Non-cardinal color mechanism elicitation by stimulus shape: Bringing the S versus L+M color plane to the table.

Neurons in the cortex typically respond best to elongated stimuli, or gratings, whereas neurons in the lateral geniculate nucleus (LGN) typically prefer circular stimuli, or spots. Further, neural mechanisms specifically tuned for non-cardinal colors largely do not emerge until the cortex; therefore, the use of gratings should better reveal non-cardinal color mechanisms. This hypothesis has been tested in the isoluminant color plane in macaque monkeys (Stoughton, Lafer-Sousa, Gagin, & Conway, 2012) and in the L-M versus L+M color plane in human subjects (Gegenfurtner & Kiper, 1992). Here, this hypothesis was tested in the third color plane, S versus L+M, in human subjects in two experiments. Experiment 1 tested 10 subjects across four directions in this color plane; Experiment 2 tested three subjects in eight to twelve color directions. Consistent with data from the other two color planes, in both experiments in the S versus L+M color plane, gratings revealed the presence of non-cardinal mechanisms more strongly than did spots.