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The Visual Capabilities of Birds

William Hodos

from "Vision, brain, and behavior in birds". Eds. Zeigler and Bischof. 1993. Cambridge, MA: MIT Press.
 

Of all the vertebrate classes, birds are the most visual dependent. Many aspects of their adaptation to their environment and their survival depend on precise and sometimes quite subtle visual discrimination. Such behaviors as foraging for food, defense of territory and the nest, selection of mates, orientation, homing, and navigation depend on a well developed and highly sensitive visual system. We have already seen in the preceding chapters how exquisitely designed is the avian eye as an optical device; in the next chapter (Varela et al.) we will see how finely tuned it is to detect the most subtle differences in color. In this chapter we shall explore the abilities of the avian visual system (eye and visual brain) for the detection and discrimination of small differences in the spatial distribution and intensity of achromatic light.

PSYCHOPHYSICS

Psychophysics is that branch of psychology that deals with the measurement of sensory events. it attempts to answer such questions as What is the dimmest light that can be detected? or What is the smallest gap between two bars that can be visualized? Although psychophysics is one of the oldest branches of human psychology, its use in the realm of animal psychology is a much more recent development that was largely made possible by the marriage of human psychophysical methodology with the sophisticated and highly automated techniques for training animals developed by B. F. Skinner. One of the pioneers of this approach was Donald Blough, who used these methods to determine the course of dark adaptation (Blough, 1956) and spectral sensitivity (Blough, 1957) in pigeons.

Psychophysicists who study humans often measure absolute thresholds and difference thresholds. The basic principle of animal psychophysics is to train the animal to respond differently in the presence of a stimulus than in its absence for the determination of an absolute threshold. To measure difference thresholds, the animals are trained to respond differently in the presence of one stimulus than in the presence of a quantitatively different stimulus. In either case, this usually is accomplished by rewarding the animal in one stimulus condition and not rewarding it in the other. The difference between the amount of behavior in the presence of the rewarded stimulus condition and the nonrewarded stimulus condition becomes a measure of the animal's ability to differentiate between the two stimuli. The threshold usually is considered to be that value of the stimulus that results in performance that is halfway between perfect detection and performance that is based on chance or random guessing. Another variant is to require the animal to select one response option (such as pecking on the right) in the presence of one stimulus and another response option (such as pecking on the left) for the other stimulus.

Once the animal has learned the "rules of the game," the experimenter begins to manipulate the intensity or some other property of the stimulus (or the difference between two stimuli) until it can no longer be detected. The stimulus may be varied according to some predetermined or random sequence or it may be adjusted according to the animal's performance. In the latter approach, known as "tracking," if the animal does well, the stimulus is weakened or the difference is made smaller; if the animal does poorly, the stimulus is strengthened or the difference increased.

BRIGHTNESS

Brightness is the psychological response to the intensity of a visual stimulus. We have investigated the abilities of pigeons to detect small differences in the intensity or luminance or visual targets (Hodos and Bonbright, 1972). Two types of targets are presented to the birds: a standard luminance of 300 cd/M² and variable stimuli that are dimmer by a specified amount. The pigeon views each target and must decide if it is seeing the standard or one of the dimmer, variable stimuli. It registers its choice by pecking to the right or to the left. If it is correct in its selection it is rewarded with grain; if it has made an error, it gets no grain for its efforts and must start again.

A plot, known as a psychometric function, is made of the percentage of correct responses as a function of the difference between the standard and the variable stimuli. The difference threshold is determined from that difference between the standard and the variable that corresponds to 75% correct, which is halfway between chance responding (50%) and perfect detection (100%). This intensity difference is known as the point of subjective equality (PSE). The difference between the PSE and the standard is the difference threshold. We determined from a large group of pigeons that were studied over the years in a number of experiments that the mean threshold for intensity differences in pigeons is 0.11 log unit (± 0.003 SEM), which corresponds to the amount of light attenuation that would be produced by two microscope cover glasses interposed in the beam path (Hodos et al., 1985). The most sensitive pigeons had difference thresholds of approximately 0.05 log unit, which corresponds to a difference of about 10% in luminance.

The reader should note that these data were collected in a successive viewing of the stimuli, i.e., the pigeon must compare each stimulus to its memory of the standard and then decide if it is looking at the standard or one of the variable stimuli. When the stimuli were viewed simultaneously, we were unable to produce (by means of optical density) differences that were small enough to be undetectable by the pigeons. By extrapolation, however, we estimated that the simultaneous discrimination difference threshold for pigeons is about 0.01 log unit.

BRIGHTNESS SCALING

Psychophysics is a process that measures sensory events in physical units such as cd/M², decibels, g/CM², nanometers, etc. In contrast, psychophysical scaling attempts to measure sensory events in psychological units. A detailed discussion of psychophysical scaling is beyond the scope of this review, but some examples from our laboratory may indicate how this approach can be used.

If we consider the size of the difference threshold as a unit of discriminability, then we can construct a psychological scale by successively adding these discriminability units together. This is feasible as Sommers (1972) showed and the result is a scale that is a linear function of the logarithm of intensity. This outcome is consistent with the psychophysical law of G. T. Fechner (1966) that relates the perceived psychological magnitude of a stimulus to the logarithm of the intensity of the physical stimulus. Thus, in the situation in which the standard (brightest) stimulus is 300 cd/M² and the dimmest variable stimulus is 50 cd/ m², the PSE is 240 cd/M² and the difference threshold would be 60 cd/ m² . By using this Fechnerian scaling technique we could state that the animal would be able to fractionate the physical continuum from 50 to 300 cd/M² into approximately seven discriminable, psychological units of brightness (Hodos and Bonbright, 1974; Hodos, 1976). The implication of such a scaling analysis is that if an animal is very sensitive (i.e., has a low difference threshold), very small changes in the size of the difference threshold translate into very large changes in the number of discriminable units. On the other hand, if the animal has low sensitivity, even fairly large changes in the difference threshold have only minor effects on the number of discriminable steps between the brightest and dimmest stimuli.

BRIGHTNESS CONTRAST

We should not lose sight of the fact that stimuli rarely exist in the absence of a background or surround condition. Moreover, the interaction between the surround and the target can have by important consequences for the subjects psychological response to the target. These effects are well known in human visual perception where the nature of the surround can have dramatic effects on the apparent brightness color or size of the target. We have demonstrated (Hodos and Leibowitz, 1978) that such effects also occur in the avian visual system. Pigeons were trained to discriminate the difference in brightness between two self-luminous disks. Each disk was surrounded by a self-luminous annulus. When the annuli were of equal luminance, the pigeons were rewarded for pecking the disc with the higher intensity, which presumably appeared brighter to them. When this discrimination was well established, the birds intermittently were presented with a situation in which the disks were equally luminous, but the annuli differed. To human observers the effect was very obvious; the disk within the less luminous (dimmer) annulus appeared brighter than the disk within the more luminous annulus. This is the well-known phenomenon of brightness contrast. In the case of the avian observers, the overwhelming majority consistently responded as did the humans and chose the disc that was within the less luminous of the two annuli.

SIZE

A second property of visual stimuli that is important to animals is the spatial extent or size of a stimulus. just as we measured the pigeons' ability to detect small differences in luminance, we were able to use the same methodology to measure their ability to detect small differences in the size of a target (Hodos et al., 1985). The standard target was an annulus with a diameter of 3.0 mm. The birds initially were trained to discriminate this annulus from one that had a diameter of 15.0 mm. Once the large vs. small discrimination was established, we systematically varied the size of the variable annulus. The mean difference threshold was 0.94 mm (± 0.08). In other words, the typical pigeon could just barely differentiate a 3.00-mm annulus from a 3.94-mm annulus. The most sensitive pigeons could detect a 3.0-mm annulus from a 3.3-mm annulus; like brightness differences, the limit of size difference discrimination also is about 10%.

SIZE SCALING

The same psychophysical scaling approach as we used for brightness was applied to stimulus size (Kertzman and Hodos, 1988). The analysis revealed that the average pigeon could fractionate the size range from 3 to 15 mm into about six or seven discriminable units.

Another important property of visual stimuli is its orientation in space. To determine how well pigeons can detect differences in orientation, we trained pigeons to discriminate a self-luminous bar that was tilted 45^o to the right from one that was tilted 45^o to the left. When this discrimination was established, we presented the birds with stimuli that were less tilted and plotted their percentages of correct detection of the degree of tilt on a psychometric function. The results indicated that the pigeons were quite readily able to detect all but the smallest differences in tilt to the right or left; indeed the actual mean difference threshold for tilt was 11.3^o (± 0.42). For comparison purposes, consider that when the hour hand of a clock moves from 1:00 to 1:05 it changes its tilt by 30^o. To detect a tilt of 11.3^owe would have to notice something that was slightly less than the difference between 1:00 and 1:02 on a clock with no numbers and no indication of the positions of any of the minutes.

ACUITY

In addition to their ability to detect small differences in the sizes of stimuli, birds also are excellent at the detection of the fine details of stimuli. Such an ability is known as visual acuity. We have tested the visual acuity of pigeons (Hodos et al., 1976) using our psychophysical procedure but with stimuli that consisted of fine, square-wave optical gratings that ranged in frequency from 1 to 20 lines/mm. The gratings were presented immediately behind an optically neutral, glass window by means of a motorized wheel. The pigeons were required to discriminate the grating from a neutral-density filter that transmitted the same intensity of light, but which was completely blank. The percentage of correct responses was plotted as a function of the spatial frequencies of the gratings to form a typical psychometric function. The threshold of detectability in lines/mm was determined from this function by observing which grating frequency corresponded to 75% correct.

The threshold of detectability in lines/mm unfortunately is of little use by itself as an indicator of the resolving power of the visual system because the size of the retinal image of a target depends on the distance of that target from the eye; the closer to the eye, the larger the image on the retina. To estimate the size of the image, we performed a photographic analysis of the pigeons as they were observing the stimulus. From this analysis we could determine the viewing distance (Hodos et al., 1991a; Macko and Hodos, 1984, 1985) from which we were able to calculate the size of the retinal image in terms of the number of cycles (line/space pairs) of the grating that subtended one degree of visual angle. The average pigeon had a visual acuity of 12.7 cycles/deg (± 0.43), which would correspond to human acuity on the familiar Snellen eye chart of approximately 20/50 or 6/14 in metric units. This corresponds well with the values reported by Fite et al. (1975) for blue jays and Blough (1971) for the acuity of pigeons that were viewing distant targets. In practical terms this means that in good illumination the typical pigeon could just barely detect a seed with a width of 0.3 mm at a distance of 50 cm. The best of our pigeons, however, had acuities of approximately 18 cycles/deg, which corresponds to 20/33 (6/10) and is not far from normal human acuity of 20/20 (6/6). Excellent as the pigeon's acuity is, however, it does not rival that of predatory birds such as hawks and eagles, which are better than that of pigeons by at least an order of magnitude (Fox et al., 1976; Reymond, 1985, 1987). Among the factors that affect visual acuity are the luminance and wavelength of the target illuminant and the adaptation level of the subject. The influence of these factors in turn is affected by the relative proportions of photoreceptor types that are present in the retina. Thus we would expect pigeons, which are diurnal birds and have a rod-poor retina, to perform less well under conditions of dark adaptation than would owls, which are nocturnal and have a rod-rich retina. Such indeed is the case; under conditions of photopic illumination pigeons have higher acuity (Hodos and Leibowitz, 1977) than do owls (Fite, 1973; Martin and Gordon, 1974), but under scotopic adaptation, the reverse is true. Likewise, when the spectral composition of the grating illuminant changes, the visual acuity of the subject changes in accordance with the spectral sensitivities of its various cone types. Hodos and Leibowitz (1977) reported that pigeons, like humans, are in the range of 525-575 nm. Longer and shorter wavelengths result in losses of acuity; the most severe losses occur at the short wavelength end of the spectrum, which suggests a paucity of blue sensitive cones.

The luminance of the acuity stimulus also has a great affect on its detectability as we all have discovered when we noticed how the finer details of an object were better visible in stronger illumination. As ill humans and other mammals, such also is the case in pigeons (Hodos et al., 1976) for which optimal acuity occurs when the stimulus luminance is in the range of 300-1000 cd/M².

VELOCITY

In the visual world, stimuli do not always remain in a fixed position. Movement, absolute or relative, thus is a fundamental property of visual stimuli. Hodos et al. (1975) investigated the movement-detection thresholds of stimuli that exhibited either linear or radial movement. The psychophysical techniques were virtually identical to those described above for intensity and spatial vision. In particular, the pigeons were required to respond differentially in the presence of a moving or a stationary target. The results of our studies indicated that the velocitydetection thresholds of pigeons ranged from 4.4 to 6.5 mm/sec which correspond to retinal velocities of 4.1-6.01 deg/sec. Similar data were reported by Mulvanny (1978). In contrast, human observers can detect retinal velocities of 3 min/sec (Graham, 1968).

Maldonado et al. (1988) report that pigeons use frontal viewing for slow or static stimuli, but adopt a lateral viewing orientation to fast moving stimuli. Since the velocity thresholds reported here were determined in frontal viewing, one must consider the possibility that the retinal regions that view the frontal and lateral visual fields may be specialized for different aspects of viewing, such as static versus dynamic acuity.

Studies that base their findings on the minimal stimulus velocity necessary to activate the optokinetic reflex may not be dealing with the same phenomenon as that in which an animal must decide whether to peck left or right depending on whether the target is moving or stationary. In other words, we do not know whether an animal "sees" the movement of the visual world that causes these reflex movements of its eyes just as we have no awareness of the reflex changes in our pupil diameter in response to changes in the level of illumination in the environment.

VISUAL SEARCH

One of the most common visual tasks for animals or humans is to find an object somewhere in the visual environment. Tasks that are designed to investigate this are known as visual-search tasks. In the commonly used procedure, the subject is presented with an array of objects that are similar, except for one. The subject's task is to locate the dissimilar object. We have devised a variant of this task for pigeons (Hodos et al., 1993a). The pigeon is confronted by a video monitor that produces a retinal image that subtended 34^o X 40^o on the retina. On this screen either a + or an = (2.7^o X 2.7^o) could appear in any one of 16 locations on the screen. The pigeon first makes an "observing response" that indicates to the experimenters that it is ready to begin a trial. The observing response causes the stimulus to appear somewhere on the monitor screen where it remains for a duration that may be as short as 150 msec or as long as 1500 msec. The pigeon's task is to detect the stimulus during the observation interval and then to indicate by pecking right or left whether the stimulus was the + or the =. By plotting a psychometric function of the percentage of correct responses as a function of the duration of the observation interval, we have determined that the mean search time for pigeons under these experimental conditions is 373.8 msec (± 20). This value compares favorably with the data of Blough (1977, 1979), who reported similar search times for pigeons but with different psychophysical methods and different stimuli.

As animals age, various changes occur in their vision. Some of these changes are due to changes in the optical media of the eye, some due to degenerative changes 'in the retina, and some due to changes in the central visual system (Bagnoli and Hodos, 1991). Among the causes of the changes in the eye are such factors as the intensity, duration, and spectral composition of the animal or human's light-exposure history (Fite et al., 1991; Werner, 1991).

In both pigeons and quail, visual acuity shows a progressive decline with age from young adulthood to senescence (Hodos et al., 1991a,b; Porciatti et al., 1991. This decline has been correlated with losses of certain classes of photoreceptors (Hodos, et al., 1991a). In addition to the acuity losses, pigeons also suffer from senile miosis, a progressive diminution in maximum pupil diameter with age that also occurs in human visual aging (Hodos et al., 1991a). These age-dependent changes should be taken into account by investigators who use the less expensive retired breeders as their subjects or who keep well-trained, experienced pigeons as subjects in their laboratories for many years. Those who use feral pigeons as their subjects should consider that animals reared in the wild age more rapidly than those reared in captivity and may not be comparable to domesticated animals of the same chronological age (Hodos, 1991).

Kurkjian and Hodos (1992) reported little change in intensity difference thresholds over the same age span in which acuity showed a serious decline. Likewise, visual search, a sensitive indicator of human visual aging (Plude and Hoyer, 1981), showed no age-related change in pigeons over the same age span that resulted in visual acuity losses. These findings are of particular interest because they indicate plainly that the acuity losses in pigeons (and probably quail as well) are specific to spatial vision and are not indicative of global age-dependent changes that could affect cognitive functions such as memory or attention.

CONTRAST SENSITIVITY

Visual acuity tells us about a bird's or a person's ability to resolve small differences between objects that have high contrast. But this does not tell the complete story of an eye's or an optical system's ability to form useful images. An image, such as a square, consists of low spatial frequency components, that give us information about global properties of the stimulus such as its size. The square also contains high spatial frequency components that tell us about the small details, such as the sharpness of the corners. High-frequency filtering of the optical image of the square would thus round the corners of the square and make its edges blurry. Visual acuity only tell us how the visual or optical system handles the fine details; high contrast is required to see these fine details. Contrast is the difference in luminance between the darkest and lightest parts of an image expressed as a percentage of the total luminance; in the case of a grating, contrast is the difference in the luminances of the dark and light bars as a percentage of their combined luminances.

Visual and optical systems also can form useful images of objects at low contrast, even though the fine details may be lost. Visual acuity tells only about the ability to resolve high-contrast, high spatial frequency objects; it tells us nothing about ability to detect the low frequency properties of stimuli, which can be accomplished quite well at relatively low levels of contrast.

A contrast-sensitivity function is an assessment of a visual system's ability to form useful images in over a wide range of spatial-frequency ranges and over a wide range of contrasts. The procedure involves determining the lowest contrast at which a grating of a given spatial frequency can be detected. A contrast-sensitivity curve thus is a plot of the reciprocal of the contrast threshold (i.e., sensitivity) as a function of the spatial frequencies of the gratings in cycles per degree of visual angle. Contrast-sensitivity functions typically have the form of an inverted U. By extrapolating the high-frequency limb of the curve to the baseline (i.e., to the highest contrast) and noting the spatial frequency at the intercept (the so-called "high-frequency cutoff "), an estimate of visual acuity can be made since visual acuity is the ability to detect high-frequency, high-contrast gratings.

Figure 4.1 shows contrast-sensitivity curves for pigeons (Nye, 1968; Hodos et al., 1993b), an eagle (Reymond and Wolfe, 1981), and a falcon (Hirsch, 1982). The data of Nye (1968) are unusual because he used a binomial probability measure of threshold that is roughly equivalent to 66% correct on a psychometric function rather than the conventional 75% correct. This may account for the unusually broad range of spatial frequencies and the very high estimate of visual acuity for pigeons. The high-frequency-cutoff data of Hodos et al. (1993b) are consistent with the visual acuity data of Hodos et al. (1976) for stimuli of this luminance. Jassik-Gerschenfeld and Hardy (1979), who recorded contrast sensitivity curves of single cells in the pigeon optic tectum using virtually the same target luminance as Hodos et al. (1993b), reported that they were unable to record high-frequency cutoffs above 7 cycles/deg, which is consistent with the observations of Hodos et al. (1993b), but not those of Nye (1968). Hodos et al. (1991a) were able to obtain visual acuities as high as the high-frequency cutoff of Nye (1968), but only with much higher target luminance. The high-frequency cutoff of the falcon is approximately 35 cycles/deg and that of the eagle is in the vicinity of 100 cycles/deg.

Figure 4.2 shows the effects of age on contrast sensitivity. Data are shown for two human subjects and four pigeons. All six subjects were tested with the same apparatus (Hodos et al., 1993b). For both species, the typical effects of age (Owsley et al., 1983) can be seen, i.e., a progressive loss of high spatial frequencies with increasing age, but little change in the low frequencies. Increasing age also results in a progressive decrease in the maximum sensitivity to contrast and in a shift of the location of this maximum to progressively lower frequencies. In other words, the elderly lose the ability to resolve high and intermediate spatial-frequency components of stimuli, but the low-frequency components are little affected.

A major difference between the contrast sensitivities of birds and humans is the maximum sensitivity. The peak pigeon contrast sensitivity was approximately 14, which corresponds to about 7% contrast. The falcon's peak sensitivity was 28, which corresponds to 3.6% contrast. The humans, on the other hand, peaked at about 150, which equals 0.7% contrast. Peak sensitivities of humans, and mammals in general typically are in the contrast-sensitivity range of 100-200. Why birds, which have visual systems so highly adapted for virtually every aspect of the visual world, should be so relatively poor at detecting low contrast targets is not clear. Without doubt birds are well adapted to the high-contrast properties of the visual world. This weaker ability to detect the low-contrast properties of the visual environment may reflect a tradeoff in the optical design of the eye to permit high acuity with relatively small eyes and relatively small pupils.

EFFECTS OF VISUAL SYSTEM LESIONS

A full survey of the effects of visual system lesions on psychophysical indicators is beyond the scope of this review. But a few general observations should be mentioned. We have compared the effects of lesions of the tectofugal and thalamofugal pathways (see chapter by Shimizu and Karten (this volume) using visual acuity, brightness difference threshold, size difference threshold, and visual search. In general we have found that lesions of the tectofugal pathway (nucleus rotundus or ectostriatum) result in elevations of intensity difference threshold, size threshold, and search time as well as losses in visual acuity (Hodos and Bonbright, 1974; Hodos et al., 1984, 1986, 1988; Kertzman and Hodos, 1988; Macko and Hodos, 1984). In contrast, lesions of the thalamofugal pathway (OPT complex or visual wulst) result in little or no change in intensity difference threshold or visual acuity (Hodos et al., 1984; Macko and Hodos, 1984; Pasternak and Hodos, 1977). A tempting explanation has been that only the tectofugal pathway processes visual information and that the thalamofugal pathway is involved in processing "something else." No convincing explanations of what that something else might be have been offered. Recently, Remy and Gunturkun (1991) have reported that the OPT complex receives very few ganglion cell axons from the retinal red field, which is the part of the retina used for viewing close objects such as the stimuli in visual-discrimination experiments. A similar result was reported by Britten (1987). Thus we must consider the possibility that our lack of effects from thalamofugal lesions may be the result of the fact that our apparatus has put the stimuli on the wrong part of the retina. Further research will be necessary to see if this is a reasonable conclusion. We should add, however, that we cannot state that OPT is totally uninvolved with the processing of information coming from the retinal red field because when lesions of the thalamofugal pathway are combined with lesions of the tectofugal pathway, the effects are much larger than those of tectofugal pathway lesions alone. Indeed they often are larger than the sum of the tectofugal and thalamofugal effects combined. Moreover, the sequence of the combined lesions can be an important factor (Riley et al., 1988). These effects suggest that we still have some way to go before we can feel that we have a complete understanding of the function of these two visual pathways in the processing of visual information.

ACKNOWLEDGMENTS

I am grateful to Mrs. Ellen Carta for assistance in manuscript preparation and to the National Eye Institute, which supported much of the research reported here through Grant EY-00735.

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