Sensation and Perception Prof David Irwin Lecture for Psych 103

Sensation and Sensation and PerceptionPerception

Prof David IrwinProf David Irwin

Lecture for Psych 103Lecture for Psych 103

IntroductionIntroduction

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

DefinitionDefinition

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

Perception is a biological Perception is a biological processprocess

• Sensory nerve fibers provide our Sensory nerve fibers provide our only link to the outside worldonly link to the outside world

• Our perceptual worlds are Our perceptual worlds are different from our physical different from our physical worlds because certain worlds because certain environmental events cannot be environmental events cannot be detected by our sensory detected by our sensory channelschannels

Perception is a Perception is a psychological processpsychological process

Perception uses symbolic Perception uses symbolic representations:representations:• Environmental stimuli trigger neural Environmental stimuli trigger neural

activityactivity

• Neural patterns are symbols for Neural patterns are symbols for

environmental events that occur in the environmental events that occur in the

physical worldphysical world• Perception is the interpretation of neural Perception is the interpretation of neural

symbolssymbols

Psychological, Biological, and Psychological, Biological, and Theoretical Approaches to Theoretical Approaches to

PerceptionPerception

Psychological Approaches (behavior)Psychological Approaches (behavior) Biological Approaches (anatomy and Biological Approaches (anatomy and

physiology)physiology) Theoretical Approaches Theoretical Approaches

(hypothetical causes and (hypothetical causes and mechanisms)mechanisms)

Example: Spectral Sensitivity Example: Spectral Sensitivity and Photoreceptor Pigmentsand Photoreceptor Pigments

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

Spectral SensitivitySpectral Sensitivity

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

Spectral SensitivitySpectral Sensitivity

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

Different spectral sensitivity Different spectral sensitivity functions are found for peripheral functions are found for peripheral and central visionand central vision

Photopigments and Photopigments and phototransductionphototransduction

Light is first registered by light-Light is first registered by light-sensitive molecules in the sensitive molecules in the photoreceptorsphotoreceptors

Photopigments and Photopigments and phototransductionphototransduction

Light is first registered by light-sensitive Light is first registered by light-sensitive molecules in the photoreceptorsmolecules in the photoreceptors

When light strikes a photopigment When light strikes a photopigment molecule, the molecule isomerizes molecule, the molecule isomerizes (changes shape)(changes shape)

When the photopigment isomerizes, the When the photopigment isomerizes, the electrical current in the photoreceptor is electrical current in the photoreceptor is disrupted; this signals the presence of disrupted; this signals the presence of lightlight

This process is called phototransductionThis process is called phototransduction

Fovea contains only cones, periphery contains mostly rods

Physiology of Spectral Physiology of Spectral SensitivitySensitivity

The photopigments in the rods and The photopigments in the rods and cones have different pigment cones have different pigment absorption spectra absorption spectra

An absorption spectrum is a plot of An absorption spectrum is a plot of the amount of light absorbed by a the amount of light absorbed by a pigment vs. the wavelength of the pigment vs. the wavelength of the lightlight

Pigment Absorption Pigment Absorption MeasurementMeasurement

Light sent in(vary wavelengthacross spectrum)

Light passed out; difference betweenintensity in and intensityout is amount absorbed


Wald and Brown (1958) chemically Wald and Brown (1958) chemically extracted photopigment from extracted photopigment from donated eyes and measured its donated eyes and measured its absorption spectrum.absorption spectrum.

This was primarily rod photopigment This was primarily rod photopigment because there are many more rods because there are many more rods than cones and each rod has more than cones and each rod has more photopigment than each conephotopigment than each cone


They found that the pigment They found that the pigment absorption spectrum perfectly absorption spectrum perfectly matched the rod spectral sensitivity matched the rod spectral sensitivity functionfunction


Later they used Later they used microspectrophotometrymicrospectrophotometry to measure the absorption spectra of the to measure the absorption spectra of the conescones

In this procedure one shines a beam of In this procedure one shines a beam of light through an individual receptor to light through an individual receptor to measure the pigment absorption measure the pigment absorption spectrumspectrum

Three kinds of pigments were found Three kinds of pigments were found (with peaks at 435 nm, 545 nm, and 580 (with peaks at 435 nm, 545 nm, and 580 nm), corresponding to the three kinds of nm), corresponding to the three kinds of conescones


So, the general cone spectral So, the general cone spectral sensitivity function reflects a sensitivity function reflects a mixture of these three cone typesmixture of these three cone types


Color blindness is usually caused by Color blindness is usually caused by a missing cone pigmenta missing cone pigment

There are individual differences in There are individual differences in pigments even when none are pigments even when none are missingmissing

Dartnall and Bowmaker found that Dartnall and Bowmaker found that individual differences in spectral individual differences in spectral sensitivity functions are sensitivity functions are accompanied by the same accompanied by the same differences in pigment compositiondifferences in pigment composition

Sensation and Sensation and PerceptionPerception

Seeks to define sensory properties of Seeks to define sensory properties of organismsorganisms

Attempts to determine relationship between Attempts to determine relationship between physical variables and perceptual experiencephysical variables and perceptual experience

Attempts to explain perceptual experience in Attempts to explain perceptual experience in terms of underlying physiological mechanisms terms of underlying physiological mechanisms

Knowledge gained is important from a basic Knowledge gained is important from a basic science standpoint, and also has practical science standpoint, and also has practical value in designing and evaluating devices that value in designing and evaluating devices that we use in the world we use in the world

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Sensation and Perception Prof David Irwin Lecture for Psych 103