When a target stimulus is embedded in a high contrast surround, the target appears reduced in contrast and is harder to detect, and neural responses in visual cortex are suppressed. We used functional magnetic resonance imaging (fMRI) and psychophysics to quantitatively compare these physiological and perceptual effects. Observers performed a contrast discrimination task on a contrast-reversing sinusoidal target grating. The target was either presented in isolation or embedded in a high-contrast surround. While observers performed the task, we also measured fMRI responses as a function of target contrast, both with and without a surround. We found that the surround substantially increased the psychophysical thresholds while reducing fMRI responses. The two data sets were compared, on the basis of the assumption that a fixed response difference is required for correct discrimination, and we found that the psychophysics accounted for 96.5% of the variance in the measured V1 responses. The suppression in visual areas V2 and V3 was stronger, too strong to agree with psychophysics. The good quantitative agreement between psychophysical thresholds and V1 responses suggests V1 as a plausible candidate for mediating surround masking.

We studied lateral interactions in the periphery by measuring how contrast discrimination of a peripheral Gabor patch is affected by flankers. In the psychophysical experiments, two Gabor targets appeared simultaneously to the left and right of fixation (4 degrees eccentricity). Observers reported which contrast was higher (spatial 2-alternative-forced-choice). In different conditions, Gabor flankers of different orientation, phase, and contrast were present above and below the two targets, at a distance of three times the spatial Gabor period. The data show that collinear flanks impair discrimination performance for low pedestal contrasts but have no effect for high pedestal contrasts. The transition between these two result patterns occurs typically at a pedestal contrast which is similar to the flanker contrast. For orthogonal flanks, we find facilitation at low pedestal contrasts, and suppression at intermediate contrasts. We account for this complex interaction pattern by a model that assumes that flankers can provide additive input to the target unit, and that they further contribute to the target's gain control, but only in a limited range of pedestal contrasts; once the target contrast exceeds a critical value, inhibition becomes subtractive rather than divisive. We further make specific propositions on how this model could be implemented at the neuronal level and show that a simple integrate and fire unit that receives time-modulated inhibition behaves in a fashion strikingly similar to the model inferred from the psychophysical data.

Complex patterns that combine pixels of different brightness and colour in a fixed spatial layout pose a challenge for the objectoriented approach to image analysis. This is because the pixels that make up the pattern cannot readily be linked together into an object as they do not share spectral or textural properties. The goal of this study was to determine whether template matching techniques can be used to identify those patterns without a prior segmentation of the image. We studied three different scenarios that pose specific challenges: the application of template matching is most straightforward in (1) the detection of symbols in scanned maps where different occurrences of a given pattern strongly resemble one another; most notably, symbols always have the same size and orientation. (2) The detection of trees in aerial images of oil palms is more challenging, since the characteristic tree pattern exhibits significant variations from tree to tree. Finally, in the (3) detection of pavement markings in aerial images objects could appear at arbitrary orientations in the image. Our results show that template matching successfully identifies many target regions. Once target locations are known, objects can be created and the full power of object-based image analysis approaches can be applied to eliminate remaining false positives.

In a visual search task, a target has to be found among distractors. For two given elements A and B, the search difficulty can depend on which of the two elements is defined as the target, a phenomenon called search asymmetry. Here, we study to what degree an element's ability to 'win' in a search asymmetry depends on its absolute contrast (first-stage signal) and to what degree it depends on its contrast difference from the background (second-stage signal). One quadrant contained a target texture (2 x 2 Gabor patches of contrast c(tg)), and the other three quadrants contained distractor textures (2 x 2 Gabor patches of contrast c(dt)). These four 'foreground textures' were embedded in a background texture consisting of patches with contrast c(bg). The task was to identify which quadrant contained the target. Quadrants are referred to as increments (foreground contrast c(fg)>c(bg)), or decrements (c(fg)

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