Unit responses to bidirectional off-vertical axes rotations in central vestibular and cerebellar fastigial nuclei

Spatiotemporal response properties of cerebellar Purkinje cells to animal displacement: a population analysis

The hypothesis that corticocerebellar units projecting to vestibulospinal neurons contribute to the spatiotemporal response characteristics of forelimb extensors to animal displacement was tested in decerebrate cats in which the activity of Purkinje cells and unidentified cells located in the cerebellar anterior vermis was recorded during wobble of the whole animal. This stimulus imposed to the animal a tilt of fixed amplitude (5 degrees) with a direction moving at a constant angular velocity (56.2 degrees/s), both in the clockwise and counterclockwise directions over the horizontal plane. Eighty-three percent (143/173) of Purkinje cells and 81% (42/52) of unidentified cells responded to clockwise and/or counterclockwise rotations. In particular, 116/143 Purkinje cells (81%) and 32/42 unidentified cells (76%) responded to both clockwise and counterclockwise rotations (bidirectional units), while 27/143 Purkinje cells (19%) and 10/42 unidentified cells (24%) responded to wobble in one direction only (unidirectional units). For the bidirectional units, the direction of maximum sensitivity to tilt (Smax) was identified. Among these units, 24% of the Purkinje cells and 26% of the unidentified cells displayed an equal amplitude of modulation during clockwise and counterclockwise rotations, indicating a cosine-tuned behavior. For this unit type, the temporal phase of the response to a given direction of tilt should remain constant, while the sensitivity would be maximal along the Smax direction, declining with the cosine of the angle between Smax and the tilt direction. The remaining bidirectional units, i.e. 57% of the Purkinje cells and 50% of the unidentified cells displayed unequal amplitudes of modulation during clockwise and counterclockwise rotations. For these neurons, a non-zero sensitivity along the null direction is expected, with a response phase varying as a function of stimulus direction. As to the unidirectional units, their responses to wobble in one direction predict equal sensitivities along any tilt direction in the horizontal plane and a response phase that changes linearly with the stimulus direction. By comparing these data with those obtained previously during selective stimulation of macular receptors by a 5 degrees off-vertical axis rotation, it appeared that the directions of maximum sensitivity to tilt were distributed over the whole horizontal plane of stimulation, in both conditions. However, co-stimulation of macular and canal receptors during wobble decreased the proportion of unidirectional units, while that of the bidirectional, namely broadly tuned units, increased. In addition, while the average gain of the Smax vector of the bidirectional units was comparable, the temporal phase of this vector tended to show a more prominent phase leading behavior during wobble with respect to off-vertical axis rotation. The possibility that the tested neurons formed a population which coded the direction of head tilt in space was also investigated using a modified version of the classical population vector analysis. It was shown that for each selected time in the tilt cycle the direction of the population vector closely corresponded to that of the head tilt, while its amplitude was related to that of the stimulus. We conclude that the broad distribution of the response vector orientation of units located in the cerebellar anterior vermis represents an appropriate substrate for the cerebellar control of vestibulospinal reflexes involving extensor muscles during a variety of head tilts. In view of their efferent projections to the vestibular and fastigial nuclei, the cerebellar anterior vermis may provide a framework for the spatial coding of vestibular inputs, giving equal emphasis to both side-to-side and fore-aft stability.

Spontaneous activity of otolith-related vestibular nuclear neurons in the decerebrate rat

The discharge properties of lateral and descending vestibular neurons responsive to constant velocity off-vertical axis rotations (OVAR) in the clockwise (CW) and counterclockwise (CCW) directions, were studied at the stationary and earth-horizontal position of decerebrate adult rats. From the coefficient of variation (CV), the spontaneous activities of OVAR-responsive neurons were classified into regular and irregular patterns. Of the neurons (n = 36) that showed symmetric and stable bidirectional response sensitivity (delta defined as CW gain over CCW gain) to OVAR (10 degrees tilt), some exhibited progressive phase shift with velocity (1.75-15 degrees/s) while others exhibited stable response phase. Most neurons of the former group (93% or 12/13) showed regular discharge pattern while only 22% (n = 5/23) of the latter group showed such a pattern. Though the phase-stable neurons showed a significantly higher average CV than the phase-shifted neurons, there was no significant difference between the mean spontaneous firing rates of these neurons. The neurons (n = 17) that showed asymmetric and variable delta to OVAR velocity can also be grouped-those that exhibited a greater gain with rotations directed towards the side of recording (I neurons) showed irregular discharge pattern while those that exhibited a greater gain with rotations directed towards the side contralateral to recording (C neurons) showed regular discharge pattern. The I and C neurons also exhibited significant difference in mean firing rates. The relationship between the response characteristics of the OVAR-responsive neurons and their spontaneous activity at the stationary and earth-horizontal position is discussed.

Response of medial medullary reticular neurons to otolith stimulation during bidirectional off-vertical axis rotation of the cat

In decerebrate cats, the extracellular activities of neurons in the medial medullary reticular formation were studied during constant velocity off-vertical axis rotations (OVAR) in the clockwise (CW) and counterclockwise (CCW) directions (at 10 degrees tilt). Spontaneously active neurons demonstrated sinusoidal position-dependent discharge modulations to OVAR which selectively stimulates the otoliths. Two features of neuronal responses to bidirectional OVAR were identified. Within the velocity spectrum tested (1.75-15 degrees/s), some neurons showed symmetric bidirectional response sensitivity (delta value) to CW and CCW rotations. The spread of the delta values of each of these neurons with velocity was small. This group of reticular neurons were described as exhibiting symmetric and velocity-stable bidirectional response sensitivity. The mathematically derived gain tuning ratios of these neurons were within the range of narrowly spatiotemporal-tuned neurons. Another group of reticular neurons, however, showed asymmetric bidirectional response sensitivity to CW and CCW rotations; a few of these neurons were responsive only to OVAR of one direction but not to both. For each of this second group of neurons, the spread of the delta values with velocity was large. These reticular neurons were described as exhibiting asymmetric and velocity-variable bidirectional response sensitivity. The gain tuning ratios of these latter neurons were found to be within the range of broadly spatiotemporal-tuned neurons. Single neurons of both groups displayed orientational tuning. Both the best response orientations of neurons that showed symmetric and velocity-stable bidirectional response sensitivity and the preferred orientations of neurons that showed asymmetric and velocity-variable bidirectional response sensitivity were found to point in all directions on the rotary plane. The response dynamics of the former group of neurons was also examined. All showed flat response gain across the entire velocity range. Some showed a flat response lead while others showed a progressive shift from small response lead at low velocity to phase close to zero at higher velocities. The functional significance of these medial medullary reticular neurons to the direction and orientation of head tilt is discussed.

Responses of Purkinje cells in the cerebellar anterior vermis to off-vertical axis rotation

Responses of 67 Purkinje cells (P-cells) and 44 unidentified neurons (U-cells) located in the cerebellar anterior vermis were recorded in decerebrate cats during off-vertical axis rotation (OVAR). This stimulus consisted of a slow constant velocity (9.4%/s) rotation in the clockwise (CW) and counterclockwise (CCW) directions around an axis inclined by 5 degrees with respect to the vertical. OVAR imposes on the animal head a 5 degrees tilt, whose direction changes continuously over the horizontal plane, thus eliciting a selective stimulation of macular receptors. A total of 27/67 P-cells (40%) and 24/44 U-cells (55%) responded to both CW and CCW rotations. For these bidirectional units, the direction of maximum sensitivity to tilt (Smax) could be identified. Smax directions were distributed over the whole horizontal plane of stimulation. Among bidirectional neurons, 48% of the P-cells and 33% of the U-cells displayed an equal amplitude of modulation during CW and CCW rotations, indicating a cosine-tuned behaviour. In these instances, the temporal phase of the unit response to a given direction of tilt remained constant, while the sensitivity was maximal along the Smax direction and declined with the cosine of the angle between Smax and the tilt direction. The remaining bidirectional units displayed unequal amplitudes of modulation during CW and CCW rotations. For these neurons, a nonzero sensitivity along the null direction was expected and the response phase varied as a function of stimulus direction. Finally, 31% and 23% of P-cells and U-cells, respectively, responded during OVAR in one direction only (unidirectional units). This behaviour predicts equal sensitivities along any tilt direction in the horizontal plane and a response phase that changes linearly with the stimulus direction. The posibility that the tested neurons formed a population which coded the direction of head tilt in space was also investigated. The data from the whole population of cells were analysed using a modified version of vectorial analysis. This model assumes that for a particular tilt each cell makes vectorial contributions; the vectorial sum of these contributions represent the outcome of the population code and points in the direction of head tilt in space. Thus, a dynamic head tilt along four representative directions was simulated. For each of the four directions, 12 population vectors were calculated at regular time intervals so as to cover an entire cycle of head tilt. The results indicate that for each selected time in the cycle the direction of the population vector closely corresponded to that of the head tilt, while its amplitude was related to the amount of head tilt. These data were particularly obtained for the P-cells. In view of their efferent connections, the cerebellar anterior vermis may provide a framework for the spatial organization of vestibulospinal reflexes induced by stimulation of otolith receptors.

Neuronal responses in the y group nucleus of unilaterally labyrinthectomized cats during off-vertical axis rotations

In cats which were decerebrated and unilaterally labyrinthectomized on the day of the experiment, the responses of neurons in the y group nucleus on the lesioned side were studied during constant velocity off-vertical axis rotations, which selectively stimulated the remaining intact otolith receptors. During 360 degrees rotations, neurons exhibited position-dependent modulation in discharge rate, thus showing that the y group neurons receive signals of head positional changes from the contralateral otoliths. Most y group neurons had their best response orientations within +/- 45 degrees of the head-up and head-down positions (i.e. antero-posterior or pitch direction) in the plane of rotation studied. This implies that the spatial coding property of these otolith neurons enables them to be particularly catered for small head tilts about the sagittal plane of the animal.

Response of medullary reticular neurons of cat to off-vertical axis rotations

To investigate whether neurons in the medullary reticular formation of decerebrate cats can encode spatial orientation during 360 degrees off-vertical axis rotations (OVAR), responses of these units were studied during constant velocity OVAR in clockwise (CW) and counter-clockwise (CCW) directions. Tilt-sensitive units, some of which were activated by antidromic stimulation of the thoracic cord, were found to respond to 360 degrees OVAR with position-and direction-dependent modulation in discharge rates. For the individual responsive unit there was a difference in the spatial location of the CW and CCW discharge maxima, while a comparable response gain was obtained for rotations in either direction. The orientation of the best response of each unit was not significantly modified by changes in the amplitude of head tilt and/or velocity of rotation. Such characterized directional axes of the medullary reticular units were found to have a fairly equal distribution over the 360 degrees. The role that neurons in the medullary reticular formation plays in encoding otolith inputs with respect to head movements is discussed.