Second-order motion discrimination by feature-tracking.
(9/1103)
When a plaid pattern (the sum of two high spatial frequency gratings oriented +/- 84 degrees from vertical) jumps horizontally by 3/8 of its spatial period its contrast envelope, a second-order pattern, moves in the opposite direction to its luminance waveform. Observers report that the pattern moves in the direction of the contrast envelope when the jumps are repeated at intervals of more than 125 ms and in the direction of the luminance profile when they are repeated at longer intervals. When a pedestal [Lu, Z.-L. & Sperling, G. (1995). Vision Research, 35, 2697-2722] is added to the moving plaid a higher contrast is required to see motion of the contrast envelope but not to see the motion of the luminance profile, suggesting that the motion of the contrast envelope is sensed by a mechanism that tracks features. Static plaids with different spatial parameters from the moving pattern are less effective at raising the contrast required to see the motion of the contrast envelope and simple gratings of low or high spatial frequency are almost completely ineffective, suggesting that the feature-tracking mechanism is selective for the type of pattern being tracked and rejects distortion products and zero-crossings. (+info)
Simultaneous color constancy: how surface color perception varies with the illuminant.
(10/1103)
In two experiments simultaneous color constancy was measured using simulations of illuminated surfaces presented on a CRT monitor. Subjects saw two identical Mondrians side-by-side: one Mondrian rendered under a standard illuminant, the other rendered under one of several test illuminants. The matching field was adjusted under the test illuminant so that it (a) had the same hue, saturation, and brightness (appearance match) or (b) looked as if it were cut from the same piece of paper (surface match) as a test surface under the standard illuminant. Matches were set for three different surface collections. The surface matches showed a much higher level of constancy than the appearance matches. The adjustment in the surface matches was nearly complete in the L and M cone data, and deviations from perfect constancy were mainly due to failures in the adjustment of the S cone signals. Besides this difference in amount of adjustment, the appearance and surface matches showed two major similarities. First, both types of matches were well described by simple parametric models. In particular, a model based on the notion of von Kries adjustment provided a good, although not perfect, description of the data. Second, for both types of matches the illuminant adjustment was largely independent of the surface collection in the image. The two types of matches thus differed only quantitatively, there was no qualitative difference between them. (+info)
Motion-transparent inducers have different effects on induced motion and motion capture.
(11/1103)
To assess the relationship among the underlying mechanisms of induced motion, motion capture, and motion transparency, directions of the former two illusions in the presence of motion-transparent inducers were examined. Two random-dot patterns (inducers) were superimposed upon a stationary disk (target), and moved in orthogonal directions. Either a high-contrast target (for induced motion) or a low-contrast target (for motion capture) was used. The task was to report the perceived direction of the target. The depth order of inducers was controlled either by adding binocular disparity or by asking the subject to report subjective depth order. For induced motion, the target appeared to move in the direction opposite to the inducer that had a disparity closer to the target; when there was no difference in disparity, induced motion occurred oppositely to the 'vector sum' of the inducers' directions. For motion capture, the target was captured by the inducer that subjectively appeared behind. These results suggest that the underlying mechanism of motion capture utilizes the output from the process for motion transparency, whereas induced motion has no clear relationship to the output of the process for motion transparency. (+info)
Comparison of color and luminance contrast: apples versus oranges?
(12/1103)
Using a spatial, forced-choice, matching protocol, we have measured observers' ability to equate the contrasts of sinusoidal gratings which vary along differing directions in a 3-dimensional color space. In a given experiment, the observer obtained a perceptual match between the contrasts of two gratings whose chromaticities or luminances varied along differing chromatic directions which were selected from among five axes: an achromatic luminance axis (lum), an isoluminant axis where only S-cone activation varied (S-axis), an isoluminant axis where L- and M-cone activation varied in a complementary manner (LM-axis), an axis where only L-cone activation varied (L-axis), and an axis where only M-cone activation varied (M-axis). Even though these chromatic axes were chosen to activate independent mechanisms involved in the early stages of spatiochromatic visual processing, and despite the distinctly differing appearance of patterns from variations along differing directions, we find that observers can reliably make such pairwise contrast matches. Furthermore there is reasonable consistency of matching contrasts among observers and the pairwise contrast matches exhibit the properties of homogeneity and transitivity. This observed homogeneity and transitivity allows, for each color direction, the specification of a single scaling factor which relates perceptual contrast to physical contrast. (+info)
Spatial sensitization of increments and decrements: a border-contrast process and a net-excitation process.
(13/1103)
We investigated the spatially local factors that adjust the sensitivity of the human visual system within a small patch of visual space. A very small adapting field was varied in diameter to map out the strength and extent of the spatially local processes that adjust sensitivity for both increments and decrements. The results demonstrated antagonistic center/surround adaptation regions with a decremental test probe comparable to those demonstrated previously for incremental probes (Westheimer, G., 1965. Spatial interaction in the human retina during scotopic vision, Journal of Physiology 81, 812-894; Westheimer, G., 1967. Spatial interaction in human cone vision, Journal of Physiology 190, 139-154) implying comparable antagonistic regions in the ON and OFF channels. In addition to spatial interactions based on light adaptation, we report a weaker effect that is based on the location of a border (luminance edge) and is governed by the contrast of this edge. Finally, we show that these effects are elicited by both highly localized edges (1' ring pairs) and radial lines (Ehrenstein figure) as well. We conclude that both a border-contrast mechanism and a net-excitation mechanism govern the spatially local adaptation of the visual system and that this view fits well with the behavior of single units reported previously. (+info)
Activation of NMDA receptors in the suprachiasmatic nucleus produces light-like phase shifts of the circadian clock in vivo.
(14/1103)
Although there is substantial evidence that glutamate mimics the effects of light on the mammalian circadian clock in vitro, it has been reported that microinjection of glutamate into the suprachiasmatic nucleus of the hypothalamus (SCN) region in vivo does not result in a pattern of phase shifts that mimic those caused by light pulses. The present study was designed to test the hypothesis that microinjection of NMDA into the SCN would induce light-like phase shifts of the circadian clock through activation of the NMDA receptor. Hamsters housed in constant darkness received microinjections of NMDA through guide cannulas aimed at the SCN region at various times throughout the circadian cycle. Wheel running was monitored as a measure of circadian phase. Microinjection of NMDA resulted in circadian phase shifts, the size and direction of which were dependent on the time of injection. The resulting phase-response curve closely resembled that of light. The circadian response showed a clear dose-dependence at circadian time (CT) 13.5 but not at CT19. Both phase delays and advances induced by NMDA were blocked by coinjection of the NMDA antagonist 2-amino-5-phosphopentanoic acid but were slightly attenuated by the non-NMDA antagonist 6-nitro-7-sulfamoylbenzo[f]quinoxaline-2,3-dione disodium. The ability of NMDA to induce phase shifts was not altered by coinjection with tetrodotoxin. These data are consistent with the hypothesis that activation of NMDA receptors is a critical step in the transmission of photic information to the SCN. (+info)
Modelling spatial contrast sensitivity functions for chromatic and luminance-modulated gratings.
(15/1103)
We extended our detection model of achromatic spatial vision (Rovamo, J., Mustonen, J., & Nasanen, R. (1994a). Modelling contrast sensitivity as a function of retinal illuminance and grating area. Vision Research, 34, 1301-1314) to colour vision by taking into account the fact that due to the spatio-chromatic opponency of retinal ganglion cells and dorsal lateral geniculate nucleus (dLGN) neurons, equiluminous chromatic gratings are not affected by precortical lateral inhibition. We then tested the extended model by using Mullen's experimental data (Mullen, K. J. (1985). The contrast sensitivity of human color vision to red-green and blue-yellow chromatic gratings. Journal of Physiology, 359, 381-400). The band-pass shape of the spatial contrast sensitivity function for luminance-modulated green and yellow gratings transformed to a low-pass shape, resembling the chromatic spatial contrast sensitivity function for red-green and blue-yellow equiluminous gratings, when the effect of precortical lateral inhibition on grating contrast was computationally removed by dividing luminance contrast sensitivities by spatial frequency (i.e. by af, where a = 1 degree). After the removal of this direct effect of lateral inhibition, there still remained a residual shape difference between the spatial contrast sensitivity functions for chromatic and luminance gratings. It was due to indirect reduction of grating visibility by quantal noise high-pass filtered by precortical lateral inhibition. When this indirect effect of quantal noise was also removed, contrast sensitivity for luminance gratings was about twice the sensitivity for chromatic gratings at all spatial frequencies. This was evidently due to the fact that the chromatic contrast of the equiluminous grating at the opponent stage (Cole, G. R., Hine, T. & McIihagga, W. (1993). Detection mechanisms in L-, M-, and S-cone contrast space. Journal of the Optical Society of America A, 10, 38-51) was about half of the luminance contrast of either of its chromatic component. Thus, if the contrast of the equiluminous chromatic grating were not expressed as the Michelson contrast of one chromatic component grating against its own background (Mullen, K. J. (1985). The contrast sensitivity of human color vision to red-green and blue-yellow chromatic gratings. Journal of Physiology, 359, 381-400) but as chromatic contrast at the opponent stage, contrast sensitivity would be the same for chromatic and luminance gratings. (+info)
A possible effect of different light sources on pregnancy rates following gamete intra-fallopian transfer.
(16/1103)
A retrospective study of 34 sequential gamete intra-Fallopian transfer (GIFT) procedures suggested a significant effect on pregnancy rates associated with the different laparoscopic light sources, with a pregnancy rate of 50% in 22 cycles using a halogen light source and 9% in 12 cycles using a xenon light source. Other explanatory variables were explored, but none was to have a significant effect on the pregnancy rate. Further investigation revealed that the xenon light source emitted more ultraviolet light than the conventional halogen light source--suggesting a possible detrimental effect of ultraviolet light on the gametes in the GIFT procedure. (+info)