Parvo versus magno isoluminance

Visual stimuli are isoluminant when they are uninterpretable by a system that is primarily sensitive to spatial variations in luminance but are interpretable by systems that are sensitive to variations in chromaticity or other visual variations that are invisible to the luminance system. Isoluminance stimuli are important for demonstrating that, while luminance information contributes to the representation of visual information in the brain, luminance is not the modality of representation. A common method for creating differently colored isoluminant patches is to alternately present them at a flicker frequency of 10 Hz, and to adjust the relative intensities to minimize perceived flicker (<1% luminance variation would be readily visible). This and other common methods create isoluminance only for the magno visual system--neurons that have large receptive fields and high temporal resolution. Parvo pathways involve neurons with small receptive fields, poor temporal resolution, but high spatial resolution for tasks like reading. Parvo isoluminance has been largely neglected. Procedures and results: To test parvo isoluminance, participants foveally viewed high spatial-frequency, yellow/red, and yellow/green gratings. The intensity of red and green was varied to find the points of minimal visibility. When the same colors were tested in alternating patches in a 15 Hz minimum flicker paradigm, the points of minimal visibility were profoundly different: Primarily, the utility of red light was much greater for spatial acuity than for flicker detection. These data demonstrate that isoluminance is a property that depends on the particular mixture of neurons involved, which in turn depends on the nature of the task (proportion of magno to parvo activation), the locations on the retina, and many other factors. Conclusion: Conditions for isoluminance vary greatly; it must be evaluated in circumstances as similar as possible to the tasks being studied.