The existence of memory molecules, on the other hand, is unlikely for purely practical reasons. Scientists now believe that memories are not to be found in any single molecule or neural group, but in an intricate and ever-shifting net of firing neurons and crackling synapses distributed throughout the brain. Memory is not, as was previously thought, some vast cerebral warehouse filled with rows and rows of neatly ordered filing cabinets. It is rather more like a labyrinth, the twistings and turnings of which rearrange themselves completely each time something is experienced and recalled. Memory is a dynamic property of the brain as a whole rather than of any one specific region.
Memory resides simultaneously everywhere and nowhere in the brain.
The scientific foundation for this hypothesis was laid by the American psychologist Karl Lashley some 50 years ago. To study learning and memory in mammals, Lashley taught rats to successfully negotiate complex mazes. He then began incrementally removing thin slices of the rats' cerebral cortex in an effort to pinpoint the memory locus for this task. But no matter which section of the brain Lashley removed, the rats were still able to run the maze. The rats' performance diminished as progressively more brain was excised, but Lashley found no one region whose ablation completely erased the memory.