Monday, 28 July 2014

Engineering a memory?

A recent Nature paper from the Malinow lab in San Diego has attracted a lot of attention. Entitled "Engineering a memory with LTD and LTP", it describes how optogenetic stimulation of auditory inputs to the rat's amygdala were paired with mild foot shocks to produce a conditioned fear response.

Optogenetics is a genetic trick which allows the neuroscientist to activate a neuron with a flash of blue laser light. It's very trendy right now. The amygdala is a brain centre concerned with emotion, and particularly fear. You may have heard it mentioned a few times on "Firefly".

The clever thing is that the investigators were able to break this conditioned link between the blue flash and the fear response by applying a protocol called LTD to the auditory inputs, using the same optogenetic flashes but arranged in a very specific time sequence that has been known for many years to produce long term depression (LTD) at an individual synapse. Moreover, another protocol called long-term potentiation (LTP) could restore the erased fear conditioning. So it seems as though an acquired memory (in this case an association between a particular auditory input and fear) can be turned off and on at will by a bit of clever manipulation of synaptic strengths. This seems very close (at first sight) to claiming that we have discovered how to create or erase memories at will by strengthening or weakening individual synapses.

I have a couple of questions about this paper. First, why is fear conditioning so popular as an experimental model of memory? Surely there must be less unpleasant (for the rat) and just as robust protocols for producing associative memory. My friend Z tells me that fear conditioning is popular because the relevant circuitry is well understood, and the association can be produced with a single trial, so maybe not.

Second: does this experiment tell us much about how or even where this associative memory is encoded? After all, we can manipulate the strength of the synapse between an incoming auditory fibre and  its postsynaptic target in the amygdala, but the actual site of the associative memory is not necessarily that synapse, it could be much deeper in the network, perhaps several synapses further along the chain, maybe not even in the amygdala at all.  All that the LTP and LTD protocols are doing is "gating" the first synapse in the chain, allowing or preventing the incoming auditory signal to proceed further into the network. But the actual locus of memory formation is most likely elsewhere in the network, further down the chain of synaptic connections.

Nabavi S, Fox R, Proulx CD, Lin JY, Tsien RY, Malinow R (2014) Engineering a memory with LTD and LTP. Nature 511:348–352.

Thursday, 3 July 2014


I feel I can finally write about this. After all, I got the certificate in the post the other day. I think that means they're not going to take it away from me.

The other day I was walking to the station with Sue, a pleasant walk along the riverbank. I remarked that this might be a nice way to start the journey to London on my viva day.  Sue had to point out that I had already done my viva a few weeks before, it was in the past, no longer in the future.

It's been like that, especially for the first few weeks after.

This thing has been on my horizon for so long, a fearful ordeal to be faced in the future, a background anxiety every day of the months I spent writing my thesis. It seemed like a permanent feature of my mental landscape. When it was finally over and done, it was unexpectedly difficulty to remove it from my future and place it where it belongs, in the past. As far as my brainstem and limbic system were concerned, it was all still to be done.

Still, I think I'm finally getting over it now. Even my reptile brain is feeling more relaxed.

The viva itself was tough. A Tuesday in early April, I was in that windowless little meeting room for 5 hours. The questioning was friendly but thorough. Every little discrepancy I had hoped might go unnoticed was ruthlessly dug out and exposed, shrinking, to the light. The folks outside, waiting with the champagne flutes and snacks, were a little restless by 6pm when I finally stumbled out, ashen-faced and mute.

Looking back, I think I pretty much lost hope halfway through. With the front of my mind I was still fielding the questions, but the back of my mind was already busy assessing options for alternative employment. I wonder if the Thirsty Meeples cafe needs another rules explainer? I was a bit gobsmacked when at the end my inquisitors exchanged a glance, then turned to me and held out their right hands. "Congratulations Dr Haslehurst!"