The electrical fields that are picked up by this technique include a variety of rhythms. Delta waves—slow brain waves that occur one to four times a second—are characteristic of deep sleep. Beta waves, which occur 12 to 30 times per second, dominate when people are actively concentrating.
At a middle tempo is the theta rhythm, which repeats three to 10 times per second. (To put this in perspective, consider that when I run along the steep trails in the San Gabriel Mountains, my heart rate plateaus at 160 beats per minute, or 2.6 beats per second.)
The theta rhythm is particularly strong when people are finding their way or looking at something novel—in other words, when they are learning. Previous experiments suggest that the stronger these oscillations are and the more often they occur during learning, the better the person will remember the new material.
So it was not a surprise that the Rutishauser team picked up prominent theta activity when the patients were memorizing the images. But their findings went deeper. Using sensitive electronics and sophisticated software, the scientists could detect the faint staccato sounds that individual neurons make as they send information to one another by way of all-or-none pulses known as spikes.
The team recorded the activity of 305 neurons in the hippocampus and the amygdala. The total number of spikes that occurred while a subject viewed an image did not predict whether or not the patient would later recall it. (On average, participants recognized two out of three of the initial pictures.) Yet the scientists found something that did predict successful recall in about one fifth of cells.
Getting into a Groove
Nerve cells do not generally operate in lockstep. They typically send out pulses irregularly, whenever their excitation levels exceed a threshold. What the Caltech team found, however, is that neuronal rhythms can be highly orchestrated at times—and that this synchrony helps people form lasting memories. Think about a freestyle swimmer. She regularly turns her head to the side to breathe within the triangle formed by her upper and lower arm and the waterline. If she takes a breath during a different phase of the crawl, she most likely will swallow water and lose her rhythm. And so it seems to be for these memory-forming neurons.
During the learning phase, the team found, if a picture flashed on the screen at a moment when neuronal spikes in the hippocampus and the amygdala lined up with the local theta clock, patients were more likely to remember the image and feel confident that their recollection was accurate. When people were viewing images that they would later fail to recognize, this coordination between individual memory-encoding neurons and overall brain activity was much reduced.
This research reveals an extra factor besides attention, novelty and emotional impact in determining what makes something memorable: timing. Neurons always spike in response to new images and experiences. But when the spikes happen to coincide with the theta rhythm, this coordinated electrical activity alters the brain’s synapses, those specialized molecular machines between neurons, enabling memories to form.
These subtle findings help to decode the mechanics of memory—how three pounds of viscous tissue produces a mind possessed of innumerable impressions, recollections and knowledge accumulated over the course of decades.