Say I define ‘in front of my body’ as the direction shared by (a) the direction in which I walk and (b) my knees move (walking ‘backward’, my knees move in the opposite direction to which I walk). I can turn my head away from that direction. Now, what is in front of my body is then not in front of my eyes. Also, this definition of the body’s direction seems quite forced to me (but I’ll put that aside for now).
Say I define ‘in front of my head’ as the side closest to the majority of sense-organs (eyes, tongue, nose, ears (just)). I can twist my eyes to the left without turning my head; what is in front of my head is not then in front of my eyes.
The Eye-Brain(or Eye-[Specific set of Neurons])
One idea might be this: the eye and something to do with the brain (the whole brain, or a particular set of neurons/neural processes) define the direction on which to judge something as being ‘in front’ and ‘behind’. To the question above: ‘What defines neural processes as being behind the eye?’, the answer is trivial: the direction is defined by the processes being behind the eye. As it were, they are the tail of the direction-defining arrow. In that answer, we might treat it as an unanalysable given that they so lie behind. There is a kind of intrinsic, basic, fundamental direction that the eye-brain system has, such that what is in front of and behind can be derived from it. And then from that, we can define what is in front of the eye, or even that the eye is at the front of the system). You better not use the eyes, as that both trivialises and complicates the definition: it becomes eye-direction = (eye+brain) direction = eye-direction.
How about this: eye+brain system has a line from the back to the front of it which can be defined as straight. But what is the ‘front-back’ here? The eyes are in front, the rest of the brain behind it.
Here is a gross counter-example I hope you never have to participate in: I can technically move one eye — say, my left — such that it faces into the connected brain (some already have had it happen as far as looking down the cheek). Now stretch the left eye so that it not only faces into the brain but passes through or over it — so that its facing out from the brain again in the opposite opposite to the right eye. If all of this was happening in the head, I would now have my left eye looking out through the hair at the back of my head.
Now, I have one eye facing backwards according to the other eye. What is ‘in front’ of each eye? An eye-brain system gives two different answers, one for each eye.
However, it does not do so independent of an understanding of the answer independent of thoughts about the brain. It takes it from their already determined direction. And that’s good — because I can’t see any good reason to suppose that either the left or right dominates here as determining eye-brain direction. Here, I’d say the eye-brain system faces in two different directions, with very different things ‘in front’ of it. And that’s because of the directions the eyes face, and what’s in front of each of them. (If still thinking about this, think also about pigeon’s eyes — and ask what’s in front of each one of them). What is said here for the brain goes also for any part of the brain, including the any specific set of neurons.