Dissertation Seminar-Sean Madigan

 DISSERTATION SEMINAR
Sean Madigan
Friday, July 15 at 10 am, in Suite 326C, 3401 Walnut St.

Title: The effect of changing object orientation on lightness representation

The reflected luminance of an achromatic object is dependent on both the reflectance of the object and the amount of light reaching the object. If the object is rotated away from a light source, a change in the reflected luminance will occur. An individual may be called lightness constant if the perceived shade of gray of the object stays constant despite changes in the reflected luminance. Previous research has shown that the human visual system is partially lightness constant. Even in cases where observers show lightness constancy, however, there is often a residual perceptual difference between an object viewed at two different orientations due to the difference in raw intensity. In our experiments, we used a task designed to evaluate the underlying representation of observers’ lightness percept and the level of lightness constancy present in observers. Across two experiments, we had observers rate the dissimilarity of a stimulus set which varied in reflectance, illumination intensity, and orientation in relation to a light source. In an additional matching experiment, observers chose matches from a palette. Data were analyzed using non-metric multidimensional scaling. In contrast to the earlier literature, our data were well-described with a one-dimensional representation. This representation was highly consistent across observers and revealed no lightness constancy with respect to slant. We conclude that the representation of lightness as object orientation changes is one-dimensional. We speculate that the lack of lightness constancy seen in observers may have been related to the stimuli chosen for the experiment.