Category Archives: human electrophysiology

On Memory and Experience

from reference 1

As reported in the New York Times, a new study has demonstrated an aspect of memory that has long been hypothesized. That being: the same neurons that fire during an experience fire during the memory of that experience. The research, published in the journal Science, relies on recordings from the brains of epileptic patients undergoing surgery to remove the parts of their brain which cause excesses of neuronal activity, essentially the only way to record the activity of neurons in awake human beings1.

The authors of the study took an approach where they recorded the activity of single hippocampal brain cells while subjects were watching a variety of video clips. Unsurprisingly, certain cells responded best to certain clips. Then, after a brief interim during which the experimenters distracted the patients, they asked the subjects to recall the video clips. Not only did the activity of the neurons during recollection correlate with activity during first viewing, the experimenters were able to predict which video clip the subjects were remembering based on the recorded activity! Interestingly, however, the hippocampus (the area of the brain being recorded from in this study) is not required for the recall of long term memories. Thus, in some ways this work further deepens the mystery of how short term versus long term memories are encoded in the brain and the involvement of hippocampus in these processes; a subject previously touched on in this forum.

Reading about this research reminded me of my favorite definition of memory, as the ability to:

“repeat a mental or physical act after some time despite a changing context…. We stress repetition after some time in this definition because it is the ability to re-create an act separated by a certain duration from the original signal set that is characteristic of memory. And in mentioning a changing context, we pay heed to a key property of memory in the brain: that it is, in some sense, a form of constructive recategorization during ongoing experience, rather than a precise replication of a previous sequence of events.

…the key conclusion is that whatever its form, memory itself is a [property of a system]. It cannot be equated exclusively with circuitry, with synaptic changes, with biochemistry, with value constraints, or with behavioral dynamics. Instead, it is the dynamic result of the interactions of all these factors acting together, serving to select an output that repeats a performance or an act.

The overall characteristics of a particular performance may be similar to previous performance, but the ensembles of neurons underlying any two similar performances at different times can be and usually are different. This property ensures that one can repeat the same act, despite remarkable changes in background and context, with ongoing experience.”2

References:

1. Gelbard-Sagiv H, Mukamel R, Harel M, Malach R, Fried I (2008) Internally Generated Reactivation of Single Neurons in Human Hippocampus During Free Recall. Science 10.1126/science.1164685
2. Edelman GM, Tononi G (2000) A Universe of Consciousness: How Matter Becomes Imagination, Basic Books, New York

On the Dynamic and Ever Changing Brain

brain sites stimulated and measured; from Ref. 1

It is well accepted that the activity in our brains is not simply defined by the anatomical connectivity therein. However, one of the great mysteries of free will (if it actually does exist) is how a conscious state (the current pattern of electrical activity) is causally linked to subsequent states (the next pattern of activity). If it is truly deterministic, then free will may truly be an illusion.

A recent paper reporting results from an experiment in which awake human cortices were stimulated and responses measured, makes an implicit comment on these matters. In this experiment, patients going in for brain surgery volunteered to have their cerebral cortices experimented on. This is not as invasive as it sounds since the experiment involved only a reanalysis of data obtained from necessary pre-surgical procedures. The experimental paradigm consisted of simply electrically stimulating the cortex at various points and measuring the evoked activity at other sites. The authors were curious how regular these responses would be, and how they would vary over time.

They found that “The likelihood of an afterdischarge at an individual site after stimulation was predicted by spontaneous electroencephalographic activity at that specific site just prior to stimulation, but not by overall cortical activity” (ref. 1). The intriguing part about this is that the overall activity does not predict the subsequent activity, supporting the notion that there is something else (free will?) that intervenes or contributes to the causation involved in moving to another state of activity. However, it must be noted that the overall activity might not be predictive for another reason. Aggregate brain activity is made up of parts devoted to distinct cognitive functions. Thus if the brain area stimulated was one devoted to motor function and the subject happened to be thinking about a movie prior to stimulation, the activity devoted to the visual experience wouldn’t necessarily be predictive of how the activity might spread through the brain from this motor area. Nonetheless, it may be the case that this sort of variation – the seemingly random, highly dynamic snatches of activity present in the brain at any given moment – contribute to our sense of free will, and the rich landscape of experience that we go through.

References:
1. Lesser RP, Lee HW, Webber WR, Prince B, Crone NE, Miglioretti DL. (2008) Short-term variations in response distribution to cortical stimulation. Brain, 131 :1528-1539