INCREMENT-THRESHOLD FUNCTIONS DIFFERENT RODENT SPECIES GERALD H. JACORSand
FOR
DAVID BIRCH
Department of Psychology. University of California, Santa Barbara. California 93106. U.S.A. (Received
I2 April 1974)
Abstract-Increment-threshold functions were measured in behavioral experiments for three species of rodent. These species had either cone-dominated (ground squirrel and tree squirrel) or rod-dominated (hooded rat) retinas. The slopesof the measuredincrement-threshold functionswere systematicallydiffercnt for the three species-the function for the rat is accurately described by Weber’s law while the functions for the squirrels are significantly lower in slope.
As a part of a recent study of visual capacity in a
rodent species having an all-cone retina (the prairie dog). increment-thresholds were measured behaviorally for a wide variety of different adaptation conditions (Jacobs and Pulham. 1973).The slopes of the increment-threshold functions for these animals (as measured with a 500-nm test light on achromatic backgrounds of several different luminance levels) were found to be unusually shallow-for three animals the functions had a mean slope of 0.59 over a range of background light levels covering three log,,, units above cone threshold. Of course. it is well known that the slopes of increment-threshold functions depend critically on a host of stimulus parameters (Barlow, 1972) but these slopes seemed remarkable in that: (a) primate subjects tested in the same experimental situation generated increment-threshold functions having slopes of approximately 1.0: and (b) the experimental conditions included large-area. long-duration test stimuli. a combination which tends to maximize the steepness of increment-threshold functions, at least when measured with human observers.These observations made us curious if shallow increment-threshold functions are a general characteristic of all-cone visual systems.To examine this possibility, we have measured increment-threshold functions for animals from three rodent species selected to be representative of the broad gamut of rodent retinal construction-animals with rod-dominated retinas and animals with conedominated retinas were studied.
METHODS Suhjccrs The rodents tested were from three different species: hooded rat (Long-Evans strain), ground squirrel (golden mantled-Spermophilus lateralis). and tree squirrel (western gray-Sciurus griseus). Although there is good evidence for the presence of two receptor systems in the rat eye (e.g. Green. 1971). it is reasonable to consider the rat as having
(at the least) a heavily rod-dominated visual system. Similarly. although there is evidence that two types of photoreceptors can be anatomically defined in at least some ground squirrel retinas (West. 1973). the ground squirrel is typically viewed as having an all-cone visual system. A majority of the receptors in the tree squirrel retina have the anatomical characteristics ofcones while a second class has some of the distinguishing characteristics usually associated with rods (Cohen. 1964). Therefore. it is not unreasonable to classify the tree squirrel as representative of a somewhat less extreme example of a cone-dominated visual system than is the ground squirrel. Extensive threshold measurements were made on a single individual from each of these three species. These are reported below. However, sufficient measurements were also made on other Ss from all three species to insure the validity of the conclusions drawn from the single subjects. Each of these three animals had Krved as a subject in similar experiments for a period of at least I yr prior to the start of this investigation. Apparatus Threshold measurements were made in a three-alternative. forced-choice psychophysical task. The test chamber was 28 x 28 x 31 cm. Along one wall of this chamber were three identical. circular. stimulus panels (2.5 cm dia) mounted 8 cm apart. As the animal was free to move about the test chamber. it is obviously not possible to specify the visual angle subtended by the test stimulus. However, observation of the animals in the chamber suggest ‘hat stimulus size could not have been less than about 14” and was frequently very much larger than that. Each stimulus panel had a response level mounted above it and a food cup mounted below it: The stimulus panels were uansluant Stimulus lights both background and test were produad by sources mounted outside the test chamber. Each stimulus panel received continuous illumination from a tungsten-filament source (the background light): in these experiments the background light was achromatic (color temperature = 2300°K) and variable in luminance. In addition the output from a Bausch and Lomb high-intensity grating monochromator could be directed by means of a turntable system. toward any one of the three stimulus panels. This monochromator provided the test stimulus; its slits were
375
GERALD
376
H. JACOBS and
adjusted so as to provide a monochromatic light having a half-energy passband of IO nm. The only light entering the test chamber was that provided by either the background or test sources. A wall-mounted speaker was used to present a tone cue. Fans were used to provide a continuous air flow through the test chamber and to effectively mask any extraneous sounds. The wavelength of the monochromator. its intensity, the duration of the test stimulus, and the location of the stimulus panel to which the monochromatic light was directed were all determined under the program control of a PDP 8S computer located in an adjacent room. The computer was also used to sense and store the responses made by the subjects.
03
All of the subjects had previously been extensively tested in the experimental situation just described. Subjects were 23-hr food-deprived prior to each test session. During each trial the test light was added to one of the three background lights. A tone cue was used to signal the occurrence of the test trial. A response made during this interval to the stimulus panel on which the background light was added resulted in the delivery of a 97-mg Noyes food pellet into the adjacent food cup. A non-correction procedure was employed; that is. a response on either the correct or incorrect bars resulted in the termination of the test light and the cueing tone. If the animal made no response within 3.6 sec. the test light and cue were terminated. In each session the animal was given a IO-min adaptation period prior to the initiation of test trials. During this period the only illumination in the test chamber was that provided by the background sources set to the luminance appropriate to the condition being tested. Thresholds were measured on nine different background luminance levels ranging from 0402 cd/m2 to 20 cd/m’. A tenth threshold measurement was made with the background lights turned off. The wavelength of the test light was 519 nm. a value selected as being reasonably close to the spectral location of peak photopic sensitivity for all three species. The intensity of the test light was varied in steps of O-3 log,,, units. Each test light intensity was presented in a block of five consecutive trials. The mean intertrial interval was 9 set; the time between blocks averaged 15 sec. The location of the positive window and the order of the test intensity blocks used were both randomized. Only a single background luminance was used in each daily test session. Each animal completed a minimum of 200 test trials per session. The order of background luminances utilized was unsystematic and varied across subjects. For each threshold determination. the test stimulus intensity was varied over a total range of 1.5 log,, units. The mean performance on 175 test trials at each stimulus intensity was used to plot a psychometric function. From these functions thresholds were taken as the test stimulus intensity necessary to support discrimination at a level of 50 per cent correct. RESCLTS
Highly reliable data were obtained from these experienced subjects. For example, a second complete set
of threshold determinations was made on the tree squirrel with the result that the single greatest deviation in derived threshold between the-initial 10 threshold measurements and their replications was 0.21
-
C-0 -
-03
-
-Ia
-
I! g
-le.-
: it
-20
-
f
-23
-
. :
: H
Procedure
DAVID BIRCH
-3.0 -33 -co -49 -
I
-3.0 -
log
bockground
lUf”tnOnC*.
cd/m2
Fig. 1. Increment-threshold functions for three different species ofrodents: golden-mantled ground squirrel (circles): western gray squirrel (crosses); hooded rat (triangles). The three functions are arbitrarily positioned on the vertical axis. Thresholds plotted at the far left are for conditions of dark adaptation. Other details are given in the text.
log, 0 units. The data obtained from the other two subjects were similar in quality. The major results of the experiment are shown in Fig. 1 where the increment thresholds for the three subjects are plotted. In this figure, the increment-threshold functions for the three Ss are arbitrarily positioned on the vertical axis for convenience in viewing. As can be seen, both of the squirrel subjects show a cone-threshold effect when the background luminance is in the region of 00%-0*02 cd/m’. Below that level. the threshold for the gray squirrel again begins to drop. Other experiments in this laboratory have shown that this second drop is accompanied by a shift in spectral sensitivity similar to that seen in other duplex mammalian visual systems. On the other hand the ground squirrel subject shows no further gain in sensitivity as long as there is any background light and only a relatively small increase when threshold is measured in complete darkness. With the combination of test and background lights used in this experiment the rat increment-threshold function is linear over the entire range of background luminance examined. Straight lines were fitted to the increment-threshold values by means of a least-squares technique. Only those values lying above cone threshold were used for fitting the functions of the squirrel subjects. Since there were no obvious discontinuities in the data, all of the light-adapted thresholds were used to determine the best-fit function for the rat subject. The &rived slopes
317
Increment-threshold functions of these functions and the regression coefficients are as follows: ground squirrel (slope = Q677, r = 0991X tree squirrel (slope = O-748, r = @996). rat (slope = 1.022. r = 0993). The slope of the increment-threshold function for the rat is clearly very much steeper than those for either of the squirrels. The slopes for the functions from the two squirrels appear considerably more similar: however, analysis reveals that these two slopes also differ significantly (P c O-05). Despite the very substantial differences in the rates at which increment threshold changes as background luminance is changed there were luminance levels at which the measured thresholds were strikingly similar for all three species. Thus when the background luminance level was set at 2-O cd/m’ the threshold values for the three animals all lay within 0 18 log,,, units of one another. At this background level the AI/l value for the three animals averages @18. This value is somewhat lower than most of the AI/l determinations previously made on a variety of rodents (Mead 1939; Muntz. Northmore and..Pragnell, 1969). And although not of direct concern to the aims of this study, it is worth noting that the dark-adapted thresholds for rat and tree squirrel were within 0.20 log,,, units of each other.
within the sciurid group the slope of the incrementthreshold function correlates with the normal photic environment of the animal. Although habitat divisions cannot be very sharply drawn. consideration of the location of origin and of some of the presumptive adaptations to diurnal existence (for example. degree of lens pigmentation-Yolton. Yolton. Renz and Jacobs 1974) suggests that average ambient light levels are greater for the prairie dog than for the golden-mantled ground squirrel which. in turn. generally encounters somewhat higher light levels than do the tree squirrels. As indicated previously, the slopes of the increment-threshold functions are species ordered in this same way. Whether or not these speculations have any utility. it does appear clear that the characteristic shallow slope for the increment-threshold function is an integral feature of the cone-dominated visual systems of the sciurids. Outside of the sciurid group, the tree shrew (Tupaia) is the only other mammal generally considered to have a cone-dominated retina. In view of other resemblances between tree shrew and sciurid visual systems (Diamond and Hall, 1969; Levey, Harris and Jane, 1973). it would be most interesting to see if the increment-threshold function for Tupaia is similar in characteristic slope to those seen in the sciurids.
DISCUSSION
Ackrlowledgemrrlrs-This research was supported by a Grant from the National Eve Institute fEY-00105). We thank Dennis McFadden for-his comments on the manuscript.
Previous electrophysiological and behavioral measurements of increment thresholds in the rat have yielded functions that have tended to closely follow Weber’s Law (Dowling. 1967; Muntz et al.. 1969; Green. 1973). A replication of this result in the present experiment lends validity to the data obtained from the sciurids in the same apparatus. In conjunction with previous behavioral results on the prairie dog (Jacobs and Pulliam. 1973). and with results based on electro-. retinography carried out on both ground and tree squirrels (Dowling, personal communication). it appears that the rate of increase in increment is unusually small for a variety of species having conedominated retinas. The mechanisms in the sciurid visual system that produce unusually shallow increment-threshold functions under conditions where other species show close adherence to Weber’s Law are not known. However, the functional utility of this capacity is perhaps related to the photic environments these animals normally in-
habit. Specifications of the ambient light levels at which antelope ground squirrels show maximal behavioral activity (Kavanau and R&her, 1972) suggests that contrast discriminations by sciurids would frequently be carried out at luminance levels considerably higher than those at which thresholds were measured in this study. With the range of normal operation extended to these higher luminances, the luminance increment required to reach threshold would be substantially lower for the sciurid than for the viewer whose increment-threshold function follows Weber’s Law. In line with this suggestion. there is at least the hint that
REFERENCES Barlow H. 8. (1972) Dark and light adaptation: Psychophysics. In Handhook ofSensoryPhysiology (edited by Jameson D. and Hurvich L. M.). Vol. VII/4 Visual Psychophysits. pp. l-28. Springer. Berlin. Cohen A. I. (1964) Some observations on the fine structure of the retinal receptors of the American Gray squirrel. Incest. Ophthal. 3, 198-2 16. Diamond 1. T. and Hall W. C. (1969) Evolution of neocortex. Science, N.Y. 164, 251-262. Dowling J. E. (1967) The site of visual adaptation. Science. N.Y. 155.273-219. Green D. G. (1971) Light adaptation in the rat retina: Evidence for two receptor mechanisms. Science, N.Y: 174. 598-600. Green D. G. (1973) Scotopic and photopic components of the rat electroretinogram. J. Physiol.. Land. 228.78 l-797. Jacobs G. H. and Pulliam K. A. (1973) Vision in the prairie dog: Spectral sensitivity and color vision. J. camp. Physiol. Psycho/.
84, 240-245.
KavanauJ. L. and Rischer C. E. (1972) Optimum illumination for ground squirrel activity. OecoIogia 8, 39 l-399. Levey N. H., Harris J. and Jane J. A. (1973) Effects of visual cortical ablation on pattern discrimination in the ground squirrel (CiteUus tridecemlineatus). Expl Neurol. 39, 270276. Mead L. C. (I 939) Thresholds of visual intensity-diserimination in phylogeny. Am. J. Psycho!. 52.465-447. Muntz W. R. A., Northmore D. P. M. and Pragnell V. (1969) increment thresholds in photopic conditions in the hooded rat. Nature. Land. 223, I28& I28 I.
378
GERALD
H. J~coas and DAVID BIRCH
West R. W. (1973) Anatomical evidence for two types of receptors in ground squirrel retina. Ahm. Neurosciences Meeting, San Diego, p. 361.
Yolton R. L.. Yolton D. P.. Rcnz J. and Jacobs G. H. (1974) Preretinal absorbance in sciurid eyes. J. Murnmul. SS, I+ 20.
FOR
DAVID BIRCH
Department of Psychology. University of California, Santa Barbara. California 93106. U.S.A. (Received
I2 April 1974)
Abstract-Increment-threshold functions were measured in behavioral experiments for three species of rodent. These species had either cone-dominated (ground squirrel and tree squirrel) or rod-dominated (hooded rat) retinas. The slopesof the measuredincrement-threshold functionswere systematicallydiffercnt for the three species-the function for the rat is accurately described by Weber’s law while the functions for the squirrels are significantly lower in slope.
As a part of a recent study of visual capacity in a
rodent species having an all-cone retina (the prairie dog). increment-thresholds were measured behaviorally for a wide variety of different adaptation conditions (Jacobs and Pulham. 1973).The slopes of the increment-threshold functions for these animals (as measured with a 500-nm test light on achromatic backgrounds of several different luminance levels) were found to be unusually shallow-for three animals the functions had a mean slope of 0.59 over a range of background light levels covering three log,,, units above cone threshold. Of course. it is well known that the slopes of increment-threshold functions depend critically on a host of stimulus parameters (Barlow, 1972) but these slopes seemed remarkable in that: (a) primate subjects tested in the same experimental situation generated increment-threshold functions having slopes of approximately 1.0: and (b) the experimental conditions included large-area. long-duration test stimuli. a combination which tends to maximize the steepness of increment-threshold functions, at least when measured with human observers.These observations made us curious if shallow increment-threshold functions are a general characteristic of all-cone visual systems.To examine this possibility, we have measured increment-threshold functions for animals from three rodent species selected to be representative of the broad gamut of rodent retinal construction-animals with rod-dominated retinas and animals with conedominated retinas were studied.
METHODS Suhjccrs The rodents tested were from three different species: hooded rat (Long-Evans strain), ground squirrel (golden mantled-Spermophilus lateralis). and tree squirrel (western gray-Sciurus griseus). Although there is good evidence for the presence of two receptor systems in the rat eye (e.g. Green. 1971). it is reasonable to consider the rat as having
(at the least) a heavily rod-dominated visual system. Similarly. although there is evidence that two types of photoreceptors can be anatomically defined in at least some ground squirrel retinas (West. 1973). the ground squirrel is typically viewed as having an all-cone visual system. A majority of the receptors in the tree squirrel retina have the anatomical characteristics ofcones while a second class has some of the distinguishing characteristics usually associated with rods (Cohen. 1964). Therefore. it is not unreasonable to classify the tree squirrel as representative of a somewhat less extreme example of a cone-dominated visual system than is the ground squirrel. Extensive threshold measurements were made on a single individual from each of these three species. These are reported below. However, sufficient measurements were also made on other Ss from all three species to insure the validity of the conclusions drawn from the single subjects. Each of these three animals had Krved as a subject in similar experiments for a period of at least I yr prior to the start of this investigation. Apparatus Threshold measurements were made in a three-alternative. forced-choice psychophysical task. The test chamber was 28 x 28 x 31 cm. Along one wall of this chamber were three identical. circular. stimulus panels (2.5 cm dia) mounted 8 cm apart. As the animal was free to move about the test chamber. it is obviously not possible to specify the visual angle subtended by the test stimulus. However, observation of the animals in the chamber suggest ‘hat stimulus size could not have been less than about 14” and was frequently very much larger than that. Each stimulus panel had a response level mounted above it and a food cup mounted below it: The stimulus panels were uansluant Stimulus lights both background and test were produad by sources mounted outside the test chamber. Each stimulus panel received continuous illumination from a tungsten-filament source (the background light): in these experiments the background light was achromatic (color temperature = 2300°K) and variable in luminance. In addition the output from a Bausch and Lomb high-intensity grating monochromator could be directed by means of a turntable system. toward any one of the three stimulus panels. This monochromator provided the test stimulus; its slits were
375
GERALD
376
H. JACOBS and
adjusted so as to provide a monochromatic light having a half-energy passband of IO nm. The only light entering the test chamber was that provided by either the background or test sources. A wall-mounted speaker was used to present a tone cue. Fans were used to provide a continuous air flow through the test chamber and to effectively mask any extraneous sounds. The wavelength of the monochromator. its intensity, the duration of the test stimulus, and the location of the stimulus panel to which the monochromatic light was directed were all determined under the program control of a PDP 8S computer located in an adjacent room. The computer was also used to sense and store the responses made by the subjects.
03
All of the subjects had previously been extensively tested in the experimental situation just described. Subjects were 23-hr food-deprived prior to each test session. During each trial the test light was added to one of the three background lights. A tone cue was used to signal the occurrence of the test trial. A response made during this interval to the stimulus panel on which the background light was added resulted in the delivery of a 97-mg Noyes food pellet into the adjacent food cup. A non-correction procedure was employed; that is. a response on either the correct or incorrect bars resulted in the termination of the test light and the cueing tone. If the animal made no response within 3.6 sec. the test light and cue were terminated. In each session the animal was given a IO-min adaptation period prior to the initiation of test trials. During this period the only illumination in the test chamber was that provided by the background sources set to the luminance appropriate to the condition being tested. Thresholds were measured on nine different background luminance levels ranging from 0402 cd/m2 to 20 cd/m’. A tenth threshold measurement was made with the background lights turned off. The wavelength of the test light was 519 nm. a value selected as being reasonably close to the spectral location of peak photopic sensitivity for all three species. The intensity of the test light was varied in steps of O-3 log,,, units. Each test light intensity was presented in a block of five consecutive trials. The mean intertrial interval was 9 set; the time between blocks averaged 15 sec. The location of the positive window and the order of the test intensity blocks used were both randomized. Only a single background luminance was used in each daily test session. Each animal completed a minimum of 200 test trials per session. The order of background luminances utilized was unsystematic and varied across subjects. For each threshold determination. the test stimulus intensity was varied over a total range of 1.5 log,, units. The mean performance on 175 test trials at each stimulus intensity was used to plot a psychometric function. From these functions thresholds were taken as the test stimulus intensity necessary to support discrimination at a level of 50 per cent correct. RESCLTS
Highly reliable data were obtained from these experienced subjects. For example, a second complete set
of threshold determinations was made on the tree squirrel with the result that the single greatest deviation in derived threshold between the-initial 10 threshold measurements and their replications was 0.21
-
C-0 -
-03
-
-Ia
-
I! g
-le.-
: it
-20
-
f
-23
-
. :
: H
Procedure
DAVID BIRCH
-3.0 -33 -co -49 -
I
-3.0 -
log
bockground
lUf”tnOnC*.
cd/m2
Fig. 1. Increment-threshold functions for three different species ofrodents: golden-mantled ground squirrel (circles): western gray squirrel (crosses); hooded rat (triangles). The three functions are arbitrarily positioned on the vertical axis. Thresholds plotted at the far left are for conditions of dark adaptation. Other details are given in the text.
log, 0 units. The data obtained from the other two subjects were similar in quality. The major results of the experiment are shown in Fig. 1 where the increment thresholds for the three subjects are plotted. In this figure, the increment-threshold functions for the three Ss are arbitrarily positioned on the vertical axis for convenience in viewing. As can be seen, both of the squirrel subjects show a cone-threshold effect when the background luminance is in the region of 00%-0*02 cd/m’. Below that level. the threshold for the gray squirrel again begins to drop. Other experiments in this laboratory have shown that this second drop is accompanied by a shift in spectral sensitivity similar to that seen in other duplex mammalian visual systems. On the other hand the ground squirrel subject shows no further gain in sensitivity as long as there is any background light and only a relatively small increase when threshold is measured in complete darkness. With the combination of test and background lights used in this experiment the rat increment-threshold function is linear over the entire range of background luminance examined. Straight lines were fitted to the increment-threshold values by means of a least-squares technique. Only those values lying above cone threshold were used for fitting the functions of the squirrel subjects. Since there were no obvious discontinuities in the data, all of the light-adapted thresholds were used to determine the best-fit function for the rat subject. The &rived slopes
317
Increment-threshold functions of these functions and the regression coefficients are as follows: ground squirrel (slope = Q677, r = 0991X tree squirrel (slope = O-748, r = @996). rat (slope = 1.022. r = 0993). The slope of the increment-threshold function for the rat is clearly very much steeper than those for either of the squirrels. The slopes for the functions from the two squirrels appear considerably more similar: however, analysis reveals that these two slopes also differ significantly (P c O-05). Despite the very substantial differences in the rates at which increment threshold changes as background luminance is changed there were luminance levels at which the measured thresholds were strikingly similar for all three species. Thus when the background luminance level was set at 2-O cd/m’ the threshold values for the three animals all lay within 0 18 log,,, units of one another. At this background level the AI/l value for the three animals averages @18. This value is somewhat lower than most of the AI/l determinations previously made on a variety of rodents (Mead 1939; Muntz. Northmore and..Pragnell, 1969). And although not of direct concern to the aims of this study, it is worth noting that the dark-adapted thresholds for rat and tree squirrel were within 0.20 log,,, units of each other.
within the sciurid group the slope of the incrementthreshold function correlates with the normal photic environment of the animal. Although habitat divisions cannot be very sharply drawn. consideration of the location of origin and of some of the presumptive adaptations to diurnal existence (for example. degree of lens pigmentation-Yolton. Yolton. Renz and Jacobs 1974) suggests that average ambient light levels are greater for the prairie dog than for the golden-mantled ground squirrel which. in turn. generally encounters somewhat higher light levels than do the tree squirrels. As indicated previously, the slopes of the increment-threshold functions are species ordered in this same way. Whether or not these speculations have any utility. it does appear clear that the characteristic shallow slope for the increment-threshold function is an integral feature of the cone-dominated visual systems of the sciurids. Outside of the sciurid group, the tree shrew (Tupaia) is the only other mammal generally considered to have a cone-dominated retina. In view of other resemblances between tree shrew and sciurid visual systems (Diamond and Hall, 1969; Levey, Harris and Jane, 1973). it would be most interesting to see if the increment-threshold function for Tupaia is similar in characteristic slope to those seen in the sciurids.
DISCUSSION
Ackrlowledgemrrlrs-This research was supported by a Grant from the National Eve Institute fEY-00105). We thank Dennis McFadden for-his comments on the manuscript.
Previous electrophysiological and behavioral measurements of increment thresholds in the rat have yielded functions that have tended to closely follow Weber’s Law (Dowling. 1967; Muntz et al.. 1969; Green. 1973). A replication of this result in the present experiment lends validity to the data obtained from the sciurids in the same apparatus. In conjunction with previous behavioral results on the prairie dog (Jacobs and Pulliam. 1973). and with results based on electro-. retinography carried out on both ground and tree squirrels (Dowling, personal communication). it appears that the rate of increase in increment is unusually small for a variety of species having conedominated retinas. The mechanisms in the sciurid visual system that produce unusually shallow increment-threshold functions under conditions where other species show close adherence to Weber’s Law are not known. However, the functional utility of this capacity is perhaps related to the photic environments these animals normally in-
habit. Specifications of the ambient light levels at which antelope ground squirrels show maximal behavioral activity (Kavanau and R&her, 1972) suggests that contrast discriminations by sciurids would frequently be carried out at luminance levels considerably higher than those at which thresholds were measured in this study. With the range of normal operation extended to these higher luminances, the luminance increment required to reach threshold would be substantially lower for the sciurid than for the viewer whose increment-threshold function follows Weber’s Law. In line with this suggestion. there is at least the hint that
REFERENCES Barlow H. 8. (1972) Dark and light adaptation: Psychophysics. In Handhook ofSensoryPhysiology (edited by Jameson D. and Hurvich L. M.). Vol. VII/4 Visual Psychophysits. pp. l-28. Springer. Berlin. Cohen A. I. (1964) Some observations on the fine structure of the retinal receptors of the American Gray squirrel. Incest. Ophthal. 3, 198-2 16. Diamond 1. T. and Hall W. C. (1969) Evolution of neocortex. Science, N.Y. 164, 251-262. Dowling J. E. (1967) The site of visual adaptation. Science. N.Y. 155.273-219. Green D. G. (1971) Light adaptation in the rat retina: Evidence for two receptor mechanisms. Science, N.Y: 174. 598-600. Green D. G. (1973) Scotopic and photopic components of the rat electroretinogram. J. Physiol.. Land. 228.78 l-797. Jacobs G. H. and Pulliam K. A. (1973) Vision in the prairie dog: Spectral sensitivity and color vision. J. camp. Physiol. Psycho/.
84, 240-245.
KavanauJ. L. and Rischer C. E. (1972) Optimum illumination for ground squirrel activity. OecoIogia 8, 39 l-399. Levey N. H., Harris J. and Jane J. A. (1973) Effects of visual cortical ablation on pattern discrimination in the ground squirrel (CiteUus tridecemlineatus). Expl Neurol. 39, 270276. Mead L. C. (I 939) Thresholds of visual intensity-diserimination in phylogeny. Am. J. Psycho!. 52.465-447. Muntz W. R. A., Northmore D. P. M. and Pragnell V. (1969) increment thresholds in photopic conditions in the hooded rat. Nature. Land. 223, I28& I28 I.
378
GERALD
H. J~coas and DAVID BIRCH
West R. W. (1973) Anatomical evidence for two types of receptors in ground squirrel retina. Ahm. Neurosciences Meeting, San Diego, p. 361.
Yolton R. L.. Yolton D. P.. Rcnz J. and Jacobs G. H. (1974) Preretinal absorbance in sciurid eyes. J. Murnmul. SS, I+ 20.
