D.R.T. Keeble

Sensitivity to orientation modulation in micropattern-based textures.

We have measured the sensitivity of the human visual system to sinusoidal modulations of orientation in micropattern-based textured stimuli. The result is the orientation modulation function, or OMF, which describes this sensitivity as a function of the spatial frequency of orientation modulation. We found that the OMF was bandpass with peak sensitivity at spatial frequencies ranging between 0.06 and 0.2 c/deg, depending on the size of the micropatterns. The OMF was found to be scale invariant, that is its position on the spatial frequency axis did not change with viewing distance when spatial frequency was measured in object rather than retinal units. This scale invariance was shown to result from the visual system taking into account the scale rather than the density of the micropatterns as viewing distance was changed. It has been argued by Bergen [(1991) Vision and visual dysfunction (Vol. 10B) New York: Macmillan] that scale invariance in textures is a consequence of the coupling of mechanisms which detect textural features with those which detect local luminance contrasts. We reasoned that Gabor micropattern textures might therefore show narrower OMFs compared to line micropattern textures. However we found no difference in OMF bandwidth between the Gabor and line micropattern textures, suggesting that the line micropatterns were acting as selectively as the Gabor micropatterns for the spatial scale of the mechanisms which detected the orientation modulation. Evidence is presented which suggests that the mechanisms which detected the orientation modulation in our stimuli are non-linear. Finally we showed similar OMFs for sine-wave and square-wave modulations of micropattern orientation, and similar OMFs for modulations of micropattern with orientation about the horizontal and about the vertical, the direction of modulation in both cases being horizontal. The implications of these findings for the mechanisms involved in orientation-defined texture processing is discussed.

A linear systems approach to the detection of both abrupt and smooth spatial variations in orientation-defined textures

Two distinct paradigms have characterized most previous studies of texture perception: one has dealt with texture segregation, the other with the processing of texture gradients. Typically, studies of texture segregation have used stimuli with abrupt textural variations, whereas studies of texture gradient processing have used stimuli with smooth textural variations. In this study we have asked whether the mechanisms which process abrupt and smooth textural variations are the same, by considering whether a simple linear model can account for the detection of orientation modulation in micropattern-based textures with three types of modulation: sine-wave (SN), square-wave (SQ) and missing fundamental (MF). The MF waveform was constructed by removing the fundamental harmonic from a square-wave. We found a clear overall ordering of sensitivity: SQ > SN > MF. We found that sensitivity to the SQ and MF stimuli could be predicted very well from the SN data if one assumed that the r.m.s. output of a single linear channel underlay the detection of the orientation modulation. This suggests that the detection of both abrupt and smooth changes in orientation-defined textures is subserved by a common mechanism which mimics the operation of a single linear channel.

Discriminating local continuity in curved figures.

We assessed whether the visual systems ability to discriminate subtle perturbations from smoothness in curved shapes was based on 1st-order properties or 2nd-order properties. We investigated which of the two would determine performance in a task where the observer had to detect spatial jitter on aligned, unaligned or unoriented Gabor patches forming either an open or enclosed path. Surprisingly, performance was no better in the conditions employing aligned micropatterns, implicating the use of 2nd-order properties. Varying the peak spatial frequency or the size, (standard deviation of the Gaussian envelope), produced little change in the jitter threshold. By contrast, increasing the spacing between the Gabor patches had a large detrimental effect. Randomizing the orientation of the Gabors also hampered performance. These results indicate that orientation linking may only aid psychophysical performance in detection tasks. If variance was imposed on the size of the blobs (a 2nd-order property), performance was degraded. Variance on the carrier spatial frequency (a 1st-order property) resulted in a smaller worsening of performance. Overall, our results imply that shape discrimination is performed by mechanisms sensitive to 2nd-order micropattern properties, although some dependence on 1st-order properties exists.

Orientation masks 3-Gabor alignment performance.

Several workers have concluded that Gabor alignment tasks are performed by using central tendencies of the micropatterns as a cue. One reason for this conclusion was that the 3-Gabor alignment task is performed equally well whether the orientations of the patches are collinear or orthogonal to the group orientation. We wished to find out if the orientation of the micropatterns has any effect on performance. We tested subjects in 3-micropattern alignment tasks using a variety of orientational conditions. If three vertically-aligned Gabor patches were vertical, horizontal or both, or if bullseye or Gaussian blobs were used, no difference in performance was found. If, however, the orientation of the patches was randomized, performance became much worse. Similarly, if the three patches were at 45 deg, thresholds were raised. The effect of orientation was maintained across different spatial frequencies. Control conditions involving randomization of the phase of the sinusoidal carrier, or jitter on the size of Gaussian blobs, confirmed that a central tendency of the micropatterns was indeed being used by subjects, indicating that the role of orientation in this task is that of a mask, rather than of a cue.

The Significance of Hair for Face Recognition

Hair is a feature of the head that frequently changes in different situations. For this reason much research in the area of face perception has employed stimuli without hair. To investigate the effect of the presence of hair we used faces with and without hair in a recognition task. Participants took part in trials in which the state of the hair either remained consistent (Same) or switched between learning and test (Switch). It was found that in the Same trials performance did not differ for stimuli presented with and without hair. This implies that there is sufficient information in the internal features of the face for optimal performance in this task. It was also found that performance in the Switch trials was substantially lower than in the Same trials. This drop in accuracy when the stimuli were switched suggests that faces are represented in a holistic manner and that manipulation of the hair causes disruption to this, with implications for the interpretation of some previous studies.

The orientation discrimination deficit in strabismic amblyopia depends upon stimulus bandwidth

We show that the previously reported orientation deficit in amblyopia (Skottun, B. C., Bradley, A., & Freeman, R. D. (1986). Orientation discrimination in amblyopia. Investigative Ophthalmology and Visual Science, 30, 532-537) also occurs for arrays of randomly positioned Gabor micropatterns for which explanations based on either neural disarray or local neural interactions would not hold. Furthermore, when using Gabors, we show that the deficit varies with the spatial frequency and orientational bandwidth of the stimuli used to measure it. We discuss two competing explanations for this, one based on a broader underlying detector bandwidth in amblyopia (both orientation and spatial frequency) and the other based on a selective deficit of first-order, as opposed to second-order orientation processing in strabismic amblyopia. Our results favour the latter interpretation.

Detection of orientationally multimodal textures

Oriented textures were produced with the use of probability density functions modulated sinusoidally over orientation. Orientational contrast sensitivity functions (OCSFs) for a task involving the discrimination of these patterns from orientationally-random textures were found for several human observers. An inverse Fourier transform of this OCSF yielded a weighting function, or filter, defined over orientation. The weighting function is broad, with a half-height full-width of 34 deg. This orientational filter was able to predict human performance in further discrimination tasks employing a variety of probability density functions over orientation.

Nulling the motion aftereffect with dynamic random-dot stimuli: Limitations and implications

We used biased random-dot dynamic test stimuli to measure the strength of the motion aftereffect (MAE) to evaluate the usefulness of this technique as a measure of motion adaptation strength. The stimuli consisted of noise dots whose individual directions were random and of signal dots moving in a unique direction. All dots moved at the same speed. For each condition, the nulling percentage (percentage of signal dots needed to perceptually null the MAE) was scaled with respect to the coherence threshold (percentage needed to perceive the coherent motion of signal dots without prior adaptation). The increase of these scaled values with the density of dots in the test stimulus suggests that MAE strength is underestimated when measured with low densities. We show that previous reports of high nulling percentages at slow speeds do not reflect strong MAEs, but are actually due to spatio-temporal aliasing, which dramatically increases coherence thresholds. We further show that MAE strength at slow speed increases with eccentricity. These findings are consistent with the idea that using this dynamic test stimulus preferentially reveals the adaptation of a population of high-speed motion units whose activity is independent of adapted low-speed motion units.

Within-texture collinearity improves human texture segmentation

Spatial arrangement has been shown to facilitate both detection of a threshold target by collinear flankers and detection of smooth chains within random arrays of suprathreshold elements. Here, we investigate the effect of alignment between texture elements on orientation-based texture segmentation. Textures composed of Gabor elements were used in a figure-discrimination task. The degree of collinearity within the texture was manipulated, and threshold figure-ground orientation differences found. A facilitative effect of collinearity on segmentation was seen, which was insensitive to Gabor carrier phase at the texture-element co-axial spacing of 3lambda used here. The pattern of results with respect to collinearity could not be attributed simply to improved linkage of local orientation contrast at figure borders in isolation, and instead suggests a role for the figure interior in texture segmentation.

Momentum space: effects of correlation in the doubly excited state of He-like ions

Momentum properties are examined for the -like systems when . Each doubly excited state (DES) is represented in momentum space by the Dirac - Fourier transform of the Aspromallis configuration-interaction (A-CI) wavefunction. Various truncations of the natural expansions of the transformed A-CI functions are used to investigate characteristics of several probability distribution functions and momentum expectation values. The sensitivity of such properties with respect to the ordered introduction of Coulomb correlation effects is of particular interest. This is especially so in the region of low momenta since, in position (or real) space, it corresponds to the outer or valence regions of an electron density. Coulomb shifts, based on distribution functions for momentum differences, and the separate angular and radial correlation effects are analysed for each Z. Comparisons are made, in momentum space, with the doubly occupied ground states and, briefly for He, with the singly excited state . Shapes and magnitudes of the Coulomb shifts are also related to the Coulomb holes in position space. Changes in statistical correlation coefficients highlight variations in the radial and angular components of electron correlation for the and states of the He-like ions as the electronic momenta decrease in magnitude.