In 3 experiments, subjects were required to detect the presence of a small region of disparate texture embedded in a larger background at a range of eccentricities. Detection performance always peaked several degrees from fixation. Experiment 1 showed that the location of the peak was not retinally specific; scaling the display changed the location of the performance peak. Experiment 2 showed that poor foveal performance could not be explained by cross-frequency interference; filtering out high spatial frequencies did not lead to improved foveal performance. Experiment 3 showed that the effect is not unique to textures comprising left and right oblique line segments. A parsimonious account of these data is that, at the fovea, there is a mismatch between the scale of the texture and the scale of the mechanisms responsible for encoding texture differences. This mismatch diminishes as the textures are moved further into the periphery.

Texton theory defines textons as such local features as elongated blobs, terminators, and line crossings. The theory states that effortless visual texture discrimination may occur between two regions only if they differ in texton density, irrespective of the spatial relationships among textons. It is argued here that line-crossing and terminator textons have not been demonstrated to function independently of configurational differences between micropatterns. Four experiments tested whether line crossings and terminators actually elicit effortless discrimination independently of configurational differences. Subjects were required to detect a disparate textured region embedded in an unpredictable quandrant of a textural display. The textural displays were presented for brief durations, ranging from 67 to 167 msec, and followed by a random dot mask. In general, when configuration was controlled, micropattems differing in terminators and line crossings elicited relatively poor discrimination. Ease of discrimination, as measured by the probability of a correct detection, was largely associated with differences in micropattem size (measured by the minimum enclosing circle). In addition, for certain texture pairs, ease of discrimination depended crucially upon which member of the pair formed the embedded region and which formed the background. This foreground/background asymmetry was also related to size differences between micropatterns forming the textures. In many cases, performance improved at longer stimulus durations, although the rate of increase and absolute level of performance depended on the particular texture pair being tested. Qualitative differences in performance between naive subjects and a highly practiced subject were also observed.Texture discrimination and segmentation have generated a great deal of interest in recent years in both the psychological literature (e.g.,

To assess the role of second-order channels in symmetry perception we measured the effects of check size, spatial frequency content, eccentricity and grey scale range on the detection of symmetrical and anti-symmetrical patterns. Thresholds for symmetrical stimuli were only moderately affected by these manipulations. Anti-symmetrical stimuli composed of large black and white checks elicited low thresholds. However, anti-symmetry became essentially undetectable at small check sizes. Removing low frequencies from large-check-size, anti-symmetrical stimuli had little effect on thresholds whereas removing high frequencies had a pronounced effect. Moving the stimuli from fixation to 8 degrees eccentricity caused a dramatic increase in thresholds for anti-symmetrical stimuli but not symmetrical stimuli. When the grey scale range was increased anti-symmetry was undetectable at any check size whereas symmetry was easily seen at all. We argue that these results and others in the literature suggest that anti-symmetry is only detected under conditions favourable to selective attention.

Bilateral or mirror symmetry is a ubiquitous feature of biological forms that the visual system could exploit for segmenting an object from a cluttered background. If this is so, the visual system may be prepared to detect symmetry at all retinal locations in parallel. Indeed, a biologically plausible model that responds optimally at axes of symmetry is quite easy to construct. Our data show, however, that if such a mechanism exists, it works with high efficiency only at the fovea. The detection of vertical bilateral symmetry embedded in random noise is very poor unless the axis of symmetry is very close to the point of fixation. This leads to the conclusion that symmetry does not play an important role in image segmentation and that it is important to the visual system only after it is fixated.

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