The triplex hypothesis of vision.
The duplex theory of vision is concerned with the light level and dual retinal function and refers only to the rod and cone photoreceptor cell systems. There are some visual functions that are not represented by the duplex theory, visual field, or the dark-adaptation curve. There is some confusion as to whether the relationship between visual threshold and bleached pigment is logarithmic or linear. I do not know how many photopigments exist and which pigment and what circuit plays a role in the photoperiod. The complexity of the retina appears to exceed its known functions. Finally, I wonder how the rate of eye growth is regulated. To clarify these concerns, I propose a new cell type and a third mechanism of vision, which has not been described previously to my knowledge. On the basis of this study and the data from the literature, it appears that early retinal ganglion cells cannot project into the visual cortex because the latter has not been formed. Therefore, their axons may progress into the diencephalic centers that are developing and are differentiating. So, there is a novel photoreceptive system with its associated ganglion cells that is the basis for the visual foundation. These are the first photoreceptors to form and are not engaged directly in the visual process, but they play a significant role in nutrition, function, and the well-being of the other parts of the visual apparatus. This, of course, requires much investigation and research. I believe there are three classes of photoreceptors and submit the following triplex hypothesis.
Kashani
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D001369:Axons / Q000502:physiology; D003712:Dendrites / Q000502:physiology; D005333:Fetus; D005865:Gestational Age; D006801:Humans; D009898:Optic Disk / Q000196:embryology; D009900:Optic Nerve / Q000196:embryology; D010786:Photoreceptor Cells / Q000196:embryology / Q000502:physiology*; D012165:Retinal Ganglion Cells / Q000502:physiology*; D014785:Vision, Ocular / Q000502:physiology*; D014793:Visual Cortex / Q000502:physiology