Evaluating the evidence for past life on Mars.

problem exacerbated by mixed surrounds (4, 7)], in agreement with the behavior of a major class of cortical cells (10), as well as with psychophysical evidence for color/luminance interactions (I 1). All of this bears on Masland's dichotomy between the multiplexing (co-coding) and parallel channel approaches. Models that do not filter parvo cells do not account for the properties of cortical cells. Moreover, the use of parvo cells for achromatic form perception without filtering to separate color is inappropriate. As Marr pointed out, the zero crossings of the P cell signal are ambiguous if the color signal is not removed (I 2). If the color signal is extractable, it makes little sense not to use it. Vincent A. Billock * Center for Complex Systems, Florida Atlantic University, Boca Raton, FL 33431-0991, USA