suggestion that adequate stimulation of the semicircular canals may contribute

Transcription

1 J. Phyiiol. (1970), 210, pp With 8 text-ftgures Printed in Great Britain RESPONSE OF SEMICIRCULAR CANAL DEPENDENT UNITS IN VESTIBULAR NUCLEI TO ROTATION OF A LINEAR ACCELERATION VECTOR WITHOUT ANGULAR ACCELERATION BY A. J. BENSON,* F. E. GUEDRYt AND G. MELVILL JONES From the Defence Research Board Aviation Medical Research Unit, Department of Physiology, McGill University, Montreal, Canada (Received 16 March 1970) SUMAARY 1. Recent experiments have shown that rotation of a linear acceleration vector round the head can generate involuntary ocular nystagmus in the absence of angular acceleration. The present experiments examine the suggestion that adequate stimulation of the semicircular canals may contribute to this response. 2. Decerebrate cats were located in a stereotaxic device on a platform, slung from four parallel cables, which could be driven smoothly round a circular orbit without inducing significant angular movement of the platform. This Parallel Swing Rotation (PSR) generated a centripetal acceleration of 4-4 m/sec2 which rotated round the head at 052 rev/sec. 3. The discharge frequency of specifically lateral canal-dependent neural units in the vestibular nuclei of cats was recorded during PSR to right and left, and in the absence of motion. The dynamic responses to purely angular motion were also examined on a servo-driven turntable. 4. Without exception all proven canal-dependent cells examined (twenty-nine cells in nine cats) were more active during PSR in the direction of endolymph circulation assessed to be excitatory to the unit, than during PSR in the opposite direction. 5. The observed changes in discharge frequency are assessed to have been of a magnitude appropriate for the generation of the involuntary oculomotor response induced by the same stimulus in the intact animal. 6. The findings suggest that a linear acceleration vector which rotates in the plane of the lateral semicircular canals can be an adequate stimulus * Royal Air Force Institute of Aviation Medicine, Farnborough, Hants, England. t U.S. Naval Aerospace Medical Institute, Pensacola, Florida, U.S.A.

2 476 A. J. BENSON AND OTHERS to ampullary receptors, though an explanation which invokes the modulation of canal cells by a signal dependent upon the sequential activation of macular receptors cannot be positively excluded. INTRODUCTION The physiological response of a semicircular canal is primarily related to angular movement of the canal in its own plane. Thus the moment of inertia of the contained endolymph causes it to flow relative to the canal during angular acceleration in the plane of that canal. In particular, largely owing to high viscous damping due to the small size of the membranous duct, the relative endolymph displacement and hence angular displacement of the cupula is probably related to instantaneous canal angular velocity (see Figs. 2 and 3A) during most natural head movements (Groen, Lowenstein & Vendrik, 1952; Mayne, 1950; Melvill Jones & Milsum, 1965). However, during steady or abnormally low frequency rotational movement, cupular elasticity exponentially restores the displaced fluid towards the neutral position with a time constant ranging from about sec in man (Groen & van Egmond, 1956) and 5-10 sec in cats (Shimazu & Precht, 1965; G. Melvill Jones & J. H. Milsum, unpublished). Thus when man or an experimental animal experiences a brief angular acceleration which is followed by rotation at a constant speed, the physiological manifestations of cupular deflexion, for example compensatory nystagmoid eye movements (Hallpike & Hood, 1953), essentially disappear after a period of steady motion in excess of two to three time constants. This pattern of response, which accords well with the established dynamics ofthe canal-cupula-endolymph system, is observed during steady motion in a horizontal plane. But recent experiments, first conducted independently by Guedry (1965) and Benson & Bodin (1966) have shown that this does not occur during steady rotation in a vertical plane (i.e. rotation about an earth-horizontal axis). On the contrary, in these circumstances the expected initial exponential decay tends to be arrested at a mean level of response which is apparently maintained as long as the steady rotation continues, although some variation within the cycle of rotation is usually present. The maintained response was ascribed by these authors to the continued re-orientation of the subject relative to the linear acceleration vector due to gravity which is inevitable when angular movement in a vertical plane occurs on the surface of the earth. This inference has subsequently been substantiated by further experiments demonstrating maintained oculomotor response to rotation of a linear acceleration vector in a horizontal plane in the absence of angular movement of the animal, both for man (Niven, Hixson & Correia, 1966; Benson, 1968) and cats

3 CANAL RESPOA7SE TO CHANGING GRAVITY VECTOR 477 (Correia & Money, 1969). Evidently rotation of a linear acceleration vector can of itself generate patterns of physiological response normally associated with stimulation of the semicircular canal receptors by angular movement of the skull. Such sustained per-rotational responses are dependent upon the adequate stimulation of the vestibular apparatus by the rotating vector, for they are absent in subjects with defective labyrinthine function (Guedry, 1965). However, the relative contribution of signals from ampullary or macular vestibular receptors in the generation of this response remains uncertain. Presumably the changing linear acceleration must excite the macular receptors; yet, because of the predominantly unidirectional nature of the sustained nystagmus, Benson & Bodin (1966) suggested that the rotating vector may be an adequate stimulus to the ampullary receptors. Theoretical support for such a mechanism was provided by Steer (1967). A previous investigation (Melvill Jones & Milsum, 1966) has suggested that the issue might be clarified by studying the response of specifically canaldependent cells in the vestibular nuclei on exposure of the animal to a rotating linear acceleration vector in the absence of angular acceleration. The following account reports the results of experiments in which this approach has been adopted. Preliminary results have been reported previously (Benson, Guedry & Melvill Jones, 1967). METHODS Fig. 1 illustrates diagrammatically the method employed for establishing rotation of a linear acceleration vector in the absence of angular movement of the animal. A horizontal platform was suspended from four parallel wire cables, each about 1 m long, to form a parallel swing system. Such a system allows approximately unidirectional linear acceleration to be imposed in a horizontal plane by establishing pendular oscillations which are constrained by the parallelogram suspensions. However, the whole system, being free to move in any direction in a horizontal plane, can also be excited to move in a natural mode whereby the centre of gravity of the platform describes a horizontal circle without introducing angular torsional oscillation of the platform. The period for one cycle of such movement is approximately the same as that of the free pendular movement. Associated with motion in a circular path is a horizontal centripetal acceleration and a corresponding centrifugal force which rotates with respect to the platform and hence the animal, in the same direction and at the same rate as the circular movement of the platform's centre of gravity. These latter features are illustrated in the diagram on the left of Fig. 1, which also defines numerical representation of vector direction relative to the cat. In all experiments this Parallel Swing Rotation, or PSR as it will be referred to below, was driven at the natural frequency of 0-52 rev/sec with the mid-skull interaural point located 0-41 m from the centre of rotation. The magnitude of the rotating linear acceleration vector at this point was thus 4-4 m/sec2 or 0-45 g. The PSR motion was maintained constant by forcing the centre of gravity of the platform-animal combination to move round a circular path defined by the eccentric armn of a servo-controlled turntable. If the time for one revolution of the turntable I6 PHY 210

4 478 A. J. BENSON AND OTHERS coincides with the natural period of the parallel swing system, frictional forces at the junction between the eccentric arm of the turntable and the platform are very small. Using this principle the PSR motion of the platform was maintained smooth and effectively free of any angular movement. Inter-collicular decerebrate cats with the forebrain removed were placed in a conventional stereotaxic device attached to the platform, with the head of the cat orientated so that the lateral canals lay in a horizontal plane. Stereotaxically positioned steel micro-electrodes of tip diameter 2-5,um were employed to record extracellularly the activity of cells in the region of the medial vestibular nucleus. Fig. 1. Diagram of the apparatus used to generate Parallel Swing Rotation (PSR) showing the suspended animal platform and driving turntable. The centre of gravity of the loaded platform was adjusted to coincide with a vertical axis which passed through the inter-aural point of the cat's head and the eccentric sprocket of the turntable drive. At the left is shown, in plan view, the angular positions of the centripetal linear acceleration vector (thick black arrows) relative to the cat during one orbital revolution of the parallel swing (cross-hatched arrows). The linear acceleration vector rotates round the animal (thin black arrows) in the same direction and at the same speed as the platform sweeps round its orbit. Action potentials were preamplified on the platform and the resulting signals fed into (1) a conventional oscilloscope display, (2) a gated speaker system for audible presentation of the neural signal, (3) a high frequency response ultra-violet (U.V.) galvanometer recorder (5 % attenuated at 5000 Hz), (4) an electronic counter and (5) a neurophysiological computer (Burns, Ferch & Mandl, 1965) employed in its averaging mode of operation. The mechanical stimulus was recorded either from a linear accelerometer fixed on the platform, or from a suitable transducer on the turntable. The turntable was driven by a servo-controlled electric motor and its associated low frequency wave form generator (Servomex motor-drive assembly No. 43 and oscillator LF 51, Servomex Controls Ltd., Crowborough, Sussex, England). Control of electrode location The stereotaxic co-ordinate system was checked by electrolytic deposition of iron from the tip of the electrode either at the site of a particular unit or at the end of the experiment. Brain stems were subsequently perfused with ferro-ferri-cyanideformalin solution and sectioned at 25 um intervals. Alternate sections were stained

5 CANAL RESPONSE TO CHANGING GRAVITY VECTOR 479 with cresyl violet (cell soma) and Luxol fast blue (myelinated nerve fibres) and the marked tip locations related to the nominal stereotaxic co-ordinate system. Appropriate corrections were then made for all relevant stereotaxic location of units. Of the twenty-nine cells successfully diagnosed and examined electrophysiologically 62 % were estimated to lie in the medial 21 % in the inferior (descending) and 17 % in the lateral nuclei. None was estimated to be in the superior vestibular nuclei. There were no consistent differences between the patterns of response found in the different nuclei. FUNCTIONAL DIAGNOSIS OF NEURAL 'UNITS Vestibular units were located which responded in phase with the angular velocity of a sinusoidal horizontal angular oscillation of the freely slung platform, driven manually at its natural frequency of angular movement (about 1 0 Hz). The platform was then subjected to oscillatory linear A~~~~~~~~~~~~~~~~~~3 n +35 D 10 3 j -35 I~. **~ - so S -2.s-,ye20. ~ 3. -.iv - 1 9cyde=4 sac"--- ~~~~~~~~~~~~~0 Fig. 2. Response of a canal-dependent cell to angular oscillation in the plane of the lateral canals. The upper half of the Figure is a sample of the photogalvanometer record and shows from above down: A, tachometer angular velocity signal; B, action potentials from micro-electrode in the left medial vestibular nucleus and C, time scale (sec). The lower half of the Figure shows the average stimulus (D) and concomitant average action potential frequency obtained over 10 cycles (E). The averaged data are written out twice to aid visual interpretation. accelerations in each of two orthogonal directions in a horizontal plane and to angular oscillations in the planes of the vertical canals, permitted by temporary replacement of diagonally opposite suspension wires with suitable springs. Quantitative studies were made only on those cells which responded solely to angular acceleration in the horizontal plane. In order to confirm the specific lateral canal-dependence of the units 16-2

6 480 A. J. BENSON AND OTHERS selected in this way, their dynamic response to angular stimuli was examined by exposing the preparation to controlled rotational stimuli in the manner described by Melvill Jones & Milsum (1970). Responses to sinusoidal angular oscillations in the plane of the lateral canals are illustrated in Figs. 2 and 3. The upper part of Fig. 2 shows the modulation of unit activity (B) produced by an oscillation having a period of 4 sec and peak angular velocity (A) of 35 /sec. The lower part shows the corresponding averaged data obtained from 10 consecutive cycles. In this and SinusoidL640/sec C Acceleration±530/sec2 A 4 sec a- 128 sec- a APs/sec ^ ~ ^ oaps/sec 40 I o_vr1 r: s :== SinusoidA1020/sec N=1 5 N=2 Impulse 51 /sec B --64 sec D) sec \. 1 l Eot N=2 N=4 Fig. 3. Averaged responses, obtained as in Fig. 2, to stimuli employed in the functional diagnosis of lateral canal-dependence. A and B show the responses to sinusoidal angular oscillations and the typical phase advancement of the firing frequency (lower curves) relative to angular velocity (upper curves) associated with change of stimulus period from 4 sec (A) to 64 sec (B). C shows the sustained patterns of responses which are characteristic of a canal response during ramp angular velocity stimuli (triangular wave form). D shows the transient response and its exponential decay following an impulsive change of angular velocity in the excitatory direction only. N gives the number of cycles averaged. subsequent Figures the averaged curves are written out twice for ease of visual interpretation. Curve E gives the averaged firing frequency of the unit throughout the averaged cycle of angular velocity stimulation (D). The response of this cell at this frequency was evidently tied closely to the stimulus angular velocity. Fig. 3 (A and B) shows how another canal-dependent cell responded in a similar way at 4 sec period of angular oscillation, but became substantially phase-advanced with respect to angular velocity when the period of oscillation was increased to 64 sec. All but three cells were exposed to this additional diagnostic measure and yielded similar phase advancement. The remaining three cells were lost before or during the long period oscillation.

7 CANAL RESPONSE TO CHANGING GRAVITY VECTOR 481 Responses to ramp and step angular velocity changes were also examined in about one third of the cells which satisfied these criteria. As illustrated in Fig. 3C, a ramp velocity input (triangular velocity waveform) induced either a sustained increase or decrease in discharge frequency according to the ramp direction. A step change of angular velocity in the excitatory direction induced a sudden increase of firing frequency followed by an exponential decay having a time constant of approximately 5-7 sec (Fig. 3D). These responses to long period sinusoidal and transient angular stimuli are in accord with established dynamics of the canal-cupulaendolymph system and imply that the cells sampled in the vestibular nuclei reflected, in a rather precise manner, the afferent signal from the ampullary receptors of the lateral semicircular canals. RESULTS Units surviving the rather rigorous functional diagnostic measures described above were examined for two main effects associated with PSR, namely the differential effect of PSR to left and to right on mean discharge frequency, and the appearance of periodicity in the response. Effect of PSR on the mean discharge frequency of canal units Spontaneous activity was compared with that obtained during PSR right and left, using an electronic counter to average action potential (AP) frequency over periods of 30 sec duration. Measures of spontaneous activity were alternated with those during PSR right and left, until four average values had been measured in each direction of PSR, or until the unit was lost. Short transient post-activation effects were excluded by allowing at least one minute between tests. Fig. 4 shows three examples of results obtained in this way. Each set of curves gives the serial values of mean AP frequency recorded from one unit during spontaneous activity and during PSR right and left, plotted in the actual order of test. The abscissae thus give an approximate indication of real time since each test-plus-rest period occupied about 90 sec, and hence a full run on one cell occupied about 25 min. Three main features are exemplified in the Figure. First, the direction of PSR had an appreciable and consistent effect on the mean discharge frequency. In the examples shown PSR right consistently produced greater cell activity than PSR left. The uniqueness of this differential effect is reflected in the fact that during functional diagnosis the three cells represented in Fig. 4 were shown to be excited by angular motion to left, and hence relative endolymph flow to right. The cells were thus preferentially

8 482 A. J. BENSON AND OTHERS excited by rotation of the linear acceleration vector in the same direction as that of the endolymph flow which was excitatory to these cells. Secondly, the differential effect of PSR to right and left was not necessarily symmetrically disposed above and below the spontaneous level. Thus some units showed an increase in frequency irrespective of the direction in which the vector rotated (Fig. 4A), some an increase in one direction but not the other (Fig. 4B) and some an alteration above and below the spontaneous level according to the direction of PSR (Fig. 4C) A BC < \ e I I.v II \\ I 0 K o..0 x Order of test Fig. 4. Comparison of spontaneous action potential frequency (--- x ---) with those obtained during PSR left ( ) and right (-*-). Each point is the mean value measured over 30 sec. The actual sequence of test is enumerated on the abscissae which therefore also give the approximate time course of each experiment, since the total inter-test interval was usually 90 sec. Results were obtained from A, a single cell on the left side excited in phase with angular velocity to the left, B, a single cell on the right side excited in phase with angular velocity to the left and C, two cells of the same type as in B. A, B and C were from three different cats. The third feature exemplified in Fig. 4 is the presence of some, often considerable, fluctuation in the level of cell activity during the 30 mi duration of an experiment, although in no instance was this sufficient to conceal the differential directional effect. Differential alteration of cell activity according to the direction of PSR was exhibited by all of the twenty-nine cells (or small cell groups) upon Av

9 CANAL RESPONSE TO CHANGING GRAVITY VECTOR 483 which quantitative measurements were made following functional diagnosis for specific lateral canal-dependence. This result is illustrated in Fig. 5, where the difference (Af) of the mean discharge frequency during PSR to the right from the spontaneous level (abscissa) is plotted against the corresponding alteration brought about by PSR left (ordinate). If the direction of rotation of the vector were without effect all the points should lie on the line of equality. However, it was found that cells, which during functional diagnosis were excited by angular motion to the right and inhibited by u60 (-41,70) <:_ ~/./ 40 / ~/*a ri_ * / 20 t0 / (104,22) % %0, 0 0 -/3 o Af PSR right APs/sc Fig. 5. Comparison of change in action potential frequency from the spontaneous level according to the direction of PSR (i.e. the direction of rotation of the linear acceleration vector). For each unit the difference (Af ) of the mean action potential frequency during PSR right from its mean spontaneous level (abscissa) is plotted against the corresponding alteration brought about by PSR left (ordinate). Symbols 0 and 0 represent cells excited by angular motion right and * and Z] cells excited by angular motion left. Filled and open symbols indicate cells in left and right vestibular nuclei respectively. angular motion to the left (termed CeR cells), lay below the line of equality; conversely, those cells which were excited by angular motion to the left (WL cells) lay above the line. This relationship was sustained irrespective of whether the cells reflected the activity of the sensory epithe-

10 484 A. J. BENSON AND OTHERS lium of the 'ipsilateral' ampulla (i.e. CR cells in the right vestibular nucleus, and u0l cells in the left vestibular nucleus) or of the 'contralateral' ampulla (i.e. W0L cells on the right side and &0R cells on the left side). The terms ipsilateral and contralateral are chosen to conform with those employed by Shimazu & Precht (1965). The average discharge frequencies obtained at rest and during PSR for the four types of cells studied are assembled in Table 1. The findings enumerated in this Table are unambiguous. When the linear acceleration vector rotated in the same direction as excitatory endolymph flow the activity of lateral canal units was greater than when the vector rotated in the opposite direction (P < 0-001). In one unit, rotation of the vector in the excitatory direction of endolymph flow was associated with a fall in discharge frequency below the spontaneous level, but rotation of the vector in the opposite direction brought about a still greater reduction in the activity of the cell. Fourteen of the analysed records showed the activity of a single cell (e.g. Figs. 2 and 7). In the other fifteen records action potentials were present from two or three active cells, which responded in a common manner, without apparent recruitment, to the rotational stimuli employed during the initial selection procedure. For each of the four cell types examined (Table 1) there were approximately equal numbers of single and multiple cell records. Although as was to be expected the multiple cell samples have on average a higher spontaneous discharge frequency (mean 38 2 AP/sec) than the single cell samples (mean 19 9 AP/sec) neither the distribution of firing rates nor the magnitude of the differential alteration of activity differed significantly between the single cells and multiple cell populations. On separating the results into those obtained from 'ipsilateral' and 'contralateral' units as defined above, it emerged that PSR in the direction of inhibitory endolymph flow produced a significant (P = 0-02) increase above the spontaneous level in the 'contralateral' units, but not in the 'ipsilateral' ones. This was the only consistent difference observed in the behaviour of ipsi- and contralateral units. Phasic components of response Early in the investigation it was apparent from the audible presentation of cell activity that the response of some units varied in a systematic way during each rotational cycle of the parallel swing. Consequently the responses from twenty-two cells were examined by means of the averaging computer as well as by the simple method of counting described above. For this the computer was triggered each time the centripetal acceleration was directed from the tail to the head of the animal (i.e. the 0 position, vide

11 CANAL RESPONSE TO CHANGING GRAVITY VECTOR o o -U c) 0 a) 0 0 c s. I _ I + + C) 4 -C P,-, 4 Ft14 M4 m d: -w OQ._ C) g 4Q_ 0 it00 w *_ C -2 0 m Z0 r- (: ct op o -I.. oq cq c 0 00 a4 &2 0 0 V c; H- r-. m ce C)0..-4._ I' 0 6 z X q O1 00 C10Il0* a + s Co. - )- EC C" - - * C) " 0 m 0 I-4 C 1) d 0 -Q -Q o._ X,s,. 0.- wo' g o; v- P4 9 C)aa lcz

12 486 - A. J. BENSON AND OTHERS Fig. 1), and the time base duration of the computer adjusted to equal the periodic time of PSR. For comparison the average spontaneous activity was measured by recycling the time base at the same settings when the platform was at rest. Measures of the response during PSR were alternated between measures of resting activity. Fig. 6 illustrates the results obtained in this way from a 'contralateral' (WL) cell showing little or no apparent periodicity in its response. From No PSR 300, - - PSR left 300, [ i #. ~~~~~10 p~~~~~ - * ~ p ; I PSR right 300 v[. 20.,,, 'p. IT,O i - 11 sec - 1 sec Fig. 6. Effect of PSR on activity of a unit on the right side shown during previous functional diagnosis to be excited by angular motion left. Markers on the horizontal lines indicate the rotational period of the parallel swing. Each record on the right hand side of the Figure shows the average response of 50 cycles and as before these are written out twice. above downwards the Figure gives samples of AP records and corresponding averaged AP frequencies obtained when the animal was stationary and during PSR to the left and to the right. At rest this cell was silent except for intermittent high frequency bursts of activity. Such discharge patterns, apparent in the upper two records of Fig. 6, are common to many vestibular units. During PSR left cell activity differed little from the resting condition, but during PSR right a continuous discharge was observed which was maintained at an approximately constant level throughout the cycle of stimulation, and as long as the PSR motion was continued. Fig. 7 shows records obtained from another 'contralateral' (WJL) cell in a different cat. This cell exhibited a slow, irregular spontaneous discharge,

13 CANAL RESPONSE TO CHANGING GRAVITY VECTOR 487 which was excited during both directions of PSR, although more so to right than to left. The general pattern of response was thus similar to that illustrated in Fig. 4A. As is evident in both the samples of original AP records and the responses averaged by the computer over thirty cycles, there was a well defined periodicity in cell activity during the cyclical stimulations. During PSR left the periodicity was manifest as alternate trains of APs and periods of silence, except for the intermittent burst activity mentioned above. Maximum AP frequency occurred when the APs/sec No PSR 30r * [400 FV PSR left I;-~i PSR right 30r* 101-.İ.. 10 L e tiliiu 20I= U J I... I-J, LI I1 Ali2 1-1 O t ; '-'.,,,,.,.'.-'e.;,... I - I1sec Fig. 7. Response of a similar cell to that shown in Fig. 6 in another cat, to illustrate periodic alteration of activity during PSR. Each average obtained over 30 cycles. linear acceleration vector was close to the 2700 position (vide Fig. 1). During PSR right the over-all level of activity was considerably raised. But again the AP frequency, this time continuous, was modulated with a periodicity tied to the imposed stimulus, the maximum response occurring in approximately the same vector direction as before, namely at 2700 relative to the cat. The particular point at which the maximum appears in the Figure depends of course upon the direction of vector rotation. Evidence of a phasic content in the response was seen in fifteen of the twenty-two records analysed. However, no definite association could be found between vector direction and the phasic response, although there was a suggestion that units in the left brain stem were most active when the acceleration vector lay in a wide sector centered on the 90 position (vide Fig. 1) and that the converse applied for units in the right brain stem. sec

14 488 A. J. BENSON AND OTHERS Relation of neural activity to the induced oculomotor response It is of interest to enquire whether the differential alteration of cell activity brought about by the rotating linear acceleration vector could account for the sustained nystagmus which is evoked by such a change in the force environment. Accordingly horizontal eye movements were recorded by electro-oculography (eog) in one unanaesthetized cat during PSR in the dark. The cat's head was fixed to the platform according to the method described by Henriksson, Fernandez & Kohut (1961). Sustained 15[_.c i1-1 3n O06 Mean PSR right 0 0~~~~~~~~~~~~~~~~~ 0 ~~~~~~~~~~x o% / % ~ ~~/Mean PSRIleft us / PSR right' / PSR left Position of acceleration vector Fig. 8. Slow phase eye angular velocity recorded from a conscious cat during PSR. The PSR motion generated a linear acceleration vector the horizontal component of which had a magnitude of 0 4 g and rotated round the animal at 0-5 rev/sec to right (-*-) or to left (--- x ---). Each point gives the mean eye velocity obtained over 6 consecutive PSR cycles. The first six points of each curve are repeated to aid visual interpretation of the phasic component of the record. The abscissae indicate the direction of the vector relative to the cat's head using the angular coordinates defined in Fig. 1. Maximum response occurred at about 3000 during PSR right and 600 during PSR left. eye movements were obtained as depicted in Fig. 8. Each point gives the mean slow phase eye angular velocity (for 6 rev) associated with the direction of acceleration vector defined on the abscissa and obtained during PSR right (continuous curve) or left (intermittent curve). Note that right-going slow phase eye angular velocity is plotted upwards relative to

15 CANAL RESPONSE TO CHANGING GRAVITY VECTOR 489 the zero ordinate and left-going velocity downwards. Since each revolution (0-360 ) occupied 1V92 sec the abscissa represents time as well as vector direction. The responses showed well defined cyclical modulations the phases of which conform well with those observed in man at this rotational frequency and level of acceleration (Correia & Guedry, 1966; Steer, 1967; Benson, 1968). During PSR right the slow-phase component of eye movement was mainly right-going and had a mean angular velocity of 5 80/sec; during PSR left a mean of 5-20/sec to left was obtained. The difference in mean slow-phase velocity during PSR right and left was thus 11-00/sec. The same stimulus brought about a mean difference in discharge frequency of 19 9 AP/sec (range I 0) in sixteen cells quantitatively examined for responses to both angular and PSR motions in seven decerebrate cats. Assuming intercollicular decerebration does not materially alter the afferent vestibular response to rotation, this suggests a ratio of 19 9/11 0 = 1 81 AP/sec per 0/sec of eye movement. Turning now to the corresponding ratio for angular head movement, a mean sensitivity of 1-73 (range ) AP/sec per 0/sec of stimulu8 angular velocity was obtained from records such as Fig. 3A for these sixteen cells. In view of the fact that angular oscillations similar to those here employed ( Hz) apparently generate compensatory eye angular velocities approximately equal to that of the stimulus (Zuckerman, 1967), the value 1*73 AP/sec per /sec may also be taken as an approximate value of the ratio of change in AP frequency per unit change in resulting eye angular velocity. The dispersion of measures was large in both sets of data and with the small number of estimates obtained from several different cats no significant correlation emerged between them. Nevertheless the closeness of ratios obtained from angular motion and PSR suggests that the differential alteration of activity in canal-dependent vestibular units brought about by the rotating linear acceleration vector here employed is of a magnitude appropriate for the generation of the sustained (unidirectional) nystagmus which is evoked in the intact animal. DISCUSSION The outstanding feature of these results is the uniqueness of the differential effect brought about by PSR right and left. Without exception all the specifically canal-dependent cells examined were more active during PSR in the direction of endolymph circulation shown to be excitatory to that unit (or units) than during PSR in the opposite direction. That is to say, rotation of a linear acceleration vector in the plane of the lateral canals apparently modulated in a systematic way the activity of all units identi-

16 490 A. J. BENSON AND OTHERS fled during functional diagnosis as responsive only to rotational stimulation of the lateral semicircular canals. Probably the observed magnitudes of the differential modulation given in Table 1 tend to underestimate the true effect for two main reasons. First, in any spontaneously silent cell, such as that in Fig. 6, only excitatory influences which raise the cell above its threshold of firing are manifest in the extracellular recording; an over-all inhibition of the cell will remain undetected. Secondly, in a highly modulated phasic response such as in Fig. 7, the measured mean discharge frequency during a generally inhibitory influence may even be increased above the spontaneous level when in reality the inhibitory influence induced a mean suppression of cell excitability below the spontaneous level. Thus the findings appear to support the view referred to in the Introduction, namely that rotation of a linear acceleration vector can, of itself, mechanically excite a semicircular canal. Of course the veracity of this conclusion is dependent upon the canal specificity of the units examined, and in this connexion it must be borne in mind that the ipsi- and contralateral cells here examined are presumably similar to those defined by Shimazu & Precht (1965) as post-synaptic to the primary vestibular neurones. Yet despite the possible accessibility ofsuch neurones to multiple afferent stimuli, it is hard to deny a substantial degree of canal specificity in cells which successfully passed the various stages of functional diagnosis here employed. A cell showing marked modulation of activity in response to an oscillating angular acceleration in the plane of only one pair of canals, but no modulation to an oscillating linear acceleration anywhere in the same plane, must surely be mainly under the influence of those canals rather than the otolith organ; particularly when the dynamic response ofthe unit to head rotation conforms with that of the mechanical components of the canal. The general conclusion drawn above raises the question of the feasibility of induction of a mechanical response in the canal which could generate a differential effect in the dependent neural signal as a result of PSR to right and to left. In this connexion a recent analytical study by Steer (1967) has shown that, due mainly to small differences in density of endolymph and perilymph, a rotating force vector might well be expected to generate pressure gradients round the canal which could deflect the cupula against its own elastic restoring force. It is of course important to realize that only cells selected for their canal specificity were examined. That otolith-dependent cells can also contribute to the generation of a reflex oculomotor response during PSR is suggested by the observations of Correia & Money (1969) and of Janeke (1968). Correia & Money mechanically obstructed the lateral canals of cats and

17 CANAL RESPONSE TO CHANGING GRA VIT Y VECTOR 491 recorded their oculomotor responses, in the absence of vision, during motion similar to the PSR motion employed in our experiments. Compensatory eye movements were apparently reduced by about 50 % as a result of the surgical obstruction to canal fluid circulation. They concluded that the residual response was probably of otolithic origin. However, it should be noted that Steer has drawn attention to the possibility that an attenuated hydrodynamic response in the canal might still deflect the cupula by a reduced amount, even after obstruction to fluid circulation at one point in the canal circuit. Janeke was unable to elicit significant oculomotor response in rabbits during rotation about a cephalo-caudal horizontal axis after bilateral section of the utricular nerves and mechanical destruction of the saccules. However, it seems these surgical measures were rather prone to interfere with the whole labyrinthine system, since only seven out of fifty operated animals yielded oculomotor responses to stimulation of the canals by angular movement. Moreover the published records from the successful preparations show considerable alteration in the pattern of oculomotor responses to angular stimuli after surgery. Although of great interest these latter observations do not therefore appear to settle the matter; particularly in view of possible species differences. Despite the apparent dependence of the cells studied on afferents from the lateral canals, it is known from the work of Duensing & Schaefer (1959) and Fredrickson, Schwartz & Korhuber (1966) that many cells in the vestibular nuclei exhibit convergence of ampullary and macular afferents. Hence, it may be argued that the linear accelerations employed in the initial functional diagnostic test procedure were not of sufficient intensity to alter, through otolithic or gravireceptor afferents, the excitability of cells, primarily innervated by the lateral canals. The influence of these pathways became overt only with the more potent and sustained stimulation afforded by PSR. However, such a stimulus is likely to engender a cyclical response in primary otolithic afferents (Lowenstein & Roberts, 1949) and their secondary neurones within the vestibular nuclei (Melvill Jones & Milsum, 1968), the magnitude of which is essentially independent of the direction of rotation of the vector; likewise it is difficult to envisage any somaesthetic gravireceptor which would show a differential response during PSR. Thus, the direct modulation of 'canal' cells by otolithic afferents, whilst an adequate explanation of the cyclical alteration of excitability does not account for the differential alteration in discharge frequency according to the direction of rotation of the vector. Such a directional effect, if it is not a manifestation of the activity of the ampullary receptors, must depend upon the neuronal integration of the sequential response of gravireceptors

18 492 A. J. BENSON AND OTHERS from which a signal related to the direction of rotation of the vector is resolved. It is pertinent at this point to recall the results reported above which suggest that the change in AP frequency associated with change in direction of the PSR stimulus appears to have been of appropriate magnitude for the generation of the associated mean angular velocities of reflex compensatory eye movement; which tends to suggest that a major contribution to that response derives from canal-dependent cells such as those examined in this series of experiments. A further feature for discussion is the fact that the differential effect described above occurred despite considerable variability of response relation to spontaneous activity. PSR in the direction of excitatory endolymph flow consistently raised the cell activity above the spontaneous level, but PSR in the opposite direction did not lower the over-all mean response below it. As indicated above, this feature suggests that the presence of PSR motion generally raised the level of cell activity in a nonspecific manner and that the differential effect was superimposed upon this non-specific one. Such non-specific activity could presumably be accounted for by the non-specific excitatory pathway through the reticular system which has been demonstrated by Shimazu & Precht (1966). These authors demonstrated such a pathway in connexion with ampullary signals only. It would be reasonable to assume a similar pathway is associated with otolith stimulation, and this would presumably further raise the general activity during the PSR motion. The presence of a phasic component of response in some cells is not easy to explain. First, it seems unlikely that it could be attributable to irregularities in anatomical structure of the canal circuit since some cells were devoid of periodicity whilst others in the same cat were not. Furthermore, there was no uniformity in the phase of cyclical components in the cells of a given animal. Secondly, any otolithic modulation of response would be expected to have manifested itself in the functional diagnostic procedures. It can only be guessed that such phasic components as were present derived from the complex interplay of widespread long range influences such as those defined and alluded to by Markham, Precht & Shimazu (1966), Fredrickson et al. (1966) and Gernandt & Gilman (1959). Whatever its origin, the phasic influence did not apparently interfere with the clear-cut outcome that, without exception all cells were more active during PSR in the direction of endolymph circulation shown to be excitatory to the individual cells, than during PSR in the opposite direction. From this result and the above discussion it seems likely that the rotating acceleration vector here employed may well provide a form of adequate mechanical stimulation to the cat's lateral semicircular canals.

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