Sensation and Perception Prof David Irwin Lecture for Psych 103

Introduction The environment is loaded with stimulation, but we are constructed in such a way that we are sensitive to only some of this stimulation

Definition Sensation and perception involves the study of how information is extracted and interpreted from the environment

Perception is a biological process Sensory nerve fibers provide our only link to the outside world Sensory nerve fibers provide our only link to the outside world Our perceptual worlds are different from our physical worlds because certain environmental events cannot be detected by our sensory channels Our perceptual worlds are different from our physical worlds because certain environmental events cannot be detected by our sensory channels

Perception is a psychological process Perception uses symbolic representations: Environmental stimuli trigger neural activity Environmental stimuli trigger neural activity Neural patterns are symbols for environmental events that occur in the physical world Neural patterns are symbols for environmental events that occur in the physical world Perception is the interpretation of neural symbols Perception is the interpretation of neural symbols

Psychological, Biological, and Theoretical Approaches to Perception Psychological Approaches (behavior) Psychological Approaches (behavior) Biological Approaches (anatomy and physiology) Biological Approaches (anatomy and physiology) Theoretical Approaches (hypothetical causes and mechanisms) Theoretical Approaches (hypothetical causes and mechanisms)

Example: Spectral Sensitivity and Photoreceptor Pigments Humans are differentially sensitive to different wavelengths of light; this is due to the fact that the chemicals (photopigments) inside our photoreceptors (rods and cones) absorb some wavelengths better than others Humans are differentially sensitive to different wavelengths of light; this is due to the fact that the chemicals (photopigments) inside our photoreceptors (rods and cones) absorb some wavelengths better than others

Spectral Sensitivity Absolute threshold (the dimmest light we can just barely detect) varies with wavelength of light Absolute threshold (the dimmest light we can just barely detect) varies with wavelength of light

Spectral Sensitivity Absolute threshold (the dimmest light we can just barely detect) varies with wavelength of light Absolute threshold (the dimmest light we can just barely detect) varies with wavelength of light Different spectral sensitivity functions are found for peripheral and central vision Different spectral sensitivity functions are found for peripheral and central vision

Photopigments and phototransduction Light is first registered by light-sensitive molecules in the photoreceptors Light is first registered by light-sensitive molecules in the photoreceptors

Photopigments and phototransduction Light is first registered by light-sensitive molecules in the photoreceptors Light is first registered by light-sensitive molecules in the photoreceptors When light strikes a photopigment molecule, the molecule isomerizes (changes shape) When light strikes a photopigment molecule, the molecule isomerizes (changes shape) When the photopigment isomerizes, the electrical current in the photoreceptor is disrupted; this signals the presence of light When the photopigment isomerizes, the electrical current in the photoreceptor is disrupted; this signals the presence of light This process is called phototransduction This process is called phototransduction

Fovea contains only cones, periphery contains mostly rods

Physiology of Spectral Sensitivity The photopigments in the rods and cones have different pigment absorption spectra The photopigments in the rods and cones have different pigment absorption spectra An absorption spectrum is a plot of the amount of light absorbed by a pigment vs. the wavelength of the light An absorption spectrum is a plot of the amount of light absorbed by a pigment vs. the wavelength of the light

Pigment Absorption Measurement Light sent in (vary wavelength across spectrum) Light passed out; difference between intensity in and intensity out is amount absorbed

Physiology of Spectral Sensitivity Wald and Brown (1958) chemically extracted photopigment from donated eyes and measured its absorption spectrum. Wald and Brown (1958) chemically extracted photopigment from donated eyes and measured its absorption spectrum. This was primarily rod photopigment because there are many more rods than cones and each rod has more photopigment than each cone This was primarily rod photopigment because there are many more rods than cones and each rod has more photopigment than each cone

Physiology of Spectral Sensitivity They found that the pigment absorption spectrum perfectly matched the rod spectral sensitivity function They found that the pigment absorption spectrum perfectly matched the rod spectral sensitivity function

Physiology of Spectral Sensitivity Later they used microspectrophotometry to measure the absorption spectra of the cones Later they used microspectrophotometry to measure the absorption spectra of the cones In this procedure one shines a beam of light through an individual receptor to measure the pigment absorption spectrum In this procedure one shines a beam of light through an individual receptor to measure the pigment absorption spectrum Three kinds of pigments were found (with peaks at 435 nm, 545 nm, and 580 nm), corresponding to the three kinds of cones Three kinds of pigments were found (with peaks at 435 nm, 545 nm, and 580 nm), corresponding to the three kinds of cones

Physiology of Spectral Sensitivity So, the general cone spectral sensitivity function reflects a mixture of these three cone types So, the general cone spectral sensitivity function reflects a mixture of these three cone types

Physiology of Spectral Sensitivity Color blindness is usually caused by a missing cone pigment Color blindness is usually caused by a missing cone pigment There are individual differences in pigments even when none are missing There are individual differences in pigments even when none are missing Dartnall and Bowmaker found that individual differences in spectral sensitivity functions are accompanied by the same differences in pigment composition Dartnall and Bowmaker found that individual differences in spectral sensitivity functions are accompanied by the same differences in pigment composition

Sensation and Perception Seeks to define sensory properties of organisms Attempts to determine relationship between physical variables and perceptual experience Attempts to explain perceptual experience in terms of underlying physiological mechanisms Knowledge gained is important from a basic science standpoint, and also has practical value in designing and evaluating devices that we use in the world