NOTICE: Posting schedule is irregular. I hope to get back to a regular schedule as the day-job allows.

Monday, March 14, 2011

The Executive

The public perception of emotion and personality is that it resides in the temporal lobe, and in the brainstem/deep brain structures of the limbic system.  Memory is thought to be strictly confined to the temporal lobe.  Decisions regarding reward and value are the province of the basal ganglia – in particular, the striatum.

… that *somewhere* in the brain, probably down deep in the middle, is a "Director" that tells the other parts of the brain when and how to do their jobs.

The truth is that all of these functions are fulfilled in part by the Frontal Lobe.  Emotions appear to be on the dorsal Frontal Lobe, just forward of where the Temporal Lobe ends.  Extremely short term memory occurs in the orbitofrontal cortex – just above the eyes (orbits) – and coordinates with the successively longer memory processed by hippocampus and stored with the assistance of the temporal Lobe.  Decision-making quite frequently involves the "prefrontal" areas (the anterior, or forward-most extent of the brain), including moral and value judgments.
The Director.  The executive function that makes the human brain… human… is the prefrontal region of the Frontal Lobes. 

Probably the least understood in terms of *how* it arises, Executive Function is at the leading edge of the Mind-Brain question:  How does the operation of billions of neurons, and trillions of synapses become the conscious brain?  This blog does not have an easy answer for that.  However, what *can* be said is that neural modeling has been surprising, quite frequently properties and features arise in a model that were not programmed in by the modelers.  We call these "emergent properties."  While we know a lot about how consciousness can be controlled or altered, we don't really know how it comes into existence except to hand-wave it away as an emergent property.
Rather than engage in that time-honored Science Fiction tradition of "handwavium," let's talk about what we, as neuroscientists, do know:

(1) Frontal Lobe is the most active during tasks which require very short term memory and decisions

If you want to remember a phone number, how do you do it?  Repeat it to yourself.  Mentally *say* it over and over again.  That involves the speech centers in Frontal Lobe, and memory.

What do you need to make a decision?  Memory, knowledge about the environment, pre-programmed motor sequences to carry out actions, value of the decision, and evaluation of consequences.  Frontal Lobe connects to all of the association cortices, to the memory centers of temporal Lobe, to the basal ganglia for reward association, to amygdala and limbic system for emotional content, and most importantly, the motor association areas of the Frontal Lobe "rehearse" motor movements and provide feedback regarding the consequences of the action.  All of these functions are integrated via connections between the Frontal Lobe and the other brain areas. 

(2) Damage to Frontal and Prefrontal areas alters personality, emotion and the ability to make decisions.

The classic head injury cliché is that an impact to the back of the skull causes unconsciousness and an impact to the front causes amnesia.  The truth is much more complicated – a blow to the *base* of the skull compresses the brainstem and subcortical areas and can cause unconsciousness – but it can just as easily cause permanent damage.  The front of the skull is well armored.  An impact here (or anywhere on the skull) is more likely to have its effect via concussion, resulting when the brain *sloshes* in its protective cushions (the outer membrane dura mater, inner membrane pia mater, and the cerebrospinal fluid between the two) and bruises the cortex on impact with the interior of the skull.  Outright damage to prefrontal and frontal areas can alter consciousness, result in blurred vision, inability to track moving (or stationary) objects, impair ability to move limbs to a defined goal.  In addition, frontal damage can alter personality, emotion and the ability to make appropriate decisions.

One of the most fascinating mental and cognitive tests is called "moral decisions."  A subject is given a situation and one of two choices.  The situation often involves something like death or serious injury to self, friends or strangers.  The decision will test whether the subject places a higher value on their own life, that of friends, strangers, one or many.  The *purpose* of the test is not *what* decision the subject makes, rather how *quickly* they make the decision (if at all) and whether they are content to stick with the choices given by the examiner or even make up their own alternatives within the rules of the test.  *Impaired* cognitive and moral decision making is characterized by snap decisions with no evaluation of alternatives or consequences.  You may have guessed that this is precisely the type of change observed in persons with Frontal Lobe brain damage.

The irony of course is that there may be no change at all.  The Frontal Lobe is highly redundant, and there is not as much lateralization observed in function.  Damage to one side can be compensated.  The remarkable healing progress of Arizona Representation Gabrielle Giffords is testament to this ability to shift functions and heal.


As we move on from Frontal Lobe, we are increasingly encountering functions that require massive connections to other brain areas, and further invoking the subcortical areas.  It may come to your attention that of the 5 primary senses, we have only discussed three – sight, sound and touch – and have not mentioned smell and taste.  The observant reader may notice two small extensions below (ventral to) the Frontal Lobe labeled "Olfactory Bulb" or "Olfactory Cortex" and figure those are part of the Frontal Lobe. 

Actually, it is not.  Olfaction and gestation – smell and taste – are the "oldest" senses from an evolutionary and developmental perspective, and project directly to the most primitive portions of the brain – the pyriform cortex (present in amphibians, reptiles and mammals), entorhinal cortex, amygdala and hippocampus in the Temporal lobe.  As such, the olfactory system is part of the "limbic system" to be discussed later this week.  The sense of taste is tightly tied to the senses of smell and touch (physical "feel" of objects tasted), and shares connections to the limbic system, as well as direct projections to somatosensory cortex.


In the weeks to come, I will be traveling on vacation.  Blogs have been prepared and uploaded to continue The Guide throughout the next two weeks, but I may be unable to respond to comments until after my return.

Also, watch for a few "surprise" columns over the next two weeks as well.

See y'all on the flip-side!

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