The article I would like to discuss briefly does not concern the retina, but memory in general. As such, it is linked to every part of the brain, including vision processes. 

Eric Kandel " The Molecular Biology of Memory Storage: A Dialog Between Genes and Synapses" (Nobel lecture, 2000). (The pagination refers to the pdf version.)

As always, I will skip all technical details in an attempt to get to the core of the matter as seen through a layman's eyes.

General patterns of memory formation according to Kandel:
1) Short-term memory, activated by a brief stimulus, does not require any changes to the neuron besides some internal reshuffling.
2) Long-term memory as the result of repeated stimuli results in the synthesis of a new protein and gene transcription.
3) The effect of (2) is the creation of new synapses on the target neuron.

This last effect is the most convincing element in the whole argumentation. The first two points are supposed  to describe processes that strengthen the connection between a stimulated neuron and its target. But, the descriptions Kandel gives of these processes do not differ from the processes as analyzed by Levitan&Kaczmarek (2002). They can be therefore, at least as far as I am concerned, considered as general processes that happen each time a neuron is stimulated, whether accompanied or not by memory changes.
What is also interesting is the first point. If, as Kandel and colleagues pretend, there is no radical change in the target neuron, then one can wonder how the brain can remember anything in the short-term. The explanation is that the changes following a stimulus take a little time before disappearing altogether. In this picture, the 'conformational changes", as Kandel calls them, are enough for short-term memory. So, in opposition to color sensation, we seem to have here a neural trace, maybe not of the original sensation itself, but at least of its memory. The problem is of course that this neural trace bears no resemblance whatsoever with the original sensation. But more importantly, that it is most probably the same for all, or at least indefinitely many sensations. After all, this internal reshuffling (as I call it), is not made of unique processes, but will be the same for different sensations in different parts of the body and the brain. So all this neural trace can tell us is that it is a memory of some sensation, and even that depends on our knowledge of the circumstances in which it was created. Just as with color sensation, external observers would be unable to reconstruct the original sensation that produced these effects.
The last point would appear to justify a little more optimism. The creation of a new synapse tells us that, whatever the sensation was, it was at least strong enough to trigger gene transcription and synaptic growth. Still, it is not what you would call a positive identification, and we could have said the exact opposite of this affirmation in the first case: that it was NOT strong enough to trigger those gene processes.
Last, but certainly not least, Kandel  studies the gill-withdrawal-reflex of Aplysia because his "radically reductionist approach" (p.3) demands a clearly delimited neural circuit that could be used to understand biological and molecular processes in the brain. He tells us that Aplysia has only 20,000 neurons, in comparison with the billion in human brains, and that "the simplest behaviors that can be modified by learning may involve less than 100 cells" (p.4). And that is exactly what he does, he studies those 100 cells and never asks himself what their place and connections are to the other 19,900 neurons. He speaks of behaviors of the snails, and is happy to tell us that one of his students succeeded in breding them in the lab, providing them with enough "material". 
That is, in my eyes, the biggest mistake of all, to forget that you are dealing with living organisms, and not with chemical systems. I will not mention the treatment of these animals, the euphemism of "training" is worthy of modern armies, but I could not help my indignation at the ease and innocence with which living creatures were subjected to what can only be described as torture. Kandel even speaks of behaviors and emotions ("We focused initially on one type of learning, sensitization, a form of learned fear in which a person or an experimental animal learns to respond strongly to an otherwise neutral stimulus", p.7), while at the same time denying them every attribute of life.
There is not a single question whether this "learned fear" may have played a role in the organism's behavior, and therefore influenced its neuronal responses. Kandel, and all his colleagues everywhere, find it perfectly normal to substitute the effects of these emotions with puffs of serotonine, and expect to understand how the brain works by studying these sanitized effects.
No wonder we know so little of the brain as yet.
That does not mean that the results Kandel and other researchers obtained are not important. They certainly are, and undoubtedly worth a prestigious price. Still, even though Kandel seems deeply conscious of the limitations of the reductionist approach (he advocates, at the end of his lecture, the fusion of bottom-up, molecular biology, and top-down, cognitive sciences, methods, I have the strong impression, reinforced by his more philosophical writings, that it is nothing more but necessary procrastination, until molecular biology can fully emancipate itself from these disciplines. He does not realize that the only way molecular biology could obtain its results were because scientists, each and every time, fulfilled the indispensable role of the brain as a whole, including, and in the first place, all its sensations and emotions. The  magical "5 puffs of serotonine" that were necessary and sufficient to trigger long-term memory processes are not just a handy trick to get things done. They are the chemical means through which the brain reacts to internal and external events, but they are not the brain itself. And however far they can go, scientists will always be able to find other magical puffs, and be reinforced in their conviction that they are closer than ever to a "biology of the mind". But will it be the final, ultimate puff? Or will these scientists one day understand that they are the brain they are so feverishly looking for?