Forgotten Memories Brought Back in Mice? Hold Up...

A study published yesterday in Cell[1] has purportedly found that memories formed as infants may be able to be retrieved as adults using optogenetic techniques, and various media outlets have enthusiastically inferred that this suggests we may be able to remember the events from our infancy, some even suggesting we may be able to remember our own birth. 

The study, led by psychologist Paul Frankland, was based on previous research by the group which found that the “forgetting” of memories during infancy may be a result of high levels of hippocampal neurogenesis at this age – i.e. the neurons representing the memories are replaced by new neurons, thus erasing the memories. 

The aim of the current study was to determine if the memories formed during infancy are permanently lost due to a failure in encoding during infancy, or become progressively inaccessible over time due to a progressive loss in the ability to retrieve them as the mice age. 

The researchers placed young mice into a box and gave them a foot-shock, so that when they are placed back into the box they freeze in anticipation of the shock - a classic fear-based training paradigm. The mice had been engineered to contain a specific set of light-sensitive neurons in a particular region of the hippocampus involved in the formation of memories - the dentate gyrus - which allowed the researchers to activate these neurons by firing a laser at them. They then placed the same mice back into the box as adults and activated this set of neurons, thus reinstating the memory and causing the mice to freeze in anticipation of the shock. 

The authors seem to suggest that this means that some hidden traces of the memories created during infancy were retained and were able to be recalled by the researchers by optogenetically activating the specific set of hippocampal neurons which were observed to be activated during the contextual fear encoding (the “dentate gyrus encoding ensemble”), positing that the reactivation of these ensembles was sufficient for “memory recovery” in adulthood. 

They then quantified the activity of an activity-regulated gene, c-Fos, in cortical and subcortical brain regions following the fear learning event. The purpose of this was to determine whether fluorescently-tagged neurons activated during the memory encoding event were preferentially reactivated during the “memory recall” in adulthood – which of course they were, implying successful recall of the memory. 

However, while they may have been able to reactivate a neural pathway they have essentially programmed into the mice’s brains during infancy, I’m not so sure it was the actual memories themselves which were “recovered”. 

It’s worth nothing that these are memories which the researchers have created in the mice by giving them foot-shocks within a particular environmental context; they are not naturally formed memories. 

“When the infant mice were placed in the box and the laser was turned on, the animals’ memories of the electric shock returned and they froze in place.” 

The fact that the mice freeze when they are placed into the same environment in which they were given a foot-shock during infancy does not necessarily mean that they remember receiving the foot-shock. It simply means that a particular behaviour (freezing) has been programmed into their brains and artificially re-instated by firing lasers at the neurons underlying this behaviour. It means that the same neural pathways resulting in a fear-based freezing response were activated – but these may be totally separate from the pathways containing the actual memory of the event (if they even exist). Besides, we are talking about a simple, conditioned response here; an instinctual behaviour in response to pain – much like you learn to quickly move your hand away from a hot plate – not an actual subjective, detailed memory of an event. It’s possible that the mice would freeze if placed in an entirely different context and the same light-sensitive neurons artificially reactivated.

“To first induce memory formation in the animals, the scientists placed the mice in a box and gave them a mild foot shock. While young adult mice retained this memory and froze when put in the box a second time, infant mice forgot this fear-related memory after a day and behaved normally when they encountered the box again” 

Percent freezing levels declined with retention delay in P17, but
not P60, mice. From Guskjolen et al., 2018, Figure 1(B).

So the infant mice were not forming the memories? This appears to contradict the conclusions of the study – i.e. that those memories are simply hard-to-retrieve; hidden deep within the brain, unable to be recovered by natural cues, and that direct stimulation of the engram (in combination with re-exposure to the training context) may reinstate the connections, leading to memory recovery. How can we say that the memories are simply difficult to retrieve when we are unsure if they were ever formed in the first place? 

“However, we found that opto-stimulation of neural ensembles that were engaged during training was sufficient to induce conditioned freezing at the same retention delays. These results suggest that the underlying engram corresponding to the fear conditioning event is not completely overwritten. Rather, this engram presumably exists in an otherwise inaccessible, dormant state, in which “natural” reminders (such as exposure to the training context) most often do not induce successful reactivation(...) This pattern of results is reminiscent of other amnestic states, including mouse models of retrograde amnesia and Alzheimer’s disease, in which opto-stimulation of tagged encoding ensembles (but not presentation of natural cues alone) permits memory recovery." 

Again, the conclusions drawn assume that the artificially activated ensembles encode the actual memory of the event itself – which is not only not confirmed, but hard to believe considering when the mice were put back in the box the second-time as infants, they had not remembered the fear-related memory supposedly created the day before. How, then, do we know that the memory was encoded at all? How do we know it is the memory that is recalled, and not simply a programmed, artificially instated fear-response? This is too simplistic of a model to draw such far-fetched conclusions, and we certainly can’t say that this type of “forgetting” in infancy is akin to other types of amnesia such as that in Alzheimer’s disease. They are completely different processes, at completely different ages. 

Furthermore, less cortical “re-engagement” was observed following optogenetic stimulation of the dentate gyrus engrams in mice trained as infants compared to those trained as adults, further highlighting the possibility that the memories which were purportedly retrieved may not have been formed at all in the infants. 

“Indeed, whereas adult contextual fear memories are successfully consolidated over the course of weeks, equivalent infant memories are being actively forgotten during this period and therefore perhaps not successfully consolidated in the cortex (…) opto-stimulation of tagged dentate gyrus ensembles leads to recovery of an engram that is qualitatively different (and likely impoverished) compared to the equivalent representation in adult animals.” 

The authors even concede that the “memory recovery” did not persist into the light OFF periods – i.e. when the trained mice were placed into the box as adults, they did not freeze unless the hippocampal engrams engineered to be light-sensitive were activated by the researchers – a pattern which has been observed in similar studies involving reactivation of tagged engram cells in the dentate gyrus [2–7]. 

While the study further adds weight to the idea that infantile forgetting is likely due to a failure of memory encoding in the infant brain (something which we knew anyway), the methods used are simply insufficient to be able to draw some of the conclusions the authors propose, and the study certainly does not suggest that we may be able to recover our infantile memories anytime soon.

 References: 

[1] A. Guskjolen, J.W. Kenney, J. de la Parra, B.A. Yeung, S.A. Josselyn, P.W. Frankland, Recovery of “Lost” Infant Memories in Mice, Current Biology. 0 (2018). doi:10.1016/j.cub.2018.05.059.

[2] X. Liu, S. Ramirez, P.T. Pang, C.B. Puryear, A. Govindarajan, K. Deisseroth, S. Tonegawa, Optogenetic stimulation of a hippocampal engram activates fear memory recall, Nature. 484 (2012) 381–385. doi:10.1038/nature11028.

[3] T. Kitamura, S.K. Ogawa, D.S. Roy, T. Okuyama, M.D. Morrissey, L.M. Smith, R.L. Redondo, S. Tonegawa, Engrams and circuits crucial for systems consolidation of a memory, Science. 356 (2017) 73–78. doi:10.1126/science.aam6808.

[4] D.S. Roy, S. Muralidhar, L.M. Smith, S. Tonegawa, Silent memory engrams as the basis for retrograde amnesia, Proc. Natl. Acad. Sci. U.S.A. 114 (2017) E9972–E9979. doi:10.1073/pnas.1714248114.

[5] T.J. Ryan, D.S. Roy, M. Pignatelli, A. Arons, S. Tonegawa, Memory. Engram cells retain memory under retrograde amnesia, Science. 348 (2015) 1007–1013. doi:10.1126/science.aaa5542.

[6] D.S. Roy, A. Arons, T.I. Mitchell, M. Pignatelli, T.J. Ryan, S. Tonegawa, Memory retrieval by activating engram cells in mouse models of early Alzheimer’s disease, Nature. 531 (2016) 508–512. doi:10.1038/nature17172.

[7] S. Ramirez, X. Liu, P.-A. Lin, J. Suh, M. Pignatelli, R.L. Redondo, T.J. Ryan, S. Tonegawa, Creating a false memory in the hippocampus, Science. 341 (2013) 387–391. doi:10.1126/science.1239073.