From cs240@cs.rochester.edu Tue Nov 4 17:59:26 2003 Date: Tue, 4 Nov 2003 17:57:07 -0500 From: cs240 To: tetreaul@vienna.cs.rochester.edu Brain Structures

Evan X. Merz
em006j@mail.rochester.edu
September 18, 2003


Engineered By Nature:

An Overview of the Subsystems of the Human Brain


Many analogies have been created to describe the structure and function of the brain. The one currently recieving the most attention is the analogy to the computer. In fact, modern computers share many traits with the human brain. Both can tackle complex computational and analytical tasks, both relay signals from one section to another, and both process input to produce more useful output. In the case of the computer, this may consist of processing strokes on a keyboard to produce a webpage. In the case of the brain, this could consist of something as simple as noticing the cracks in the sidewalk and avoiding them, or something as complex as improvising music by following a group of other performers. Even for simple signals, however, the input to a computer goes through many different systems to be finally transformed to the appropriate output. What subsystems exist in the brain to do similar processing?

The Road to Intelligence

The evolution of the human brain is a process which has occurred over many millions of years, and continues today. Hence, the brain is built layer upon layer, such that each layer depends on the previous one. The basis of the whole system, however, is the spinal cord.

The first vertebrate animal was probably a fishlike creature which housed the most basic of spines. Sensory information probably entered the top of the spine, and resulted in an automatic response, or reflex, on the opposite end. Over time, the front end of the spine became more specialized in order to handle more complex sensory input. This end probably developed because early vertebrates swam forward, head first. However, the basic task of the spine remains the same today as it was long ago; it recieves sensory input from the environment, generates simple reflex responses, and transmits them to the rest of the body.

The first primitive brain that developed, and is still present in humans today, is what is now known as the hindbrain. The hindbrain is made up of the medulla oblongata, the cerebellum, and the reticular formation. A major task of the hindbrain is connecting the rest of the brain to the spine, however, it is responsible for many life-sustaining functions as well. The medulla regulates heartrate, circulation, and respiration. The cerebellum is responsible for chaining well-sequenced, smooth movements, as well as maintaining balance and posture. The reticular formation is primarily aimed at regulating consciousness and arousal levels. Additionally, the reticular formation controls the activity of neurons throughout the rest of the central nervous system, such as the midbrain.

The midbrain is composed of the tectum and tegmentum. The midbrain allows humans to orient themselves according to sensory stimuli. In fact, much of the AI and Computer Vision research conducted up to this point has been aimed at reproducing the work done by the midbrain. Localization and optical flow, for instance, are problems which have only recently been somewhat satisfactorily tackled by researchers; the midbrain has been doing this for many millions of years.

The human hindbrain and midbrain are strikingly similar to the hindbrains and midbrains of many lesser animals. In fact, "the human brainstem (which includes most of the structures below the cerebrum) is almost identical to the brainstem of a sheep" (Westen 100). It is the layers built upon the brainstem which separate the intellectual abilities of humans from those of most other animals. These layers are grouped together into what is generally referred to as the forebrain.

Basic Bahaviors: the Forebrain

The forebrain is composed of the hypothalamus, thalamus, and cerebrum, which can be broken down even further. The cerebrum can be broken down into the cortex, and the sub-cortical structures. The cortex will be examined in a section of it's own; the hypothalamus, thalamus, and sub-cortical structures comprise the components that create basic human bahaviors.

The hypothalamus is a tiny structure located directly next to the pituitary gland. It's primary function is to maintain homeostasis in the body. This entails regulating body temperature, blood-sugar and metabolism. It is also responsible for necessary behaviors such as eating, sleeping, sexual activity, and even emotion. This differs from species to species, however, since the hypothalamus regulates many behaviors which are species-specific. For example, in lower species, the hypothalamus controls an animal's reaction to a predator.

For computer scientists, the thalamus is a very interesting part of the brain. It takes raw sensory data, processes it, and passes it on to the higher sections of the brain. It's function "is not only to route messages to the appropriate structures, but also to filter them, highlighting some and de-emphasizing others" (Westen 108). It doesn't take a PhD to see the natural connection to computer science; the thalamus can be easily compared to the parser component of a compiler. It organizes data in such a way that it can be easily handled by the more complicated systems.

The sub-cortical structures can be broken down into the limbic system and the basal ganglia. The limbic system can be broken down even further into it's components, which are the septal area, the amygdala, and the hippocampus. These three structures serve a wide array of purposes.

Although it has only recently been explored by researchers, the septal area seems to be connected to the experience of pleasure. It also helps animals learn to avoid things which may cause unwanted experiences. As with any part of the brain which is involved in learning, the septal area might be modeled on a computer with a neural net. This makes sense, since the idea of neural nets was derived from the functioning of neurons, the cells which make up most of the brain. For many years, computer scientists have been using neural nets to train computers to perform certain tasks. The idea is to derive the weights of a decision making network of nodes by rewarding nodes which help the computer arrive at the correct decision.

The amygdala's primary function is to assign emotional significance to events, especially when the emotion involved is fear. The amygdala triggers the fight-or-flight reaction, whose symptoms include "increased heart rate and force of each beat; increased muscle tension that can even cause tremors; sweaty but cold palms; even nausea and diarrhea" (http://www.psycheducation.org/emotion/amygdala.htm). It also plays a part in memory, so far as it passes emotionally significant events on to the hippocampus.

The hippocampus is responsible for creating new memories. There are numerous examples in fiction (Memento) and medical history (H. M.) of people who have damaged their hippocampus and been stuck in the moment before their hippocampus was removed. In terms of modern computers, having no hippocampus would be like having a full hard drive. A user can browse the internet, where pages will be stored in the computer's RAM, however, the user cannot save anything to the hard drive for long term storage. After the user has left a webpage, it's content can no longer be retrieved.

The final sub-cortical structure is the basal ganglia. Located near the thalamus and hypothalamus, the basal ganglia assists in coordinating movement. Although much is still unclear about it, it also plays a part in mood and memory.

Complex Thought: the Cerebral Cortex

Although the above cartoon is silly, and clearly intended only for humor, it demonstrates the quality which many scientists say is what makes humans intelligent, the ability for symbolic thought. In this case, the symbolic thought is the communication of humor through text and an illustration. The cerebral cortex is largely responsible for this. Westen describes the job of the cerebral cortex quite gracefully,

"First, it allows the flexible construction of sequences of voluntary movements ... Second, it permits subtle discrimination among complex sensory patterns ... Third, the cortex makes possible symbolic thinking ..." (Westen 110).
The cerebral cortex accomplishes all this through coordination of subdivions called lobes. Each half of the cortex is made up of four lobes: occipital, parietal, frontal, and temporal.

The occipital lobes' main job is interpreting sensory data coming from the eyes. The primary regions perform elementary preprocessing, such as recognizing lines, and the association regions group features into distinct objects. This region is of special interest to those researchers in the field known as computer vision. It must be incredibly frustrating to see that the brain can easliy accomplish so many tasks which have been struggled with for decades by academics! However, recently many scientists have made significant progress in imitating the actions of this area of the brain. Here at the U of R, for instance, Randall Nelson's object recognizer locates objects in a similar manner to the occipital lobes. After extracting significant lines and edges in the image, much like the primary regions in the occipital lobes, his recognizer integrates these features into a complex map of the image, much like the association regions. Like human vision, his system is also somewhat view, size, and rotation invariant.

The parietal lobes also integrate and process sensory information. They handle the sense of touch, detection of movement, localization of objects, and the sense of one's own body. The parietal lobes accomplish this gargantuan feat by utilizing data from nearly every sense.

The frontal lobes may be the hardest working part of the human brain. Not only do they assist with "movement, attention and planning", but they also help with "social skills, abstract thinking, memory, and some aspects of personality" (Westen 112). The frontal lobes help to coordinate fine motor movements, such as those made by the fingers or the mouth. Of especial importance to computer scientists is Broca's Area, which is specialized in speech production. Damage to Broca's Area "prevents a person from producing speech" (http://faculty.washington.edu/chudler/lang.html). This task has been largely conquered by academics. In fact, the version of Fruityloops on my computer comes bundled with an excellent speech synthesizer.

The last important region of the cortex, and the last which we will include in this whirlwind tour of the brain, is the temporal lobes. The temporal lobes also play an important role in hearing and language; Wernicke's Area is primarily responsible for language understanding. This function has yet to be reproduced by a computer, though many existing systems can emulate a limited understanding. In fact, the undergrad robotics team implemented a basic natural language understanding system in Mabel, just this past semester.

The Big Picture

The brain is an incredibly complex organ which we have only just begun to understand. Although it can be broken down into simpler parts which perform more basic functions, even the simplest functions can be unimaginably tough to reproduce on a computer. Luckily, the more we know about the brain, the more clear our job as computer scientists becomes. This can be seen in the tremendous work being done in the fields of artificial intelligence, computer vision, and natural language understanding. The gauntlet has been laid though, and only when it has been fully explored will the computer become and outdated model of the brain.



Notes

It should be kept in mind that the subdivisions of the brain described here are natural subdivisions. They are not strict subdivisions as might occur in a computer. The regions are often difficult to identify, and the borders between them are not always easily distinguishable.


Bibliography

The primary source used to compile this paper was my psych 101 textbook, which is listed first. The websites were used conservatively as additional sources.

Westen, Drew. Psychology: Mind, Brain, & Culture, second edition. John Wiley & Sons Inc. New York. 1996.

http://www.psycheducation.org/emotion/introduction.htm#Tours. Brain Tours

http://faculty.washington.edu/chudler/introb.html#bb. Explore the Nervous System

http://ifcsun1.ifisiol.unam.mx/Brain/segunda.htm. A Brief Introduction to the Brain