VISUAL RESOLUTION IN HUMANS FLUCTUATES OVER THE 24H PERIOD*

The present experiment was designed to assess daily fluctuations of visual discriminability, a function reflecting the resolution power of the visual sensitivity by measure of a differential threshold. Sixteen subjects underwent a visual discrimination threshold task (using the constant method) in a protocol allowing one point every 2h over the 24h period. The results show that the visual discrimination threshold is low in the morning and increases progressively over the day, reaching a first peak at 22:00. During the night, the same pattern occurs, with low threshold levels at the beginning of the night and high levels at the end. This profile is quite different from that of detection threshold variations, suggesting that the two visual functions are under the control of different underlying mechanisms. Two interpretations could account for this discrepancy. The first relates to different oscillators in the eye for detection and discrimination. The second refers to a possible linkage of visual discriminability with the sleep-wake cycle since threshold measures were systematically low (i.e., high resolution power) after long sleep periods. (Chronobiology International, 17(12), 187–195, 2000)     

Variation of visual detection over the 24-hour period in humans

A circadian rhythm for visual sensitivity has been intensively assessed in animals. This rhythm may be due to the existence of a circadian clock in the mammalian eye, which could account for fluctuating sensitivity to light over the day in certain species. However, very few studies have been devoted to the human visual system. The present experiment was designed to assess a possible rhythm of visual sensitivity using a psychophysical method over the whole 24h period. Twelve subjects underwent visual detection threshold measures in a protocol that allowed one point every 2h. The results show that the visual detection threshold changes over the 24h period, with high thresholds in the morning, a progressive decrease over the day and the early night, and an increase during the last part of the night. These data suggest that a circadian rhythm influences visual sensitivity to mesopic luminance in humans. (Chronobiology International, 17(6), 795-805, 2000)     

VARIATION OF VISUAL DETECTION OVER THE 24-HOUR PERIOD IN HUMANS

A circadian rhythm for visual sensitivity has been intensively assessed in animals. This rhythm may be due to the existence of a circadian clock in the mammalian eye, which could account for fluctuating sensitivity to light over the day in certain species. However, very few studies have been devoted to the human visual system. The present experiment was designed to assess a possible rhythm of visual sensitivity using a psychophysical method over the whole 24h period. Twelve subjects underwent visual detection threshold measures in a protocol that allowed one point every 2h. The results show that the visual detection threshold changes over the 24h period, with high thresholds in the morning, a progressive decrease over the day and the early night, and an increase during the last part of the night. These data suggest that a circadian rhythm influences visual sensitivity to mesopic luminance in humans. (Chronobiology International, 17(6), 795–805, 2000)     

Artificial vision with wirelessly powered subretinal electronic implant alpha-IMS

This study aims at substituting the essential functions of photoreceptors in patients who are blind owing to untreatable forms of hereditary retinal degenerations. A microelectronic neuroprosthetic device, powered via transdermal inductive transmission, carrying 1500 independent microphotodiode-amplifier-electrode elements on a 9 mm² chip, was subretinally implanted in nine blind patients. Light perception (8/9), light localization (7/9), motion detection (5/9, angular speed up to 35 deg s⁻¹), grating acuity measurement (6/9, up to 3.3 cycles per degree) and visual acuity measurement with Landolt C-rings (2/9) up to Snellen visual acuity of 20/546 (corresponding to decimal 0.037 or corresponding to 1.43 logMAR (minimum angle of resolution)) were restored via the subretinal implant. Additionally, the identification, localization and discrimination of objects improved significantly (n = 8; p < 0.05 for each subtest) in repeated tests over a nine-month period. Three subjects were able to read letters spontaneously and one subject was able to read letters after training in an alternative-force choice test. Five subjects reported implant-mediated visual perceptions in daily life within a field of 15° of visual angle. Control tests were performed each time with the implant''s power source switched off. These data show that subretinal implants can restore visual functions that are useful for daily life.     

Stimulus discrimination by horses under scotopic conditions

Scotopic vision in horses (Equus caballus) was investigated using behavioral measurements for the first time. Four horses were tested for the ability to make simple visual discriminations of geometric figures (circles and triangles) under various brightness levels within an enclosed building. Measurements of brightness ranging from 10.37 to 24.12 magnitudes per square arcsecond (mag/arcsec²; in candelas per square meter—7.70 to 2.43E-05 cd/m²) were taken using a Sky Quality Meter. These values approximated outdoor conditions ranging from twilight in open country to a dark moonless night in dense forest. The horses were able to solve the discrimination problems in all brightness settings up to 23.77 mag/arcsec² (3.35E-05 cd/m²). Moreover, they easily navigated their way around obstacles located within the testing area in extremely dim light (>23.50 mag/arcsec²; 4.30E-05 cd/m²), which were in conditions too dark for the human experimenters to see. These findings support physiological data that reveal a rod-dominated visual system as well as observations of equine activity at night.     

Understanding bird collisions with man‐made objects: a sensory ecology approach

Sensory ecology investigates the information that underlies an animal’s interactions with its environment. A sensory ecology framework is used here to seek to assess why flying birds collide with prominent structures, such as power lines, fences, communication masts, wind turbines and buildings, which intrude into the open airspace. Such collisions occur under conditions of both high and low visibility. It is argued that a human perspective of the problems posed by these obstacles is unhelpful. Birds live in different visual worlds and key aspects of these differences are summarized. When in flight, birds may turn their heads in both pitch and yaw to look down, either with the binocular field or with the lateral part of an eye’s visual field. Such behaviour may be usual and results in certain species being at least temporarily blind in the direction of travel. Furthermore, even if birds are looking ahead, frontal vision may not be in high resolution. In general, high resolution occurs in the lateral fields of view and frontal vision in birds may be tuned for the detection of movement concerned with the extraction of information from the optical flow field, rather than the detection of high spatial detail. Birds probably employ lateral vision for the detection of conspecifics, foraging opportunities and predators. The detection of these may be more important than simply looking ahead during flight in the open airspace. Birds in flight may predict that the environment ahead is not cluttered. Even if they are facing forward, they may fail to see an obstacle as they may not predict obstructions; perceptually they have no ‘prior’ for human artefacts such as buildings, power wires or wind turbines. Birds have only a restricted range of flight speeds that can be used to adjust their rate of gain of visual information as the sensory challenges of the environment change. It is argued that to reduce collisions with known hazards, something placed upon the ground may be more important than something placed on the obstacle itself. Foraging patches, conspecific models or alerting sounds placed a suitable distance from the hazard may be an effective way of reducing collisions in certain locations. However, there is unlikely to be a single effective way to reduce collisions for multiple species at any one site. Warning or diversion and distraction solutions may need to be tailored for particular target species.     

ON THE RELATION BETWEEN MEASUREMENTS OF INTENSITY DISCRIMINATION AND OF VISUAL ACUITY IN THE HONEY BEE

1. Bees respond by a characteristic reflex to a movement of their visual field. By confining the field to a series of parallel stripes of two alternating different brightnesses it is possible to determine for any width of stripe, at any brightness of one of the two sets of stripes, the brightness of the second at which the bee will first respond to a displacement of the field. Thus the relations between visual acuity and intensity discrimination can be studied. 2. For each width of stripe and visual angle subtended by the stripe the discrimination power of the bee''s eye for different brightnesses was studied. For each visual acuity the intensity discrimination varies with illumination in a characteristic, consistent manner. The discrimination is poor at low illuminations; as the intensity of illumination increases the discrimination increases, and reaches a constant level at high illuminations. 3. From the intensity discrimination curves obtained at different visual acuities, visual acuity curves can be reconstructed for different values of ΔI/I. The curves thus obtained are identical in form with the curve found previously by direct test for the relation between visual acuity and illumination.     

The costs of becoming nocturnal: feeding efficiency in relation to light intensity in juvenile Atlantic Salmon

1. Most animals are active by day or by night, but not both; juvenile salmonids are unusual in that they switch from being predominantly diurnal for most of the year to being nocturnal in winter. They are visual foragers, and adaptations for high visual acuity at daytime light intensities are generally incompatible with sensitive night vision. Here we test whether juvenile Atlantic Salmon Salmo salar are able to maintain their efficiency of prey capture when switching between diurnal and nocturnal foraging.
2. By testing the ability of the fish to acquire drifting food items under a range of manipulated light intensities, we show that the foraging efficiency of juvenile salmon is high at light intensities down to those equivalent to dawn or dusk, but drops markedly at lower levels of illumination: even under the best night condition (full moon and clear sky), the feeding efficiency is only 35% of their diurnal efficiency, and fish will usually be feeding at less than 10% (whenever the moon is not full, skies are overcast or when in the shade of bankside trees). Fish were unable to feed on drifting prey when in complete darkness.
3. The ability of juvenile salmon to detect prey under different light intensities is similar to that of other planktivorous or drift-feeding species of fish; they thus appear to have no special adaptations for nocturnal foraging.
4. While winter drift abundance is slightly higher by night than by day, the difference is not enough to compensate for the loss in foraging efficiency. We suggest that juvenile salmon can nonetheless switch to nocturnal foraging in winter because their food requirements are low, many individuals adopting a strategy in which intake is suppressed to the minimum that ensures survival.     

Measurement and modeling of peripheral detection and discrimination thresholds

This report describes experimental measurements of threshold contrasts as a function of the angle to the visual axis (peripheral threshold contrasts). The visual tasks consist in detection (perception of presence) and discrimination (perception of a form feature) of simple visual signs during a fixation period realistic observing conditions being chosen. Proceeding from the experimental findings a model for forecasting off-axis threshold contrast functions on different visual conditions is developed based upon spatial frequency filters. Further with the aid of a known model visibility fields are calculated.     

THE VISIBILITY OF SINGLE LINES AT VARIOUS ILLUMINATIONS AND THE RETINAL BASIS OF VISUAL RESOLUTION

The visual resolution of a single opaque line against an evenly illuminated background has been studied over a large range of background brightness. It was found that the visual angle occupied by the thickness of the line when it is just resolved varies from about 10 minutes at the lowest illuminations to 0.5 second at the highest illuminations, a range of 1200 to 1. The relation between background brightness and just resolvable visual angle shows two sections similar to those found in other visual functions; the data at low light intensities represent rod vision while those at the higher intensities represent cone vision. With violet light instead of white the two sections become even more clearly defined and separated. The retinal image produced by the finest perceptible line at the highest brightness is not a sharp narrow shadow, but a thin broad shadow whose density distribution is described in terms of diffraction optics. The line of foveal cones occupying the center of this shadow suffers a decrease in the light intensity by very nearly 1 per cent in comparison either with the general retinal illumination or with that on the row of cones to either side of the central row. Since this percentage difference is near the limit of intensity discrimination by the retina, its retinal recognition is probably the limiting factor in the visual resolution of the line. The resolution of a line at any light intensity may also be limited by the just recognizable intensity difference, because this percentage difference varies with the prevailing light intensity. As evidence for this it is found that the just resolvable visual angle varies with the light intensity in the same way that the power of intensity discrimination of the eye varies with light intensity. It is possible that visual resolution of test objects like hooks and broken circles is determined by the recognition of intensity differences in their diffracted images, since the way in which their resolution varies with the light intensity is similar to the relation between intensity discrimination and light intensity.     

Two-dimensional pattern discrimination by the honeybee

For a reward of sugar, bees will learn to prefer a pattern rather than an alternative similar one. This visual discrimination allows us to measure resolution, and to search for the cues that the bees remember and later use to recognize the rewarded pattern. Two systems in parallel, analogous to low pass and high pass filters, are distinguished. The first system discriminates the location and size of at least one area of contrast on each side of the target, with inputs from blue and green receptors, but the ability to discriminate the location of colour depends upon fixation. The bees remember less than a low resolution copy of the image, even when they fixate on a vertical pattern. The second system amplifies the contrast at edges in the pattern, ignoring the direction of contrast, and controls fixation upon the target. Edges are discriminated according to their orientation and radial or tangential arrangement. An axis of bilateral symmetry is detected. However, the relative locations of cues within the image are lost, apparently because the relevant neurones have very large fields. Only the cues, not the whole patterns, are preserved in memory. This system is colour blind because its input is restricted to the receptors with peak sensitivity in the green. The two systems together discriminate many simple patterns, but not all, because the filters are limited in variety.     

Temporal mechanisms of multimodal binding

The simultaneity of signals from different senses—such as vision and audition—is a useful cue for determining whether those signals arose from one environmental source or from more than one. To understand better the sensory mechanisms for assessing simultaneity, we measured the discrimination thresholds for time intervals marked by auditory, visual or auditory–visual stimuli, as a function of the base interval. For all conditions, both unimodal and cross-modal, the thresholds followed a characteristic ‘dipper function’ in which the lowest thresholds occurred when discriminating against a non-zero interval. The base interval yielding the lowest threshold was roughly equal to the threshold for discriminating asynchronous from synchronous presentations. Those lowest thresholds occurred at approximately 5, 15 and 75 ms for auditory, visual and auditory–visual stimuli, respectively. Thus, the mechanisms mediating performance with cross-modal stimuli are considerably slower than the mechanisms mediating performance within a particular sense. We developed a simple model with temporal filters of different time constants and showed that the model produces discrimination functions similar to the ones we observed in humans. Both for processing within a single sense, and for processing across senses, temporal perception is affected by the properties of temporal filters, the outputs of which are used to estimate time offsets, correlations between signals, and more.     

ANOXIA AND BRIGHTNESS DISCRIMINATION

1. Brightness discrimination has been studied with individuals breathing oxygen concentrations corresponding to 7 altitudes between sea level and 17,000 feet. The brightnesses were 0.1, 0.01, and 0.001 millilambert involving only daylight (cone) vision. 2. At these light intensities, brightness discrimination begins to deteriorate at fairly low altitudes. The deterioration is obvious at 8,000 feet, and becomes marked at 15,000 feet, where at low brightness, the contrast must be increased 100 per cent over the sea level value before it can be recognized. 3. The impairment of brightness discrimination with increase in altitude is greater at higher altitudes than at lower. The impairment starts slowly and becomes increasingly rapid the higher the altitude. 4. Impairment of brightness discrimination varies inversely with the light intensity. It is most evident under the lowest light intensities studied, but shows in all of them. However, it decreases in such a way that the deterioration is negligible in full daylight and sunlight. 5. The thresholds of night (rod) vision and day (cone) vision are equally affected by anoxia. 6. The quantitative form of the relation between brightness discrimination ΔI/I and the prevailing brightness I remains the same at all oxygen concentrations. The curve merely shifts along the log I axis, and the extent of the shift indicates the visual deterioration. 7. The data are described in terms of retinal chemistry. Since anoxia causes only a shift in log I it is shown that the photochemical receptor system cannot be affected. Instead the conversion of photochemical change into visual function is impaired in such a way that the conversion factor varies as the fourth power of the arterial oxygen saturation.     

Sensory capacities and the nocturnal habit of owls (Strigiformes)

Behavioural studies show that in the eye of the Tawny Owl Strix aluco both absolute visual sensitivity and maximum spatial resolution at low light levels are close to the theoretical limit dictated principally by the quantal nature of light and the physiological limitations on the structure of vertebrate eyes. However, when the owl's visual sensitivity in relation to naturally occurring ligh levels is analysed, it is concluded that at night there will often be occasions when vision can only be used to control the owl's behaviour with respect to large objects.
Owls are capable of detecting and catching prey by hearing alone. However, absolute auditory sensitivity is not superior to that of mammals (including Man), but does appear to have reached the absolute limit on sensitivity in the aerial environment, which is dictated by the minimum ambient sound level.
An explanation of the owl's ability to be active at night based only upon high sensory sensitivity is thus untenable. Many features of the natural behaviour of the Tawny Owl (e.g., the high degree of territoriality, prey catching technique, dietary spectrum) may be interpreted as reflections of an additional requirement for the nocturnal habit beyond high sensory sensitivity: detailed knowledge of local topography.     

THE VISUAL ACUITY OF THE HONEY BEE

1. Bees respond by a characteristic reflex to a movement in their visual field. By confining the field to a series of parallel dark and luminous bars it is possible to determine the size of bar to which the bees respond under different conditions and in this way to measure the resolving power or visual acuity of the eye. The maximum visual acuity of the bee is lower than the lowest human visual acuity. Under similar, maximal conditions the fineness of resolution of the human eye is about 100 times that of the bee. 2. The eye of the bee is a mosaic composed of hexagonal pyramids of variable apical angle. The size of this angle determines the angular separation between adjacent ommatidia and therefore sets the structural limits to the resolving power of the eye. It is found that the visual angle corresponding to the maximum visual acuity as found experimentally is identical with the structural angular separation of adjacent ommatidia in the region of maximum density of ommatidia population. When this region of maximum ommatidia population is rendered non-functional by being covered with an opaque paint, the maximum visual acuity then corresponds to the angular separation of those remaining ommatidia which now constitute the maximum density of population. 3. The angular separation of adjacent ommatidia is much smaller in the vertical (dorso-ventral) axis than in the horizontal (anterio-posterior) axis. The experimentally found visual acuity varies correspondingly. From this and other experiments as well as from the shape of the eye itself, it is shown that the bee''s eye is essentially an instrument for uni-directional visual resolution, functional along the dorso-ventral axis. The resolution of the visual pattern is therefore determined by the vertical angular separation of those ocular elements situated in the region of maximum density of ommatidia population. 4. The visual acuity of the bee varies with the illumination in much the same way that it does for the human eye. It is low at low illuminations; as the intensity of illumination increases it increases at first slowly and then rapidly; and finally at high intensities it becomes constant. The resolving power of a structure like the bee''s eye depends on the distance which separates the discrete receiving elements. The data then mean that at low illuminations the distance between receiving elements is large and that this distance decreases as the illumination increases. Since such a moving system cannot be true anatomically it must be interpreted functionally. It is therefore proposed that the threshold of the various ommatidia are not the same but that they vary as any other characteristic of a population. The visual acuity will then depend on the distance apart of those elements whose thresholds are such that they are functional at the particular illumination under investigation. Taking due consideration of the angular separation of ommatidia it is possible to derive a distribution curve for the thresholds of the ommatidia which resembles the usual probability curves, and which describes the data with complete fidelity.     

Saccadic tracking of targets mediated by the anterior-lateral eyes of jumping spiders

The modular visual system of jumping spiders (Salticidae) divides characteristics such as high spatial acuity and wide-field motion detection between different pairs of eyes. A large pair of telescope-like anterior-median (AM) eyes is supported by 2-3 pairs of 'secondary' eyes, which provide almost 360 degrees of visual coverage at lower resolution. The AM retinae are moveable and can be pointed at stimuli within their range of motion, but salticids have to turn to bring targets into this frontal zone in the first place. We describe how the front-facing pair of secondary eyes (anterior lateral, AL) mediates this through a series of whole-body 'tracking saccades' in response to computer-generated stimuli. We investigated the 'response area' of the AL eyes and show a clear correspondence between the physical margins of the retina and stimulus position at the onset of the first saccade. Saccade frequency is maximal at the margin of AL and AM fields of view. Furthermore, spiders markedly increase the velocity with which higher magnitude tracking saccades are carried out. This has the effect that the time during which vision is impaired due to motion blur is kept at an almost constant low level, even during saccades of large magnitude.     

THE VISUAL INTENSITY DISCRIMINATION OF THE HONEY BEE

1. Bees respond by a characteristic reflex to a movement in their visual field. By confining the field to a series of parallel stripes of different brightness it is possible to determine at any brightness of one of the two stripe systems the brightness of the second at which the bee will first respond to a displacement of the field. Thus intensity discrimination can be determined. 2. The discriminating power of the bee''s eye varies with illumination in much the same way that it does for the human eye. The discrimination is poor at low illumination; as the intensity of illumination increases the discrimination increases and seems to reach a constant level at high illuminations. 3. The probable error of See PDF for Equation decreases with increasing I exactly in the same way as does See PDF for Equation itself. The logarithm of the probable error of ΔI is a rectilinear function of log I for all but the very lowest intensities. Such relationships show that the measurements exhibit an internal self-consistency which is beyond accident. 4. A comparison of the efficiency of the bee''s eye with that of the human eye shows that the range over which the human eye can perceive and discriminate different brightnesses is very much greater than for the bee''s eye. When the discrimination power of the human eye has reached almost a constant maximal level the bee''s discrimination is still very poor, and at an illumination where as well the discrimination power of the human eye and the bee''s eye are at their best, the intensity discrimination of the bee is twenty times worse than in the human eye.     

A model for visual image-background discrimination by relative movement

Biologically plausible electronic neural network setup for real time processing motion image informa-tion was built. Using this setup the first part of the model was examined and real time discrimination of moving object image was realized from complex background in high resolution. Afterimages may play an important role in filtering moving object image and the aperture problem should be separated into two parts: the first part, i.e. the incomplete filtered moving object image, can be better resolved by parallel integration of multi-channel visual information, howev-er, the second part, i.e. the inaccurate measurement results for movement direction, may only get certain compensa-tion by visual integration.     

Spatial, colour and contrast response characteristics of mechanisms which mediate discrimination of pattern orientation and magnification

We have measured response times for the detection of a single target presented against a set of reference elements which are characterised by combinations of four different stimulus parameters; colour, contrast polarity, magnification and orientation. The aim of the experiments was to determine the response characteristics of visual mechanisms which mediate target detection through the discrimination of orientation and magnification. In the first experiments, we determined sensitivity to differences in colour and contrast polarity, and show that the mechanisms responsible for the discrimination of orientation and of magnification are both selective in their responses to colour and to contrast polarity. There are, nonetheless, residual interactions between patterns of different contrast polarities and between those of different colour, and in the latter case, weak interactions persist under equiluminance conditions. In a second set of experiments, we examined the interactions between orientation and magnification. We conclude that the responses of visual mechanisms which mediate target detection through discrimination of orientation are markedly dependent on stimulus magnification whereas those which mediate detection through discrimination of magnification are, in contrast, relatively insensitive to stimulus orientation.     

Critical flicker detection frequency and double pulse recognition threshold do not depend on the spectral properties of stimuli

We have measured the critical flicker detection frequency (CFDF) and double pulse recognition threshold (DPT) using three LEDs with power peaks at 460, 525 and 625 nm for target illumination. Brightness equalization was performed by customized heterochromatic flicker photometry (cHFP). Reference luminance levels were 170 cd/m² (blue LED, 60 subjects), 4 cd/m² (green LED, 20 subjects), and 1 cd/m² (green LED, 20 subjects). The measurement at 1 cd/m² was preceded by 15 min of dark adaptation. The angle of view for the target was 3°, and the duration of stimuli was 1 ms. An experimental pulse generator with three channels and a projector was used. No differences in CFDF at different spectral properties of stimulus were observed at all three levels of luminance. Thus, it is concluded that temporal vision resolution does not depend on the spectral properties of visual stimuli.