Showing papers in "Journal of Vision in 2001"

Compensation of corneal aberrations by the internal optics in the human eye.

Abstract: The objective was to study the relative contribution of the optical aberrations of the cornea and the internal ocular optics (with the crystalline lens as the main component) to overall aberrations in the human eye. Three sets of wave-front aberration data were independently measured in the eyes of young subjects: for the anterior surface of the cornea, the complete eye, and internal ocular optics. The amount of aberration of both the cornea and internal optics was found to be larger than for the complete eye, indicating that the first surface of the cornea and internal optics partially compensate for each other's aberrations and produce an improved retinal image. This result has a number of practical implications. For example, it shows the limitation of corneal topography as a guide for new refractive procedures and provides a strong endorsement of the value of ocular wave-front sensing for those applications.

Photopigment basis for dichromatic color vision in the horse.

Abstract: Horses, like other ungulates, are active in the day, at dusk, dawn, and night; and, they have eyes designed to have both high sensitivity for vision in dim light and good visual acuity under higher light levels (Walls, 1942). Typically, daytime activity is associated with the presence of multiple cone classes and color-vision capacity (Jacobs, 1993). Previous studies in other ungulates, such as pigs, goats, cows, sheep and deer, have shown that they have two spectrally different cone types, and hence, at least the photopigment basis for dichromatic color vision (Neitz & Jacobs, 1989; Jacobs, Deegan II, Neitz, Murphy, Miller, & Marchinton, 1994; Jacobs, Deegan II, & Neitz, 1998). Here, electroretinogram flicker photometry was used to measure the spectral sensitivities of the cones in the domestic horse (Equus caballus). Two distinct spectral mechanisms were identified and are consistent with the presence of a short-wavelength-sensitive (S) and a middle-to-long-wavelength-sensitive (M/L) cone. The spectral sensitivity of the S cone was estimated to have a peak of 428 nm, while the M/L cone had a peak of 539 nm. These two cone types would provide the basis for dichromatic color vision consistent with recent results from behavioral testing of horses (Macuda & Timney, 1999; Macuda & Timney, 2000; Timney & Macuda, 2001). The spectral peak of the M/L cone photopigment measured here, in vivo, is similar to that obtained when the gene was sequenced, cloned, and expressed in vitro (Yokoyama & Radlwimmer, 1999). Of the ungulates that have been studied to date, all have the photopigment basis for dichromatic color vision; however, they differ considerably from one another in the spectral tuning of their cone pigments. These differences may represent adaptations to the different visual requirements of different species.

Driving in the future: temporal visuomotor adaptation and generalization.

Abstract: Rapid and accurate visuomotor coordination requires tight spatial and temporal sensorimotor synchronization. The introduction of a sensorimotor or intersensory misalignment (either spatial or temporal) impairs performance on most tasks. For more than a century, it has been known that a few minutes of exposure to a spatial misalignment can induce a recalibration of sensorimotor spatial relationships, a phenomenon that may be referred to as spatial visuomotor adaptation. Here, we use a high-fidelity driving simulator to demonstrate that the sensorimotor system can adapt to temporal misalignments on very complex tasks, a phenomenon that we refer to as temporal visuomotor adaptation. We demonstrate that adapting on a single street produces an adaptive state that generalizes to other streets. This shows that temporal visuomotor adaptation is not specific to a single visuomotor transformation, but generalizes across a class of transformations. Temporal visuomotor adaptation is strikingly parallel to spatial visuomotor adaptation, and has strong implications for the understanding of visuomotor coordination and intersensory integration. Language: en

Surround modulation of perceived contrast and the role of brightness induction.

Abstract: We studied iso- and cross-orientation surround modulation of perceived contrast (contrast-contrast phenomenon) with a contrast-matching method. Our results indicate (1) iso-oriented surrounds at all contrasts suppress perceived contrast of the test pattern. Cross-orientation surrounds, however, tend to enhance the perceived contrast of the test, particularly for high-contrast test patterns. Iso-orientation modulation acts over larger distances than does cross-orientation modulation. Surround modulation of perceived contrast is not accompanied by a simultaneous change of discrimination threshold. (2) Iso-orientation surround suppression is phase insensitive when brightness induction due to local luminance contrast is eliminated by a small center-surround gap. (3) Perceived contrast is similarly affected when the surround spatial frequency is equal to or higher than the center spatial frequency, but lower spatial frequency surrounds markedly enhance perceived contrast as a result of brightness induction. These data indicate that the contrast-contrast phenomenon is often mixed with brightness induction when it is measured with sinusoidal grating stimuli, and we suggest that this may account for some of the individual differences. After excluding the role of brightness induction, surround modulation of perceived contrast appears to be a second-order process that is phase independent and not tuned or very broadly tuned to spatial frequency.

Rod-cone interactions assessed in inferred magnocellular and parvocellular postreceptoral pathways.

Abstract: Interactions between receptor-isolating rod and long (L)- or middle (M)-wavelength-sensitive cone modulations at 2 Hz and 10 Hz were analyzed in terms of underlying inferred magnocellular (MC) and parvocellular (PC) postreceptoral pathways. Stimuli originated from a colorimeter with 4 primaries in both the center and surround fields. The first experiment employed a phase paradigm in which the thresholds for mixed rod and cone modulations were measured as a function of relative phase. The amplitudes of the rod and cone modulations, equated in threshold units, were varied in tandem. In the second experiment, thresholds for mixed rod and cone modulations were measured as a function of the ratio of the rod and cone modulation amplitudes for 2 fixed phase offsets. Both experiments yielded similar interpretations of rod and L- (or M-) cone interactions. At 1 and 10 troland (td), rod and L- (or M-) cone interactions varied depending on the postreceptoral pathways underlying the detection. When cone thresholds were mediated by the inferred MC pathway, rod and cone thresholds showed almost linear summation. When cone thresholds were mediated by the inferred PC pathway, rod and cone thresholds showed probability summation. Assuming that signals within the same pathway follow linear summation, and signals traveling in different pathways follow probability summation, we concluded that the rod thresholds were mediated by the inferred MC pathway for both the 2-Hz and 10-Hz conditions.

Local and global visual grouping: tuning for spatial frequency and contrast.

Abstract: Glass patterns are visual textures composed of a field of dot pairs (dipoles) whose orientations are determined by a simple geometrical transformation, such as a rotation. Detection of structure in these patterns requires the observer to perform local grouping (to find dipoles) and global grouping to combine their orientations into a percept of overall shape. We estimated the spatial frequency tuning of these grouping processes by measuring signal-to-noise detection thresholds for Glass patterns composed of spatially narrow-band elements. Local tuning was probed by varying the spatial frequency difference between the two elements comprising each dipole. Global tuning was estimated using dipoles containing one spatial frequency and then estimating masking as a function of the spatial frequency of randomly positioned noise elements. We report that the tuning of local grouping is band-pass (ie, it is responsive to a narrow range of spatial frequencies), but that tuning of global grouping is broad and low-pass (ie, it integrates across a broader range of lower spatial frequencies). Control experiments examined how the contrast and visibility of elements might contribute to these findings. Local grouping proved to be more resistant to local contrast variation than global grouping. We conclude that local grouping is consistent with the use of simple-oriented filtering mechanisms. Global grouping seems to depend more on the visibility of elements that can be affected by both spatial frequency and contrast.

Perceiving slant about a horizontal axis from stereopsis.

Abstract: Rotating a surface about a horizontal axis alters the retinal horizontal-shear disparities. Opposed torsional eye movements (cyclovergence) also change horizontal shear. If there were no compensation for the horizontal disparities created by cyclovergence, slant estimates would be erroneous. We asked whether compensation for cyclovergence occurs, and, if it does, whether it occurs by use of an extraretinal cyclovergence signal, by use of vertical-shear disparities, or by use of both signals. In four experiments, we found that compensation is nearly veridical when verticalshear disparities are available and easily measured. When they are not available or easily measured, no compensation occurs. Thus, the visual system does not seem to use an extraretinal cyclovergence signal in stereoscopic slant estimation. We also looked for evidence of an extraretinal cyclovergence signal in a visual direction task and found none. We calculated the statistical reliabilities of slant-from-disparity and slant-from-texture estimates and found that the more reliable of the two means of estimation varies significantly with distance and slant. Finally, we examined how slant about a horizontal axis might be estimated when the eyes look eccentrically.

Detecting faces in impoverished images

Abstract: The ability to detect faces in images is of critical ecological significance. It is a pre-requisite for other important face perception tasks such as person identification, gender classification and affect analysis. Here we address the question of how the visual system classifies images into face and non-face patterns. We focus on face detection in impoverished images, which allow us to explore information thresholds required for different levels of performance. Our experimental results provide lower bounds on image resolution needed for reliable discrimination between face and non-face patterns and help characterize the nature of facial representations used by the visual system under degraded viewing conditions. Specifically, they enable an evaluation of the contribution of luminance contrast, image orientation and local context on face-detection performance. Research reported in this paper was supported in part by funds from the Defense Advanced Research Projects Agency and a Sloan fellowship for neuroscience to PS.

The flash-lag effect as a spatiotemporal correlation structure.

Abstract: The flash-lag effect refers to the phenomenon in which a flash adjacent to a continuously moving object is perceived to lag behind it. Phenomenally, the flash appears to be spatially shifted relative to the moving stimulus, and the amount of lag has often been quantified as the flash's nulling position, which is the physical spatial offset needed to establish perceptual alignment. The present study offers a better way to summarize flash-lag data. Instead of plotting data in terms of space, the psychometric function of the observer's relative-position judgment is drawn on spatiotemporal plot. The psychological process underlying illusory lag is formulated as spatiotemporal bias and uncertainty and their estimate as a spatiotemporal convolution kernel that best explains the spatiotemporal psychometric function. Two empirical procedures of kernel estimation are described. One procedure is to fit the free parameters of the kernel to experimental data for continuous motion trajectory. The second is to give an analytical solution to the kernel using experimental data for random motion trajectory. The two procedures yield similar kernels, with negligible spatial bias and uncertainty and substantial temporal bias and uncertainty. In addition, it is demonstrated that an experimental manipulation of temporal predictability of the flash can change the temporal bias in the estimated kernel. The results of this novel analysis reveal that the flash-lag effect is viewed as a spatiotemporal correlation structure, which is largely characterized by the tendency to compare the position of the flash in the past with the position of the moving item in the present.

Brightness induction from rods.

Abstract: Rod modulation of an annular surround can produce brightness contrast in a test field centered at 100 from the fovea. In our research, stimuli originated from a colorimeter that provided 4 primaries in both the circular test and the annular surround fields, and allowed independent modulation of the rods and each of the short (S)-, middle (M)-, and long (L)-wavelength-sensitive cone types. The chromaticity was set so fields had the same appearance as the equal energy spectrum. At 1 photopic troland (td), rod-induced modulation in the test field could be cancelled by either a rod- or a cone-nulling modulation added to the test field. The best cone nulling of rod induction showed residual flicker. Nulling was more effective, though still imperfect, with a cone-nulling stimulus of higher S-cone modulation contrast. Rod induction with square-wave, on-pulse, and off-pulse temporal profiles was closely similar. At higher light levels, 10 and 100 td, rod contrast could not be nulled by rod or cone modulation. The failure to achieve nulls may have been caused by either or both of the following hypotheses: (1) there is a mismatch between the rod and cone temporal waveforms; (2) there is strong rod input to the magnocellular pathway, but negligible rod input to the parvocellular pathway, as shown by single-unit electrophysiological data.

Odd-men-out are poorly localized in brief exposures.

Abstract: Signal detection theory (SDT) asserts that sensory analysis is limited only by noise, and not by the number of stimuli analysed. To test this claim, we measured the accuracy of visual search for a single tilted element (the target) among 7 horizontal elements (distractors) using several different exposure durations, each terminated by a random noise mask. In the uncued condition, each element was a potential target. In the cued condition only 2 were. SDT predicts that location errors should be evenly distributed among all distractors. For long exposures (eg, 5.0 seconds), this prediction was confirmed, and SDT could simultaneously fit uncued and cued accuracies. For short exposures (eg, 0.1 seconds), errors were concentrated among distractors adjacent to the target, and, unless modified to account for this, SDT underestimated the difference between uncued and cued accuracies. Therefore, when the time available for search is brief, odd-men-out (ie, featural discontinuities) can be seen, but their positions can be only roughly estimated.

Asymmetries in contrast polarity processing in young human infants.

Abstract: Luminance increments and decrements of equal magnitude are processed asymmetrically in the adult visual system. At detection threshold, decrements are slightly easier to detect than increments. At suprathreshold contrast levels decrements appear to have more contrast than increments when both differ from the background by the same absolute amount. Two experiments are reported with 3.5-month-old human infants examining the processing of luminance increments and decrements. Using two different methods to measure the relative salience of positive and negative polarity high contrast bars, we found consistent evidence that dark bars appeared more salient to infants than light bars when both differed from the background by the same absolute amount. The asymmetry may be explained by noting that when luminance increments and decrements have the same Weber contrast, the decrements will have greater Michelson contrast. Perceived contrast in adults follows Michelson contrast more closely than Weber contrast, and a similar metric may characterize the relations between negative and positive contrasts in young human infants.

The symmetry magnification function varies with detection task.

Abstract: Detection of the presence of bilateral symmetry was investigated at various retinal eccentricities for static and dynamic noise reflected around a vertical axis. At a low detection criterion (60% correct), peak duration sensitivities were high and varied little (<0.2 log units) from 0 masculine eccentricity to 10 masculine eccentricity for either static or dynamic targets. Duration thresholds for symmetry in dynamic noise fields were significantly higher (about 100 ms) than those for static symmetry detection (about 40 ms), despite the fact that the information was refreshed many times during the threshold presentation period. The spatial summation width for symmetry processing was evaluated with randomization around the axis of symmetry. The estimated summation width for static symmetry detection was approximately constant with eccentricity for short duration stimuli. For long duration stimuli, the summation width was substantially greater in central vision but decreased with eccentricity, the first known visual function to exhibit such reverse magnification behavior (Tyler, 1999). These findings suggest that static and dynamic symmetry detection are supported by different neural mechanisms and that these mechanisms are relatively invariant across the retina, unlike known mechanisms of spatial processing.