Sensation

1
Sensation

• How do we know what is real?
• Locke and the empiricists Is sensation where
  knowledge begins?
• The fear of the Lord is the beginning of
  wisdom. (Proverbs 17)

2
Sensation and bottom-up processing

• How do we know what is real? Empiricism and
  epistemology
• Experience is traditionally divided into two
  parts
• Sensation
• Perception
• But they are intimately connected.
• Sensation is bottom-up perception is top-down.
• Sensation is experienced perception is
  constructed.

3
Berkeley and the limits of sensations

• Distal stimulus
• Proximal stimulus

4
A nativist critique

• Kant Sensations are sorted by pre-existing
  (innate) categories of space, time, and causality
• We cannot help but understand our sensations in
  terms of these categories.

5
Three parts of the psychology of sensation

• 1. Psychophysics What is the relationship
  between the physical characteristics of a
  stimulus and the psychological experience of it?
  Is it the connection between body and mind, as
  Fechner thought?
• 2. Sensory physiology How do sense organs,
  receptor cells, and neural circuits respond to
  physical stimuli, to tell our brains what is out
  there?
• 3. Transduction and coding
• Place or anatomical coding
• Temporal coding Rate and pattern

6
Psychophysics and thresholds Scaling and
measurement.

• How do we measure how strong a stimulus is?
• The absolute threshold (Reiz Limen or RL) in
  classical psychophysics serves as the zero point.
• The absolute threshold is the intensity or
  duration of a stimulus that is sensed 50 of the
  time (the median).
• Stimuli below the threshold (limen) are
  subliminal.

7
More measures of RL The tachistoscopic method
8
Relative thresholds Scale units

• Difference thresholds Weber, Fechner, and the
  jnd (just noticeable difference)
• Method of limits
• Method of right and wrong cases
• Method of adjustment
• Which is louder?
• The Weber fraction

9
Subliminal stimulation

• Priming emotions and perceptions are the only
  reliable effects of subliminal stimulation.
• Subliminal stimulation does not increase the
  likelihood that you will buy a product, vote for
  a candidate, or break bad habits.
• Some people make money by warning against
  subliminal persuasion.
• Some people use subliminal persuasion as an
  excuse for their own irrational acts.

10
Sensory adaptation

• Receptor fatigue
• Habituation
• Adaptation and contrast
• Overcoming adaptation to see what is really
  there The flashlight experiment

11
Habituation

• Saccadic movement
• Stabilizing the retinal image

Mounting an LED or a miniature projector on a
contact lens produces a fixed retinal image.
12
The physics of light

• Light energy characteristics
• Waves and particles
• Frequency
• The visible spectrum 380nm to 760 nm

• Ultraviolet and infrared
• Amplitude or intensity
• Purity

13
The structure of the eye and the physics of light
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Physics of light and the visual system

• Sensitivity and reliability
• Retinal cells
• Rods and cones
• The fovea
• The blind spot
• Accommodation
• Binocular disparity

15
Color vision

• Young Helmholtzs trichromatic theory
  Different colors are sensed by cones containing
  different photopigments
• Green photopigment, in 50 of cells
• Red photopigment, in 45 of cells
• Blue photopigment, in 5 of cells
• Sensed color depends on which combinations of
  cones are absorbing light in their photopigments.

16
Opponent process theory

• Herings theory of ganglion cells
• Red/Green cells
• Yellow/ Blue cells
• Sensed color is coded by rate of firing Faster
  for red and yellow slower for green and blue.
• Habituation of ganglion cells produces negative
  afterimages.

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Negative afterimages
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Color Blindness

• Sex-linked conditions Genes on X chromosome, so
  more common in men.
• Protanopia, missing red photopigment
• Deuteranopia, missing green photopigment
• Non-sex-linked condition
• Tritanopia, missing blue photopigment or blue
  cones

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What do you see?
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What do you see?