The “null” is the “no signal” point between the differential signals.
More specifically, the bulk of the evidence supports the idea, as wonderfully laid out by Gilchrist, that the sensory apparatus is not a photometer1, or “measurement of quantity” device; it’s entirely differentials based. At the lowest level apparatus, beginning with Hartline and Kuffler, and later culminating in the famous Hubel and Wiesel work, our signals are only spatiotemporal temporal differences across the On-Off, and Off-On cells2, 3, 4, even though we think we can evaluate “quantities”.
The “null” is the “no signal” point. Given the underlying system is purely differential, without motion or (field based) difference or change, the signal becomes a “null”.
In turn, the idea of a given cognition of “colour” seeming like it is “not part of” the “region”, would be in the increment direction from the null / no signal. “Part of the region” would be a decrement, from the null / no signal. “Polarity” could be used here, with the increment being considered “positive” and the decrement being considered “negative”, but that might be a bit of a bridge too far to try. They are different signals, and could indeed be subject to an orthogonal ontology.
If one is able to withstand cat torture, the videos are online showcasing the neuronal firing with audio from Kuffler, and Hubel and Wiesel. I won’t link them here.
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1 Gilchrist, Alan, Stanley Delman, and Alan Jacobsen. “The Classification and Integration of Edges as Critical to the Perception of Reflectance and Illumination.” Perception & Psychophysics 33, no. 5 (September 1983): 425–36. The classification and integration of edges as critical to the perception of reflectance and illumination - Attention, Perception, & Psychophysics.
2 Hartline, H. K. “THE RESPONSE OF SINGLE OPTIC NERVE FIBERS OF THE VERTEBRATE EYE TO ILLUMINATION OF THE RETINA.” American Journal of Physiology-Legacy Content 121, no. 2 (January 31, 1938): 400–415. https://doi.org/10.1152/ajplegacy.1938.121.2.400.
3 Kuffler, Stephen W. “DISCHARGE PATTERNS AND FUNCTIONAL ORGANIZATION OF MAMMALIAN RETINA.” Journal of Neurophysiology 16, no. 1 (January 1, 1953): 37–68. https://doi.org/10.1152/jn.1953.16.1.37.
4 Hubel, D. H., and T. N. Wiesel. “Receptive Fields of Single Neurones in the Cat’s Striate Cortex.” The Journal of Physiology 148, no. 3 (October 1, 1959): 574–91. https://doi.org/10.1113/jphysiol.1959.sp006308.