REFRACTIVE STATE AND VISUAL ACUITY IN THE HOODED RAT Zst_zs.~~A Section of Neurobiolo~ (Rrcriced
WIESEXELD’ and THERESA BRGCHEK
and Behavior, Cornell University, Ithaca. NY 14853. U.S.A. 25 July
1975: in recisedform
17 Nouembrr
1975)
Abstract-Rats were trained to discriminate horizontal vs vertical stripes in a maze. and tested on stripes subtending 0.5’. 1’. 2’ and 1’ of visual angle at various distances from the choice point. The limit of acuity for most rats was 1’ at a distance of 20 and 30cm from the choice point. The majority of the animals performed best at onl; one distance from the choice point and the rest over a limited range of distances. Transfer of leammg of an easy discrimination was better at the distances associated with maximal acuity than at others.
S%THODS
I?iTRODUCTt03
many electrophysiologica1 studies of various aspects of the visual pathway of the rat have been conducted (Brown and Rojas, 1965; Montero. Brugge and Beitel, 1968; Montero, Rojas and Torrealba, 1973; Sefton and Bruce, 1971; Siminoff, Schwassmann and Kruger, 1966; Humptiey, 1968; Adams and Forrester, 1968; Shaw, Yinon and Auerbath, 1975; Wiesenfeld and Kernel, 1975), important questions about the visual capacity of the rat remain unanswered. The first question is what is the refractive state of the rat‘s eye? Some authors have found the rat to be myopic (Lashley, 1932; Brown and Rojas. 1965; Montero et nl., 1968) while others report it to be hypermetropic (Siminoff er al., 1966; Block, 1969; Massof and Cbang, 1972; Shaw er al., 1975). The range of obtained values is from + 17 D hyper- 1 metropia (Massof and Chang, 1972) to -12.5 D myopia (Lashley, 1932). Retinoscopy is the most commonly used technique for establishing the refractive state during an acute experiment, but this technique has been shown to give systematica~iy erroneous rest&s with small eyes, such as the rat’s (Glickstein and Millodot. 1970). Therefore, the electrophysiologist must rely on data obtained under different circumstances or use indirect techniques to determine refractive state so that the visual stimuli can be projected at the proper distance from the rat’s eye for maximum sharpness of focus. No such data have been available and as a consequence, a variety of distances have been used, thereby introducing possible errors in describing receptive field sizes and characteristics. A related question concerns the acuity of the hooded rat. LashIey (1930) found it to be between 0.5’ and 1’ of visual angle in his jumping stand experiment. The present experiment was designed to re-examine this problem, as it is important to know the capacity of the animal for visual resolution in order to design behavioral or physiological experiments testing any type of visual function. Although
Subjects were 11 male, naive hooded rats of rhe LongEvans strain whose weights ranged between 270 and 32Og at the start of the experiment. Their weights were reduced to SOY;by restricting food intake. Water was available at all times in the home cages. The rats were maintained on a 12 hr light-12 hr dark cycle. After the rats reached their target weights. they were trained in a maze (Fig. 1) to discriminate between horizontal and vertical black and white stripes. The maze consisted of a start box with a guillotine door. an alley with a similar door at the end of it, a funnel-shaped choice area and two alleys. with a narrow wall separating them. with guillotine doors at the alley entrances. The walls and floor of the maze were painted gray and the top of the maze was covered with clear plastic. The alleys past the choice area had slots placed at IO-cm intervals into which the stimulus cards could be placed. Round metal food cups, 1in. deep and 1.5 in. dia, were placed in front of the stimulus cards. The maze was uniformly illuminated from above with fluorescent light fixtures. The level of illumination of a white paper anywhere in the maze was 34.3&m’ (EM1 photometer). A background level of 70 dB white noise was present during the ex~riment. The stimulus cards were 10 x IOcm photoaraphic enlargements of Ronchi gratings (Edmund Scien&) printed on high contrast paper, dry mounted on a 10 x 1Scm piece of white cardboard. The black and white stripes were of equal width. The contrast between the black and white stripes was 1.0 log,, unit. Great care was taken to ensure that no local features of the stimulus cards could be used for discrimination bv the animals. The rats were trained to discriminate between pairs of stimulus cards. one with horizontal stripes and the other with vertical stripes. Six of the rats were trained to approach horizontal stripes and five were trained to approach verticai stripes. Training consisted of 20 trials; day until a criterion of 909/, correct choices on two consecutive days was achieved. The training took an average of 14.3 + 3.1 days, with no difference between the animals trained to horizontal or vertica1 stripes. The “correct” side was arranged in pseudorandom series with half the correct choices on the right and half on the IeR. A non-correction technique was used and reinforcement consisted of a 94 meY Noyes~pellet. Initial training was discrimination of stripes subtending 4’ of visual angle (or 8’ .oer cvcle of black and whitistripes) placed in slots 20cm from the choice point. When the rats reached criterion. they were tested on stripes subtending 4’. 7’ _ . 1’ and O.j-’ of visual angle. I
’ Present address: Department of Physical Biology. College of Veterinary Medicine. Cornell University. Ithaca, NY 14853. U.S.A. 823
Fig. I. Maze used in training and testing of disc~mination of horizontal and v-ertical stripes. Dimensions are in cm. Start box is on the right. The tests consisted of 20 trials, day where the order of presentation of both the stripe orientation and stripe width were scrambled with equal probabitity of appearing on either side. No stripe of the correct orientation of one size was presented on one side more than twice in succession. Two hundred trials were run at 30cm from the choice point. resuiting in a total of 50 responses to each of the four stripe widths. After the completion of testing at 20 cm. ah the rats were trained to run to cards placed 4Ocm from the choice point. with stripes subtending -t’ of visual angle, to the same criterion as in initial training. The “correct” orientation for each animal remained the same as before. only the distance of the target changed. Of course. the physical dimension of the stripes was doubled, as a stripe subtending 4’ at 20 cm subtends only 3’ of visual angle at 40 cm. Although the distance from the choice point changed, the degree of visual angle the stripes subtended has kept constant. After reaching criterion again. the rats were tested with stripes of 4’. 2’. I’ and 02’ (at 4Ocm). for a total of 200 trials. After testing at 40cm from the choice point the same procedure of training and testing was carried out at 10 cm and then 3Ocm from the choice point. This sequence of distances (20. 40, 10 and 3Ocm) constituted the entire experiment for six rats as these subjects exhibited a definite peak of acuity at one or more distances that were less than 40 cm from the choice point. The other five rats were further trained and tested with stripes subtending the same degrees of visual angle as stated above at 6Ocm and four of these rats were also trained and tested at SOcm from the choice point. RESULTS
Table 1 summarizes the results for all Ii subjects. numbers of correct trials out of the total of 50 at each degree of visual angle at all distances from the choice point are presented. With an :I’ of 50, the number of correct trials above chance level (P < O.Ol), from the binomial distribution (Darlington. 1974), is 34 or more. All performances above chance level are underlined. The smallest visual angle at which performance is above chance level is the limit of acuity, as tested in the present experiment. It may be seen that IO of the 11 rats performed at chance level at 0.5’ at all distances from the choice point. One rat (12 VI had above chance performance for 0.5’ stripes at a distance of 30 cm from the choice point. Performance
The
with 2’ and 4’ stripes was always si~ificantl~ above chance level at all distances for 10 of the I1 rats. One rat (%I) was able to discriminate 1’ stripes at 2O~m from the choice point. but was unable to discriminate i’ or 2’ stripes at 10, 30 and 40 cm from the choice point. Rurlgr of’best disrances for grenresr
ctcuif>
Performance significantly above chance level at the limit of acuity was found at only one distance from the choice point for five rats and at more than one distance for the other six rats. In order to test whether the performance of these six rats was better at some of the distances where performance was above chance level than at others. z2 was calculated for the number of correct responses at the limit of acuitv (I “) for thde six rats at all distances from the choke point. The results are presented in Table 2. The performance of rat 4H was significantly better at 2Ocm than at any other distance. The same was true of rat III at 4Ocm. There was no single best distance for the other four rats. Subjects 9V and 11V presumably had their best distance between 70 and 3Ocm from the choice point. whereas rat 7V performed equally well at 10, 20, 30 and 40 cm from the choice point and rat 8V at 20. 30. 10 and 6Ocm from the choice point. Table 3 summarizes the best distance(s) for all 1i rats. The distances associated with the highest acuity are 20 and 3Ocm from the choice point. As the distances of the stimulus cards from the choice point were not at equal diopter intervals. it is not possible to pinpoint the exact degree of myopia Trwsjer of leurning It has been repeatedly observed that rats trained to discriminate between horizontal and vertical stripes will better transfer this learning to larger than to smaller stripes. even if the smaller stripes subtend a visual angle larger than the limit of acuity of the rat (Sutherland, 1961 a. b). It is possibie that the reason for this observation is that the stimuli are presented at a distance where the stripes would not be in focus on the rat‘s retina, making dis~iminat~on of the smaller stripes difficult. To test this hypothesis the number of correct responses to the different sized stripes at the distance(s) associated with maximal acuity were compared with responses at the other dis-
Refractive state and visual acuity in the hooded rat
83
Table I. Number of correct responses for ail animals at all distances from the choice point at which
they were tested for O.Y, 1’. 2- and i stripes. The numbers that are underlined represent correct responses above chance Ievet {P c 0.01) with an .\; of 50. X represents distances at which an animal was not tested.
taxes. The data for all 11 animals were combined. The average numbers of correct responses-in the two conditions are presented in Fig. 2. At the optimal distances, the average number of correct responses to the 4’ stripe (to which the rats were trained) was 48.6/50 (97.2%) and the 2’ stripes 46.3/50 (92.6%). The amount of transfer was calculated with the formula (lr’ - jOiT - 50) where r is the per cent correct responses to the 4” stripes (to which the rats were trained) and T’ is per cent correct responses to the stripe to which transfer was made. Therefore complete transfer yields a value of 1.00 and no transfer a value of 0.00. At the best distances, the amount of transfer from 4’ to 2” was 0.90. The mean number of correct responses to the I” stripe was 39.4/50 (78.80,b).which represents 0.61 transfer of the response from the 4’ stripes. At 0.5 the number of correct responses is 27.6/50 (55.2%). representing 0.11 transfer. The average number of correct responses to the 4’ stripes at suboptimai distances was 47.4;50 (95.80/b) and 42.4/50 (84.8%) for the 2” stripes, which is 0.76 transfer. The mean number of correct responses to the 1’ stripes was 29.SjSO (59.6”/,) and to 0.5’ stripes Table 1. x2 of the number of correct responses to 1’ stripes for those rats that had significantly above chance performance at more than one distance from the choice point. The numbers with asterisks are significantly larger than those without an asterisk. X indicates that that rat was not run at that distance.
was 25.9;50 (51.8%), representing transfers of 0.21 and 0.04 respectively. In order to test which of these rates of transfer were significantly different from each other a 2-factor analysis of variance for unequal iVs (Winer, 1962) was calculated, with width of stripe as one factor (f,, LB&= 322.0. P < 0.001) and best vs other distances as the second factor (F,, IB1 = 58.3, P < 0.001). The interaction was also significant (F3, ,BJ = 12.83, P c 0.001). The Newman-Keuls test (Winer, 1962) indicates that the average number of correct responses differed significantly from each other at all four stripe sizes at both optima1 and suboptimal distances. The interaction was tested by calculating t between the pairs of mean correct responses for the two sets of distances for each size of stripe. The four values obtained were f = 1.53 (no sig. diff.) for the 4” stripes, Table 3. Summary of the best distances from the choice point. associated with highest acuity, for all rats.
PC
.Ol
chance
Fig. 1. Mgon number of correct respoases of all rats for the various stripe sizes at the dkrances nssociated with maximal acuity (solid lines) and the other distances (broken lines). The bars represent _i=1 S.D. Lines are draw through the number of correct responses representing chance Ievel of performance (330) and significantly {P c &al) above chance levet of performance (33.33).
t = 5.09 (P -c 0.001) for the 2” stripes, 1 = 12.62 Funher evidence af the correctness of the observed (P a= ~.~~) far the I’ stripes, and t = 2.30 (P < 0,05f best distance is the data on transfer of learning. Disfor the 0.5’ stripes. Therefvre, the interaction irtdi- crimination of the il” stripes was equally good at all cates that the number of correct rerpvnsa to the 4” distances from the choice point, but transfer to the stripes did not differ at the twv sets of distances and 3’ stripes was significantly better at the --best” disfor the 0.5’” stripes differed on@ marginalty. But for tances than at other distances. in spire of the tact the other two stripe sizes the differences were highly that 10 of the 11 rats cvuld discriminate the 2’ stripes significant. Of course, the differences betweeD the twv at all distances. This means that transfer of a Aativefy easy dj~Tirni~at~o~ can be affected by the quameans for the i” stripes were to be expected because lity of the retinal image. Tn fact, the transfer uaiue that was fhe limit of acuity for 10 animals and that of 0.90 from the 4” to the 3” stripes at the best disperformance defined optimal vs suboptimal distances. tances is very similar tv the value of 0.91 obtained But performance fvr 10 animals was above chance level at ali distances for the 2’” stripes. (Xfthe value .by Sutherland (t96fa) for transfer from smaller to of t was calculated without including the data for larger stripes and the value of 0.76 obtained in the rat .%I, the difference between the two means fvr the present study of transfer at the subuprimaf distances is similar to Sutb~rla~d~s value of 0.79 for transfer 2’ stripes was still significant at P < 0.OQl.f to smaller stripes. DlSCKSSION With respect tv v&al acuity the results of the presThe major aim of this experiment was to assess ent experiment are in good agreement with those of the genera1 refractive start: of the hooded rat. The Lashky f1930f. who found that on a jumping stand. indirect method of behavioral testing was used Bcm from the srimutus cards, pigmented rats {strain stripes because we found that even if the rat’s eye was not stated) could be trained to desalinate assumed to be emmetropi~ at i-9 D (GGfickstein and s~btend~g 52’ ofarc. but not 26’. In the present work Miliodot, 19701,there was stilt a large range (usually one rat out of 11 could discrimiraate 30’ of arc to between -+-10 and +ZO D) of equivocal measure- a criterion of P < 0.01. In a Iater experiment, Lashlcy ments with a retinoscope. It is evident that hooded (1938) found that most rats that were a cross between rats are myopic. rather than h~pe~~~~vp~~. Xf they Wistar albinos and trapped wild stock could be were the ratter, a continuous improvement of per- trained to discriminate Jo’ of arc. The difference may formance with increased distances from the choice be due to variability in capacity among strains and point would have been observed. Testing was carried perhaps criterion Level. Large differences in acuity out over a considerable range (between 7.5 and 8.5 D have been observed among various species of mice for the various animal& so such an effect would cer- (Rahmann. Rahmann and King, 1968; Vestal, 1973). Hermann (195’8)WZLS abIe to train pi_emented rats to tainly have been seen. In fact, the Ievd of performance of all the rats fell below the levef of significance at discriminate stripes U’ wide from Fay cards of tquai the limit of acuity at 50 cm or fess from the choice average iuminanee in a maze at distances of between 63 and 85 cm from the choice point. Tt is diffcult point and most rats had their highest acuity between to compare his results with thoss in the present stud? 10 and JOem.
Refractive state and visual acuity- in the hooded rat
because of the unclear presentation of the methods and data in that study. It is possible that in Hermann’s work the rats were not responding to the stripes, but to some other uncontrolled features of the stimuli because they could not be trained initially to discriminate stripes from gray and “learned to discern between striped patterns and gray papers of equal average brightness only after they had been trained for black-and-white discrimination” (p, 517). Seven of the 11 rats exhibited best performance at only one distance from the choice point. Two rats (9V and 11V) performed equally well at 20 and 30 cm from the choice point, with worse performance at 20 and 40cm. These two animals probably had their peak of acuity (1’) between 20 and 30 cm, at a distance that was not tested. Tw-o rats performed equally well over a broader distance-rat 8V between 20 and 6Ocm and 7V between 20 and 40 cm. For 8V this represents a range of 3.3 D and for 7V a range of 7.5 D. It is unlikely that the rat is capable of accommodation (Walls, 1942). Lashley (1932) found no trace of ciliary musculature in the eyes of either albino or pigmented rats. The axial length of the eye has been measured to be about 6mm and the thickness of the lens almost 4 mm (Lashley. 1932; Massof and Chang,
1972). The large lens size in comparison to the size of the eye also is strong evidence against the possibility of accommodation. The reasons for the anomalous observations for rats 7V and 8V are’ not clear. A more direct assessment of lack of accommodation on performance in the maze would be to run animals after the application of atropine eye drops. clcknowledgeme,lt-The comments and suggestions of the referees are gratefully acknowledged.
REFERESCES Adams A. D. and Forrester J. hf. (1968) The projection of the rat’s visual field on the cerebral cortex. Q. JI exp. Phvsioi. 53, 327-336.
Block M. T. (1969) A note on the refraction and image formation of the rat’s eye. Vision Res. 9. 7Os7 Il. Brown J. E. and Rojas J. A. (1965) Rat retinal ganglion cells: receptive field organization and maintained activity. J. Neurophysiol.28. 1073-1090.
Darlington R. B. (1971)Radicals and Squares and Other Sratisrical Procedures for the Behnviorai Sciences. Logan Hill. Ithaca, N.Y.
827
Clickstein M. and Millodot \I. (1970) Retinoscopy and eye size. Science 168. 605-606. Hermann G. (1958) Beitrage zur Physiologie des Rattenough. 2. Tirrps)chol. 15. 162-j 18. Humphrey N. K. (1968) Responses to visual stimuli of units in the superior colliculus of rats and monkeys. Expl 9nrroi. 20. 3 12-310. Lashley K. S. (1930) The mechanism of vision. III: The comparative visual acuity of pigmented and albino rats. J. yen. Psyhol. 37. J8I-W. Lashleg K. S. (1932) The mechanism of vision. V: The structure and image-forming power of the rat’s eye. J. camp. Psycho/. 13. 173-200. Lashley K. S. (19%) The mechanism of vision. XV: Preliminary studies of the rat’s capacity for detail vision. .I. gm. Ps,vchol. 18. 123-193. Massof R. W. and Chang F. W. (1972) A revision of the rat schematic eye. Vision Res. 12. 793-796. Montero V. til.. Brugge J. F. and Beitel J. E. (1968) Relation of the visual field to the lateral geniculate body of the albino rat. J. Nruroph_vsiol. 31. 221-236. Montero J. M.. Rojas A. and Torrealba F. (1973) Retinotopit organization of striate and peristriate visual cortex in the albino rat. Brnin Rrs. 53. 197-201. Rahmann H., Rahmann M. and King J. A. (1968) Comparative visual acuity (minimum separable) in five species and subspecies of deermice (Peromysctrs). Physio/. Zool. 41. 295-j 12. Sefton A. J. and Bruce I. S. L. (1971) Properties of cells in the lateral geniculate nucleus. Vision Res. (Suppl. $0. 3). 239-252.
Shaw C.. Yinon V. and Auerbach E. (1975) Receptive fields and response properties of neurons in the rat visual cortex. L’ision R;s. is. 203-208. Siminoff R.. Schwassmann H. 0. and Krueer L. (1966) An electrophysiological study of the visual projection of the superior colliculus of the rat. J. camp. LVeurol. 127. 435-t-U. Sutherland N. S. (1961a) Visual discrimination of vertical and horizontal rectangles by rats on a new discrimination training apparatus. Q. J1 exp. Psq‘chol. 13. 117-1~1. Sutherland N. S. (1961b) The methods and findings of experiments on the visual discrimination of shape by animals. Q. J1 esp. Psychol. (Motlogr. No. I). l-65
Vestal B. IM.(1973) Ontogeny of visual acuity in two species of deermice (Peromyscus). Anim. Behar. 21. 71 l-719. Walls G. L. (1912) The Verrebrate Eye and ifs Adaprice Rudintion. Cranbrook Institute of Science, Bloomfield Hills. Mich. Wiesenfeld Z. and Kornel E. E. (1975) Receptive fields of single cells in the visual cortex of the hooded rat. Brain Res. 94. -lOl-112. Winer B. J. (1962) Statistical Principles of Experimental Design. McGraw-Hill. New York.
WIESEXELD’ and THERESA BRGCHEK
and Behavior, Cornell University, Ithaca. NY 14853. U.S.A. 25 July
1975: in recisedform
17 Nouembrr
1975)
Abstract-Rats were trained to discriminate horizontal vs vertical stripes in a maze. and tested on stripes subtending 0.5’. 1’. 2’ and 1’ of visual angle at various distances from the choice point. The limit of acuity for most rats was 1’ at a distance of 20 and 30cm from the choice point. The majority of the animals performed best at onl; one distance from the choice point and the rest over a limited range of distances. Transfer of leammg of an easy discrimination was better at the distances associated with maximal acuity than at others.
S%THODS
I?iTRODUCTt03
many electrophysiologica1 studies of various aspects of the visual pathway of the rat have been conducted (Brown and Rojas, 1965; Montero. Brugge and Beitel, 1968; Montero, Rojas and Torrealba, 1973; Sefton and Bruce, 1971; Siminoff, Schwassmann and Kruger, 1966; Humptiey, 1968; Adams and Forrester, 1968; Shaw, Yinon and Auerbath, 1975; Wiesenfeld and Kernel, 1975), important questions about the visual capacity of the rat remain unanswered. The first question is what is the refractive state of the rat‘s eye? Some authors have found the rat to be myopic (Lashley, 1932; Brown and Rojas. 1965; Montero et nl., 1968) while others report it to be hypermetropic (Siminoff er al., 1966; Block, 1969; Massof and Cbang, 1972; Shaw er al., 1975). The range of obtained values is from + 17 D hyper- 1 metropia (Massof and Chang, 1972) to -12.5 D myopia (Lashley, 1932). Retinoscopy is the most commonly used technique for establishing the refractive state during an acute experiment, but this technique has been shown to give systematica~iy erroneous rest&s with small eyes, such as the rat’s (Glickstein and Millodot. 1970). Therefore, the electrophysiologist must rely on data obtained under different circumstances or use indirect techniques to determine refractive state so that the visual stimuli can be projected at the proper distance from the rat’s eye for maximum sharpness of focus. No such data have been available and as a consequence, a variety of distances have been used, thereby introducing possible errors in describing receptive field sizes and characteristics. A related question concerns the acuity of the hooded rat. LashIey (1930) found it to be between 0.5’ and 1’ of visual angle in his jumping stand experiment. The present experiment was designed to re-examine this problem, as it is important to know the capacity of the animal for visual resolution in order to design behavioral or physiological experiments testing any type of visual function. Although
Subjects were 11 male, naive hooded rats of rhe LongEvans strain whose weights ranged between 270 and 32Og at the start of the experiment. Their weights were reduced to SOY;by restricting food intake. Water was available at all times in the home cages. The rats were maintained on a 12 hr light-12 hr dark cycle. After the rats reached their target weights. they were trained in a maze (Fig. 1) to discriminate between horizontal and vertical black and white stripes. The maze consisted of a start box with a guillotine door. an alley with a similar door at the end of it, a funnel-shaped choice area and two alleys. with a narrow wall separating them. with guillotine doors at the alley entrances. The walls and floor of the maze were painted gray and the top of the maze was covered with clear plastic. The alleys past the choice area had slots placed at IO-cm intervals into which the stimulus cards could be placed. Round metal food cups, 1in. deep and 1.5 in. dia, were placed in front of the stimulus cards. The maze was uniformly illuminated from above with fluorescent light fixtures. The level of illumination of a white paper anywhere in the maze was 34.3&m’ (EM1 photometer). A background level of 70 dB white noise was present during the ex~riment. The stimulus cards were 10 x IOcm photoaraphic enlargements of Ronchi gratings (Edmund Scien&) printed on high contrast paper, dry mounted on a 10 x 1Scm piece of white cardboard. The black and white stripes were of equal width. The contrast between the black and white stripes was 1.0 log,, unit. Great care was taken to ensure that no local features of the stimulus cards could be used for discrimination bv the animals. The rats were trained to discriminate between pairs of stimulus cards. one with horizontal stripes and the other with vertical stripes. Six of the rats were trained to approach horizontal stripes and five were trained to approach verticai stripes. Training consisted of 20 trials; day until a criterion of 909/, correct choices on two consecutive days was achieved. The training took an average of 14.3 + 3.1 days, with no difference between the animals trained to horizontal or vertica1 stripes. The “correct” side was arranged in pseudorandom series with half the correct choices on the right and half on the IeR. A non-correction technique was used and reinforcement consisted of a 94 meY Noyes~pellet. Initial training was discrimination of stripes subtending 4’ of visual angle (or 8’ .oer cvcle of black and whitistripes) placed in slots 20cm from the choice point. When the rats reached criterion. they were tested on stripes subtending 4’. 7’ _ . 1’ and O.j-’ of visual angle. I
’ Present address: Department of Physical Biology. College of Veterinary Medicine. Cornell University. Ithaca, NY 14853. U.S.A. 823
Fig. I. Maze used in training and testing of disc~mination of horizontal and v-ertical stripes. Dimensions are in cm. Start box is on the right. The tests consisted of 20 trials, day where the order of presentation of both the stripe orientation and stripe width were scrambled with equal probabitity of appearing on either side. No stripe of the correct orientation of one size was presented on one side more than twice in succession. Two hundred trials were run at 30cm from the choice point. resuiting in a total of 50 responses to each of the four stripe widths. After the completion of testing at 20 cm. ah the rats were trained to run to cards placed 4Ocm from the choice point. with stripes subtending -t’ of visual angle, to the same criterion as in initial training. The “correct” orientation for each animal remained the same as before. only the distance of the target changed. Of course. the physical dimension of the stripes was doubled, as a stripe subtending 4’ at 20 cm subtends only 3’ of visual angle at 40 cm. Although the distance from the choice point changed, the degree of visual angle the stripes subtended has kept constant. After reaching criterion again. the rats were tested with stripes of 4’. 2’. I’ and 02’ (at 4Ocm). for a total of 200 trials. After testing at 40cm from the choice point the same procedure of training and testing was carried out at 10 cm and then 3Ocm from the choice point. This sequence of distances (20. 40, 10 and 3Ocm) constituted the entire experiment for six rats as these subjects exhibited a definite peak of acuity at one or more distances that were less than 40 cm from the choice point. The other five rats were further trained and tested with stripes subtending the same degrees of visual angle as stated above at 6Ocm and four of these rats were also trained and tested at SOcm from the choice point. RESULTS
Table 1 summarizes the results for all Ii subjects. numbers of correct trials out of the total of 50 at each degree of visual angle at all distances from the choice point are presented. With an :I’ of 50, the number of correct trials above chance level (P < O.Ol), from the binomial distribution (Darlington. 1974), is 34 or more. All performances above chance level are underlined. The smallest visual angle at which performance is above chance level is the limit of acuity, as tested in the present experiment. It may be seen that IO of the 11 rats performed at chance level at 0.5’ at all distances from the choice point. One rat (12 VI had above chance performance for 0.5’ stripes at a distance of 30 cm from the choice point. Performance
The
with 2’ and 4’ stripes was always si~ificantl~ above chance level at all distances for 10 of the I1 rats. One rat (%I) was able to discriminate 1’ stripes at 2O~m from the choice point. but was unable to discriminate i’ or 2’ stripes at 10, 30 and 40 cm from the choice point. Rurlgr of’best disrances for grenresr
ctcuif>
Performance significantly above chance level at the limit of acuity was found at only one distance from the choice point for five rats and at more than one distance for the other six rats. In order to test whether the performance of these six rats was better at some of the distances where performance was above chance level than at others. z2 was calculated for the number of correct responses at the limit of acuitv (I “) for thde six rats at all distances from the choke point. The results are presented in Table 2. The performance of rat 4H was significantly better at 2Ocm than at any other distance. The same was true of rat III at 4Ocm. There was no single best distance for the other four rats. Subjects 9V and 11V presumably had their best distance between 70 and 3Ocm from the choice point. whereas rat 7V performed equally well at 10, 20, 30 and 40 cm from the choice point and rat 8V at 20. 30. 10 and 6Ocm from the choice point. Table 3 summarizes the best distance(s) for all 1i rats. The distances associated with the highest acuity are 20 and 3Ocm from the choice point. As the distances of the stimulus cards from the choice point were not at equal diopter intervals. it is not possible to pinpoint the exact degree of myopia Trwsjer of leurning It has been repeatedly observed that rats trained to discriminate between horizontal and vertical stripes will better transfer this learning to larger than to smaller stripes. even if the smaller stripes subtend a visual angle larger than the limit of acuity of the rat (Sutherland, 1961 a. b). It is possibie that the reason for this observation is that the stimuli are presented at a distance where the stripes would not be in focus on the rat‘s retina, making dis~iminat~on of the smaller stripes difficult. To test this hypothesis the number of correct responses to the different sized stripes at the distance(s) associated with maximal acuity were compared with responses at the other dis-
Refractive state and visual acuity in the hooded rat
83
Table I. Number of correct responses for ail animals at all distances from the choice point at which
they were tested for O.Y, 1’. 2- and i stripes. The numbers that are underlined represent correct responses above chance Ievet {P c 0.01) with an .\; of 50. X represents distances at which an animal was not tested.
taxes. The data for all 11 animals were combined. The average numbers of correct responses-in the two conditions are presented in Fig. 2. At the optimal distances, the average number of correct responses to the 4’ stripe (to which the rats were trained) was 48.6/50 (97.2%) and the 2’ stripes 46.3/50 (92.6%). The amount of transfer was calculated with the formula (lr’ - jOiT - 50) where r is the per cent correct responses to the 4” stripes (to which the rats were trained) and T’ is per cent correct responses to the stripe to which transfer was made. Therefore complete transfer yields a value of 1.00 and no transfer a value of 0.00. At the best distances, the amount of transfer from 4’ to 2” was 0.90. The mean number of correct responses to the I” stripe was 39.4/50 (78.80,b).which represents 0.61 transfer of the response from the 4’ stripes. At 0.5 the number of correct responses is 27.6/50 (55.2%). representing 0.11 transfer. The average number of correct responses to the 4’ stripes at suboptimai distances was 47.4;50 (95.80/b) and 42.4/50 (84.8%) for the 2” stripes, which is 0.76 transfer. The mean number of correct responses to the 1’ stripes was 29.SjSO (59.6”/,) and to 0.5’ stripes Table 1. x2 of the number of correct responses to 1’ stripes for those rats that had significantly above chance performance at more than one distance from the choice point. The numbers with asterisks are significantly larger than those without an asterisk. X indicates that that rat was not run at that distance.
was 25.9;50 (51.8%), representing transfers of 0.21 and 0.04 respectively. In order to test which of these rates of transfer were significantly different from each other a 2-factor analysis of variance for unequal iVs (Winer, 1962) was calculated, with width of stripe as one factor (f,, LB&= 322.0. P < 0.001) and best vs other distances as the second factor (F,, IB1 = 58.3, P < 0.001). The interaction was also significant (F3, ,BJ = 12.83, P c 0.001). The Newman-Keuls test (Winer, 1962) indicates that the average number of correct responses differed significantly from each other at all four stripe sizes at both optima1 and suboptimal distances. The interaction was tested by calculating t between the pairs of mean correct responses for the two sets of distances for each size of stripe. The four values obtained were f = 1.53 (no sig. diff.) for the 4” stripes, Table 3. Summary of the best distances from the choice point. associated with highest acuity, for all rats.
PC
.Ol
chance
Fig. 1. Mgon number of correct respoases of all rats for the various stripe sizes at the dkrances nssociated with maximal acuity (solid lines) and the other distances (broken lines). The bars represent _i=1 S.D. Lines are draw through the number of correct responses representing chance Ievel of performance (330) and significantly {P c &al) above chance levet of performance (33.33).
t = 5.09 (P -c 0.001) for the 2” stripes, 1 = 12.62 Funher evidence af the correctness of the observed (P a= ~.~~) far the I’ stripes, and t = 2.30 (P < 0,05f best distance is the data on transfer of learning. Disfor the 0.5’ stripes. Therefvre, the interaction irtdi- crimination of the il” stripes was equally good at all cates that the number of correct rerpvnsa to the 4” distances from the choice point, but transfer to the stripes did not differ at the twv sets of distances and 3’ stripes was significantly better at the --best” disfor the 0.5’” stripes differed on@ marginalty. But for tances than at other distances. in spire of the tact the other two stripe sizes the differences were highly that 10 of the 11 rats cvuld discriminate the 2’ stripes significant. Of course, the differences betweeD the twv at all distances. This means that transfer of a Aativefy easy dj~Tirni~at~o~ can be affected by the quameans for the i” stripes were to be expected because lity of the retinal image. Tn fact, the transfer uaiue that was fhe limit of acuity for 10 animals and that of 0.90 from the 4” to the 3” stripes at the best disperformance defined optimal vs suboptimal distances. tances is very similar tv the value of 0.91 obtained But performance fvr 10 animals was above chance level at ali distances for the 2’” stripes. (Xfthe value .by Sutherland (t96fa) for transfer from smaller to of t was calculated without including the data for larger stripes and the value of 0.76 obtained in the rat .%I, the difference between the two means fvr the present study of transfer at the subuprimaf distances is similar to Sutb~rla~d~s value of 0.79 for transfer 2’ stripes was still significant at P < 0.OQl.f to smaller stripes. DlSCKSSION With respect tv v&al acuity the results of the presThe major aim of this experiment was to assess ent experiment are in good agreement with those of the genera1 refractive start: of the hooded rat. The Lashky f1930f. who found that on a jumping stand. indirect method of behavioral testing was used Bcm from the srimutus cards, pigmented rats {strain stripes because we found that even if the rat’s eye was not stated) could be trained to desalinate assumed to be emmetropi~ at i-9 D (GGfickstein and s~btend~g 52’ ofarc. but not 26’. In the present work Miliodot, 19701,there was stilt a large range (usually one rat out of 11 could discrimiraate 30’ of arc to between -+-10 and +ZO D) of equivocal measure- a criterion of P < 0.01. In a Iater experiment, Lashlcy ments with a retinoscope. It is evident that hooded (1938) found that most rats that were a cross between rats are myopic. rather than h~pe~~~~vp~~. Xf they Wistar albinos and trapped wild stock could be were the ratter, a continuous improvement of per- trained to discriminate Jo’ of arc. The difference may formance with increased distances from the choice be due to variability in capacity among strains and point would have been observed. Testing was carried perhaps criterion Level. Large differences in acuity out over a considerable range (between 7.5 and 8.5 D have been observed among various species of mice for the various animal& so such an effect would cer- (Rahmann. Rahmann and King, 1968; Vestal, 1973). Hermann (195’8)WZLS abIe to train pi_emented rats to tainly have been seen. In fact, the Ievd of performance of all the rats fell below the levef of significance at discriminate stripes U’ wide from Fay cards of tquai the limit of acuity at 50 cm or fess from the choice average iuminanee in a maze at distances of between 63 and 85 cm from the choice point. Tt is diffcult point and most rats had their highest acuity between to compare his results with thoss in the present stud? 10 and JOem.
Refractive state and visual acuity- in the hooded rat
because of the unclear presentation of the methods and data in that study. It is possible that in Hermann’s work the rats were not responding to the stripes, but to some other uncontrolled features of the stimuli because they could not be trained initially to discriminate stripes from gray and “learned to discern between striped patterns and gray papers of equal average brightness only after they had been trained for black-and-white discrimination” (p, 517). Seven of the 11 rats exhibited best performance at only one distance from the choice point. Two rats (9V and 11V) performed equally well at 20 and 30 cm from the choice point, with worse performance at 20 and 40cm. These two animals probably had their peak of acuity (1’) between 20 and 30 cm, at a distance that was not tested. Tw-o rats performed equally well over a broader distance-rat 8V between 20 and 6Ocm and 7V between 20 and 40 cm. For 8V this represents a range of 3.3 D and for 7V a range of 7.5 D. It is unlikely that the rat is capable of accommodation (Walls, 1942). Lashley (1932) found no trace of ciliary musculature in the eyes of either albino or pigmented rats. The axial length of the eye has been measured to be about 6mm and the thickness of the lens almost 4 mm (Lashley. 1932; Massof and Chang,
1972). The large lens size in comparison to the size of the eye also is strong evidence against the possibility of accommodation. The reasons for the anomalous observations for rats 7V and 8V are’ not clear. A more direct assessment of lack of accommodation on performance in the maze would be to run animals after the application of atropine eye drops. clcknowledgeme,lt-The comments and suggestions of the referees are gratefully acknowledged.
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