Linking appearance to neural activity through the study of the perception of lightness in naturalistic contexts M Maertens1, R Shapley2 |
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1Modelling of Cognitive Processes, Technische Universität Berlin, Germany |
| We address the classical question how a psychological experience, in this case apparent lightness, is linked by intervening neural processing to physical variables. We address two issues: how does one know the appropriate physical variable to look at, and how can behavioral measurements be used to probe the internal transformation that leads to psychological experience. We measured lightness transfer functions (LTFs), that is the functions that map retinal luminance to perceived lightness for naturalistic checkerboard stimuli. The LTFs were measured for different illumination situations: plain view, a cast shadow, and an intervening transparent medium. Observers adjusted the luminance of a comparison patch such that it had the same lightness as the test patches. When the data were plotted in luminance-luminance space, we found qualitative differences between mapping functions in different contexts. These differences were greatly diminished when the data were plotted in terms of contrast for which the data were compatible with a single linear generative model. This result indicates that, for the naturalistic scenes used here, lightness perception depends mostly on local contrast. We further discuss that, one may find it useful to consider also the variability of observers adjustments in order to infer the true luminance-to-lightness mapping function. |
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