The Biological Level of Analysis: Studying the Brain

Transcription

1 The Biological Level of Analysis: Studying the Brain In the past the study of the brain was limited to people suffering from head injuries and the effects of accidental damage. It was only possible to study the brain by carrying out an autopsy. Laboratory research with animals eventually resulted in important discoveries about causes and possible treatments of neurological and mental disorders. Research using live animals must now follow ethical rules. Physiological psychologists have a range of research techniques to study the function of the brain and body that could not have been imagined just a few decades ago- for example, ways to identify neurotransmitters and taking pictures of particular ions entering neurons when the appropriate ion channels open. It is also possible to study the activity of the brain through neuro-imaging. Broca and Wernicke Pioneers in Brain Research Broca (1861) found that people suffering from damage in an area that was eventually called the Broca s area are unable to understand and make grammatically complex sentences. Their speech consists almost entirely of content words. Wernicke (1874) first described the area that appears to be crucial for language comprehension. People who suffer from damage to Wernicke s area are unable to understand content words while listening, and they are unable to produce meaningful sentences. Both researchers used autopsies to study the brain in patients who had suffered from strokes. Auditory and speech information is transported from the auditory area to Wernicke's area for evaluation of significance of content words, then to Broca's area for analysis of syntax. In speech production, content words are selected by neural systems in Wernicke's area, grammatical refinements are added by neural systems in Broca's area, and then the information is sent to the motor cortex, which sets up the muscle movements for speaking. Broca s patients had problems producing speech but could understand it. Wernicke s patients could produce speech but could not understand speech. Post-mortem autopsies revealed areas shown in the picture above had been damaged, and the conclusion was that these areas somehow were involved in understanding and production of speech.

2 The Story of Phineas Gage Phineas Gage accidentally received a steel bar through his forehead in 1848 while working on the railroad. He survived but consequently his personality changed. An autopsy revealed damage to his frontal lobes. Subsequent research has established the important connection between social behaviour and the frontal lobes. The case study of Gage was one of the first important examples of how researchers worked before the new techniques of brain investigation. Research Techniques The earliest research method of physiological psychology involves correlating a behavioural deficit with damage to a specific part of the nervous system. This method can be used in two ways: 1. Study of the effects of brain damage on function, e.g. effect of damage to the frontal lobes on the ability to create and adhere to plans (Code et al. 1996). 2. Experimental brain lesioning, i.e. an injury to a particular part of the brain - only in an animal brain. The techniques used for observing activities of the brain are either: Non-invasive that is, the technique does not involve breaking the skin or injecting anything into the body. Invasive where the skin is broken or injections are given, for example with a radioactive isotope. Invasive methods: ablation/lesioning, electrical stimulation, micro-electrode recording Brain lesioning involves the deliberate surgical removal of brain tissue in order to study behavioural change. You can cut, burn with electrodes or suck part of the brain tissue away. Ablation/lesioning was introduced by Pierre Flourens in the 1820 s. He cut slices of the cerebellum in rabbits, birds and dogs and found that animals had problems in muscular co-ordination and poor sense of balance after the lesion. Flourens correctly hypothesized that the removed parts were involved in muscular coordination and balance.

3 Advantages of ablation/lesion in animal research Possibility to understand how the brain functions without having to wait for naturally occurring damages. Under anaesthetic, an animal s head can be held in a fixed position in what is termed a stereotaxic apparatus to insert an electrode into a particular location in the brain so that you can investigate an exact correlate of behaviour by comparing behaviour before and after the brain damage. The term stereotaxic refers to the ability to manipulate an object in three-dimensional space. The researcher passes an electrical current through the electrode, which produces heat that destroys a small portion of the brain around the tip of the electrode. After a few days, the animal recovers from the operation, and the researchers can assess its behaviour. Disadvantages of ablation/lesion in animal research: Limited what such studies can tell about the human brain One cannot be absolute sure that behavioural changes are only due only to ablation There are ethical issues in using this technique Electrical stimulation of the brain (ESB) This method implies insertion of electrodes into the brain of a living animals and sending a weak electric current into the brain to mimic a nerve impulse - that is, a false nerve impulse makes the brain react as if it were a real impulse from a sensory receptor. In the picture of the monkey it is demonstrated that the diameter of the pupil can be electrically controlled as if it were the diaphragm of a photographic camera lens. See the constriction of the right pupil evoked by stimulation of the hypothalamus (Delgado). The other pupil is big. Delgado demonstrated that he could also control a bull by sending impulses via a transmitter into the brain. Olds & Milner (1954) found that electrical stimulation of the hypothalamus in rats seemed extremely pleasurable and that animals who were stimulated in this area were only interested in continuing to stimulate the area. It was suggested that this centre was perhaps a pleasure center in the brain. They also located a different center that the animals would avoid activating and suggested that this was a pain centre.

4 Using the electrical stimulation method, James Olds and Peter Milner (1954) observed that some animals seem to behave in a manner that increased the amount of intracranial stimulation that they received. Further investigation demonstrated that rats will press a lever as rapidly as 2000 times each hour to obtain electrical brain stimulation, and they will continue responding at this rate for twenty-four hours or longer. They will ignore other rewards, such as water or food, to continue working for electrical stimulation. These very powerful results led to the adoption of the intracranial self-stimulation method for investigating the "reward system" in the brain and remain up to now the principle tool. Walter Penfield performed surgery on epileptics during the 1940 s and 1950 s. Before surgery he tried to stimulate the cortex. He found that there were no pain receptors in the brain. The patient was conscious during the stimulation, and Penfield mapped the cortex in this way. Evaluation of ESB Provides insight into functioning of brain for example, argued that if stimulation in one area produces a behaviour then the site is involved in that behaviour. May be efficient in treatment of certain psychological disorders such as schizophrenia. Efficient in blocking pain for example, in cancer patients. Evaluation by Valenstein (1977) No single area of the brain is the only source of a behaviour/emotion; ESB-provoked behaviour is compulsive + stereotypical that is, it does not perfectly mimic natural behaviour; ESB effects may depend on other factors; not everybody reacts in the same way to stimulation. Non-invasive methods: CAT, MRI, and PET Scanning The development of several different diagnostic machines which can be used to investigate the brain s structure and activity has revolutionized neuropsychological research. Since the 1970 s it has been possible to study the human brain in living individuals, by using X-rays. Sophisticated techniques called neuro-imaging techniques now allow researchers to visualize and obtain images of brain function and structure. These techniques include CAT, PET, MRI and fmri scanning.

5 CAT (computerized axial tomogram): In this technique a number of X-ray pictures are taken from different locations. The scanner sends a narrow beam of X-rays through a person s head. The beam is moved around the patient s head, and a computer calculates the amount of radiation that passes through it at various points along each angle. The result is a three- dimensional image of a slice of the brain s structures. CAT scans help determine if behavioural problems have physiological determinant, and they can help surgeons to determine how to proceed in an operation. It can also determine the effects of therapy, e.g. radiation on brain tumors. MRI (Magnetic Resonance Imaging) gives a three-dimensional picture of the brain structures in greater detail than the CAT scanner. It uses magnetic fields and radio waves instead of X-rays. It exploits the fact that some substances that make up the body have intrinsic magnetic properties and respond to being in a magnetic field, rather as does a compass needle. For example, water, a major component of the body is made up of hydrogen and oxygen and the hydrogen atoms exhibit such a magnetic property. When a magnetic field is passed over the head, reverberations are produced by hydrogen molecules, and these are picked up by the scanner which converts the activity into a structural image. CAT vs. MRI Harmless radio waves are used in MRI More sensitive than the CAT scan and thus more accurate pictures PET (Positron Emission Tomography) invasive measure of brain metabolism, glucose consumption and blood flow. The person is injected in the arm with a harmless dose of radioactive glucose that enters the brain and goes to its active parts. PET measures brain activity by examining the amount of oxygen consumed by, or blood flow traveling to, neurons. The radioactive parts of glucose emit positrons, which are detected by PET scanner and this activity is represented in the form of coloured maps. In the image above, a PET scan depicts the effects of Alzheimer s disease on metabolism. The arrow indicates areas of low activity in the parietotemporal cortex - a region important for processing language and memory.

6 PET scans can Diagnose abnormalities like tumours or changes in brain function like in Alzheimer s. Compare brain differences in normal individuals and individuals with psychological disorders for example, neural activity is different in persons with schizophrenia. The greatest advantage of PET compared to MRI is that it can record ongoing activity in the brain, e.g. thinking. Some empirical research Restak (1984) used the PET to show how the front of the brain and the part that produces movement became active when a person was asked to move the right hand. When a person is asked to think about moving the hand, only the front part is active and not the part involved in actual movement. Martin & Brust (1985) showed that participants asked to listen to and recall a story had activity in the part of the brain responsible for processing auditory information, as well as in the hippocampus. It is possible to use MRI in a functional capacity, i.e. to examine the brain s function as well as its structure (functional magnetic resonance imaging or fmri). MRI and fmri are noninvasive methods compared to PET where radioactive substances are introduced in the body. MRI and fmri have been used to investigate similar functions to those investigated using PET: language, attention, vision, and memory. PET and MRI scans can be used in combination. They have the advantage of good spatial resolution that is, images and structures are seen very precisely - but the disadvantage of a poor temporal resolution that is, it is difficult to match the psychological and neural event in time precisely. The reason for this is that in PET and MRI scanning, a number of scans are taken and these are averaged in time.

7 Questions for discussion 1. How did researchers study the brain before the new techniques? 2. What are the advantages of the new techniques compared to the old techniques? 3. Discuss ethical implications of using animals in neurological research: Consult the APA guidelines for ethical conduct in animal research, available at: anguide.html 4.Compare two different methods for investigating the brain and evaluate their strengths and limitations.

8

9

10

11

12