But to this we need to add the identity relation between the neuronal activations and the perceptual categorisations they realise.
Here we can say that the material impacts of photons on the retina betoken perceptual values realised in neuronal tokens in the visual cortex (and beyond).
Epistemological Digression
Now, stepping back for a moment, we can see that this is the first stage in the explanation of what Bernstein termed ‘how the “outside” becomes the “inside”’.
It can also be seen as the beginning of an expansion of the point made by Halliday & Matthiessen (1999: 609) that:
… what is being construed by the brain is not the environment as such, but the impact of that environment on the organism and the ongoing material and semiotic exchange between the two.
You’ll notice that here we have both material doing, photons impacting retinas, and relational being, the identifying of retinal activations with neuronal activations.
You may have noticed also that most models of brain function tend to conceptualise only in terms of material processes, and doing so typically requires that “information” — in the sense of (pre-existing) “categories of the world” — travel into the brain and then along neural pathways into regions where they are somehow “processed”.
The view we offer here is more in line with Bateson’s view of information (or better: experience) as ‘difference that makes a difference’, and Halliday & Matthiessen’s view that experience is construed as meaning in semiotic systems.