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If you don't know who Karl Lashley is the whole story is like this: Karl Lashley systematically tried and failed to find how memories are stored by ablating cortical tissue at varying locations in rats after maze-learning, and concluded that “This can only mean that the retention of the habit is conditioned by the total amount of functional tissue in the cortex and not, primarily, by the inherent properties of the synapses themselves” He destroyed tissue in different parts of the brain and realized that no matter where he destroyed the tissue the function remained the same. He couldn't find the actual memory itself and where it is stored.
https://pubmed.ncbi.nlm.nih.gov/19286560/
>Memories are thought to be encoded by sparsely distributed groups of neurons. However, identifying the precise neurons supporting a given memory (the memory trace) has been a long-standing challenge. We have shown previously that lateral amygdala (LA) neurons with increased cyclic adenosine monophosphate response element-binding protein (CREB) are preferentially activated by fear memory expression, which suggests that they are selectively recruited into the memory trace. We used an inducible diphtheria-toxin strategy to specifically ablate these neurons. Selectively deleting neurons overexpressing CREB (but not a similar portion of random LA neurons) after learning blocked expression of that fear memory. The resulting memory loss was robust and persistent, which suggests that the memory was permanently erased. These results establish a causal link between a specific neuronal subpopulation and memory expression, thereby identifying critical neurons within the memory trace.
>suggests
The funny part about this study is that they claim to have erased a memory by altering chemicals in the brain when in reality they removed a fear response. Fear is not the same as memory. You can't think to yourself that "now I will become afraid!" and as a result you experience fear. Fear is attached to danger. Fear of death when you see a bomb, a starving white shark, a poisonous snake or a man with a gun in his hand is not the same as the memory itself. If you remove fear from a memory you remove a feeling and not the experience. The shark is still there, the snake is still there, the bomb is still there and the man with the gun is still there but you lack fear. They don't even explain what a memory consists of and how you construct a specific memory from scratch.
https://pubmed.ncbi.nlm.nih.gov/22441246/
>A specific memory is thought to be encoded by a sparse population of neurons. These neurons can be tagged during learning for subsequent identification and manipulation. Moreover, their ablation or inactivation results in reduced memory expression, suggesting their necessity in mnemonic processes. However, the question of sufficiency remains: it is unclear whether it is possible to elicit the behavioural output of a specific memory by directly activating a population of neurons that was active during learning. Here we show in mice that optogenetic reactivation of hippocampal neurons activated during fear conditioning is sufficient to induce freezing behaviour. We labelled a population of hippocampal dentate gyrus neurons activated during fear learning with channelrhodopsin-2 (ChR2) and later optically reactivated these neurons in a different context. The mice showed increased freezing only upon light stimulation, indicating light-induced fear memory recall. This freezing was not detected in non-fear-conditioned mice expressing ChR2 in a similar proportion of cells, nor in fear-conditioned mice with cells labelled by enhanced yellow fluorescent protein instead of ChR2. Finally, activation of cells labelled in a context not associated with fear did not evoke freezing in mice that were previously fear conditioned in a different context, suggesting that light-induced fear memory recall is context specific. Together, our findings indicate that activating a sparse but specific ensemble of hippocampal neurons that contribute to a memory engram is sufficient for the recall of that memory. Moreover, our experimental approach offers a general method of mapping cellular populations bearing memory engrams.
>activation of cells labelled in a context not associated with fear did not evoke freezing in mice that were previously fear conditioned
>suggesting that light-induced fear memory recall is context specific
Same thing with this study. They see mice freeze due to a fear response but they never explain what the memory itself is made of. They can measure activity within the brain and see that mice react to stimulus but they have been conditioned to react to light in a fearful manner so this doesn't prove that memories are stored in a specific part of the brain and that you can somehow affect the actual memory since they haven't explained what a memory actually is. There is only physical action and reaction and nothing more.
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