It is almost philosophical to think that our mental representations, imagery, reasoning, and reflections are generated by electrical activity of interconnected brain cells. And even more so is to think that these abstract phenomena of the mind could be enhanced by passing electricity through specific cellular networks in the brain.
Yet, it turns out these tenets can be subjected to empirical experimentation. Groundbreaking discoveries made by Dr. Wilder Penfield with his patients undergoing open-brain surgeries to treat epilepsy opened a new field of brain research exploring physiological substrates of human memory. By applying electrical current in discrete regions of the brain’s cortex, Penfield induced a conscious experience of facts and events from the recent and distant past. For instance, one of his patients described re-living a childhood episode of a circus show with the vividness of detail associated with such a unique and captivating event. This phenomenal effect of electrical stimulation on memory experience was described in a series of patients stimulated in different brain regions.
In summary, the effect was found in defined areas of the cortex when electrical current was applied with optimal frequency and amplitude parameters for inducing the memory experiences. This led Penfield and colleagues to hypothesize that the stimulation worked by activating some specific physiological substrates storing memory traces of the remembered events. The idea is brilliantly simple. Stimulating networks of brain cells at a set of parameters, which match their physiological activity when memories are encoded, can reverberate a memory trace and result in its conscious experience. Waves of the applied electrical current would in a way entrain and re-activate groups of connected cells supporting specific mental representations. Around the time of these experiments, Polish neurophysiologist Dr. Jerzy Konorski and Canadian neuropsychologist Dr. Donald Hebb independently proposed a mechanism through which such connected groups of cells, aka neuronal assemblies, could form the physiological substrate for our mental constructs and their memory traces.
Since these initial ideas and findings, technological progress has been made to modulate these hypothetical substrates in attempt to enhance human memory performance. Several studies of individual cases or small groups of patients reported both positive and negative effects of electrical stimulation in a range of brain regions. The positive effect was manifested in improved reaction time or response accuracy in various tests probing spatial, semantic, and other forms of declarative memory for facts and events. Despite the mixed effects and other limitations in these pioneering studies, improving mental processes supporting memory performance with electrical stimulation became a tangible reality.
This fascinating research can now be more robust and reproducible in the era of collaborative brain research projects. In our recent study, we used data collected from 22 patients stimulated in a range of brain regions during performance of a verbal memory task completed at multiple clinical centers. Four brain regions, which were previously implicated in declarative memory functions, were investigated for the effect of electrical stimulation on remembering lists of common words like “fish” or “rose.” We found that more words were remembered when one of the studied regions was stimulated (overlapping with the cortical area originally identified by Penfield). Electrical current passed through that region enabled the patients to recall more words as their abstract mental representation (or concepts, if you like) were generated. This time it was not the reaction time or response accuracy that was improved, but the memory for additional concepts of words when electrically entraining that brain region. And so was the subjective experience of one of the patients who reported that it was “easier to picture those words” in his mind.