EFFECT OF RETICULAR STIMULATION UPON SYNAPTIC TRANSMISSION IN CAT'S LATERAL GENICULATE BODY

Transcription

1 EFFECT OF RETICULAR STIMULATION UPON SYNAPTIC TRANSMISSION IN CAT'S LATERAL GENICULATE BODY Hisao SUZUKI AND Norio TAIRA* Department of Physiology, Tohoku University, Sendai The visual signals transmitted in the geniculo-striate system are relayed at the lateral geniculate synapses, and it is likely that they are modified considerably at this level. Some evidence for this has been obtained from the previous experiment23), in which unit discharges were recorded from the optic tract and optic radiation. It was found that the discharge rate of radiation fibers was decidedly lower than that of tract fibers. On the other hand, HUBEL14) reported that striking differences were seen in the discharge pattern of geniculate neurons depending upon the cat's waking state, and on the basis of such observation he considered that some extraretinal afferent to the lateral geniculate might influence the activity of geniculate neurons. BRINDLEY6) also proposed this possibility as one of his alternative hypotheses on the function of the lateral geniculate. Many experiments have been carried out on the controlling influence of the reticular formation of the brain stem upon the visual system. HERNANDEZ-PEON et al. observed that the photically evoked potential of the lateral geniculate was greatly reduced in amplitude by stimulation of the reticular formation and that a similar attenuation of the geniculate evoked potential occurred during animal's attention to acoustic or olfactory stimuli11,12,13). In contrast with the depressive effect observed by HERNANDEZ-PEON et al., LoNG18) described that the geniculate evoked potential by optic nerve stimulation increased in amplitude with reticular stimulation. DUMONT and DELL8,9) also observed that the initial positive spike of the visual cortical response to stimulation of the optic chiasm was augmented in amplitude during reticular iterative stimulation, and assumed some reticular facilitatory effect on geniculate neurons. The purpose of the present experiment is to provide further information concerning the effect of reticular stimulation and its controlling mechanism on coding of visual messages at the geniculate level. METHODS Experiments were carried out on cats. Under ether anesthesia the trachea was Received for publication, June 15,

2 642 H. SUZUKI AND N. TAIRA cannulated, and the animal was transferred to a Horseley-Clarke apparatus. The dorsal surface of the skull was widely removed unilaterally with a dental burr. After these procedures had been finished, pressure points and wound margins were infiltrated with procaine, and then ether was interrupted. The animal was immobilized with d-tubocurarine (Flaxedil in the later experiments) administered through an inlying tubing in the saphenous vein and maintained on artificial respiration. Then, the pial surface of the cerebral cortex was exposed so as to permit the electrodes to be arranged as desired. The exposed surface of the brain was kept moist by frequent application of warm saline or covered with a thin layer of saline agar. The posterior cistern was opened to control the respiratory and pulsatory movements of the brain. For stimulation of the optic tract and the reticular formation and for recording geniculate evoked responses bipolar electrodes were used. They consisted of two silver wires, 100ƒÊ in diameter, insulated except 0.5mm. from their tips, the exposed tips of which were separated by a distance of about 1mm. Since the silver wires were too flexible to be inserted into the brain, an insulated stainless steel wire 100ƒÊ in diameter was cemented to them so that the electrodes were stiff enough for insertion. These electrodes were oriented stereotaxically. The reticular formation was stimulated at the level of the mesencephalon (F+3) ipsilateral to the recording site, and the optic tract near the optic chiasm. For recording unit discharges from the optic radiation or the visual cortex 3 M-KCl glass micropipettes or tungsten microelectrodes were used. A reference electrode was placed over the frontal sinus or nearby position so as to minimize shock artefact. Since stereotaxic placement of the stimulating electrodes onto the optic tract spared both eyes, the visual system could be activated by photic stimulation of either eye as well as by electrical stimulation of the optic tract. When necessary, the eyes were stimulated photically every two seconds. Two stimulators were arranged so that after application of a conditioning shock to the reticular formation, or other structures, a test shock could be delivered to the optic tract at an interval which could be varied systematically from zero to one second. A single pulse of 3 msec. duration was used for the conditioning stimulus, and that of 0.03 msec. duration for the test one. An RC amplifier with a time constant of 10 msec. and a dual beam cathode ray oscilloscope were used. To observe the detailed figures of test responses the sweep of one beam of the oscilloscope was set so that the test response appeared at a fixed position on the oscilloscope whatever the interval between the conditioning and test shocks might be. The other beam was used as a monitor with which the interval could be measured precisely. After experiments the brain was perfused with formalin and frozen sections were made to identify the position of the gross electrode. The stereotaxic atlases of JASPER and AJMONE-MARSAN16) were used as guides. RESULTS The bipolar recording electrodes were inserted vertically into the brain from the suprasylvian gyrus towards the lateral geniculate in steps of 1mm. and a

3 RETICULAR FORMATION AND LATERAL GENICULATE RESPONSES 643 response to a single shock stimulus to the optic tract was explored at each position of the electrode tip. The response obtained near/in the lateral geniculate consisted of two components or two elevations, although they varied in shape and polarity depending upon recording sites. Taking FIG. 1C as an example the first component occurred immediately after the shock artefact and was recorded negative in sign with the deeper electrode (in this record an upward deflection denotes negativity at the deeper electrode). The second component is longer in duration, but of the same polarity. This response pattern resembles that obtained by VASTOLA24) just above the lateral geniculate. Most experiments to be described below were carried out on such responses obtainable just above the lateral geniculate, because the effect of reticular stimulation was generally more conspicuous upon these responses than upon geniculate potentials from deeper parts of the geniculate, although there seemed to be no qualitative difference depending upon recording sites. Single shock stimulation of the mesencephalic reticular formation produced conspicuous modification of the succeeding geniculate potential evoked by a single shock stimulus to the optic tract; the second component of the response increased markedly in amplitude while the first one remained unaltered or was reduced slightly. An example of such differential effects upon the first and second components of the geniculate potential is shown in FIG. 1. As can be seen in this figure, the second component of the response evoked 30 msec. after the conditioning stimulus to the reticular formation (F+3, L+3, H-1) is clearly enhanced above that of control one (C in FIG. 1). Enhancement of the second component became more marked as the interval between the conditioning and test stimuli was increased. Maximal augmentation occurred between 70 and 90 msec. after the conditioning shock. The amount and duration of facilitation of the second component by reticular stimulation was not consistent even in one and the same animal. However in most cases the maximal facilitation occurred about 100 msec. after the conditioning stimulus, and facilitation continued over 500 msec. (FIG. 2). The maximal increase in amplitude produced by reticular stimulation was about 30 per cent in most cases, though it was over 100 per cent in a few cases. In general with Flaxedil more intensive and longer facilitation was produced than with curare, for example, in a cat immobilized with the former the facilitatory state could persist over one second after conditioning reticular stimulation. The conditioning shock alone did not evoke any response detectable in almost all the experiments, but occasionally an ill-defined long-latency and long-duration response similar to that described in LONG's report18) occurred following a conditioning shock alone. The effect of reticular stimulation upon the second component of the geniculate evoked response was perceived even when supramaximal test stimuli were used. Such an example is shown in FIG. 3. In this experiment the conditioning reticular stimulus of a certain definite intensity (10V in this case) preceded the test optic

4 644 H. SUZUKI AND N. TAIRA FIG. 1. Effect of reticular stimulation upon geniculate evoked potential to optic tract stimulus. C, control responses. Others are preceded by single shock stimuli (5 msec., 10V) to mesencephalic reticular formation (F+3, L+3, H-1) by intervals indicated in numerals in msec. Records are formed with superposition of five sweeps. Upward deflection denotes negativity at deeper electrode. Voltage calibration, 500ƒÊ/V. Time maker, 1 msec. These records were taken in cat immobilized with d-tubocurarine. FIG. 2. Time course of effect of reticular stimulation upon geniculate evoked response. Ordinates give increase in amplitude of response conditioned by reticular stimulation as per cent of that of control one. Abscissae give intervals between conditioning and test stimuli in msec. Open circles refer to first and solid ones to second component of response. Graph was constructed from records formed by superposition of three sweeps. These data were taken in cat immobilized with Flaxedil. tract stimulus by 100 msec., and the latter intensity was varied systematically. As seen in this figure, facilitatory effects can certainly be observed upon maximal as well as submaximal responses. This finding may be accounted for as follows: Many geniculate neurons remained subliminal due to the stimulation technique

5 RETICULAR FORMATION AND LATERAL GENICULATE RESPONSES 645 FIG. 3. Effect of reticular stimulation upon second component of geniculate evoked response to sub- and supramaximal tract stimuli. Ordinates give amplitude of second component of response in arbitrary scale. Abscissae give intensity of optic tract stimuli in V. Open circles refer to control response and solid ones to conditioned response. FIG. 4. Relation between location of tip of conditioning electrode and effect of stimulation upon geniculate evoked response. Ordinates give increase in amplitude of response to test stimulus following conditioning stimulation at interval of 100 msec. as per cent of amplitude of control response. Abscissae give location of tip of conditioning electrode shown by stereotaxic vertical coordinates. Conditioning electrode was moved in parallel with vertical coordinate axis on stereotaxic planes (F+3, L+3). Open circles refer to first component of response and solid ones to second component respectively. A and B refer to different animals. employed even when the intensity of the stimulus was set supramaximal, and these neurons could fire only with the aid of activation from the reticular formation. The structures in which stimuatlion produced facilitation had a rather restricted localization within the mesencephalon. The effective place corresponded roughly to the mesencephalic reticular formation. When the conditioning electrodes were withdrawn or advanced from the depth at which the conditioning stimulus evoked the most dominant facilitatory effect, the effect was gradually reduced and finally disappeared (FIG. 4A). In some cats, however, stimulation after withdrawal of the electrodes up to the level of the superior colliculus brought about a depressive effect upon the second component (FIG. 4B). This depressive effect, taking the result of the next experiment into consideration, seems to be elicited by stimulation of fibers in the superior colliculus originating from the optic tract. The mesencephalic reticular formation which was stimulated in the present

6 646 H. SUZUKI AND N. TAIRA experiment lies just below the superior colliculus. Thus, it is questionable whether the result obtained from the above experiment is a true effect resulting from stimulation of the reticular formation itself or a false one from stimulation of fibers from the optic tract running in the superior colliculus. If these fibers were stimulated, impulses produced in them would proceed to the lateral geniculate by axon reflex and change the responsiveness of a geniculate neuron pool to the test stimulus. In order to exclude such a possibility the effect conditioned by reticular stimulation was compared to that by optic tract stimulation in one and the same animal. In contrast to the facilitatory effect obtained with reticular stimulation (left column in FIG. 5) the conditioning stimulus to the optic tract always produced depression of the second component of the test response over wide ranges of intensity of conditioning stimuli (right column in FIG. 5). The depression occurred maximally about 50 msec. after the conditioning stimulus and persisted up to about 100 msec. This depression may be compared to that in the excitability cycle of geniculate neurons reported by previous workers3,4). The first component suffered only a slight reduction. The natural light stimulus to the eye as a conditioning one brought about similar results to those obtained with conditioning stimulation of A1 B1 A2 B 2 A3 B3 A4 FIG. 5. Effect of reticular stimulation (left column) and that of conditioning optic tract stimulation (right column) upon geniculate evoked response. Intensity of conditioning stimuli was decreased in steps from Al to A4 (10.0, 7.5, 5.0, 2.5V) in left column and from B1 to B3 (7.5, 5.0, 2.5V) in right column. Other conditions are same as in Fig. 2.

7 RETICULAR FORMATION AND LATERAL GENICULATE RESPONSES 647 the optic tract. Depression of the second component of the test response started msec. after the onset of illumination of both eyes and continued over several seconds. Important differences in the effects between conditioning photic and optic tract stimulation were as follows: With conditioning photic stimulation diminution of the second component of the test response was more intense, and the first component also suffered reduction in amplitude though to a lesser extent as compared with that of the second component. Augmentation of the second component of the geniculate evoked potential with reticular stimulation seemed due to the situation that geniculate neurons became prone to fire to the test volley from the optic tract after reticular stimulation; an increase in number of firing neurons would result in increase in amplitude of the second component. This was evidenced by the following experiments using unit activity of the optic radiation as an indicator. In order to isolate a radiation unit a microelectrode was inserted deeply into the visual area while both eyes were illuminated with a white light of 0.5 sec. duration every two seconds. When isolation of the unit was satisfactory the photic stimulus was replaced by the single shock stimulus to the optic tract. The radiation unit was easily identified according to the following criteria besides its discharge pattern to photic stimulation23). The radiation unit responded with a relatively fixed and short latency to a single shock and discharged only once to the supraliminal stimulus to the optic tract. When the intensity of the stimulus to the optic tract was decreased gradually to the threshold the radiation unit could not respond to successive stimuli each time but only to some of them (cf. FIG. 6A and C). At such an intensity level of the test stimulus the radiation unit became capable of responding to each stimulus when the conditioning reticular stimulus preceded the test stimulus (FIG. 6B). The above-mentioned phenomenon was studied more quantitatively using the so-called firing index which was originally devised by LLOYD et al.17) to describe the reflex behavior of a motoneuron pool in terms of observed behaviors of a number of individual motoneurons. In the present study the firing index was defined as n/10 ~100 (%) when n responses were obtained to 10 successive stimuli. The sigmoid relation was obtained between the firing index and the intensity of the stimulus (FIG. 7A). The intensity of stimuli to the optic tract was adjusted at a certain definite value so that the firing index of a given radiation unit was scores per cent. Under such a stimulating condition of the optic tract the firing index of the unit changed beyond ranges of its spontaneous fluctuation when optic tract stimulation was preceded by reticular one. Changes in the firing index of an optic radiation unit after reticular stimulation are exemplified in FIG. 7B. The average firing index without reticular conditioning was about 32 per cent in this case and shown a straight line parallel to the axis of abscissae (FIG.

8 648 H. SUZUKI AND N. TAIRA FIG. 6. Responses of single optic radiation fiber to ten successive threshold tract stimuli. A and C give control responses. B gives responses conditioned by reticular stimulation. Reticular stimuli preceded tract ones by 100 msec. Positive deflection upward. Voltage calibration, 5mV. Time marker, 1 msec. 7B). The upper and lower extremes of the firing index without reticular stimulation were about 50 and 30 per cent respectively and shown by dashed lines. In this unit the firing index was raised beyond the limits of its spontaneous fluctuation 10 msec. after reticular stimulation. Then, the firing index remained in the control range or showed a tendency to be lowered at intervals of 20 `40 msec. after the conditioning stimulus. From 50 msec. on the firing index was again raised beyond the control value. In other words, this unit received combined influence of facilitation and inhibition in temporal pattern from the mesencephalic reticular formation although facilitation prevailed. The temporal pattern of facilitation and inhibition following reticular stimulation was found to be different from one radiation unit to another. Some examples are shown in FIG. 8. The num-

9 RETICULAR FORMATION AND LATERAL GENICULATE RESPONSES 649 A B FIG. 7A. Relation between firing index of single radiation fiber and intensity of tract stimuli. Ordinates give firing indices. Abscissae give intensity of tract stimuli. B: Changes in firing indices of single radiation fiber to threshold tract stimuli following reticular stimulation. Ordinates, firing indices; abscissae, intervals between reticular and tract stimuli in msec. Mean firing index is given by straight line which crosses ordinate axis at about 30 per cent. Upper and lower extremes of spontaneous fluctuation of firing indices are shown by dashed lines. A B C D FIG. 8. Four examples of changes in firing indices of single radiation fibers following reticular stimulation. Their mean firing indices without reticular conditioning are shown by dashed lines. Other conditions are same as in Fig. 7B. ber of units in which facilitation prevailed far exceeded that of inhibitory units. This finding runs parallel to the augmentation of the geniculate evoked potential, which, as a mass response, represents a dominant effect, that is, facilitation only. An example of reticular influence upon a visual cortical neuron is shown in FIG. 9. The criteria used for identification of the cortical unit were as follows: (1) The unit was isolated at a rather shallow level within the cortex. (2) It responded with a relatively long latency, i.e. over 3 msec. to the stimulus to the optic tract. (3) It showed a tendency to respond with repetitive discharges to the supraliminal stimulus, but when the intensity of the stimulus to the optic tract was reduced to a certain level such a unit lost the tendency to repetitive discharges and even failed in responding to some of sequential stimuli (FIG. 9A). When the stimulus to the reticular formation preceded such a liminal stimulus to the optic tract by 100 msec. the unit became prone to repetitive firing again (FIG.

10 650 H. SUZUKI AND N. TAIRA FIG. 9. Responses of single neuron of visual cortex to ten successive threshold tract stimuli. A and C give control responses. B gives responses conditioned by reticular stimulation. Time marker, 1 msec. 9B). Another important phenomenon was that the latency of such repetitive responses was strikingly short as compared with that of the control single response (cf. B to A and C in FIG. 9). Although the number of cases was not sufficiently large with respect to the cortical units explored so far, it is almost certain that some cortical neurons receive facilitatory influence from the reticular formation. DISCUSSION In the present experiment the reticular controlling influence upon transmission of visual inflow at the geniculate level was investigated mainly by the use of the geniculate evoked potential elicited by the stimulus to the optic tract as an indicator. As the influence is manifested as augmentation or depression of some component of the response, it will be necessary to discuss the nature of the geniculate evoked response. The geniculate evoked potential elicited by the stimulus to the optic nerve has been analyzed in detail by P. O. BISHOP and his coworkers3,4,5). According to their interpretation the first component of the response is to be ascribed to the presynaptic activity of the geniculate and the second one to the postsynaptic activity; in other words, the former represents the activity of the group of tract fibers with larger diameter and the latter that of

11 RETICULAR FORMATION AND LATERAL GENICULATE RESPONSES 651 geniculate neurons. In the present experiment the bipolar technique was employed to avoid picking up electrical phenomena far from the recording site. Thus, our response resembles rather that obtained by VASTOLA24) with a similar technique to ours and differs in some respects from that obtained by P. O. BISHOP et al.5). Nevertheless, the first component of our response was resistant to anoxia in contrast with the second one susceptible thereto22) as was describes by P. O. BISHOP et al.5). Therefore, it may be reasonable to consider that the first and second components of our response represent the pre- and postsynaptic activities of the lateral geniculate respectively. The second component which represents the activity of geniculate neurons was augmented with reticular stimulation without significant alteration of the first one which represents the presynaptic event. From this finding we might draw the conclusion that the mesencephalic reticular formation facilitates afferent transmission at the geniculate synapses. However, before we evaluate the observed result we must scrutinize whether the result is actually due to stimulation of the reticular formation or not, because spread of current from the stimulating electrodes can stimulate other structures and lead to misinterpretation of the result. In the first place the possibility should be ruled out that the lateral geniculate may be activated from the superior colliculus which can be stimulated with spread of current from the conditioning electrodes placed within the mesencephalon. BARRIS2) described that the collaterals of some fibers of the optic tract passing towards the superior colliculus make synaptic contacts with cells of the lateral geniculate. If such fibers were stimulated in the superior colliculus it would be possible for a volley elicited therein to reach the lateral geniculate and influence the responsiveness of geniculate neurons to the test stimulus to the optic tract. In this connection the effect of a conditioning stimulus to the optic tract deserves attention, because it is equivalent to the effect of a conditioning stimulus to the fibers in question. As has been described above, the conditioning shock to the optic tract always brought about depression of the second component of the test response while that to the reticular formation produced facilitation. Moreover, conditioning stimulation made directly at the level of the superior colliculus produced depression (cf. FIG. 4B). In the second place the possible effect resulting from stimulation of the retina should be excluded because the retina is very sensitive to electrical stimulation19); it may be considered that retinal excitation has some influence upon the excitability of geniculate neurons irrespective of the way of stimulation. As a matter of fact the effect of reticular stimulation could still be observed after electrical coagulation of both optic discs22), and thus the possible effect from retinal stimulation was certainly ruled out. In the third place a centrifugal or antidromic effect of stimulation of the visual cortex and/or its surrounding cortical areas must be considered, but this possibility may be dismissed on the basis of the fact that stimulation of the visual and/or nearby

12 652 H. SUZUKI AND N. TAIRA cortical areas exerted depression alone upon the geniculate evoked potential24,25) and the single unit activity of the lateral geniculate26). Further possible effect of direct stimulation of the lateral geniculate can also be ruled out because of rather restricted localization of the effective place within the mesencephalon. On the basis of the above discussion it may be concluded that facilitation of the second component of the geniculate evoked potential was actually due to stimulation of the reticular formation. The mechanism of the augmentation of the second component would be that the synaptic transmission in the lateral geniculate of the incoming volley from the optic tract is facilitated by the conditioning impulses from the reticular formation. Evidence hereof has been provided by the experiments on the effect of reticular stimulation upon the responsiveness of individual radiation units to optic tract stimulation. The criteria used for identification of radiation units in the present experiment need some comments. Our radiation units responded with only one spike discharge to a supraliminal stimulus to the optic tract. Among postsynaptic units recorded by WIDEN and AJMONE-MARSAN26) there were some units with a tendency to repetitive discharges to tract stimuli and they showed rather long latencies as compared with those without such a tendency. So, there is some doubt whether such repetitive firing units were actual radiation units; they might be units projecting to some other structures than the visual cortex. But it is also possible that differences in response behavior between their radiation units and ours may be in part due to differences in experimental conditions. In the present experiment radiation units received facilitatory and/or inhibitory effects from the reticular formation, though the facilitatory effect was dominant in most cases. These effects were displayed most clearly with liminal stimulation of the optic tract, and this fact leads to the speculation that the reticular activation acts mainly on geniculate neurons remaining within a subliminal fringe. Evidence for the existence of the subliminal fringe in a geniculate neuron pool may be provided by structural organization of the lateral geniculate10,20). Stimulation of the reticular formation may modify the subliminal fringe in a geniculate neuron pool with its long-lasting effect so that the number of neurons firing to tract stimuli will increase following reticular stimulation, and this results in the augmentation of the second component of the geniculate evoked potential. As has been shown above, some radiation units certainly received inhibitory or combined effects of facilitation and inhibition following reticular stimulation. The question as to whether or not there is any functional differentiation in the mesencephalic reticular formation so as to produce facilitatory and inhibitory effects according to stimulating loci has, however, to be determined by further experiments. There is anatomical evidence that geniculate neurons also receive afferent fibers other than those of retinal origin. GLEES10) described that there remained apparently normal terminal rings after section of one optic nerve and POLYAK21)

13 RETICULAR FORMATION AND LATERAL GENICULATE RESPONSES 653 also described fibers differing in terminal arbolization from those of retinal origin. These authors considered that they originate from neurons in the brain. These anatomical findings are in line with our physiological findings, but we are not in a position to mention the pathways from the reticular formation to the lateral geniculate, for there are no reliable anatomical and physiological data concerning this problem. The long-lasting effect of reticular stimulation seems to suggest that the pathways from the reticular formation are multineuronal and form axodendritic synapses with cells of the lateral geniculate. Many investigators have assumed convergence of the afferent fibers from the optic tract and those of the non-specific reticular system onto the geniculate neurons8,9,11,12,13,18) This assumption has fully been confirmed by the present experiment. There seem to be some other systems influencing the synaptic transmission at the lateral geniculate, for example the corticifugal extrareticular system studied by WIDEN and AJMONE-MARSAN26). IWAMA and YAMAMOTO reported a facilitatory effect of the diffuse thalamic projection system upon the geniculate evoked potential in deeply nembutalized cats15). AKIMOTO and CREUZTFELDT suggested the possibility that the specific and nonspecific system converge onto cortical neurons1,7), and DUMONT and DELL showed that the visual cortex and the lateral geniculate could be activated by reticular stimulation independently of each other8,9). This problem will deserve further confirmation. SUMMARY The effect of single shock stimulation of the mesencephalic reticular formation upon the geniculate evoked potential and the response probability of optic radiation units to optic tract stimulation was studied in locally anesthetized and curarized cats. 1. The geniculate evoked potential following optic tract stimulation consisted of two components, i.e. the pre- and postsynaptic components respectively. By conditioning stimulation of the mesencephalic reticular formation the postsynaptic component alone was augmented for a few hundred msec. without significant alteration of the presynaptic one. 2. Remarkable facilitatory effects were obtainable only when a certain limited region of the mesencephalon was stimulated. 3. Discharges of single radiation units were explored, and it was found that they received facilitatory and/or inhibitory effects following reticular stimulation, the facilitatory effect being predominant in most units. These effects were manifested as changes in responsiveness of radiation units to liminal optic tract stimuli after conditioning reticular stimulation. 4. From the result mentioned above it was concluded that transmission at most geniculate synapses was facilitated following reticular stimulation.

14 654 H. SUZUKI AND N. TAIRA The authors wish to thank Prof. K. Motokawa for his invaluable discussion and suggestion throughout the course of the experiment and the preparation of the manuscript. REFERENCES 1) AKIMOTO, H. UND CREUTZFELDT, O. Reaktionen von Neuronen des optischen Cortex nach elektrischer Reizung unspezifischer Thalamuskerne. Arch. Psychiat. Zeitschr. Neurol., 196: , ) BARRIS, R. W. Disposition of fibers of retinal origin in the lateral geniculate body. Arch. Ophthal., 14: 61-70, ) BISHOP, P. O. AND DAVIS, R. The recovery of responsiveness of the sensory synapses in the lateral geniculate nucleus. J. Physiol., 150: , ) BISHOP, P. O. AND EVANS, W. A. The refractory period of the sensory synapses of the lateral geniculate nucleus. J. Physiol., 134: , ) BISHOP, P. O. AND MCLEOD, J. G. Nature of potentials associated with synaptic transmission in lateral geniculate of cat. J. Neurophysiol., 17: , ) BRINDLEY, G. S. Physiology of the retina and the visual pathway. London, Edward Arnold, 298 pp., xi, ) CREUTZFELDT, O. UND AKIMOTO, H. Konvergenz und gegenseitige Beeinflussung von Impulsen aus der Retina und den unspezifischen Thalamuskernen an einzelnen Neuronen des optischen Cortex. Arch. Psychiat. Zeitschr. Neurol., 196: , ) DUMONT, S. ET DELL, P. Facilitations specifiques et non-specifiques des reponses visuelles corticales. J. Physiol., Paris, 50: , ) DUMONT, S. ET DELL, P. Facilitation reticulaire des mechanismes visuelles corticaux. EEG Clin. Neurophysiol., 12: , ) GLEES, P. The termination of optic fibres in the lateral geniculate body of the cat. J. Anat., 75: , ) HERNANDEZ-PEON, R. Central mechanisms controlling conduction along central sensory pathways. Acta Neurol. Latinoamer., 1: , ) HERNANDEZ-PEON, K., SCHERRER, H. AND VELASCO, M. Central influences on afferent conduction in the somatic and visual pathways. Acta. Neurol. Latinoamer., 2: 8-22, ) HERNANDEZ-PEON, R., GUZMAN-PLORES, C., ALCARAZ, M. AND FERNANDEZ-GUARDIOLA, A. Sensory transmission in visual pathway during "attention" in unanesthetized cats. Acta. Neurol. Latinoamer., 3: 1-8, ) HUBEL, D. H. Single unit activity in lateral geniculate body and optic tract of unrestrained cats. J. Physiol., 150: , ) IWAMA, K. AND YAMAMOTO, C. Nature of the secondary discharge of negative polarity in the cerebral cortex of cats and dogs. Tohoku J. Exp. Med., 75: 43-54, ) JASPER, H. H. AND AJMONE-MARSAN, C. A stereotaxic atlas of the diencephalon of the cat. Ottawa, Canada, The National Research Council of Canada, 15 pp., 54 pl., ) LLOYD, D. P. C. AND MCINTYRE, A. K. Monosynaptic reflex responses of individual motoneurons. Gen. Physiol., 38: , ) LONG, R. G. Modification of sensory mechanism by subcortical structures. J. Neurophysiol., 22: , ) MOTOKAWA, K. Retinal processes and their role in color vision. J. Neurophysiol., 12: , ) O'LEARY, J. L. A structural analysis of the lateral geniculate nucleus of the cat. J. Comp. Neurol., 73: , ) POLYAK, S. The vertebrate visual system. Chicago, The University of Chicago, 1390 pp., xviii, 1957.

15 RETICULAR FORMATION AND LATERAL GENICULATE RESPONSES ) SUZUKI, H. AND TAIRA, N. (to be published) 23) SUZUKI, H., TAIRA, N. AND MOTOKAWA, K. Spectral response curves and receptive fields of pre- and postgeniculate fibers of the cat. Tohoku J. Exp. Med., 71: , ) VASTOLA, E. F. Antidromic action potentials in lateral geniculate body. J. Neurophysiol., 20: , ) VASTOLA, E. F. After-positivity in lateral geniculate body. J. Neurophysiol., 22: , ) WIDEN, L. AND AJMONE-MARSAN, C. Effects of corticipetal and corticifugal impulses upon single elements of the dorsolateral geniculate nucleus. Exptl. Neurol., 2: , 1960.