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Symposium on the Role of the Vestibular Organs in Space Exploration (1970)

Chapter: CORTICAL PROJECTION OF LABYRINTHINE IMPULSES: STUDY OF AVERAGED EVOKED RESPONSES

« Previous: EVOKED POTENTIAL AND MICROELECTRICAL ANALYSIS OF SENSORY ACTIVITY WITHIN THE CEREBELLUM
Suggested Citation:"CORTICAL PROJECTION OF LABYRINTHINE IMPULSES: STUDY OF AVERAGED EVOKED RESPONSES." National Research Council. 1970. Symposium on the Role of the Vestibular Organs in Space Exploration. Washington, DC: The National Academies Press. doi: 10.17226/18593.
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Suggested Citation:"CORTICAL PROJECTION OF LABYRINTHINE IMPULSES: STUDY OF AVERAGED EVOKED RESPONSES." National Research Council. 1970. Symposium on the Role of the Vestibular Organs in Space Exploration. Washington, DC: The National Academies Press. doi: 10.17226/18593.
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Suggested Citation:"CORTICAL PROJECTION OF LABYRINTHINE IMPULSES: STUDY OF AVERAGED EVOKED RESPONSES." National Research Council. 1970. Symposium on the Role of the Vestibular Organs in Space Exploration. Washington, DC: The National Academies Press. doi: 10.17226/18593.
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Suggested Citation:"CORTICAL PROJECTION OF LABYRINTHINE IMPULSES: STUDY OF AVERAGED EVOKED RESPONSES." National Research Council. 1970. Symposium on the Role of the Vestibular Organs in Space Exploration. Washington, DC: The National Academies Press. doi: 10.17226/18593.
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Suggested Citation:"CORTICAL PROJECTION OF LABYRINTHINE IMPULSES: STUDY OF AVERAGED EVOKED RESPONSES." National Research Council. 1970. Symposium on the Role of the Vestibular Organs in Space Exploration. Washington, DC: The National Academies Press. doi: 10.17226/18593.
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Suggested Citation:"CORTICAL PROJECTION OF LABYRINTHINE IMPULSES: STUDY OF AVERAGED EVOKED RESPONSES." National Research Council. 1970. Symposium on the Role of the Vestibular Organs in Space Exploration. Washington, DC: The National Academies Press. doi: 10.17226/18593.
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Suggested Citation:"CORTICAL PROJECTION OF LABYRINTHINE IMPULSES: STUDY OF AVERAGED EVOKED RESPONSES." National Research Council. 1970. Symposium on the Role of the Vestibular Organs in Space Exploration. Washington, DC: The National Academies Press. doi: 10.17226/18593.
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Suggested Citation:"CORTICAL PROJECTION OF LABYRINTHINE IMPULSES: STUDY OF AVERAGED EVOKED RESPONSES." National Research Council. 1970. Symposium on the Role of the Vestibular Organs in Space Exploration. Washington, DC: The National Academies Press. doi: 10.17226/18593.
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Suggested Citation:"CORTICAL PROJECTION OF LABYRINTHINE IMPULSES: STUDY OF AVERAGED EVOKED RESPONSES." National Research Council. 1970. Symposium on the Role of the Vestibular Organs in Space Exploration. Washington, DC: The National Academies Press. doi: 10.17226/18593.
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Suggested Citation:"CORTICAL PROJECTION OF LABYRINTHINE IMPULSES: STUDY OF AVERAGED EVOKED RESPONSES." National Research Council. 1970. Symposium on the Role of the Vestibular Organs in Space Exploration. Washington, DC: The National Academies Press. doi: 10.17226/18593.
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Suggested Citation:"CORTICAL PROJECTION OF LABYRINTHINE IMPULSES: STUDY OF AVERAGED EVOKED RESPONSES." National Research Council. 1970. Symposium on the Role of the Vestibular Organs in Space Exploration. Washington, DC: The National Academies Press. doi: 10.17226/18593.
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Suggested Citation:"CORTICAL PROJECTION OF LABYRINTHINE IMPULSES: STUDY OF AVERAGED EVOKED RESPONSES." National Research Council. 1970. Symposium on the Role of the Vestibular Organs in Space Exploration. Washington, DC: The National Academies Press. doi: 10.17226/18593.
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Cortical Projection of Labyrinthine Impulses: Study of Averaged Evoked Responses1 E. A. SPIEGEL, E. G. SZEKELY, H. MOFFET, AND J. EGYED Temple University Medical School SUMMARY While the observation of the eyeballs, of the reactions of the trunk, and of the extremities permits only a study of the vestibulo-ocular and of the vestibulospinal reflex arcs, the perrotatory or post- rotatory recording of the electroencephalogram or electrocorticogram may help one to ascertain the conduction of labyrinthine impulses and their projection to the cerebral cortex. The cortical responses to single rotations were summed by a Mnemotron computer. After cessation of rotation, long-latency, slow, sometimes multiphasic responses appeared in human subjects and in cats. In man they were either diffuse or were noted chiefly or exclusively in the region of the area preoccipitalis and/or para- striata. They are probably due to excitation of the diffuse thalamic projection system. Short-latency responses in the cat's cerebral cortex at the start of rotation were not limited to the second somatic sensory area, but were found also in parts of the auditory cortex and in the so-called association cortex; in some experiments they were also close to, or in parts of, the second visual area. The initial as well as the postrotatory reactions in posterior parts of the cerebral cortex were not prevented by bilateral ablation of the second sensory area: they depended on a functioning labyrinth. The usual methods of testing the excitability of the labyrinth are limited to observations or records of reflex reactions to the muscles of the eyes, the trunk, and the extremities. Such tests do not give any information about the state of the cortical projections of the labyrinth and their afferents above the mesencephalon, though they may be influenced by corticofugal impulses. It seemed, therefore, of interest to develop methods of testing these corticopetal labyrinthine systems. In selecting a method of stimulation that would be applicable to man, electric stimulation seemed inadvisable, since it is not possible to stimulate the labyrinth selectively by using surface elec- trodes without affecting adjacent receptors and nerves. Caloric stimulation may induce cortical responses; usually, however, these are too weak and inconstant to be useful for systematic studies. It would seem, therefore, that production of an 1 Aided by grant 04418, National Institute of Neurological Diseases and Blindness, MH. LSPHS. endolymph flow by rotation would be the prefer- able method of labyrinthine stimulation. The usual type of stimulation by 10 rotations in 20 seconds on a Barany chair is too intense for a study of possible localized cortical responses be- cause the excitation quickly affects the whole cortex. An attempt was made, therefore, to use single rotations only. By employing an averaging Mnemotron computer, the responses in constant- time relationship to the stimulus (cessation or onset of the rotation) could be summed and the discharges unrelated to the stimulus diminished (refs. 1 to 4). Two groups of experiments were performed: studies of the responses following the cessation of rotation and of those at its onset. The first series was carried out in 37 cats maintained in bulbocapnine catalepsy, and in 43 human sub- jects who showed no signs or symptoms of impairment of the inner ear or of the nervous system; the second series consisted of 26 cats only.

260 THE ROLE OF THE VESTIBULAR ORGANS IN SPACE EXPLORATION TEST SERIES 1 Procedure In the first series, mostly unipolar, but in some instances also bipolar, derivations were used. In cats the recording electrodes were epidural stainless-steel balls; the reference electrode was the head holder or a plate fixed above the frontal sinus. In the human subjects. Grass electrodes were applied, with the reference electrode placed on the chin. The wires connecting the electrodes to the am- plifiers and the averaging Mnemotron computer were conducted over the subject's head in the axis of the rotating chair. Each rotation lasted 3 seconds; as soon as possible after its cessation, the computer was turned on. After the recording of the response, the chair was slowly rotated in the opposite direction (e.g., counterclockwise after clockwise rotation) so that an undesirable twisting of the wires by repeated rotations in the same direction was avoided. The rotation was repeated after a pause of 1 minute. After 10 rotations the averaged responses were displayed on an X-Y plotter (model 500, Electro Instruments Inc.), and this process was repeated, so that the effects of 10 to 30 or even more rotations were averaged. Thus, both the summated responses and possible changes of the excitability, for example, those due to habituation, could be studied. In one test, only the reactions to rota- tions in one direction were studied. The reac- tions to the other direction were recorded after a pause of several hours or on another day. To prevent interference of retinal impulses, the eyelids of the experimental animals and human subjects were kept closed during the rotations. Sp»t IV* B FIGURE 1. — Human subject without organic disease. Aver- aged electronystagmograms (E) and monopolar EEG (scalp) records. A: Before rotations. B: After 15 single clockwise rotations (averaged). Similar responses in the frontal (F,), temporal (T,), and occipital (O,) leads. Time signal in figures I to 8: 0.4 sec. (From ref. 3.) tar — ... a W ~J 4^ ^^A/T^ <^/v ^ VU\ f^f r*$r v/V V^ ^/^N^ ^W1- •rtlr FicURE 2. —Human subject without organic disease. Postro- tatory responses to 10 single clockwise rotations (averaged). F:I, frontal, Ts, temporal, Hn, parietal, Oi, occipital leads. Response in (), prolonged and with higher amplitude than in other leads. (From ref. 3.) Results and Discussion The responses observed in man had a long latency; they were either diffuse (fig. 1), or they prevailed in (fig. 2) or were restricted to (fig. 3) the region of areas 19 and/or 18 of Brodman (preoccipital and parastriate regions). Some- times they were multiphasic. To ascertain whether eye movements or contractions of neck muscles interfered, electronystagmograms (fig. 1) were recorded in some instances, and in others, electromyograms of the neck muscles were recorded. Only rarely did such potentials play a part and invalidate the records. In most records there was no such interference; it seems justifiable to assume a cerebral genesis of the responses. The experiments on cats (figs. 4 to 8) served as a further analysis of these responses. After peak latencies of 0.3 to 0.6 second, one finds slow, sometimes multiphasic responses. They were recorded not only from anterior parts of the cere-

CORTICAL PROJECTION OF LABYRINTHINE IMPULSES 261 FIGURE 3. — Human subject without organic disease. Bipolar scalp EEG with phase reversal in Oi. Averaged postrota- tory responses to 10 single clockwise rotations. Fa, frontal; d, central; Tit, temporal; and Oi, occipital leads. (From ref. 3.) bral cortex (anterior part of the gyri ectosylvius and suprasylvius) but also from posterior parts (posterior and middle ectosylvian and suprasyl- vian and lateral gyri) (fig. 4). The gyms ectosylvius anterior and the gyrus suprasylvius anterior are regarded by some as the "vestibular" cortex. It seemed, therefore, of interest to determine whether unilateral or bi- lateral ablations of these areas prevent the appearance of these responses in the posterior parts of the cerebral cortex. This is not the case (figs. 5 to 7); therefore, the responses recorded from the posterior areas cannot be caused simply by spread or propagation from the anterior part of the suprasylvian and ectosyl- vian gyri. Whether impulses of extralabyrinthine origin, e.g., proprioceptive impulses from striated mus- cles, are responsible for these reactions was the subject of further study. The fact that the cortical responses are independent of the post- rotatory nystagmus (fig. 5) excludes propriocep- tive impulses from the eye muscles as the source of the cortical responses. Furthermore, neither muscular paralysis induced by gallamine tri- ethiodide (Flaxedil) nor high transverse section of the spinal cord at C, (fig. 4), which interrupted ascending impulses also from the body surface, from joints, and from viscera, interfered with these responses. In contrast, they no longer appeared after chemical destruction of the re- ceptors of the labyrinth by 70 percent alcohol or 4 percent formaldehyde (fig. 4); so, the laby- rinthine origin of these responses can be inferred. Acoustic stimuli of low intensity (persistence or Ml billobyctctV 20 in LEsa potto LEsm LSsmOStp. FIGURE 4. — Cat. Transverse section of cord at C,. Compare averaged spontaneous discharges of cortex, 20 averaged postrota- tory responses before and after bilateral Inbyrinthectomy. L.Es.a.post.p.: Left gyrus ectosylvius anterior, posterior part. L.Es.m.: Left gyrus ectosylvius medius. L.Ss.m.: Left gyrus suprasylvius medius. Ss.p.: Gyrus suprasylvius posterior. L.L.p.: Left gyrus lateralis posterior. (From ref. 3.)

262 THE ROLE OF THE VESTIBULAR ORGANS IN SPACE EXPLORATION Both Eta cot Spun LLp FIGURE 5. — Comparison of electronystagmograms (E) anrf cortical postrotatory responses in a cat with gyrus ecto- and suprasylvius anterior bilaterally extirpated. Cat in bulbo- capnine catalepsy. A: Records in resting state (averaged). B: Records following 30 single clockwise rotations (aver- aged). L.Es.a.: Area of extirpation of left gyrus ecto- sylvius and suprasylvius anterior. L.Ss.p.: Left gyrus suprasylvius posterior. L.Lp.: Left gyrus lateralis, pos- terior part. (From ref. 3.) exclusion of the noise from an adjacent room) did not significantly alter these responses. With regard to the afferent pathways by which the vestibular impulses elicit these slow cortical waves, the long latency of the responses sug- gests a multisynaptic system. The same con- clusion is suggested by the sensitivity of these responses to anesthesia; pentothal anesthesia, for instance, is able to diminish or to abolish them (fig. 8). Obersteiner (ref. 5), as early as 1912, anatomically traced fibers from the vestibular nuclei to the reticular formation, and Held (ref. 6) and Godlowski (ref. 7) later described reticulo- thalamic pathways. Thus it may be inferred that vestibular impulses activate the diffuse tha- lamic projection system and that the slow waves appearing after a long latency are so-called secondary evoked responses. We have seen that these reactions were often FIGURE 6. — Averaged postrotatory cortical responses in cat kept in cataleptic state by bulbocapnine (brain shown in fig. 7); left gyrus ectosylvius anterior and suprasylvius anterior ablated. A: Effect of 10 single counterclockwise rotations. B: Effect of 30 single counterclockwise rotations. L.Es.a.: Area of ablation of left gyrus ectosylvius anterior. L.Ss.m.: Left gyrus suprasylvius medius. L.L.p.: Left gyrus lateralis posterior. (From ref. 3.) more marked and/or longer lasting in the pos- terior part of the cerebral cortex. This suggests that it is particularly those parts of the areas close to the visual cortex which are stimulated. That only a certain part of the diffuse thalamic system is stimulated is not an unusual situation. Moruzzi (ref. 8) has already indicated that sensory stimuli of low intensity may activate only part of the reticular system, and the same may be postulated for the diffuse thalamic system that receives im- pulses from the reticular activating system. The question arises whether, besides the activa- tion of parts of the diffuse thalamic system, there exists a more circumscribed cortical projection of the labyrinth. Recordings made after cessa- tion of the rotation hardly permit one to answer this question; since acceleration induces an excitation of the cortex, the corticopetal impulses induced.by the deceleration do not act upon a resting cortex but upon one already invaded by impulses of labyrinthine origin. In further experiments, therefore, an attempt was made to develop a method for averaging the cortical responses to the onset of single rotations by a Mnemotron computer.

CORTICAL PROJECTION OF LABYRINTHINE IMPULSES 263 SpcnI FIGURE 7.— Cat, bilateral destruction of gyrus ectosylvius anterior and gyrus suprasylvius anterior. (From re/. 3.) LS.H FIGURE 8. — Averaged spontaneous discharges and responses following 30 single counterclockwise rotations. Upper two records: Cat in bulbocapnine (25 mglkg) catalepsy. Lower two records: Same cat under sodium pentobarbital anes- thesia. L.Ss.m.: Left gyrus suprasylvius medius. L.Lp.: Left gyrus lateralis posterior. (From ref. 3.) TEST SERIES 2 Procedure The rotation was produced by a weight of 100 grams that acted by means of a string carried over an upper and lower pulley and along the grooved periphery of a rotating disk (radius 35.5 cm) to which its free end was attached (fig. 9). The animal board was fixed to this disk so that the cat's head was at the center of the disk. The experiments were performed on 26 cats; most of them were under Nembutal (3 to 12 mg/kg) and chloralose (25 mg/kg) anesthesia. The triggering circuit consists essentially of a hand switch (//) and a mercury foot switch (M) in series with a 3-volt battery (B) (fig. 10). If both switches are closed, the sweep of the computer is triggered. The hand switch consists of a rigid straight spring (A) and, parallel to it, a shorter plate carrying a screw (S) that is in contact with A. The spring (A) of the hand switch is held by the experimenter against the animal board that is fixed to the rotating disk, thus preventing the rotation of the disk by the pull of the weight. The hand switch is opened by pressure exerted on A. In this stage the previously opened mercury foot switch (M) is closed. If the experimenter now discontinues the pressure of A against the animal board, rotation of the disk by the pull of the weight is started, and the spring (A) comes in contact with the screw (S) so that the circuit triggering the sweep of the computer is closed. FIGURE 9.— Rotating disk (r). Weight (w) acts upon it over pulleys (p). Light from electric bulb (b) reaches photocell (ph) through slits (si). (For further details, see text.) "JJ. , ) : 5 A V ( J to compuUr v '• T i — ilil 3- M FIGURE 10.-Circuit triggering computer and rotating table. H: Hand switch with rigid straight spring (\)and screw (S). M: Mercury foot switch. B: 3-volt dry-cell battery. T: Triggering switch {for control experiments without rota- tion). V: voltmeter. (For further details, see text.)

264 THE ROLE OF THE VESTIBULAR ORGANS IN SPACE EXPLORATION To determine the angular acceleration, a small stationary electric bulb was placed below the rotating disk close to its periphery (fig. 9). Two weights totaling 4 or 5 pounds, depending on the weight and position of the cats used, were fixed at the center of the disk. In the early experi- ments a plastic strip with slits at distances of 0.5° was attached to the part of the periphery at which the electric bulb was placed. The light of this bulb passed through the slits and activated a photocell; its current was conducted to one of the inputs of the computer. For measurement of the acceleration in the beginning of the rota- tion, the plastic strip was replaced by a photo- graphic film on which a series of equidistant vertical lines had been photographed, so that distances representing angular displacements (4>) of 0.05° were obtained. From the records showing the fluctuations of the output of the photocell, the angular displace- ments (4>), the angular velocities (o>), and the angu- lar accelerations (a) were determined (fig. 11). As can be noted in figure 11, the acceleration gradually increased until a constant value of 40°/sec2 was reached; after 10, 20, 40, 60, and 80 msec, respectively, the angular accelerations were 5°, 10°, 18.5°, 26°, and 30°/sec2. The above-described arrangement served also for determination of the time interval between the triggering of the sweep of the computer and the beginning of the rotation in that the intensity of the light passing to the photocell —and thus the current emerging from the photocell in the resting state of the disk—was altered when the rotation started. An average time interval of 4.5 msec was measured between the start of the sweep and the onset of the rotation. Bipolar derivations were used; the stainless- steel-ball electrodes were placed in straight rows (see fig. 12) equidistant as far as possible. Results and Discussion The cortical responses obtained by the aver- aging technique at the onset of rotation had volt- ages varying between 10 piV and 100 /iV and lasted between 20 and 60 msec. Their peak latencies, i.e., from the beginning of rotation to the peak of the response, were between 10 and M M i If FIGURE 11.— Angular displacements (i£), angular velocities (o>), and angular accelerations (a) in the beginning of rotation. FIGURE 12.— Diagram showing electrode placement cor- responding to figure 13 in anterior part and to figure 19 in posterior part. Anterior row of electrodes: 1, gyr. sylv. post.; 2. gyr. sylv. ant. upper part; 3. corner betuven gyr. ectosylv. ant. and med.; 4, corner between gyr. supra- sylv. ant. and med.; 5. ant. part of gyr. lateral. Posterior row of electrodes: 10, gyr. ectosylv. post.; 11 and 12. gyr. suprasylv. post.; 13, gyr. lateral, post. {From ref. 4.)

CORTICAL PROJECTION OF LABYRINTHINE IMPULSES 265 60 msec. In microelectrode studies of the corti- cal responses to labyrinthine stimulation by polarization, Kornhuber and da Fonseca (ref. 9) distinguished specific primary cortical responses with latencies of 5 to 30 msec and specific associa- tive responses with latencies of 25 to 150 msec. If one accepts this classification according to the latent period, the responses obtained by the averaging technique would have to be considered as belonging partly to the specific primary and partly to the specific associative responses. There was no definite difference in the re- sponse, whether the rotation was directed toward or away from the hemisphere on which the re- cording electrodes had been placed. Muscle paralysis induced by gallamine triethiodide (Flaxedil) in waking or anesthetized cats did not prevent the reactions. Triggering of the sweep of the computer 10 times without acceleration produced only minimal fluctuations of the baseline. With regard to the areas from which these responses could be led off, it does not seem pos- sible to define a region from which they could be recorded exclusively. They could be ob- tained from £2, the second somatic sensory area in the anterior part of the ectosylvian and supra- sylvian gyri (fig. 13). Phase reversals were also observed from this zone (six times), i.e., deflections in opposite directions in two adjacent electrode pairs (1-2, 2-3), indicating that the area below the common electrode 2 corresponded to the site of the evoked potentials. It seems hardly justifiable, however, to designate the region of the gyri ectosylvius and suprasylvius anteriores as "vestibular" cortex as Kornhuber and da Fonseca do (ref. 9). One has to bear in mind that this region receives also impulses from other receptors (tactile, nociceptive, pro- prioceptive, acoustic) so that vestibular impulses can interact here with other modalities. The designation, composite sensory projection area or polysensory area (refs. 10 and 11), seems best to reflect these facts. Furthermore, responses showing the above- mentioned characteristics could also be obtained from areas behind £2, particularly from parts of the auditory cortex and of Buser's association cortex. Phase reversals were recorded from the FIGURE 13. — Averaged responses of cat's left cortical areas to onset of 10 counterclockwise rotations. Electrode place- ments are shown in anterior part of figure 12. Chloralose- sodium pentobarbital anesthesia was used in this and subsequent experiments if not otherwise indicated. Analy- sis sweep time, 500 msec; time marked 50 msec. Calibra- tion, 25 (iV (applies also to subsequent records). (From ref. 4.) gyrus sylvius anterior (twice), from the gyrus sylvius medius (six times), from the gyrus sylvius posterior (once), from the gyrus ectosylvius medius (six times), and from the gyrus supra- sylvius medius (three times). The most posterior areas exhibiting responses to the onset of rotation of relatively short latency (peak latencies 10 to 60 msec) were the gyrus ectosylvius posterior (figs. 14 and 15) and the most posterior part of the gyrus suprasylvius medius. This corresponds to the so-called second visual area. In 80 percent of the experiments (16 out of 20), the responses in the gyrus ectosylvius posterior had a higher amplitude than those in the adjacent parts of the gyrus suprasylvius. Phase reversals were observed four times from the sulcus ectosylvius posterior, three times from the anterior part of the gyrus ectosylvius posterior, but not at all from its posterior part. Once they were observed from the corner be- tween the sulcus suprasylvius medius and posterior, and once from the sulcus suprasylvius posterior. Unilateral or bilateral extensive lesions or ablations of the anterior part of the ectosylvius and suprasylvius gyri (figs. 16 to 18) did not prevent the appearance of the responses in

266 THE ROLE OF THE VESTIBULAR ORGANS IN SPACE EXPLORATION FIGURE 14. — Averaged responses to onset of 10 counterclock- wise rotations. Electrode positions are shown in figure 15. Gallamine triethiodide paralysis besides chloralose-sodium pentobarbital anesthesia. a=artifact. (From ref. 4.) FIGURE 15.— Diagram showing electrode positions correspond- ing to record in figure 14. 1, sulc. ectosylv. post.: 10 and 11, gyr. ectosylv. post.; 12, corner between gyr. suprasylv. med. and post.; 13, corner between gyr. lateral, med. and post. (From ref. 4.) the posterior parts of the cortex, while such responses did not appear after bilateral labyrinth- ectomy (fig. 19A before, fig. 19B after, labyrinthectomy). CONCLUSIONS Thus it would seem that the onset of rotation may induce so-called specific primary and specific associative responses not only in S-i but also in posterior parts of the cortex as far back as close to the second visual area. This may be the basis for a cortical integration of labyrinthine and retinal impulses. A comparison of our findings with the micro- electrode studies of Griisser et al. (ref. 13) and of Kornhuber and da Fonseca (ref. 9) shows that the responses recorded by these authors from the visual and paravisual regions showed chiefly a long and variable latent period; there- fore, they were regarded as unspecific reactions. Yet a specific vestibular influence upon the optic cortex could be demonstrated by Griisser et al. (refs. 12 and 13) in that the discharge rate of cells of the visual cortex could be influenced only by vestibular stimuli but not by trigeminal or acoustic stimuli. They assumed that afferent fibers with a specific function exist within the unspecific system. Our present findings seem to indicate that angular acceleration may, at least in some instances, also induce responses with relatively short latency; i.e., primary specific and/or specific associative responses in the vicinity of the visual cortex. This is in agreement with earlier experiments of Spiegel (refs. 14 and 15) in which labyrinthine stimulation by rotation was able to elicit epilepti- form convulsions in dogs and cats in which strychnine had been administered to the region of the gyrus ectosylvius posterior. Rotation producing an endolymph flow in both labyrinths is, of course, a much stronger stimulus than unilateral polarization of the labyrinth as used by Griisser et al. and by Kom- huber and da Fonseca, particularly since these authors had to apply rather weak currents in order to avoid an excitation of other nerves, such as the cochlear and intermedius. Furthermore, the averaging technique permits one to visualize responses that may be masked by the background activity of the cortex. These technical details may explain why it was possible to demonstrate responses with relatively short latency in the vicinity of the visual cortex, which did not appear

CORTICAL PROJECTION OF LABYRINTHINE IMPULSES 267 FIGURE 17. — Averaged response to onset of 10 counterclock- wise rotations I hour after the ablations shown in figure 18. The electrode positions correspond to the posterior row marked by ink dots in figure 18. (From ref. 4.) FIGURE 16. — Cat with bilateral ablation of the gyrus ecto- and syprasylvius anterior. Averaged responses to onset of 10 counterclockwise rotations, with phase reversal at the corner between the middle and posterior part of the sulc. suprasylvius (at 11). Other electrode positions: 1, at the gyrus sylvius posterior close to the lower part of the sulcus ectosylvius posterior; 10, gyrus ectosylvius posterior, upper part; 12, gyrus suprasylvius, corner between middle and posterior pan; 13, gyrus lateralis, corner between middle and posterior part. (From ref. 4.) FIGURE 18. — Bilateral ablation of the gyrus ecto- and supra sylvius anterior. (From ref. 4.) on electric stimulation of the vestibular nerve (refs. 10 and 11) or on unilateral polarization of the labyrinth (refs. 9, 12, and 13).2 '* We wish to express our deep appreciation to Dr. G. Henny, Dr. G. Stewart. G. V. Jacoby, and Ch. Zanes for their advice regarding the physical problems. FIGURE 19. — Influence of bilateral labyrinthectomy upon averaged responses to onset of 10 counterclockwise rota- tions. A: Responses before elimination of both labyrinths. B: Responses 2 hours after elimination of both labyrinths. Electrode positions are shown in figure 12 (posterior part). (From ref. 4.)

268 THE ROLE OF THE VESTIBULAR ORGANS IN SPACE EXPLORATION REFERENCES 1. SPIEGEL, E. A.; SZEKELY, E. G.; MOFFET, R.; GILDEN- BERG, P.; LEHMAN. R.; AND BOBBINS, A.: Projection of the Labyrinth to Posterior Areas of the Cortex. Proc. 8th Inter. Congr. Neurol., vol. 3, 1965, pp. 119- 122. 2. SPIEGEL, E. A.; SZEKELY, E. G.; MOFFET, R.; GILDEN- BERG, P.; LEHMAN, R.; AND ROBBINS, A.: Cortical Projections of the Labyrinth. Proc. Intern. Sym- posium, Vestibular and Oculomotor Problems, Univ. of Tokyo, 1965, pp. 9-14. 3. SPIEGEL, E. A.; SZEKELY, E. G.; AND MOFFET, R.: Cortical Responses to Rotation. I. Responses Recorded After Cessation of Rotation. Acta Oto- Laryngol., vol. 66, July-Aug. 1968, pp. 81-88. 4. SPIEGEL, E. A.; EGYED. }.; AND SZEKELY, E. G.: Cortical Responses to Rotation. II. Responses Recorded at the Onset of Rotation From the Second Somatic Sensory and Posterior Areas. Acta Oto-Laryngol., Sept. 1968, vol. 66, pp. 261-272. 5. OBERSTEINER, H.: Anleitung beim Studium des Baues der nerviisen Zentralorgane. 5th ed., Deuticke (Vienna), 1912. 6. HELD, H.: Die anatomische Grundlage der Vestibularis- funktionen. Beitr. Anat., etc., Ohr., vol. 19, 1923, pp. 305-312. 7. GODLOWSKI, W.: Les Centres Sous-corticaux du Regard et des Mouvements Associes des Globes Oculaires. Trav. Clin. mal. Nerv. de 1'Univ. Cracovie, 1936. 8. MORUZZI, G.: The Physiological Properties of the Brain DISCUSSION Parker: One of the most impressive aspects of this meeting has been the cataloging of the great wealth of neural con- nections with the labyrinth. In light of this information, and keeping in mind the facts that (1) your data show no associa- tion between evoked cortical responses and eye movements and (2) these evoked cortical responses are very widespread, might we consider that there are other possibilities, rather than strict vestibular afferent pathways, to account for your Stem Recticular System. Brain Mechanisms and Consciousness, J. F. Delafresnaye, ed., Blackwell (Oxford), 1954, pp. 21-53. 9. KORNHUBER, H. H.: AND DA FONSECA, J. S.: Optovestib- ular Integration in the Cat's Cortex. The Oculomotor System, M. B. Bender, ed., Hoeber Medical Division, Harper & Row, 1964, pp. 239-277. 10. MICKLE, W. A.; AND ADES, H. W.: A Composite Sensory Projection Area in the Cerebral Cortex of the Cat. Am. J. Physiol., vol. 170, 1952, pp. 682-689. 11. MICKLE, W. A.; AND ADES, H. W.: Rostral Projection Pathway of the Vestibular System. Am. J. Physiol.. vol. 176, 1954, pp. 243-246. 12. GRUSSER, O. J.; GRUSSER-CORNEHLS, U.; AND SAUR, 0.: Reaktionen einzelner Neurone im optischen Cortex der Katze nach elektrischer Polarisation des Labyrinths. Pfliigers Arch. ges. Physiol., vol. 269, 1959, pp. 593- 612. 13. GRUSSER, O. J.; AND GRUSSER-CORNEHLS, U.: Mikro- elektrodenuntersuchungen zur Konvergenz vestibu- larer und retinaler Afferenzen an einzelnen Neuronen des optischen Cortex der Katze. Pflugers Arch. ges. Physiol., vol. 270, 1960, pp. 227-238. 14. SPIEGEL, E. A.: Rindenerregung (Auslosung epileptiformer Anfalle) durch Labyrinthreizung. Versuch einer Lokalisation der corticalen Labyrinthzentren. Z. ges. Neurol. Psychiat., vol. 138, 1932, pp. 178-196. 15. SPIEGEL, E. A.: Cortical Centers of the Labyrinth. J. Nerv. Ment. Dis., vol. 75, 1932. pp. 504-513. data? For example, could we account for your data, perhaps, by cardiovascular influences? Szekelv: It could not be. but we did not study it. Prescott: Did you have an opportunity to look at the evoked potentials in the frontal cortex? Szekely: No; we did not. Lowy: Have you used or are you planning to use cats with chronic implants? Szekely: No.

SESSION IX Chairman: WILLIAM E. COLLINS Civil Aeromedical Institute

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