Lecture (Sensation & Perception)

Introduction to Perception

Sensation & Perception

Guest Lecturer: Michael Engles, Ph.D.

Sensation, perception, cognition

1. Sensation: Physical stimulation

2. Perception: Interpretation/meaningfulness

3. Cognition: “Use” of perception

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

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

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

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

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

Studying the perceptual process

Behavioral approach

1. Phenomenology: What do you see?

2. Psychophysical approach: Thresholds

a. Measuring the absolute threshold: minimum

detectable energy

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

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

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

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

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.

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

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

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

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


• 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


• 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

Brindley (1960)

• Class A Observation

Stimulus A -> Sensation X

Stimulus B -> Sensation X

• Class B Observation

Stimulus A -> Sensation A’

Stimulus B -> Sensation B’

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

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

Absolute Threshold

Difference Threshold

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

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

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0 10 20 30 40 50

Δφ

Stimulus Intensity (φ)

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

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

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)

0.0

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0 20 40 60 80 100 120

Stimulus Intensity

Sen

sati

on

un

its

∆I∆I∆I∆I

ψ

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

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

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

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

Stevens’ Law suggests that psychological intensity is a function of

physical intensity raised to some power.

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


• 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)


• 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


• Let us begin with the concept of the absolute threshold

– Minimum amount of energy necessary to generate a response reliably


Φcrit = 4 units

Theoretical Threshold Function


• 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


Φcrit = 4 units

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)

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Stimulus Intensity

Pro

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yes"

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• 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


• 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

Psychophysical Methods

• Method of Constant Stimuli

• Method of Limits

• Method of Adjustment

• Staircase Method

• Method of Forced Choice

• Theory of Signal Detection

Transmission Characteristics of the Eye

Transmission Characteristics of the Eye

• The Retina

– Before light reaches the photoreceptors, it must pass through a jungle of neural tissue

– Prereceptoral filters

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

The Macular Pigment

1.25-deg

Macular Pigment

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?


• 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


• Consequently, the two lamps are considered radiometrically equal, but photometrically different

– The 555nm light is bright, but the 400nm light is dark

Radiometric

Photometric

Dark Adaptation

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

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

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

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)

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

Rods & Cones

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

Rods & Cones

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)

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

Light Adaptation

• Increment Thresholds

BackgroundTest FlashIncrement

ΔI

Intensity

Distance

IB

ΔI

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

Spatial Summation

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

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

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

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

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

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

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

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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

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

Lateral Antagonism

Inhibitory-center-excitatory-surround

Excitatory-center-Inhibitory-surround

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

Receptive Fields & Inhibition

• Implications for visual perception

– Mach bands

• The perception of light and dark bands near the borders between light and dark areas

Mach Bands

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

Physical

Optical

Perceived

Inte

nsi

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ten

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Inte

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tySpace

Space

Space

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

Temporal SummationTemporal Summation Period

No FlashSeen

ONE FlashSeen

TWO FlashesSeen

ONE FlashSeen

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

Stiles-Crawford Effect


• Two kinds:

– First kind (brightness)

– Second kind (chromatic)


• 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


• 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


• 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)


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


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


• 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

Lecture (Sensation & Perception)

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