Abstract
BackgroundWhen we are viewing natural scenes, every saccade abruptly changes both the mean luminance and the contrast structure falling on any given retinal location. Thus it would be useful if the two were independently encoded by the visual system, even when they change simultaneously. Recordings from single neurons in the cat visual system have suggested that contrast information may be quite independently represented in neural responses to simultaneous changes in contrast and luminance. Here we test to what extent this is true in human perception.Methodology/Principal FindingsSmall contrast stimuli were presented together with a 7-fold upward or downward step of mean luminance (between 185 and 1295 Td, corresponding to 14 and 98 cd/m2), either simultaneously or with various delays (50–800 ms). The perceived contrast of the target under the different conditions was measured with an adaptive staircase method. Over the contrast range 0.1–0.45, mainly subtractive attenuation was found. Perceived contrast decreased by 0.052±0.021 (N = 3) when target onset was simultaneous with the luminance increase. The attenuation subsided within 400 ms, and even faster after luminance decreases, where the effect was also smaller. The main results were robust against differences in target types and the size of the field over which luminance changed.Conclusions/SignificancePerceived contrast is attenuated mainly by a subtractive term when coincident with a luminance change. The effect is of ecologically relevant magnitude and duration; in other words, strict contrast constancy must often fail during normal human visual behaviour. Still, the relative robustness of the contrast signal is remarkable in view of the limited dynamic response range of retinal cones. We propose a conceptual model for how early retinal signalling may allow this.
Highlights
During the viewing of a natural visual environment, every saccade abruptly changes both the mean luminance and the contrast structure falling on the fovea or any other region on the retina
Statistical analyses of natural images [5] suggest that mean luminance and local contrast at different retinal locations are only modestly correlated [1,2, but see, 6]
In experiment 1, the mean luminance of the entire screen increased by seven-fold simultaneously with the onset of the target grating
Summary
During the viewing of a natural visual environment, every saccade abruptly changes both the mean luminance and the contrast structure falling on the fovea or any other region on the retina. Statistical analyses of natural images [5] suggest that mean luminance and local contrast at different retinal locations are only modestly correlated [1,2, but see, 6] This has led researchers to look for an analogous independence in the visual system. To probe the time course of the effect of the luminance change, the target stimuli were presented at various delays in relation to the luminance step We find that both upward and downward luminance steps attenuate the perceived contrast of a simultaneously presented target, the upward step somewhat more than the downward step. We suggest that the failure of contrast-luminance independence under simultaneous changes of both parameters reflects the fact that luminance adaptation in the retina cannot be instantaneous
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