Summary of Discussion of Mental Code

Prepared By Brandon Schakola (addtions by Frawley)

We discussed two theories of the structure of cognitive processes. Using the second chapter of Green, the symbolic and the connectionist approaches were discussed as methods of dealing with the problem of determining and simulating the nature of cognition. We focused on the clarification of the nature of the machinery of the mind/brain. Is the mind/brain a finite state or push-down automaton, or even a Turing or Von Neuman machine?

Our method of attempting to resolve this question was to first look at the extent of the limitations of the mind. We used examples from linguistic studies to determine that the mind/brain could not be as limited as a finite state automaton, but not as unlimited as a Turing machine either.

After dealing briefly with the problem of classifying of the basic machinery of the mind/brain, we worked on developing our understanding of the symbolic and associative (connectionist) theories of cognition. We then listed the distinctions between these two: symbolic networks are language-like, productive, contain innate or prespecified information, and are abstract with respect to the outside world; associative networks have emergent patterns, are subsymbolic, and recoverable (representations close to outside world). Finally, we discussed the debate between connectionist and symbolic models and questioned their compatibility, because both models have their advantages within cognitive science.

Summary of discussion of Ch. 2 and 3 of Churchland's Neurophilosophy

Prepared by James B. Witkoskie (additions and comments by Frawley)

The Neuron

The neuron is the functional unit of the nervous system. It is composed of a cell body called the soma and projections called processes. There are two types of processes: the dendrite that receives signals and the axon that sends signals. A neuron only has one axon but can have thousands of dendrites. There are also "helper" cells called neurogliain the nervous system. These include Schwann cells and oligodendrocytes that form the myelin sheath, astrocytes and ependymal cells that control nutrient exchange between the blood and the neuron, and microglia that remove dead neurons.

The neuron sends messages to other neurons via an impulse or spike that results from the depolarization of the neuron's membrane along the axon. This process happens via the following:

1. Sodium-potassium pump removes Na+ from the cell and adds K+ to cell.

2. The concentration gradient allows the K+ ions to leave the cell, but Na+ and the negative ions can't because they are not permeable to the membrane. This causes the cell have a negative charge on the inside of the cell and a positive charge on the outside of it. This is called resting potential.

3. Because a stimulus to the neuron causes a change in the permeability of the cell membrane, the Na+ ions enter the cell and reverse the polarity. The charge on inside of the membrane goes from -70 to +55 millivolts. This happens in a wave down the axon.

4. The myelin sheath made by the Schwann cells accelerates the movement of the impulse down the axon by making it skip from one gap in the sheath to the next.

5. When the impulse reaches the synapse (junction between neurons), it causes the release of neurotransmitters or ions which enter the next neuron and causes a reaction that results in the post-synaptic cell depolarizing or hyperpolarizing. Neurons can excite or inhibit other neurons. If the ion or neurotransmitter excites a cell, it causes depolarization. If it inhibits a cell it hyperpolarizes the cell.

Question: Is this neurochemical activity an exchange of information?

Habituation

When an organism changes its response to a stimulus that it is subjected to repeatedly, it habituates. The sea hare, when gently stroked, recoils at first, but after a short time of continued stimulus, it no longer has this reflex. It is shown that a temporary build up of Ca+ ions in neurons may play a role in the memory of a frequently encountered stimulus.

Question: if we say that memory is the temporary buildup of Ca+ ions, does this change our view of what memory is and does?

Neurotransmitters

There are 40 known naturally occurring substances that directly affect the nervous system , but only 11 are classified as neurotransmitters. To be a transmitter a substance must (1) be produced in the pre-synaptic cell, (2) be released from synapse, (3) cause depolarization of hyper-polaiization, (4) have a mechanism for removal.

Hormones also play a role in neurological development. For example, the difference between males and females mental development is caused by hormone interaction vith the developing brain. Epinephrine is another hormone that affects the nervous system. However, epinephrine and sex hormones are not neurotransmitters.

The Brain and Periphery

Structure and Function

The brain is divided into 5 structures:

1. Telencephalon -- the cerebral cortex and hemispheres
2. diencephalon -- thalamus and hypothalamus
3. mesencephalon -- mid brain along with thalamus
4. metencephalon -- cerebellum and pons
5. myelencephalon -- medulla oblongata or brain stem
The gray matter of the brain is where the soma of the neurons are; the white matter is the axons. The gray matter of the cerebral cortex is only a few millimeters in thickness. It is folded to increase the surface area of this thin gray sheet. Subcortical gray structures play a role in linking stimuli Aith the cerebral cortex, memorizing, coordinating body function and performing basic biological functions. These structures include: hippocampus and amygdala (memory), thalamus (major connector of periphery to cortex), basal ganglia (motor function), reticular formation (arousal and control, circadian rhythms), cerebellum (balance), medulla (most basic functions, breathing and heart rate)

NOTE: talk of the brain's cognitive activities usually implicates the cortex only. But this is misleading given the whole structre of the brain and the interconnectedness of the areas.

Pathways and Processing

Neurological pathways and tracts have been revealed through the use of dyes and radioactive amino acids to trace neurons. Through this research, two paths for the neurons that play a role in touch have been revealed. The lemniscal system goes to cortex and is used for detecting a minute stimulus, such as the moving of a hair. The spinothalanlic system is used to detect pain.

Once information has been collected by the senses the brain must process it. There are currently two theories about this information processing. The parallel system theory says that multiple information processors operate simultaneously to relay information and bring it together. An example is the optic nerve. It has three different tracts to the visual cortex. So the information is processed by the different parts of the visual cortex before being brought together to form the image. The systematic system theory proposes that as the stimulus enters the different processing centers of the brain the information gets processed a little at a time until it is all brought together. This system is hierarchical (and less neurologically accepted than the parallel).

Another method of tracking a system is the use of neurochemicals. Because certain systems use certain neurochemicals, a scientist can deduce what system a neuron belongs to based on the neurochemical it uses.

Horizontal and Vertical Organization

The cerebral cortex has a laminar or horizontal organization. It has six layers distinguished by the characteristics of the cells in that layer: density, type of cell, and connection in neurological networks. Gray subcortical structures also have a laminar organization.

Scientists are able to map the parts of the brain that perform different functions because (1) neurons in the cerebral cortex with similar functions are in close proximity to each other, and (2) the distance between neurons in the cerebral cortex is proportional to the distances between the body regions that they get information from. (The brain is thus said to have a topographic structure.) Because of this, neurons that process information from fingers on the same hand vall be closer together than neurons that process information from the opposite hand. However, the neurons from the two hands will be closer together than the neurons that process information for the eyes. This concept allows scientists to map which part of the brain performs what function.

The six laminae are also aligned so that the neurons directly above or below a specific neuron are basically processing the same data. The more a particular area is used the more interconnected the neurons in that area become.

Neurons of the different strata form columns that are 0.5 to I mm in thickness. This is a method of vertical organization. These columns allow a stimulus sent to one of the strata to affect the other strata. The effect can be excitatory or inhibitory, This is beneficial because the neurons in the same column have similar reception and response properties. The horizontal (1aminar) organization represents the neurons in the body and their relative distances from each other, and the vertical (colunmar) organization represents the modularity of the systems. This system is believed to be input-output in nature. The columns may be sub-units to macro-columns or subsystems.

Neural Development

The development of the nervous system begins with a cell called the neuroblast. This cell is the father of all cells in an organisms body. When the organism first creates its neurological network, it makes a surplus of neurons. But shortly after fomiing 85 percent of these cells will die because they have no use. The objective behind making so many neurons is to allow natural selection to assure that only the best neurons produced are used.

After fanning out of the neuroblast The neurons begin to migrate to the arae of the organism in which they will function. Scientists believe that a chemical affinity determines where the neurons migrate. In an experimental transplanting of an eye from one embryonic frog to another, the optic nerves that would originally migrate to the two eyes end up migrating to all three. Because no new neurons are formed the eyes have to compete for dominance over the neurons that are there.