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Velitchko Manahilov, William A. Simpson, Julie Calvert; Classification images for second-order patterns. Journal of Vision 2005;5(8):229. doi: 10.1167/5.8.229.
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Vision is sensitive to first-order luminance patterns and second-order modulations of carrier contrast. The response classification technique has shown that the observers' template for detecting first-order patterns resembles the waveform of the luminance modulations. We sought to determine the relevant stimulus features used by human observers to detect contrast modulations. Two types of carrier were used: a 10-c/deg sinusoidal grating and one-dimensional static binary noise. The carrier contrast was modulated by a sinusoidal signal of 2 c/deg. The stimuli were embedded in one dimensional Gaussian static noise. The observers detected the second-order patterns in 2 alternative forced choice experiments.
We found that the classification images for contrast modulations of a sinusoidal carrier resembled the side-band component of the contrast modulations (the product of the carrier and the modulation signal). The classification images for contrast modulations of binary noise contained a component at the modulation frequency. These findings correspond to two models for detecting contrast modulations. The first one consists of a matching device whose template is a copy of the difference between the stimuli presented in both stimulation fields, which is the side-band component of the contrast modulations. The second model is based on the idea that second-order patterns are detected by a distinct second-order pathway consisting of a non-linearity followed by a matching device which extracts the modulation signal.
The results suggest that the observers may use different strategies to detect second-order patterns depending on the nature of the carrier. When the carrier is a sinusoidal grating, the template is the side-band component of the contrast modulations which could be implemented by a combination of luminance channels. When the carrier is binary noise, observers seem to use a non-linear second-order pathway whose template is a copy of the modulation signal.
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