Chromatic input to motion processing in the absence of attention.

While several previous psychophysical and neurophysiological studies have demonstrated chromatic (red/green) input to motion processing, the nature of this input is still a matter of debate. In particular, recent controversy has developed regarding whether chromatic motion processing relies on lower-level processes [J. Neurosci. 14 (1994) 4854; 19 (1999) 6571] versus higher-level, attention- or salience-based mechanisms [Science 257 (1992) 1563; Proc. Natl. Acad. Sci. 96 (1999a) 8289; 96 (1999b) 15374]. In this study, we investigated the degree to which chromatic motion is influenced by attentional mechanisms by employing a dual-task paradigm, which allowed us to compare the strength of chromatic motion under conditions of poor versus full attention. Here, we found that for equiluminant red/green gratings, chromatic motion processing is as robust in poor, as in full, attention conditions. This lack of an attentional effect suggests that chromatic motion processing must rely, at least in part, on lower-level (i.e., pre-attentive) motion mechanisms. For non-equiluminant (e.g., red brighter than green) gratings, however, attention significantly modulates chromatic motion strength. Possible explanations for this latter result are discussed in the context of inherent salience differences between the bright-red and dim-green stripes of the heterochromatic grating.