Biomedical subjects

D R Keeble

Publications and source records attributed to D R Keeble.

10 recordsLinked to original sources

PubMed2001

Micropattern orientation and spatial localization.

A current, popular, theory of spatial localization holds that the visual system represents the location of simple objects by a single positional tag, the accuracy of which is largely independent of the internal properties of the object. We have already presented evidence of the limitations of such a view (Keeble & Hess (1998). Vision Research, 38, 827-840) in that 3-micropattern alignment performance was found to be dependent on the orientation of the micropatterns. We tested whether this was caused by a local anisotropy in positional coding by conducting 3-micropattern bisection experiments with varying patch orientation. No corresponding effect of patch orientation was found, implying a difference in the mechanisms used for the two tasks. In a further experiment we show that alignment task performance is very similar to the otherwise identical 2-patch orientation discrimination task. We conclude that the 3-micropattern alignment task is mediated by orientational mechanisms. We therefore present a 2nd-order orientation model for 3-patch alignment.

Anisotropy↗

PubMed2000

Luminance spatial frequency differences facilitate the segmentation of superimposed textures.

Do superimposed textures segregate on the basis of a difference in their luminance spatial frequency? We addressed this question using orientation-gratings, which consist of dense arrays of Gabor micropatterns whose orientations vary sinusoidally across space. Two orientation gratings of the same texture spatial frequency were combined in anti-phase, to produce a 'dual-modulation' orientation grating. Thresholds for detecting the dual-modulation gratings were measured as a function of the difference in Gabor spatial frequency between the two grating components. When the two components were made from the same Gabors, thresholds were relatively high. However a one octave difference in Gabor spatial frequency between the components caused thresholds to fall close to those of single-modulation orientation gratings. The fall in threshold was accompanied by a change in appearance of the stimulus; to that of two transparent, interwoven, flow patterns. We show that these results are incompatible with current Filter-Rectify-Filter models of 'second-order' pattern detection. Rather, they favour the idea that feature analysis precedes texture analysis, with the visual system encoding local orientation content prior to the texture stage.

Humans↗

PubMed1999

On the mechanism for scale invariance in orientation-defined textures.

Texture perception is generally found to be scale invariant, that is, the perceived properties of textures do not change with viewing distance. Previously, Kingdom, F. A. A., Keeble, D. R. T., & Moulden, B. (Vision Research, 1995, 35, 79-91) showed that the orientation modulation function (OMF), which describes sensitivity to sinusoidal modulations of micropattern orientation as a function of modulation spatial frequency, was scale invariant--peak sensitivity occurred at a modulation spatial frequency which was invariant with viewing distance when modulation frequency was plotted in object units, e.g. cycles cm-1. We have attempted to determine the mechanism underlying the scale invariant properties of the OMF. We first confirmed that the OMF was scale invariant using Gabor-micropattern textures. We then measured OMFs at a number of viewing distances, while holding constant various stimulus features in the retinal image. The question was which stimulus feature(s) disrupted scale invariance when manipulated in this way. We found that the scale (size) of the micropatterns was a critical factor and that the most important scale parameter was the micropatterns' carrier spatial frequency. Micropattern length and density were shown to have a small influence on scale invariance, while micropattern width had no influence at all. These results are consistent with the idea that scale invariance in orientation-defined textures is a consequence of 'second-stage' texture-sensitive mechanisms being tied in spatial scale selectivity to their 'first-stage' luminance-contrast-sensitive inputs.

Humans↗

PubMed1999

Discriminating local continuity in curved figures.

We assessed whether the visual system's 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.

Discrimination, Psychological↗

PubMed1999

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.

Amblyopia↗

PubMed1998

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.

Cues↗

PubMed1997

The orientational resolution of human texture perception.

A major determinant of human texture segregation and discrimination is the orientational content of the stimuli used. We have investigated the ability of observers to resolve features defined in the orientation domain in a variety of textures. It was found that features had to be separated by at least 13 deg for subjects to discriminate orientationally bimodal textures from same-variance unimodal textures. For larger separations, the determinant of performance was the magnitude of the central "dip" in the probability density functions determining the bimodal textures. Resolution performance can be modelled by assuming that a filtering process over orientation demodulates the central dip in the bimodal texture and that discrimination depends on criterion depth in the resulting function. Such modelling produces relatively broad estimates of the bandwidth ranging from about 10-20 deg. Performance was similar for both line and Gabor micropattern stimuli.

Discrimination, Psychological↗

PubMed1996

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.

Humans↗

PubMed1995

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.

Contrast Sensitivity↗

PubMed1995

The perceived orientation of aliased lines.

The use of raster display devices for the display of graphics causes problems of aliasing when edges or lines are produced. This can be significant in those psychophysical experiments where the orientational properties of the stimulus are important. We have assessed the perceived orientation of a selection of aliased lines by comparing them with the orientation of pairs of dots. It is found that the perceptual orientation is modelled well by a least-squares metric on the pixels that compose the line. Small deviations from this metric were found, and were also found in a control experiment employing anti-aliased lines. They appear to be due to range effects. Averaged across subjects, orientational acuity was only slightly lower for aliased lines.

Data Display↗