Unitary analysis of retino-geniculate response time in rats

l’kion Ref. Vol. 7, pp. 205-213.

UNITARY

Pcrgamon

Press 1967. Primed in Great Britain.

ANALYSIS RESPONSE

OF RETINO-GENICULATE TIME IN RATS

HIROHARUNODA and KITSUYA IWAMA Laboratory of Neurophysiology, Institute of Higher Nervous Activity, Osaka University Medical School, Kitaku, Osaka, Japan (Received 13 July 1966)

THEREhas been presented physiological

and anatomical evidence that at least two groups of fibers are recognized in the optic tract (OT) (BISHOP et al., 1953; BISHOP and CLARE, 1955; CHANG, 1956). Similarly to the OT, the pathway from the lateral geniculate nucleus (LGN) to the visual cortex (VC) may be composed of fibers with a wide range of conduction velocity. The primary object of the present experiment is to answer the question whether or not there might be some rules in the connection between the pre- and postgeniculate pathways, both comprising fibers of different sizes and conduction velocities, This paper will report the finding in rats that, at least with respect of the P cells of the LGN (SEFTONand BURKE, 1965), there is a rule of the peripheral and central connection; faster conducting OT fibers are destined to faster P cells whereas slower conducting ones are similarly related to slower P cells. Further, the finding will be reported suggesting that intraretinal transmission time of impulses varies according to whether the impulses finally impinge upon faster P cells or slower ones. The results of a similar analysis made on the I cells of the LGN will also be described. METHODS

They were Adult albino rats, weighing 250-400 g, were used in all the experiments. anesthetized with urethane (1 g/kg3 intraperitoneally). The animals head was fixed to a stereotaxic apparatus. Both eyes were left intact. Electrodes used for stimulating the OT were two insulated steel wires of O-2 mm in diameter, cemented together with bared tips 0+5-07 mm apart. They were inserted into the 01: at the chiasm stereotaxically and were positioned at a point giving a response to a single flash stimulus with the highest amplitude. Electrodes for stimulating the VC, insulated steel wires of O-2 mm in diameter with exposed tips of 05 mm, were thrust about 1 mm deep and separated horizontally by 1SO-2.0 mm. For keeping a constant distance between the LGN and the stimulating point of the VC, the VC electrodes were placed at approximately the same cortical point in all the experimental animals. Their position was O-5-1 ‘0 mm anterior to the lambda suture and 2-3 mm lateral from the midline (this cortical point usually showed a high-amplitude evoked potential to a single shock stimulation of the 01: with the standard wave form). The line connecting the two VC electrodes was made parallel to the lambda suture. Stimulation of the OT and the VC was made with single square pulses of 0~01-0~05 msec duration with the intensities mostly supramaximal. In one series of experiments, activation 205 0

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of the LGN cells was made by applying a single flash from a xenon discharge bulb (FT 100, Tokyo-Shibaura Electric Comp.) which was driven by a condenser of 1 PF charged with a voltage of 600 V. The discharge bulb was placed 20-30 cm in front of one eye. The experimental room was kept half-dark. In most of the experiments activity of the LGN cells was recorded as unit spikes with glass capillary microelectrodes filled with 3 M KC1 (d.c. resistance, 5-20 MQ). In a few experiments the LGN response was recorded as mass activity with electrodes of the same type as those inserted to the OT. RESULTS

According to SEPTONand BURKE (1965) there are two types of the LGN cells, P and I. The P cell responds with a single spike to stimulation of both the OT and the VC. The response of this type of the cell to VC stimulation is of true antidromic origin. In contrast to this, the I cell is fired repetitively by single shock stimulation of both the OT and the VC, and the response to VC stimulation in this case is most probably elicited transsynaptically. In the present experiment we have fully confirmed the findings of the abovementioned authors. Among a total of 141 units, we identified 109 units (77%) as the P cells and 32 units (23 %) as the I cells. Sample records showing the peripherally and corticaily elicited reponses of the P and I ceils are mounted in Figs. 1 and 6-&C. Experiments on the P cell consisted of the following two series. The first series was aimed at determining how the latencies to OT stimuiation are related to those to VC stimulation in the individual P cell responses. In the second series a similar measurement was made by combining VC stimulation with a flash stimulus. With the I cells, examination of the latencies in the individual units was made by a combination of VC and OT stimulation only. Relation between the latencies to peripheral and cortical stimdation in the P cells Figure 1 illustrates two examples of the P cell which responded with single positivenegative spikes to electrical stimulation of both the OT (1-O and 2-O) and the VC (1-A and 2-A). Recordings were made by superimposing 15 sweeps. As can be seen, a fluctuation of the latency was negligibly small (less than 0.1 msec) in VC stimulation. It was

FIG. 1. Eexampksof ortho- and antidramk unit responses of the same P cells. Unit 1 (left) had shorter i&ncics than unit 2 (right) to both o&o- (0) and antidrunk (A) atimuuon. RecOlXlhg!4w#e~&bySU p4mposiag 15 sweeps. No& nq@#bly sxmdlv&t&ion of the latency in antidromk rcspo~ (1-A and 2-A) and small waves of the EPSPs at stuting point of positive phase (downward dbkction] in orthodromic responses (1-O and 2-O). Voltage calibration, 5 mV. Time calibration. 1000 cps.

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Response Time

slightly large in OT stimulation but was less than O-5 msec. In the orthodromic responses small steps signaling the EPSPs were seen preceding rapid large excursions of positive polarity, whereas the antidromic responses had no such step-like deflections. The latency to OT stimulation was measured as a time from a stimulus artefact to the point from which a sharp downward stroke started (the point designated as S-A step by BISHOPet al. (1962b)). The latency to VC stimulation was measured simply as a time between a shock artefact and the beginning of a sharp positive deflection. In both cases the latencies were measured with 5 consecutive responses and they were averaged. In Fig. 2A are presented all the data of the latencies obtained from a total of 48 P cells in a form of correlation diagram where the latencies to VC stimulation were plotted against those to OT stimulation as abscissae. The latencies were distributed between O-6 and 6.2

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FIG. 2. A. correlation diagram for 48 P units between the latencies to OT stimulation (abscissae) and those to VC stimultation (ordinates). Correlation coefficient is 080. A straight line was drawn according to regression equation Y=O*38 X+090. B. histogram showing distribution of the orthodromic latencies. Two groups ranging from 0.5 to 3.0 msec and from 3-O to 6.5 msec are noticeable.

msec with respect to OT stimulation and between 0.7 and 3.6 msec with respect to VC stimulation. It is clearly seen that the P cells responding with longer latencies to orthodromic OT stimulation have a tendency to respond with longer latencies to antidromic VC stimulation. With these data the correlation coefficient was calculated as 0.80 and the regression equation was determined as Y=O*38 X+0*90 where Y means the latency to VC stimulation and X the latency to OT stimulation (a line in the correlation diagram was drawn according to this regression equation). That there is an approximately linear relation between the latency to orthodromic stimulation and that to antidromic stimulation is well exemplified by the records of Fig. 1. Unit 1 which responded to VC stimulation with a latency of 1e4 msec had a latency of 2-4 msec to OT stimulation. The corresponding figures in the unit 2 were 2.5 and 4.3 msec, respectively, both being about 1a8 times as large as in the unit 1.

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With 48 P cell responses assembled in the correlation diagram of Fig. 2A, one can see that these units were divided into two groups with regard to the fatency to OT stimulation. This is clearly shown in the latency histogram of Fig. 2B. The histogram has two peaks, the one at 1G2.0 msec and the other at 4.0-4.5 msec, The average latency of the faster ruining group, which includes the units with Iatencies ranging from O-5 to 3-O msec, is 1.9 msec and that of the slower responding group (3.0-6.5 msec) is 4.7 msec. BEHOP and MCLEOD (1954) introduced the term “ti and tZ” to designate the two groups of the OT fibers with different conduction velocities and “ri and rz” to designate the mass responses following cl and t2 impulses, respectively. After these authors we shall call the faster responding P units “tr-q group” and the slower responding ones “t2-r2 group”. This classification of the t-r group, based upon the unitary recording, may be justified by the following fact. We made several observations of the rl and r2 responses with gross recording electrodes under the same condition of OT stimulation as in the unitary recordings and found that the peak time of the rl response is 2.0 msec (1.7-2.9 msec) and that of the r2 response is 45 msec (36-5.2 msec). These values of the peak times are in good agreement with the times of the peaks in the latency histo~am of Fig. 28. Lutencies of the P cell responses to photic and VC stimulation In the second series of experiments on P cells, a total of 61 units were sampled as responding to both photic and VC stimulation. Three examples of the responses elicited by flashes, repeated at 0*5-0*8 c/s, are shown in Fig. 3. The response to a single flash appeared usually as a train of 3-5 spikes (IQ. Some units responded with more than 5 spikes (C). It was rather rare to encounter the units responding with a single spike (A).

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FIG. 4. A. axrelation diypam for 61 P u&s between the &en&a to pbotk stin&&on ~a~)~~to~~of~V~~~~~. act is 030. A straight line was drawn aooQtdis0 tqmgwaion equatkn Y-O.36 X + 044. B. histogram of latency distribution to photic stimulation. A single peak is at the range of

FIG. 3. Examples of P cell responses to photic stimulation. A. unit responded with a single spike. B. unit showed a train of 3-5 spikes with very small variation in the latency of the first spike. C. unit responded with multiple discharges with some variation of the latency and spike number. In A a flash stimulus was given at the point (marked with small upward deflection) slightly later than the start of sweep, and in B and C it was given at the start of sweep.

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Usually there was seen some fluctuation in the latency and number of spikes from stimulation to stimulation and this was particularly marked in the units yielding repetitive spikes. In such cases we sampled from a series of more than 20 records a group of 5-10 responses which appeared with a consistent pattern allowing us to calculate the average latency of the response (the latency in the case of multiple spike response means the latency to the first spike). The measurement of the latency to VC stimulation was conducted as in the series 1 experiment. All the data of the latencies obtained in this series of experiments were treated in the same way as in the series 1 experiment. It is seen in the diagram of Fig. 4A that the latency to VC stimulation is positively correlated with that to OT stimulation. From these data the correlation coefficient was calculated as 0.80 and the latency to VC stimulation (Y) was expressed as a function of the latency to OT stimulation (X) with a regression equation Y=O*36 X+0*84. Thus, the fact that the antidromic latency is an approximately linear function of the orthodromic latency is again demonstrated by extending the peripheral pathway so as to include the retina. Corresponding to the histogram of Fig. 2B, we constructed a histogram showing the distribution of the latency to photic stimulation (Fig. 4B). It is seen that the latencies scattered widely from 14 to 86 msec with a mode at 20-25 msec. It is difficult to find a clear-cut tendency to grouping.

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FIG. 5. Histogram of latencies of antidromic responses in 109 P cells. Black bars; units obtained in series 1 experiment. Stippled bars; units obtained in series 2 experiment. A peak is seen at 1.0-l .5 msec range.

Assembling all the data obtained in the series 1 and 2 experiments, a distribution of the latencies of the antidromic P cell response is shown in Fig. 5. The latency ranged from 0.7 to 5.3 msec with the average of 2.03 msec. The histogram has a sharp mode at 1.0-l 05 msec without clear gaps which permit us to divide the units into two groups or more. Lntency studies on the I cells As described above, the response of the I cells to OT stimulation with a single shock appeared as a train of repetitive discharges of mostly 5-8 spikes. In response to VC stimu-

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IWAMA

lation this type of the LGN cell also fired repetitively with about the same number of spikes as in the case of peripheral stimulation. This is seen in Fig. 6B,C which shows that the same I cell responded to both VC and OT stimulation with about the same discharge pattern. It was remarkable that in both cases of stimulation the latency and the

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FIG. 6. A. correlation diagram of latencies for 23 I cell responses. Abschae; Wencies to OT stimulation. Ordinates; latmcies to VC stimulation. Two groups (Ia and Ib) are rccognizableasindicatcdbycirduofdottedlines. AlineinthcIagroupisayprssionlinccahlated from the data of Fig. 2A. B and C; rssponsa of the same I cell to OT stimulation (B) and to VC stimulation (C).

number of spikes varied considerably from sweep to sweep. Therefore, the latency of the I cell response, which means here the latency of the first spike of the discharge train, was measured in about the same way as in measuring the latencies of the P cell response with photic stimulation. In Fig. 6A are presented the data obtained from a total of 23 I cells. The late&es to VC stimulation were plotted on the ordinates aguinst those to OT stimulation as abscissae. In contrast to the P cell response, it seems di5cult to find a single linear relationship between the two kinds of the latency covering the entire range of distribution. However, if we divide the plotted points into two groups @I and Ib) as indicated in the figure, there seems to be a linear correlation within each group. Moreover, it is notable that if the regression equation derived from the data of Fig. 2A is transferred here, it well fits with the distribution of the group Ia as indicated by a straight line in Fig. 6. This will be discussed later. DISCUSSION The fhdhgs by SBKIWIaad RWUCE(1965) that them are two types of the cells (P and I) within the LGN has been con&m& in our expuriment. Since the P cells send their axons to the VC and the I celis do not, one can say that impulse tmnsfm from the OT to the VC is a primary role of the P cells and the I cells may exert some regulatory effect upon the former. Restricting our attention to the unit activity of the P cell, we have established that the latencies to orthodromic stimulation, both photic and electrical, are positively correlated with those to antidromic stimulation with high correlation coefficients. This fact may

Retino-geniculate

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simply mean that the P cells having faster conducting axons are connected to the faster retino-optic-tract-units while those having slower conducting axons are to the slower peripheral units. The latency histogram of the P cell response to electrical stimulation of the OT clearly indicates that there are two groups of the optic-tract-geniculate connection which are properly called “1~-rl” and “tz-r2”, following the nomenclature by BISHOPand MCLEOD (1954). The average latency to electrical stimulation of the OT in each group coincides very well with the peak times of the rl and r2 mass responses, respectively, which were obtained by the same method of stimulation as in the unit recordings. From this we may conclude that the rl and r2 mass responses are produced by the different groups of the P cells which are distinguishable from each other in the average latency to antidromic stimulation of the VC. It is found from the data of Fig. 2 that the average latencies of the antidromic responses are 1.4 and 2.7 msec, respectively, in the rt-producing P cells and the r2-producing ones. The measurement of the latencies of the individual LGN cells to electrical stimulation of the optic nerve has been conducted with cats (BISHOPet al., 1962a; WIDEN and AJMONE MARSAN,1960; AJMONEMARSANand MORILLO, 1961). So far as the published data are concerned, such a clear-cut separation of the two groups of the t-r connection as demonstrated here cannot be found. This discrepancy in the results may be ascribed partly to the difference in species of experimental animal and probably to the situation that the previous authors constructed their latency histograms without regard to the cell types of the LGN SEFTONand BURKE(1965) and H. SUZUKI (personal communication) have been successful with cats to identify the two types of the LGN cell which correspond to the P and I cells in rat LGN). As a matter of fact, if we add the data obtained from the I cells (Fig. 6) to the latency histogram of the P cells, there results a histogram which has a single peak without any gap permitting us to identify the two groups of r-r. It is obvious that the difference in the response time to OT stimulation between the ti-r1 and t2-r2 groups arises, at least partly, from the difference in the conduction velocity of fl and 12 fibers. However, it must be pointed out that there is also a difference in the synaptic delay between the two groups, this making a rather large contribution to the difference of the orthodromic response time. SEFTONand SWINBURN(1964) showed in rats that the mass response of the LGN to electrical stimulation of the optic nerve appears with a wave form analysable into the sequential occurrence of the nerve terminal activity (ti, r2 and sometimes 1~) and postsynaptic cellular response (rr and r2). The time difference between the tr and t2 activities resulted from the difference in the average conduction velocity between the tt and ?2 fibers, while the time difference between the corresponding t and r activities may give us the average synaptic delay in each t-r group. This latter measurement, made on the published record of SEFTONand SWINF~JRN(1964) (Fig. 11 in their paper), clearly shows that the average synaptic delay in the tl-rt group is shorter than in the t2-r2 group. We made a systematic study on this point with our own records of the mass LGN response. It was found that the average synaptic delay is 0.9 msec (10.06 msec, S.D.) in the rl-rl group and 2.6 msec (ho.33 msec, SD.). Thus, it is concluded that the faster conducting tl-rl group has a shorter synaptic delay, on the average, than the slower conducting t2-rp group. In the second series of experiments on the P cells, it has been shown that the latencies of the P cell response to a single flash are distributed in the range of 14-86 msec with a single mode. The time of this mode, which was found to be 20-25 msec in our experiment,

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may be taken as corresponding to the latency or the peak time of the photically evoked, mass LGN response. Actually KIMURA(1962) reported the value of 22-25 msec as the latency of the photically evoked response of the rat VC, being comparable to the latency of the response seen at the LGN. The latencies of the P cell response to photic stimulation have much larger values and a much wider range of distribution than those determined with electrical stimulation of the OT at the chiasm. Though we are not in a position to discuss the problem on the basis of numerical values, it is not unreasonable to suppose that the largest part of the photically determined latency is consumed until the impulses reach the exit point of the optic nerve from the eye. Then, we may be led to a supposition that photically evoked impulses which are finally to be transferred to the faster r--r-units consume shorter intraretinal transmission time than those destined to the slower r-r-units. Lastly a discussion will be made on the data obtained from the I cells. It was mentioned in the section of RESULTS that the points in the correlation diagram of Fig. 6A are divided into the two groups, Ia and Ib, and that in each group the latencies to VC stimulation are positively correlated to those determined by OT stimulation. In regard to the distribution of the Ia group, an explanation may be possible in the following way. Firstly, we assume that an I cell of the Ia group is connected to a P cell through a recurrent axon collateral of the latter, and secondly that activation of this I cell is made through the same axon collateral in both VC and OT stimulation. Then, the correlation of the latencies between the two modes of activation, determined with many I cells of the above-mentioned type, is supposed to be about the same as in the case of the P cell. This seems to be supported by the fact that, as shown in Fig. 6A by a straight line, the regression equation derived from the data of the P cell in Fig, 2A, well fits the distribution of the Ia group. The I cells belonging to the Ib group are distinctly different from those of the Ia group. It may be said that they have relatively shorter latencies to OT stimulation, or that they have relatively longer latencies to VC stimulation. It may be possible to consider several types of neuronal organization for explaining the latency distribution of the Ib group. However, a discussion on this point is beyond the scope of the present paper. SUMMARY

In anesthetized rats, two types of unitary responses (P and I) were recorded from the lateral geniculate nucleus (LGN) after electrical stimulation of the visual cortex (VC). In individual units, the latencies of the cortically elicited unitary responses were compared with those of the responses elicited by peripheral stimulation. (2) With 109 P cells, a positive correlation was demonstrated between the antidromic latencies and the latencies to orthodromic stimulation with a tIash and a single electroshock to the optic tract (OT). This fact means that faster responding P cells are connected with peripheral pathways having larger conduction velocities. (3) With 23 I cells, the correlation of the latencies between electrostimulation of the OT and the VC suggested that there are two groups of the I cells with different modes of activation. The one group of the I cells is probably connected to the P cells through axon collaterals. (1)

Acknowledgemem-Our

sincere thanks are due to Dr. TOMIOARJKUNI for his tech&al

help.

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REFERENCES AJMONEMARSAN,C. and MORILLO,A. (1961). Cortical control and callosal mechanisms in the visual system of cat. EEG din. Neuro~liysioI. 13, 553-563. BISHOP,G. H. and CLARE, M. H. (1955). Organization and distribution of fibers in the optic tract of the cat. J. camp. Neural. 103, 269-304. BISHOP,P. O., BURKE,W. and DAMS, R. (1962a). The identification of single units in central visual pathways. J. Physiol. 162,409-431. BISHOP,P. O., BURKE,W. and DAMPS, R. (1962b). The interpretation of the extracellular response of single lateral geniculate cells. J. Physiol. 162, 451-472. BIS~IOP,P. O., JEREMY,D. and LANCE,J. W. (1953). The optic nerve. Properties of a central tract. J. Ph_vsiol. 121, 415-432.

BISHOP,P. 0. and MCLEOD,J. G. (1954). Nature of potentials associated with synaptic transmission in lateral geniculate of cat. J. Neurophysiol. 17, 387-414. CHANG,H-T. (1956). Fiber groups in the primary optic pathway of cat. J. Neurophysiol. 19,224-231. KIMURA,D. (1962). Multiple reponse of visual cortex of the rat to photic stimulation. EEG din. Neurophysiol. 14, 115-122. SEFTON,A. J. and BURKE,W. (1965). Reverberatory inhibitory circuits in the lateral geniculate nucleus of the rat. Nom-e, Lord 205, 1325-1326. SEFWN, A. J. and SWINBURN,M. (1964). Electrical activity of lateral geniculate nucleus and optic tract of the rat, Vision Res. 4, 315-328. WIDEN, L. and AJMONE MARSAN, C. (1960). Effects of corticipetal and corticifugal impulses upon single elements of the dorsolateral geniculate nucleus. Expl. Neurol. 2, 468-502. Abstract-In rats, unitary responses were recorded from lateral geniculate nucleus (LGN). Latencies were compared between the responses to cortical and peripheral stimulation with the same units. In one type of the LGN cells (P cells of Sefton and Burke), a positive correlation was found between the two kinds of latency. It was suggested that faster responding P cells were connected with peripheral pathways having larger conduction velocities. In another type of the LGN cells (I cells), two groups with different modes of activation were found. R&n&-On enregistre sur le rat les reponses unitaires du noyau lateral genouille (LGN). On compare les latences entre les r&ponses aux stimulations cotticale et p&iph&ique sur les memes unites. Avec un type de cellule du LGN (cellules P de Sefton et Burke), on obtient une correlation positive entre les deux types de latences. On suggere que les cellules P a rtponse rapide sont en liaison avec la p&ipherie par des fibres de grande vitesse de conduction. Dam un autre type de cellules du LGN (cellules I), on trouve deux groupes avec des modes differents d’activation. Z~~e~~u~-In Ratten wurden die ~ei~~ionen an Zellen im Kniehticker registriert. Latenzen zwischen den Reaktionen auf eine kortikale und periphere Reizung der gleichen, Zellen wurden verglichen. In einem Typ der LGN-Zellen (P-Zellen von Sefton und Burke) konnte eine positive Korrelation zwischen den beiden Liatenzen festgestellt werden. Es wird behauptet, daD die schneller reagierenden P-Zellen mit den peripheren Leitungs verbunden sind, die eine hohere Leitungsgeschwindigkeit besitzen. Bei einer andemn Art von LGN-Zellen (I-Zellen) wurden zwei Gruppen mit einem unterschiedlichen Erregungsmodusen gefunden. Pe3robse -

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