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ES Kachinsky, VC Smith, J Pokorny; Contrast Detection and Polarity Identification in inferred On- and Off-Pathways . Invest. Ophthalmol. Vis. Sci. 2002;43(13):2841.
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Purpose: Previous work showed that the inferred PC-pathway has similar contrast detection and polarity identification thresholds for both color and luminance. Thresholds for polarity identification for the inferred MC-pathway were circa 0.2 log unit higher than for contrast detection. The goal here was to ask whether polarity identification for an inferred MC-pathway stimulus matched detection when the stimulus conditions isolated either on- or off-pathways. Methods: The observer's task was to identify which 1 deg square in a 4-square array differed from the others (contrast detection), and whether the difference was an increment or decrement in luminance (polarity identification). We measured luminance increments and decrements using Pedestal-Δ;-Pedestal stimuli in a four-alternative spatial forced choice design. The 4-square array of 182 Td was presented in an 8 deg surround of 115 Td. During the test period, the entire 4-square array changed in retinal illuminance (the Δ;-pedestal) varying between 91 and 363 Td with the test square incremented or decremented from the other three by a different amount. The temporal presentation of the test was a 26.6 ms rectangular pulse. Four interleaved staircases were used to assess increment and decrement detection and identification. Thresholds for each run were estimated from the average of 10 reversals/staircase. The final threshold values for each condition were calculated from the average of three repetitions. Results: Contrast detection thresholds matched polarity identification when the decrement direction was measured with an increment Δ;-pedestal or the increment direction was measured with a decrement Δ;-pedestal. Detection of the increment direction on an incremental pedestal or the decrement direction on a decremental pedestal revealed sub-threshold summation. For these conditions, polarity identification thresholds were higher than detection. Conclusion: When a Δ;-pedestal is used to stimulate on- or off-pathways, polarity identification can be as good as detection in the inferred MC-pathway. Deficits persist in the region of sub-threshold summation.
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