Lecture 2c_ Sensation .pdf

what is the role of sensation in perception? sensations are the response of receptors to energy patterns in the world sensory receptors are cells that produce a neural response when they are stimulated regardless of how a receptor is stimulated,...

what is the role of sensation in perception? sensations are the response of receptors to energy patterns in the world sensory receptors are cells that produce a neural response when they are stimulated regardless of how a receptor is stimulated, it produces the same neural response responses are specific to neurons, not stimulation sensations are physiological responses that measure pieces of the world sensations are not perception, but they form the building blocks of perception how does physical energy map onto psychological experiences? (psychophysics) to understand perception, we first need to understand what the raw sensory input is what are receptors and how do they measure the world? the pupil of the eye can dilate (expand) to allow more light into the eye this can also occur for a number of other reasons (e.g., emotional response) pupil dilation is a relatively small and very fast contribution to overall light adaptation when current passes through a wire, it glows more brightly the more current is passed through it, allowing us a quantitative measurement of light intensity Aubert (1865) measured how much current it took to perceive the wire every minute after exposure to bright light what are receptors and how do they measure the world? the resulting dark adaptation curve intensity to perceive intensity to perceive has been replicated in many ways 1 1 typically shows two distinct pieces, suggesting two distinct processes 2 2 initially rapid decrease (adaptation) followed by a slowing, then another rapid decrease after 5-8 minutes time (min) time (min) these two curves are produced by the presence of two different types of receptors the back of the eye (retina) is an array of many closely packed photoreceptors periphery retina fovea optic nerve what are receptors and how do they measure the world? what are receptors and how do they measure the world? in the human retina, there are two distinct types of photoreceptors: rod cells (~100m, mostly in periphery) cone cells (~6-7m, mostly in fovea) (three types, each responding to a different color of light) both respond to light, but the responses of many adjacent rod cells are combined in a single output neuron, while the response of each cone cell causes activity in a different output neuron given this pattern of connectivity, which type of cell is likely responsible for each part of the dark adaptation curve? I t what are receptors and how do they measure the world? consider the case where there are two small, weak sources of light positioned so that would only activate a single receptor, for both rod and cone cells in rod cells, weak signals are combined, meaning ✓ less light is required for cones activation (i.e., detection) I rods in cone cells, signals remain ✖ separate, so weak signals will t ✖ not activate the connected neurons (not detected) what are receptors and how do they measure the world? consider the case where there are stronger sources of light with small differences in rod cells, patterns are ✓ detected, but any light on ✓ them produces the same neural response ✓ in cone cells, small ✓ ✓ ✓ ✖ differences in patterns and ✖ ✓ structure can be detected ✖ ✓ because signals are separate ✖ what we perceive depends on whether it is detected by rods or cones what are receptors and how do they measure the world? psychophysics is the quantitative exploration of the effects of stimuli on a perceiver the absolute threshold is the minimum energy required to detect a stimuli on which step do you see a grey square? 1 2 3 4 5 6 7 8 a difference threshold is the minimum change required to distinguish two stimuli (just noticeable difference, or JND) these thresholds are different for different stimuli as well as different participants what can psychophysics explain about stimuli and sensations? Weber (1834) observed that the JND changed systematically with stimulus intensity “In observing the disparity between things that are compared, we perceive not the difference between the things, but the ratio of this difference to the magnitude of things compared.” 30g 30g 1g JND 60g 60g 2g JND doubling the intensity or magnitude of the stimulus doubles the JND Weber’s Law ΔI the ratio of a change in intensity (ΔI) to the initial intensity (I) is a =k I constant (k) (with individual stimuli types having different constants) the relationship between a physical stimulus in the world and our perception of that stimulus is systematic, but it is not a perfect, one-to-one relationship what can psychophysics explain about stimuli and sensations? Weber’s Law example Weber fractions: higher k values (less sensitive).1 for light intensity ΔI ΔI.1 for sound loudness =k.05 for saltiness I lower k values.001 for line length I (more sensitive).0003 for sound pitch breaks down at the limits for very intense or very weak stimuli, but holds over a broad range of stimuli and sets the stage for quantifying sensation and perception Weber fractions have meaning beyond measures of sensitivity to stimuli quantification of JNDs also allows us to measure mental states energy (intensity) response (sensation) the relation of intensity to sensation gives us insight into physiology and mind what can psychophysics explain about stimuli and sensations? Fechner’s Law (1860) can be derived from Weber’s Law, and directly relates the intensity of a stimulus to the sensation it causes increasing intensity in a multiplicative, geometric way (I2 = C × I1) increases sensation in an I7 additive, arithmetic way (S2 = S1 + C) intensity increases in a multiplicative way 1, 2, 4, 8, 16, 32… intensity sensation increases in an additive way 1, 3, 5, 7, 9, 11… I6 sensation and intensity are I5 related logarithmically S = k log I I4 I Stevens (1961) used a large number of magnitude estimation I2 I3 1 experiments to derive a power-law relationship similar to this S1 S2 S3 S4 S5 S6 S7 sensation psychophysics gives us a quantitative relationship between stimuli and sensations, a psychological snapshot of the world what are the other receptors and how do they measure the world? we find a similar receptor configuration in the auditory system in the ear, on the other side of the eardrum, is the cochlea, a fluid-filled, multi-layered, spiral-shaped organ sound waves are transmitted to the fluid by the eardrum, where they cause vibrations in the basilar membrane (tough, flexible membrane rolled up in the cochlea) vibrates differently depending on the frequency of the incoming sounds, with lower frequencies vibrating the base more, and higher frequencies vibrating the tip more more flexibility the actual receptors are hair cells sitting on the membrane what are the other receptors and how do they measure the world? because the basilar membrane differs in thickness along its length, it responds to vibrations in different ways higher frequency sounds vibrate the thinner part more, lower frequency sounds vibrate the thicker part more so based on which hair cells are producing a stronger response, receptors can distinguish different frequencies down the center of the basilar membrane are ~3500 inner hair cells in a single row, with about ~90-95% of the outgoing neural connections on the edges, ~50k outer hair cells in three rows, with the remaining neural connections what does this structure suggest about their roles in responding to auditory stimuli? what are the other receptors and how do they measure the world? Meissner free nerve corpuscles (RA1) endings (texture, slow (pain) vibrations) Ruffini Merkel discs (SA1) endings (SA2) (sustained pressure) (stretching) Pacinian corpuscles types 1 (small) and 2 (large) refer to the (RA2) size of the receptive field, the region of (deep pressure, skin that that causes a response fast vibrations) some are slowly adapting others are rapidly (SA, respond as long as a adapting (RA, respond stimulus is present) to onset and offset) what are the other receptors and how do they measure the world? resolution of touch receptors varies (sensory) (motor) considerably over the body significantly more neurons dedicated to processing the face, lips, tongue and hands than the legs or torso on both the sensory and motor cortices touch receptors are also responsible for detecting pain and temperature, largely handled by nociceptors temperature and pain are much more complex and much less understood than other areas of touch part of this stems from the relative nature of temperature and pain, as well as differences in how nociceptors are distributed through the skin what are the other receptors and how do they measure the world? what are the other receptors and how do they measure the world? the chemical senses are defined by a central issue: the diversity and complexity of the stimuli they are intended to detect both light and sound are relatively simple combinations of wave patterns chemical senses are capable of detecting a huge variety of complex molecules chemical stimuli can combine into even more complex stimuli + ? as one of the oldest senses in evolutionary terms, chemical senses also connect to older neural and behavioral systems involved in survival, sustenance, and nutrition chemical senses also seem to be much more subjective than other sensory systems further complicating the chemical senses are interactions with other senses, e.g., taste can be affected by visual, textural, temperature, olfactory and other sensory information what are the other receptors and how do they measure the world? cognitive psychology has largely focused on vision because the other senses are much more complicated and vision is our primary source of information about the world when we see something, an array of ~130 million receptors transforms a pattern of light energy into neural responses we end up with a retinal image that doesn’t match the world inverted made up of different in vertically and individual different parts horizontally points of the retina how do sensations at receptors relate to our experience of the world? receptor responses are… discrete and pointillistic (each individual receptor produces a single response) momentary and transient (on the time scale of neural activity, milliseconds) dependent on physiology and by themselves, meaningless our experience of the world is… wholistic, extended (the smallest change we can perceive involves many receptors) continuous (on the scale of behavior, seconds to minutes or more) dependent on the world and inherently meaningful mismatch between receptor responses and experience must be fixed the mind transforms meaningless* sensations into meaningful perceptual experiences indirect perception requires us to reconstruct the world from our sensations