The purpose of this lecture is to describe the func2on (and anatomy) of the pre frontal cortex. I would like to be able to do that by poin2ng to certain areas and saying, this func2on is carried out here. But the truth is our frontal cortex is perhaps the most complex of all our neural structures and is con2nually relying on communica2on with other neural structures. Thus, to describe the anatomical component of a func2on is not possible. Rather, one must describe circuits. As well, the PFC is more unique to the individual than other structures. It represents our personality and our individual strengths and limita2ons which further complicates the simple anatomicalfunc2on paradigm. 1
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Transcript
The purpose of this lecture is to describe the func2on (and anatomy) of the pre-‐frontal cortex. I would like to be able to do that by poin2ng to certain areas and saying, this func2on is carried out here. But the truth is our frontal cortex is perhaps the most complex of all our neural structures and is con2nually relying on communica2on with other neural structures. Thus, to describe the anatomical component of a func2on is not possible. Rather, one must describe circuits. As well, the PFC is more unique to the individual than other structures. It represents our personality and our individual strengths and limita2ons which further complicates the simple anatomical-‐func2on paradigm.
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I assume that you have all heard the story of Phineas Gage. In the mid 1800’s Phineas was a railway worker who managed to have a rod lodged in his frontal lobe. It is remarkable that he survived considering the high likelihood of infec2on to exposed brain 2ssue. However, the new Phineas Gage was different than before the injury. Most notable: he became much more aggressive toward others. He went from being someone everyone liked to someone who seemed to care liRle for others. He was even described as arrogant. Is arrogance in the PFC? Maybe not arrogance but the ability to take the perspec2ve of others (empathy) is in the PFC and the absence of empathy is arrogance.
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The frontal cortex is largely about “outputs”, whereas the other lobes are primarily involved with inputs (senses). The major func2ons of the PFC are generally referred to as execu2ve func2ons and include planning, reasoning, awareness, problem solving, judgment and decision making. One of my goals is to iden2fy how these func2ons are described, assessed and related to one another.
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The motor strip and the pre-‐motor cortex will be discussed in subsequent lectures. However, please keep in mind that “behavior”, which represents the primary output of our neural ac2vity results from pre-‐motor/motor strip ac2va2on. These components are considered part of the frontal lobes and the pre-‐frontal cortex is merely an extension of these more basic aspects of output (namely behavior) whereas the pre-‐frontal cortex is involved in the more complex aspects of output such as planning, problem solving etc.
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For the sake of simplicity we are going to divide the PFC into two basic components, the Dorsolateral region and the Orbitofrontal/Ventromedial region. The Dorsolateral region is immediately anterior to the premotor areas and as such is more involved in func2ons associated directly with behavior. The Orbitofrontal/Ventromedial regions are anterior to the limbic cortex and have more to do with outputs (and as it turns out, inputs) of an emo2onal nature.
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At least five different pathways have been iden2fied in the Dorsal regions (and many more pathways will be iden2fied in the coming years). All of the Dorsal pathways involve the temporoparietal regions. Thus ac2vity of the Dorsolateral cortex depends on input of a visual/auditory/somatosensory nature. As stated earlier, motor outputs are generally the end result of planning etc. in the Dorsal region and for the most part the informa2on required for planning is of a logical nature (as opposed to an emo2onal nature). Conversely, ventral pathways rely on input of an emo2onal nature (from the limbic cortex and somatosenory cortex). Outputs from the ventral streams may also be behavior (e.g. a smile), or they may be visceral (feeling sad, nausea).
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In order for the PFC to benefit from informa2on from other parts of the brain, that informa2on must be temporarily stored in the Dorsal pfc. There are 5 circuits iden2fied just from Brodmann areas 9/10 primarily involved in working memory. Informa2on from the DLPFC is mediated by Glutamate which is excitatory. Glutamate is a neurotransmiRer that is involved in learning. Inhibitory pathways are mediated by GABA. The circuitry from the Caudate Nucleus generally includes the globus pallidus interna and the substan2a nigra. The mediodorsal region of the thalamus handles the dorsal pathways.
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The circuitry involved in working memory is complex as is the circuitry for focused aRen2on. I will return to working memory in a moment. A second major aspect of managing all of the inputs from other brain regions is aRen2on to those details that maRer. In other words focused aRen2on.
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The Occipital, Parietal and Temporal cortex are constantly aRending to the environment. This is called reflexive aRen2on. The brain has developed mechanisma to suppress aRen2on to all of the possible inputs such as when we are sleeping. It is also cri2cal to suppress aRen2on so that we may focus out aRen2on (e.g when reading or speaking on the phone). The inability to suppress reflexive aRen2on is a cardinal characteris2c of aRen2on deficit disorder. Focused aRen2on is a func2on of the dorsolateral PFC. During an assessment of aRen2on we are not only interested in whether aRen2on can be focused, but how long aRen2on may be sustained, whether the individual can alternate between tasks, or mul2-‐task. The average adult can sustain aRen2on for approximately 50 minutes.
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ARen2on to visual s2muli is ini2ated in the DLPFC (Frontal Eye Fields); spa2al maps are recorded in the lateral intraparietal cortex. The superior colliculi play a role in causing the saccadic movement of the eyes and goal directed visual scanning. The FEF are located just superior to the Broca’s area.
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There are various tests for aRen2on. To some extent the Mini Mental Status exam assesses aRen2on although not specifically. The Stroop test is one of the more common tests of aRen2on.
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As we age “aRen2on” is more ocen the culprit than actual memory loss as older people complain (and complain ocen) about memory loss. You ocen hear the phrase, “he remembers what he wants to remember”, and actually that is quite true. Although this might be beRer stated “ he remembers what he pays aRen2on to”.
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Working memory: The capacity to retain informa2on no longer present in the environment, to manipulate or transform this informa2on and to use it to guide behavior. Working memory is generally assessed using a delayed response. Research has demonstrated con2nued ac2vity in the PFC (during the delay) but the actual storage of informa2on may be in the parietal regions associated with visual spa2al, auditory and other s2muli.
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Visual working memory appears to be right DPFC but func2onal imaging reveals the circuitry that must be intact for an image to remain in short term memory.
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Spa2al working memory is where we store informa2on about where we are in space. It is almost impossible to separate visual/spa2al memory from spa2al memory since our knowledge of where we are in space is so dependent on our visual input. This picture also gives you a general idea idea of where you might find the frontal eye field (FEF) and the working memory for faces. We are social creatures so it is important that we remember faces (even if most of us are not good at remembering names)..
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Working memory for verbal informa2on, not surprisingly, is located near the Broca’s area and also near working memory for objects. We remember objects much easier if they have a name. In addi2on to being able to remember something of a verbal or visual nature we need to be able to manipulate the informa2on. It may be as simple a task as being given three names and reorder them into alphabe2cal order. At a more complex level we may be looking at various bits of unconnected informa2on and being able to solve a puzzle or mystery. This we call analy2c reasoning. Please not the loca2on near the ventral region of the DPFC.
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The Orbital Frontal PFC is best known as the communicator between the VMPFC and the DLPFC.
Nucleus accumbens is more than “reward” or feel good. The NA is about mo2va2on. What keeps a medical student on track and mo2vated when the reward of comple2ng medical school, residency and gegng established in a prac2ce seems so far away? The answer is dopamine and the NA for the steps iden2fied along the way. Learning itself can produce a feeling of accomplishment. It is also necessary that the student be able to see relevance of each step (each class?) to the ul2mate goal. This laRer process is more Dorsal that Ventral, applying logic to the process, also requires the Orbitofrontal PFC for communica2on.
The primary role of the vetromedial PFC is social processing. I will describe this in more detail in an upcoming lecture.
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This is an example of a ventral circuit. Please note the close 2e to the hippocampus. Emo2on plays a large role in making events memorable. Also note the insula, which has a major role in taste and is therefore well connected to the stomach. For lower organisms this reflects the connec2on between emo2on-‐food-‐memory. For humans, it reflects the visceral response to emo2on.
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The ventral circuitry has other func2ons more closely aligned with execu2ve func2on. The ability to use cues to predict future rewarding or aversive events. Flexibility/problem solving is another important func2on. Autobiographical memory… what happened when. The ability to sequence, put things in proper order.
Olfactory processing is also carried out in the ventral circuitry and has considerable overlap with emo2on regula2on.
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I have covered this already.
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When the Orbitofrontal cortex is damaged it has the greatest impact on the individual’s ability to make decisions. This makes sense when you consider the communica2on role of the OFC vis a vis the Ventromedial PFC and the Dorsolateral PFC.
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Another aspect of execu2ve func2on is awareness. Awareness starts with basic awareness of your physiologic state… the answer to “how are you”. Informa2on about abili2es and disabili2es is the next step up. Lack of awareness of disabili2es is called Anosognosia. At another level is informa2on about body needs like hunger, thirst, body temperature etc. Finally there is informa2on of self esteem, self image. 1,2,3 Somatosensory cortex, 1,2 Anterior Insula, 3 Posterior Insula. 4? Ventromedial PFC.