Retinal and geniculate neurons respond best to circular spots (like the delphinium blossom), whereas cortical neurons respond better to gratings, lines, or bars (like the horizon between the blue sky and the amber grains) ( Conway, Chatterjee, Field, Horwitz, Johnson, Koida, & Mancuso, 2010). Not only do neurons at the different levels of processing (retina and LGN, versus cortex) show differential color tuning, but they also prefer stimuli of different shapes. The ability to reveal the presence of mechanisms tuned to respond to non-cardinal colors has varied across studies (for reviews, see Eskew, 2009 Gunther, 2014a). Thus, S– LGN neurons may be responding to non-cardinal colors.) (Note that the S+/S– pathway may be comprised of distinct S+ and S– pathways, and that S– LGN neurons have been shown to respond to a wide range of colors around 90° in Derrington, Krauskopf, and Lennie (DKL) space (e.g., Tailby, Solomon, & Lennie, 2008 Wool, Packer, Zaidi, & Dacey, 2019). All color directions beyond these three axes (including the bright blue sky and deep amber grains) are known as non-cardinal, and their neural mechanisms largely do not emerge until the cortex ( DeValois, Cottaris, Elfar, Mahon, & Wilson, 2000 Gegenfurtner, 2003). These six colors are designated as “cardinal” because there are cell types early in visual processing, in the retina and lateral geniculate nucleus of the thalamus, that respond best to these colors: L–M and –L+M are primarily underlain by the midget retinal ganglion cells (RGCs) and parvo lateral geniculate nucleus (LGN) of the thalamus cells S+ and S– are primarily underlain by the small bistratified RGCs and konio LGN cells and –L–M and L+M are underlain by the parasol RGCs and magno LGN cells or, in some cases, the midget/parvo pathway. These colors can be represented by their cone inputs: reddish as L–M, greenish as –L+M, bluish as S+, yellowish as S–, black as –L–M, and white as L+M, with L representing the long-wavelength-sensitive cones, M representing the medium-wavelength-sensitive cones, and S representing the short-wavelength-sensitive cones. Cardinal colors are reddish, greenish, bluish, yellowish, black, and white. In nature, how do our brains perceive a round bluish delphinium blossom or the horizon between a bright blue sky and the amber waves of grain? The bluish-purple of the delphinium blossom is considered to be a “cardinal” color, whereas bright sky blue and deep amber yellow are considered to be “non-cardinal” colors. 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. Experiment 1 tested 10 subjects across four directions in this color plane Experiment 2 tested three subjects in eight to twelve color directions. Here, this hypothesis was tested in the third color plane, S versus L+M, in human subjects in two experiments. 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). 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. In fact, we’re so confident in the quality of our products that we offer a lifetime warranty.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.
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