Post by Anastasia Sares
What's the science?
The hippocampus is the ‘seat’ of memory. From experiments with animals, scientists have been able to determine many aspects of its function, including the roles of its different cell layers and the growth of new cells over time. One interesting element of memory formation is called the sharp-wave ripple complex; a burst of synchronized neuronal activity that happens during memory consolidation and also during memory retrieval. However, animals can’t communicate their cognitive experience in detail, so we can’t be sure whether these retrieval-related ripples are accompanied by conscious memories. This week in Science, Norman and colleagues were able to connect these sharp-wave ripple complexes to reported memories in humans.
How did they do it?
The authors used intracranial electroencephalography, a rare opportunity to measure human brain activity in patients with electrodes directly implanted into the brain tissue. These patients have the electrodes implanted for unrelated medical reasons, often to monitor brain activity before surgery. The participants in this experiment had electrodes in both the hippocampus and the visual cortex, so the sharp-wave ripples could be measured along with visual activity.
Participants saw a series of images, then were blindfolded and asked to recall as many of the images as possible (there was another task in between these learning and recall sessions to prevent people from mentally rehearsing what they had just seen). This is known as a “free recall” task. The entire time, the electrodes were recording brain activity and a microphone was recording what the participant said.
What did they find?
The rate of ripples was highest when participants were initially viewing the images and while at rest (presumably, at rest, memories are being consolidated). During the free recall, there was a very specific increase in the rate of ripple events about one second before the participant verbally recalled a memory. On top of this, images that produced a higher number of ripples when viewing them for the first time were more likely to be successfully recalled. The number of ripples elicited during this initial viewing stage predicted the participants' performance in the subsequent free recall stage.
The authors also identified sites in the visual cortex that had a preference for (i.e. responded to) certain types of images during viewing (for example, preferring faces over places). Then, during recall, when the participant reported an image, they observed that the ripples from the hippocampus were coupled with activity in the visual sites that were selective to that image. The authors interpreted this as a “reactivation” of the visual memory. They could even train an algorithm to predict the image based on the neural activity of these two areas alone. The authors argue that this is consistent with a two-stage recollection process, involving a fast subconscious stage (the ripple) and a slower conscious one, in which the brain uses the retrieved content to re-create an experience.
What's the impact?
Because this study was able to use human reports of memories along with precise measurements of neuronal activity, it presents a strong case that these sharp-wave ripple complexes from the hippocampus are indeed involved in conscious memory retrieval. More work is needed to explain exactly how the hippocampus interacts with the rest of the brain during this process, however, these findings represent an important step forward in understanding the role of sharp-wave ripples in visual memory.
Norman et al. Hippocampal sharp-wave ripples linked to visual episodic recollection in humans. Science (2019). Access the original scientific publication here.