Sensory processing delays measured with the eye-movement correlogram.
Explore the source record for details and available documents.
Acoustic Stimulation↗
Biomedical subjects
J B Mulligan
Publications and source records attributed to J B Mulligan.
Application of M-JPEG compression hardware to dynamic stimulus production.
Inexpensive circuit boards have appeared on the market which transform a normal micro-computer's disk drive into a video disk capable of playing extended video sequences in real time. This technology enables the performance of experiments which were previously impossible, or at least prohibitively expensive. The new technology achieves this capability using special-purpose hardware to compress and decompress individual video frames, enabling a video stream to be transferred over relatively low-bandwidth disk interfaces. This paper will describe the use of such devices for visual psychophysics and present the technical issues that must be considered when evaluating individual products.
Computer Storage Devices↗
Image processing for improved eye-tracking accuracy.
Video cameras provide a simple, noninvasive method for monitoring a subject's eye movements. An important concept is that of the resolution of the system, which is the smallest eye movement that can be reliably detected. While hardware systems are available that estimate direction of gaze in real-time from a video image of the pupil, such systems must limit image processing to attain real-time performance and are limited to a resolution of about 10 arc minutes. Two ways to improve resolution are discussed. The first is to improve the image processing algorithms that are used to derive an estimate. Off-line analysis of the data can improve resolution by at least one order of magnitude for images of the pupil. A second avenue by which to improve resolution is to increase the optical gain of the imaging setup (i.e., the amount of image motion produced by a given eye rotation). Ophthalmoscopic imaging of retinal blood vessels provides increased optical gain and improved immunity to small head movements but requires a highly sensitive camera. The large number of images involved in a typical experiment imposes great demands on the storage, handling, and processing of data. A major bottleneck had been the real-time digitization and storage of large amounts of video imagery, but recent developments in video compression hardware have made this problem tractable at a reasonable cost. Images of both the retina and the pupil can be analyzed successfully using a basic toolbox of image-processing routines (filtering, correlation, thresholding, etc.), which are, for the most part, well suited to implementation on vectorizing supercomputers.
Algorithms↗
The barberplaid illusion: plaid motion is biased by elongated apertures.
The perceived direction of motion of plaids windowed by elongated spatial Gaussians is biased toward the window's long axis. The bias increases as the relative angle between the plaid motion and the long axis of the window increases, peaks at a relative angle of approximately 45 deg, and then decreases. The bias increases as the window is made narrower (at fixed height) and decreases as the component spatial frequency increases (at fixed aperture size). We examine several models of human motion processing (cross-correlation, motion-energy, intersection-of-constraints, and vector-sum), and show that none of these standard models can predict our data. We conclude that spatial integration of motion signals plays a crucial role in plaid motion perception and that current models must be explicitly expanded to include such spatial interactions.
Adult↗
Nonlinear combination rules and the perception of visual motion transparency.
Experiments were performed to elucidate the decomposition performed by the human visual system in the segregation of complex motion stimuli into distinct moving surfaces. Subjects were presented with achromatic patterns consisting of four types of elements, generated from two random binary luminance patterns (random-dot checkerboards). The luminance of each of the four region classes was under program control. Animated sequences of such images were produced by displacing each of the two generating patterns in opposite directions on a frame by frame basis. These displays evoke a wide variety of percepts, depending on the programmed luminance values, including motion in a single direction, simultaneous motions of transparent sheets in opposite directions, dynamic noise with no directional component, or any combination of the above percepts. A theory is presented which relates the strengths of these percepts to the amplitudes of the components in the perceptual decomposition. The experiments described measured thresholds for seeing noise or "twinkling" in addition to the multiple motions, with the goal of determining the particular signal transformations preceding motion analysis. The results are consistent with a motion extraction mechanism which operates on a linear representation of the input imagery. These results extend a similar finding due to Anstis and Mather [(1985) Perception, 14, 167-179] and call into question the interpretation of a recent study by Stoner, Allbright and Ramachandran [(1990) Nature (London), 344, 153-155].
Contrast Sensitivity↗
Anisotropy in an ambiguous kinetic depth effect.
A set of animated stimuli (Lissajous figures), each element of which is physically consistent with two different three-dimensional shapes undergoing rigid rotations about orthogonal axes, is described. Human observers typically show a preference for one shape or the other; this preference may be biased by manipulating various parameters of the stimulus. Fairly good predictions of which shape will be seen are made by using an adaptation of Hildreth's smoothest-velocity-field computation. When a given stimulus is rotated 90 degrees in the picture plane, the resolution of the ambiguity is often different, demonstrating anisotropy in the processing of the figures. The nature of this bias is such that for certain figures subjects see a three-dimensional object rotating about a vertical axis regardless of which two-dimensional orientation is used to present the stimulus. This bias is not predicted by the Hildreth model. One interpretation of the results is that the ambiguity in two-dimensional visual motion (i.e., the aperture problem) is not resolved before the interpretation of the three-dimensional structure.
Anisotropy↗
Visual sensitivity to spatially sampled modulation in human observers.
Thresholds were measured for detecting spatial luminance modulation in regular lattices of visually discrete dots. Thresholds for modulation of a lattice are generally higher than the corresponding threshold for modulation of a continuous field, and the size of the threshold elevation, which depends on the spacing of the lattice elements, can be as large as a one log unit. The largest threshold elevations are seen when the sample spacing is 12 min arc or greater. These results are similar to those observed by Burr, Ross and Morrone [Vision Research, 25, 717-727 (1985)], who proposed an explanation based on a compressive point nonlinearity. Although their explanation is not consistent with the present data, the results may be explained in terms of nonlinear saturation of a spatially opponent stage early in the visual pathway. Theories based on response compression cannot explain the further observation that the threshold elevations due to spatial sampling are also dependent on modulation frequency: the greatest elevations occur with higher modulation frequencies. The idea that this is due to masking of the modulation frequency by the spatial frequencies in the sampling lattice is considered.
Contrast Sensitivity↗
Consent, compulsion and confidentiality in relation to testing for HIV infection: the views of WA doctors.
A survey was undertaken of all of the consultant staff members of Perth's major teaching hospitals together with all the fellows of The Royal Australian College of General Practitioners in Western Australia in order to define their views on the issues of informed consent, compulsion in relation to surgery, and confidentiality in a particular circumstance, when testing for infection with the human immunodeficiency virus (HIV). Of the 701 individuals surveyed, 548 (78.2%) responded. Of these, 74.3% considered that it was not always necessary to gain informed consent, 22.0% believed that it was always necessary to do so, while 2.3% were undecided. General practitioners (38.4%) were more likely to think it necessary to obtain consent than were hospital consultants (19.0%), but otherwise the field of specialty had little effect on opinion. Of the respondents, 39.0% believed that testing before elective surgery is mandatory for all patients, while 53.0% considered that it should be compulsory in high-risk groups. Similar views were held about compulsory HIV antibody testing after emergency surgery. Similar responses were obtained from all specialty groups. When asked about whether they would tell a sexual partner of a patient's HIV status when the patient refused, 10.5% of doctors stated they would never advise the partner, 24.7% of doctors would on some occasions, 41.0% of doctors would tell a partner, and 23.8% always were undecided. Many individuals commented that they failed to see why HIV infection was being treated differently from other serious diseases. We have found that the majority opinion of the most senior members of the medical profession is that specific, informed consent should not always be required, that there is great support for compulsory testing, and that confidentiality may be broken under certain circumstances. These views must be recognized by administrators and legislators when framing measures to control this infection.
Acquired Immunodeficiency Syndrome↗
Effect of contrast on the perceived direction of a moving plaid.
We performed a series of experiments examining the effect of contrast on the perception of moving plaids. This was done to test the hypothesis put forth by Adelson and Movshon (1982) that the human visual system determines the direction of a moving plaid in a two-staged process: decomposition into component motion followed by application of the intersection of constraints rule. Although there is recent evidence that the first tenet of their hypothesis is correct, i.e. that plaid motion is initially decomposed into the motion of the individual grating components (Movshon, Adelson, Gizzi & Newsome, 1986; Welch, 1989), the nature of the second-stage combination rule has not as yet been established. We found that when the gratings within the plaid are of different contrast, the perceived direction is not predicted by the intersection of constraints rule. There is a strong (up to 20 deg) bias in the direction of the higher-contrast grating. A revised model, which incorporates a contrast-dependent weighting of perceived grating speed as observed for 1-D patterns (Thompson, 1982), can quantitatively predict most of our results. We discuss our results in the context of various models of human visual motion processing and of physiological responses of neurons in the primate visual system.
Adolescent↗
Reciprocity between luminance and dot density in the perception of brightness.
Thresholds were measured for detecting perturbations in a regular lattice of dots by modulating local dot density, local dot luminance, or some combination of the two. For high mean densities (dot spacing less than or equal to 15 min of arc), perturbations in local density increase the perceived brightnesses of the individually resolved elements in the more densely filled regions, and appear (at near threshold levels) as modulations of brightness rather than density. This illusory brightness modulation may be nulled by applying a real luminance modulation to make the lattice elements appear equally bright. Once this is done, thresholds for detecting any nonuniformity in the array are elevated compared to thresholds for detecting uncompensated density modulation. This result suggests that uncompensated density modulation is detected via the illusory brightness variations. This interpretation suggests that dot brightness is determined on the basis of the space average luminance of an area a substantial fraction of 1 deg in diameter. To test this hypothesis, thresholds were measured for detecting luminance modulation in a regular array of dots viewed against a comparatively dim background, where the modulation was applied to the dots themselves, to the background alone, or to both the dots and the background in either reinforcing or cancelling relative phase. For small, closely spaced dots, the threshold for modulation of luminance can be predicted on the basis of the amplitude of the Fourier component at the modulation frequency, regardless of whether it is carried by dots, the background, or both. The threshold is greatly elevated when modulation in the dots cancels the background modulation, so that there is contrast modulation of the dots, but no net energy at the fundamental frequency (zero amplitude of the Fourier component). For large, coarsely spaced dots, on the other hand, thresholds for conditions which contain energy at the fundamental modulation frequency are higher. The threshold increase is much greater when the modulation is applied to the dots than when it is applied to the background. This result suggests that the coarsely spaced dots are saturating the response of spatially opponent units. This hypothesis was confirmed by tests using backgrounds with the same luminance as the dots; threshold elevations selective for dots or background were abolished.
Form Perception↗
Optimizing stereo separation in color television anaglyphs.
A method is described for minimizing the ghost images which normally appear when anaglyphs are presented on color television screens. This is done by careful adjustment of the phosphor levels in each of the anaglyph regions.
Color↗
Adaptation to polarized light in humans.
Recent investigators reported large adaptational effects in human vision dependent on the plane of linearly polarized light. In the present study, such adaptational effects were observed to be small or insignificant. Our observers adapted to intense homogenous, foveal fields of linearly polarized, orange-red light produced by a beam that entered the side of the pupil. Periodically the plane of polarization of the beam suddenly shifted 90 degrees without there being any change in the spectral distribution of the light or position of the beam in the pupil plane. The change of radiance of the beam could be adjusted to be as low as approx. 0.5%. Signal detection experiments revealed that observers typically could not detect a change of polarization of the field. Results obtained with the method of adjustment showed that the threshold of an increment flash presented on the intense field was not affected by changing the polarization of the field. Physiological implications of these results are discussed.
Adaptation, Ocular↗