Monday, September 1, 2014

Welcome to the Connectome

Why does everyone's brain function so differently? Some of us are extroverted, some not; some of us are experts at language, some not; some of us are afflicted with pathologies like schizophrenia, some not; some of us are compassionate, some not. 

According to MIT neuroscientist Dr. Sebastion Seung, author of Connectome: How the Brain's Wiring Makes Us Who We Are, the differences are all due to our neural wiring. Personality, IQ and memories are encoded by our connectomes - the neural wiring which is every bit as individual as a fingerprint, but on a massively more complex scale.

The connectome describes both the brain's overall wiring, and how genes organize and express the proteins that form neural connections.

The brain's architecture gives rise to specific capabilities and tendencies, including perception, evaluation, behavioral selection, and personal traits. Much of it seems to be based upon a hierarchical system. 

For example, neurons related to perception comprise one such hierarchical network. Neurons at the bottom of the visual hierarchy respond to the simplest stimuli - individual spots of light. In your eye, each photoreceptor on your retina responds to a tiny spot of light at a specific location, much like a digital camera's multiple sensors, which detect light in terms of individual pixels. Moving upward in the hierarchy, neurons process progressively more complex data, with those at the top detecting the most complex stimuli, such as a person's identity. The neurons which detect parts send excitatory signals to neurons which eventually perceive the entirety.

UCLA's Dr. Itzhak Fried discovered one such "top neuron" able to respond specifically to images of actress Halle Berry. Interestingly, the subject's Halle Berry neuron also responded to the actress' written name, suggesting this cell participates in both perceiving and thinking about the actress, meaning it corresponds to an abstract representation of her.

According to Dr. Seung, such neurons are interconnected in cell assemblies, which hold the associations used in forming thoughts. These cell assemblies also interconnect and overlap.

Philosophers say specific principles govern how these associations are learned. The main method is through coincidence detection, finding a contiguity (sequence or series) in time or place. For example, since you often see toast eaten with butter, you have learned to mentally connect the two. 

Repetition is another method for learning associations. As a baby, the first time you saw your parents buttering their toast, perhaps your brain didn't form a permanent association, but after you saw it every morning at the breakfast table, you eventually formed a permanent mental association - and a synaptic connection. 

Sequences are also important for building associations. Reciting the days of the week and the months of the year repeatedly eventually allowed you to learn them by heart. Since each day and each month always followed its predecessor, you learned to associate them in sequence. Episodic memories, for example, involve a sequence of events, the synapses must be activated in one direction, allowing for memories to be recalled in chronological order.

This type of association will be linear, but if you always see two things appearing together, the association will be bidirectional.

Perception may seem effortless, but memory often seems difficult. If your brain only contained one cell assembly with a single memory, recall would undoubtedly be simple, but since a huge number of these cell assemblies overlap, it creates the potential for memory errors. 

Imagine the first time you ever rode a ferris wheel. You were in the fairgrounds surrounded by the cacophony of rides, electronic games and screams of delight, the smells of smoke, cotton candy and hot dogs wafting on the air. 

If you have a second memory which includes hot dogs - perhaps a Fourth of July barbecue with your family - both memories will differ, but the cell assemblies will share hot dog neurons, so when one cell assembly is activated, it can trigger the second. Delighted squeals might trigger a mixup of both memories. Perhaps this is what leads to faulty memory retrieval.

Says Dr. Seung, a high firing threshold might prevent such haphazard activation spread: if a given neuron can't activate without two excitatory inputs, two cell assemblies which only share one neuron would not be able to have such indiscriminate firing. But such a protective measure becomes problematic because it makes memory recall more difficult. To trigger an entire memory would require a minimum of two cell assembly neurons firing, so recalling your ferris wheel ride might require both the ringing of electronic bells and the smell of hot dogs cooking together. 

This means sometimes your memory may not work even when you need it to, because memory requires a delicate balance:  if there's too much activity, your memories may be hazy, but if there's too little, you may not remember at all. 

In forming associations, synapses "reweight" - either strengthening or weakening, and this is the physical basis of memory in the brain. Strengthening (long term potentiation) occurs as synapses grow more neurotransmitter-filled sacs (vesicles) on the transmitting neuron and more neurotransmitter-sensitive receptors on the receiving neuron. Synaptic weakening (long term depression) or dendritic atrophy occurs when neural pathways fall into disuse. Synapses may also be synthesized or eliminated, in the process called reconnection.

When two neurons repeatedly fire in sequence, the connection from the first to the second will strengthen; if they repeatedly fire simultaneously, connections will strengthen in both directions. This strengthening is the long-lasting basis of memory. These "Hebbian principles" of synaptic plasticity are activity-dependent, because the change in synaptic strength (plasticity) is triggered by the repeated firing of neurons. The changes last for weeks or even a lifetime, depending upon repetition and the subjective importance of the information.

A collection of such strengthened synapses acts as a cell assembly, a group of excitatory neurons interconnected together with strong synapses. There will be a number of additional weak synapses, but they aren't part of the cell assembly, having not been fired remained, and thus remaining unaltered. These weak synapses won't affect recollection, because firing will spread among the neurons in the cell assembly, but not to the unrelated neurons, because the synapses are too weak to activate the unrelated neurons.

Dr. Seung summarizes the idea thusly: "Ideas are represented by neurons, associations of idea by connections between neurons, and a memory by a cell assembly or synaptic chain. Memory recall happens when activity spreads after ignition by a fragmentary stimulus. The connections of a cell assembly or synaptic chain are stable over time, which is how a childhood memory can persist into adulthood."

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