Journal Reveiw!

The exclamation points are not out of sarcasm. Cognitive Neuroscience genuinlly excites me.

Internally Generated Reactivation of Single Neurons in Human Hippocampus During Free Recall was, in my opinion interesting not  as much because of the conclusions as the fact that the because the firing rates of single neuron were measured, the method of collecting data was necessarily more intrusive than usual. The authors of the study used patients with recurring epilepsy, all of whom had depth electrodes implanted into presumably, but not explicitly, the Medial Temporal Lobe (from here on referred to as the MTL) as treatment for there disorder.  The aforementioned detail allows them to use more temporal and spatial acuity on humans without any ethical issues being raised.  The study stands out for this reason if nothing else. The Hypothesis correlation between activity of neurons in the Hippocampus and adjacent regions as it takes in a visual stimulus with the reactivation of many of the same neurons when said stimulus is being recalled free from any outside stimulus, represents a measurable physical correlate to the subjective experience of remembering a stimulus.

For the actual experiment, subjects where to view clips of television shows as the firing rates of neurons in the MTL and more specifically the hippocampus, where monitored. A total of 857 units were measured from 13 different subjects. Of these neurons, 54.9% showed a significant response to at least one clip. Of those that responded, 9.7% displayed a sustained response for one entire clip, of which 44 where in the MTP and only two in the Medial Frontal Lobe. Interestingly, one neuron in the Left Anterior Hippocampus responded to only certain segments of one specific clip. The authors speculate that this suggests a preference for a certain episode or feature of the clip.

The subject were then instructed to recall the stimuli from earlier in the experiment with the same kind of data being collected from the  the hippocampus and medial temporal lobe. As it turned out, the same areas where activated during recall as where activated during the viewing of the stimuli, and units that responded to one particular stimulus reacted most when that stimuli was recalled. These spikes in activity persisted up to 3 seconds after the offset of the stimulus, and interestingly enough began 150 ms before verbal recall of the stimulus. They where also on average far more pronounced that the increases in activity in neurons that did not respond only to that stimulus.

The study seems to show that the neurons that are active during the formation of a memory are reactivated during recall, but it also shows that certain neurons are geared toward forming memories of and remember certain stimuli, which is interesting in that there isn’t any real suggestion as to what constitutes a stimuli having neurons specific to it.  As for the examples of the Simpsons and Seinfeld clips, there is no indication as to what was going on on the screen. The only real similarities are that they are both situational comedies and arguably funny. The authers say that this may be due to “some complex association rule”. I think future studies should focus on these episode-specific neurons with a more diverse range of stimuli that present characters displaying behaviors that it would be evolutionarily beneficial to recognize, such as aggression.

 

Other Studies Concerning the Hippocampus.