Substantial evidence has accumulated for the notion that modulations of second-order properties in the visual scene are processed by mechanisms which detect contrast variations within narrow orientation/spatial frequency channels. It has also been suggested that mechanisms exist which detect contrast modulations across all orientations. Many naturally occurring texture variations (e.g., modulations in orientation and/or spatial frequency) involve simultaneous contrast modulations in multiple channels. Contrasting conclusions have been drawn regarding the manner in which the information carried in multiple channels is combined. In a series of two experiments it is shown that simultaneous contrast modulations in two narrow orientation bands are detected by three mechanisms, two of which detect contrast modulations within the modulated bands only, the third of which integrates contrast across orientations in order to detect modulations of overall contrast. The three mechanisms combine their efforts by probability summation.