Engineering topics

Mulligan, Jeffrey B.

Publications and source records attributed to Mulligan, Jeffrey B..

54 records · Page 3

NASA NTRS1996

Speed Judgements of Transparent Stimuli

When two moving patterns are combined additively, observers often perceive two transparent surfaces, even when there are no cues supporting this segmentation in a frozen snapshot. We examined the ability of observers to make quantitative judgments about the speed of one of the patterns under these conditions. The component patterns consisted of band-pass filtered random noise presented in a spatial Gaussian contrast envelope, presented for 250 ms. On each trial a standard pattern appeared on one side of the fixation point, while a test pattern appeared on the other. The test pattern moved in the same direction as the standard, but with a speed which varied from trial to trial using a staircase procedure. The subjects' task was to report the side of the fixation point on which faster motion was seen. In some conditions the test stimulus was made to appear transparent by adding a mask pattern. When the mask was stationary, or moved slowly with respect to the test, no significant biases were introduced and discrimination performance was comparable to the no-mask condition (typically 3%). If the mask moved over the test with similar speed, however, the task became much harder, regardless of whether the mask moved opposite or orthogonal to the test. (Some subjects commented on a perceived directional repulsion between tests and orthogonally moving masks.) These results suggest the use of non-directional temporal channels in the performance of the speed discrimination task.

Mulligan, Jeffrey B.↗

NASA NTRS1995

Analysis of the Trial-By-Trial Correlation Between Eye Movement and Perceptual Responses to Moving Plaids

We have previously shown that windowing a drifting plaid with an asymmetric spatial Gaussian produces systematic biases of similar magnitude in both the perceived direction of motion and the direction of the eye movement response. To further investigate this, we simultaneously measured the pyschophysical and eye-movement responses (with an ISCAN RK426 IR tracker)to drifting plaids in a direction discrimination task. Three observers were instructed to track a plaid (Type I 90 deg.; TF = 4 Hz, SF = 0.6 c/d; windowed by a circular spatial gaussian) and to respond whether the motion was to the right or left of pure vertical. Three plaid directions were presented (-2, 0, 2 deg. with respect to straight down), so there was uncertainty in the perceptual judgements. This enabled us to examine the trial-by-trial relationship between the eye movements and psychophysical responses. The eye-movement direction was computed to be the slope of the best fitting line to the initial 300 ms of saccade-free tracking (near open-loop). Again, we found the mean psychophysical and oculomotor responses were correlated. The mean percentages of rightward responses were 6, 27, 87%, and the mean normalized eye-movement directions were -1.5, 0, and 1.0 deg. for the -2, 0, and 2 deg. stimuli respectively. We also computed the trial-by-trial correlation by performing an SOC analysis. The mean correlations were 0.52, 0.66, and 0.67 for our three observers. Because of eye-tracker measurement noise, the above results are lower limits on the actual correlation. These results support the view that the oculomotor and perceptual system share common motion inputs.

Beutter, Brent R.↗

NASA NTRS1995

The Barberplaid illusion: Even Perceived Plaid Motion is Biased by Asymmetric Apertures

Our results show that the perceived direction of motion of plaids windowed by asymmetric spatial Gaussians is biased toward the long axis of the window. The bias increases as the relative angle between the plaid motion and the window increases, peaks at a relative angle of about 40 degrees, and then decreases. The peak bias was 14 degrees for a spatial frequency of 0.6 cpd and a window aspect ratio of 4.0. The biases increase as the window is elongated and decrease as the component spatial frequency increases. We tested the predictions of several models of human motion processing (cross correlation, motion energy, intersection of constraints, and vector sum), and show that none of these can predict our data. These results suggest that spatial integration of motion signals plays a crucial role in the perception of plaid motion.

Beutter, Brent↗

NASA NTRS1995

Reflexive OKN is Biased Like Perception

The classic "barber-pole illusion" demonstrates how the perceived motion of an ambiguous stimulus is influenced by the shape of the viewing aperture: the motion is preferentially seen in the direction of the long axis of the aperture. We have previously reported (ARVO 94) that we are able to continuously vary the strength of this effect by varying the aspect ratio of the window. In the present study we examined the motion of the eyes while subjects viewed such stimuli in an attempt to discover whether the eye movement control system performs the same computation underlying the perceptual judgments.

Mulligan, Jeffrey B.↗

NASA NTRS1995

Temporal MTF of Motion Perception

Although human motion sensing mechanisms can be stimulated by rapid changes in the visual input the response of the motion mechanisms themselves is rather sluggish, i.e. they cannot track rapid changes in the speed or direction of a moving stimulus when there are no positional cues to the variation. We tested the ability of human observers to detect sinusoidal perturbations in the velocity of a moving stimulus at a variety of temporal frequencies and drift velocities; the results show a low-pass temporal characteristic, and performance is degraded at high velocities.

Mulligan, Jeffrey B.↗

NASA NTRS1994

In Search of an Optoretinogram

A sensitive apparatus was constructed to measure the infrared reflectance of human retinas in vivo. Changes in the light scattering properties of rod outer segments have been observed both in excised retinas and photoreceptor suspensions, motivating the present work. Unlike results of vitro studies, no visually evoked changes were observed, despite a very low noise level. Methods for improving the technique using modem developments in CCD cameras are discussed.

Mulligan, Jeffrey B.↗

NASA NTRS1994

Eye-Movement Tracking Using Compressed Video Images

Infrared video cameras offer a simple noninvasive way to measure the position of the eyes using relatively inexpensive equipment. Several commercial systems are available which use special hardware to localize features in the image in real time, but the constraint of realtime performance limits the complexity of the applicable algorithms. In order to get better resolution and accuracy, we have used off-line processing to apply more sophisticated algorithms to the images. In this case, a major technical challenge is the real-time acquisition and storage of the video images. This has been solved using a strictly digital approach, exploiting the burgeoning field of hardware video compression. In this paper we describe the algorithms we have developed for tracking the movements of the eyes in video images, and present experimental results showing how the accuracy is affected by the degree of video compression.

Mulligan, Jeffrey B.↗

NASA NTRS1992

Anisotropy in an ambiguous kinetic depth effect

A set of animated stimuli (Lissajous figures), each element of which is physically consistent with two different 3D shapes undergoing rigid rotations about orthogonal axes is presented. Predictions of which shape will be seen are made by utilizing an adaptation of Hildreth's smoothest-velocity-field computation. Results indicate that the ambiquity in 2D visual motion, i.e., the aperture problem, is not resolved before the interpretation of the 3D structure.

Mulligan, Jeffrey B.↗

NASA NTRS1992

Principled halftoning based on human vision models

When models of human vision adequately measure the relative quality of candidate halftonings of an image, the problem of halftoning the image becomes equivalent to the search problem of finding a halftone that optimizes the quality metric. Because of the vast number of possible halftones, and the complexity of image quality measures, this principled approach has usually been put aside in favor of fast algorithms that seem to perform well. We find that the principled approach can lead to a range of useful halftoning algorithms, as we trade off speed for quality by varying the complexity of the quality measure and the thoroughness of the search. High quality halftones can be obtained reasonably quickly, for example, by using as a measure the vector length of the error image filtered by a contrast sensitivity function, and, as the search procedure, the sequential adjustment of individual pixels to improve the quality measure. If computational resources permit, simulated annealing can find nearly optimal solutions.

Mulligan, Jeffrey B.↗

NASA NTRS1991

Half-Tone Video Images Of Drifting Sinusoidal Gratings

Digital technique for generation of slowly moving video image of sinusoidal grating avoids difficulty of transferring full image data from disk storage to image memory at conventional frame rates. Depends partly on trigonometric identity by which moving sinusoidal grating decomposed into two stationary patterns spatially and temporally modulated in quadrature. Makes motion appear smooth, even at speeds much less than one-tenth picture element per frame period. Applicable to digital video system in which image memory consists of at least 2 bits per picture element, and final brightness of picture element determined by contents of "lookup-table" memory programmed anew each frame period and indexed by coordinates of each picture element.

Mulligan, Jeffrey B.↗

NASA NTRS1991

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. 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.

Mulligan, Jeffrey B.↗

NASA NTRS1991

Network compensation for missing sensors

A network learning translation invariance algorithm to compute interpolation functions is presented. This algorithm with one fixed receptive field can construct a linear transformation compensating for gain changes, sensor position jitter, and sensor loss when there are enough remaining sensors to adequately sample the input images. However, when the images are undersampled and complete compensation is not possible, the algorithm need to be modified. For moderate sensor losses, the algorithm works if the transformation weight adjustment is restricted to the weights to output units affected by the loss.

Ahumada, Albert J., Jr.↗

NASA NTRS1990

Vision Science and Technology at NASA: Results of a Workshop

A broad review is given of vision science and technology within NASA. The subject is defined and its applications in both NASA and the nation at large are noted. A survey of current NASA efforts is given, noting strengths and weaknesses of the NASA program.

Watson, Andrew B.↗

NASA NTRS1990

Factors affecting the perception of transparent motion

It is possible to create a perception of transparency by combining patterns having different motions. Two particular combination rules, have specific interpretations in terms of physical phenomena: additive (specular reflection) and multiplicative (shadow illumination). Arbitrary combination rules applied to random patterns generate percepts in which the motions of the two patterns are visible, but have super-imposed noise. It is also possible to combine the patterns (using an exclusive-OR rule) so that only noise is visible. Within a one-dimensional family of combination rules which include addition and multiplication, there is a range where smooth motions are seen with no superimposed noise; this range is centered about the additive combination. This result suggests that the motion system deals with a linear representation of luminance, and is consistent with the analysis of motion by linear sensors. This research gives tentative validation the use in beam splitters (which combine images additively) in the construction of heads-up aviation displays. Further work is needed to determine if the superiority of additive combination generalizes to the case of full-color imagery (there are results in the literature suggesting that subtractive color mixture yields the best legibility of overlapping alphanumerics).

Mulligan, Jeffrey B.↗

NASA NTRS1990

Learning receptor positions from imperfectly known motions

An algorithm is described for learning image interpolation functions for sensor arrays whose sensor positions are somewhat disordered. The learning is based on failures of translation invariance, so it does not require knowledge of the images being presented to the visual system. Previously reported implementations of the method assumed the visual system to have precise knowledge of the translations. It is demonstrated that translation estimates computed from the imperfectly interpolated images can have enough accuracy to allow the learning process to converge to a correct interpolation.

Ahumada, Albert J., Jr.↗

NASA NTRS1990

Digital halftoning methods for selectively partitioning error into achromatic and chromatic channels

A method is described for reducing the visibility of artifacts arising in the display of quantized color images on CRT displays. The method is based on the differential spatial sensitivity of the human visual system to chromatic and achromatic modulations. Because the visual system has the highest spatial and temporal acuity for the luminance component of an image, a technique which will reduce luminance artifacts at the expense of introducing high-frequency chromatic errors is sought. A method based on controlling the correlations between the quantization errors in the individual phosphor images is explored. The luminance component is greatest when the phosphor errors are positively correlated, and is minimized when the phosphor errors are negatively correlated. The greatest effect of the correlation is obtained when the intensity quantization step sizes of the individual phosphors have equal luminances. For the ordered dither algorithm, a version of the method can be implemented by simply inverting the matrix of thresholds for one of the color components.

Mulligan, Jeffrey B.↗

NASA NTRS1989

Halftoning method for the generation of motion stimuli

This paper describes a novel computer-graphic technique for the generation of a broad class of motion stimuli for vision research, which uses color table animation in conjunction with a single base image. Using this technique, contrast and temporal frequency can be varied with a negligible amount of computation, once a single-base image is produced. Since only two-bit planes are needed to display a single drifting grating, an eight-bit/pixel display can be used to generate four-component plaids, in which each component of the plaid has independently programmable contrast and temporal frequency. Because the contrast and temporal frequencies of the various components are mutually independent, a large number of two-dimensional stimulus motions can be produced from a single image file.

Mulligan, Jeffrey B.↗

NASA NTRS1988

Efficient use of bit planes in the generation of motion stimuli

The production of animated motion sequences on computer-controlled display systems presents a technical problem because large images cannot be transferred from disk storage to image memory at conventional frame rates. A technique is described in which a single base image can be used to generate a broad class of motion stimuli without the need for such memory transfers. This technique was applied to the generation of drifting sine-wave gratings (and by extension, sine wave plaids). For each drifting grating, sine and cosine spatial phase components are first reduced to 1 bit/pixel using a digital halftoning technique. The resulting pairs of 1-bit images are then loaded into pairs of bit planes of the display memory. To animate the patterns, the display hardware's color lookup table is modified on a frame-by-frame basis; for each frame the lookup table is set to display a weighted sum of the spatial sine and cosine phase components. Because the contrasts and temporal frequencies of the various components are mutually independent in each frame, the sine and cosine components can be counterphase modulated in temporal quadrature, yielding a single drifting grating. Using additional bit planes, multiple drifting gratings can be combined to form sine-wave plaid patterns. A large number of resultant plaid motions can be produced from a single image file because the temporal frequencies of all the components can be varied independently. For a graphics device having 8 bits/pixel, up to four drifting gratings may be combined, each having independently variable contrast and speed.

Mulligan, Jeffrey B.↗