Tuesday, July 8, 2014

Why Is Human Memory So Variable and Subjective?

I can mobilize an eidetic recall of the events at my last party in my apartment in New Orleans in May, 1969 (including the brew I was drinking), and yet a brother who visited me with a friend can't recall even being there. (I showed him the photos, and told him he was the one taking them.)  How is it I can recall this event including that he was there, in person, doing the photography, but he can't?

How is it that other events also may slip off the memory grid for him, and not for me, and in other cases slip off my memory grid but not his?  How is it my wife can exactly recall a movie we went to a year ago, but I can't? How is it I can recall exactly where my wife and I ate at Dallas-Ft. Worth airport in May, 2003 - while in transit to Barbados -but my wife can't?

All these questions and more occurred to me lately, and I began to inquire into why human memory is such a fragile, imperfect, subjective and variable thing. What we do know is that the hippocampus (named after its resemblance to the seahorse, from the Greek hippos meaning "horse" and kampos meaning "sea monster") plays important roles in the consolidation of information from short-term memory to long-term memory and spatial navigation. The hippocampus is located under the cerebral cortex . In primates it is located in the medial temporal lobe, underneath the cortical surface. It contains two main interlocking parts: Ammon's horn and and the dentate gyrus.

In Alzheimer's disease, the hippocampus is one of the first regions of the brain to suffer damage; memory loss and disorientation are included among the early symptoms. Damage to the hippocampus can also result from oxygen starvation (hypoxia), encephalitis, or medial temporal lobe epilepsy. People with extensive, bilateral hippocampal damage may experience anterograde amnesia—the inability to form or retain new memories.

In the case of Janice's cousin Desmond,  who died of Alzheimer's disease two years ago, the first memories lost were of distant events - while he retained memories of more recent ones. This is actually in contrast to how the disease usually presents, with long past events much more easily recalled than recent ones.

But let's leave out Alzheimer's and simply ask why memory varies so much in normal people, and how it is we differ so markedly in what events we can recall.  Some hints were presented in the Scientific American article 'The Language of the Brain' (October, 2012). What we learn is interesting to say the least.  This is in terms of "sequences of synchronized spikes"  in the particular cortical region to enable a critical factor in perception and memory.

As the article observes (p. 59):

"It appears that such synchrony acts to emphasize the importance of a particular perception or memory passing through conscious awareness."

In addition, evidently the degree to which we pay attention determines the frequency of the synchronized spikes in the 'gamma band' of frequencies (30 - 80 Hz).  The higher the frequency, the more numerous the synchronized spikes, the better a memory will be recalled. Further, gamma band signaling between distant cortical areas is also involved - as discovered by Pascal Fries of the Ernst Strugman Institute for Neuroscience.

Another interesting aspect of subjective memory I've found is that in places where I've actually lived over time I have much better memories of events than for the odd times I visit other locations - for relatively brief times. In the latter case, my memory is often patchy and unreliable. This is one reason whenever I travel now I keep a daily journal of events. In this way I can easily grab a journal kept over any past vacation, or brief interlude, and see what happened on such and such a day without having to play craps with my memory. (Which I admit is variable: excellent and eidetic in some realms and not so much in others!)

Research by Margarita Behrens of the Salk Institute - cited in the article - has also traced signal deficits to a neuronal cell called a "basket cell" - which is involved in synchronizing spikes in nearby circuits. An imbalance of either inhibition or excitation of these basket cells can then lead to reduced synchronized activity in the gamma band. Interestingly, this anomaly can also indicate neurological disorders such as autism and schizophrenia. These circuit anomalies may also be responsible for the thought disorders that often accompany schizophrenia.

We also learn (ibid.):

"When it comes to laying down memories, the relative timing of spikes seems to be as important as the rate of firing. In particular, the synchronized firing of spikes in the cortex is important for increasing the strengths of synapses- an important process in forming long term memories."

A synapse is "strengthened" whenever the firing of a neuron on one side of a synapse incites the neuron on the other side to register a stronger response. See e.g. the diagram below:

How critical is this timing aspect? In 1997, researchers at the Max Planck Institute for Medical Research in Heidelberg, Germany,  found that a 20 milliseconds variance is all that's needed to throw a memory off. Thus, assuming a frequency in the gamma range, if an input at a synapse is consistently followed - within 10 milliseconds- by a spike from the neuron on the other side, then the memory is likely sealed. However, if the converse occurs, i.e. the neuron on the other side fires 10 milliseconds before the first one  all bets are off. The strength of the synapse between the cells decreases and the memory is then either fuzzy or lost.

The article ends by noting: that some of the strongest evidence that synchronized spikes may be important for memory comes from research by Gyorgy Buzsacki of New York University on the hippocampus. He found that the spiking of neurons in the hippocampus and other cortical regions it interacts with is strongly influenced by synchronous oscillations of brain waves in a range of frequencies from 4 to 8 hertz (the theta band). It seems that these theta band oscillations can coordinate the timing of spikes and have a "more permanent effect on the synapses."

This means the memory produced will be more long term, and better, more reliable.

In the end, memory is seen to be very dependent on synapse strengths and synchronization of specific types of waves (gamma and theta bands). If any one element is missing or off, then the memory simply can't be depended upon to yield accurate information - or maybe any information.

The moral of the story is that if we intend to trust our memories we better have those which resemble  the "memory masters" who possess the ability to recall what events happened on every day of their lives, e.g.

Failing that, we better learn to keep meticulous journals or take photographs of every major event we attend!

As for the key question: Why can't I remember that? The fault may not lie in one's 'stars' but in his synchronous spikes in the gamma band!

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