Unipolar chromatic contrast mechanisms behave similarly to, but independently from, classically conceived bipolar chromatic contrast mechanisms
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The encoding of chromatic contrast is traditionally viewed as a bi-directional, opponent process wherein two opposite colors are compared by a given set of contrast sensitive neurons. While there is substantial evidence for neural systems from the retina to the cortex that are inhibited by one hue and excited by another (bipolar) there is also strong evidence for cortical neurons whose level of activation is largely affected by a single hue (unipolar). The presence of both unipolar and bipolar mechanisms has been demonstrated by chromatic contrast adaptation and masking studies. The results of the present experiments show that unipolar and bipolar chromatic contrast mechanisms share the same properties of transitivity and homogeneity. This is particularly interesting given evidence for asymmetries between opposite unipolar mechanisms, differences that are ignored when treating contrast as being bipolar only. The experiments detailed herein use a traditional contrast matching technique used for bipolar stimuli to show that unipolar stimuli have similar properties while also showing significant deviations from an assumed bipolar symmetry present in classical models. Visual evoked potentials (VEP) elicited by unipolar stimuli also show similar patterns to those elicited by bipolar stimuli. Evidence for unipolar mechanisms such as asymmetry and selective chromatic adaptation from VEP data are non-significant, however the data hint at possible effects that may be revealed with greater statistical power. There is great potential for further exploration of the differences and similarities in perceived contrast between unipolar and bipolar stimuli and of the selective effects of short term adaptation to unipolar stimuli.