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Chapter 2

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PSYC 2390
Lana Trick

PSYC 2390: SENSATION AND PERCEPTION Chapter 2 – pg. 34 The Brain: The Mind’s Computer • Brain: the structure responsible for mental functions such as memory, thoughts, language, and perceptions Brief History of the PhysiologicalApproach • Early thinking of the physiology of the mind focused on determining the anatomical structures involved in the operation of the mind • Early HypothesesAbout the Seat of the Mind → Aristotle stated that the hear was the seat of the mind and the soul → The Greek physician Galen saw human health, thoughts, and emotions as being determined by four different “spirits” flowing from the ventricles- cavities in the center of the brain. This idea was accepted all the way through the middle ages and into the renaissance in the 1500s and early 1600s → Then philosopher Rene Descartes, although still accepting idea of following spirits, specified the pineal gland, which was though to be located over the ventricles, as the seat of the soul • The Brain as the seat of the mind → Thomas Willis published a book titled The Anatomy of the Brain, which was based on dissections of the brains of humans, dogs, sheep and other animals. → Willis concluded that the brain was responsible for mental functioning, that different functions were located in different regions of the brain, and that disorders of the brain were disorders of chemistry • Signals Traveling in Neurons → In the 1800s, there were two opposing ideas about the nervous system. → One idea, called reticular theory, held that the nervous system consisted of a large network of fused nerve cells. → The other idea, neuron theory, stated that the nervous system consisted of distinct elements of cells → An important development that led to the acceptance of neuron theory was the discovery of staining, a chemical technique that caused nerve cells to become colored so they stood out from surrounding tissue → By the late 1800s, researchers had shown that a wave of electricity is transmitted in groups of neurons, such as optic nerve. → To explain how these electrical signals result in different perception, Johannes Mueller proposed the doctrine of specific nerve engines, which stated that our perceptions depend on “nerve engines” reaching the brain and that the specific quality we experience depends on which nerves are stimulated. Thus activity in the optic nerve results in seeing, activity in the auditory nerve depends on hearing and so on → This idea had expanded to conclude that nerves from each of these senses reach different areas of the brain. PSYC 2390: SENSATION AND PERCEPTION • Recording From Neurons → The ability to record electrical signals from individual neurons ushered in the modern era of brain research, and in the 1950s and 1960s development of more sophisticated electronics and the availability of computer and the electron microscope made more detailed analysis of how neurons function possible Basic Structure of the Brain • Much of the research on the connection between the brain and perception has focused on activity in the cerebral cortex, the 2-mm thick later that covers the surface of the brain and contains the machinery for creating perception, as well as for other functions, such as language, memory and thinking • Abasic principle of cortical function is modular organization – specific functions are served by specific areas of the cortex → One example of modular organization is how the sense are organized into primary receiving areas, the first areas in the cerebral cortex to receive the signals initiated by each sense’s receptors → The primary receiving area for vision occupies most of the occipital lobe; the area for hearing is located in part of the temporal lobe; and the area for skin senses- touch, temperature, and pain- is located in an area in the parietal lobe → The frontal lobe receives signals from all of the senses, and plays an important role in perception that involves the coordination of information received through two or more senses Neurons: Cells That Create and Transmit Electrical Signals • One purpose of neurons that are involved in perception is to respond to stimuli from the environment, and transduce these stimuli into electrical signals (transduction) • Another purpose is to communicate with other neurons, so that these signals can travel long distances (transmission) Structure of Neurons • The cell body contains mechanisms to keep the cell alive; dendrites branch out from the cell body to receive electrical signals from other neurons; and the axon, or nerve fiber, is filled with fluid that conducts electrical signals • There are variations of this basic nerve structure: some neurons have long axons; others have short axons or none at all • Especially important for perception are a type of neuron called receptors, which are specialized to respond to environmental stimuli such as pressure for touch • All receptors for vision, hearing, touch, smell and taste look different but they all have something in common: part of each receptor reacts to environmental stimuli and triggers the generation of electrical signals, which eventually are transmitted to neurons with axons Recoding Electrical Signals in Neurons • It is important to distinguish between single neurons and nerves PSYC 2390: SENSATION AND PERCEPTION • Anerve, such as the optic nerve, which carries signals out the back of the eye, consists of the axons (or nerve fibers) of many neurons • Thus recording from an optic nerve fiber involves recording not from the optic nerve as a whole, but from one of the small fibers within the optic nerve • METHOD: RECORDING FROMANERUON → Microelectrodes, small shafts of glass or metal with very fine tops, are used to record signals from single neurons → The key principle for understanding electrical signals in neurons is that we are always measuring the difference in charge between two electrodes → One of these electrodes located where the electrical signals will occur, is the recording electrode → The other one, located some distance away so it is not affected by the electrical signals, is the reference electrode → The difference in charge between the recording and reference electrodes is displayed on an oscilloscope, which indicates difference in charge by a small dot that creates a line as it moves across the screen • When the nerve fiber is at rest, the oscilloscope records a difference in potential of -70 millivolts. This value, which stays the same as long as there are no signals in the neuron, is called the resting potential. In other words, the inside of the neuron is 70 mV negative compared to the outside, and remains that way as long as the neuron is at rest • When the neuron’s receptor is stimulated so that a signal is transmitted down the axon.As the signal passes the recording electrode, the charge inside the axon raises to +40 millivolts compared to the outside.As the signal continues past the electrode, the charge inside the fiber reverses course and starts becoming negative again, until it returns to the resting level. This signal, which is called the action potential, lasts about 1 millisecond Chemical Basis ofAction Potentials • The electrical signals in neurons are created by and conducted through liquid • The key to understanding the “wet” electrical signals transmitted by neurons is understand the components of the neuron’s liquid environment • Neurons are surrounded by a solution rich in ions, molecules that carry an electrical charge • Ions are created when molecules gain or lose electrons, as happens when compounds are dissolved in water. For example, adding table salt (sodium chloride NaCl) to water creates positively charged sodium ions (Na ) and negatively charged chlorine ions (Cl). • The solution outside the axon of a neuron is rich in positively charged sodium ions, whereas the solution inside the axon is rich in positively charged potassium
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