Monday, February 6, 2012

The Cortical Factor

I just finished viewing Lecture 18 in a 25-part video series by Robert Sapolsky of Stanford. Dr Sapolsky is to human behavior what Carl Sagan was to astronomy. There is no one better at explaining the topic to the general public than this man. It’s not even close. I first stumbled into Sapolsky in early 2003 and I’ve been something of a groupie ever since.

The video series I am watching is an actual undergraduate course in human behavior. I was looking forward to blogging on various highlights once I’d completed the series, but Lecture 18 represents the best exposition I’ve ever come across concerning how different parts of the brain talk to each other, so let’s get straight into it ...

Lower mammals have an olfactory hotline to the amygdala, the part of the brain in the limbic system that mediates fear and arousal and kickstarts fight or flight. Furry creatures are literally primed to react when they smell something funny. Mammalian limbic systems are standard issue for humans, except ours are wired to respond mainly to visual stimuli.

Ordinarily, the cortical areas of the brain are responsible for visual processing and sending info to the amygdala, but we don’t always have time to wait. There is a short-cut in the brain through the lateral geniculate (LG). The trade-off is that this information is less accurate. We’re more likely to make mistakes. As Dr Sapolsky explains, there is now strong evidence that this pathway is hyper-excitable in those with PTSD.

According to Dr Sapolsky, the frontal cortex should be regarded as part of the limbic system. Its role, essentially, “is getting you to do the harder thing when it’s the right thing to do,” as in behaving appropriately. Below is a screenshot of Dr Sapolsky illustrating in a very simple way two contrasting neural circuits. The circuit on the left has more axonal inputs going into its target neuron than the right. This makes it easier to activate this particular neural pathway.

But what if this pathway represents doing the wrong thing? To offset this, the frontal cortex essentially massages both circuits, inhibiting the left and biasing (rather than causing) excitation in the right. To accomplish enhanced excitation, the frontal cortex gets a boost from dopamine via projections shooting out of the ventral tegmental area and nucleus accumbens. Dopamine acts as the fuel in goal-directed behavior.

One illustration of doing the harder thing would involve reciting the months backward. The frontal cortex needs to be on its toes in processing the task, but those with damage in this area may have trouble over-riding the more habitual forward-recitation response.

Over time, learned behavior becomes automatic and gets stored elsewhere in the brain. Thus, someone with Alzheimer’s may not know what decade it is but still knows how to knit. This brings us to the famous example of Phineas Gage.

According to Dr Sapolsky, they take away your neurobiology license if you fail to mention this guy. In 1848, while tamping down blasting power during the construction of a railroad line, the powder exploded, sending the tamping rod through the side of Gage’s skull and out the top, taking out his left eye and emptying out most of his frontal cortex. (See top left image.) Amazingly, because the rod cauterized his blood vessels, Gage was able to get up and walk a mile-and-a-half to the nearest doctor.

Gage achieved a partial recovery, but experienced major problems controlling his behavior, leading his physician to conclude that this part of the brain “reins in our animal energies.” Interestingly enough, about a quarter of those on death row have a history of concussive trauma to the head.

Doing the harder and right thing tends to involve delaying gratification and not giving into temptation. Consider the m&m test. You hold five morsels in one hand and one in the other. The rule is you reach for five you get one, and vice-versa. People with cognitive impairments, even knowing the rule, may still reach for the five - they just can’t help it.

Lack of cortical input explains why our dreams make no sense. In REM sleep our frontal cortices are at their least active. That’s why in our dreams we do all sorts of things we would never want to do in real life. Thank heaven we're merely dreaming. Imagine, if we did some of that stuff in while awake. Oops - sometimes we mess up, and we find ourselves living with the consequences.

Much more to come ...

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