons had response vector orientations that were closer to the pitch plane than the roll or vertical canal planes, although the remaining cell was preferentially activated by roll rotations.
Figure 6 Characteristics of neuronal responses to vertical rotations following a bilateral vestibular neurectomy. A: examples of the responses of one neuron to 15° pitch rotations at frequencies of 0.02-0.2 Hz. The number of sweeps averaged to generate (more ...)
Despite the fact that few CVN neurons responded to moderate-amplitude vertical tilts in animals lacking vestibular inputs, many cells quickly regained spontaneous activity following a bilateral vestibular neurectomy. A caveat is that we were not able to detect silent neurons, and thus could not ascertain whether the fraction of neurons that lacked spontaneous discharges changed following the peripheral vestibular lesions. Recordings resumed the day after the bilateral vestibular neurectomy in animals 2 and 3, when considerable spontaneous activity could be detected in the vestibular nuclei. For animal 2, the mean firing rates for all units examined were 31±2 spikes/s when the labyrinths were intact, 47±6 spikes/s in the first week after the bilateral vestibular neurectomy, and 29±2 spikes/s subsequently. In animal 3, the spontaneous firing rates were as follows: 20±1 spikes/s before the vestibular neurectomies, 25±4 spikes/s in the first week after the surgery, and 33±4 spikes/s during the following three weeks. Recordings were not initiated in animal 1 until two weeks subsequent to the peripheral vestibular lesions, when the mean firing rate of CVN units was 17±2 spikes/s. It is unclear why the firing rates of CVN units varied between animals, although this could be due to differences in the subregions of the CVN that were sampled. Removal of vestibular inputs did not result in any significant changes (p>0.4, Mann Whitney test) in the regularity of neuronal firing. In animal 2, the mean CV of spontaneous firing rate was 0.77±0.06 before the removal of vestibular inputs and 0.76±0.05 subsequently. In animal 3, the average CV of firing rate was 0.86±0.05 when the labyrinths were intact and 0.91±0.07 following the bilateral vestibular neurectomy.
Histological confirmation of peripheral vestibular lesions
A histological analysis of temporal bone sections was performed for animals 1 and 2 to determine whether the labyrinthectomy and eighth cranial nerve transections were complete. In both of the cases, we confirmed that the vestibular labyrinth had been opened, thereby producing a functional lesion by permitting the perilymph and endolymph to escape. Furthermore, both eighth cranial nerves were completely severed; the cut ends of the nerves were degenerated and surrounded with glial scars, and all vestibular endorgans appeared to be necrotic.