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Modeling approaches have contributed in various ways to our understanding of the neural principles and mechanisms of color vision. We review computational models addressing aspects of color vision at different levels, from the photoreceptors to neural processing and color perception, and we consider the kinds of insights that have been enabled by different kinds of models.

We document the performance of 26 human observers practicing a motion discrimination task in foveal vision. Two blocks of 1960 apparent motion stimuli each were presented in succession. Stimuli in the two blocks were tailored to activate either the ON-pathway (ON-stimulus) or the OFF-pathway (OFF-stimulus). Initial performance of about half of the subjects was rather weak but improved with practice. Initial performance of the other subjects remained unaffected for the first block. Once performance had improved in one of the tasks it transferred to the other tasks. Improvement in performance to the ON-stimulus was found to extend over many more presentations than that to the OFF-stimulus.

To examine the effect of reducing luminance contrast in human foveal vision, discrimination thresholds were measured in four tasks and also a numerical measure of two visual illusions were obtained by a nulling technique. The patterns used for all tasks were made very similar to facilitate comparison between them-all featured luminance step edges whose contrast could be varied from near unity down to the detection threshold. Orientation, vernier and blur discrimination thresholds rise on average 5-6-fold when the contrast is reduced from near unity to a Michelson value of 0.03. Jump displacement thresholds are somewhat more robust to contrast reduction, and the curve of separation discrimination versus contrast is much shallower, rising by a factor of about 2. The magnitude of the Poggendorff and tilt illusions changes very little until the inducing contours are barely detectable. (C) 1999 Elsevier Science Ltd. All rights reserved.

The strength of the Craik–O'Brien–Cornsweet illusion was measured for different values of spatial and temporal stimulus parameters, in the traditional achromatic version, and in an isoluminant colour version. It was found that the illusion is much weaker with isoluminant colour stimuli than with achromatic luminance stimuli. The illusion depends on the spatial parameters of the stimulus in a way that yields an approximate scale invariance: The strength of the illusion is similar for different stimulus sizes, as long as the ratio of the width of the transition region around the edge, where luminance or colour change, to the total stimulus width is preserved. In both the achromatic and the chromatic case, the strength of the illusion decreases with increasing presentation time. The similarity of the differences between brightness and colour effects on one hand and the differences in sensitivity for colour and luminance changes in humans on the other suggests that a lack of gradient detection underlies the Craik–O'Brien–Cornsweet illusion.

The threshold for detecting a change in orientation away from the vertical of a briefly presented foveal line target is raised when there are immediately following visual presentations. This masking effect was examined by measuring the capacity of a variety of patterns to act as masks. When patterns were made of exactly the same number of light pixels, masking was least when they formed random dots and progressively became stronger as they formed lines of decreasing curvature from full circles to straight lines. The longer the lines, the stronger the masking. Threshold elevation was highest when the masking pattern was spatially superimposed on the line and was lessened when a large surround area was included, but there was still considerable masking when the interfering patterns were confined to the surround. By placing masks and test lines in different eyes, or by giving them opposite contrast polarity, almost complete interocular and interpolarity transfer was demonstrated. Relating these results to anatomical and electrophysiological findings about neurons in the primary visual cortex leads to the conclusion that the masking effects could have their substrates in interaction between cells in V1.

Humans can see fine structures with excellent precision even if these are moving (1). To achieve this, the visual system may also compensate for the fixational image drift projected on the retina (2). In a recent issue of PNAS, Burak et al. (2) proposed that the effects of such eye movements may be compensated for by two parallel mechanisms possibly implemented in V1. One mechanism, called where, is tracking the image; the other retrieves the image as such and is called what. The authors write about their model: “A salient prediction is that in high acuity tasks, fixed features within the visual scene are beneficial because they provide information about the drifting position of the image. Therefore, complete elimination of peripheral features in the visual scene should degrade performance on high acuity tasks involving very small stimuli.”

Two dots may be aligned vertically with a precision much higher than that expected from two-point resolution provided they are separated by a visual angle of 3–5 min of arc. This precision suffers when the two dots are not exposed synchronously. Neither onset nor offset asynchronies can be tolerated; exposure differences of the two components of the vernier task as low as 30 ms can lead to a reduction in performance when the total exposure is below 90 ms. This effect cannot be compensated for by synchronizing the onset of one stimulus component with the offset of the other, even when the two are of opposite contrast. The data suggest that vernier acuity may be subserved by a dynamical linking of cortical excitation generated by the synchronous arrival of signals within a range of locations in the cortex whose spatial separation is critical for optimal hyperacuity performance. The evidence presented in this paper must be taken into account when a physiological substrate for hyperacuity is considered.

A small gray test field superimposed on a large colored background appears tinted in a color complementary to that of the surround. We tested the hypothesis whether photoreceptor sensitivity in the test field is altered in the presence of a colored surround. We investigated this effect using dichoptic viewing conditions. With the left eye, subjects viewed a small gray target superimposed on a large colored background. The right eye saw a gray target superimposed on a large gray background. When the two images were fused, observers perceived one homogeneous background and two targets. Observers matched the color of the target seen by the right eye to that seen by the left eye. A modified two-stage model for chromatic induction assuming that both receptor and postreceptor mechanisms contribute to the shift in color was fitted to the matched settings. We find that the dichoptic viewing effects presented here are well explained by an approximately equal contribution of receptor and postreceptor processes to the perceived shift in color.

Gori and Stubbs (2006 Perception35 1573–1577) have published some visual illusions elicited in observers who fixate very blurred disks while they move their head towards and away from them. We interpret these illusions as afterimages. We support this with examples of eccentric, colored, and striped afterimages.

Two briefly flashed lines shown in rapid succession evoke a sensation of motion in human observers. This is examined quantitatively such that line separation, temporal offset, and contrast polarity are varied. Line pairs are presented on a gray background and are either both bright or both dark (equal contrast polarity) or one line is bright and the other is dark (opposite contrast polarity). Observers are instructed to indicate the perceived direction of motion. With foveal viewing, perceived direction is veridical for line pairs with equal contrast polarity but is reversed for line pairs with opposite contrast polarity, with spatial separations between 0 and 12 arcmin, and with temporal offsets between 8 and 33 ms. When separations, temporal offsets, or both are further increased, perception reverts to veridical. When lines of opposite contrast polarity are presented in the parafovea, reversal of perceived direction is also observed. The psychophysical results correlate well with those of recent intracellular recordings from directionally selective simple cells in the primary visual cortex of anesthetized cats (N. J. Priebe & D. Ferster, 2005).

The ability of human observers to detect and discriminate a single feature of a visual image deteriorates markedly when the targeted feature is surrounded by others of a similar kind. This perceptual masking is mirrored by the suppressive effects of surround stimulation on the responses of neurons in primary visual cortex (area V1). Both perceptual and neuronal masking effects are partially relieved, however, if the targeted image feature is distinguished from surrounding features along some dimension, such as contour orientation. Masking relief is likely to play an important role in perceptual segmentation of complex images. Because dissimilar surfaces usually differ along multiple feature dimensions, we tested the possibility that those differences may influence segmentation in an invariant manner. As expected, we found that the presence of surrounding features resulted in perceptual masking and neuronal response suppression in area V1, but that either orientation or contrast polarity differences between the target and surrounding features was sufficient to partially relieve these effects. Simultaneous differences along both dimensions, however, yielded no greater relief from masking than did either difference alone. Although the averaged neuronal effects of orientation polarity cues were thus invariant, the time course over which these effects emerged after each stimulus appearance was different for the two cues. These findings refine our understanding of the functions of nonclassical receptive fields, and they support a key role for V1 neurons in surface segmentation.

We investigated the effect of 16 isoluminant chromatic surrounds on the perceived colour of an enclosed grey test-field at photopic (43 cd/m2) conditions. Stimuli were shown on a grey background identical to the test-field. Use of these stimuli implies that activations of receptoral (cS, cM and cL) and postreceptoral (cM−cL, cS−(cM+cL)) mechanisms by surround colours are known quantitatively. This allows to predict shifts in colour of the test-field in terms of receptoral (adaptation) as well as postreceptoral (contrast) mechanisms assuming a standard two-stage model. Predictions are tested using matching and hue compensation procedures. Both procedures yield comparable results that are consistent with the assumption that postreceptoral mechanisms explain the observed shifts in perceived colour.

We were interested in the question of how cones contribute to the detection of brightness, red-green, and blue-yellow. The linear combination of cone signals contributing to flicker detection was determined by fitting a plane to 64 points (colors) of equal heterochromatic flicker brightness. A small S-cone contribution to flicker brightness of similar amplitude in all five subjects was identified. The ratio of L- to M-cone contribution was found to vary considerably among subjects (1.7-4.1). Chromatic detection thresholds were determined for small patches in the isoluminant plane defined by flicker brightness. These stimuli were presented at an eccentricity of 40 arc min. By using color naming at the detection threshold, one can attribute different segments of the resulting detection ellipses to different chromatic mechanisms. Linear approximation of these segments provided an estimate for the contribution of the different cone types to the detection of red-green and blue-yellow. The results are consistent with the hypothesis that S cones contribute to the red-green mechanism with the same sign as that of the contribution from L cones. The blue-yellow mechanism very probably subtracts S-cone contrast from luminance contrast. The detection ellipse can be mapped into a circle in cone difference space. The base of this canonical transformation is a set of three cone fundamentals that differs from previously published estimates. Projecting the circle onto the three cone difference axes produces sinusoidal changes within the respective excitations. We propose that simultaneous sinusoidal changes of equal increment in the three cone difference excitations generate stimuli differing by equal saliency.

We studied the effects of various patterns as contextual stimuli on human orientation discrimination, and on responses of neurons in V1 of alert monkeys. When a target line is presented along with various contextual stimuli (masks), human orientation discrimination is impaired. For most V1 neurons, responses elicited by a line in the receptive field (RF) center are suppressed by these contextual patterns. Orientation discrimination thresholds of human observers are elevated slightly when the target line is surrounded by orthogonal lines. For randomly oriented lines, thresholds are elevated further and even more so for lines parallel to the target. Correspondingly, responses of most V1 neurons to a line are suppressed. Although contextual lines inhibit the amplitude of orientation tuning functions of most V1 neurons, they do not systematically alter the tuning width. Elevation of human orientation discrimination thresholds decreases with increasing curvature of masking lines, so does the inhibition of V1 neuronal responses. A mask made of straight lines yields the strongest interference with human orientation discrimination and produces the strongest suppression of neuronal responses. Elevation of human orientation discrimination thresholds is highest when a mask covers only the immediate vicinity of the target line. Increasing the masking area results in less interference. On the contrary, suppression of neuronal responses in V1 increases with increasing mask size. Our data imply that contextual interference observed in human orientation discrimination is in part directly related to contextual inhibition of neuronal activity in V1. However, the finding that interference with orientation discrimination is weaker for larger masks suggests a figure-ground segregation process that is not located in V1.

International audience; We measured detection of a thin vertical line (target) in the presence of a slightly thicker collinear, adjacent line (inducer). Sign and strength of contrast of the inducer were varied. Test lines could be either bright or dark. Detection thresholds were obtained through a temporal two-alternative forced-choice (2AFC) procedure with the method of constant stimuli. When target and inducer had equal contrast polarity, low thresholds of target lines were observed for low inducer contrasts and increased with increasing inducer contrast. With opposite contrast polarity of target and inducer, thresholds were high for low inducer contrasts and decreased for increasing contrast thereof. Our results support the hypothesis that cortical mechanisms with different sensitivity to the sign and strength of contrast participate in the detection facilitation of line contours.

The strength of the Poggendorff illusion has been determined by a nulling method for the classical as well as other configurations of the central inducing region. Compared to a uniform field, an inducing rectangle with very low contrast produces a marked illusion, which saturates at a Michelson contrast of about 0.1. With virtual borders of the Kanizsa type there is a weak illusion and this effect is attenuated when the ‘pacman’ sectors are occluded. Texture borders without luminance contrast induce a stronger illusion. These results are discussed in relation to earlier data for contrast dependence of Vernier acuity and for the orientation discrimination and tilt illusion with real and virtual borders.

We developed a physiologically plausible model of the first steps of spatial visual information processing in the fovea of the human retina. With the predictions of this model we could support the hypothesis that, for moderate contrasts (less than or equal to 40), hyperacuity is mediated by the magnocellular (MC-) pathway. Despite the lower sampling density in the MC pathway, as compared to the parvocellular (PC-) pathway, the information that is transferred by the MC ganglion cells is sufficient to achieve thresholds comparable to those of human subjects in psychophysical tasks. This is a result of the much higher signal-to-noise ratio of the MC pathway cell signals. The PC pathway cells do not transfer enough information for hyperacuity thresholds.

Luminance and color of surfaces provide fast and reliable means for the visual recognition and identification of objects (Dobkins & Albright, 1994; Sun & Perona, 1996). The perceptions of both color and luminance seem to share many properties. They are established essentially on the basis of local contrasts but subject to variations due to more global variations of illumination (Daw, 1984; Shapley & Reid, 1985; Reid & Shapley, 1988). Long range phenomena like constancy and induction appear to modify the perception of both luminance and color in a comparable way.

Responses of parafoveal macaque ganglion cells were measured as a function of the contrast and position of an edge flashed within their receptive fields. The goal was to determine the ability of different cell types to signal edge location. For comparison, parafoveal vernier thresholds of human observers were measured with pairs of flashed edges. Cells of the magnocellular (MC-) pathway gave larger responses than cells of the parvocellular (PC-) pathway. Neurometric analyses comparing a cell's response at different edge positions were performed. The positional signal from single MC-pathway cells was more precise than from PC-pathway cells, especially at lower contrasts. In a second analysis, based on the neurophysiological results, responses from a matrix of ganglion cells were generated. Using a simple model, vernier performance expected from such a matrix was predicted as a function of edge length and contrast. Again, the MC-pathway gave a more precise positional signal than the PC-pathway despite the latter's numerical advantage. At contrasts of 20% and below, only the MC-pathway would appear capable of supporting vernier performance with our stimuli. At higher contrasts either the MC- or PC-pathway could provide an adequate signal.

1. Differences as low as 0.5 degrees can be discriminated in the direction of motion of a single spot of light moving with optimum speed and seen in the fovea for < 250 ms. There is no improvement for a cloud of random dots or a short line. 2. For high velocities the thresholds approach those for the discrimination of orientation of a single line, when the length of the line is equal to the excursion of the dot and when the line is shown for the same duration. 3. The sensitivity for orientation of line of motion of a moving spot also shares two other attributes with that for the orientation of a single solid line of similar temporal and spatial extents: discrimination is seriously impaired when flanked by related close-by stimuli, and sensitivity is subject to simultaneous orientation contrast. 4. It is suggested that the orientation both of features and of lines of motion is processed by the same mechanism.

We have tested the hypothesis that in humans the signals carried by ON- and OFF-pathway respectively are processed for the perception of motion by two distinct physiological substrates. In vertebrates, onset of a bright visual stimulus is signaled to the CNS by ON-center retinal ganglion cells; onset of a dark stimulus is transmitted by OFF-center cells. We chose apparent motion generated by successive presentation of two adjacent lines as a stimulus. Lines presented on a bright background were either darker or brighter than this background. Delayed onset of a pair of bright or dark lines elicits apparent motion at the same time fulfilling the constraint of stimulating either ON- or OFF- center ganglion cells, respectively. We determined the threshold delay needed for subjects to perceive the temporal order of the onset of the two lines for various angular separations. The threshold delay for a pair of bright lines stayed low for separations from 2′ to 7′. The threshold delay for a pair of dark lines was low only within a narrow range of separations centered around 3′. The variation of thresholds with line distance must reflect the existence of a limited processing zone for the perception of motion. The diameter of the processing zone for bright lines is about twice as large as that for dark lines. This suggests that in humans the separation of ON- and OFF-pathways extends to the early stages of motion perception. To test this hypothesis independently, thresholds were determined when a bright and a dark line were presented in succession. This was done for a separation of 3′ where thresholds for a pair of lines with equal contrast are similarly low. Temporal order was perceived correctly only when the delay was at least two to four times as high as the threshold delays found for the equal contrast stimuli.

Two recent experiments contributing to the analysis of temporal resolution in human foveal vision are reviewed. First, the critical duration for the detection of a single line on a bright ground is determined. When the line is brighter than the ground, critical duration amounts to 4·10 −3 s as opposed to 12·10 ‒3 s when the line is darker than the ground. This difference is discussed in connection with ON- and OFF- centre retinal ganglion cells in primates. Relations to possible physiological substrates of feature binding are sketched. Second, apparent motion generated by successive onset of two adjacent lines on a bright ground is studied. Psychophysical thresholds for the detection of temporal order of the onset of two dark lines are determined for different distances of the lines. The minimum of the distance function found is very narrow centering around 3 \ This is small compared to the distance function measured in earlier experiments and when the lines are brighter than the background. This difference indicates that the signals of ON- and OFF- centre ganglion cells are processed separately in the perception of local motion. Findings and conclusions are discussed with respect to earlier anatomical and physiological findings in the visual cortex of primates.

Thresholds were determined for the perception of the motion of a single bar moving at different positions in the field of view. Performance in the temporal hemified was slightly superior to that in the nasal hemifield and depended on the orientation as well as on the direction of the motion. The perception of horizontal motion was better than that of vertical motion. In spite of large variations, centrifugal motion was significantly more readily perceived than centripetal motion.

Vernier acuity was measured by finding the just discriminable offset for an edge separating fields of different luminances. The contrast of this stimulus is easily specified by the formula c = (L stim — L sur )(L stim + L sur ). Vernier thresholds are about 4–5 sec of arc for contrasts 0.22 and higher, but increase exponentially with decreasing contrast (Fig. 1). By comparison, the presence of the stimulus could be detected at a contrast of 0.016. The possible role of the magnocellular and parvocellular pathways in carrying the input signals to the fine localization process is discussed.

It has been suggested (Livingstone and Hubel, 1988 Science240 740 – 749) that the ‘colour-blind’ magnocellular pathways generate the neurophysiological basis of surfaces with illusory contours since the latter do not seem to be perceived in inducing configurations of a given colour which is isoluminant with regard to the colour of the background (equiluminant colour contrast). However, psychophysical data allowing us to assess the relative visibility of illusory surfaces in coloured stimuli with luminance contrast compared to configurations with equiluminant colour contrast are not yet available. We designed a colour-matching experiment where ten naive observers had to adjust the intensity of a red illusory surface so that it appeared to match the intensity of the red background. The configurations used were Kanizsa squares with green inducing elements, isoluminant or not with regard to the background. Isoluminance was assessed individually for each observer by means of a classical flicker test. A brightness-matching procedure was applied to configurations of achromatic inducers on a grey background. In this case, the inducers had either all the same contrast polarity (light), or both polarities (light and dark) within a given configuration. Luminance contrast in the achromatic configuration with only one polarity was the same as in the non-isoluminant colour condition. Luminance contrasts of light and dark inducers in the mixed-polarity condition were physically balanced. The results show that the mean point of subjective equality (PSE) of the test surface corresponds to the physical intensity of the background with equiluminant colour contrast only, indicating the absence of an apparent surface in this condition. This result supports the idea that magnocellular pathways in the human visual system mediate the neurophysiological genesis of illusory surfaces. In all the other stimulus conditions, the PSE does not correspond to the physical intensity of the background. Matching ‘errors’ are significantly stronger in the achromatic conditions, but, paradoxically, strongest in the condition with balanced contrasts of opposite polarity. This finding suggests that luminance contrast is not the only determinant of the perceived strength of illusory surfaces.

Thresholds for temporal-order detection of two successively displayed vertical stripes were determined for monocular and binocular stimulation. Thresholds are lower for binocular stimulation by roughly a factor of 1.4 compared with those found under monocular stimulation. This corresponds to quadratic summation. This is found for angular separations of the two stripes amounting to 5 and 40 min of arc.

The landing response of tethered flying housefliesMusca domestica elicited by motion of periodic gratings is analysed. The field of view of the compound eyes of a fly can be subdivided into a region of binocular overlap and a monocular region. In the monocular region the landing response is elicited by motion from front to back and suppressed by motion from back to front. The sensitivity to front to back motion in monocular flies (one eye covered with black paint) has a maximum at an angle 60°–80° laterally from the direction of flight in the equatorial plane. The maximum of the landing response to front to back motion as a function of the contrast frequencyw/λ is observed at around 8 Hz. In the region of binocular overlap of monocular flies the landing response can be elicited by back to front motion around the equatorial plane if a laterally positioned pattern is simulataneously moved from front to back. 40° above the equatorial plane in the binocular region the landing response in binocular flies is elicited by upward motion, 40° below the equatorial plane in the binocular region it is elicited by downward motion. The results are interpreted as an adaptation of the visual system of the fly to the perception of a flow field having its pole in the direction of flight.

The optomotor yaw torque response of fixed flying female houseflies, Musca domestica to three different types of visual stimuli is analyzed. In contrast to most previous investigations, the stimuli were displayed for short time intervals only in order to approximate transiently occuring visual stimuli, which mainly govern the torque generation during free flight. Monocular stimulation with a periodic pattern moving in different positions in the equatorial plane of the compound eyes reveals that (1) flight torque responses are mainly induced by progressive (front to back) motion; regressively moving stimuli are significantly less effective. (2) the strength of the response to motion in the horizontal direction depends on the position of the stimulus and (3) vertical motions do not elicit flight torque responses. Correspondingly the response to a single vertical black stripe moving clockwise in a cylindrical panorama centered around the fly is small if the stripe is in the visual field of the left eye but becomes large and strongly depending on position if the stripe enters the visual field of the right eye. The response to counterclockwise motion of the stripe is small if the stripe is in the visual field of the right eye but becomes large and strongly depending on position if the stripe enters the visual field of the left eye. Torque responses to two adjacent stripes whose intensities are modulated in time with a rectangular function can be elicited if apparent motion is generated by means of a phase difference between the intensity modulations of the two stripes. Apparent progressive motion elicits strong torque responses, apparent regressive motion is less effective. Synchronous flicker of both stripes does not elicit torque responses. The extraction of positional information from the incoming visual signals has been considered to play an important role in the orientation behaviour, and especially in the tracking behaviour of flies. The results of the experiments indicate, that under transient stimulation the evaluation of positional information is in general not mediated by formerly postulated flicker detectors but is bound to the computation of motion. These findings are implemented in a model, describing the free flight tracking behaviour of a female fly on the horizontal plane. It is shown that tracking can be achieved by a mechanism whose sensitivity to motion is parametrized in the stimulus position as outlined above. The results of the behavioural experiments are interpreted in view of electrophysiological and anatomical data on giant interneurons in the third optic ganglion of the fly.

The motion-sensitive horizontal cells in the lobula plate of the fly are assumed to play a key role in the sensory control of yaw torque generated by the flying animal during course-stabilization maneuvers and the fixation of objects. This inference results from comparisons of electrophysiological data obtained from blowflies (Calliphora erythrocephala) and behavioral data obtained mainly from houseflies (Musca domestica) and fruitflies (Drosophila melanogaster). Apart from few exceptions, the compatibility of these physiological and behavioral data has not been critically tested. In the present study, the responses of the equatorial horizontal cell HSE of Calliphora and the yaw torque responses of Calliphora and Musca were recorded under identical visual stimulation with moving periodic gratings. The goal of the experiments was to obtain electrophysiological and behavioral data on Calliphora, on the one hand, and behavioral data on Calliphora and Musca, on the other hand, that allow direct comparisons between the physiological properties of the HSE and the visually induced torque responses in both species. The dependence of the HSE responses and the yaw torque responses on the direction, contrast frequency, and brightness of a moving periodic grating were evaluated quantitatively. The results of the electrophysiological recordings and torque measurements are in close agreement and thus represent strong evidence that the horizontal cells are, in fact, involved in yaw torque control in both species. Measurements of the cellular and behavioral responses as function of the stimulus position in the visual field, however, reveal differences between the spatial sensitivity of the horizontal cells and the sensory input to the motor system.(ABSTRACT TRUNCATED AT 250 WORDS)

The visually controlled height orientation of fixed flying flies (Musca domestica) was investigated. The flight lift force measured by a transducer drives the vertical motion of a panorama. The dynamical conditions of the free flight are electronically simulated for the fly with respect to this degree of freedom of motion. In most of the experimentally investigated cases the panorama consists of a horizontally oriented narrow dark stripe on a bright background. The fly orientates with respect to the stripe, transporting it into a stable fixation position just below the equatorial plane of its compound eyes. It is experimentally demonstrated that the formalism of the linearized theory of the pattern induced flight orientation -- Poggio and Reichardt (1973a) -- can be applied to describe the height orientation of the fly. The experimental evidence concerning the simultaneous perception of stripes moving in a well defined manner in front of each of the two compound eyes is consistent with the hypothesis that the two halves of the visual system are perceptually additive.

Color vision in humans is independent over a wide range of the spectral composition of the illuminating light (Young 1807; Hering 1879). The retinex theory accounts for this color constancy by assuming that for each of the three waveband channels determined by the retinal cones a global lightness record of the scene is first computed by the visual system. The three records then serve to generate color at every point (Land 1983). Where do these computations take place? In this report a scene consisting of fourteen colored fields was viewed while one band of wave-lengths enters one eye and a different band enters the other (dichotpic case) or while both bands enter both eyes (normal case) under otherwise identical conditions. The perceived color of every field is very similar in both cases although the physical stimulation of the eyes differs. It is also found that color constancy is maintained under dichoptic conditions. The results show that the cortex is crucial for the computation of color.

Flies were filmed simultaneously from above and from the side. Their flight tracks were analyzed frame by frame. Male and female flies were found to chase other flies. But female chases are brief and poorly controlled as compared to male chases. Female flies use the lower frontal part of their visual field for tracking other flies. Male flies use the upper frontal part of their visual field for that purpose. Male flies are capable of controlling their forward velocity roughly proportional to the distance to their target. Implications for the function of recently found sexdimorph visual interneurones are discussed.

Flies are able to track small, fast moving objects, a task which requires high spatial and temporal resolution in information processing of the visual system and aerobatic capabilities of the flight motor system. This review briefly outlines which basic perceptual principles are used by flies during tracking.

The lift response of houseflies Musca domestica in fixed flight to periodic gratings movins in 12 different orientations has been measured. Two projectors were arranged symmetrically to the flies stimulating successively 18 circular patches of 50 degrees (25 degrees) diameter (9 for each eye) in their visual field. The shapes of the lift responses measured as a function of the orientation of the moving grating varied when different patches in the visual field were stimulated. A qualitative comparison of these response curves leads to the conclusion that the orientation of the movement detecting substrate acting on the flight lift response varies as a function of the stimulated area in the visual field. A straightforward correlation between the geometry of the ommatidial pattern and the orientation of the movement detecting substrate valid for all stimulated areas of the compound eyes does not seem very likely.

The flight torque responses of tethered flying houseflies to motion and presentation or removal of a vertical dark stripe on a bright background were recorded in real time. Motion with constant speed of 100° s-1 from front to back elicits a strong fast response following the diraction of the stimulus motion. Motion from back to front elicits a weaker response. Instantaneous presentation and removal of a stationary stripe elicit weak, slow response. Apparent motion from front to back and from back to front elicit weak responses with a fast, directionally selective, transient peak followed by a slow response component oriented towards the stripes position. The fast transient peak response is not elicited if the animals were stimulated before with real movement of the stripe. The results are discussed and an earlier proposed model for free flight tracking and fixation is extended.

Visually evoked torque and lift responses in fixed flying houseflies Musca domestica, were measured under open loop conditions. The visual stimuli were: a) Vertical stripes (60°×5°), moving horizontally in a range±30°. b) Horizontal stripes (60°×5°), moving vertically in a range±30°. c) Vertical stripes (30°×5°), moving horizontally in a range±55° in five different planes relative to the equatorial plane (θ=0) of the fly's eye. d) Horizontal stripes (30°×5°), moving vertically in a range±45° in seven different planes (ψ=-45°,-30°,-15°, 0°, 15°, 30°, 45°) relative to the symmetry line (ψ=0) between the two compound eyes. e) Periodic gratings displayed by two projectors at each side of the test animals (the middle part was situated at ψ=±50°). The stimulated area was roughly 52°×80° (∼2000 ommatidia). This stimulus was used only in lift experiments. The results are: 1) The preferred direction of the direction sensitive torque response corresponds with the z-direction (Braitenberg, 1971). 2) The direction sensitive torque response is elicited by stimulating above and below the equatorial plane. 3) The direction insensitive torque response is only elicited by stimulating close to and below the equatorial plane. 4) The preferred direction of the direction sensitive lift response has an angle tilted about 30° to the back relative to the vertical axis in the region described in e). 5) The magnitude of the direction sensitive lift response varies considerably over θ. 6) The w/γ dependence of the direction sensitive lift response corresponds qualitatively to the known w/γ dependence of the direction sensitive torque responses. 7) The direction insensitive lift response has its maximum at ψ=±15° and decreases with increasing ψ. 8) The findings reported in 3) and 8) indicate the existence of a two-dimensional potential from which the attraction towards a stripe in the two considered degrees of freedom can be derived. Implications for the visually induced orientation behaviour and connections with electrophysiological experiments are discussed.

The spatial distribution of light intensity received by the eyes changes continually when an animal moves around in its environment. These retinal activity patterns contain a wealth of information on the structure of the environment, the direction and speed of self-motion, and on the independent motion of objects1,2. If evaluated properly by the nervous system this information can be used in visual orientation. In a combination of both behavioural and electrophysiological analysis and modelling, this article establishes the neural mechanisms by which the visual system of the fly evaluates two types of basic retinal motion patterns: coherent retinal large-field motion as induced by self-motion of the animal, and relative motion between objects and their background. Separate neuronal networks are specifically tuned to each of these motion patterns and make use of them in two different visual orientation tasks.

The orientation of walking flies with blinded compound eyes was tested. By using only their ocelli, flies orient tow ards edges and relatively small bright objects situated in the frontal equatorial p art of the visual field

A dark target on a bright background is fixated1 and tracked2,3 by a flying fly. The delay (dead time) in the fly's behavioural response to a change in the retinal image of the target is very small (about 20 ms)3–5. An artificial increase of this delay may give useful insight into the kind of control systems used by the tracking fly. Comparable psychophysical tracking experiments have been performed. For instance, when a televised display of a person's own behaviour in (closed-loop) pursuit tracking is delayed, his performance, as measured by time on target, is seriously impaired6. It is possible to delay the fly's response in artificial closed-loop conditions, and we have therefore used the effects of delayed visual feedback on tracking performance to test quantitatively a theoretical description of the visual orientation behaviour of flies1,7,8.

Positional information is required in solving orientation tasks. In open loop experiments it was found that a resting stripe does not influence the time averaged (over 120 s) yaw torque signal of a tethered flying fly irrespective of the position at which it is presented. An open loop response was elicited when either the stripe was moved or the head of the fly was free to move (Reichardt 1973, Wehrhahn 1980). Thus light flux changes are necessary for the extraction of positional information. It is known that flies are able to perceive motion and a mechanism responsible for the computation of motion proposed earlier (Hassenstein and Reichardt 1956), was found to apply also for the fly visual system (Fermi and Reichardt 1963). This mechanism selectively extracts the phase relations between the signals of neighbouring photoreceptors and thus enables the fly to follow a moving object. The direction of the response is inverted when this phase relation is inverted. The existence of a mechanism selectively extracting the position of an object can be tested by stimulating a fly with a resting object, whose luminosity is modulated in time. The result of the first of such experiments where the time averaged torque response to a flickered stripe has been determined, was negative. Thus it was concluded that no separate mechanism exists for the computation of position (Reichardt 1973). However, the motion and flicker stimuli were different in these experiments.