The evolution of selectively similar electrophoretically detectable alleles in finite natural populations.

Most of the models of population genetics are not realistic when applied to data on electrophoretic variants of proteins because the same net charge may result from any of several amino acid combinations. In the absence of realistic models they have, however, been widely used to test competing hypotheses about the origin and maintenance of genetic variation in populations. In this paper I present a general method for determining probability generating functions for electrophoretic state differences. Then I use the method to find allelic state difference distributions for selectively similar electrophoretically detectable alleles in finite natural populations. Predicted patterns of genetic variation, both within and among species, are in reasonable accord with those found in the Drosophila willistoni group by Ayala et al. (1972) and by Ayala and Tracey (1974).

Perceptual learning of apparent motion mediated through ON- and OFF-pathways in human vision.

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 presentation than that to the OFF-stimulus.

Detection facilitation by collinear stimuli in humans: dependence on strength and sign of contrast.

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.

Contrast dependency of foveal spatial functions: orientation, vernier, separation, blur and displacement discrimination and the tilt and Poggendorff illusions.

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.

The spatial precision of macaque ganglion cell responses in relation to vernier acuity of human observers.

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.

The angular orientation of the movement detectors acting on the flight lift response in flies.

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.

Comparison of color sensation in dichoptic and in normal vision.

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 wavelengths enters one eye and a different band enters the other (dichoptic 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.

Temporal asynchrony interferes with vernier acuity.

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.

Motion perception in the peripheral visual field.

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.

Discrimination of direction of motion in human vision.

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.

How vernier acuity depends on contrast.

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 = (Lstim - Lsur)(Lstim + Lsur). Vernier thresholds are about 4-5 sec of arc for contrasts 0.22 and higher, but increase exponetially with decreasing contrast. 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.

The Craik-O'Brien-Cornsweet illusion in colour: quantitative characterisation and comparison with luminance.

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.

Real and virtual borders in the Poggendorff illusion.

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.

Neural circuits mediating visual flight control in flies. I. Quantitative comparison of neural and behavioral response characteristics.

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)

Neural circuits mediating visual flight control in flies. II. Separation of two control systems by microsurgical brain lesions.

The role of 2 sets of interneurons in the optic lobes of blowflies in visual course control was studied by means of brain lesions. The first set comprises the cells HS and H2, which respond to global horizontal motion. The second set are the FD-cells, which respond selectively to local horizontal motion. All these cells are output neurons of the third optic ganglion of flies and are thought to be coupled via descending neurons to the flight motor system. In 2 series of experiments specific cells of these 2 sets were inactivated by microsurgical brain lesions L1 and L2 respectively. The effects of the lesions on visual course control were tested by measuring the yaw torque responses of the animals in restrained flight before and after the operation. The flies were stimulated in these tests with monocular and binocular motion of periodic gratings moving in either the horizontal or the vertical direction. Lesion L1 in the right side of the brain inactivates the right HS-cells and the left H2- and FD-cells. This leads to a complete block of the response to binocular clockwise horizontal motion and a reduction of the response to monocular motion from front to back on the right side of the animal. Application of L1 also leads to a pronounced response to binocular motion from front to back not observed in normal animals. The response to monocular vertical motion is unaffected. Lesion L2 reduces all responses to monocular and binocular horizontal motion present in normal animals. The behavioral effects of the lesions are highly specific and consistent with predictions based on the well-known anatomical and physiological properties of the neural circuitry investigated. The results demonstrate directly that the HS-, H2-, and FD-cells control motion-induced steering maneuvers in flight.

ON- and OFF-pathways form separate neural substrates for motion perception: psychophysical evidence.

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.

Evidence for the contribution of S cones to the detection of flicker brightness and red-green.

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.

Neuronal responses from beyond the classic receptive field in V1 of alert monkeys.

Responses of primary visual cortex (V1) neurons to stimuli inside the classic receptive field (CRF) can be modulated by stimuli outside the CRF. We recently reported that responses of most V1 neurons to a line in the CRF center are inhibited by large surround-stimuli and that this modulation is stimulus selective. Here we report that a significant proportion of V1 neurons in alert monkeys respond directly to stimuli outside the CRF with very long latency and much reduced selectivity. When surround stimuli are presented alone, three response patterns can be distinguished in 153 single- or multiunits tested: (1) 31.4% have no significant response; (2) 50.3% show excitatory responses that are significantly higher than spontaneous activity. The average latency of these responses is about 145 ms, 2-3 times longer than center responses; (3) 18.3% show suppressed spontaneous activity after stimulus onset. The direct surround responses are found to be only weakly selective for the orientation of contextual lines, and not selective for other contextual patterns tested. While the outburst of responses to stimuli within the CRF is not affected by reducing stimulus duration from 500 ms to 50 ms, late excitatory surround responses are virtually eliminated. We propose that the late excitatory surround responses to extra-CRF stimulation alone are the reflection of feedback from higher cortical areas and may contribute to reduced contextual inhibition of cells in V1. This could play a role in figure-ground segregation.

Patterns that impair discrimination of line orientation in human vision.

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 VI.