Publications

1999
Bex PJ, Metha AB, Makous W. Enhanced motion aftereffect for complex motions. Vision Research [Internet]. 1999;39 :2229–38. Publisher's VersionAbstract
We measured the magnitude of the motion after effect ({MAE)} elicited by gratings viewed through four spatial apertures symmetrically positioned around fixation. The gratings were identical except for their orientations, which were varied to form patterns of global motion corresponding to radiation, rotation or translation. {MAE} magnitude was estimated by three methods: the duration of the {MAE;} the contrast required to null the {MAE} and the threshold elevation for detecting an abrupt jump. All three techniques showed that {MAEs} for radiation and rotation were greater than those for translation. The greater adaptability of radiation and rotation over translation also was observed in areas of the display where no adapting stimulus had been presented. We also found that adaptation to motion in one direction had equal effects on sensitivity to motion in the same and opposite directions.
Bex PJ, Baker CL. Motion perception over long interstimulus intervals. Perception & Psychophysics [Internet]. 1999;61 :1066–74. Publisher's VersionAbstract
Recent studies using moving arrays of textured micropatterns have suggested that motion perception can be supported by two mechanisms, one quasilinear and sensitive to the motion of luminance-defined local texture, the other nonlinear and coding motion of contrast-defined envelopes of texture (Baker & Hess, 1998; Boulton & Baker, 1993b). Here we used similar patterns to study motion perception under conditions previously shown to isolate the nonlinear mechanism (low micropattern densities and positive interstimulus intervals [{ISIs].} We measured direction discrimination for two-flash apparent motion over a much larger range of {ISIs}, and susceptibility to masking by incoherently moving "distractor" micropatterns. The results suggest that two nonlinear mechanisms can support motion perception under these conditions. One operates only for relatively short {ISIs} (less than c. 100 msec), is sensitive to small spatial displacements, and is relatively insensitive to distractor masking. The other operates over much longer {ISIs}, is insensitive to small spatial displacements, and is highly disrupted by distractor masking. These results are in line with previous studies suggesting that three mechanisms support motion perception.
Lu Z-L, Lesmes LA, Sperling G. The mechanism of isoluminant chromatic motion perception. Proceedings of the National Academy of Sciences. 1999;96 :8289–8294.
1998
Hess RF, Bex PJ, Fredericksen ER, Brady N. Is human motion detection subserved by a single or multiple channel mechanism?. Vision Research [Internet]. 1998;38 :259–66. Publisher's VersionAbstract
Two recent versions of a single channel model of motion perception have had impressive success in explaining direction discrimination by human observers for spatially filtered noise images in two-flash apparent motion. It has been argued that the dramatic breakdown in motion perception which occurs when one image in the two-flash sequence is low-pass filtered can be explained only by a single channel model. We show that neither version of the single channel model which has been proposed can explain performance for noise images chosen to provide comparable stimulation in the spatial channels known to subserve human vision. A multi-channel model of motion perception has little difficulty in explaining these results.
Bex PJ, Metha AB, Makous W. Psychophysical evidence for a functional hierarchy of motion processing mechanisms. Journal of the Optical Society of America [Internet]. 1998;15 :769–76. Publisher's VersionAbstract
Current models of motion perception typically describe mechanisms that operate locally to extract direction and speed information. To deal with the movement of self or objects with respect to the environment, higher-level receptive fields are presumably assembled from the outputs of such local analyzers. We find that the apparent speed of gratings viewed through four spatial apertures depends on the interaction of motion directions among the apertures, even when the motion within each aperture is identical except for direction. Specifically, local motion consistent with a global pattern of radial motion appears 32% faster than that consistent with translational or rotational motion. The enhancement of speed is not reflected in detection thresholds and persists in spite of instructions to fixate a single local aperture and ignore the global configuration. We also find that a two-dimensional pattern of motion is necessary to elicit the effect and that motion contrast alone does not produce the enhancement. These results implicate at least two serial stages of motion-information processing: a mechanism to code the local direction and speed of motion, followed by a global mechanism that integrates such signals to represent meaningful patterns of movement, depending on the configuration of the local motions.
1997
Fredericksen RE, Bex PJ, Verstraten FA. How big is a Gabor patch, and why should we care?. Journal of the Optical Society of America [Internet]. 1997;14 :1–12. Publisher's VersionAbstract
We propose a two-parameter model for the perceived size (spatial extent) of a Gaussian-windowed, drifting sinusoidal luminance pattern (a Gabor patch) based on the simple assumption that perceived size is determined by detection threshold for the sinusoidal carrier. Psychophysical measures of perceived size vary with peak contrast, Gaussian standard deviation, and carrier spatial frequency in a manner predicted by the model. At suprathreshold peak contrasts Gabor perceived size is relatively unaffected by systemic noise but varies in a manner that is consistent with the influence of local contrast gain control. However, at and near threshold, systemic noise plays a major role in determining perceived size. The data and the model indicate that measures of contrast threshold using Gaussian-windowed stimuli (or any other nonflat contrast window) are determined not just by contrast response of the neurons activated by the stimulus but also by integration of that activation over a noisy, contrast-dependent extent of the stimulus in space and time. Thus, when we wish to measure precisely the influence of spatial and temporal integration on threshold, we cannot do so by combining contrast threshold measures with Gaussian-windowed stimuli.
Brady N, Bex PJ, Fredericksen RE. Independent coding across spatial scales in moving fractal images. Vision Research [Internet]. 1997;37 :1873–83. Publisher's VersionAbstract
We compared observers' ability to discriminate the direction of apparent motion using images which varied in their spatial characteristic; white or flat spectrum noise, and 1/f noise which has an amplitude spectrum characteristic of natural scenes. The upper spatial limit for discrimination (dmax) was measured using a two-flash random dot kinematogram ({RDK)}, which consisted either of a pair of bandpass filtered images or of a bandpass filtered image and its broadband counterpart. Six bandpass central frequencies were used, ranging from 0.25 to 5.66 cyc/deg. Subjects could perform the direction discrimination task for all six central frequencies in both the bandpass-bandpass and bandpass-broadband sequences for the 1/f images, and dmax values were found to be approximately equal in these two conditions at all spatial scales. However, for the white noise images, direction discrimination was not possible at the lowest central frequencies in the bandpass-broadband task. These data show that information from a wide range of spatial scales is equally salient to the human motion system in images whose amplitude spectra fall as 1/f. However, for white noise images, information at the higher spatial frequencies is more salient and dominates performance in the direction discrimination task. These results are consistent with a model in which spatial frequency filters in the input lines of motion detectors have octave constant spatial frequency bandwidths and equal peak sensitivity. In line with a number of recent studies, this suggests that the spatial properties of motion sensitive cells are matched to the statistical properties of natural scenes.
Mareschal I, Ashida H, Bex PJ, Nishida S, Verstraten FA. Linking lower and higher stages of motion processing?. Vision Research [Internet]. 1997;37 :1755–9. Publisher's VersionAbstract
The spatial frequency selectivity of motion detection mechanisms can be measured by comparing the magnitude of motion aftereffects ({MAEs)} as a function of the spatial frequency of the adapting and test gratings. For static test gratings, narrow spatial frequency tuning has been reported in a number of studies. However, for dynamic test patterns, reports have been conflicting. Ashida & Osaka [(1994). Perception, 23, 1313-1320] found no tuning whereas Bex et al. [(1996) Vision Research, 36, 2721-2727] reported a narrow tuning. The main difference between the two studies was the temporal frequency of the test pattern. In this study we measured the spatial frequency tuning of the {MAE} using test patterns for a range of temporal frequencies. The results confirmed that there was narrow spatial frequency tuning when the test pattern was counterphasing at a low temporal frequency. However, the spatial frequency selectivity broadened as the temporal frequency of the test pattern was increased.
Hess RF, Demanins R, Bex PJ. A reduced motion aftereffect in strabismic amblyopia. Vision Research. 1997;37 :1303–1311.
Bex PJ, Baker CL. The effects of distractor elements on direction discrimination in random Gabor kinematograms. Vision Research. 1997;37 :1761–1767.
Bex PJ, Makous W. Radial motion looks faster. Vision Research [Internet]. 1997;37 :3399–405. Publisher's VersionAbstract
Current models of motion perception depend on unidirectional motion-sensitive mechanisms that provide local inputs for complex pattern motion, such as optic flow. To test the generality of such models, we asked observers to compare the speed of radial gratings with the translational speed of vertical gratings. The speed of the radial gratings was consistently overestimated by 20-60% relative to that of translating gratings that were identical in all other respects. The speed bias was not associated with a general spatial or temporal processing bias, nor with the high relative speed of points about the center of expansion/contraction. The bias increased non-linearly with the size of sectors of the radiating pattern exposed. As the motion of the two patterns was locally identical but judged differently, the apparent speed of both kinds of motion cannot be served by any mechanism, nor described by any model, that is based entirely on local motion signals. We speculate that the greater apparent speed of the radial motion has to do with apparent motion in depth.
1996
Hammett ST, Bex PJ. Motion sharpening: evidence for the addition of high spatial frequencies to the effective neural image. Vision Research [Internet]. 1996;36 :2729–33. Publisher's VersionAbstract
The perceived blur of drifting sinusoidal gratings was compared to that of static, blurred "square wave" gratings before and after adaptation to a missing fundamental ({MF)} pattern. The results indicate that the perceived blur of a drifting sine grating is inversely related to its drift speed. However, after adaptation to a {MF} pattern, this effect is reduced. The adaptation effect is most profound for low contrast gratings. The results provide tentative evidence for a non-linear stage in motion processing which serves to introduce higher frequencies into the neural image which are not present in the original signal.
Bex PJ, Edgar GK. Shifts in the perceived location of a blurred edge increase with contrast {II}. Perception & Psychophysics [Internet]. 1996;58 :31–3. Publisher's VersionAbstract
Perceived brightness is nonlinearly related to luminance. Consequently, any mechanism operating on the (transformed) luminance profile of a blurred edge to detect its location should make errors, and the magnitude of these errors should increase with contrast. The perceived location of a blurred edge was measured at a range of contrasts and a range of blur space constants in a vernier alignment task. It was found that the perceived location of a blurred edge was affected by the contrast and the blur space constant of the edge. At low contrasts, the apparent location of the blurred edge was near the calculated location of the edge, assuming the linear transduction of luminance. At higher contrasts, the perceived location of a blurred edge was shifted toward the dark side of the edge, and the shift increased with contrast.
Bex PJ, Verstraten FA, Mareschal I. Temporal and spatial frequency tuning of the flicker motion aftereffect. Vision Research [Internet]. 1996;36 :2721–7. Publisher's VersionAbstract
The motion aftereffect ({MAE)} was used to study the temporal and spatial frequency selectivity of the visual system at supra-threshold contrasts. Observers adapted to drifting sine-wave gratings of a range of spatial and temporal frequencies. The magnitude of the {MAE} induced by the adaptation was measured with counterphasing test gratings of a variety of spatial and temporal frequencies. Independently of the spatial or temporal frequency of the adapting grating, the largest {MAE} was found with slowly counterphasing test gratings (at approximately 0.125-0.25 Hz). The largest {MAEs} were also found when the test grating was of similar spatial frequency to that of the adapting grating, even at very low spatial frequencies (0.125 c/deg). These data suggest that {MAEs} are dominated by a single, low-pass temporal frequency mechanism and by a series of band-pass spatial frequency mechanisms. The band-pass spatial frequency tuning even at low spatial frequencies suggests that the "lowest adaptable channel" concept [Cameron et al. (1992). Vision Research, 32, 561-568] may be an artifact of disadvantaged low spatial frequencies using static test patterns.
1995
Bex PJ, Brady N, Fredericksen RE, Hess RF. Energetic motion detection. Nature [Internet]. 1995;378 :670–2. Publisher's Version
Bex PJ, Edgar GK, Smith AT. Multiple Images Appear When Motion Energy Detection Fails. Journal of Experimental Psychology: Human Perception and Performance. 1995;21 :231–238.
Bex PJ, Edgar GK, Smith AT. Sharpening of drifting, blurred images. Vision Research [Internet]. 1995;35 :2539–46. Publisher's VersionAbstract
The perceived blur of moving images is less than expected given the sluggish temporal response of the visual system. This suggests that a motion deblurring mechanism may exist to preserve the positional acuity and sharpness of moving images. Furthermore, when sequences of blurred stills are presented, observers report that the moving image is in sharp focus raising the possibility that there is a mechanism which may sharpen the appearance of moving, blurred images. We have measured the effects of velocity and contrast on the perceived blur of drifting, blurred images (sine gratings and blurred edges). Subjects matched the perceived blur of drifting, blurred images to that of static, blurred images in a dimly lit room. It was found that perceived blur was inversely related to drift speed and contrast. The results confirm that moving, blurred images may appear sharper than when they are static. This finding is not consistent with some models of motion deblurring since these account only for the preservation of sharp contours that are present in the image and not for the sharp appearance of images that are in fact blurred.
Bex PJ, Edgar GK. Shifts in the perceived location of a blurred edge increase with contrast I. Perception & Psychophysics [Internet]. 1995;57 :1187–9. Publisher's VersionAbstract
Perceived brightness is nonlinearly related to luminance. Consequently, any mechanism operating on the (transformed) luminance profile of a blurred edge to detect its location should make errors, and the magnitude of these errors should increase with contrast. The perceived location of a blurred edge was measured at a range of contrasts and a range of blur space constants in a vernier alignment task. It was found that the perceived location of a blurred edge was affected by the contrast and the blur space constant of the edge. At low contrasts, the apparent location of the blurred edge was near the calculated location of the edge, assuming the linear transduction of luminance. At higher contrasts, the perceived location of a blurred edge was shifted toward the dark side of the edge, and the shift increased with contrast.
Edgar GK, Bex PJ. Vision and Displays. In: Carr K, England R Simulated and Virtual Realities: Elements of Perception. London {UK}: Taylor and Francis ; 1995. pp. 85–101. Publisher's Version

Pages