This post is a slightly modified version of my answer to a Quora question:
This is a very interesting question. I can add some neuroscientific flesh to the skeleton you have already laid out.
seems to involve the ability for memories to be accessed via sensory “triggers”. This may occur via a proposed neural mechanism called Through this mechanism, “cells that fire together wire together.” In other words, if two neurons are connected together, then if they both happen to to fire at the same time, their synaptic connections are strengthened.
So to use Proust’s famous example from, let’s say you are eating a piece of cake at teatime in your aunt’s house. A neuron — or group of neurons — linked to the taste of the cake will presumably fire. Let’s call it neuron A. Similarly a neuron that is linked to the sight of your aunt will also fire at the same time — let this be neuron B. Let’s assume that neuron A projects to neuron B. Then according to Hebb’s rule, the strength of the connection between A and B increases. This in turn improves the ability of A to cause B to fire.
The more connections there are, the more opportunities for Hebbian learning. So while you are listening to a lecture or reading a book, connections with other experiences and memories are being made, rendering the information easier to access. Synesthesia may give you more “storage space”, and as a bonus, it may give you more ways of accessing that storage space.
The memory system may be like a labyrinthine mansion. Your memories are locked away in the rooms of this vast maze of a building, and to remember something is to find a way to get to the the room where it is stored. The memory mansion of someone without synesthesia may be full of rooms that each have only a single entrance. Without knowing the way to the right entrance — the right recollection strategy — the memory may be present but inaccessible. The memory system of a synesthete, by contrast, may be like a mansion whose rooms have several entrances, as well as secret passageways linking wings of the building that are usually far apart. So a synesthete may have more ways to navigate the maze of his or her own memory!
This is of course speculation, and careful experimentation and theory will be needed to come up with a more solid explanation!
There are documented cases of synesthesia co-occurring with exceptional memory:
Some more relevant citations:
This paper suggests synesthesia may not always confer an advantage. The authors say “The results indicate that synesthesia per se does not seem to lead to a strong performance advantage. Rather, the superior performance of synesthetes observed in some case-report studies may be due to individual differences, to a selection bias or to a strategic use of synesthesia as a mnemonic.”
So synesthesia may not itself be a memory-enhancing condition, but a basis from which to discover or create improved memory-recovery strategies — a way to build new doors and secret passageways in your memory mansion! 🙂