As a neuroscientist, when I think of the retina I am trained to think of a precise set of neurons that functions like a machine, grinding out the visual basis of the world and sending it on to the brain. It operates independently of the rest of the system with the only feedback coming from muscles that move the eye around and dilate the pupils. So when someone [Philipp Berens] casually mentioned to me that yes, the retina does in fact receive signals from the brain? Well, I was floored.
I suppose I should not have been surprised. In fruit flies, there has been a steady accumulation of evidence that the brain sends signals to the eye to get it ready to compensate for any movement the animal will make. Intuitively, that makes a lot of sense. If you are trying to make sense of the visual world, of course you would want to be able to compensate for any sudden changes that you already know about.
It turns out that there is a huge mass of feedback connections from the brain to the retina in birds and mammals, something termed the centrifugal visual system. And inputs are sent via this system from both visual areas and non-visual areas (olfactory, frontal, limbic, and so on). So imagine – your eye knows about what you are smelling. Why? In order to do what?
The answer, it turns out, is that we don’t know. It sends all sorts of neurotransmitters and neuromodulators. The list of peptides it sends are long (GnRH, NPY, FMRF, VIP, etc) as is the list of regions that send feedback to the retina. It seems as if which regions send feedback to the retina is very species-specific, suggesting something about the environment each animal is in. But why?
This is a post long on questions and short on answers. It is more a reminder that the nice, feedforward systems that we have simple explanations for are really complex, multimodal systems designed to create appropriate behaviors in appropriate circumstances. Also it is a reminder to myself about how little I know about the brain, and how mistaken I am about even the simplest things…
I would love someone more knowledgable than me to pipe up and tell me something functional about what these connections do?
Repérant J, Médina M, Ward R, Miceli D, Kenigfest NB, Rio JP, & Vesselkin NP (2007). The evolution of the centrifugal visual system of vertebrates. A cladistic analysis and new hypotheses. Brain research reviews, 53 (1), 161-97 PMID: 17059846
Vereczki, V. The centrifugal visual system of rat. Doctoral Thesis. PDF.