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OK, perhaps not exactly like Eternal Sunshine, but a new paper from Sheena Josselyn's group at U. Toronto has just been published in Science demonstrating rather convincingly true selective memory erasure.  Until now, we in the neuroethics community and many in the PTSD community had been considering the implications of memory dampening - modifying the emotional impact of a memory by the use of such agents as propanalol, but in all of the experiments carried out to date, the memory itself was intact. Using an elegant set of molecular tools, specifically by coupling an inducible form of diptheria toxin to overexpression of the transcription factor CREB, Han et al. selectively ablated both a subset of neurons in the lataral amygdala and the fear memory that would normally be associated with the behavioral manipulation.  The effects were selective, robust, and long lasting.

The results are important for a number of reasons, but perhaps most importantly, they confirm a theory that has been widely held by neuroscientists for quite some time: that memories are encoded by ensembles of neurons, and that the specific neurons that encode a particular memory are sparsely distributed in the brain.  Han et al.'s results fully validate that perspective - ablating a random set of neurons in the lateral amygdala had no effect upon memory - an extremely important piece of data for our understanding of 'the memory trace'. 

The results also provide compelling data to buttress arguments which suggest that the resolution of fMRI is far too crude to successfully probe memory, given its resolution.  So mind reading with fMRI is, in my view, dead unless cellular level activity becomes discernible.  Long live fMRI.  

At the same time as providing stunningly elegant data on this important issue, this result underscores why we have little to fear in the way of specific memory erasure along the lines of Eternal Sunshine - identifying the neurons that encode a particular memory is still far off.  What Han et al. did was develop transgenic mice that allowed them to ablate the set of neurons that would be activated at some future date when fear memory was developed, rather than going in after the fact and finding the neurons that encode a particular memory and killing them. Unless we begin to infect our brains with viral vectors that change important parts of our genetic code, this experiment is not moving from the laboratory bench to the field any time soon.  Moreover, most neuroscientists believe that the memory trace is not encoded in the neurons as a whole but rather in the strength of specific synapses (once again, sparsely distributed between many neurons), and killing entire sets of neurons as Han et al. did would likely affect, although not ablate, other memories as well.

So we are still a ways off from true memory erasure.  But this result is still remarkable, and anyone interested in the issue would do well to pay attention.  That is, if your memory of this post is still intact.

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