2nd-order summation experiments indicate multiple 2nd-order channels

L. Chukoskie, M. S. Landy

Research output: Contribution to journalArticle

Abstract

Purpose. Summation experiments have been used to determine spatial frequency channel shape and interactions between channels in the detection of low contrast patterns. Here, we perform analogous summation experiments where the components gratings and their sums are 2nd-order modulations of texture, as a means of elucidating 2nd-order texture analysis mechanisms. Methods. Stimuli were texture patterns of the form T(x,y) = √1 + Cm(x) V(x,y) + √1 - Cm(x) H(x,y), where C is the 2nd-order contrast and H and V are independent, horizontally- and vertically-oriented filtered noise textures (4 cpd peak frequency, 1 octave spatial frequency bandwidth, 30° orientation bandwidth). The square root ensures that expected Ist-order contrast power is constant across the stimulus. The spatial modulators m(x) were either single low-frequency sine wave gratings or sums of two such gratings with various contrast ratios. Absolute spatial phase was randomized across stimuli, but relative phase between the two summands was fixed. Subjects discriminated a pattern of contrast C from one of zero contrast, i.e. a noisy plaid (2IFC pattern detection). Results. In previous work (Landy & Ternes, OSA '95; Landy, ARVO '96), we found that threshold modulation contrast as a function of modulation frequency / was very broadband and scaleinvariant, and that increment threshold showed evidence of the downward sloping portion of the "dipper" function. Here, we find little or no summation between sine wave grating modulators that differ in spatial frequency by 1-2 octaves. Conclusions. 2nd-order pattern detection may involve multiple spatial frequency channels, but little or no probability summation across channels was observed in our data.

Original languageEnglish (US)
JournalInvestigative Ophthalmology and Visual Science
Volume38
Issue number4
StatePublished - 1997

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2nd-order summation experiments indicate multiple 2nd-order channels. / Chukoskie, L.; Landy, M. S.

In: Investigative Ophthalmology and Visual Science, Vol. 38, No. 4, 1997.

Research output: Contribution to journalArticle

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abstract = "Purpose. Summation experiments have been used to determine spatial frequency channel shape and interactions between channels in the detection of low contrast patterns. Here, we perform analogous summation experiments where the components gratings and their sums are 2nd-order modulations of texture, as a means of elucidating 2nd-order texture analysis mechanisms. Methods. Stimuli were texture patterns of the form T(x,y) = √1 + Cm(x) V(x,y) + √1 - Cm(x) H(x,y), where C is the 2nd-order contrast and H and V are independent, horizontally- and vertically-oriented filtered noise textures (4 cpd peak frequency, 1 octave spatial frequency bandwidth, 30° orientation bandwidth). The square root ensures that expected Ist-order contrast power is constant across the stimulus. The spatial modulators m(x) were either single low-frequency sine wave gratings or sums of two such gratings with various contrast ratios. Absolute spatial phase was randomized across stimuli, but relative phase between the two summands was fixed. Subjects discriminated a pattern of contrast C from one of zero contrast, i.e. a noisy plaid (2IFC pattern detection). Results. In previous work (Landy & Ternes, OSA '95; Landy, ARVO '96), we found that threshold modulation contrast as a function of modulation frequency / was very broadband and scaleinvariant, and that increment threshold showed evidence of the downward sloping portion of the {"}dipper{"} function. Here, we find little or no summation between sine wave grating modulators that differ in spatial frequency by 1-2 octaves. Conclusions. 2nd-order pattern detection may involve multiple spatial frequency channels, but little or no probability summation across channels was observed in our data.",
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