A presentation I gave as a guest lecturer to S&P students enrolled at the University of Kansas

1. Introduction to Perception Sensation & PerceptionGuest Lecturer: Michael Engles, Ph.D.

2. Sensation, perception, cognition 1. Sensation: Physical stimulation 2. Perception: Interpretation/meaningfulness 3. Cognition: “Use” of perception

3. Historical overview 1. Greek philosophers: eye receives “copies” of objects 2. Empiricists – we learn to perceive a. Berkeley: How do we perceive depth with 2D receptors??? b. James: “blooming, buzzing confusion” 3. Gestaltists – we are tuned to perceive organization

4. Historical overview 4. Ecological approach a. 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 computation a. Develop mathematical models of how information can be processed

5. 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 not light energy) 4. Neural processing: computations

6. 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

7. 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

8. Studying the perceptual process Behavioral approach 1. Phenomenology: What do you see? 2. Psychophysical approach: Thresholds Measuring the absolute threshold: minimum detectable energy

9. The physiological approach Doctrine of specific nerve energies (Mueller) Perception results from specific nerve stimulation 2. Neuroanatomy Nerves cell body, dendrites, axons, synapse Synapse presynaptic and postsynaptic, receptor sites, neurotransmitters, excitation and inhibition

10. Psychophysics Elemente der Psychophysik (1860) Authored by Gustav Theodore Fechner Marked the transition from philosophy to science for psychology by introducing objective measurement techniques Modern scientists still appeal to it’s basic principles for the purpose of understanding the nature of perception

11. 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

12. 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

14. The patient behaviorally indicates alterations in their experience when some dimension of the stimulus is changed

15. The clinical inspection of the eye (e.g., retina or lens) is also “psychophysical”

18. Provides a specification of sensory capability

19. E.g., Threshold changes across wavelength

20. Analytical

21. Testing of hypotheses about the nature of biological mechanisms underlying sensory experience

24. Declares that identical neural events give rise to identical psychological events

25. When stimulus A and stimulus B produce the same sensory experience, they produce the same neural response

27. 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)

28. Action SpectrumAction spectrum of Rods as measured psychophysically, photochemically, &electrophysiologically

29. 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

30. Brindley (1960) Class A Observation Stimulus A -> Sensation X Stimulus B -> Sensation X Class B Observation Stimulus A -> Sensation A’ Stimulus B -> Sensation B’

31. 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

32. 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

33. Absolute Threshold

34. Difference Threshold

35. 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

36. 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/φ

37. 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

39. The validity of this law requires that the constant ‘c’ be the same across a wide range of stimulus intensities

41. Sensation is proportional to the log of the Intensity (φ)Ψ = kx log(φ)

42. ψ ∆I ∆I ∆I ∆I

43. 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

44. Durup & Piéron (1933) X 10 JNDs ≠ = Red & Green lights matched for luminance Red & Green lights no longer matched Fechner’s second assumptionholds that the two lightswill remain equally brightif matched at a low luminance

45. 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

46. 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

47. Stevens’ Law suggests that psychological intensity is a function of physical intensity raised to some power.

48. Obtaining Thresholds in Human Observers

49. 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

50. 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)

51. 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

52. 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

53. Obtaining Thresholds in Human Observers Theoretical Threshold Function Φcrit = 4 units

54. 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 momentarythreshold is exceeded

55. Obtaining Thresholds in Human Observers Φcrit = 4 units

56. 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)

57. 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

58. 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

59. Psychophysical Methods Method of Constant Stimuli Method of Limits Method of Adjustment Staircase Method Method of Forced Choice Theory of Signal Detection

60. Transmission Characteristics of the Eye

61. Transmission Characteristics of the Eye

62. Transmission Characteristics of the Eye The Retina Before light reaches the photoreceptors, it must pass through a jungle of neural tissue Prereceptoral filters

63. The Retina The vertebrate retina has ten distinct layers. Inner limiting membrane 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

65. The Macular Pigment 1.25-deg Macular Pigment

66. 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?

67. 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

68. 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

69. Dark Adaptation

70. 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

71. 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 pigmentbleaching 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 visualpigmentregeneration

72. 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

74. Relates the bleaching of photopigment to the change in threshold for the observerLog(It/I0) = kP Where: It = threshold intensity of the test flash I0 = dark-adapted threshold intensity P = proportion of pigment in the bleached state k = a constant (20 for rods, 3 for cones)

76. More pigment = more sensitive

77. A plot of visual sensitivity across time spent in a dark room

78. 2 stages

79. An initial rapid phase that lasts ~7-10min (maximum cone sensitivity)

81. Rods & Cones

82. Dark Adaptation Rods and cones donot have the same spectral sensitivities Therefore, the shape of the dark adaptation curve is a function the test wavelength

83. Rods & Cones

84. 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)

85. 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

86. Light Adaptation Increment Thresholds Test Flash IncrementΔI Background ΔI Intensity IB Distance

87. 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

88. Spatial Summation

89. 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

90. 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

91. Spatial Summation The concept of the ReceptiveField 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

92. 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

93. 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 peripheralretina has larger receptive fields, and poor acuity

103. 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

104. 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 Number of Quanta at Threshold Spot Diameter (arc-min)

105. Ricco’s Law Total spatial summation is represented by: IA = K Where: I = stimulus intensity (quanta/area) A = Stimulus Area K = Constant It should be clear that the photopic (cone dominated) system has a much smaller critical diameter relative to the scotopic (rod dominated) system

106. 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

107. Lateral Antagonism Excitatory-center-Inhibitory-surround Inhibitory-center-excitatory-surround

108. 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

109. Receptive Fields & Inhibition Implications for visual perception Mach bands The perception of light and dark bands near the borders between light and dark areas

111. Mach Bands

112. 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

113. Physical Intensity Space Intensity Optical Space Intensity Perceived Space

114. 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

115. Temporal Summation Temporal Summation Period No FlashSeen ONE FlashSeen TWO FlashesSeen ONE FlashSeen

117. A series of flashes presented within the critical duration cannot be independently resolved, but rather are seen as a single flashIt = K Where: I = stimulus intensity (quanta / time) t = Stimulus duration K = Constnat

118. Stiles-Crawford Effect

119. Stiles-Crawford Effect Two kinds: First kind (brightness) Second kind (chromatic)

120. 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

121. 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

123. This makes the angle of entry of the light ray critical

124. Deviations from an orthogonal entry angle result in a reduced visual efficiency

125. The cone photoreceptors tend to aim their long-axes toward the center of the pupil

127. Stiles-Crawford Effect Light entering the periphery of the pupil encounters some refraction and will strike the cone photoreceptors at an angle relative to their long axis pupil Consequently, some of the light is refracted out of the receptor and is lost Spot of light

128. 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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