Every experience leaves a trace in the brain, and some traces seem to become permanently attached to its tissue. Yet your brain’s event library is highly selective, producing edited highlights rather than a complete recording of your life and experiences to date.
More than half a century ago, the Canadian neuroscientist Wilder Penfield presented remarkable evidence that seemed to show that all our experiences might be recorded somewhere in the brain. Pen Field was treating patients for epilepsy and had the opportunity to stimulate their exposed brains just prior to surgery. Because brain tissue does not have pain receptors, brain surgery can be carried out without a general anesthetic: the wide-awake patient can help guide the surgeon to some extent by reporting how he or she feels when various parts of the brain are touched.
Memory ‘replays’
Penfield wanted to find out what would happen if an exposed brain was stimulated directly with electrodes, so he asked his patients if they would agree to have their brains touched more than usual. As he probed, he appeared to trigger snatches of memories:
‘ Yes, doctor! Now I hear people laughing .. . my friends in South Africa … yes, they’re two cousins, Bessie and An .’
‘ Oh gee, gosh – robbers are coming at me with guns!’
‘My nephew and niece… visiting me at home… getting ready to go home, putting on their coats and hats, the dining room. My mother. .. talking to them. She [is] rushed in a hurry.’
Pen field found that when he stimulated certain areas of the cortex – especially in the temporal lobes – his patients reported fragmented ‘replays’ of what seemed like long-forgotten events. This suggested to him that memories might be imprinted in the brain and, if that were the case, everything a person had ever experienced might be laid down and waiting for some stimulation to bring it back to mind.
Reinterpreting Penfield’s results
The Pen field experiments have since been replicated many times, but the original interpretation – that memories are stamped into the brain as immutable traces – has changed. Subsequent research has shown that memories are reassembled from many different fragments stored throughout the brain. That reassembly is always a new version of an event rather than a perfect replay (see page 174). Furthermore, the patients who took part in Pen Field’s experiments were probably not so much recounting memories as describing dream-like hallucinations made up of past recollections and imaginings bound together into a familiar–seeming scene. Robbers had never attacked the young boy who reported robbers corning at him – but he had often fantasized about it. What he reported while he lay on the operating table was a replay of that fantasy, not a memory of a real event.
Faint Traces and Enduring Patterns
Every experience – whether it is a real or perceived event, a thought, a feeling, an act of imagination, or a recollection of a previous experience – involves the activation of a unique neural firing pattern. Each pattern (and countless patterns are passing through the brain at any one time) leaves a faint trace in that it makes the participating neurons more likely to join together in that pattern again. The vast majority of patterns are so fleeting that their trace disappears almost immediately. So, most of what we experience is not available for recall.
However, particularly impressive events produce strong and long-lasting patterns, and these tend to pop up again and again. Every time they are reproduced, they become more likely to be reproduced in the future, so over time, they become reinforced as ‘ habits’ of mind and firmly established as memories. Those memories that are not replayed in this way fade away and are forgotten.
Experiences, then, are never frozen in memory. Rather, they leave an impression on the brain – like the outline of a body on a feather bed – which the recall may consolidate, merge into other impressions, or simply leave to fade away.
Alien Abduction
Neuroscientist Michael Persinger has devised a ‘helmet’ that uses magnetic pulses to stimulate the temporal lobes. The effect is similar to that seen in Penfield’s experiments – people describe vivid ‘memories,’ such as being abducted by aliens. Persinger suggests that these are not memories but fantasies constructed out of fragmented recollections. The other-worldly feeling may be due to the fact that the prefrontal cortex, which has an important role in checking the reality of perceptions, is bypassed.
