Lawrence K. Cormack - Academia.edu (original) (raw)

Papers by Lawrence K. Cormack

Journal of Vision

Binocular stereo cues are important for discriminating 3D surface orientation, especially at near... more Binocular stereo cues are important for discriminating 3D surface orientation, especially at near distances. We devised a single-interval task where observers discriminated the slant of a densely textured planar test surface relative to a textured planar surround reference surface. Although surfaces were rendered with correct perspective, the stimuli were designed so that the binocular cues dominated performance. Slant discrimination performance was measured as a function of the reference slant and the level of uncorrelated white noise added to the test-plane images in the left and right eyes. We compared human performance with an approximate ideal observer (planar matching [PM]) and two subideal observers. The PM observer uses the image in one eye and back projection to predict a test image in the other eye for all possible slants, tilts, and distances. The estimated slant, tilt, and distance are determined by the prediction that most closely matches the measured image in the other eye. The first subideal observer (local planar matching [LPM]) applies PM over local neighborhoods and then pools estimates across the test plane. The second suboptimal observer (local frontoparallel matching [LFM]) uses only location disparity. We find that the ideal observer (PM) and the first subideal observer (LPM) outperforms the second subideal observer (LFM), demonstrating the additional benefit of pattern disparities. We also find that all three model observers can account for human performance, if two free parameters are included: a fixed small level of internal estimation noise, and a fixed overall efficiency scalar on slant discriminability.

Image differences between the eyes can cause millisecond-scale interocular differences in process... more Image differences between the eyes can cause millisecond-scale interocular differences in processing speed. For moving objects, these differences can cause dramatic misperceptions of distance and 3D direction. Here, we develop a continuous target-tracking paradigm that shows these tiny differences in the speed of visual processing are preserved in the movement dynamics of the hand. Human observers continuously tracked a target stimulus with various luminance differences across the eyes as it underwent Brownian motion in the horizontal plane. We show that suitable analysis recovers the time course of the visuomotor response, and comparisons across luminance conditions reveal the temporal evolution of visual processing differences between the eyes. Additionally, using a direct within-observer comparison, we show that target tracking and traditional psychophysics provide scalar estimates of interocular delays that agree on average to within a fraction of a millisecond. Thus, target tra...

Journal of Vision, 2016

Some animals with lateral eyes (such as bees) control their navigation through the 3D world using... more Some animals with lateral eyes (such as bees) control their navigation through the 3D world using velocity differences between the two eyes. Other animals with frontal eyes (such as primates, including humans) can perceive 3D motion based on the different velocities that a moving object projects upon the two retinae. Although one type of 3D motion perception involves a comparison between velocities from vastly different (monocular) portions of the visual field, and the other involves a comparison within overlapping (binocular) portions of the visual field, both compare velocities across the two eyes. Here we asked whether human interocular velocity comparisons, typically studied in the context of binocularly overlapping vision, operate in the far lateral (and hence, monocular) periphery and, if so, whether these comparisons were accordant with conventional interocular motion processing. We found that speed discrimination was indeed better between the two eyes' monocular visual fields, as compared to within a single eye's (monocular) visual field, but only when the velocities were consistent with commonly encountered motion. This intriguing finding suggests that mechanisms sensitive to relative motion information on opposite sides of an animal may have been retained, or at some point independently achieved, as the eyes became frontal in some animals.

Perception & Psychophysics, 2007

Current research sometimes generalizes findings from largely horizontal distance-perception studi... more Current research sometimes generalizes findings from largely horizontal distance-perception studies to all distance perception without considering that height perception may result from processes not shared with other distance perception (

Alcoholism: Clinical and Experimental Research, 2013

Background-There is a strong relationship between circadian rhythms and ethanol responses. Ethano... more Background-There is a strong relationship between circadian rhythms and ethanol responses. Ethanol consumption has been shown to disrupt physiological and behavioral circadian rhythms in mammals (Spanagel et al., 2005b). The Drosophila central circadian pacemaker is composed of proteins encoded by the per, tim, cyc, and Clk genes. Using Drosophila mutant analysis we asked whether these central components of the circadian clock make the equivalent contribution towards ethanol tolerance and whether rhythmicity itself is necessary for tolerance.

Annual Review of Vision Science, 2017

The visual system must recover important properties of the external environment if its host is to... more The visual system must recover important properties of the external environment if its host is to survive. Because the retinae are effectively two-dimensional but the world is three-dimensional (3D), the patterns of stimulation both within and across the eyes must be used to infer the distal stimulus—the environment—in all three dimensions. Moreover, animals and elements in the environment move, which means the input contains rich temporal information. Here, in addition to reviewing the literature, we discuss how and why prior work has focused on purported isolated systems (e.g., stereopsis) or cues (e.g., horizontal disparity) that do not necessarily map elegantly on to the computations and complex patterns of stimulation that arise when visual systems operate within the real world. We thus also introduce the binoptic flow field (BFF) as a description of the 3D motion information available in realistic environments, which can foster the use of ecologically valid yet well-controlled...

Journal of vision, 2017

Estimating an accurate and naturalistic dense depth map from a single monocular photographic imag... more Estimating an accurate and naturalistic dense depth map from a single monocular photographic image is a difficult problem. Nevertheless, human observers have little difficulty understanding the depth structure implied by photographs. Two-dimensional (2D) images of the real-world environment contain significant statistical information regarding the three-dimensional (3D) structure of the world that the vision system likely exploits to compute perceived depth, monocularly as well as binocularly. Toward understanding how this might be accomplished, we propose a Bayesian model of monocular depth computation that recovers detailed 3D scene structures by extracting reliable, robust, depth-sensitive statistical features from single natural images. These features are derived using well-accepted univariate natural scene statistics (NSS) models and recent bivariate/correlation NSS models that describe the relationships between 2D photographic images and their associated depth maps. This is ac...

2015 IEEE Global Conference on Signal and Information Processing (GlobalSIP), 2015

Journal of vision, 2016

The now well-known motion-silencing illusion has shown that salient changes among a group of obje... more The now well-known motion-silencing illusion has shown that salient changes among a group of objects' luminances, colors, shapes, or sizes may appear to cease when objects move rapidly (Suchow & Alvarez, 2011). It has been proposed that silencing derives from dot spacing that causes crowding, coherent changes in object color or size, and flicker frequencies combined with dot spacing (Choi, Bovik, & Cormack, 2014; Peirce, 2013; Turi & Burr, 2013). Motion silencing is a peripheral effect that does not occur near the point of fixation. To better understand the effect of eccentricity on motion silencing, we measured the amount of motion silencing as a function of eccentricity in human observers using traditional psychophysics. Fifteen observers reported whether dots in any of four concentric rings changed in luminance over a series of rotational velocities. The results in the human experiments showed that the threshold velocity for motion silencing almost linearly decreases as a fun...

Perception, 2009

Recognition memory for fixated regions from briefly viewed full-screen natural images is examined... more Recognition memory for fixated regions from briefly viewed full-screen natural images is examined. Low-level image statistics reveal that observers fixated, on average (pooled across images and observers), image regions that possessed greater visual saliency than non-fixated regions, a finding that is robust across multiple fixation indices. Recognition-memory performance indicates that, of the fixation loci tested, observers were adept at recognising those with a particular profile of image statistics; visual saliency was found to be attenuated for unrecognised loci, despite that all regions were freely fixated. Furthermore, although elevated luminance was the local image statistic found to discriminate least between human and random image locations, it was the greatest predictor of recognition-memory performance, demonstrating a dissociation between image features that draw fixations and those that support visual memory. An analysis of corresponding eye movements indicates that im...

Journal of Vision, 2007

The human visual system is remarkably adept at finding objects of interest in cluttered visual en... more The human visual system is remarkably adept at finding objects of interest in cluttered visual environments, a task termed visual search. Because the human eye is highly foveated, it accomplishes this by making many discrete fixations linked by rapid eye movements called saccades. In such naturalistic tasks, we know very little about how the brain selects saccadic targets (the fixation loci). In this paper, we use a novel technique akin to psychophysical reverse correlation and stimuli that emulate the natural visual environment to measure observers' ability to locate a low-contrast target of unknown orientation. We present three main discoveries. First, we provide strong evidence for saccadic selectivity for spatial frequencies close to the target's central frequency. Second, we demonstrate that observers have distinct, idiosyncratic biases to certain orientations in saccadic programming, although there were no priors imposed on the target's orientation. These orientation biases cover a subset of the near-cardinal (horizontal/vertical) and near-oblique orientations, with orientations near vertical being the most common across observers. Further, these idiosyncratic biases were stable across time. Third, within observers, very similar biases exist for foveal target detection accuracy. These results suggest that saccadic targeting is tuned for known stimulus dimensions (here, spatial frequency) and also has some preference or default tuning for uncertain stimulus dimensions (here, orientation).

Journal of Vision, 2006

Visual search experiments have usually involved the detection of a salient target in the presence... more Visual search experiments have usually involved the detection of a salient target in the presence of distracters against a blank background. In such high signal-to-noise scenarios, observers have been shown to use visual cues such as color, size, and shape of the target to program their saccades during visual search. The degree to which these features affect search performance is usually measured using reaction times and detection accuracy. We asked whether human observers are able to use target features to succeed in visual search tasks in stimuli with very low signal-to-noise ratios. Using the classification image analysis technique, we investigated whether observers used structural cues to direct their fixations as they searched for simple geometric targets embedded at very low signal-to-noise ratios in noise stimuli that had the spectral characteristics of natural images. By analyzing properties of the noise stimulus at observers' fixations, we were able to reveal idiosyncratic, target-dependent features used by observers in our visual search task. We demonstrate that even in very noisy displays, observers do not search randomly, but in many cases they deploy their fixations to regions in the stimulus that resemble some aspect of the target in their local image features.

Spatial vision, 2009

DOVES, a database of visual eye movements, is a set of eye movements collected from 29 human obse... more DOVES, a database of visual eye movements, is a set of eye movements collected from 29 human observers as they viewed 101 natural calibrated images. Recorded using a high-precision dual-Purkinje eye tracker, the database consists of around 30 000 fixation points, and is believed to be the first large-scale database of eye movements to be made available to the vision research community. The database, along with MATLAB functions for its use, may be downloaded freely from http://live.ece.utexas.edu/research/doves, and used without restriction for educational and research purposes, providing that this paper is cited in any published work. This paper documents the acquisition procedure, summarises common eye movement statistics, and highlights numerous research topics for which DOVES may be used.

Perception & Psychophysics, 2007

We propose a novel variant of the classification image paradigm that allows us to rapidly reveal ... more We propose a novel variant of the classification image paradigm that allows us to rapidly reveal strategies used by observers in visual search tasks. We make use of eye tracking, 1/f noise, and a grid-like stimulus ensemble and also introduce a new classification taxonomy that distinguishes between foveal and peripheral processes. We tested our method for 3 human observers and two simple shapes used as search targets. The classification images obtained show the efficacy of the proposed method by revealing the features used by the observers in as few as 200 trials. Using two control experiments, we evaluated the use of naturalistic 1/f noise with classification images, in comparison with the more commonly used white noise, and compared the performance of our technique with that of an earlier approach without a stimulus grid.

Vision Research, 2007

Analysis of the statistics of image features at observers' gaze can provide insights into the mec... more Analysis of the statistics of image features at observers' gaze can provide insights into the mechanisms of fixation selection in humans. Using a foveated analysis framework, in which image patches were analyzed at the resolution corresponding to their eccentricity from the prior fixation, we studied the statistics of four low-level local image features: luminance, RMS contrast, and bandpass outputs of both luminance and contrast, and discovered that the image patches around human fixations had, on average, higher values of each of these features at all eccentricities than the image patches selected at random. Bandpass contrast showed the greatest difference between human and random fixations, followed by bandpass luminance, RMS contrast, and luminance. An eccentricity-based analysis showed that shorter saccades were more likely to land on patches with higher values of these features. Compared to a full-resolution analysis, foveation produced an increased difference between human and random patch ensembles for contrast and its higher-order statistics.

Journal of Vision

Binocular stereo cues are important for discriminating 3D surface orientation, especially at near... more Binocular stereo cues are important for discriminating 3D surface orientation, especially at near distances. We devised a single-interval task where observers discriminated the slant of a densely textured planar test surface relative to a textured planar surround reference surface. Although surfaces were rendered with correct perspective, the stimuli were designed so that the binocular cues dominated performance. Slant discrimination performance was measured as a function of the reference slant and the level of uncorrelated white noise added to the test-plane images in the left and right eyes. We compared human performance with an approximate ideal observer (planar matching [PM]) and two subideal observers. The PM observer uses the image in one eye and back projection to predict a test image in the other eye for all possible slants, tilts, and distances. The estimated slant, tilt, and distance are determined by the prediction that most closely matches the measured image in the other eye. The first subideal observer (local planar matching [LPM]) applies PM over local neighborhoods and then pools estimates across the test plane. The second suboptimal observer (local frontoparallel matching [LFM]) uses only location disparity. We find that the ideal observer (PM) and the first subideal observer (LPM) outperforms the second subideal observer (LFM), demonstrating the additional benefit of pattern disparities. We also find that all three model observers can account for human performance, if two free parameters are included: a fixed small level of internal estimation noise, and a fixed overall efficiency scalar on slant discriminability.

Image differences between the eyes can cause millisecond-scale interocular differences in process... more Image differences between the eyes can cause millisecond-scale interocular differences in processing speed. For moving objects, these differences can cause dramatic misperceptions of distance and 3D direction. Here, we develop a continuous target-tracking paradigm that shows these tiny differences in the speed of visual processing are preserved in the movement dynamics of the hand. Human observers continuously tracked a target stimulus with various luminance differences across the eyes as it underwent Brownian motion in the horizontal plane. We show that suitable analysis recovers the time course of the visuomotor response, and comparisons across luminance conditions reveal the temporal evolution of visual processing differences between the eyes. Additionally, using a direct within-observer comparison, we show that target tracking and traditional psychophysics provide scalar estimates of interocular delays that agree on average to within a fraction of a millisecond. Thus, target tra...

Journal of Vision, 2016

Some animals with lateral eyes (such as bees) control their navigation through the 3D world using... more Some animals with lateral eyes (such as bees) control their navigation through the 3D world using velocity differences between the two eyes. Other animals with frontal eyes (such as primates, including humans) can perceive 3D motion based on the different velocities that a moving object projects upon the two retinae. Although one type of 3D motion perception involves a comparison between velocities from vastly different (monocular) portions of the visual field, and the other involves a comparison within overlapping (binocular) portions of the visual field, both compare velocities across the two eyes. Here we asked whether human interocular velocity comparisons, typically studied in the context of binocularly overlapping vision, operate in the far lateral (and hence, monocular) periphery and, if so, whether these comparisons were accordant with conventional interocular motion processing. We found that speed discrimination was indeed better between the two eyes' monocular visual fields, as compared to within a single eye's (monocular) visual field, but only when the velocities were consistent with commonly encountered motion. This intriguing finding suggests that mechanisms sensitive to relative motion information on opposite sides of an animal may have been retained, or at some point independently achieved, as the eyes became frontal in some animals.

Perception & Psychophysics, 2007

Current research sometimes generalizes findings from largely horizontal distance-perception studi... more Current research sometimes generalizes findings from largely horizontal distance-perception studies to all distance perception without considering that height perception may result from processes not shared with other distance perception (

Alcoholism: Clinical and Experimental Research, 2013

Background-There is a strong relationship between circadian rhythms and ethanol responses. Ethano... more Background-There is a strong relationship between circadian rhythms and ethanol responses. Ethanol consumption has been shown to disrupt physiological and behavioral circadian rhythms in mammals (Spanagel et al., 2005b). The Drosophila central circadian pacemaker is composed of proteins encoded by the per, tim, cyc, and Clk genes. Using Drosophila mutant analysis we asked whether these central components of the circadian clock make the equivalent contribution towards ethanol tolerance and whether rhythmicity itself is necessary for tolerance.

Annual Review of Vision Science, 2017

The visual system must recover important properties of the external environment if its host is to... more The visual system must recover important properties of the external environment if its host is to survive. Because the retinae are effectively two-dimensional but the world is three-dimensional (3D), the patterns of stimulation both within and across the eyes must be used to infer the distal stimulus—the environment—in all three dimensions. Moreover, animals and elements in the environment move, which means the input contains rich temporal information. Here, in addition to reviewing the literature, we discuss how and why prior work has focused on purported isolated systems (e.g., stereopsis) or cues (e.g., horizontal disparity) that do not necessarily map elegantly on to the computations and complex patterns of stimulation that arise when visual systems operate within the real world. We thus also introduce the binoptic flow field (BFF) as a description of the 3D motion information available in realistic environments, which can foster the use of ecologically valid yet well-controlled...

Journal of vision, 2017

Estimating an accurate and naturalistic dense depth map from a single monocular photographic imag... more Estimating an accurate and naturalistic dense depth map from a single monocular photographic image is a difficult problem. Nevertheless, human observers have little difficulty understanding the depth structure implied by photographs. Two-dimensional (2D) images of the real-world environment contain significant statistical information regarding the three-dimensional (3D) structure of the world that the vision system likely exploits to compute perceived depth, monocularly as well as binocularly. Toward understanding how this might be accomplished, we propose a Bayesian model of monocular depth computation that recovers detailed 3D scene structures by extracting reliable, robust, depth-sensitive statistical features from single natural images. These features are derived using well-accepted univariate natural scene statistics (NSS) models and recent bivariate/correlation NSS models that describe the relationships between 2D photographic images and their associated depth maps. This is ac...

2015 IEEE Global Conference on Signal and Information Processing (GlobalSIP), 2015

Journal of vision, 2016

The now well-known motion-silencing illusion has shown that salient changes among a group of obje... more The now well-known motion-silencing illusion has shown that salient changes among a group of objects' luminances, colors, shapes, or sizes may appear to cease when objects move rapidly (Suchow & Alvarez, 2011). It has been proposed that silencing derives from dot spacing that causes crowding, coherent changes in object color or size, and flicker frequencies combined with dot spacing (Choi, Bovik, & Cormack, 2014; Peirce, 2013; Turi & Burr, 2013). Motion silencing is a peripheral effect that does not occur near the point of fixation. To better understand the effect of eccentricity on motion silencing, we measured the amount of motion silencing as a function of eccentricity in human observers using traditional psychophysics. Fifteen observers reported whether dots in any of four concentric rings changed in luminance over a series of rotational velocities. The results in the human experiments showed that the threshold velocity for motion silencing almost linearly decreases as a fun...

Perception, 2009

Recognition memory for fixated regions from briefly viewed full-screen natural images is examined... more Recognition memory for fixated regions from briefly viewed full-screen natural images is examined. Low-level image statistics reveal that observers fixated, on average (pooled across images and observers), image regions that possessed greater visual saliency than non-fixated regions, a finding that is robust across multiple fixation indices. Recognition-memory performance indicates that, of the fixation loci tested, observers were adept at recognising those with a particular profile of image statistics; visual saliency was found to be attenuated for unrecognised loci, despite that all regions were freely fixated. Furthermore, although elevated luminance was the local image statistic found to discriminate least between human and random image locations, it was the greatest predictor of recognition-memory performance, demonstrating a dissociation between image features that draw fixations and those that support visual memory. An analysis of corresponding eye movements indicates that im...

Journal of Vision, 2007

The human visual system is remarkably adept at finding objects of interest in cluttered visual en... more The human visual system is remarkably adept at finding objects of interest in cluttered visual environments, a task termed visual search. Because the human eye is highly foveated, it accomplishes this by making many discrete fixations linked by rapid eye movements called saccades. In such naturalistic tasks, we know very little about how the brain selects saccadic targets (the fixation loci). In this paper, we use a novel technique akin to psychophysical reverse correlation and stimuli that emulate the natural visual environment to measure observers' ability to locate a low-contrast target of unknown orientation. We present three main discoveries. First, we provide strong evidence for saccadic selectivity for spatial frequencies close to the target's central frequency. Second, we demonstrate that observers have distinct, idiosyncratic biases to certain orientations in saccadic programming, although there were no priors imposed on the target's orientation. These orientation biases cover a subset of the near-cardinal (horizontal/vertical) and near-oblique orientations, with orientations near vertical being the most common across observers. Further, these idiosyncratic biases were stable across time. Third, within observers, very similar biases exist for foveal target detection accuracy. These results suggest that saccadic targeting is tuned for known stimulus dimensions (here, spatial frequency) and also has some preference or default tuning for uncertain stimulus dimensions (here, orientation).

Journal of Vision, 2006

Visual search experiments have usually involved the detection of a salient target in the presence... more Visual search experiments have usually involved the detection of a salient target in the presence of distracters against a blank background. In such high signal-to-noise scenarios, observers have been shown to use visual cues such as color, size, and shape of the target to program their saccades during visual search. The degree to which these features affect search performance is usually measured using reaction times and detection accuracy. We asked whether human observers are able to use target features to succeed in visual search tasks in stimuli with very low signal-to-noise ratios. Using the classification image analysis technique, we investigated whether observers used structural cues to direct their fixations as they searched for simple geometric targets embedded at very low signal-to-noise ratios in noise stimuli that had the spectral characteristics of natural images. By analyzing properties of the noise stimulus at observers' fixations, we were able to reveal idiosyncratic, target-dependent features used by observers in our visual search task. We demonstrate that even in very noisy displays, observers do not search randomly, but in many cases they deploy their fixations to regions in the stimulus that resemble some aspect of the target in their local image features.

Spatial vision, 2009

DOVES, a database of visual eye movements, is a set of eye movements collected from 29 human obse... more DOVES, a database of visual eye movements, is a set of eye movements collected from 29 human observers as they viewed 101 natural calibrated images. Recorded using a high-precision dual-Purkinje eye tracker, the database consists of around 30 000 fixation points, and is believed to be the first large-scale database of eye movements to be made available to the vision research community. The database, along with MATLAB functions for its use, may be downloaded freely from http://live.ece.utexas.edu/research/doves, and used without restriction for educational and research purposes, providing that this paper is cited in any published work. This paper documents the acquisition procedure, summarises common eye movement statistics, and highlights numerous research topics for which DOVES may be used.

Perception & Psychophysics, 2007

We propose a novel variant of the classification image paradigm that allows us to rapidly reveal ... more We propose a novel variant of the classification image paradigm that allows us to rapidly reveal strategies used by observers in visual search tasks. We make use of eye tracking, 1/f noise, and a grid-like stimulus ensemble and also introduce a new classification taxonomy that distinguishes between foveal and peripheral processes. We tested our method for 3 human observers and two simple shapes used as search targets. The classification images obtained show the efficacy of the proposed method by revealing the features used by the observers in as few as 200 trials. Using two control experiments, we evaluated the use of naturalistic 1/f noise with classification images, in comparison with the more commonly used white noise, and compared the performance of our technique with that of an earlier approach without a stimulus grid.

Vision Research, 2007

Analysis of the statistics of image features at observers' gaze can provide insights into the mec... more Analysis of the statistics of image features at observers' gaze can provide insights into the mechanisms of fixation selection in humans. Using a foveated analysis framework, in which image patches were analyzed at the resolution corresponding to their eccentricity from the prior fixation, we studied the statistics of four low-level local image features: luminance, RMS contrast, and bandpass outputs of both luminance and contrast, and discovered that the image patches around human fixations had, on average, higher values of each of these features at all eccentricities than the image patches selected at random. Bandpass contrast showed the greatest difference between human and random fixations, followed by bandpass luminance, RMS contrast, and luminance. An eccentricity-based analysis showed that shorter saccades were more likely to land on patches with higher values of these features. Compared to a full-resolution analysis, foveation produced an increased difference between human and random patch ensembles for contrast and its higher-order statistics.