Abstract
PURPOSE: Directing attention toward a stimulus has been shown to enhance processing for various types of stimuli, including motion stimuli. It has been suggested that attention may act to change the strength of a stimulus by increasing its “effective contrast” (Treue & Trujillo, 1999). This predicts that attention should shift the contrast response functions of underlying neural mechanisms horizontally, enhancing processing for stimuli of intermediate luminance contrast the most. Alternatively, if the effects of attention operate separately from the encoding of stimulus contrast, the magnitude of attentional modulation is predicted to be independent of stimulus contrast. To test these two alternative hypotheses, we measured the effect of attention on the strength of motion processing, assessed by the duration of the motion aftereffect (MAE), as a function of contrast. METHODS: Stimuli were achromatic sine wave gratings presented at 11 different contrasts (4–80%). For each stimulus contrast, MAE duration was obtained for two conditions: 1) Full Attention- subjects were instructed to attend to the adapting motion stimulus, and 2) Poor Attention- subjects were instructed to perform an attentionally demanding vowel counting task in the center of gaze while the adapting motion stimulus was presented. Thus, subjects' attention to the adapting stimulus was greatly diminished in condition #2. In both conditions, a static test stimulus was presented after 30 seconds of adaptation. Subjects responded with a key press to indicate when the MAE was no longer perceived. RESULTS & CONCLUSIONS: The effect of attention on MAE duration was relatively constant (∼ 2-fold) over a wide range of stimulus contrasts, even those for which MAE duration had saturated (at roughly 33 – 80% contrast). These results are consistent with the notion that attention and stimulus contrast influence motion processing independently.