Exp. Brain Res. 27, 35-50 (1977) Experimental

Brain Research

�9 Springer-Ver[~g 1977

Dark Adaptation and Receptive Field Organisation of Cells in the Cat Lateral Geniculate Nucleus

V. Virsu 1, B.B. Lee and O.D. Creutzfeldt

Max-Planck Institute for Biophysical Chemistry, Department of Neurobiology, Am Fassberg, D - 3400 G/Sttingen-Nikolausberg, Federal Republic of Germany

Summary. The receptive fields of LGN cells were investigated with station- ary light and dark spot and annulus stimuli. Stimulus size and background intensity were varied while stimulus/background contrast was kept con- stant.

The speed of dark adaptation varied considerably from cell to cell. Dark adaptation made responses more sustained in all neurones and eliminated the oscillatory on-responses evoked under some conditions in the light- adapted cells. Dark adaptation led also to a disappearance of early phasic inhibition in on-responses, and increased response rise time and latency.

The power of surround responses to inhibit centre responses decreased slightly at low levels of light adaptation in LGN cells but much less than in retinal ganglion cells. Some other traces of changing retinal surround ef- fects also appeared in the LGN on dark adaptation. For example, the func- tional size of receptive fields increased at low levels of illuminance as has been observed in retinal ganglion cells and the receptive fields as estimated from response peaks were larger than those estimated from sustained com- ponents.

Key words: Lateral geniculate nucleus - Cat - Dark adaptation

Introduction

The responses of retinal ganglion cells to light stimuli depend on the level of light adaptation. At low levels of retinal illuminance the power of the receptive field (RF) surround to antagonise centre responses decreases and the apparent size of RF centre increases (Barlow et al., 1957b; Rodieck and Stone, 1965; Enroth-Cugell and Robson, 1966; Gouras, 1967; Andrews and Hammond, 1970; Maffei et al., 1971; Cleland et al., 1973; Hammond, 1975; Enroth- Cugell and Lennie, 1975), response latencies get longer (Levick, 1973), and

Trainee of the European Training Programme in Brain and Behaviour Research, 1975. Present address: Department of General Psychology, University of Helsinld, SF00170 Helsinld 17, Fin- land

36 v. Virsu et al.

responses become m o r e sustained (Cleland et al., 1973). These events appear to explain various concomi tan t psychophysical effects which indicate losses o f spatial and tempora l resolut ion and contrast sensitivity, and gains in summa- t ion area and t ime (Smith, 1973).

The effects of dark adapta t ion on the R F organisat ion o f ceils in the lateral geniculate nucleus ( L G N ) have no t been studied systematically. The results of Wiesel and Hube l (1966) and Mar rocco (1972) indicate that m o n k e y L G N cells react to dark adapta t ion like retinal ganglion cells: the relative effective- ness o f the antagonist ic sur round decreases and an increase in the apparen t size o f the R F centre is seen in Mar rocco ' s results. D a t a on the cat L G N cells are conflicting. Poggio et al. (1969) found a decrease in the inhibi tory effec- tiveness of the sur round at a low level of i l luminance but Mal le i and Fiorent ini (1972) did not observe this effect.

Several recent studies suggest that the centre responses of retinal ganglion cells domina te bo th the centre and the sur round responses of cells in the L G N of the cat (Singer and Creutzfeldt , 1970; Singer et al., 1972; Maffei and Fiorentini , 1972; Glezer et al., 1972; H a m m o n d , 1972; 1973). This implies tha t there should be much less decrease in the effectiveness of the sur round in L G N cells than in retinal gangl ion cells at low levels of retinal i l luminance. There could be several o ther differences be tween the effects o f dark adapta- t ion on L G N ceils and retinal gangl ion cells also, depending on the specific or- ganisation of the RFs in L G N cells.

In order to find out how the effects of dark adapta t ion are t ransferred th rough the L G N , we studied the extracellularly recorded responses of single L G N cells to spots and annuli at various scotopic and mesopic levels of light adaptat ion.

Methods

Recording

LGN recordings were done from 10 adult cats. In control experiments, the activity of the axons of retinal ganglion cells was recorded from the optic tract of 2 additional cats. The animals were anaesthetised with an initial intraperitoneal 35 mg/kg injection of Nembutal. In experiments last- ing longer than 16 hrs, small additional intravenous doses of Nembutal were occasionally added. Wounds and pressure points were infiltrated with a local anaesthetic (Xylocain). Our aim was to keep the pentobarbital anaesthesia light during the recordings and sometimes recording was post- poned by several hrs in order to allow the obvious effects of the initial dose of Nembutal on the firing pattern of neurones to wear off. Strong bursting, low maintained activity level, and short and peaked responses were signs of deep Nembutal anaesthesia. The firing patterns judged normal were similar to those obtained in an experiment with 65% N20/35% 02 anaesthesia after an ini- tial dose of 30 mg/kg Brevimytai.

The animals were paralysed with a continuous infusion of Flaxedil (gallamine triethiodide, 10 mg/ml) through a cannula in the cephalic vein (about 1.3 ml Flaxedil, 1 ml Ringer solution, and 0.7 ml Laevosan 40% per hr). Artificial respiration was adjusted so that the CO2 content of the expired air stayed between 3 and 4 %. A heating pad was used for keeping the rectal temperature between 37 and 38.5 ~ C.

The pupils were dilated by means of atropine and the nictitating membranes were retracted with Neosynephrine. The refraction of the eyes was determined with a Rodenstock refractometer and corrected with contact lenses so as to focus on a tangent screen 1 m from the animal. Stimuli

Dark Adaptation in the Cat LGN 37

were shown and RFs mapped on the screen. The positions of the optic discs were mapped a s de- scribed by Fernald and Chase (1971). In long experiments, the contact lenses were removed now and then for checking the condition of the eyes and for preventing the anoxia of the corneae. Ac- tihaernyl was applied between the lens and the cornea.

The recording electrodes were inserted stereotaxically through a 3 mm agar-filled hole in the skull and dura above the left LGN. Glass-insulated tungsten electrodes (Merrill and Ainsworth, 1972) having a 4-8 ~m long exposed tip, 1-3 ~tm in diameter, were used. Neural activity was am- plified and displayed in the conventional manner. Peristimulus time histograms (PSTHs) were pro- duced and displayed on line by a PDP12 computer. The recordings were made extracellularly from layers A and A1 of the LGN. The layers were identified on the basis of distance from the surface of cortex, and on the basis of the location sequences of RFs in the visual fields of each eye utilising the maps prepared by Bishop et al. (1962). Histological controls made with one cat indicated that the identification method was reliable.

Altogether 50 LGN cells and 4 retinal ganglion cell fibres from the optic tract were studied in detail. The average recording time per unit was about 1.5 hrs. Cells were distinguished from retinal fibres in the LGN on several criteria, of which the most important ones were the wave-form of the action potentials, firing pattern and maintained activity level, and poor response to sudden changes of overall illumination. The LGN recordings were also limited to units which had a circular RF with a clear centre surround organisation and central location in the visual field (distance from area centralis less than 18~

Stimulation and Experimental Procedures

Both stationary and moving stimuli were used. The results obtained with moving slits, bars, and edges will be presented in the subsequent paper (Lee et al., 1976). The stationary stimuli were series of annuli with a centre spot, annuli alone, and discs of various sizes. The annulus stimuli were either brighter than background ("bright stimuli") or darker than background ("dark stimu- li"). In annuli with spot the centre spot had a constant diameter of 0.5 ~ and the size of the annulus concentric with the spot was varied approximating a geometric series. The ratio of the outer diameter of the annulus to the inner diameter was about 1.3 and the mean diameters of succes- sively larger annuli increased by a constant factor, varying from 0.95 to 13.6 ~ in 9 steps; an illus- tration of the annulus with spot stimuli is shown in Figure 3D. The spot series consisted of homogeneously illuminated round spots whose diameter varied from 0.5 to 10 ~ in 11 steps.

Each stimulus was shown for 0.5 sec and its presentation was repeated at 2.5 see intervals 20 or 40 times for one PSTH. A projector transilluminated the tangent screen to provide a large homogeneous field of light forming the adapting background for the stimuli. All stimulus and background lights were white with colour temperatures comparable to that of a 60 W tungsten lamp (2800~ The luminance levels of the stimuli were adjusted by means of Kodak Wratten Gelatin ND filters in glass frames; similar filters in front of the eye were used for obtaining the de- sired levels of adaptation. A round artificial pupil, 4 mm in diameter, was placed in the middle of the natural pupil next to the cornea of each eye. Stimulation was always monocular; the non- dominant eye was covered. No stray light could enter the eyes from the sides.

The luminance levels of the stimuli were measured with an SEI photometer from the direction of the cat's eye. All stimulus intensities below are expressed in photopic trolands (td). Troland is the most commonly used correlate of retinal illuminance (pupillary area in mm 2 multiplied by stimulus luminance in cd/m2). The reader can assess the stimulus luminances of our experiments in cd/m 2 by multiplying the td values by 8• -2, and the true retinal illuminances in lrn/m 2 by mul- liplying the td values by 12.5 -2, assuming complete transparency for the ocular media and a value of 12.5 mm for the posterior nodal distance. All values can be converted into scotopic quantities through multiplication by 1.4. For characterising the adaptation level of the eye while stimulated we use the temporal average of the stimulus and background illuminances.

After establishing a stable recording of a cell of the desired type the RF was mapped with a small spot of light and marked on the screen. All stimuli were presented concentric with the RF center. The location of the centre was frequently remapped to reveal possible eye movements and the stability of responses for each cell was checked by frequent repetitions of earlier stimulus con- ditions for the same cell; unstable recordings were rejected.

38 V. Virsu et al.

Altogether, we studied the range of retinal illuminances from 3x10 -s to 5)<103 td for the background and 5x 10-3-5)< 103 for the stimulus. The weakest stimuli were selected so that some response was obtained to the 0.5 deg reference spot. With a 5)< 10 -a td stimulus illuminance the spot produced a luminous flux equivalent to about 610 quanta (507 nm)/sec incident at the cornea. During each 0.5 sec presentation this stimulus adapted the retina by an equivalent of about 230 quanta (507 nm) if 75% of quanta reached the retina. The flux from a background of 10 -4 td is equivalent to 62 quanta (507 nm)/deg2sec. According to the results of Daw and Pearlman (1969) the cat cone threshold is about 4-25 photopic td (pupil diameter about 11 rnm) and rod saturation level 3• 102-3)< 10 a td; hence, our stimuli covered the scotopic and mesopic levels of adaptation.

In order to maximise the accuracy of RF size comparisons, the same stimulus size was studied at two adaptation levels before changing to the next size in most experiments. The time allowed for dark adaptation was then about 5 min, and 3 min were allowed for light adaptation; the stimulus/background contrast was the same, 10:1 or 1:10, for each level of adaptation in these cases. In some experiments, a complete series of stimulus sizes was studied before and after 0.5-1.5 hrs dark adaptation at a very low level of illuminance. Thestimulus/background contrast in these cases was not the same for the various adaptation levels: contrast was increased to about 2 log units at the low level in order to evoke some response by the 0.5 deg reference spot.

It should be noted that when stimulus contrast is constant the amount of stray light stays in constant proportion to the background light at various levels of adaptation. Under equal-response conditions stimulus/background contrast has to be increased at low levels of adaptation so that the relative amount of stray light increases and the spatial characteristics of RFs may become con- founded with the effects of stray light.

Results

1. Time Course of Dark Adaptation

According to Barlow et al. (1957a), dark adaptation in the cat retinal ganglion cells is a slow process, requiring up to 3 hrs for completion. The initial level of adaptation was about 8 • 104 scotopic td in their experiments, and therefore, this slow recovery reflects, at least in part, the regeneration rate of rhodopsin (cf. Bonds and MacLeod, 1974). Very little rhodopsin was bleached in our ex- periments. Therefore, we expected complete dark adaptation of the LGN cells in a few minutes, for the full sensitivity of the rat E R G (Dowling, 1963; Weinstein et al., 1967) or human detection thresholds (Haig, 1941) is reached in less than 5 minutes when pigment bleaching is minimal. As a control, we re- corded the time course of response stabilisation over a period of a few minutes in 6 cells. We found that the responses of the cells did not become stable in the expected time. Therefore, we ran another series of experiments with a sample of 7 LGN cells and 1 retinal fibre, which were studied over an extended period of time in 3 cats. There was a considerable variation of time course between cells, some cells requiring 10 times more time than expected for complete re- covery of visually-evoked responses to a stable level in the dark.

Figure 1 illustrates the range of the fastest and slowest examples of on- and off-centre cells; the other units behaved similarly but with intermediate time

courses. The fastest cell was an off-centre cell having a small RF: it dark adapted in about 2 rain to a level of illuminance more than 4 log units below the initial level, which was 10-20 photopic td. This behaviour agrees with the expected time course. However, the slowest cell required about 50 min to

Dark Adapta t ion in the Cat L G N 39

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Pig. l . T ime course of response stabilisation in the dark in four L G N cells of the cat. Prior to clark adaptat ion measurements the cells were light adapted for at least 20 rain during stimulation with 102 td spots and annuli presented on a 10 td background. At t ime = 0 the intensity of the s t imulus was decreased to 10 -2 td and that of the background to about 10 .4 td. A PSTH based on 40 periods of 0.5/2.5 sec st imulation with a spot + annulus st imulus covering only the centre of RF was recorded at 2 rain intervals. The responses shown in the graphs indicate the average number of spikes/sec recorded during 0.5 sec st imulation (on-response of on-centre cells) or during 0.5 sec after st imulation (off-response of off-centre cells). The responses evoked by the background dur- ing 0.5 sec before st imulus presentat ion have been subtracted from the responses shown in the fig- ure. The straight lines have been fitted visually. The number next to a unit designation indicates the approximate size of RF centre

reach its full sensitivity. Hal f of the 8 units required more than 10 min for al- most complete stabilisation of the responses.

As our initial level of adaptat ion was less than 20 td, less than 0 .5% of rhodopsin was in the bleached state at the onset of dark adaptat ion (according to our calculations, 0.45 % would be bleached in 20 min by a 20 td stimulus without regeneration; a linear interpolation f rom the measurements of Bonds and MacLeod yields a value of 0.35 %). Thus, the lack of sensitivity at the on- set of adaptation was almost completely based on the neural component of light adaptation. This is confirmed by the fact that some cells recovered very quickly and also by the fact that a large variation occurred between cells. The state and time course of photochemical adaptat ion should have been the same for the same cat under similar conditions. Nevertheless, in spite of the same rectal temperature, anaesthesia level and stimulation a slowly adapting unit could occur either before or after the occurrence of a quickly adapting unit in the same cat.

This result suggests that the neural component of adaptation is not neces- sarily fast. What caused the differences between neurones is unclear. There may be some correlation between receptive field centre size a n d speed of

40 V. Virsu et al.

adaptation (Enroth-Cugell and Shapley, 1973), but the trend in this direction is weak: the Spearman rank correlation coefficient between centre size and rapidity of recovery was -0.45 for the 8 units. Another possible source of dif- ferences could be the rod/cone balance of RFs. Since the cone threshold is about 4-25 photopic td (Daw and Pearlman, 1969; Barlow and Levick, 1968), the contribution of cones in the results of Figure 1 is negligible, however.

2. Effects of Dark Adaptation on the Response Patterns of L G N Cells

Cells varied considerably in their response to spot and annulus stimuli, particu- larly when the annuli were large enough to inhibit the centre responses. Fig- ure 2 shows a set of PSTHs obtained from an on-centre cell at two relatively high levels of adaptation (0.2 and 200 td). The cell is quite typical of the units considered in this section (984 PSTHs from 30 LGN cells) in that it illustrates the features prominently affected by dark adaptation in other cells also.

The stimulus-evoked events at the on and off of bright spots and annuli (on for 0.5 sec) in the RF on an on-centre cell at a relatively high level of light adaptation (low photopic and high mesopic) could be classified as follows for most cells. At stimulus on, a phasic activation (A1) with a minimum onset la- tency of about 30 msec occurred. It was followed by a phasic inhibition (I1), onset latency about 60 msec (see Fig. 2). These two components correspond respectively to the primary excitatory on-response and the secondary inhibition of Singer and Creutzfeldt (1970). There follows then a more sluggish activa- tion (A2), latency about 80 msec, and a slow inactivation (I2), latency about 100 msec. Oscillations (0) in responses strongly inhibited by annuli occurred often and consisted of two or three short-duration peaks. The time interval be- tween the peaks was not constant but varied from 40 to 110 msec depending on the size of stimulus. Off-centre cells (14 in this sample) displayed a similar sequence of response components if stimulated with dark spots and annuli, and behaved similarly to on-centre cells also in other respects discussed below.

At stimulus off, an inhibition (I3 in Fig. 2) occurs, latency about 30 msec and duration 20-300 msec. It is followed by an activation (A3) the strength and duration of which varied considerably. These components correspond to the primary inhibition and secondary excitation of Singer and Creutzfeldt (1970). Later fluctuations in firing level occurred sometimes but they dis- appeared usually in less than 1 sec. A phasic inactivation I4 sometimes inter- rupted A3.

As a function of annulus size, A1, I1, and A2 increased for sizes smaller than RF centre. A1 increased up to a larger size than A2, and therefore, the RFs of cells if estimated from the phasic response were larger than if estimated from measures which include this later component. Stimulation of RF surround by an annulus larger than RF centre could suppress component A2 completely and leave A1 virtually unaltered.

The latency of components A1 and I1 decreased as a function of annulus size, and the decrease continued at larger annulus sizes than the one evoking a maximum response. In fact, the shortest onset latency of responses was typi-

Dark Adaptation in the Cat LGN 41

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Fig. 2. Peristimulus time histograms of the responses of an on-centre cell (U 16 on) to bright spot + annulus stimuli as a function of annulus mean diameter at two levels of light adaptation. The stimulus/background contrast was the same 10:1 at both levels of adaptation. The dimensions of four stimuli are illustrated in the insets. The first vertical bar below each PSTH indicates stimulus onset and the second stimulus offset 0.5 sec later. The whole stimulus cycle was 2.5 sec; only a part is seen in the PSTHs. Binwidth 5 msec, 20 summations. See text for further details

cally caused by the same stimulus that evoked the response indicating maximal sur round inhibition. The behaviour o f L G N ceils in this respect is similar to that observed by BiJttner et al. (1975) in retinal ganglion cells.

The magni tude of off- inhibit ion I3 did no t change in a systematic manne r as a funct ion o f annulus size. The strength of the secondary excitation A3 and of the possible phasic inact ivat ion I4 increased as a funct ion of annulus size, reaching a max i m um often at a size which al lowed an a lmost uninhibi ted re- sponse to the centre spot. This behaviour resembles that observed by Hickey et al. (1973) in their G r o u p / retinal gangl ion cells. Characterist ic of off- responses was their apparen t unpredictabi l i ty in size and shape, and variability be tween cells and exper imental conditions.

The off- responses of off -centre cells evoked by d isappearance of bright stimuli can be character ized by componen t s A1, I1, A2 , and I2. The disc

42 v. Virsu et al.

stimuli of various sizes evoked responses similar to those evoked by spots and annuli but response components and their dependence on stimulus size were less clear.

Dark adaptation strikingly changed the appearance of PSTHs. Superficial- ly, responses became more sustained, phasic components being attenuated or lost. With stimuli having a constant 1 log-unit contrast, the primary excitation A1 decreased in size, inhibition I 1 disappeared, and no oscillations were elicited by large annuli. Secondary excitation A3 diminished or disappeared and I4 did not occur at a low level of illuminance. Response latencies and the rise time of responses increased in all cells in the dark. All these changes in PSTHs were clearly visible at relatively high levels of adaptation of 0 .2-2 td. Primary inhibition I3 at stimulus off did not usually disappear although it often became less prominent, and at no level of dark adaptation did the surround of RF become incapable of inhibiting the responses of RF centre to the reference spot. The maintained discharge rate (MDR) increased in the dark in off-centre cells but both increases and decreases occurred in various on-centre cells.

We did not run formal tests for distinguishing between sustained and tran- sient cells (Cleland et al., 1971). One on-centre cell (U 46 on T), however, was most probably a transient cell, for its light-adapted responses were very phasic, it responded to fast movements of a stimulus spot and with some stimulus combinations its off-responses (A3) were much stronger than on-responses (A1). Two other on-centre cells (including U 16 on of Fig. 2) and two off- centre cells had some features suggesting the transient category but it would be unsafe to classify them as transient. The rest of the cells considered in this sec- tion (13 on-centre and 12 off-centre cells) probably represent the sustained type. The description above fits both types of cells, but it should be kept in mind that we did not record from cells more peripheral than 18 ~ from the area centralis and all cells had a clear centre-surround organisation. In a dark adapted state the differences between the response patterns of various neurones were much smaller than in a light adapted state.

3. Effects of Dark Adaptation on the Spatial Organisation of Receptive Fields

The RFs of 13 on-centre and 11 off-centre cells of the LGN were studied in detail with spot and annulus stimuli at two or three levels of adaptation. The size of RF centre of the cells as determined with a small spot in a light-adapted state varied from 0.3 ~ to 2.0 ~ . Each cell was under observation for 2 -4 hrs. Re- sponses to bright spot + annulus stimuli were analysed for 10 on-centre cells and 3 off-centre cells, to dark spot + annulus stimuli for 7 off-centre cells, and area summation curves were recorded with bright discs for 4 on-centre and 2 off-centre cells. In the first experiments, responses to annuli alone were re- corded for one on- and one off-centre cell. As stray light and the low MD R of many LGN cells make it difficult to interpret results obtained with annuli alone, we did not continue their use in later experiments. In control experi- ments, the RFs of 2 on-centre retinal ganglion cells were investigated in detail with spot and spot + annulus stimuli.

Dark Adaptat ion in the Cat LGN 43

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Fig. 3 A - F . Responses of six LGN cells to spot + annulus stimuli at various levels of light adapta- tion. The diameter of the stimulus spot was 0.5 ~ and the mean diameter of the accompanying an- nulus was varied as shown on the abscissae; the first value of each curve refers to the response evoked by the spot alone. The relative dimensions of the stimuli are illustrated in Fig. D. Adapta- tion levels are indicated in the graphs: the first number refers to retinal illuminance produced by the background and the second to that of the stimulus; filled-in symbols and heavy lines refer to low levels of adaptation. The response is the average firing rate evoked by the stimulus during its whole 0.5 sec presentation time, averaged over 20 or 40 stimulus cycles. Firing rate evoked by the background has been subtracted and each curve has been normalised so that the largest response is equal to 1. A - D Stimulus brighter than background. E - F Stimulus darker than background. In D, adaptation levels are the same as in A and the off-response of the cell is shown in this case, evaluated as the average over 3 consecutive 5 msec bins yielding the maximum firing rate

Figures 3 and 4 present a few spatial response profiles of individual LG N cells; average data are shown in Figure 5. The individual profiles are shown for rather large RFs in order to illustrate summation in the RF centre, but smaller RFs behaved similarly to larger ones in all respects considered below.

The inhibition exerted by the RF surround on the centre response disap- peared under none of the conditions we employed, being visible after a 1.5 hr dark adaptation and with stimuli that were at the absolute threshold for the 0.5 ~ reference spot. A comparison of Figures 3 and 4 suggests that the disc stimuli were somewhat more effective than spot + annulus stimuli in revealing the suppressive action of the surround. Surround inhibition is apparently weak in the cells depicted in Figure 3A and 3D, but Figure 4A and 4C, displaying the area summation curves of the same cells, indicate strong inhibition.

The quantitative characteristics of responses varied somewhat from cell to cell, and there was some change of centre/surround balance on the average in

44 V, Virsu et al.

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Fig. 4A-C. Area summation curves for three LGN cells. The stimuli were homogeneous spots of light of various sizes presented at two levels of light adaptation. The diameters of the stimuli are shown on the abscissae. Other data as in Fig. 3

the dark. In the spot + annulus data, we estimated the effectiveness of sur- round inhibition by calculating percentage suppression, S = 100 ( 1 - Rmin/Rs), where Rs is the average firing rate during the exposure of the centre spot and Rmin the firing rate when a maximally-inhibiting annulus ac- companied the spot. For a 500 msec response count (A1 + A2), the value of S was 68% (S.D. = 18%) in the light-adapted state, averaged from the 20 LGN cells studied with spot + annulus stimuli (see the legend of Fig. 5 for details). The corresponding value in the dark-adapted state was 56 % (S.D. = 23 %). If a 15 msec peak response (A1) is considered, the average value was 67 % in the light and 56 % in the dark. In both cases there is a tendency for the surround to be less effective in the dark, but the differences were statistically only almost significant (p < 0.05, two-tailed t-test for dependent observations). The con- trol experiments indicated much larger losses of surround inhibition in retinal ganglion cells under similar conditions, particularly for the peak responses.

Small changes of RF size occurred in most cells. In general, the apparent RF became larger in the dark even when the same stimulus/background con- trast or dark stimuli on a brighter background were employed. This can be seen especially well in Figures 3A, 3F, and 4A. The apparent size of RF was larger also for the peak response A1 than for the response including both A1 and A2.

Figure 5 presents a summary of the effects of dark adaptation on the RF properties of LGN cells separately for the peak and the whole response. Figure 5A shows the averages of normalised spot + annulus data for 20 cells. The first and last point in each curve depicts the average relative response evoked by the 0.5 ~ reference spot and by a 10.5 ~ spot + annulus stimulus, respectively. The intermediate points represent the average stimulus sizes that evoked re- sponses having a constant relative strength; thus, the maxima of the curves show the average size that evoked the maximum response, and the minima of the curves indicate the average strength of the minimum relative response and the average size that caused the minimum response. Figure 5B shows the aver- age relative response of 4 on-centre cells stimulated with spots only: direct re-

Dark Adaptation in the Cat LGN 45

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Fig. 5A and ll. The effect of dark adaptation on the average spatial properties of RFs in LGN cells. A Responses to spot + annulus stimuli. The data are the average relative responses (MDR subtracted) of 11 on- and 9 off-centre cells; the spatial averaging has been explained in the text. B Responses to spots of light. The data are based on direct averaging of the relative responses evoked by each size in 4 on-centre cells whose RFs were similar in the light-adapted state. Open symbols refer to a light-adapted state. Background intensity was then 101-102 td for on-centre cells and 101--103 td for off-centre cells. Filled-in symbols refer to a dark-adapted state in which the temporal average adaptation level was typically about 10 -3 times the light-adapIcd level of each cell. Circle symbols indicate sustained responses averaged over a period of 0.5 s~e; star sym- bols show the peak responses averaged over the three highest bin values

sponse averaging without spatial normalisation was justified for this subgroup of cells because the spatial properties of the RFs of the cells were similar in the light adapted state.

Statistical features of the spot + annulus data were analysed by means of two-tailed t-tests (dependent observations) made with respect to the stimulus size evoking a criterion response under different conditions, and by means of 3-way analyses of variance (neurones x adaptation levels (or type of response counted) • 5 levels of response strength from the maximum to the minimum). The analyses indicated statistically highly significant changes in the stimulus size that caused a constant relative response in the light v s . in the dark, and indicated that peak responses give a larger estimate of the RF centre than sustained responses (p < 0.001). On the average, the annulus size evok- ing a maximum response in the dark-adapted state was 17.5 % larger than the size evoking a maximum response in the light-adapted state and the transition from maximum to minimum was shifted by the same amount, if the 500 msec responses were considered (cf. open and filled circles in Fig. 5A). The corre- sponding change of the transition zone was 10 % for the peak responses, but this change was not statistically significant (open v s . filled stars in Fig. 5A). In the light-adapted state, the size evoking a maximum phasic response was 9 %

46 v. Virsu et al.

larger than the size evoking a corresponding sustained response; the average shift of the whole transition zone from maximum to minimum was 26 % (open circles vs. open stars in Fig. 5A). In the dark-adapted state, the average shift of the transition zone was 18%; this change was statistically almost significant (p < 0.05). The average area-summation curves shown in Figure 5B indicate similar but somewhat larger changes of RF size. In addition, they show clearly more inhibition of sustained than peak responses.

Discussion

1. The Organisation of Receptive Fields in the Cat L G N

The change of functional spatial organisation in retinal ganglion cells at low levels of retinal illuminance is a gradual process, which seems to begin at about 50 td level of adaptation in the cat (see Barlow et al., 1957b; Andrews and Hammond, 1970; Maffei et al., 1971). Various estimates are available for the illuminance level that leads to a complete disappearance of surrounds, depend- ing on the criteria used. The absolute threshold of surround antagonism seems to be about 0.3-0.5 log units higher than the absolute threshold of the RF centre (Enroth-Cugell and Lennie, 1975).

Some adaptation levels of the present experiments were so low that a com- plete disappearance of surrounds was likely to occur in retinal ganglion cells, and we observed that the off-responses evoked by cessations of surround stimulation disappeared at the low levels of illuminance in most LGN cells. Nevertheless, there was very little decrease in the power of the surround to an- tagonise the centre responses of LGN cells on the average, as compared with responses obtained at 103-104 times higher levels of adaptation, and a reliable decrease was seen only in a few cells. Evidently dark adaptation has a much stronger effect on the centre/surround balance in retinal ganglion cells than in LGN cells of the cat. In sum, our results seem to indicate somewhat stronger effects of dark adaptation on the centre/surround balance of LGN cells than the results of Maffei and Fiorentini (1972) in the cat and somewhat less than the results of Marrocco (1972) in the monkey.

The neurones we studied were probably relay cells. In all recent models of their RF organisation one or only a few retinal ganglion cells are assumed to converge into the RF centre (Singer and Creutzfeldt, 1970; Cleland et al., 1971). Different assumptions have been made regarding the structure of the RF surround. The surrounds of retinal RFs contribute to the surrounds of LGN cells, but since peripheral suppression is stronger in the LGN cells than in retinal ganglion cells (Hubel and Wiesel, 1961), some additional surround mechanism has to be postulated.

In what could be called the "zone" models, it is assumed that retinal ganglion cells neighbouring the main excitatory cells project to the relay cell directly or via interneurones so as to strengthen the surround of the LGN cell (Maffei and Fiorentini, 1972; Hammond, 1973). Very little or no overlap of excitation and inhibition in the RF centre is assumed in these models. In other

Dark Adaptation in the Cat LGN 47

models, which could be called the "pool" models, it is assumed that many ganglion cells excite interneurones, which in turn inhibit the relay cells (Singer and Creutzfeldt, 1970; Singer et al., 1972; Glezer et al., 1972; Levick et al., 1972; Cleland and Dubin, 1976). In these models excitation and inhibition are coextensive in the RF centre of relay cells (for evidence from intracellular re- cordings, see McIlwain and Creutzfeldt, 1967; Singer et al., 1972).

An acceptable model of the RF of relay cells should explain the following aspects of the present results, for they were valid for most cells and are not likely to be trivial consequences of the behaviour of retinal ganglion cells:

(a) the persistence of surround inhibition in the dark; (b) the enlargement of functional RF centre in the dark; (c) the larger RF size displayed by the peak response than by the sustained

response; (d) the occurrence of secondary inhibition I1 in the on-response and its

disappearance in the dark; (e) oscillations of strongly inhibited on-responses and their disappearance

in the dark. ,~ The zone models explain (a) but (b) and (c) are difficult to reconcile with

the zone models, Enlargements of retinal ganglion cell RFs were observed in control experiments, but it is difficult to see how these retinal effects could be transferred to relay cells because at the border between the centre and sur- round the enlargements of the retinal centres should cancel each other. On the other hand, the oscillatory responses suggest a negative feedback, which has not been postulated in the zone models; the present zone models are not able to account for (e).

The pool models are in good agreement with the present results. If the ret- inal RF stands in the middle of an inhibitory pool, any changes of the func- tional size of the retinal RF centre are transferred to the relay cell but the in- hibitory pool does not transmit size changes and allows only minor losses of surround inhibition, such as observed in the present study. The oscillatory re- sponses suggest that at least a part of the inhibition exerted by the pool is re- current. It is interesting that the oscillations occurred only when a large an- nulus stimulated the cell; they were not evoked by small stimuli. It appears that in order to evoke oscillations, strong inputs were required. This suggests that a combined activity of many neurones is necessary in order to activate the pool sufficiently for oscillations. At a low level of retinal illuminance responses of cells are smaller and the rise time of responses is longer so that the pool does not receive sufficiently coherent and strong input that could start oscilla- tion. What remains to be explained, however, is the origin of the secondary inhibition I1, for it is evoked also by stimuli covering only the RF centre. A parsimonious explanation is that its origin is also recurrent as suggested by Singer and Creutzfeldt (1970); then I1 and A1 would correspond to one cycle of weak oscillations. One could entertain also the possibility that secondary in- hibition reflects an input into the inhibitory pool from transient retinal cells (se e Hoffmann et al., 1972; Singer and Bedworth, 1973). Since transient reti- nal ganglion cells respond in a sustained manner in the dark (Cleland et al., 1973; iakiela et al., 1976), the lack of a sharp input from them may explain

48 V. Virsu et al.

the disappearance of I1 and other phasic inhibitory phenomena at a low level of illuminance in the LGN cells.

2. Psychophysical Correlates of LGN Responses

Intrageniculate inhibition sharpens the temporal response of sustained cells at high levels of adaptation and prevents the loss of centre-surround structure at low levels of adaptation. Even though the reorganisation of visual inputs in the LGN is not very radical, it requires consideration when psychophysical effects are explained in terms of retinal ganglion cell physiology.

At a low level of adaptation, human contrast sensitivity decreases. This cor- relates with the decrease in sensitivity of retinal ganglion cells (Sakmann and Creutzfeldt, 1969) and the decrease of responses to a constant contrast, ob- served in all LGN cells in this study at a low level of illuminance. Various spa- tial effects are related to this decrease, for the decrease of contrast sensitivity is strongest for high spatial frequencies and small stimuli: low-frequency attenua- tion seen in the contrast sensitivity curves disappears in the dark and optimum sensitivity shifts towards larger stimuli (Arden and Weale, 1954; Barlow, 1958; Daitch and Green, 1969; Kelly, 1972;. Smith, 1973). An explanation commonly given for these changes is the drop-out of retinal RF surrounds in the dark. This explanation cannot be entirely correct because the surrounds of RFs in the LGN cells suppress responses to large stimuli almost equally at high and low levels of adaptation even if the surrounds of retinal ganglion cells do not. However, a partial explanation for these phenomena is the increase of RF size in the dark. This may explain also the changes of perceived spatial fre- quency and size of small stimuli at low levels of illuminance (Virsu, 1974; Virsu and Vuorinen, 1975), for the average changes of RF size agree in mag- nitude and direction with the psychophysical effects.

The effects of dark adaptation on the temporal properties of LGN ceils were dramatic because of the drop-out of phasic inhibition in the on-responses of sustained cells, and dark adaptation also made the responses generally more sluggish. These effects correlate with the increase of summation time for threshold and the decrement of temporal resolution at low levels of adaptation (Barlow, 1958; Kelly, 1972; Smith, 1973). It is also known that brief exposure and flicker increase perceived spatial frequency (Tynan and Sekuler, 1974; Virsu and Nyman, 1974). These phenomena have been explained by postulat- ing an increase of RF size. The result that the functional RF of the initial phasic on-response is larger than the RF of the later on-response supports these explanations.

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Received May 11, 1976