lecture (sensation & perception)
DESCRIPTION
A presentation I gave as a guest lecturer to S&P students enrolled at the University of KansasTRANSCRIPT
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Introduction to Perception
Sensation & Perception
Guest Lecturer: Michael Engles, Ph.D.
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Sensation, perception, cognition
1. Sensation: Physical stimulation
2. Perception: Interpretation/meaningfulness
3. Cognition: “Use” of perception
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Historical overview
1. Greek philosophers: eye receives
“copies” of objects
2. Empiricists – we learn to perceivea. Berkeley: How do we perceive depth with 2D
receptors???
b. James: “blooming, buzzing confusion”
3. Gestaltists – we are tuned to perceive
organization
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Historical overview
4. Ecological approacha. Gibson: active observer seeks out information from a rich
visual world – we perceive environment directly
5. Information-processing approach – processing
stages
6. Computational approach – perception as
computationa. Develop mathematical models of how information can be
processed
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The perceptual process
Stages
1. Distal stimulus: our environment
2. Proximal stimulus: information which stimulates the receptors (i.e. image light striking the retina)
3. Transduction: transforming proximal stimulus into another form of energy (i.e. our brains process electrochemical notlight energy)
4. Neural processing: computations
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The perceptual process
5. Perception: organization of computations
6. Recognition: meaningfulness of perception
7. Action: response to meaning
Another view
Direct Perception: we act to create lawful
changes in our environment which we
perceive which tell us about the structure
of our environment
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Studying the perceptual process
Overview of relationships
1. Stimulus-perceptual: change the physical stimulation and ‘ask’ how perception changes (behavioral)
2. Stimulus-physiological: measure brain/neuron activity
in response to stimulation
3. Physiological-perceptual: measure how physiological
capabilities correlate with perception
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Studying the perceptual process
Behavioral approach
1. Phenomenology: What do you see?
2. Psychophysical approach: Thresholds
a. Measuring the absolute threshold: minimum
detectable energy
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The physiological approach
Doctrine of specific nerve energies (Mueller)
Perception results from specific nerve stimulation
2. Neuroanatomy
Nerves
cell body, dendrites, axons, synapseSynapse
presynaptic and postsynaptic, receptor sites, neurotransmitters, excitation and inhibition
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Psychophysics
• Elemente der Psychophysik (1860)– Authored by Gustav Theodore
Fechner
– Marked the transition from philosophy to science for psychology by introducing objectivemeasurement techniques
• Modern scientists still appeal to it’s basic principles for the purpose of understanding the nature of perception
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What is Psychophysics?
G. T. Fechner
“Psychophysics should be understood here as an exact theory of the functionally dependent relations of body and soul or, more generally, of the material and the mental, of the physical and the psychological worlds.”
From: Elemente der Psychophysik (1860);Translated from the original German
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What is Psychophysics?
– Measurement of a ‘psychological’ response to a ‘physical’ stimulus
– Stimulus
• Something that when applied to a an object of interest results in a measureable response or change in that object
• Anything that affects or influences a system
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The Value of Psychophysics
• Examination of the eye and the visual system often relies on psychophysical testing– The patient behaviorally indicates alterations in their
experience when some dimension of the stimulus is changed
• The clinical inspection of the eye (e.g., retina or lens) is also “psychophysical”– You are trying to identify subtle differences/changes in
optical clarity, retinal color, etc.
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Clinical Application
• Psychophysical evaluation of visual function among normal, healthy adults provides baseline data
– Significant deviations from these established norms may indicate the presence of a pathology
– The effectiveness of a treatment can be evaluated/tracked by re-measuring thresholds
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Psychophysics
• Descriptive– Provides a specification of sensory capability
– E.g., Threshold changes across wavelength
• Analytical– Testing of hypotheses about the nature of
biological mechanisms underlying sensory experience
– E.g., Neural activity
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Descriptive Psychophysics
• Wald (1945)
– Described the spectral sensitivity of cones and rods in the human eye
– One can see how rods are more sensitive than cones
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Analytical Psychophysics
• Principle of Nomination– Declares that identical neural events give rise to
identical psychological events
– When stimulus A and stimulus B produce the same sensory experience, they produce the same neural response
– Provides a valuable link between psychophysics and biology
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Analytical Psychophysics
• Criterion Response Technique– The investigator seeks to
discover, not how the response changes as the stimulus is varied, but rather the combinations of stimulus variables that generate identical responses (e.g., 50% detection)
– Action Spectrum
Action spectrum of Rods as measured psychophysically, photochemically, &
electrophysiologically
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Analytical Psychophysics
• Brindley (1960)
– Two general types of psychophysical observations:
• Class A – observations in which the two stimuli are adjusted so that they elicit the same response from the observer
– Principle of Nomination
• Class B – Any identity that cannot be expressed as the identity or nonidentity of two sensations is Class B
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Brindley (1960)
• Class A Observation
Stimulus A -> Sensation X
Stimulus B -> Sensation X
• Class B Observation
Stimulus A -> Sensation A’
Stimulus B -> Sensation B’
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The Concept of the Threshold
• Herbart (1824)
– Originally conceived of the idea by assuming that mental events had to be stronger than some critical amount in order to be experienced
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Psychophysical Thresholds
– Absolute threshold (RL - Reiz Limen)
• The minimum amount of energy needed to elicit a response
– Difference threshold (DL - Differenz Limen)
• Required change in the amount of a stimulus to result in a Just Noticeable Difference (JND) (Δφ)
• A plot of JNDs across stimulus energies = Stimulus critical value function
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Absolute Threshold
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Difference Threshold
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Threshold examples
sight: candle flame at 30 miles on dark night
sound: tick of watch at 20 feet in quiet room
sound: tick of watch at 20 feet in quiet room
taste: 1 teaspoon of sugar in 2 gallons of water
smell: 1 drop of perfume in a 3 bedroom
apartment
touch: wing of a bee falling on cheek from 1 cm
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Difference Threshold
• Ernst Weber (1834)
– First to generate a stimulus critical value function
– Increases in stimulus intensity that werejust noticeably differentto the observer were always a constant fractionof the stimulus intensity
– Δφ/φ0
1
2
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0 10 20 30 40 50
Δφ
Stimulus Intensity (φ)
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Weber’s Law
• The change in stimulus intensity that can just be discriminated (Δφ) is a constant fraction (c) of the starting intensity of the stimulus (φ)
Δφ = cφ or Δφ/φ = c
Weber’s Fraction
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Weber’s Law
• Weber’s fraction is different for different sensory modalities (e.g., smell, touch, etc.) and test conditions (e.g., intensity, quality, etc.)
• The validity of this law requires that the constant ‘c’ be the same across a wide range of stimulus intensities
– Unfortunately, Weber’s Fraction tends to increase at very low intensities
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Fechner-Weber Law
• On the assumption of the validity of Weber’s Law and that each JND represented a discrete increment in sensory magnitude (ψ) Fechner derived the logarithmic relationship from Weber’s fraction
• Sensation is proportional to the log of the Intensity (φ)
Ψ = k x log(φ)
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0.0
1.0
2.0
3.0
4.0
5.0
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0 20 40 60 80 100 120
Stimulus Intensity
Sen
sati
on
un
its
∆I∆I∆I∆I
ψ
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Validity of Fechner-Weber Law
• 1st Fechner’s Law is only valid to the extent that Weber’s law is valid– Not always true, especially at very low intensity
levels
• 2nd Fechner’s Law assumes that each JND is an equal increment in sensation at every level of stimulus intensity– SS Stevens (1936) contested this assumption
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Durup & Piéron (1933)
Red & Green lights matched for luminance
X 10 JNDs
≠=Red & Green lights no longer matched
Fechner’s second assumptionholds that the two lights
will remain equally brightif matched at a low luminance
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Stevens (1957)
• Magnitude Estimation
– Subject is shown a standard and given a value to correspond to its magnitude
– New stimuli are judged relative to the standard
– The results of these experiments produce a power function
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To Honor Fechner and Repeal his Law
• SS Stevens (1961) critiqued Fechner’s law and proposed the Power law in its place
– Ratios of sensation are related to ratios of stimulation
Ψ = k (Φ)a --- OR --- Ψ = k (Φ – Φo)a
Using magnitude estimation experiments, Stevens was able to show that a constant ratio of sensation is produced by a constant ratio of stimulation
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Stevens’ Law suggests that psychological intensity is a function of
physical intensity raised to some power.
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Obtaining Thresholds in Human Observers
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Obtaining Thresholds in Human Observers
• Presenting a stimulus to an observer and asking them to report whether or not it was detected is the basic procedure for measuring thresholds
– Physical (and biological) systems, however, are dynamic
– An observer presented with the same intensity stimulus multiple times will report a “yes” on some occasions and a “no” on others
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Obtaining Thresholds in Human Observers
• The threshold cannot be defined quite so strictly as Herbart’s original concept
– It is still a useful concept
– Affords us the ability to evaluate the sensitivity various sensory modalities
• The threshold can be expressed as a statistical value (e.g., 50% detection)
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Obtaining Thresholds in Human Observers
• Fechner recognized the statistical nature of the threshold
– Developed the methodologies to measure them in this way (more on this in a moment)
• The modern concept of the threshold has been only slightly modified
– E.g., sources of variance
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Obtaining Thresholds in Human Observers
• Let us begin with the concept of the absolute threshold
– Minimum amount of energy necessary to generate a response reliably
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Obtaining Thresholds in Human Observers
Φcrit = 4 units
Theoretical Threshold Function
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Obtaining Thresholds in Human Observers
• However, an additional primary assumption in classical threshold theory is that the threshold varies across time
– An observer’s threshold may be a sharp boundary at a particular instant in time, but during an experiment, would appear to be changing
– These fluctuations are assumed to be randomly distributed
– Detection of the stimulus is accomplished only on those trials when the momentary threshold is exceeded
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Obtaining Thresholds in Human Observers
Φcrit = 4 units
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Classical Psychophysical Theory
• The Psychometric Function:
– The proportion of trials where the momentary threshold is exceeded should increase as function of stimulus intensity if the variation of the momentary thresholds is normally distributed (i.e. random)
– Ogive function (cumulative probability function)
-0.1
0.0
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1 2 3 4 5 6 7
Stimulus Intensity
Pro
po
rtio
n o
f "
yes"
resp
on
ses
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Classical Psychophysical Theory
• That psychometric functions can be fit by an ogive is supported by theory & experimental findings
– Variation of biological and psychological measurements tend to be normally distributed
– The ogive curve is a cumulative form of the normal distribution
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Classical Psychophysical Theory
• One useful technique for fitting ogive functions is by the process of transformation
– An ogive function becomesa linear function when thedetection probabilities ateach stimulus magnitude are converted to z-scores
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Psychophysical Methods
• Method of Constant Stimuli
• Method of Limits
• Method of Adjustment
• Staircase Method
• Method of Forced Choice
• Theory of Signal Detection
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Transmission Characteristics of the Eye
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Transmission Characteristics of the Eye
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Transmission Characteristics of the Eye
• The Retina
– Before light reaches the photoreceptors, it must pass through a jungle of neural tissue
– Prereceptoral filters
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The Retina
The vertebrate retina has ten distinct layers. (1) Inner limiting membrane (2) Nerve fiber layer (3) Ganglion cell layer
– Layer that contains nuclei of ganglion cells and gives rise to optic nerve fibers
(4) Inner plexiform layer (5) Inner nuclear layer (6) Outer plexiform layer
– In the macular region, this is known as the Fiber layer of Henle
(7) Outer nuclear layer (8) External limiting membrane
– Layer that separates the inner segment portions of the photoreceptors from their cell nuclei
(9) Photoreceptor layer – Rods / Cones
(10) Retinal pigment epithelium
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The Macular Pigment
1.25-deg
Macular Pigment
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Spectral Sensitivity
• Imagine that you are shown two lights, one 400nm and one 555nm, and that they are both equated for physical energy
– Do they both appear equally bright?
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Spectral Sensitivity
• Although both lights have equal energy, the produce different effects on the visual system
– The visual efficiency at 400nm = 0.0
– The visual efficiency at 555nm = 1.0
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Spectral Sensitivity
• Consequently, the two lamps are considered radiometrically equal, but photometrically different
– The 555nm light is bright, but the 400nm light is dark
Radiometric
Photometric
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Dark Adaptation
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Dark Adaptation
• Dark adaptation is a progressive increase in sensitivity that takes place while one is in the dark
– Photochemical Hypothesis (Hecht, 1937)
• Visual recovery in the dark is mediated by the regeneration of photopigment
• Increased photopigment density = increase quantal catching probability
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Dark Adaptation
• Pigment regeneration– After the retinal molecule has absorbed a photon
(isomerized), it breaks free of the opsin and (because it has a new conformation) becomes transparent
– This is known as pigment bleaching
– Before the pigment can be receptive to light again, it must return to its original conformation and rejoin with the opsin molecule• This process is known as visual pigment regeneration
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Dark Adaptation
• Photochemical Hypothesis
– Predicts that if 50% of the photopigment is bleached (isomerized) that the visual threshold should increase by a factor of 2
– In fact, the threshold increases by 10 log units!
– Therefore, the process of dark adaptation includes both photochemical and neural components
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Dark Adaptation
• The Dowling-Rushton equation:– Relates the bleaching of photopigment to the change
in threshold for the observer
Log(It/I0) = kP
Where:It = threshold intensity of the test flashI0 = dark-adapted threshold intensityP = proportion of pigment in the bleached statek = a constant (20 for rods, 3 for cones)
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Dark Adaptation
• The sensitivity of a person’s eye is related to the amount of pigment in their retina– More pigment = more sensitive
• A plot of visual sensitivity across time spent in a dark room
• 2 stages– An initial rapid phase that lasts ~7-10min (maximum
cone sensitivity)– A second slower phase that takes ~30-40min to reach
maximum (rod) sensitivity
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Rods & Cones
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Rods & Cones
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Dark Adaptation
• Rods and cones do not have the same spectral sensitivities
• Therefore, the shape of the dark adaptation curve is a function the test wavelength
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Rods & Cones
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Importance of the Rod/cone break
• The rod/cone break is:
– most pronounced when using short wavelengths (blue-ish light)
– only observable in the peripheral retina where rods and cones co-exist
• The rod/cone break is NOT:
– observable when using very long wavelengths (red-ish light) irrespective of retinal location
– observable in the fovea (rod free zone)
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Light Adaptation
• Light adaptation is a relatively fast process that occurs when one moves from a dark environment into a well illuminated one
• Studied using Increment Thresholds
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Light Adaptation
• Increment Thresholds
BackgroundTest FlashIncrement
ΔI
Intensity
Distance
IB
ΔI
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Light Adaptation
• Increment thresholds are measured using backgrounds of varying intensity (weak to strong)
– When an object is sufficiently more intense than it’s background, it is detectable
– This JND increases as the intensity of the background increases
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Spatial Summation
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Spatial Summation
• 126 million photoreceptors, but only 1 million ganglion cells
• In some parts of the eye, a single ganglion cell is receiving inputs from many photoreceptors
• This has implications for visual sensitivity and acuity
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Spatial Summation
• The degree of convergence among photoreceptors onto ganglion cells increases with retinal eccentricity (moving from the center out into
the periphery)
• Foveal cones have a 1:1 or near 1:1 relation with ganglion cells
• Receptors in the far periphery can have up to a 400:1 relation with a ganglion cell
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Spatial Summation
• The concept of the Receptive Field
– A region of space (visual, auditory, somatosensory) that is associated with a particular response
• In this case, the “region of space” is a section of retina, and the “response” is ganglion cell activity
– As the degree of convergence increases, so does the size of the receptive field
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Spatial Summation
• Implications for visual sensitivity– With a larger receptive field, more light can be
caught• With more photoreceptors active and pooling their
collective responses onto a single ganglion cell, it is much more likely for the ganglion cell to fire
– With a smaller receptive field, less light can be caught• With fewer photoreceptors active, it is less likely for the
ganglion cell to fire
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Spatial Summation
• Implications for visual acuity
– Large receptive fields pool light information from a large area• Small details get lost
• It does not matter where in the receptive field light has fallen, only that enough of it has
– The fovea has very small receptive fields, and good acuity
– The peripheral retina has larger receptive fields, and poor acuity
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Ricco’s Law
• Classic demonstration of spatial summation
– An observer is shown a small spot of light
– The threshold number of quanta necessary for seeing is recorded
– The spot size is increased, and the experiment is repeated
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Ricco’s Law
• The threshold number of quanta needed for seeing remains constant up to a critical diameter
This means that the threshold number of quanta could all be delivered to a small spot within the critical diameter or spread out across the entire critical diameter to result in the same visual response
Spot Diameter (arc-min)
Nu
mb
er o
f Q
uan
ta a
t Th
resh
old
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Ricco’s Law
Total spatial summation is represented by:
IA = K
Where:I = stimulus intensity (quanta/area)A = Stimulus AreaK = Constant
It should be clear that the photopic (cone dominated) system has a much smaller critical diameter relative to the scotopic (rod dominated) system
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Receptive Fields & Inhibition
• Recall the concepts of excitation and inhibition
– Excitation = more activity
– Inhibition = less activity
• Within a receptive field, some photoreceptors are excitatory, whereas others are inhibitory to the ganglion cell
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Lateral Antagonism
Inhibitory-center-excitatory-surround
Excitatory-center-Inhibitory-surround
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Center-Surround Antagonism
• Lateral Antagonism
– As you increase the size of the spot of light on the entire receptive field, the firing rate of the ganglion cell changes
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Receptive Fields & Inhibition
• Implications for visual perception
– Mach bands
• The perception of light and dark bands near the borders between light and dark areas
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Mach Bands
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What is the benefit?
• Although LA can lead to some interesting visual illusions, it does serve very important functions– E.g. Contrast enhancement at edges
• Your eye is not a perfect optical instrument– Edges are blurred to some degree
– By enhancing the difference between a light and dark region, your visual system is able to compensate for the blurring
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Physical
Optical
Perceived
Inte
nsi
tyIn
ten
sity
Inte
nsi
tySpace
Space
Space
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Temporal Summation
• Integration of information over a discrete period of time
– Time analog to spatial summation
• Rods tend to integrate light over longer periods of time than do cones
– Increased sensitivity, poorer temporal resolution
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Temporal SummationTemporal Summation Period
No FlashSeen
ONE FlashSeen
TWO FlashesSeen
ONE FlashSeen
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Bloch’s Law
• Temporal equivalent to Ricco’s Law
– A series of flashes presented within the critical duration cannot be independently resolved, but rather are seen as a single flash
It = K
Where:
I = stimulus intensity (quanta / time)
t = Stimulus duration
K = Constnat
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Stiles-Crawford Effect
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Stiles-Crawford Effect
• Two kinds:
– First kind (brightness)
– Second kind (chromatic)
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Stiles-Crawford Effect
• Stiles and Crawford observed that light entering the center of the pupil had greater luminous efficiency than light that entered at the periphery of the pupil
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Stiles-Crawford Effect
• Initially, the cause of the effect was unknown, but largely attributed to the lens
• It is currently understood to be related to the waveguide properties of the cone photoreceptors
– Rods do not exhibit a strong Stiles-Crawford effect
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Stiles-Crawford Effect
• The diameter of a cone photoreceptor is very near that of the wavelength of light– This makes the angle of entry of the light ray critical
– Deviations from an orthogonal entry angle result in a reduced visual efficiency
• The cone photoreceptors tend to aim their long-axes toward the center of the pupil– In fact, the cones are phototropic, and like plants will
follow the light if it is displaced (decentered pupil)
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Stiles-Crawford Effect
pupil
Spot of light
Light entering the center of the pupil
encounters minimal refraction and will
strike the cone photoreceptors
orthogonal to their long-axis
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Stiles-Crawford Effect
pupil
Spot of light
Light entering the periphery of the pupil
encounters some refraction and will
strike the cone photoreceptors at an angle relative to their
long axis
Consequently, some of the light is
refracted out of the receptor and is lost
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Stiles-Crawford Effect
• It is thought that this effect reduces the visual efficiency of scattered light in the eye
– Light that has been scattered encounters the photoreceptors at oblique angles
– If the receptors were equally receptive to oblique light as they are to direct light, then everything would appear fuzzy in dim light and “glary” in bright light
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