In ‘Self-knowledge as an imperative with comments by David Parker’’ (on my website) I write: ‘contrast your most vivid dreams with your taking a walk through the countryside. Observe how the scenery changes with every step you take, how trees and their branches and leaves, the blades of the grass on which you walk, move relative to you and to each other with every step – it all is there, in front of you, behind you, around you, with every step, with every breath you perceive its real existence – and yet it all is you in so far as you see it, hear it, touch it. It is neurophysiology that enables us to fully appreciate this fascinating experience, firmly embedded as it is in the physical world, in physics, chemistry, biology; the way in which the stimuli from the external world affect our sensory receptors is one of its most important areas of study.’ David remarks: Arguably it is not so much the transduction process that is important, but how the encoded information is translated into some representation of the external world. Sensory transduction is arguably the aspect of neurophysiology about which we know most, so this is maybe an area where we are more complete in terms of understanding.
If I understand David correctly, he assumes that neurophysiology can, at least in principle, answer the question how the encoded information is translated into some representation of the external world.
In ‘Self-knowledge’ I ask whether the brain can create the world of our experience: ‘This question must be asked, for the information on the basis of which we perceive the outside world is stored and structured in the brain in a completely different way from the way in which the world we see is structured. The biochemical and electric activities of nerve cells, by means of which information in the brain is processed, proceed completely differently from the way in which our consciousness perceives movements and cessation of movements of objects and of living beings we see around us.’ David remarks: Do we know that this is completely different? We are ignorant of much of the “basic” aspects of neurophysiology, and we don’t know how these elementary effects give rise to even simpler functional effects in much simpler systems than humans (even in insects, snails etc… we lack clear links between cellular properties and behaviours). So from this I am not sure that we can start to make strong or sensible claims of how brain processing relates to our awareness of the external world.
This will not do. Let me quote Wikipedia’s succinct description of the nervous system: ‘At the cellular level, the nervous system is defined by the presence of a special type of cell, called the neuron, also known as a "nerve cell". Neurons have special structures that allow them to send signals rapidly and precisely to other cells. They send these signals in the form of electrochemical waves traveling along thin fibers called axons, which cause chemicals called neurotransmitters to be released at junctions called synapses. A cell that receives a synaptic signal from a neuron may be excited, inhibited, or otherwise modulated. The connections between neurons can form neural circuits and also neural networks that generate an organism's perception of the world and determine its behavior.’
Anatomically, I learn about neurons linked by synaptic clefts, organized into neural networks; physiologically, I learn about neurotransmitters and action potentials. Neurotransmitters move from a presynaptic neuron to a postsynaptic one through the synaptic cleft (and back), action potentials move along the axons. Both must follow the anatomical constraints of the neural network in which they take place.
As I am writing this, my eyes follow the movements of my hands and of my fingers. I cannot see how neural networks with all their imaginable complexity and intricacy can generate or model my perception of these movements. Neurophysiology informs me that I cannot see my hands and fingers moving along the key-board without photoreceptors affected by electromagnetic ways scattered by the key-board and by my hands moving along it. Photoreceptors convert these effects into signals passed along the optic nerve and processed in the visual cortex. But however many times and from however great authorities I may read that the visual system ‘detects and interprets information from visible light to build a representation of the surrounding environment’, I cannot see how the visual nervous system can do so. There must be involved in all this an entity that receives the information processed by the visual nervous system and transforms it into what I can see in front of me.