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Feb 27, 2021
  

[Brain Image]    

PSY 340 Brain and Behavior

Class 11: Cerebral Cortex

   

Cerebral Cortex

[Lobes of the Brain]The outer surface of the cerebral hemispheres is known as the cerebral cortex ("tree bark" in Latin). The surface of the cortex has many folds and contains a great deal of surface packed into a small area. The surface is also called the brain's "gray matter" because it consists mostly of the bodies of neurons (see [Gray vs. White Matter]diagram on right where the gray area is above the green line). From these neurons, connecting axons stretch inward away from the surface of the cortex. These form the "white matter" of the cortex beneath the surface (below the green line).

 

 Each of the two cerebral hemispheres communicates with the other via several bundles of axons. The largest is the corpus callosum (see diagram on the right and left below). A second, smaller connection is the anterior commissure (see diagram on the left below).

    [Commissures]

Organization of the Cerebral Cortex

[Layers of
            Cortex] [Columns of Cortex]

Laminae: There are six layers or laminae (singular = lamina) of neurons which comprise the surface of the cerebral cortex. The thickness of these layers varies in different sections of the cortex, but averages 2.6 mm (= size of 2 dimes stacked one on the other).

Columns: The cortex's neurons are also arranged as columns with cells of similar properties grouped perpendicular to the surface of the cortex. For example, in the "Somatosensory Strip" (the Postcentral Gyrus noted below), if a neuron responds to pressure upon the elbow, other neurons in its column will also respond to pressure on the elbow. Or, in the primary visual area of the occipital lobe, a neuron which responds to a particular light pattern will find other neurons in its column also responding.

[Lobes of the Brain] Central Sulcus: Note in the diagram of the cerebral cortex to the left the label "central sulcus". A sulcus (sulci, pl.) is a fissure, groove, or depression on the surface of the cortex. These sulci normally separate ridges or protrusions of cortical tissue called gyri (gyrus, singular).

The central sulcus separates the Frontal from the Parietal Lobe of the brain. Anterior to (in front of) the Central Sulcus lies the Precentral Gyrus. We have already seen this area under the label of the brain's "Motor Strip." Posterior to (in back of) the central sulcus lies the Postcentral Gyrus. We have already seen this area under the label of the brain's "Somatosensory Strip."

 


The Lobes of the Cerebral Cortex

Occipital Lobe

In the posterior of the cerebral hemispheres lies the occipital lobe of the brain.

Somatorsensory StripParietal Lobe

Somatosensory Strip. Anterior to the occipital lobe and posterior to the Central Sulcus lies the parietal lobe of the brain (see diagram above). The Postcentral Gyrus ("Somatosensory Strip") receives sensations from the rest of the body. The information arriving in this area involves touch (both light and heavy), temperature, and the position of the limbs of the body (telling where your arms, legs, hands, head, etc. are located). 

Visuo-spatial Processing & Sensory Integration. The parietal lobe maintains representations of the body's and of the head's position in space. This permits the rest of the brain to interpret what it is experiencing when it sees and feels things happening in actual space in and around ourselves. Thus, the parietal lobe integrates (brings together) sensory information (especially touch and vision) from multiple parts of the body. It interprets where objects are in space and analyzes visual motion. E


Temporal Lobe

[Temporal Lobe]The temporal lobe lies on the side (lateral) of each cerebral hemisphere. On the superior margin (top) of the temporal lobe lies the primary auditory cortex, the tissue which is central to the understanding of spoken language. The temporal lobe is central to hearing.

Other parts of the temporal lobe are involved in the recognition of faces and objects.

Other emotional and motivational behaviors are associated with the temporal lobe. A very rare disorder, Klüver-Bucy Syndrome, is associated with destruction of the inferior temporal cortex and part of the amygdala. Its symptoms include

  • putting many types of objects in the mouth,
  • emotional placidity,
  • hyper/inappropriate sexual advances, and
  • distractibility caused by small objects.
The original investigators, Heinrich Klüver (1897-1979) and Paul Bucy, experimented on monkeys who received lesions bilaterally to this section of the brain. They noted the monkey's seemed to become "psychically blind," i.e., they were unable to recognize objects any longer. This may be why they would handle snakes or lighted matches which otherwise they would avoid.

Frontal Lobe

[Frontal Lobe]The frontal lobe lies anterior to the central sulcus and superior to the temporal lobe. It contains the "Motor Strip" or Precentral Gyrus from which motor instructions (particularly for fine motor control) are sent to the voluntary muscles of the body.

[Brain Sizes]Prefrontal Cortex: Executive Control. The tissue in the frontmost part of the lobe is called the "prefrontal cortex." In humans, this is a very large area of the brain (compared to what is found in other animals; see graphic on left).  This cortical area receives projections from the entire cortex and contains neurons with many dendrites and dendritic spines (16x more numerous than in other areas). This allows the prefrontal area to integrate a large amount of data. During the rest of the semester we will be looking at various functions performed by the prefrontal cortex (e.g., involving "executive" planning and judgmental roles, emotional modulation, and others). However, we might note here some important notions:

  • Involved with working memory: short-term information (rather than long-term factual data) & responding to delayed-response tasks (needing to respond to what is remembered after a short delay)
     
  • Control of behavior that depends upon context or setting. Damage shows itself in impulsivity or inappropriate responses. For example, do you look through the desk drawers in someone else's home? What difference does it make to your behavior if you are in a sports arena or your local church?
     

[Lobotomy] [Leukotomy] Lobotomy/Psycho-surgery. From the late-1930s to the mid-1950s, physicians performed prefrontal lobotomies, an operation in which the surgeon would cut some of the nerve tracts between the prefrontal tissue and the rest of the brain. It was believed that this would "calm" individuals with severe mental disorders (especially schizophrenia) without impairing motor skills or sensory capacities.

The technique was developed in Europe, but pioneered in the U.S. by a physician, Dr. Walter Freeman, It was used with about 40,000 patients. The operation did not usually result in any significant help to the patient. We know today, for example, that the frontal lobes of schizophrenic patients are under-aroused and, thus, the operation harmed an area that was already functioning poorly.


Putting It Together: The Binding Problem
 

Consider the following images. On the left is Vincent Van Gogh's The Harvest and on the right is Escher's Relativity.

[The
          Harvest]    [Relativity]
 Which one of these pictures is easier to view as a whole?

Our interactions with the world -- via sight, sound, movement, etc. -- is processed in quite different parts of the brain. But, we normally experience the world and what's in it (the objects around us) each as unified or whole. How does the brain do that? How does the brain take sound, vision, and other sensory qualities and bring them together so that we perceive every object as a unified and complete whole? This is what is called in neuroscience the binding problem (or, "the large scale integration" problem).

We used to think that the brain "associated" different sense modalities and joined sight, sound, etc. in cerebral cortex tissue called "association areas." Now we know that that does not happen. What does happen?  We are not really sure. We do know that the various sensations (e.g., sound, vision) need to come from the same place at the same time.

Ramachandran has developed a treatment for phantom limb pain based upon these insights. He created a "mirror box" which allows arm amputees to "fool" themselves and achieve relief from itching and other pains.

Treating Phantom Pain with Mirror Therapy (YouTube)


RESOURCES

Klüver H, Bucy PC: Preliminary analysis of functions of the temporal lobes in monkeys. Arch Neurol Psychiatry 1939; 42:979­1000

 

This page was first posted February 8, 2005