Week 4 Summary - Brain and Behaviour PDF

WEEK 4: Summary Sensation Week Objective 1: Describe the key elements of a sensory system and the flow of information from a stimulus in the environment to the higher centres of the brain The first step in sensation involves accessory structures, which collect and modify sensory stimuli. The second...

WEEK 4: Summary Sensation Week Objective 1: Describe the key elements of a sensory system and the flow of information from a stimulus in the environment to the higher centres of the brain The first step in sensation involves accessory structures, which collect and modify sensory stimuli. The second step is transduction, the process of converting incoming energy into nerve cell activity; it is accomplished by neural receptors, cells specialised to detect energy of some type. Sensory adaptation takes place when receptors receive unchanging stimulation. Nerve cell activity is transferred through the thalamus (except in the case of olfaction) and on to the cortex. Weekly Objective 2: Explain the terms 'transduction' and 'encoding' Transduction is the process of converting incoming energy into nerve cell activity. This ‘translates’ the ‘language’ of the environment into the ‘language’ of the nervous system. Encoding is the translation of a stimulus’ physical properties into a pattern of neural activity that specifically identifies those properties. Weekly Objective 3: Describe the following for vision, hearing, taste, smell, touch, temperature and the body senses • • • • the nature of the physical stimulus detected by each sense the form/function of the accessory structure (i.e. the anatomy of the sense organ) the receptor cells the psychological correlates of the stimulus dimensions Vision. The human visual system combines great sensitivity and great sharpness. Although our night vision is not as acute as that of some animals, our colour vision is excellent. Visible light is electromagnetic radiation that has a wavelength from just under 400 nanometres to about 750 nanometres (a nanometre is one-billionth of a metre). • Light intensity refers to how much energy the light contains and determines the brightness of light. • What colour you sense depends mainly on light wavelength, with different wavelengths producing sensations of different colours. Light rays enter the eye through a transparent, protective layer called the cornea. Light then passes through the pupil, the opening behind the cornea. The iris, which gives the eye its colour, adjusts the amount of light allowed into the eye by constricting to reduce the size of the pupil or relaxing to enlarge it. The lens is directly behind the pupil. Both the cornea and the lens are curved so that they bend light rays. Light rays are focused into an image on the retina, the surface at the back of the eye. The lens bends light rays from various angles so that they meet on the retina. Photoreceptors are the specialised cells in the retina that convert light energy into nerve cell activity. The retina has two basic types of photoreceptors: rods and cones. These cells differ in shape, in composition, and in response to light. • Cones use one of three varieties of iodopsin photopigments, each of which is differently sensitive to different light wavelengths. • Rods use the photopigment rhodopsin, which is much more light-sensitive than cones’ iodopsin. Hearing. Sound is a repetitive fluctuation in the pressure of a medium, such as air. Vibrations of an object produce the fluctuations in pressure that create sound. A wave is a repetitive, rhythmic variation in pressure that spreads out in all directions. Physical Characteristics of Sound Sound is represented graphically by waveforms. A waveform represents a wave in two dimensions. Waveforms have three characteristics: • Amplitude or intensity is the difference in air pressure from a wave’s baseline to its peak. • Wavelength is the distance from one peak to the next. • Frequency is the number of complete waveforms, or cycles, that pass by a given point in space every second. One cycle per second is 1 hertz (Hz). Auditory Accessory Structures The pinna is the crumpled part of the ear visible on the outside of the head that funnels sound down through the ear canal. At the end of the ear canal, the sound waves reach the middle ear, where they strike a tightly stretched membrane known as the eardrum or tympanic membrane. The vibrations of the tympanic membrane are transferred through a chain of three tiny bones named for their shapes, the malleus (hammer), incus (anvil) and the stapes (stirrup). These bones amplify the changes in pressure produced by the original sound waves by focusing the vibrations of the tympanic membrane onto the oval window, a smaller structure. Auditory Transduction After sound passes through the oval window, it enters the inner ear reaching the cochlea. The cochlea is the structure in which transduction actually occurs. The cochlea is wrapped into a coiled spiral. A fluid filled ‘tube’ runs the length of it. The asilar membrane forms the floor of this long tube. When a sound wave passes through the fluid in the tube, it moves the basilar membrane, and this movement deforms hair cells of the organ of Corti, a group of cells that rests on the membrane. These hair cells make connections with fibres from the acoustic nerve (auditory nerve), a bundle of axons that go into the brain. When the hair cells bend, they stimulate neurons in the auditory nerve to fire in a pattern that sends the brain a coded message about the amplitude and frequency of the incoming sound waves, senses as loudness and pitch. Psychological Dimensions of Sound The physical characteristics of sound waves produce the psychological dimensions of sound. Loudness is determined by the amplitude of the sound wave. Waves with greater amplitude produce sensations of louder sounds. Pitch, how ‘low’ or ‘high’ a tone sounds, depends on the frequency of sound waves. Highfrequency waves are sensed as high-pitched sounds. • Humans can hear sounds from about 20 hertz to about 20,000 hertz. Almost everyone experiences pitch as relative – being able to tell which note is higher. A few people have absolute pitch (more commonly known as perfect pitch) – or are able to identify specific frequencies and the notes they represent. Some people appear to be ‘tone deaf’ in that they are not good at discriminating among musical tones. Timbre is sound’s quality. It is caused by complex wave patterns that are added onto the lowest, or fundamental, frequency of a sound. This enables you to tell the difference between a note played on a flute and a note played on a clarinet. Taste & Smell. Taste perception (gustatory perception or sense of taste) is the chemical sense system in the mouth. The receptors for taste are in taste buds, which are grouped together as papillae in the mouth and throat. Olfactory perception (or sense of smell) detects chemicals that are airborne, or volatile. Accessory structures include the nose, the mouth and the upper part of the throat. Odour molecules can reach receptors either through the nose or through an opening in the palate at the back of the mouth. Smell and taste act as two components of one system, flavour. Most of the properties that make food taste good are actually odours detected by the olfactory system. Anosmia, the inability to distinguish smells, also interrupts the ability to determine flavour, although the gustation system is in working order. The olfactory and gustatory pathways converge in the orbitofrontal cortex where neurons also respond to the sight and texture of food. Touch & Temperature. The cutaneous senses (somatic senses or somatosensory systems) are located throughout the body, rather than in a localised, specific organ. These include the skin senses of touch, temperature, and pain as well as proprioception, the sense that tells the brain about body position and movements. The vestibular system (sense of equilibrium) tells the brain about the position and movements of the head. Touch is vitally important. It would be difficult to survive without a sense of touch. Touch is even needed to swallow food. Stimulus and Receptors for Touch The energy detected by the sense of touch is physical pressure on tissue, usually the skin. The skin covers about 1–2 square metres of surface and weighs nearly 10 kg. Hairs on the skin bend and deform the skin beneath them. The receptors that transduce pressure into neural activity are in, or just below, the skin. Proprioception: Sensing body position The proprioceptive senses provide information about the position of the body and what each part of the body is doing. Weekly Objective 4: Outline the trichromatic and opponent-process theories of colour perception, and give examples of the phenomena they explain The Trichromatic Theory of Colour Vision The trichromatic theory or Young-Helmholtz theory of colour vision states that there are three types of visual elements in the eye, each most sensitive to a different wavelength, and information from these three elements combines to produce the sensation of colour. Short-wavelength cones respond most to light in the blue range. Medium-wavelength cones respond most to light in the green range. Long-wavelength cones respond most to light in the reddish-yellow range, although these have traditionally been called ‘red cones’. No single cone, by itself, can signal the colour of light, but it is the ratio of the three cone types’ activity that determines what colour will be sensed. Colour vision is the result of the pattern of activity of the different cones. The trichromatic theory was applied in the creation of colour television screens, which contain microscopic elements of red, green and blue. The Opponent-Process Theory of Colour Vision The trichromatic theory alone cannot explain all aspects of colour vision. The opponentprocess theory of colour vision holds that the colour-sensitive elements in the eye are grouped into three pairs, where each pair member opposes, or inhibits, the other. The three pairs are a red–green element, a blue–yellow element, and a black–white element. Each element signals one colour or the other, but never both. This theory explains afterimages. Complementary colours stimulate opposite ‘sides’ of the same opponent-process colour element. Two colours are complementary, if grey results from mixing lights of the two colours. Weekly Objective 5: Outline some of the mechanisms that the auditory system uses to determine the frequency and location of sound Locating Sounds The brain analyses the location of sound based partly on the difference in time the sound takes to reach each ear and on the sound intensity at each ear. Coding Intensity and Frequency The auditory system generally codes intensity by the speed of the firing of the given neuron. The more intense the sound, the more rapid the firing. Weekly Objective 6: Explain the gate theory of pain, and outline the effects of culture, expectation and analgesics on pain The Gate Control Theory of Pain The gate control theory of pain says that there is a ‘gate’ in the spinal cord that either lets pain impulses travel to the brain or blocks their progress. Input from other skin senses may ‘take over’ pathways that pain impulses would have used. This may explain why rubbing the area around a wound eases pain. The brain can close the gate by sending signals down the spinal cord. The result is analgesia, an absence of pain sensation in the presence of a normally painful stimulus. Natural Analgesics At least three chemicals that are released by the body during stress play a role in the brain’s ability to block pain signals. They are the neurotransmitter serotonin, natural opiates called endorphins, and endocannabinoids Emotional Aspects of Pain All senses can have emotional components, most of which are learned responses. However, pain is more direct. Specific pathways carry an emotional component of the painful stimulus to areas of the hindbrain and reticular formation, as well as the cingulate cortex via the thalamus. Cognition affects emotional responses to pain. Knowing about the nature of pain and when to expect it seems to make it less aversive even though the sensations are reported to be just as intense. The use of pain-reducing strategies, such as focusing on distracting thoughts, also affects emotional responses to pain.