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Chapter 2

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Phillip Servos

Chapter 2 – Introduction to the Physiology of Perception The Brain: The Mind’s Computer Brief History of the Physiological Approach  Early thinking about the physiology of the mind focused on determining the anatomical structures involved in the operation of the mind Early Hypotheses About the Seat of the Mind  In the fourth century B.C. the philosopher Aristotle (384-322 B.C) stated that the heart was the seat of the mind and the soul  The Greek physician Galen (ca. A.D. 130-200) saw human health, thoughts, and emotions as being determined by four different “spirits” flowing from the ventricles – cavities in the center of the brain  In the early 1630s the philosopher Rene Descartes, although still accepting the idea of flowing spirits, specified the pineal gland, which was thought to be located over the ventricles, as the seat of the soul The Brain As the Seat of the Mind  In 1664 Thomas 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 Travelling in Neurons  One of the most important problems to be solved was determining the structure of the nervous system  In the 1800s, there were two opposing ideas about the nervous system o One idea, called reticular theory, held that the nervous system consisted of a large network of fused nerve cells o The other idea, neuron theory, stated that the nervous system consisted of distinct elements or 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  Camillo Golgi (1873) developed a technique in which immersing a thin slice of brain tissue in a solution of silver nitrate created pictures in which individual cells were randomly stained  By the late 1800s, researchers had shown that a wave of electricity is transmitted in groups of neurons, such as the optic nerve o To explain how these electrical signals result in different perceptions, Johannes Mueller in 1842 proposed the doctrine of specific nerve energies, which stated that our perceptions depend on “nerve energies” reaching the brain and that the specific quality we experience depends on which nerves are stimulated  Thus, he proposed that activity in the optic nerve results in seeing, activity in the auditory nerve results in hearing, and so on Recording From Neurons  In the 1920s Edgar Adrian (1928, 1932) was able to record electrical signals from single sensory neurons  Just as listening to individual people provides valuable information about what is happening in a large crowd, recording from single neurons provides valuable information about what is happening in the nervous system  It is important to record from as many neurons as possible because just as individual people may have different opinions about the speech, different neurons may respond differently to a particular stimulus or situation 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 layer 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  A basic 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 senses are organized into primary receiving areas, the first areas in the cerebral cortex to receive the signals initiated by each senses 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 the skin senses – touch, temperature, and pain – is located in area in the parietal lobe  The frontal lobe receives signals from all of the senses, and plays an important role in perceptions that involve the coordination of information received through two or more sense 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 transducer these stimuli into electrical signals  Another purpose is to communicate with other neurons, so that these signals can travel long distances 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 fibre, is filled with fluid that conducts electrical signals  Especially important for perception are a type of neuron called receptors, which are specialized to respond to environmental stimuli such as pressure for touch Recording Electrical Signals in Neurons  It is important to distinguish between single neurons and nerves  A nerve, such as the optic nerve, which carries signals out the back of the eye, consists of the axons (or nerve fibres) of many neurons, just as many individual wires make up a telephone cable o Thus, recording from an optic nerve fibre involves recording not from the optic nerve as a whole, but from one of the small fibres within the optic nerve  Microelectrodes, small shafts of glass or metal with very fine tips, are used to record signals from single neurons o The key principle for understanding electrical signals in neurons is that we are always measuring the difference in charge between two electrodes o One of these electrodes, located where the electrical signals will occur, is the recording electrode o The other one, located some distance away so it is not affected by the electrical signals, is the reference electrode  When the nerve fibre is at rest, the oscilloscope records a difference in potential of -70 millivolts (where a millivolt is 1/1000 of a volt) o This value, which stays the same as long as there are no signals in the neuron, is called the resting potential  When the neurons receptor is stimulated so that a signal is transmitted down the axon: o As the signal passes the recording electrode, the charge inside the axon rises to +40 milivolts compare to the outside o As the signal continues past the electrode, the charge inside the fibre reverses course and states becoming negative again until it returns to the resting level o This signal, which is called the action potential, lasts about 1 millisecond Chemical Basis of Action 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 understanding the components of the neuron’s liquid environment  Neurons are surrounded by a solution rich in ions, molecules that carry an electrical chare o Ions are created when molecules gain or lose electrons, as happens when compounds are dissolved in water  Remember that the action potential is a rapid increase in positive charge until the inside of the neurons is +40 mV compared to the outside, followed by a rapid return to the baseline of -70 mV o These changes are caused by the flow of sodium and potassium ions across the cell membrane o First, sodium flows into the fibre, then potassium flows out, and this sequence of sodium-in, potassium-out continues as the action potential travels down the axon  The upward phase of the action potential – the change from -70 to +40 mV – occurs when positively charged sodium ions rush into the axon  The downward phase of the potential – the change from +40 back to -70 mV – occurs when positively charged potassium ions rush out of the axon  Once the action potential has passed the electrode, the charge inside the fibre returns to the resting potential of -70 mV  The changes in sodium and potassium flow that create the action potential are caused by changes in the fibre’s permeability to sodium and potassium o Permeability is a property of the cell membrane that refers to the ease with which a molecule can pass through the membrane o Selective permeability occurs when a membrane is highly permeable to one specific type of molecule, but not to others  Before the action potential occurs, the membranes permeability to sodium and potassium is low, so there is little flow of these molecules across the membrane o Stimulation of the receptor triggers a process that causes the membrane to become selectively permeable to sodium, so sodium flows into the axon o When the action potential reaches +40 mV, the membrane suddenly becomes selectively permeable to potassium, so potassium flows out of the axon o The action potential, therefore, is caused by changes in the axon’s selective permeability to sodium and potassium Basic Properties of Action Potentials  An important property of the action potential is that it is a propagated response – once the response is triggered, it travels all the way down the axon without decreasing in size o Another property is that the action potential remains the same size no matter how intense the stimulus is. We can demonstrate this by determining how the neuron fires to different stimulus intensities  Changing the stimulus intensity does not affect the size of the action potentials but does not affect the rate of firing  Although increasing the stimulus intensity can increase the rate of firing, there is an upper limit to the number of nerve impulses per second that can be conducted down an axon o This limit occurs because of a property of the axon called the refractory period – the interval between the time one nerve impulse occurs and the next one can be generated in the axon o Because the refractory period for most neurons is about 1 ms, the upper limit of a neurons firing rate is about 500 to 800 impulses per second  The action potentials that occur in the absence of stimuli from the environment is called spontaneous activity o This spontaneous activity establishes a baseline level of firing for the neuron o The presence of stimulation usually causes an increase in activity above this spontaneous level, but under some conditions it can cause firing to decrease below the spontaneous level  The action potential’s function is to communicate information o Increasing the stimulation of a receptor can cause a change in the rate of nerve firing, usually an increase in firing above the baseline level, but sometimes a decrease below the baseline level o These changes in nerve firing can therefore provide information about
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