We studied the effects of contextual modulation in area V1 of anesthetized macaque monkeys. In 146 cells, responses to a single line over the center of the receptive field were compared with those to full texture patterns in which the center line was surrounded by similar lines at either the same orientation (uniform texture) or the orthogonal orientation (orientation contrast). On average, the responses to single lines were reduced by 42% when texture was presented in the surround. Uniform textures often produced stronger suppression (7% more, on average) so that lines with orientation contrast on average evoked larger responses than lines in uniform texture fields. This difference is correlated with perceptual differences between such stimuli, suggesting that physiological mechanisms contributing to the saliency ("popout") of textural stimuli operate, at least to some degree, even under anesthesia. Significant response modulation by the texture surround was seen in 112 cells (77%). Fifty-three cells (36%) responded differently to the two texture patterns; response preferences for orientation contrast (35 cells; 24%) were seen more often than preferences for uniform textures (18 cells; 12%). The remaining 59 cells (40%) were similarly suppressed by both texture surrounds. Detailed analysis of texture modulation revealed two major components of surround effects: (1) fast nonspecific ("general") suppression that occurred at about the same latency as excitatory responses and was found in all layers of striate cortex; and (2) differential response modulation that began about 60-70 ms after stimulus onset (about 15-20 ms after the onset of the excitatory response) and was less homogeneously distributed over cortical layers.

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Texture arrays of line elements at various orientations were used to study three phenomena of preattentive vision. Subjects were asked (1) to discriminate texture areas and to distinguish their form (experiments on texture segmentation); (2) to detect salient or vertical line elements (experiments on pop-out); and (3) to identify configurations of similar or dissimilar targets (experiments on grouping). Within the patterns, line orientation was systematically varied to distinguish the effect of differences between areas from the effect of similarity within areas. In all of the experiments, performance was found to depend on local orientation contrast at texture borders rather than on the analysis ofline orientation itself. Texture areas were correctly identified only when the orientation contrast at the border well exceeded the overall variation of line orientation in the pattern. Similarly, only target elements with high local orientation contrast were detected fast and "in parallel." Targets with an orientation contrast lower than background variation required serial search. Preattentive grouping was found to depend on saliency, as defined by local orientation contrast, but not on the similarity of line elements. In addition to local orientation contrast, which played an important role in all of the visual phenomena studied, influences from the alignment of line elements with the outline of a figure were also seen.Three spontaneous perceptual phenomena have been described as occurring in the visual inspection of line arrays with differently oriented elements: (1) The pattern may segregate into distinct areas, each of which appears to be separated from neighboring areas by a clearly visible (but physically nonexistent) texture border (texture segmentation; Beck, 1966bBeck, , 1972Beck, , 1982Julesz, 1975Julesz, , 1984Olson & Attneave, 1970). (2) Individual line elements at an orientation different from that of the rest of the pattern appear to be particularly salient and are instantaneously detected in search experiments (pop-out;Treisman, 1985Treisman, , 1986Treisman & Gormican, 1988). (3) Lines at similar orientations may be seen as grouping together, forming distinct ensembles of elements within the line pattern (grouping; Beck, 1966a Beck, , 1967. Although these phenomena have been discovered independently of each other, and although they have been quantitatively assessed in different ways, they share important properties. They can all be achieved by preattentive visionthat is, without detailed analysis of the pattern-and they all seem to recruit from the same set of visual features. In general, elements that do not segregate in texture segmentation tasks also do not pop out from each' other or provide perceptual grouping. Controversial reports, ac-I would like to thank Peter Ruhlender for a suggestion concerning Experiment 5 and Jacob Beck, Rick Gurnsey. Lester Krueger. Jeremy Wolfe. and an anonymous referee for helpful comments on an earlier version of the manuscript. I am grateful to all of the subjects ...