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George Mather's Lecture Notes PERCEPTION TOPIC NOTES: Introduction to the Senses 1. Classification There are five major groups of senses: Sense Stimulus Receptor Sensory Structure Vision Electromagnetic energy (light) photoreceptors Eye Hearing Air pressure waves mechanoreceptors Ear Touch Tissue distortion mechanoreceptors Skin, muscles, etc. thermoreceptors Balance Gravity, acceleration mechanoreceptors vestibular organs Taste/smell Chemical composition chemoreceptors nose, mouth Certain basic physical and perceptual principles apply to all the senses. 2. General physical principles 1) Transduction All senses need to convert ('transduce') environmental energy into electrical energy. Receptors in each sense are specialised for transducing particular forms of energy into electrical signals for transmission to the brain. 2) Sensory pathways Electrical signals from all the senses are transmitted ultimately to the cerebral cortex via relays in the thalamus. All senses involve both cortical and sub-cortical processes, but differ in the area of cortex involved, and the involvement of sub-cortical structures. Cortical analysis is thought to be linked with conscious experience, and sub-cortical analysis is thought to be involved in unconscious events eg. reflexes. 3) Selectivity Individual neurons in each sensory system are very selective in their response to a sensory stimulus; each has a RECEPTIVE FIELD, a restricted area of the sensory space within which the stimulus must fall for the cell to respond. Different cells have receptive fields in different parts of the space (eg. spatial position on the body, position in the visual or auditory field, colour, sound frequency). 4) Sensory maps Cells with similar receptive fields tend to be found near to each other in the sense organ or cortex, so that a map of the sensory surface is formed on the surface of the organ or cortex. These maps tend to exhibit distortions favouring certain areas of the visual field, or parts of the body, or certain sound frequencies. Sensory coding requires interactions between cells with similar properties, so proximity simplifies wiring. 5) Specific nerve energy All sense organs generate the same kind of electrical signals. How can they evoke different sensations? Differences between senses are not reflected in the sensory signals, but in their destination in the brain. During neurosurgery, small electrical signals are often applied directly to the cortex of an awake (anaesthetised) patient. This stimulation evokes sensations associated with the particular sense organ connected to that part of the cortex. 3. General perceptual principles 1) Thresholds All sense organs require a certain minimum amount of stimulation ('absolute threshold') before they evoke a perceptual sensation. Note that detection threshold is not the same as threshold for discrimination or identification. We can also measure the 'difference threshold' - minimum difference between two stimuli that can be detected (look up Weber's Law). 2) Sensory magnitude Above absolute threshold, there is a range of stimulus intensity levels which generates corresponding variations in sensory magnitude, providing valuable information (eg. proximity, force). The variation in sensory magnitude with stimulus intensity is not linear (look up Stevens' Power Law). 3) Adaptation Sensitivity must be matched to the physical stimulation available. Extreme sensitivity would be undesirable (eg. itching, deafening). Lack of sensitivity is obviously also undesirable. All modalities have the ability to shift their operating range ie. shift the range of physical intensity levels which generate a response. This allows us to adapt to prevailing environmental conditions and remain sensitive to change (light and noise level, skin stimulation). Within modalities, many subtle adaptation effects can also be generated, which tell us a lot about how the system operates. 4. Coverage of the senses in this course Vision will receive the most coverage, followed by hearing. The other senses will be covered in much less detail. Why? Because we know most about vision, and because it is the most important sense in several respects: 1) It has the greatest range of operation. 2) Cortical areas devoted entirely to vision cover about 60% of the total cortical surface (in primates). 3) Experiments demonstrate that when visual information is set into conflict with other senses, vision dominates eg. McGurk effect, ventriloquism, Lee and Aronson (1974), Rock and Harris (1967). PERCEPTION TOPIC NOTES: Touch and balance 1. Touch 1) Physiology Our sense of touch is based on a set of receptors which respond to deformation of tissue. There are four basic types: - Meissner corpuscles (most superficial, punctate) - Merkel discs - Ruffini endings - Pacinian corpuscles (deepest, diffuse) The type of the receptor affects the adaptive properties of its nerve fibre (corpuscles tend to give transient responses), and its location affects the fibre's spatial properties (punctate vs. diffuse). In addition, there is a network of free nerve endings (with no end receptor) below the skin surface and wrapped around the base of hair follicles, which respond to slight bending of the hair. The afferent fibres from receptors enter the spinal cord. Some connect with motor neurons which travel back to muscles in the body region where the afferent fibres originated. This in-out circuit mediates reflex reactions. Signals from other fibres are relayed to the brain (lemniscal pathway), where they arrive eventually in the somatosensory cortex. The cortical surface is arranged in an orderly way, with some body areas covering more cortical surface than others (an property known as cortical magnification). 2) Perception Quantitative aspects Our sense of touch differs markedly in its sensitivity and in its acuity in different body regions. Sensitivity is highest at the lips and fingertips, and lowest in the back and stomach. Similarly, two-point acuity is only 2mm on the finger, 30mm on the forearm, and 70mm on the back. This variation in response corresponds with the variation in cortical area devoted to different body parts. Qualitative aspects Our sense of touch varies qualitatively - we can perceive roughness, pressure, warmth, and pain. Early theories proposed that different receptors were associated with different sensory qualities, but experiments have not supported them (some investigators mapped sensory spots on their skin and then dissected it to find what receptors were underneath). A single event will generate responses in several receptor types simultaneously. Our perception of the event arises from a combination of all responses. 2. Balance The vestibular sense provides information about the head's attitude relative to gravitational vertical, which is used to maintain posture, stabilise eye position, and compensate for image motion introduced by head movement. Unlike other senses, the information generated by the vestibular sense does not usually impinge on consciousness unless unusual or violent body movements are involved (eg. abrupt halt after spinning). Individuals who suffer loss of vestibular function (due to toxic drugs or disease) are not aware of an obvious deficit, but they have difficulty in maintaining balance on uneven or compliant surfaces, and experience visual disturbance (movement of the visual field) during irregular head movement (eg. travelling over uneven surfaces). 1) Physiology The vestibular apparatus (about the size of a pea) contains three fluid-filled curved tubes (semicircular ducts) at right-angles to each other. Each duct contains a sheet of sensory hair cells (cilia) stretched across it to form a diaphragm. The cilia are connected to sensory cells which respond to deflection of the cilia. The three ducts are arranged so that angular acceleration of the head in any plane will deflect at least one set of cilia, and so generate a sensory response. The vestibular afferents terminate primarily in the vestibular nucleus, which also receives information from joint receptors and from the eyes. Complex interconnections are made with other sub-cortical brain structures, such as the cerebellum, and a small area of the cortex receives projections from the vestibular nucleus (via the thalamus as usual). The complex sub-cortical processing and relatively small cortical projection probably explains why vestibular signals have little conscious representation. However, we do have some conscious awareness of body movement, and direct brain stimulation in the appropriate area evokes sensations of body rotation and displacement. 2) Perception It is very difficult to study the vestibular sense in isolation from other senses, because there are many non-vestibular cues to orientation which must be excluded. Apart from vision, pressure receptors in supporting tissue, joint position receptors, and muscle feedback all give information about forces acting on the body. Elaborate precautions are needed, such as total immersion in water, so instead most studies examine the interactions between vestibular and non-vestibular sensations. The clearest example of this interaction is motion sickness, characterised by nausea and vomiting, which is thought to arise from a mismatch of sensory cues from vestibular and non-vestibular senses. For example, the movement of ships or planes can generate vestibular signals which are not accompanied by visual information about the movement. The problem is worsened by the tendency for the changes in direction and magnitude of acceleration to be abnormally
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