It's been known for a long time that electrical stimulation of the brain's temporal lobe can sometimes evoke vivid memories.
The famous neurosurgeon Wilder Penfield first noticed this effect as part of his pioneering stimulation experiments, but he believed that it was both uncommon and haphazard with any given stimulation able to evoke any memory, more or less at random. A new paper, however, says different. Philadelphia's Joshua Jacobs et al report that they found a spot in the left temporal lobe of a male patient, stimulation of which evoked memories of the man's time at high school. The guy was in his 30s at the time, so these are quite distant memories. When it first happened, the patient is reported to have said:
I'm, like, remembering stuff from, like, high school…. Why is this suddenly popping in my head?
Repeated stimulation of the same electrode - but not nearby electrodes - caused other high school memories to emerge. Even more interestingly, when the same stimulating electrode was used to record activity during memory retrieval, the "high school spot" was found to be significantly less active when high school was being remembered, compared to when various other kinds of memories were being accessed. This graph shows that all kinds of memories evoked high-frequency activity in the high-school zone, but high-school memories did so less:
No other electrode location caused the same effects (or indeed, any detectable memory effects), although as you can see on the image at the top, the electrode coverage was not huge. A little background: the guy had these electrodes in place because he suffered from epilepsy, resistant to medication, which was believed to originate in the temporal lobe. Temporal lobe epilepsy can cause memory phenomena rather like this, but this patient had never experienced that, and the electrically-evoked memories were experienced as entirely novel. It's a nice case report and it raises many questions. Why is the high-school spot less active during memory retrieval? That seems the wrong way around (I did a double-take to make sure I was reading it properly). And what would happen if you somehow disabled (or overactivated) this area, and asked him to remember a particular school memory? Would he draw a blank, or would he remember it but without the "high-school-ness"? If the latter, what would that feel like? Either way, this case suggests that memories are stored in the brain "by topic", in the sense that "similar" memories are associated with nearby areas of the brain. At least sometimes. But then, why didn't nearby electrodes evoke other memories? If there's a high-school spot, why not a kindergarten spot, a my-first-job spot? Maybe those spots lay in areas with no electrode coverage... but the fact that many temporal electrodes didn't bring back any memories suggests that there's lots of cortex which isn't part of a "spot". Perhaps those areas are "spare", waiting to be used up? Clearly, he wasn't born with a high school spot. It must have emerged during high school. But in that case there had to be a "blank" area first.
Jacobs J, Lega B, and Anderson C (2011). Explaining How Brain Stimulation Can Evoke Memories. Journal of cognitive neuroscience PMID: 22098266
