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Chapter 5

Chapter 5 - sensation and perception.docx

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Dan Dolderman

Chapter 5: Sensation and Perception Sensation = the sense organs’ responses to external stimuli and the transmission of these responses to the brain. Perception = the processing, organization and interpretation of sensory signals; it results in an internal representation of the stimulus. Sensory coding = our sensory organs’ translation of stimuli’s physical properties into neural impulses. Transduction = a process by which sensory receptors produce neural impulses when they receive physical or chemical stimulation. Connecting neurons then transmit information to the brain in the form of neural impulses. Most sensory information first goes to the thalamus, a structure in the middle of the brain. Neurons in the thalamus then send information to the cortex, where incoming neural impulses are interpreted as sight, smell, sound, touch or taste. Sense Stimuli Receptors Pathways to the brain Gustation (Taste) Molecules Taste cells in taste Portions of facial, dissolved in fluid buds on the tongue glossopharyngeal, on the tongue and vagus nerves Olfaction (Smell) Molecules Sensitive ends of Olfactory nerve to dissolved in fluid olfactory neurons the olfactory bulb on mucous in the olfactory below the frontal membranes in the epithelium lobes nose Haptic sense Pressure on the Sensitive ends of Trigeminal nerve (Touch) skin touch neurons in for touch above skin neck, spinal nerves for touch elsewhere Audition (Hearing) Sound waves Pressure-sensitive Auditory nerve hair cells in cochlea of inner ear Vision Light waves Light-sensitive Optic nerve to the rods, cones, primary visual bipolar, amacrine cortex in the and horizontal occipital lobe at cells in retina of the back of the eye. Then one head million ganglion cells Our brains need qualitative and quantitative information about a stimulus. Different sensory receptors respond to qualitatively different stimuli. In contrast, quantitative differences in stimuli are coded by the speed of a particular neuron firing – a more rapidly firing neuron is responding at a higher frequency. Coarse coding = sensory qualities are coded only be a few different types of receptors, each of which responds to a broad range of stimuli. Psychophysics = a subfield developed during the nineteenth century by the German researchers Ernst Weber and Gustav Fechner, examines our psychological experiences of physical stimuli. It assesses how much physical energy is required for our sense organs to detect a stimulus. To test this, researchers present very subtle changes in stimuli and observe how participants respond. They study the limits of humans’ sensory systems. The absolute threshold = the minimum intensity of stimulation that must occur before you experience a sensation, or the stimulus intensity detected above chance. The difference threshold = the just noticeable difference between two stimuli – the minimum amount of change required for a person to detect a difference. Weber’s Law states that the just noticeable difference between two stimuli is based on a proportion of the original stimulus rather than on a fixed amount of difference. Signal Detection Theory (SDT) = a theory of perception based on the idea that the detection of a faint stimulus requires a judgment – it is not an all-or-none process. Sensory adaptation = a decrease in sensitivity to a constant level of stimulation. Gate control theory = for us to experience pain, pain receptors must be activated and a neural “gate” in the spinal cord must allow the signals through the brain. Pain signals are transmitted by small-diameter nerve fibres, which can be blocked at the level of the spinal cord by firing of larger sensory nerve fibres. Lateral inhibition = a visual process in which adjacent photoreceptors tend to inhibit one another. Subtractive color mixing = a way to produce a given spectral pattern in which the mixture occurs within the stimulus itself and is actually a physical, not psychological, process. Additive color mixing = a way to produce a given spectral pattern in which different wavelengths of light are mixed. The percept is determined by the interaction of these wavelengths with receptors in the eye and is a psychological process. A receptive field = the region of visual space to which neurons in the primary visual cortex are sensitive. Simultaneous contrast = an optical illusion in which identical stimuli appear different when presented against different backgrounds. One theory to explain this effect is that lateral inhibition in the retina emphasizes the difference between an object and its background. Kinesthetic sense = refers to sensations we gather from receptors in muscles, tendons and joints, that pinpoint the position in space and the movements of both our body and our limbs. Vestibular sense = uses data from receptors in the semicircular canals of the inner ear. These canals contain a liquid that moves when the head moves, bending hair cells at the ends of the canal. The bending generates nerve impulses that inform us of the head’s rotation and is thus responsible for a sense of balance. Nonhuman animals’ sensory systems: - sonar senses - electroreception Senses bases on electrical fields. Perception occurs in the Brain With the exception of olfaction, all sensory information is relayed to cortical and other areas of the brain from the thalamus. From the thalamus, the information from each sense is projected to a specific region of the cerebral cortex. In these primary sense areas, the perceptual process begins. Hearing – auditory neurons in the thalamus extend their axons to the primary auditory cortex (A1), which is in the temporal lobe. Neurons in this region code the frequency of auditory stimuli. Neurons towards the rear of A1 respond best to lower frequencies, such as that of a foghorn, whereas those toward the front of A1 respond best to higher frequencies, such as that of a train whistle. Touch – touch information from the thalamus is projected to the primary somatosensory cortex (S1), which is in the parietal lobe. Neighboring body parts tend to be represented next to each other in S1, so that the body is effectively mapped out there according to physical proximity. More sensitive body parts have relatively larger amounts of cortical tissue dedicated to them. Vision – the primary visual cortex (V1) is in the occipital lobe. The neural pathway from the retina to the occipital lobe preserves spatial relationships, so that adjacent areas of the retina correspond to adjacent areas in V1. Some neurons in the V1 respond more to lines of particular orientations than to any other type of stimulus, including lines tilted at a different angle. For example, some neurons increase their firing rate when a vertical line segment is presented in their receptive field. The firing rate of these cells, termed simple cells by Hubel and Wiesel, decreases as the orientation of the line segment is rotated away from the preferred orientation. Neurons in different parts of the brain tend to have different types of recep
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