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PSYC 100
Ingrid Johnsrude

SENSATION AND PERCEPTION Sensation is process by which our sensory systems gather information about the environment; turns chemical, mechanical, sound and light energy into electrical signals in the brain (through the process of transduction where the physical or chemical stimulation is converted into nerve impulse that is relayed to the brain) Perception: is the selection, organization and interpretations of sensations; extracts meaning from what you sense, tying it to know and understand Sensing and perceiving begin with the detection of a stimulus by one of our senses. receptors convert the stimulus into a neural impulse, a process called transduction. our perception of the stimulus takes pale in higher, specialized regions of the brain 5 Senses - Vision, audition (hearing) - gustation (taste), olfaction (smell) are chemical senses - touch is broken down into the somatosenses, which include the skin senses of touch, temperature and pain, kinaesthetic senses (how you know the position of your limbs, trunk and head in space) Sensory Adaption • sensory adaption is the reduction of activity in sensory receptors with repeated exposure to a stimulus • neurons do fatigue and stop sending signals across synapses • sensory adaptation is a change, usually a decrease, in sensitivity that occurs when a sensory system is repeatedly stimulated in exactly the same way • all sensory systems have specialized receptor cells that detect the energy and conveys signals to the brain about the presence of environmental stimuli • if a neuron isn't fatigued, then the rate of firing indicates the intensity of a stimulus • if a neuron becomes fatigued as a result of adaption, its rate of signalling may fall below the spontaneous rate, or minimum detectable level and your body no longer notices the stimuli Cranial Nerves • information from the senses travel along cranial nerves (body has twelve pairs of nerve fibres that travel into and out of the skull and carry all sensory information from parts of the body below the neck to the brain Light • the adequate stimulus for vision is light • light is a form of electromagnetic energy ◦ the brightness we see depends on the intensity of the light ◦ wave lengths are characterized by wavelength and amplitude Anatomy of the Eye The Retina • rods and cones contain photopigments • rods are sensitive to dim light (contains three types of photopigments) and cones are sensitive to bright light (contains one photopigment) • most of the light is focused onto the fovea ◦ highly concentrated with cones, no rods ◦ outside the fovea is where we see fine detail which means that every point has a receptor cell with a dedicated ganglion cell • colour vision is the result of the activity of three different colour (red, blue, green) receptors in the retina • people with defects in colour vision are generally missing some or all of one or more types of cone • dark adaption is the process by which the rods and cones become increasingly sensitive to light under low levels of illumination • nearsightedness occurs when the eye is slightly elongated, causing the image hat the cornea and lens focus on to fall short of the retina • if the length of the eye is shorter than the normal from front to back, the result is farsightedness Eye Movements • eye movements are necessary because of the design of the human eye ◦ the resolution of our sight varies by orders of magnitude across the visual field Sound • sound results from the vibration of air molecules • composed of three characteristics: frequency (pitch), amplitude (loudness) and complexity (timbre) Anatomy of the Ear Outer Ear • pinna (channels the sound waves), ear canal (which conducts sound waves to the eardrum) and ear drum (membrane that vibrates in response to sound waves) Inner Ear • sound waves vibrate the ear drum which causes vibration of three ossicles (smallest bones in body) • ossicles act as an amplification system - they push all the energy of the in-out vibration of the eardrum into an in-out vibration of the small oval window of the inner ear • loud sounds can damage the delicate cilia of the inner ear • sound localization is the process of identifying where sound comes from Habituation and Adaption • gustatory and olfactory system show a loss of sensitivity due to sensory adaption and habituation ◦ habituation is a diminished response to a stimulus, as a result of leaning that is meaningless or irrelevant ◦ adaption is a change is a behaviour to a repeatedly presented stimulus that results from fatigue Touch • skin conveys three classes of sensations: touch, temperature and pain ◦ each sensation class has three specialized cells (receptors) that detect the intensity of different types of sensation • one way to test sensitivity is the two-point threshold test ◦ regions with high acuity, such as the fingertips, can detect the two separate but closely spaced pressure points of the device, whereas less sensitive regions such as lower back will perceive the same stimuli as only one pressure point • pain is perceived by the brain ◦ Nociception is the activity of nerve pathways that respond to uncomfortable stimulation ◦ our skin, teeth and internal organs contain nerve endings called nociceptors which are receptors that initiate pain messages that travel to the central nervous system Just Noticeable Difference • skin surface is projected to the primary somatosensory cortex, which has map of the body called the homunculus Each body part is represented next to adjacent areas - somatotopic representation Body parts with highest sensitivity to touch (face and hands) receive more cortical representation that those with less sensitivity (trunk and legs) • we sense temperature through thermoreceptors in the skin made of free nerve endings - they help regulate our body temperature to keep homeostasis • sensory receptors for pain are also free nerve endings in the body ◦ peripheral receptors react to extreme pressure, extreme temperatures and to tissue damage by signalling pain Taste and Smell Taste • the gustatory system functions in the sensation and perception of taste • receptors involved in taste sensations are chemical compounds that are water soluble • sensory neurons that transmit signals from the taste buds respond to different types of stimuli, but respond best to particular taste ◦ receptors for taste are the small bumps on your tongue ◦ the bundles of nerves that register taste at the taste buds send the signal through the thalamus Smell • the olfactory system is involved in smell - the detection of airborne particles with specialized receptors located in the nose • the sensation of smell begins with nasal air flow bringing in molecules that bind with receptors at the top of the nasal cavity ◦ receptors are called cilia (tiny hair projections) Perception Week 9 Objectives • compare and contrast sensation and perception • summarize the way that the brain perceive sound • analyze the structuralist, Gestalt, and constructivist concepts of perception Differences and Thresholds • a difference threshold is the just-noticeable difference between two stimuli - the minimum change required in the intensity of something in order to detec t that is now stronger or weaker than it was • Weber's Law: the size of the just-noticeable difference of a stimulus divided by its initial intensity is a constant • Fechner's Law: in every sensory domain, each just-noticeable difference represents an equal step in the psychological magnitude of a sensation. this means that changes in stimulus can be compared across sensory domains ex: between vision and touch, although each different domain could have a different JND • Stevens Power Law proposed relationship between the magnitude of a stimulus and its perceived intensity or strength • absolute threshold is the minimum intensity of stimulation that must occur before we can experience a sensation - ex: the faintest sound a person can hear, the gentlest touch a person can feel or the weakest odour someone can smell Detecting Faint Signals • ones ability to detect absolute stimulus does not just depend on ones sensitivity, but also depends on their expectations (which is a cognitive factor) Bias: • response bias is a persons tendency to say yes or no when he is not sure whether he detected the stimulus Noise: • external noise is the noise coming from outside a persons body • internal noise is the spontaneous random firing of neurons; same as neural noise Signal Detection Theory • these two factors - response bias and the presence of noise - led some psychophysicists to formulate signal detection theory (SDT) and to develop a new way to describe observer sensory sensitivity ◦ proposed that there is no such thing as absolute threshold because what is measured as the threshold changes depends no the persons ability to detect the stimulus against a background of continually changing noise and the persons willingness to guess ◦ the big contribution of signal detection theory is that it allows independent assessment of sensitivity and bias ◦ bias is shown in receiver-operating characteristic plots, which illustrate how different sensitivities and biases will change performance Applied Signal and Noise • pitch corresponds to the physical characteristic of frequency of sound • how pitch is coded: ◦ the cochlea analyzes complex sounds, breaks them down into their component frequencies. the presence of each component and its relative intensity are relayed to the brain through the individual fibres of the auditory nerve. each auditory nerve fibre is most sensitive to a particular frequency tonotopic organization is the separation of frequencies in the air every time there is a compression in the sound wave, the hair cell depolarizes, and an action potential is generated ◦ brain codes pitch by place code or temporal code • volley principle: an auditory nerve as a whole produces volleys of impulses for sounds up to about 5000 per second, even though no individual axon can fire that fast • timbre perception is a kind of pattern recognition • sound intensity (loudness) is coded by the degree to which each auditory nervee fibre fires on every cycle of a stimulating waveform ◦ if a sound is intense, then each nerve fibre attached to the responsive place on the basilar membrane will fire as much as it can ◦ psychophysical studies have measured the sensory relationship between frequency and the perceived loudness of a sound • your eyes record spatial location because every position on the retina corresponds to a position in
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