FLICKER FUSION IN NORMAL AND BINOCULARLY DEPRIVED CATS*

MICHAEL S. Loop, SI~VXA PEKJCHOWSKI~ and DOUGLAS C. Snnm: Department of Phys~ologicai Optics, The Medical Center/School of ~tome~y,

University of Alabama in Birmingham Birmingham AL 35294. U.S.A.

(Recriwd 1 February 1979)

Abstract-CFF thresholds were determined for 3 normal and 3 binocularly deprived (BD) cats utilizing a simultaneous two-choice discrimination procedure. For all cats when adaptation level was steady-state for stimulus luminance. the CFF/luminana: function showed a rod-cone break which took the form of a mesopic null. When the cats were maintained in a light-adapted state CFF declined as a monotonic function of luminance. Under most luminance and adaptation levels the BD cats’ CFF was slower than that of the normal cats. The BD cats’ depressed CFF is interpreted as a result of their LGNd Y-cell loss.

EXPERIMENT I

The ability to discriminate temporal light fluctuations constitutes one of the fundamental operating charac- teristics of the visual system. Although most of the physiological studies of visua1 system response to flicker have utilized the cat (Enroth, 1952; Dodt and Enroth, 1953; Ogawa er al.. 1966; Fukada and Saito, 1971; Foerster er al, f977a. b; Saunders, 1977 and others) reIatively little psychophysical data is avail- able for this species’ perception of flicker. The results of those studies which have investigated this question (Kappauf, 1936; Pautler and Clark, 1961; Taravella and Clark, 1963: Norton and Clark, 1963a, b; Schwartz and Lindsley, 1964; Schwartz and Cheney, 1966: Loop and Berkley, t975; Blake and Cam&, 1977) are idiosyncratic to the stimulus conditions which varied considerably. It is thus presently not possible to say anything about the influence of impor- tant stimulus variabies such as brightness, wavelength or adaptation level on feline critical flicker frequency (CFF).

We have undertaken to examine the influence of these parameters on cat CFF with three goals in mind. Our first goal was to determine the cat’s CFF under a variety of stimulus conditions so as to pro- vide psychophysical data relatable to some of the physiological data. Our second goal was to define behaviorally photopic, mesopic and scotopic lumin- ance levels by determining the rod-cone break in the cats’ CFF/luminance function. Although several elec- trophysiological studies have investigated this ques-

l We thank MS Sally Trotter and Mr Olen Mackey for their exceptional care of the animals and Dr D. Sparks and Dr C. Oyster for helpful comments on the manuscript. This work was supported by PHS Grant ROI MrW 30936. D.C.S. was supported by Training Grant EYO 7005.

t Present address: Department of Psychology. Univer- sity of Maryland, College Park, MD 20742, U.S.A.

$ Present address: Department of Psychology, Southern Illinois University. Carbondaie, IL 62901, U.S.A.

tion (Ogawa et af., 1966; Daw and Pearlman, 1969; Andrews and Hammond, 197Oa, b; Hammond and James, 1971; Ahmed er al., 1977) little psychophysical data is available for the cat (Kappauf, 1937; LaMotte and Brown, 1970). Our final goal was to provide nor- mative data against which to evaluate the influence of visual deprivation upon CFF.

Methods

Animals

Experiments were performed on 3 adult cats which were donated to us by encumbered owners. CTB was a neutered female blue American short-hair; CTC was a male silver tabby short-hair with white patches; CTD was a female calico (Loxton. 1976). All three had participated in a previous color disaimination study (Loop et al., I979). The cats were housed indivi- dually with a 12 hr light period and free access to water. Each cat was reduced to 80% of its free feeding weight and maintained at that level of food depriva- tion with reinforcement, dry cat chow, and daily vitamins.

Behavioral apparatus and stimuli

Our simultaneous two-choice discrimination appar- atus, modeled after Berkley (1970), consisted of a re- straining box with a head chamber at one end into which the cat put its head (detaiIs in Loop et al., 1979). The head chamber contained two clear plexi- glas response keys through which the cat could view the stimulus screen. The reinforcement of dilute beef baby food was delivered through a hole in the floor of the head chamber just below and between the re- sponse keys. The testing apparatus was housed in a larger light-tight box and faced the stimulus screen upon which stimulus pairs were projected. Response contingencies and data collection were accomplished with conventional el~tromech~ical devices.

49

50 hl. s. LOOP er ul.

All stimuli were rear projected from a Kodak car- ousel slide projector with a 300 W tungsten lamp (GE, ELH) onto a rear projection screen (Polacoat glass- cat, 3M Co.). The projector lamp was powered by 12OVDC (Power Mate PSX-E-120) and current was continuously monitored. Stimuli were calibrated with a Tektronix J16 photometer-radiometer for irra- diance (probe J6502) and luminance (probe J6503).

Stimulus pairs for the flicker-steady discrimination were achieved by placing a piece of film polarizing material and ftlm neutral density filter (Kodak No. 96) side by side on standard 35 mm double frame slides. Sinusoidal flicker (Michelson contrast 8&85p/,) was imparted to one side of the projected image by continuously rotating a large disc of film polarizing material in front of the slide projector’s lens. The full beam of the slide projector was masked at the projec- tion side of the viewing screen producing two circular stimuli (diameter 2.5 cm) centered behind the two re- sponse keys. With a viewing distance of 3 cm at the moment of response each stimulus subtended a visual angle of 45”. Stimulus surround was dark. The maxi- mum mean luminance of the flicker field was 2.40 log cd/m’ and was paired with one of three steady (Michelson contrast ~2%) intensities (2.45, 2.38 or 2.27 log cd/m’) so that the brightness of the steady field varied randomly with respect to the flicker field. Variation in flicker frequency was accomplished by changing the rotational speed of the polarizing disc. Flicker frequency was continuously monitored by dis- playing the output of a phototransistor on an oscillo- scope and counting voltage transitions on an electro- nic counter.

When stimuli were not present, i.e. during intertrial intervals, a second projector (500 W tungsten lamp, GE, DAK, 120 VAC) was used to replace the stimulus pair with steady fields so as to prevent the cat from dark-adapting during a testing session. Each day the luminance of the flicker stimuli and these adapting lights was matched at 2.4 log cd/m*. Luminance was then varied by placing neutral density filters at the viewing screen, thereby attenuating the luminance of the flicker-steady stimuli and the adapting lights; or by placing neutral density filters in front of the stimu- lus projector lens, thereby attenuating only the flicker-steady stimuli. Consequently when adapting light luminance was the same as stimulus luminance a steady-state CFF/luminance function was determined. When adapting light luminance was maintained at 2.4 log cd/m2 a light-udapted CFF/luminance function was obtained.

Monochromatic stimuli were also utilized but required some modification of the procedure. During photopic monochromatic testing the flicker-steady stimuli were presented at 450, 500, 560. 600 and 670nm of equal energy (6.0 x 10-l PWlcm’) while adapting light luminance was 2.4 log cd/m2. During scotopic mon~hromatic testing the same wave- Iengths were presented at an energy of 6.0 x 1O-s pW/cm’ while adapting light luminance

w-as - 1.6 log cd:m’. Restricted spectral output was achieved with Corion interference filters and energy measurements refer to irradiance of the J6502 probe by the stimuli on the viewing side of the screen with the probe flush against the screen.

Behariornl training

The reinforcement contingencies were as follows: correct responses were followed by reinforcement (dilute beef baby food). a brief tone. and a 3 set inter- trial interval. Incorrect responses were followed by no reinforcement and a 6 set intertrial interval. During any intertrial interval the stimuli were off, white noise and adapting lights were on and responses reset the intertrial interval to 6 sec. At the end of the intertrial interval a I set “observation” period began during which the stimuli were on (adapting lights off) but responses during this period reset the observation period to 1 sec. At the end of the observation period the white noise terminated and the cat was free to choose between the stimuli. The Ieft,‘right position of the correct stimulus was organized in a haphazard order and determined by the sequence of slides with a total sequence length of 120. No more than 3 identical slides were allowed to appear in a row. A correction procedure was used wherein only a correct response advanced the sequence to the next position. This cor- rection procedure is a powerful deterrent to the cat’s initial and recurrent habit of responding predomi- nantly to one key. This procedure, however, also pro- vides a nonvisual positional solution. Changing re- sponse position after every error results in scores as high as 677; correct (Loop and Sherman, 1977a). Con- sequently, the percent correct of all responses pre- ceded by a correct response (i.e. responses following a slide change) was used as the measure of discrimi- nation performance. This measure represents an unbiased assessment of the cat’s stimulus-based per- formance.

All 3 cats required no reshaping to press the re- sponse keys with their noses and were retrained to criterion accuracy on a light-dark discrimination, light correct, and then trained to criterion on the flicker (20 c/s)-steady di~riminatio~ flicker correct (luminance 2.4 log cd/m*). Criterion performance was 4 consecutive days at >90% correct with 200 trials per day for 7 days per week.

Once criterion level performance had been estab- lished a cat’s CFF for a particular stimulus condition could be determined in one testing session by a modi- tied tracking procedure (Blough, 1955). Daily each cat was tested for 40 trials at one frequency beginning at a frequency which could be easily discriminated (90% carrect). The frequency was then increased by 7 Hz and another 40 trials conducted. This regime was con- tinued until a frequency was attained at which per- formance fell to 64% correct or below. A final block of 40 trials was then conducted at the starting fre-

CFF in normal and binocularly deprived cats 51

quency to determine if fatigue, satiation, or any other non-stimulus variable could account for the decline in response accuracy. The frequency of the first block of trials was varied somewhat to accotnmodate the cats’ lower CFF at dim luminances. A typical testing ses- sion included 6 blocks of trials. CFF was then esti- mated from a daily function to be that frequency at which the percent correct,Jgicker frequency function crossed the 70Y0 correct line. As the hnaf Mock of trials aIways elicited a response accuracy of > 80% we attributed the decline in performance to stimulus fre- quency.

Following an initial ten sessions of CFF testing at 2.4.log cd/m2 determination of the cats’ sreudy-state CFF/iuminance was commenced. In order to ensure a steady-state adaptation level the animals were dark- adapted in a light-tight box for at least 30min before testing and then quickly transferred to the testing chamber in very dim illumination. During these ex- periments the mean luminance of the stimulus pair and the adapting lights was equal. Our rationale for this procedure was that the cat would rapidly light- adapt to the mean luminance of the stimuli and then be held at that level by the adapting lights. Conse- quently their adaptation level would be steady-state for that stimulus luminance by the time the blocks of trials which actually determined their CFF for the day were conducted. Stimulus luminance was reduced in 1 log unit steps across days from 2.4 logcd/m’ to -4.6 logcd/m’. The luminance IeveI was counter- balanced across days and each cat was tested 5 (Cf’D) or 3 (CTB, CTC) days at each luminance.

Following this experiment the procedure was modi- fied in two ways: (1) The cats were not dark-adapted before testing and (2) the luminance of the adapting lights was always 2.410gcd/mr. The purpose of this procedure was to maintain light-adaptation for 2.4fogcd/m2, above rod saturation, thereby permit-

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60

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c 50 2

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8 30

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i 20

zr” u IO i-

B

/- /’

1 , K, , I J_

-5 -4 -3 -2 -I 0 I 2 3

LUMINANCE (log cd/m’) Fig. 1. Critical flicker frequency for normal cats as a func- tion of stimulus luminance when light adaptation level is steady-state for stimulus luminance (solid circles) or Iight- adapted for a luminance of 2.4 log cd/m2 (open squares).

Bars represent 0.5 SD. Y.R. m;l--o

ting a determination of the lighr-adapted CFF/lumin- ante function based solely upon cone function. As before, the luminance of the test stimuli was dimmed across days from 2.4 log cd/m’ to - 2.6 log cd,‘m2 and each cat was tested three or five times at each stimu- lus luminance.

As the results of these experiments indicated a rod- cone break around 0.5 log cd/m’ we wished to cor- roborate this point by examining the influence of monochromatic light on CFF at two adaptation levels; photopic and scotopic. Under photopic condi- tions (no dark adaptation, 2.4 log cd/m2 adapting lights luminance) each cat was tested with equal energy stimuli of 450. 500 and 560 nm. CTD was also tested at 600 and 670nm. For scotopic testing the above procedures were repeated but the cats were dark-adapted and the intensity of the monochromatic stimuhts and the adapting lights was reduced by 4.OIog units. Each cat was tested for 5 days at each wavelength under both conditions. Wavelength sequence was counterbalanced across days.

Results

All three cats rapidly reached criterion performance on the light/dark discrimination and required a mean of 11 sessions to reach criterion on the flicker/steady discrimination. Each cat’s mean CFF over the next 10 days ranged from 53.6 to 56.9 Hz for the 2.4 tog cd/m2 luminance.

Figure 1 shows the influence of varying stimulus luminance on CFF when stimulus and adapting lights luminance were equal (solid circles) and when the adapting lights luminance was 2.4logcdjm* (open squares). Each data point represents the mean of each cat’s mean CFF. When stimulus and adapting lumin- ance were equal the function shows a clear disconti- nuity from 1.4 to -0.6logcd/m*. Above this lumin- ance range CFF rises at a rate of approximately 30 Hz/log cd/m2 and below this range CFF declines at a rate of approx 10 Hz/log cd/m’.

In man, the break in the CFF/luminance function observed for eccentrically fixated test fields (Hecht and Verrijp, 1933X or large centrally fixated test fields (Hecht and Smith, 1936), has been attributed to the shift from cone to rod vision as luminance decreases. This interpretation is based, in part, upon the fact that no break is observed if the test field is restricted to fovea1 stimuiation thereby activating only cones (Hecht and Verrijp, 1933; Hecht and Smith, 1936). We have attempted the same logic while side-stepping the anatomical fact that the cat has rods and cones mixed together across the retina (Steinburg et al., 1973). Thus we attempted to eliminate rod function by exposing the cats to rod saturating adapting lights between trials. As Fig. 1 illustrates when stimulus luminance was decreased but adapting luminance was maintained at 2.4 log cd/m2 CFF declined in a mono- tonic fashion at a rate of approx 20 Hz/log cd/m’.

Another basis for attributing the discontinuity in the CFF~uminance function to rods and cones is a

hf. s. LOOP ‘2 al.

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E 20 - .g .- ; Nwmol Cots

IO - (n=3)

.

Wavelength (nm)

Fig. 1. Critical flicker frequency for equal energy mono- chromatic stimuli at photopic (solid circles) and scotopic (open squares) adaptation ievels. Solid circles at 600 aid 670nm represent the CFF of CTD only. Bars represent

0.5 SD.

difference in luminous efficiency across the spectrum as luminance decreases (Hecht and Shalaer, 1936). Figure 2 shows the inRuence of wavetength on CFF for adaptation light levels of 2.4 (solid circles) and - 1.6 (open squares) log cd/m’. Each data point at 450,500 and 560 nm represents the mean of each cat’s mean CFF (5 determinations). The data points at 600 and 670 nm represent the mean CFF of cat CTD for an adaptation level of 2.4 fog cd/m’. The stimuli were matched for equal energy. slightly dimmer than the adapting lights. and adapting light luminance was selected to hold the cats’ adaptation level at apparently photopic (2.4 log cd/m’) or scotopic (- 1.6 log cd/m21 levels. Consequently the influence of stimulus wavelength should reflect luminous efficiency and hence photopic and scotopic spectral sensitivities. As can be seen in Fig. 2 for scotopic luminance CFF was highest at 5OOnm (open squares) which is in keeping with behavioral (Gunter. 19.52: LaMotte and Brown, 1970) and physiological (Granit 1943; Daw and Pearlman. 1969) determinations of the cats’ sco- topic spectral sensitivity. Physiological determina- tions of photopic mechanisms indicate that the cat has at least two cone pigments with i.,, around 560 and 450nm (Pearlman and Daw, 1970; Hammond, 1978) and perhaps a long-wavelength cone (Granit, 1943: Saunders, 1977) and a SOOnm cone (Ring0 et ut., 1977). Behavioral determinations of the cat’s pho- topic spectral sensitivity have been highly variable (Bonaventure. 1962; Gunter. 1954; LaMotte and Brown, 1970). The results obtained by Gunter (1954), in best agreement with the physiological findings, indicate comparatively flat luminous et?iciency from 450 to 550 nm and is compatible with our observation that at an adaptation level of 2.4 log cd,‘m’ (solid cir- cles) the cats’ CFF is unaffected by wavelength vari- ations from 450 to 560 nm. Although scattered reports (Grdnit. 1943; Saunders. 1977) have implied a red-

sensitive cone our data for cat CTD indicate that at photopic luminance levels CFF declines at wave- lengths longer than 560 nm.

Discussion

The cat’s CFF~iuminance function showed a clear discontinuity in slope between 1.3 and -0.6 log cd/m’ when stimulus and adapting light luminance were equal. We attribute this discontinuity to a shift from cone to rod function because (1) rod-saturating adapt- ing lights between trials eliminate the break, (2) the luminous efficiency of monochromatic stimuli brighter or dimmer than the luminance at which the break occurs is consistent with the cat’s photopic (Gunter. 1954) and scotopic (Gunter, 1952; LaMotte and Brown, 1970) spectral sensitivities and (3) the luminosity range of the break is consistent with pre- vious determinations of the cat’s mesopic zone @au: and Pearlman. 1969: Andrews and Hammond. 1970a. b; Hammond and James, 1971: Ahmed er al., 1977).

The cat’s CFF/luminance function differs from that of man in several respects. First. the cat’s rods are able to support the discrimination of flicker at fre- quencies as high as 35 Hz whereas human scotopic CFF is generally no more than I3 Hz (Hecht and Verrijp, 1933; Hecht and Shalaer, 1936; Hecht and Smith, 1936: Crozier et (II.. 1937: Crozier and Wolf, 1941). However, Conner and MacLeod (1977) have shown that under certain circumstances human rods can be shown to support CFF up to 28 Hz. Second. the luminosity at which the rod-cone break occurs in cats is considerably brighter than in man. We and Kappauf (1937) find the break around 0.5 logcd/m2 for cat whereas in man it occurs at around - 1 logcd/m’ (Crozier et al., 1937; Crozier and Wolf. 1941) under comparable viewing conditions.

Finally. and perhaps the most intriguing difference. the rod-cone break in the cat’s CFF,luminance func- tion is more a rod-cone dip. That is to say. the cat’s CFF is lower for mesopic than for brighter or slightly dimmer stimuli. Although not as prominent a feature of man’s CFFjiuminance function. an apparently identical effect has been observed by MacLeod (1972) and van den Berg and Spekreijse (1977) which they attributed to a cancellation of ganglion cell output by rod and cone signals arriving 180’ out of phase. The effect of this rod-cone interaction, or mesopic null (MacLeod. 1972). is to create a luminosity zone in which a particular frequency seen as flickering at sco- topic and-photopic levels is seen as fused for mesopic levels. The mesopic null is apparently a far more powerful effect in cat presumably due to a large rod contribution and the virtually complete mixing of rod and cone signals at the ganglion cell (Daw and Pearl- man, 1969; Hammond and James, 1971; Hammond 1978) in a cancellation interaction (Rod&k and Rushton, 1976).

Physiological data on the cat indicate that neural CFF (usually defined as that stimulus frequency at

CFF in normal and binocularly deprived cats 53

which the response fails to follow the stimulus in a 1: 1 fashion) increases with stimulus intensity. Thus horizontal cell (Foerster er ai.. 1977a. b). eIectroretino- gram (Dodt and Enroth. 1953) and ganglion cell (Ogawa et al.. 1966; Fukada and Saito. 1971) CFF all show a tendency to rise with increases in stimulus intensity. Furthermore. under appropriate conditions a rod-cone break is apparent (Dodt and Enroth, 1954; Ogawa er nl., 1966) with CFF increasing rapidly above and falling more gradually below mesopic tuminances. Dodt and Enroth (1954) found the cat’s rod-cone break occurred at a frequency of 30 Hz with a retinal illumination of 250 Im/m’ while Ogawa er al. (1966) found the break at 25 Hz for 3 Imim’. This illumination difference is most likely due to the facts that Dodt and Enroth (1954) did not dilate the cats’ pupils and allowed relatively little time for dark-adap tation when moving from bright to dim stimuli. Both the frequency (25 Hz) and luminance (approx 0.9 logcd/m’) of the rod-cone break reported by Ogawa et af. (1966) is in good agreement with our results. Also consistent with our behavioral data Ogawa er al. (1966: Fig. IOC) and Enroth (1952: Fig. 7 and Fig. 38) have observed that for some cat ganglion cells CFF decreases as luminance is increased from scotopic to mesopic levels. Although Enroth (1952) supposed these units to be responding at odds to the populations behavior we feel the oppo- site more likely to be the case.

EXPERIMENT II

Numerous studies have demonstrated that bilateral (BD) and unilateral (MD) eyeiid ctosure during the first 3+ months of a cat’s life causes a variety of anatomical (Wiesel and Hubet, 1963; Guillery and Stelzner, 1970; LeVay and Fester, 1977). physiological (Wiesel and Hubel, 1965a. b; Sherman et al.. 1972; Btakemore and Van Stuyters. 1974, 1975) and behav- ioral (Ganz and Fitch, 1968; Dews and Wieset, 1970; Ganz er af.. 1972; Sherman, 1973; Griffin and Mit- chell, 1978) abnormalities.

The principle impetus for previous behavioral studies of visually deprived cats’ spatial discrimi- nation abilities has been the physiological abnormali- ties at the cortical level. Thus area 17 neurons are virtually inaccessible to the deprived eye of the MD cat and exhibit much less receptive field specificity for stimulus orientation and direction of movement in both MD and BD cats (Wiesel and Hubel, 1965a).

However, another major physiological abnormality of visually deprived cats is apparent at the level of the dorsal lateral genie&ate nuckus (LGNd) where an abnormally small number of Y-cells are encountered in contrast to an apparently normal representation of X-cells. In BD cats this Y-cell “loss” is apparent throughout the LGNd’s visual field but in MD cats the Y-cell reduction is confined to that portion of LGNd which receives its input from temporal retina of the deprived eye, i.e. the binocular segment (Sher-

man er al., 1972; Hoffman and Cynader. 1977; Wilson er al., 1977; Hoffman and Holtander. 1978).

The physiological distinctions of X- and Y-cells have been detailed in several studies (Enroth-Cugeli and Robson, 1966; Cleland er al.. 1971; Fukada and Saito, 1971). The general consensus is that X-cells with their small receptive fields, continuous response to steady contrast. and stow conducting axons are best suited for conveying spatial information. Y-cells, on the other hand. by virtue of their large receptive fields, transient response to steady contrast and fast conducting axons, are best suited for conveying tem- poral information (Fukada and Saito, 1971; Ikeda and Wright, 1972. Kulikowski and Tolhurst, 1973; Tothurst, 1973).

These functional aspects of X- and Y-cells coupled with the differential consequences of neonatal lid suture upon these two populations of LGNd efferents suggested to us that visuatly deprived cats might show abnormally poor discrimination of flicker. In keeping with this hypothesis, Jones and Berktey (1977) have shown that the visually evoked potential to high fre- quency flicker in area 18 of MD cats is lower in amplitude through the deprived eye than through the nondeprived eye.

Methods

Animals and rearing condirions

Experiments were performed with 3 BD (two tor- toiseshell females and one white male; Loxton, 1976) cats taken from 3 litters. Before natural eye opening the eyelids were sutured cfosed under methoxyfiurane anesthesia. At the onset of testing BD-I8 possessed a 1 mm lid hole 13 mm from the nasal comer in the right eye. This hole developed at 5 months of age and did not overlie the cats’ pupil. None of the other lids had any openings at the time we parted them for post-deprivation testing.

The kittens were housed with their mothers until weaned at 8-10 weeks. Thereafter they were housed two per standard-sized dog cage and were permitted the run of the colony room during cage cleaning. The colony room was illuminated with fluorescent lights with a 12 hr period. Routine vaccination against pan- leukopenia was performed when the animals were 9 and 12 weeks old.

Behaoiorul resting

These visually deprived cats were trained and tested concurrently with the normal cats. Consequently, all procedural details were identical.

At 26291 days of age the BD cats were initially trained to criterion on both the light-dark (light cor- rect) and flicker (21 Hz)-steady discriminations with their lids still closed. Steady was correct for SD-12 flicker was correct for the others. Over the next 3-4 months CFF was determined daily (except for a 4 week hiatus for panieuko~nia vaccination) across a wide range of stimulus luminance. This was done so

that the cats would be well rehearsed in the testing procedure allowing CFF measurements immediately following lid opening. The cats’ lids were then parted bilaterally at 371 to 414 days of age and CFFilumin- ante functions determined for (1) equal stimulus and adapting luminance and (2) adapting luminance main- tained at 2.4 log cd/m’.

Al! three BD cats fulfilled criterion on the light/ dark (mean = 4.3 sessions) and flicker-steady dis- crimination (mean = 7.8 sessions) with their eyelids still closed. These rates of acquisition were not relia- bly different from the normal cats. Over the first 10 days individual mean CFF determinations for the 2.4 log cd/m” stimulus ranged from 31 to 47 which did not overlap the normal cats CFF’s.

The outcome of the steady-state and light-adapted CFFiluminance determinations for the BD cats is presented in Fig. 3. BD- t Z was tested five times at each data point white BD-15 and BD-18 were tested three times at each data point. As for the normal cats, the mean of each cat’s mean CFF for a particutar stimulus condition is plotted. When stimulus and adapting light luminance were equal (solid circles) the CFFjluminance function shows a break. taking the shape of a mesopic null, centered at 1.4 logcdjm”. When stimulus luminance was decreased but adapting light tuminance maintained at 2.4 log cd/m’ (open squares) CFF declined as a monotonic function of stimulus luminance at a rate of approx I4 Hz/log cdim’.

Because the 2.4 log cd/m’ stimulus and adapting light recurred throughout the determination of all functions before and after lid openings it was possible to determine any increase in CFF resulting from time andior lid opening. Table 1 presents the mean CFF for each BD cat determined from the first 3 days fot- lowing criterion performance on the flicker-steady discrimination (eyes closed); the first 3 days of 2.4 log cd/m’ testing after lid opening (early eyes open); and the last 3 days of 2.4 log cd/m’ testing (late eyes open). The number of days elapsed between the second CFF determination in each triad is given between the CFF scores. As can be seen in Table I there was a general trend toward increasing CFF across testing which was statistically reliable (p = 0.03: Friedman’s test. S = IS: Bradley. 1968).

F 60

2 BINOCULARLY DEPRIVED

-5 -4 -3 -2 -I 0 I 2 3

LUMINANCE flog cd/m’)

Fig. 3. Critical flicker frequency for binocularly deprived cats as a function of stimulus luminance when light-adap- tation level is steady-state for stimulus luminance (solid circles) or light-adapted for a luminance of 2.410gcd/m2

(open squares). Bars represent 0.5 SD.

These BD cats learned a discrimination of Ricker (21 Hz)-steady with their lids still closed. This finding replicates and extends the observatiol) of Loop and Sherman (1977b) and further emphasizes that (1) the visual world of a lid-sutured cat is not devoid of vis- ual stimulation and (2} these cats are not blind for any period of time foltowing deprivation. The fact that the neural mechanism to detect Bicker is comparatively intact during lid closure deprivation may explain why Wilson et a[. (1977) were unable to mitigate any of the physiological consequence of lid closure by ancillary temporal stimulation. Likewise, Singer er al. (1977) were unable to produce any shift in the ocular domin- ance distribution of cat visual cortex neurons by pro- viding an imbalance in only temporal stimulation to one eye,

The influence of stimulus luminance and adapta- tion level upon CFF is very similar in BD and normal cats. If adaptation level is steady-state for stimulus luminances (solid symbols in Fig. 3) the CFF/tumin- ante function shows a rod-cone break, taking the shape of a mesopic null, with a rapid increase in CFF as luminance rises to photopic levels and a more gra- dual decline as luminance falls to scotopic levels. If

Table I. CFF as a function of visual experience in BD cats

Eyes closed Early eyes open Late eyes open

BD-12 32 (2+.5) C4g3 33 (l4) 3g(-?,2) BD-IS -wt+2

I::; 4x(*9) ‘tz:3

BD-18 34(23) 43 f+ 14) I:31 56(*3) Jg(_t6)

Mean 37 [91] JI [73] 37

Mean CFF (Hz) for the 1.4log cdim’ stimulus and adapting light luminance determined during lid closure (eyes closed). immediately following lid opening (early eyes open) and at the end of CFF/luminance measurements (late eyes open). The number of days elapsed between each condition is presented in brackets.

CFF in normal and binocularly deprived cats jj

adaptation leve! is maintained above rod saturation (open squares in Fig. 3) CFF declines as a monotonic function of stimulus luminance.

There is, however, an effect of neonatal lid closure upon the cat’s ability to discriminate Bicker. A com- parison of Figs 1 and 3 indicates that when stimulus and adapting luminance are equal (solid circles) BD cats have a lower CFF than normal cats for photopic (2.4 log cd/m2) and low scotopic (- 2.6 log cd/m’ and below) luminances.

Through the mesopic luminance range of the nor- mal cat, however, BD cats evidence a somewhat higher CFF. A difference is also apparent in the CFF/ luminance functions where the adapting light was maintained at 2.4 logcd/m’ (open squares of Fig. 1 and 3) but here the normal and BD cats differ at every luminance. level.

The BD cat’s lower CFF at most stimulus lumin- antes and adaptation conditions is certainly consis- tent with the view that Y-cells are an important path- way for the detection of rapid visual events. However, there is excellent correspondence with the normal cats for both steady-state and light-adapted CFF/lumin- ante functions if the BD curves are slid 1 log unit down the luminance axis. That is to say, the BD cat’s CFFjluminance functions could be mimicked by a normal cat wearing I log unit sunglasses. This sug- gests that the BD cat’s problem is also one of general light sensitivity. In keeping with this observation a I log unit difference is also apparent for absolute and increment threshold when comparing the non- deprived eye of an MD cat with the previously deprived eyes of a BD cat (Loop and Sherman, 1977b: Fig. 2). This view of the perceptual deficit of a BD cat is also consistent with impaired Y-cell input if one supposes that Y-cells are more sensitive than X-cells due to their larger retinal summation areas. This interpretation of our results suggests that binocularly deprived cats may well show deficits in any percep- tion that is affected by stimulus luminance.

Neurophysiological studies indicate that although lid suture results in a reduction of the Y-cell popula- tion in LGNd (Sherman et al., 1972) the temporal contrast sensitivity of the remaining Y-cells (and X-cells) is normal (Lehmkule er al., 1978). These find- ings might indicate that the CFF deficit of lid-sutured cats is the result of a simple reduction in the,Y-cell population. However, the temporal contrast sensi- tivity data of Lehmkule et al. (1978) were collected at a luminance of 1.5 log cd/m*. At this luminance level a BD cat shows no CFF deficit and the possibility must be entertained that deprived Y-cells might exhi- bit abnormal temporal resolution for brighter or dim- mer stimuli.

The exact magnitude of the BD cat’s CFF deficit is difficult to estimate because as Table 1 indicates CFF tended to increase following eyelid opening. This fact accounts for the BD cat’s higher CFF at 2.4 log cd/m2 in the Light-adapted (open squares) CFF/luminance function than in the steady-state CFF/luminance

function (solid circles) apparent in Fig. 3. Although the stimulus and adapting light luminance was identi- cal in both cases the steady-state function was

determined first. A similar effect was not observed with the normal cats (Fig. 1) because their CFF did not increase across testing. In reverse sutured IMD cats Hoffman and Cynader (1977) and Hoffman and Hollander (1978) have reported an increase in the encounter frequency of Y-cells following l-3 years of visual experience. Our results suggest a Similar effect may occur in BD cats although electrophysiological data are needed to confirm this point.

In man, amblyopia is defined as reduced visual acuity in the absence of refractive error or ophthal- moscopically detectable anomalies of the fundus and may be precipitated by several conditions (Von Noor- den, 1967; Awaya et al., 1973). In man this reduced visual acuity is usually reported to be accompanied by reduced CFF (Feinberg, 1956; Alpern et al., 1960). As neonatal lid closure in the cat produces reduced visual acuity in both MD (Griffin and Mitchell, 1978) and BD cats (Smith et al., 1978) without any consis- tent refractive error (Gollander et al., 1976) these cats’ symptoms closely resemble those of amblyopic humans. This similarity is furthered by the finding of this study that neonatal lid closure in the cat also reduces CFF. In detail, however, the BD cats’ CFF deficit does not exactly match the amblyopic human CFF deficit. In strabismic amblyopic humans the CFF deficit is apparent only at high retinal illumin- ante (Alpern et al., 1960) while in BD cats it is present at high and low stimulus luminance but absent at mesopic levels. Whether this represents a difference between cat and man or a difference in the conditions which precipitated the amblyopia remains to be determined.

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