Details 1995-2002

Braun, D., Wichmann, F. A. and Gegenfurtner, K. R. (2002)
Phase information in the recognition of natural images
European Conference on Visual Perception (ECVP), Glasgow, UK (poster)

Fourier phase plays an important role in determining global image structure. For example, when the phase spectrum of an image of a flower is swapped with that of a tank, we usually perceive a tank, even though the amplitude spectrum is still that of the flower. Similarly, when the phase spectrum of an image is randomly swapped across frequencies, that is, its Fourier energy is randomly distributed over the image, the resulting image becomes impossible to recognize.

Our goal was to evaluate the effect of phase manipulations in a quantitative manner. Subjects viewed two images of natural scenes, one of which contained an animal (the target) embedded in the background. The spectra of the images were manipulated by adding random phase noise at each frequency. The phase noise was the independent variable, uniformly distributed between 0 degree and +/-180 degrees.

Subjects were remarkably resistant to phase noise. Even with +/-120 degree noise subjects were still 75% correct. The proportion of correct answers closely followed the correlation between original and noise-distorted images. Thus it appears as if it was not the global phase information per se that determines our percept of natural images, but rather the effect of phase on local image features.

Henning, G. B., Wichmann, F. A. and Bird, C. M. (2002)
Pulse-train detection and discrimination in pink noise
Vision Sciences Society (VSS), Sarasota, FL, USA (poster)

Much of our information about spatial vision comes from detection experiments involving low-contrast stimuli. Contrast discrimination experiments provide one way to explore the visual system's response to stimuli of higher contrast. We explored both detection and contrast discrimination performance with sinusoidal and "pulse-train" (or line) gratings. Both types of grating had a fundamental spatial frequency of 2.09-c/deg but the pulse-train, ideally, contains, in addition to its fundamental component, all the harmonics of the fundamental. Although the 2.09-c/deg pulse-train produced on the display was measured and shown to contain at least 8 harmonics at equal contrast, it was no more detectable than its most detectable component; no benefit from having additional information at the harmonics was measurable. The addition of broadband "pink" noise, designed to equalize the detectability of the components of the pulse train, made it about a factor of four more detectable than any of its components. However, in contrast-discrimination experiments, with an in-phase pedestal or masking grating of the same form and phase as the signal and 15% contrast, the noise did not improve the discrimination performance of the pulse train relative to that of its sinusoidal components. In contrast, a 2.09-c/deg "super train," constructed to have 8 equally detectable harmonics, was a factor of five more detectable than any of its components. We discuss the implications of these observations for models of early vision in particular the implications for possible sources of internal noise.

Rosas, P., Shepers, J., Wichmann, F. A. and Wagemans, J. (2002)
Surface-slant and surface-curvature from texture
European Conference on Visual Perception (ECVP), Glasgow, UK (poster)

The contribution of texture to the perception of slant and curvature was studied in two experiments in which we collected the judgments of human subjects by means of probe adjustment. For the slant perception experiment our stimuli consisted of flat surfaces receding away from the subject, while for the curvature experiment we used surfaces depicting half elliptic cylinders whose major semi- axis pointed towards the subject. Both types of stimuli were generated under perspective projection using purpose written software. The surfaces were mapped with different types of textures: uniform dot lattices, polka dots, Voronoi tessellations, orthogonal sinusoidal plaid patterns, fractal or 1/f noise, "coherent" noise and a "diffusion-based" texture (leopard skin-like). Although we observed differences in performance for different textures, in general terms the results generally show a higher correlation between the perceived and depicted slant of a plane than the correlation between perceived and depicted curvature. These results suggest, which would indicate that texture may be cue is more informative for perceiving slant than curvature perception. For Nevertheless, the plaid textures, however, plaid elicit -qualitatively- similar results were obtained for between in both experiments. Since this is not the case for a textures comparatively similar in uniformity (e.g. dot lattice), further spatial- frequency-based comparisons might show relevant differences for a spectrum-based shape-from-texture mechanism.

Rosas, P., Wichmann, F. A. and Wagemans, J. (2002)
Surface-slant-from-texture discrimination: Effects of slant level and texture type
Vision Sciences Society (VSS), Sarasota, FL, USA (poster)

The problem of surface-slant-from-texture was studied psychophysically by measuring the performances of five human subjects in a slant-discrimination task with a number of different types of textures: uniform lattices, randomly displaced lattices, polka dots, Voronoi tessellations, orthogonal sinusoidal plaid patterns, fractal or 1/f noise, "coherent" noise and a "diffusion-based" texture (leopard skin-like). The results show: (1) Improving performance with larger slants for all textures. (2) A "non-symmetrical" performance around a particular slant characterized by a psychometric function that is steeper in the direction of the more slanted orientation. (3) For sufficiently large slants (66 deg) there are no major differences in performance between any of the different textures. (4) For slants at 26, 37 and 53 degrees, however, there are marked differences between the different textures. (5) The observed differences in performance across textures for slants up to 53 degrees are systematic within subjects, and nearly so across them. This allows a rank-order of textures to be formed according to their "helpfulness" - that is, how easy the discrimination task is when a particular texture is mapped on the surface. Polka dots tended to allow the best slant discrimination performance, noise patterns the worst up to the large slant of 66 degrees at which performance was almost independent of the particular texture chosen. Finally, our large number of 2AFC trials (approximately 2800 trials per texture across subjects) and associated tight confidence intervals may enable us to find out about which statistical properties of the textures could be responsible for surface-slant-from-texture estimation, with the ultimate goal of being able to predict observer performance for any arbitrary texture.

Wichmann, F. A. (2002)
Modelling Contrast Transfer in Spatial Vision
Optical Society of America (OSA) Annual Meeting, Orlando, FL, USA (poster)

Much of our information about spatial vision comes from detection experiments involving low-contrast stimuli. Contrast discrimination experiments provide one way to explore the visual system's response to stimuli of higher contrast, the results of which allow different models of contrast processing (e.g. energy versus gain-control models) to be critically assessed (Wichmann, 1999, OSA Annual Meeting, p. 62). Studies of detection and discrimination using pulse train stimuli in noise, on the other hand, make predictions about the number, position and properties of noise sources within the processing stream (Henning, Bird & Wichmann, JOSA A 19, in press). Here I report modelling results combining data from both sinusoidal and pulse train experiments in and without noise to arrive at a more tightly constrained model of early spatial vision.

Wichmann, F. A. (2002)
Application of Monte Carlo Methods to Psychometric Function Fitting
European Mathematical Psychology Conference (EMPG), Bremen, FRG (talk)

The psychometric function relates an observer's performance to an independent variable, usually some physical quantity of a stimulus in a psychophysical task. Here I describe methods to (1) fitting psychometric functions, (2) assessing goodness-of-fit, and (3) providing confidence intervals for the function's parameters and other estimates derived from them. First I describe a constrained maximum-likelihood method for parameter estimation. Using Monte-Carlo simulations I demonstrate that it is important to have a fitting method that takes stimulus-independent errors (or "lapses") into account. Second, a number of goodness-of-fit tests are introduced. Because psychophysical data sets are usually rather small I advocate the use of Monte Carlo resampling techniques that do not rely on asymptotic theory for goodness-of-fit assessment. Third, a parametric bootstrap is employed to estimate the variability of fitted parameters and derived quantities such as thresholds and slopes. I describe how the bootstrap bridging assumption, on which the validity of the procedure depends, can be tested without incurring too high a cost in computation time. Finally I describe how the methods can be extended to test hypotheses concerning the form and shape of several psychometric functions. Software describing the methods is available (http://www.bootstrap-software.com/psignifit/), as well as articles describing the methods in detail (Wichmann&Hill, Perception&Psychophysics, 2001a,b).

Wichmann, F. A. and Henning, G. B. (2002)
Detection and discrimination in pink noise
Tübingen Perception Conference (TWK), Tübingen, FRG (poster)

Much of our information about early spatial vision comes from detection experiments involving low-contrast stimuli, which are not, perhaps, particularly "natural" stimuli. Contrast discrimination experiments provide one way to explore the visual system's response to stimuli of higher contrast whilst keeping the number of unknown parameters comparatively small. We explored both detection and contrast discrimination performance with sinusoidal and "pulse-train" (or line) gratings. Both types of grating had a fundamental spatial frequency of 2.09-c/deg but the pulse-train, ideally, contains, in addition to its fundamental component, all the harmonics of the fundamental. Although the 2.09-c/deg pulse-train produced on our display was measured using a high-performance digital camera (Photometrics) and shown to contain at least 8 harmonics at equal contrast, it was no more detectable than its most detectable component; no benefit from having additional information at the harmonics was measurable. The addition of broadband 1-D "pink" noise made it about a factor of four more detectable than any of its components. However, in contrast-discrimination experiments, with an in-phase pedestal or masking grating of the same form and phase as the signal and 15% contrast, the noise did not improve the discrimination performance of the pulse train relative to that of its sinusoidal components. We discuss the implications of these observations for models of early vision in particular the implications for possible sources of internal noise.

Henning, G. B., Wichmann, F. A. and Bird, C. M. (2001)
The pedestal effect with a pulse train and its constituent sinusoids
Annual Interdisciplinary Conference (AIC), Teton Village, WY, USA (talk)

Curves showing "threshold" contrast for detecting a signal grating as a function of the contrast of a masking grating of the same orientation, spatial frequency, and phase show a characteristic improvement in performance at masker contrasts near the contrast threshold of the unmasked signal. Depending on the percentage of correct responses used to define the threshold, the best performance can be as much as a factor of three better than the unmasked threshold obtained in the absence of any masking grating. The result is called the pedestal effect (sometimes, the dipper function). We used a 2AFC procedure to measure the effect with harmonically related sinusoids ranging from 2 to 16 c/deg - all with maskers of the same orientation, spatial frequency and phase - and with masker contrasts ranging from 0 to 50%. The curves for different spatial frequencies are identical if both the vertical axis (showing the threshold signal contrast) and the horizontal axis (showing the masker contrast) are scaled by the threshold contrast of the signal obtained with no masker. Further, a pulse train with a fundamental frequency of 2 c/deg produces a curve that is indistinguishable from that of a 2-c/deg sinusoid despite the fact that at higher masker contrasts, the pulse train contains at least 8 components all of them equally detectable. The effect of adding 1-D spatial noise is also discussed.

Wichmann, F. A. (2001)
Plaid maskers revisited: asymmetric plaids
Tübingen Perception Conference (TWK), Tübingen, FRG (poster)

A large number of psychophysical and physiological experiments suggest that luminance patterns are independently analysed in "channels" responding to different bands of spatial frequency. There are, however, interactions among stimuli falling well outside the usual estimates of channels' bandwidths. Derrington & Henning (1989) first reported that, in 2-AFC sinusoidal-grating detection, plaid maskers, whose components are oriented symmetrically about the signal orientation, cause a substantially larger threshold elevation than would be predicted from their sinusoidal constituents alone. Wichmann & Tollin (1997a,b) and Wichmann & Henning (1998) confirmed and extended the original findings, measuring masking as a function of presentation time and plaid mask contrast. Here I investigate masking using plaid patterns whose components are asymmetrically positioned about the signal orientation. Methoden: Standard temporal 2-AFC pattern discrimination experiments were conducted using plaid patterns and oblique sinusoidal gratings as maskers, and horizontally orientated sinusoidal gratings as signals. Signal and maskers were always interleaved on the display (refresh rate 152 Hz). Ergebnisse: As in the case of the symmetrical plaid maskers, substantial masking was observed for many of the asymmetrical plaids. Masking is neither a straightforward function of the plaid's constituent sinusoidal components nor of the periodicity of the luminance beats between components. Schlussfolgerung(en): These results cause problems for the notion that, even for simple stimuli, detection and discrimination are based on the outputs of channels tuned to limited ranges of spatial frequency and orientation, even if a limited set of nonlinear interactions between these channels is allowed.

Wichmann, F. A., Willems, B., Rosas, P. and Wagemans, J. (2001)
Perception of planar shapes in depth
Vision Sciences Society (VSS), Sarasota, FL, USA (poster)

We investigated the influence of the perceived 3D-orientation of planar elliptical shapes on the perception of the shapes themselves. Ellipses were projected onto the surface of a sphere and subjects were asked to indicate if the projected shapes looked as if they were a circle on the surface of the sphere. The image of the sphere was obtained from a real, (near) perfect sphere using a highly accurate digital camera (real sphere diameter 40 cm; camera-to-sphere distance 320 cm; for details see Willems et al., Perception 29, S96, 2000; Photometrics SenSys 400 digital camera with Rodenstock lens, 12-bit linear luminance resolution). Stimuli were presented monocularly on a carefully linearized Sony GDM-F500 monitor keeping the scene geometry as in the real case (sphere diameter on screen 8.2 cm; viewing distance 66 cm). Experiments were run in a darkened room using a viewing tube to minimize, as far as possible, extraneous monocular cues to depth. Three different methods were used to obtain subjects' estimates of 3D-shape: the method of adjustment, temporal 2-alternative forced choice (2AFC) and yes/no. Several results are noteworthy. First, mismatch between perceived and objective slant tended to decrease with increasing objective slant. Second, the variability of the settings, too, decreased with increasing objective slant. Finally, we comment on the results obtained using different psychophysical methods and compare our results to those obtained using a real sphere and binocular vision (Willems et al.).

Wichmann, F. A. and Henning, G. B. (2000)
Contrast discrimination using periodic pulse trains
Tübingen Perception Conference (TWK), Tübingen, FRG (poster)

Understanding contrast transduction is essential for understanding spatial vision. Previous research (Wichmann et al. 1998; Wichmann, 1999; Henning and Wichmann, 1999) has demonstrated the importance of high contrasts to distinguish between alternative models of contrast discrimination. However, the modulation transfer function of the eye imposes large contrast losses on stimuli, particularly for stimuli of high spatial frequency, making high retinal contrasts difficult to obtain using sinusoidal gratings. Methoden: Standard 2AFC contrast discrimination experiments were conducted using periodic pulse trains as stimuli. Given our Mitsubishi display we achieve stimuli with up to 160% contrast at the fundamental frequency. Ergebnisse: The shape of the threshold versus (pedestal) contrast (TvC) curve using pulse trains shows the characteristic dipper shape, i.e. contrast discrimination is sometimes "easier" than detection. The rising part of the TvC function has the same slope as that measured for contrast discrimination using sinusoidal gratings of the same frequency as the fundamental. Schlussfolgerung(en): Periodic pulse trains offer the possibility to explore the visual system's properties using high retinal contrasts. Thus they might prove useful in tasks other than contrast discrimination. Second, at least for high spatial frequencies (8 c/deg) it appears that contrast discrimination using sinusoids and periodic pulse trains results in virtually identical TvC functions, indicating a lack of probability summation. Further implications of these results are discussed.

Henning, G. B. and Wichmann, F. A. (1999)
The masking effect of pulse trains on trains of pulses
European Conference on Visual Perception (ECVP), Trieste, ITA (poster)

It is important to know for theoretical reasons how performance varies with stimulus contrast. But, for objects on CRT displays, retinal contrast is limited by the linear range of the display and the modulation transfer function of the eye. For example, with an 8 c/deg sinusoidal grating at 90% contrast, the contrast of the retinal image is barely 45%; more retinal contrast is required, however, to discriminate among theories of contrast discrimination (Wichmann, Henning and Ploghaus, 1998). The stimulus with the greatest contrast at any spatial-frequency component is a periodic pulse train which has 200% contrast at every harmonic. Such a waveform cannot, of course, be produced; the best we can do with our Mitsubishi display provides a contrast of 150% at an 8-c/deg fundamental thus producing a retinal image with about 75% contrast. The penalty of using this stimulus is that the 2nd harmonic of the retinal image also has high contrast (with an emmetropic eye, more than 60% of the contrast of the 8-c/deg fundamental ) and the mean luminance is not large (24.5 cd/m2 on our display). We have used standard 2-AFC experiments to measure the detectability of an 8-c/deg pulse train against the background of an identical pulse train of different contrasts. An unusually large improvement in detetectability was measured, the pedestal effect or "dipper," and the dipper was unusually broad. The implications of these results will be discussed.

Ploghaus, A., Clare, S., Wichmann, F. A., and Tracey, I. T. (1999)
Unexpected and anticipated pain: Identification of specific brain activations by correlation with reference functions derived from conditioning theory
Society for Neuroscience, Miami Beach, FL, USA (poster)

Wichmann, F. A. and Henning, G. B. (1999)
Implications of the pedestal effect for models of contrast-processing and gain-control
Optical Society of America (OSA) Annual Meeting, Santa Clara, CA, USA (talk)

Understanding contrast processing is essential for understanding spatial vision. Pedestal contrast systematically affects slopes of functions relating 2-AFC contrast discrimination performance to pedestal contrast. The slopes provide crucial information because only full sets of data allow discrimination among contrast-processing and gain-control models. Issues surrounding Weber's law will also be discussed.

Hill, N. J. and Wichmann, F. A. (1998)
A bootstrap method for testing hypotheses concerning psychometric functions
Computers in Psychology Conference, York, UK (poster)

Whenever psychometric functions are used to evaluate human performance on some task, it is valuable to examine not only the threshold and slope values estimated from the original data, but also the expected variability in those measures. This allows psychometric functions obtained in two experimental conditions to be compared statistically. We present a method for estimating the variability of thresholds and slopes of psychometric functions. This involves a maximum-likelihood fit to the data using a three-parameter mathematical function, followed by Monte Carlo simulation using the first fit as a generating function for the simulations. The variability of the function's parameters can then be estimated (as shown by Maloney, 1990), as can the variability of the threshold value (Foster & Bischof, 1997). We will show how a simple development of this procedure can be used to test the significance of differences between (a) the thresholds, and (b) the slopes of two psychometric functions. Further, our method can be used to assess the assumptions underlying the original fit, by examining how goodness-of-fit differs in simulation from its original value. In this way data sets can be identified as being either too noisy to be generated by a binomial observer, or significantly "too good to be true." All software is written in MATLAB and is therefore compatible across platforms, with the option of accelerating performance using MATLAB's plug-in binaries, or "MEX" files.

Wichmann, F. A. and Henning, G. B. (1998)
Masking by plaid patterns: Effects of presentation time and mask contrast
Tübingen Perception Conference (TWK), Tübingen, FRG (poster)

Most current models of early spatial vision comprise of sets of orientation- and spatial-frequency selective filters with our without limited non-linear interactions amongst different subsets of the filters. The performance of human observers and of such models for human spatial vision were compared in experiments using maskers with two spatial frequencies (plaid masks). The detectability of horizontally orientated sinusoidal signals at 3.02 c/deg was measured in standard 2AFC-tasks in the presence of plaid patterns with two-components at the same spatial frequency as the signal but at different orientations (+/- 15, 30, 45, and 75 deg from the signal) and with varying contrasts (1.0, 6.25 and 25.0% contrast). In addition, the temporal envelope of the stimulus presentation was either a rectangular pulse of 19.7 msec duration, or a temporal Hanning window of 1497 msec.Threshold elevation varied with plaid component orientation, peaked +/- 30 deg from the signal where nearly a log unit threshold elevation for the 25.0% contrast plaid was observed. For plaids with 1.0% contrast we observed significant facilitation even with plaids whose components were 75 deg from that of the signal. Elevation factors were somewhat lower for the short stimulus presentation time but were still significant (up to a factor of 5 or 6). Despite of the simple nature of the stimuli employed in this study-sinusoidal signal and plaid masks comprised of only two sinusoids-none of the current models of early spatial vision can fully account for all the data gathered.

Wichmann, F. A., Henning, G. B. and Ploghaus, A. (1998)
Non-linearities and the pedestal effect
European Conference on Visual Perception (ECVP), Oxford, UK (poster)

Psychophysical and physiological evidence suggests that luminance patterns are independently analysed in "channels" responding to different bands of spatial frequency. There are, however, interactions among stimuli falling well outside the usual estimates of channels' bandwidths (Henning, Hertz, and Broadbent, (1975). Vision Res., 15, 887-899). We examined whether the masking results of Henning et al. are consistent with independent channels. We postulated, before the channels, a point non-linearity which would introduce distortion products that might produce the observed interactions between stimuli two octaves apart in spatial frequency.

Standard 2-AFC masking experiments determined whether possible distortion products of a 4.185 c/deg masking sinusoid revealed their presence through effects on the detection of a sinusoidal signal at the frequency of the second harmonic of the masker-8.37 c/deg. The signal and masker were horizontally orientated and the signal was in-phase, out-of-phase, or in quadrature with the putative second-order distortion product of the masker. Significant interactions between signal and masker were observed: for a wide range of masker contrasts, signal detection was facilitated by the masking stimulus. However, the shapes of the functions relating detection performance to masker contrast, as well as the effects of relative phase, were inconsistent with the notion that distortion products were responsible for the interactions observed.

Wichmann, F. A. and Tollin D. J. (1997)
Masking by plaid patterns is not explained by adaptation, simple contrast gain-control or distortion products
ARVO Annual Meeting, Fort Lauderdale, FL, USA (poster)

Purpose. Orientation- and spatial-frequency-selective channels are a common feature of models of early spatial vision. Contrary to the predictions of such models, Derrington & Henning (1989, Vision Research 29: 241-246) report that, in 2-AFC sinusoidal-grating detection, a plaid pattern, whose components are oriented symmetrically about the signal orientation, causes a larger threshold elevation than would be predicted from its sinusoidal constituents alone. The present study investigated whether the unexpectedly strong masking using certain plaid-masks is explicable in terms of adaptation or simple nonlinear interactions between channels. Methods. 2-AFC experiments measured the detectability of 3.02-c/deg, horizontally oriented sinusoidal signals. Detection thresholds were determined in the presence of either a single masking grating, with orientations ranging from 5 to 90 deg from that of the signal, or in the presence of a two-sinusoid masking grating (plaid) with component orientations covering all orientations. Three temporal conditions were studied: 1.) Low temporal frequency - Hanning-windowed, 1497 msec. duration; 2.) Counterphase temporal modulation - sinusoidal flicker at 4.34 Hz within the same Hanning window; 3.) Broadband temporal frequency - rectangular window, 79 msec duration. Results. Masking was independent of temporal variations and of the relative phase of the signal and masker, arguing against the observed masking's being an early, global adaptation phenomenon or arising from a distortion product. Certain plaid patterns produce very little masking, arguing against a (Heeger-type) divisive-normalization gain-control mechanism pooling across different orientations. Conclusions. These results cause problems for the notion that, even for simple stimuli, detection and discrimination are based on the outputs of channels tuned to limited ranges of spatial frequency and orientation, even if a limited set of nonlinear interactions between these channels is allowed.

Wichmann, F. A. and Tollin, D. J. (1997)
Masking by plaid patterns: Spatial-frequency tuning and contrast dependency
OSA Annual Meeting, Long Beach, CA, USA (talk)

The detectability of horizontally orientated sinusoidal signals at different spatial-frequencies was measured in standard 2AFC - tasks in the presence of two-component plaid patterns of different orientation and contrast. The shape of the resulting masking surface provides insight into, and constrains models of, the underlying masking mechanisms.

Wichmann, F. A. and Henning, G. B. (1996)
Does motion-blur facilitate motion detection?
OSA Annual Meeting, Rochester, NY, USA (poster)

Retinal-image motion induces the perceptual loss of high spatial-frequency content - motion blur - that affects broadband stimuli. The relative detectability of motion blur and motion itself, measured in 2-AFC experiments, shows that, although the blur associated with motion can be detected, motion itself is the more effective cue.

Gegenfurtner, K. R., Sharpe, L. T. and Wichmann, F. A. (1995)
The contribution of colour to recognition memory in normal and colour-deficient observers
European Conference on Visual Perception (ECVP), Tübingen, FRG (talk)

The role of chromatic information in visual cognition is still unclear. Considerable neural resources appear to be devoted to the analysis of chromatic information. Yet roughly 8% of the male population are at least partially colour blind and many remain unaware of their deficiency until formally tested. We used a recognition task to assess the importance of colour information for recognition memory. During the presentation phase a series of 48 images of natural scenes was presented to subjects on a CRT display for exposure durations between 50 and 1000 msec. Display of the images was followed by a random noise mask. Half of the images were presented in colour; half in black & white. The luminance component of the images remained constant. In the subsequent query phase the same 48 images were intermixed with 48 new images, and the subjects had to indicate which one of the images they had already seen in the presentation phase. We found a significant effect of exposure duration and colour on recognition performance. Subjects performed 5-10% better for the coloured than for the black & white images at all exposure durations, even the shortest ones. Surprisingly, performance was not impaired for a comparison group of 31 dichromats (17 protanopes and 14 deuteranopes), who were also better for colour images. The high speed of colour processing indicates that its usefulness might lie in fast image segmentation. Colour-deficient observers seem to be able to compensate their reduced chromatic information range when viewing and analysing complex scenes.

Wichmann, F. A. and Gegenfurtner, K. R. (1995)
Colour in recognition memory and image understanding
ARVO Annual Meeting, Fort Lauderdale, FL, USA (talk)

Purpose. The role of chromatic information in visual cognition is not well understood. While many neurons in retina, lateral geniculate nucleus and visual cortex respond to variations in the spectral composition of light, many of the 2.5 % of the human population who are partially colour blind remain unaware of their deficiency. Recent experimental investigations of the role of colour in object recognition have yielded equivocal results (Biederman & Ju, 1988; Wurm et al., 1993). We used a recognition task to assess the importance of colour information for recognition memory at various presentation times. Methods. During the presentation phase, 48 images of natural scenes were presented on a CRT display for exposure durations between 50 and 1000 msec., immediately followed by a random noise mask. Half the images were in colour, half in black & white. The luminance component of the images was identical between the colour and the black & white conditions. The same 48 images were subsequently intermixed with 48 new images (distractors), and the subjects were required to indicate which images they had already seen. Results. Exposure duration and colour significantly affected recognition; subjects performed 10% better with coloured images at all durations. Conclusions. Colour information can be used in recognition memory; further, colour processing is as fast as luminance processing, at least down to the shortest duration used (50 msec.). The superior recognition at short durations is not inconsistent with a role for colour in early image segmentation, as has recently been proposed for machine vision algorithms (Klinker et al. 1990; Healey, 1992).

Wichmann, F. A. and Henning, G. B. (1995)
Image segmentation from motion: Just the loss of high-spatial-frequency content?
European Conference on Visual Perception (ECVP), Tübingen, FRG (poster)

The human contrast sensitivity function (CSF) is bandpass for stimuli of low temporal frequency but, for moving stimuli, results in a low-pass CSF with large high spatial-frequency losses. Thus the high spatial-frequency content of images moving on the retina cannot be seen; motion perception could be facilitated by, or even be based on, the selective loss of high spatial-frequency content. 2-AFC image segmentation experiments were conducted with segmentation based on motion or on form. In the latter condition, the form difference mirrored that produced by moving stimuli. This was accomplished by generating stimulus elements which were spectrally either broadband or low-pass. For the motion used, the spectral difference between static broadband and static low-pass elements matched the spectral difference between moving and static broadband elements. On the hypothesis that segmentation from motion is based on the detection of regions devoid of high spatial-frequencies, both tasks should be similarly difficult for human observers. However, neither image segmentation (nor, incidentally, motion detection) was sensitive to the high spatial-frequency content of the stimuli. Thus changes in perceptual form produced by moving stimuli appear not to be used as a cue for image segmentation.