CEREBELLAR INTERACTION with the ACOUSTIC REFLEX Abstract

CEREBELLAR INTERACTION WITH THE ACOUSTIC REFLEX

Pawel J. JASTREBOFF

Department of Neurophysiology, Nencki Institute of Experimental Biology Pasteura 3, 02-093 Warsaw, Poland

Key words: cerebellum, acoustic reflex, plasticity

Abstract. The involvement of the cerebellar vermis in the acoustic reflex was analyzed in 12 cats, decerebrated or in pentobarbital anesthesia. Anatomical data suggested the existence of a connection of lobulus VIII with the ventral cochlear nucleus. Single cell recording and evoked potential techniques demonstrated the existence of the acoustic projection to lobulus VIII. Electrical stimulation of this area changed the tension of the middle ear muscle and caused evoked potential responses in the caudal part of the ventral cochlear nucleus. Electrical stimulation of the motor nucleus of the facial nerve evoked a slow wave in the recording taken from the surrounding of the cochlear round window. A hypothesis is prqposed which postulartes hhe involvement of the acoustic reflex in space localization of acoustic stimuli and the action of cerebellar vermis in wder to assure the sta,bility and plasticity of the acoustic reflex arc.

INTRODUCTION

One of the first scientists analyzing the role of the cerebellum was Rolando, who in 1809 published the results of his ablation experiments (18). On the \basis of observed 'disturbances he postulated involvement of the cerebellum in motor functions. This concept was further extended by Flourens (18) who proposed that the cerebellum is responsible for the coordination of movements, but not for their initiaition. Furthermore, after paAial lesions of the cerebellum, the remaining part compensates for the induced motor disturbances. The observation of patients led to the specification of characteristical features of cerebellar disorders - intentions tremor, asymmetry, decomposition of movement, diskinesia, adiadocholkinesis - and also demonstrated the posdbility of a conside- rable degree of compensation. Snider and Stomell (60) starte~dthe analysis of potentials evoked on the cerebellar surface and showed that there (is a localization of various representations of stimulus modality. Introduction of single cells recording (11) permited an analysis of the cerebellar cortex cells activity and the manner in which the cerebellum performs the function of data processing. The alsrton.ishing xeguhriity of the oe~eibellar anatomy suggested analogies with computer functions (21), leading to theories in which the cerebellum is treated as a specialized data processing computer that integrates information from various sources and is responsible for the correction of the movements by predicting the future positions of bodily limbs (53). The cerebellar cortex iis composed of five types of cells (granule, basket, stellate, Golgi, Purikinje cells) and two tyyes of afferent fibers (climbing and mossy fibers). Both afferent fibers are excitatory, while all but the granule cells are inhilbitory. Independent of such anatomical classification of afferent fibers, from the functional point of view, two nonspecific inhibitory systems of fibers can be distinguish - nonadr- energic and serotonergc afferents (5, 42, 63). The excitation travelling through mossy fibers (MF), granule cells, and their axons (pa~allelfibers) cause the discharge of Punkinje cells whose axons are the sole output of the cerebellar cortex. Spikes caused by MF excitation are called "simple spikes" and have an average frequency of approximately 60 Hz. The excitation travelling through climfb- ing fibers (CF) has completely different features - it always produces a short series of 3 to 5 high frequency spikes in Puakinje cells called the "complex spike". The spontaneous rate of complex spi!ke is around one ,per second. The stellate, basket and Golgi cells are involved with the additional shaping of cerebellar cortex responses and with the spatial focusing of excitation. Detailed descdption of bhe cerebellar anatomy has been presented in many papers (21, 49-52). The most recent ~themiesthat atrtempt to explain the furnotion and role of the cerebellum stress its learning capabilities, as initially proposed by Luciani, which are (based on Rosenblaitt's simple pecrcepltron (2, 20, 27, 41). The simple perceptron is composed of three layers, receptor, association and effector, to whiah the cells of mossy fiber origin, granule c&Is and Rurkimje cells are reqxcrtively related. In bhis mded it is assumed that information travelling from the first to (the second layer is processed, and that learning can occur between the second and third layer. This learning process is strictly under the control of the "teacher" who, using information about the final results of perceptual action, changes the weight of connections between the second and third layer in such a manner as to obtain maximum efficiency of the system. In the case of the cerebellum, the inferior olive (the only source of climbing fibers) is put in the teacher position with the following assumptions: (i) the inferior olive obtains information of system per- formance; (ii) synaptic transmission from granule cells to Purkinje cells cam be modified and ~(iii)this moidifiability is under tlhe control of the activity of climbing fibers. Marr (41) assumed that simultaneous excitation of MF and CF causes an enhancement of synaptic efficiency from granule to Purkinje cells; Albus (2) postulated the decline of synaptic efficiency. Recent data (19, 25, 31a) support Albus' theory. In addition, recent data has presented evidence that the system of climbing fibers influences Purkinje cell activity to a much greater extent than was previously assumed (14, 31). Recently, the assumptions (ii) and (iii) have acquired experimental support (31a, 44). It is possible to show three features commonly connected with the function of the cerebellum: the control of open loop systems, the integration of information from different places and modalities, and plasticity. These features will be demonstrated using cerebellar interaction with the vestibulo-ocular reflex (VOR). The VOR is of (great physiollogical significance because it assures the stability of the retinal image in spite of head and body movements. For example, in the case of head movement of 20' to bhe left, the eye rotates to the right with the same angular amplitude. The feedback from the visual system to the centre controlling the eye movement is fast enough only for slow, pursuit eye movements, and it is at least 10 times too slow for stabilization of the retinal image during normal, physiological head movements (56). Stabidimtion is due to information from the vestibular system. The input signal from the semicil-cular canals passes through the vestibular nuclei and are relayed to the motor nuclei whose motoneurons cause eye muscle movements (27). This short arc has a latency of only 10 ms, including delay in the muscles and, because of this, guarantees stabilization of the image on the retina in the physiological range of angular velocities. But this system has one basic drawback, namely it does not have any feedback and thus cannot alter the transmission of the signal from the vestibular organ to eye muscles in order to insure appropriate activity of the system. The la& of such correction in the cases of changes in e T e muscle stiffness, microlesions in the VOR arc or, for human beings, wearing glasses, can lead to the loss of clear vision d~uringhead movements. Physiological anld behavioral experiments have shown that the VOR is susceptible to plastic changes, even to t'he reversal of the direction of eye movement (26-28, 43). These results .imply that it is necessary to add new elements to the VOR arc. Ito (27) proposed that the cerebellum is responsible for the o,bserved plastic changes in the VOR. The cerebel. lurn is part of a secondary feedforward loop between the vestibular organ and vestibular nuclei. Afferent signals rmch the cerebellar cortex via mossy fibers, whlle some Purkinje cell axons terminate in the vestibular nuclei. According to Ito's hypothesis, the error signal of the VOR, the velocity of movement on the retina, is sent through climbing fibers to the cerebellum wihere, according to Marr's theory, it modifies the transmission of vestilbular signals reaching the cerebellar cortex. The detailed parametrical analysis of the horizontal vestibulo-ocular reflex (HvOR) and the optakinetic response (OKR) a'llowed analyses of their interactions, which occur during simultaneous vestibular and visual stimulation. It was shown that by knowing the parameters oi pure HVOR and OKR, it is possible to predict the results of combined stimulation (4). Training based on the com~binedvesti,bular and visual stimulation leads to changes of HVOR parameters, whereas the OKR remains constant (28). An intact cerebellar flocculus is necessary for evoking plastic changes of the ipsihteral HVOR. Unikateral lesions of this struc- .ture, even chemical ones that spare the input fibers, cause irreversible abolishion of the plasticity and a decrease of gain of the ipsilateral HVOR, while the plasticity of the contralateral reflex is not influenced by such lesions. Lesions of the other parts of the cerebellum have no effect on the plasticity and caused only mirror changes of the HVOR parameters (29, 35). Some interesting resullts were olbtained after separate disruption 01 two different pathways by means of which visual informlation reaches the flocculu cortex (30, 44). Severance sf any of these pathways had no influence on the parameters of HVOR. Disruption of the mossy fibers pathways caused a strong decrease of the gai,n of the contralateral OKR and a,bolished the rapid modification of the eye movement observed during the interaction of vestibular and visual stimulation, whereas long term plastic changes of HVOR parameters were not affected. The severance of climbing fiber pathways had just opposite effects: no changes of the OKR parameters and rapid response modifications, but totally removed the possibility of plastic changes. The analysis of the filoccula~,single PuPkinje cell activity during chronic experiments revealed that only a part of the cells reacting to horizontal vestibular stimulation is involved in controlling horizontal eye movements, but the cells that control the HVOR have the tendency to react to vestibular stimulation with the phase opposite to the time course of head velocity. The adaptation training leading to the increase or the decrease of the HVOR gain caused >parallel alternations in the populations of cell responses in the direction predicted by the theory, i.e., for training which enhance.d the gain, th'e relative amplitude of modulation of cell response from the out phase quadrant increased, and for attenuating training it decreased (19, 36). The proportion of reacting cells and their mean frequency of firing were not influenced by the training. The most recent of Ito's experiments with electrical stimulation of mossy and climbing fibers (31a) have shown that parallel stimulation of both fibers cause.s the decrease of synaptic transmission between the parallel fibers and the Purkinje cells. Other connections in the cerebellar cortex did not show any changes due to conjunctive stimulation. Thus, on the basis of these data it is possi'ble to state that the cerebellar flocculus is the essential structure for the plasticity of HVOR and is closely involved in the controlling the HVOR using vesti~bularand visual information. In addition, the mechanism of the ,plasticity is based on the Albus modification of Marr's theory. Marr's tlheory can ,be applied to other reflexes such as postural and righting reflexes (56), but these hypotheses remain unproven to date. It is possible to lo& for such an interaction of the cerebellum with a reflex that has to be performed accurately, and so rapidly that the negative feedback loop is too slow, and when the necessity of plastic changes of an arc appear. In the follow'ing part of this paper an hypothesis concerning the interaction of the cerebellar vermis with the acoustic reflex arc will be proposed. The acoustic reflex (AR) is defined as a contraction of the mi'ddle ear muscle ,evoked )by an acoustic stimulus. 11ncats and huma~ns,AX malinly results from the action of the stapedius muscle. Acoustic information from the cochlea reaches the ventrall part of the cochlear nucleus, and is relayed to the motor nuclleus of the facial nerve whose motoneurons cause bhe stapedizls muscular reaction, that in turn attenuates the sound transmission thr,ough the middle ear by up to 20 dB (47). The latency of this reflex is approximately 10 ms. Neither the pole of the acoustic reflex nor the stapedius m'wle are fully explained. One possible role of the reflex may be to protect the cochlea from damage due to ex- cessive sound. However, the low threshold of the AR, as bw as 20 + 40 dB SPL, 1(8, 22) and its sensitivity to small changes in stimulus intensity (2 dB), 'even near 100 dB SPL (57), argue agaiamt such a role. It has been shown in several papers (7, 8) that the AR arc works on the border of stability, and even drops into damped oscillations when a strong stimuli are applied. In such instances, even small changes in the arc parameters may cause instability. Since these oscillations are due to existing feedback from the acoustic system (7), this finding provoked the postulaitiion of bhe exilstence of an atdlditiona~lstmucItu~e, which can improve the frequency characteristics of the arc and can modify its parameters to obtain bhe stability of the arc. The stapedius muscle controls the input of acoustic signals to the cochlea, changing their levels. Thus, knowledge of this muscle's state is essential for judgement of the absolute level of a stimulus and, because of sapid changes in its tens'ion, for evaluating changes of the intensity of a sound. Another aspect that should be taken into acount concerns psycho- acoustic data related to space localization of acoustic stimulus. This localization is based on time, phase, or level differences between the two ears-depending on stimulus frequency. It is iworthwhile to note that time differences for the cat are below 200 ps, and level differences are between 0.6 dB to albout 10 dB (38, 46). herborders are on the threshold of discrimination. In such situations it is necessary to assure ex- tremely good control of the transmission of the stimulus travelling through the external and the middle ear to the cochlea with the pur(pse of aalowing not only the discrimination of interaural1 differences but also their proper intenpretation. Desmedt (17) compared the effects of centrifugal olivo-cochlear inhibition and of the middle ear muscle conrtracbon. He has shown that contracti~onof the stapedius muscle can produce not only 10-20 dB attenuation, lbut also that the latency of the N1 wave of the cochlear neural response is shifted about 200-400 ps. Both effects are well within the significant range for binaural interaction. Desmedt concluded that "Sound localization would no doubt be influenced by the middle ear m~wles,especially if their cmtacttioms were asyrnmet~ical" p. 381. However, asymmetrical sound transmission could be due to random causes, such as partial damage of the ear, infection, or microlesions in amustic pa$hways, whieh change khe characteristics of lhe acoustic information transmission and need adequate compensation. Ilt is possible to assume that the asyununhy can be i'ntmtional. Lawrence (40) offered the following proposal: since interaural phase and intensity differences are under the control of the middle ear muscle, the joint action of two sets of muscles mlay serve to direct and alternate attention to specific sound sources, among many sound sources occuring simultaneously. This hypothesis demands that the fast aooustic reflex undergoes the plastic changes in a time scale of seconds or minutes. The amount and direction of changes can be evaluated only on the basis of comparison of acoustic with visual and prioproceptive information concerning space localization of stimulus. I't is k'nown Uhart ithe AR can be mdiltioned by weak acoustic, visual or electrical sti#mulahi,on, ad iks esfici mcy increases 'Wh'm amustic stimuli a're r~peatedseverail times (13, 59, 61), s~uggesttin~grtihart the AR is suscqtii&18erto plaistic ciha~nges.T!h~e ac~ou,&ic reflex a'rc has no anoldif iabLity itself. Therefore it seems reasonable to postulate the existence of a structure Ohat obtains data from inputs of several modalities, is able to form feedforward arcs parallel to main acoustic reflex arcs and has the possibility of f plastic changes. It is well 'known that the cerebellum receives information from the acoustic, visual, and motor nuclei of cranial nerves as well as afferent system (15, 16, 23, 32-34, 39, 45, 58, 60). Ilt has been show~nals'o $hat the cerebellar responses were sensitive to space localization of a stimulus (1, 3). Moreover, (there are data suggesting involvement of both middle ear muscles (54) and cerebellar vermis (37) in echo-location. On the basis of experi,ments with the acoustic representation on the cerebellar vermis, we postul~atedin 1973 th'at part of these connect~ionsare involved with the action of the mi~dldleear muscles (32-34). Data from the analysis of the interaction of flocculus with HVOR showed that the cerebellum can play the postulated role for the VOR, im~provingaction of the reflex and introducing the plastic changes. There are similarities in the organization of the HVOR and the AR inclu'ding findings that the Purkinje cell axons from lobuli VIII, IX, X of the cerebellar vermis directly reach the posteroventral part of the cochlear nucleus (24), which is part of the acoustic reflex arc (47). On the basis of these argum'ents, the following hypothesis is proposed. The cerebellum is involved in the control of the acoustic reflex, improving the frequ,ency characteristics, assuring the stability and equalization of both arcs, and causing neces,sary plasitic changes of the reflex. These functions should be in close connection with space localization of stimuli, where integration of acoustic and visual information shlould enable one to find the almount of changes in the work of the acoustic system necessary for proper space localization. Addition- ally, it is postulated that the meclhanism of the plastic changes is based on the Albus Ithemy, 4.e., &hie infomatiom via olimbl~gf~ibeax modulaltes transmi'ssion occurring in the cerebellar contex depending on the coincidence of excitation in mossy and climbing filber afferents. The activity of climbing fibers should be correlated with the discordance of acoustic and visual signals about space localization and/or imbalance of middle ear muscle action. The existing literature supporting the above hypothesis is very limited. It was found that miographic activity of the stapedius muscle Increases after decerebellation (12). Some anatomical wo~ksare postulat- lng the -possibility of connections of the dorsal cochlear nucleus with the cerebellar vermis (48, 62), and lobuli VIII, IX, X of the vermis with the posteroventral part of the cochlear nucleus. There IS also a suggestion of a connection of the cochlear nucleus with the cerebellum via the subtrigeminal portion of the lateral reticular nucleus of Brodal (10). Accordingly as the first step, it was decided to analyze connections ot the cerebellar vermis with different levels of the acoustic reflex pathway using elect~phy!siologicalmethods. Anatomical data suggest the existence of a connection of lobuli VIII, IX, X with the ventral cochlear nucleus. As these regions of vermis are outside of the classic acoustic region on vermis, it was initially checked whether in given experimental situation, the acoustic information reaches this part of the cerebellum. The next experiments were based on the prediction that if the cerebellum is involved in the control of the acoustic reflex, its stimulation should cause the changes of the activity of the stapedius muscle. The anatomical dab suggest the ventral cochlear nucleus as a site for the intemwtion of the cerebellum with the acoustic reflex arc. Therefore an attempt was made to confirm this supposition by means of recording responses from different ,parts of the cochlear nucleus evoked by electrical stimulation of the vermis. During previous experimental work (32, 33), the slow waves were recoded firom the round windlow in alddition to the cochlear microphonic potential. The interesting feature of these waves was their cmelation with waves recorded from the cerebellar vermis. As one of the sources of these potentials, the activity of the stapedius muscle and its nerve was proposed (32). As was menti~oned(earlier, this muscle exhibits activity not only to amustic stimulation, but also to stimulation of the face, or the activity correlated with the movement of a body ox vocalization. To analyze the problem of the origin of the slow waves, experiments wlth stimulation of the motor nucleus of the stapedius muscle and recording from the round window were done.

METHOD This study wals performed on 12 cats. Half of them were decerlebrated under short-acting anesthesia (Ketalar 10 mg/kg). The remaining cats were examined under pentolba~bitalalnesthesia '(Nemlbutad, initial dose 40 mglkg ~ntramusculary).During the experimental session, additional doses of 5 mgkg i.v. were given when necessary to maintain the anesthesized state. Paralyzing agents were not used. The temperature of the an~malswas kept constant at approximately 38°C. An 0.5 mm silver ball electrode was placed near the round window of the cochlear using a dorsolateral approach, for the punpose of recording the microphonic potential and slow waves. The bone overlying the cerebellar vermis was widely removed exposing not only all lobuli down from the fisura prima bubt the entire cerebellum to the right of the middle-line. This operation allowed the motor nucleus of the stapedius and the cochlear nuclei to be reached through the cerebellum. After the recording of evoked potentials from the surface, the exposed area was covered by 2010 agar to prevent drying and strong pulsation. Recopding fxm the cerebellar cortex was performed under visual control of the electrode position. The stapedius and cochlear nuclei were located stereotaxically and approached either vertically or at 50" and 30" back fmthe vertical, reqpecrtively. The positions of these electrodes were adjusted to obtain the best response to electrical stimulation of the cerebellar vermrs and to acoustic stimulation. A single click, caused by a rectangular pulse of 0.5 ms duration applied to ear-phones (Type SN 50), was used as a standard acoustic stimulus. This stimulus was applied to the right ear through a coupling device and a hollow stereotaxic ear-bar at a rate of one every 3 s. This stimulus represents a narrow band noise with dominant frequency around 2.5 kHz and peak amplitude approximately 100 dB SPL (Impulse Precision Sound Level Meter and Octave Filter Set, Bruel and Kjar Type 2209, 1619). In addition, trains of click (train frequency 1,000 Hz) of dumtlons from 5 to 100 ms, )pure tones or white noise were used. For electrical stimulation of the vermis, the single pulses (duration 0.2 ms) or short trains of three to five pulses (frequency 300 Hz) were apphed through metal mono- or bipolar electrodes. The peak current value was monitored and kept in the range 10 to 200 PA. For single cell recordings metal micnoelectmdes were used. Electrodes with more exposed tips were used to record evoked potentials in the cochlear nucleus. For recording from the cerebellar surface and round window of the cochlea, silver ball electrodes with diameters of about 0.5 mm were employed. The resulting signals were amplified and fed into an ANOPS 101 analyzer, which allowed averaging of the potentials and construction of post-stimulus time histograms. To obtain the envelope of the microphonic potential evolked by conlstant tone stimulation, the signal was full-wave rectified before the averaging process. RESULTS

The responses recorded from the cerebellar vermis on acoustic stimulation. Examples of evoked potentials in the cerebellum from click are shown in Fig. 1A-C. Notice that the potential evoked on lobulus VIII is clear, thugh smaller than on the lobulus VI. The latency is similar

Fig. I. Cerebellar responses to acoustic stimulation. A, B, C, averaged evoked potentials taken from lobuli VI, VIII of the vermis and paraflocculus respectively; 32 repetitions of single click stimulation; decerebrated cats; positivity upward. In this and the following figures, stimuli were given at the begining of an analysis. D-G, PSH of Purkinje cells; vertical bars represent 10 impulses per bin. D-F, simple spike responses from decerebrated cats; G, complex spike response in pentobarbital anesthesia; histograms from the right side of each pair represent expanded initial parts of the left side the histogram. D, lobulus VIII, 64 repetitions of 100 ms train of clicks; E, lobulus VI, 128 repetitions of 20 ms train; F, lobulus VIII, 128 repetitions of the single click; G, lobulus VII, 32 repetitions of 10 ms train. For details see Method.

to the classic cere~bellar acoustic response of about 10 ms. No clear response is observed when the electrode was outside the vemis or paravermis. The simple spike activity of 64 Purkinje cells from the cerebellar vermis was investigated; 22 (34OIo) .of them show a response with latency around 15 ms. Examples of post-stimulus-time histograms are shown in Fig. ID-G. About half of the cells were recorded from lobuli VIII and IX; the ratio of responding cells in this region was approximately the same as in the classic acoustic area of the vermis. Climbing fiber responses were analysed in four cases (Fig. 1G). Two cells had no response to click, and two showed excitation. The influence of the electrical stimulation of the vermis on the end results of the acoustic reflex. It was shown that the activity of the middle ear muscle can be described with nearly the same accuracy by the measurement of the miographic activity, changes of the acoustic impendance or changes of the microphonic potential (9). The last method was choosen, and changes of the ampli'tude of the microphonic potential evoked by 1,500 Hz tone of constant intensity (about 90 dB SPL) was taken as a reflection of middle ear muscle activity. This intensity of the acoustic stimulus is above the threshold of evoking additional tension of the stapedius muscle. ?vhe white matter of the lobulus VIII of the cerebellar vermis was stimulated via the bipolar electrode using a single pulse with a duration of 0.2 ms and peak amplitude of 50 PA. As shown in Fig. 2, such stilmulation causes an initial increase of the microphonic potential amplitude followed by a latter decrease. These changes represent the initial relaxation of the stapedius muscle followed

~ -- Fig. 2. The influence of electrical stimulation of the lobulus VIII of the cerebellar vermis on microphonic potential. The left column shows the single sweep taken from the osciloscope; right, averaged, full wave rectified potentials of 32 repetitions. Pentobarbital anaesthesia. A, cochlear microphonic evoked by constant level (100 dB SPL) pure tone stimulation of 1,500 Hz. B, records from round window on sole electrical stimulation of lobulus VIII (single pulse; t = 0.2 ms 50 pA). Marked on lower beam. C, combined re- H sults of above described stimulation. 100 m~ by the contraction. The latency is about 15 ms, and the entire effect lasted up to about 125 ms. To obtain an envelope of the signal and to avoid a cancelation of the signals coming with different phases, full wave rectification of the signal was done before averaging. The sole acoustic stimulation gives a horizontal 16ne (Fiig. 2A) became in (this case the ampliltude of the colclhlea~ microphonic is constant. The observed noise is a result of the incomplete averaging of random values of the phase at the begining of the sweeps.

3 - Acta Neurobiol. Exp. 3/81 The sole el&~ic stimulation of the lobdlus VIII caused only an electrical artifact of the stimulus and some short small waves (Fig. 2B). However, the combined stimulation showed clearly the initial increasing of the cochlear microphonic amplitude followed by its decrease (Fig. 2C). Connections of the vermis with the cochlear nuclei complex. To determine if there is any specificity of the connection of the cerebellar vermis with the cochlear nucleus, the following experiments were performed. The position of the electrodle inserted into the different parts of the cochlear nucleus and its surrounding was changed during the recording of the ,potentials evoked by the click or the electric stimulation of 'the cerebellar vermis. The resulb are pesente~din Fig. 3. It may be

electrical atoustlc stimulation "------A,, Fig. 3. Responses recorded from the cochlear nucleus (CN) evoked by the

".- ".- f acoustic stimulation and electrical stimulation of lobulus VIII of the cerebellar vermis. Pentobarbital anesthesia; acoustic C--- C--- C stimulation-single click; electrical stimulation-single pulse, t = 0.2 ms, 100 PA; averaged 32 repetitions; positivity upward. Points of recordings were placed at approximately: A, 1 mm caudal and 0.5 E mm ventral from the CN. B, caudal bor- - '4- der of the dorsal CN. C, caudal part of the ventral CN. D, middle part of the C--( ventral CN. E, rostra1 part of the CN. seen that hr both types of stimulation, there was some reson of best response. This area was smaller for the electrical than for the acoustic stimulation, and the change of an electrode position caused stronger alternation of the response for the electrical than for the acoustic stimulation. Therefore it seas possible to postulate the existence of the particular parit of (the coch&ear nucleus which is conn& with the cembellar vexunis. Round window responses evoked by acoustic and nonacoustic stimulation. The potentials from the round window were recorded during acoustic stiimulation and during electirical stimulating via an electrode which went through the motor nuclei of the facial nerve. The acoustic stimulus ev&d the microphonic potentials and the slow wave (Fig. 4A and B). The low1 electrical stimulation of the nucleus of the facial nerve by a single pulse of the weak current (50 yA) caused the slow wave witlh thle latency approximately 8 ms lasting to 20 ms (Fig. 4C). Subsequently, the initial tension of the stapedius muscle was increased by applying the constant white noise with an intensity of abvrut 90 dB SPL parallel with electrical stimulation. As a result, the response increased conside- rably (Flg. 40). Next, the influence of the ,position of the stimulating electrode on the response recorded from the round window was investigated (Fig. 4E-I). It appeared that when the stimulating electrode was three mili-

Fig. 4. Averaged responses recorded from the surrounding of the cochlear round window to a single click stimulus (A, B) or electrical stimulation of the nucleus of the facial nerve (C-I). A, B, pentobarbital anaesthesia; C-I, decerebrated animals. A, coch- 9E ' lear microphonic potential to the click, 32 repetitions, potential filtrated by high pass A *w-- E "1 1- w filter (500 Hz). B, the same as A except the sum frequency band was from 1 Hz to 10 kHz L and a different time scale. C, full wave rec- ,,y~ tified responses for a single pulse (t = 0.2 B ms, 50 PA) electrical stimulation of the nuc- '- leus of the facial nerve. 64 repetitions. The 25- time scale is kept constant for the remaining " 6 $L recordings. D, the same as C but record was taken with the background of constant level white noise acoustic stimulation (90 dB SPL). c fk'"L- E-I, the influence of the position of the H wms iY's, stimulating electrode on the recorded response, stimulation conditions as for D. 64 repetitions. E, F, G, I, 3 mm, 2 mm, 1 mm above, and 1 mm below the optimal point. I- H, note the disappearance of ,fast components parallel with the appearance of late waves. Distortion of the first few milisecond of records is due to artifact from the electrical stimulation. meters above the previously described point, the slow wave disappeared completely and new, with a latency of 3 ms and lasting to 10 ms, highe~ frequency components may be observed. Parallel with desce~nding the electrode, the fast components decreased to vanishing, and the slow waves appeared. One milimeter below the point optimal for slow waves, all responses receded from view. The change of the position of tlhe stimulating electrode caused some additional effect. Namely, the excitation of the point optimal for evoking of the slow waves caused mall but clear flick of the lower lip. If the electnode position was changed, this response decreased to vanishing and eventually a jmk of other parts of facial musculatuxe was observed. This observation is of some interest because the same part of the motor nuclei of the facial nerve su~ippiliesthe stape~diusmuwle and the muscles of lower lw (47).

DISCUSSION

In this paper an interaction of the cerebellar vermis with the acoustlc reflex arc was analysed by means of elec~~physiologicalmethods. Special attention was paid to the regions suggested by anatomical work, i.e., lobuli VIII, IX, X of the cerebellar vemnis and cochlear nucleus (24). It was initially shown that from this part of the vemis, which isl outside of the classic acoustic region, it was possible to register both evoked potentials and single cell reactions to an acoustic stimulus. Because of the danger of contamination of the responses by the stading reaction, the chloralosa anesthesia was discarded and the decerebration preparation or pento- bambital anaesthesia were used. Im adId!i%ion, the la& of a pa~ralysilng agent allowed us to notice any symptom of mlotor activity evoked by stimulation. Such a procedure has offered the possibility of keeping the level of penbbarbital anaesthesia ~lativelyconstant, and to observe twitches of groups of muscles caused by electrical stimulation of the moto~nuclei of the facial nerve. The decerebrated preparation has an additional advantage of damping the activity of tensor tympani muscle (12, 13). The single cell recording of Punkinje cells have documented the existence of clear reactions, although of smaller magnitude and ratio of occurrence than in chloralosa anesthesia. There was no difference in the proportions of the responding cells in the lobuli VI, VII, VIII and IX. The most frequent response consis'bed of emitation but other types of histograms were observed, for example multipeak excitation. The laten- cies were around 10 ms for evolked potentials and above 15 ms for single cell recording. Flor four cases the climbing fiber response was teste~d,and in two ot $hem excitatory rea&im were observad. These data indicate that acoustic information reaches the lobuli VIII, IX of the cerebellar vermis in addition to the higher part of the ve~xais.The smalller percentage of reacting cellh tihan in ch~lwaloseanes- thesia, or in the case of oonscious animal, may be due to the depressing action of the preparations employed. Previously reported data j(32, 33) revealled the existence of slow waves in the recordings from the round window evoked by acoustic stimulation. These potentials were of special interest because of their correlation with the potentials observed on the cerebellar surface. As an explanation of the origin of the slow waves, the integrated activity of the stapedius muscle and its nerve was suggested (32). In an attempt to prove this hypothesis recordings from the round window and observat- ions of the facial musmlature were done during the electrical stimulation of motor nuclei of the facial nerve and its surroundings. To stimulate only the smal region (around 200 + 500 ym) (55) the pulses of the low amplitude (50 PA) were used. The results showed that slow waves could be evoked by stimulation of the limited part of the motor nuclei of tlhe facial nervle. It is worth to note that excitation of this rcegion caused {the flidkw of the lower lip. It is known that the stapedius muscle is innervated by the same part of the motor nuclei of the facial nerve as the lower lip. The small displacement of the electrode up or down caused the rapid disappearance of the slow waves and the twitches of the lower lip. The excitation of the shallow part of the brain stfem evoked a series of short latency, high frequency waves. On the bases of anatomical data, it seems possible to postulate the vestibular nuclei or oliwcochlear bundle as structures for which excitation causes short latency mves. Thus, it seems pmbable that the slow waves observed in the round window can be due to the activity of the stapedius muscle and its nerve. In the Intmduc'tion sedion it was proposed that the cerebellar vermis interads with the acoustic reflex arc. If yo, one may expect that not only responses of the cerebellar cortex ev~okedby acoustic stimulation, but electric stimulation of the proper part of the cerebellum should cause changes at the different llevels of this reflex arc, i.e., on bhe stapedius muscle, the cochlear nucleus and the motor nuclei of the facial nerve. To detect xellaxatiom as (weill as stretching of the stapedius muscle, the constant level pure tone was used for evoking initial tension of the muscle. The cochlleacr mimophomic fpohenhila~l,evoked iby tihis stilm1ulus caln be used as a measure of the muscle activity. It was shown that cochlear microphonlc potential can be used, with the same accuracy, as a measurement of miographic activity of the muscle or the acoustic impendance of an ear (9). So, the enhancement of cochlear microphonic means muscle relaxation and its decrease - stretching. The w~eaiksingle pulse stimul- atlon of the lobuli VIII was observed to have a clear influence on the cochlear mlcrophonic. The response consisted of initial enhancement of the cochlear microphonic amplitude followed by its suppression. Such a time course of the effect of the stimulation can be due to initial inhibition of the transmission of infiormation in the AR an: which caused the decrease of stapedius muscle excitation. The initial tension of the stapedius muscl'e woultd decrealsle, causing a decrease of an attsenuation of the cochlear microphonic. Th~elatter effect may be due to the swb- sequent dishhibition of AR arc caused by the action of inlhibitory inter- newm in the cereMllar co&x or by antydrmic stimulation of the climbing fibers. It is known that collaterals of the climbing fibers reaching the given awa of cortex are gioinlg to the same nuclew as Pur- hisn je cells axm from this area. This ,pr,qposal is im ffull a,@eement with the fact that the cerebellum always has inhi!bitory action on the target nuclei (20, 21), and with the anatomical data which indicate the direct conn,ectim of plurkmje ce& wieh v'enltral c.ochl,ear inuoleus (24). The experiments in which the responses in CN were observed after the electrical stimulation of tihe clerebellar vermis, offer addition support fo~the hypoth~esisabout the internotion of the cerebellum with the AR arc, demonstrating tht the ce~ebellumis oonnected wiltih this nuclei of acoustic pathways, which is part of the acoustic r'efliex am. In addition, the localization of responses to electric stimulation was olbserve,d inside of the' CN complex, pointing to the ventral part of the CN, which is already involved in the AR arc. Taken collectively, these data suggest the existence of the inter- connections between the ce8re;bellarvermis and the acoustic reflex arc. Harli,er it was shown Ohat the milddle ear muscl~es,and panticulary stapedius muscle, act in a manner fax beyond the simple protlection of the cochllea agaiamt strong acoustilc stimuli. ghe proposed h~.porthesis suggested the important role of the starpedius muscle in the juldgement of the absolute level of atmuistic stimuli, their changes, and in the localization in space of an acoustic po'sition, pointing out the necessity of both rapid acti'on and plasticity of the acoustic reflex arc. The cerebellar vermis was suggested as a structul-e which allows improvement of frequency characteristics of the AR, integrated visual and acoustic information ajbout space localization, and enlables the plastic changes of AR arc. The data presented in this papeT imdicat,e that it is possi1b8leto show the existence of responses of the lower part of the cerebellar vermis on acoustic stimulation outside of the classic Snilder's and St'owell's acoustic regi'on of th'e cerebelllas vmis. Morleover, $he weak el~ed~icalsti~mulatisn of the same part of the cerebellar vermils has a strong influence on the acoustic ilnput, as was shown by analyzing th'e alhernations of the ampli- tudse of cochlear microphonic potentials during such stimulation. The data from the analysis of the HVOR have indicated that the cerebellum is alble to improve th,e clharacteristics of the reflex to assure its plasticity and integrate information of different mcodalities necessary for per- forming such actions. The similarity of construction, the characteristics and demand from the HVOR and the AR suggest the analogical role played by cerebellum for both reflexes. The tension of the st~aped~iusmuscle undergoes ra,pid alternations due to changes of the acoustic input. These changes have to be of short lat'ency because the rapid alt'ernations of the acoustic stilmuli level observed under normal con~ditions,anld because of the great physiological impoI.lta'nce of quick deiterminaition of stiunul~us,posiiti,o~n an,d carny into effect the obtlained information. This process is possi'ble because tshe main arc coasists of only three neurons, th~esame as for the HVOR, so that the latency is arounld 10 ms. By mews of property adjusting the transmission through the left and right arcs, it is possible to a.ssure the inter- au~albalaince of the acoustic i'nput. The. cerebellar vermils obtains the acoustic inf~ormarti~onfrom the cochlear nuclei complex sending, in turn, fibers to the ventral part of tlhe CN, whiah is pad of the AR an. This information in the vermis i's integrated with that of the origin from the viisual input, neck muscles, and highw levels of the aooudic system. AE a results of these int'egrat- ions, the parameters of an additional arc via the vermis can be changed allowing the plastic changes of the AR. Parallel with this role, the adtdit- ional arc, working in a feed-forward manner, should improve the frequency characteristics of the AR and allow for its work even on the bord'er of stlabiliity. Thus, it is postulated thart the 'cerebellar vermis acts i~na similar manner hr acoustic reflexels as the fEocculus for the HVOR.

The autor wants to thank Dr. S. Kasicki for assistance during the initial part of experiment. This investigation was supported by Project 10.4.1.01.3 of the Polish Academy of Sciences.

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Accepted 20 February 1981