1. Sensation is the detection of environmental stimuli by the sense organs, while perception is the process of organizing and interpreting these stimuli. 2. Sensory processing involves the transduction of stimuli into nerve impulses, feature detection in the brain, and the reconstruction of stimuli from neural representations matched with prior experience. 3. Psychophysics studies the relationship between physical stimulus characteristics and sensory capabilities. Signal detection theory holds that perception involves uncertainty and decision-making rather than fixed thresholds.

1. SENSATION & PERCEPTION

2. Sensation & Perception Defined
• Sensation – stimulus detection process by
which our sense organs respond to and
translate environmental stimuli into nerve
impulses that are sent to the brain
• Perception – the active process of organizing
this stimulus input and giving it meaning

3. Stages of Sensory Processing
1. Sensation is received by sensory receptors.
2. Receptors translate stimulus properties into
nerve impulses (transduction).
3. Feature detectors analyse stimulus features.
4. Stimulus features are reconstructed into neural
representation.
5. Neural representation is compared with
previously stored information in brain.
6. Matching process results in recognition and
interpretation of stimulus.

5. SENSORY PROCESSES
Psychophysics – scientific study of physical characteristics of stimuli and
sensory capabilities

6. Stimulus Detection: The Absolute
Threshold
• The lowest intensity at which a stimulus can
be detected 50% of the time
• < Absolute threshold  > sensitivity

7. Approximate Absolute Threshold for
YOU
Sensory Modality Absolute Threshold
Vision Candle flame seen at 30 mi (48.2 km) on a clear, dark night
Hearing Tick of a watch under quiet conditions at 20 ft (6 m)
Taste 1 teaspoon of sugar in 2 gal (3.7 L) of water
Smell 1 drop of perfume diffused into the entire volume of a large
apartment
Touch Wing of a fly or bee falling on a person’s cheek from a distance of
1 cm

8. Signal Detection Theory

9. Signal Detection Theory
• Before: “each person has a more or less fixed
level of sensitivity for each sense”
• Now: fixed AT is inaccurate
• Why?
• Range of uncertainty (decision criterion) – a
standard of how certain people must be that a
stimulus is present before they will say they
detect it

10. Signal Detection Matrix

11. Conclusions in SDT
1. At low stimulus intensities, both the P’s and
situation’s characteristics influence the decision
criteria
2. Bold Ps (people who frequently say “yes”): hits =
false alarms than conservative Ps
3. Rs can influence Ps to become bolder or more
conservative by manipulating the rewards &
costs for giving correct or incorrect responses
4. Perception is, in part, a decision.

12. Subliminal Stimuli: Can They Affect
Behavior?
• Subliminal stimulus – stimulus that is so weak
or brief that although it is received by the
senses, it cannot be perceived consciously
• YES, to a limited extent.

13. The Coca-Cola-popcorn-subliminal-
message controversy
• James Vicary (late
50s) – arranged to
have subliminal
messages flashed
on a theater screen
during a movie
• Popcorn sales –
50%
• Coke sales – 18%

14. • Influence to consumer behavior: persuasive
stimuli the perceptual threshold > subliminal
stimuli sneaking into the subconscious mind
BUT(T)!
• They affect more subtle phenomena (i.e.,
judgment, attitudes & behaviors)

15. The Difference Threshold
• The smallest difference between two stimuli
that people can perceive 50% of the time (aka
just noticeable difference or jnd)
• Weber’s law – the difference threshold/jnd is
directly proportional to the magnitude of
stimulus with which the comparison is being
made

16. Weber Fractions for Various Sensory
Modalities
Sensory Modality Weber Fraction
Audition (tonal pitch) 1/333
Vision (brightness, white light) 1/60
Kinesthesis (lifted weights) 1/50
Pain (heat produced) 1/30
Audition (loudness) 1/20
Touch (pressure applied to skin) 1/7
Smell (India rubber) ¼
Taste (salt concentration) 1/3

17. Sensory Adaptation
• The diminishing sensitivity to an unchanging
stimulus (aka habituation)
• Reduces overall sensitivity
• Frees our senses from the constant and
mundane (important to our well-being &
survival)

18. THE SENSORY SYSTEMS

19. Transduction
• The process by whereby the characteristics of
a stimulus are converted into nerve impulses
(electrical by nature)
• Electromagnetic energy/light waves – normal
stimulus for vision

20. Vision

21. The Human Eye

22. Photoreceptors: The Rods & Cones

23. Facts about Photoreceptors
• Rods & cones  bipolar cells  1 million
ganglion cells (axons form the optic nerve)
• Light-sensitive ends point away from the
direction of the entering light
• The connection of rods & cones to bipolar
cells account for the importance of rods in
dim light & visual acuity caused by cones

24. Facts about Photoreceptors
• Many rods & cones  single/same bipolar
cells: additive effect of many signals
• Cones in the fovea have “private lines” to
single/same bipolar cell: visual acuity – ability
to see fine details
• Blind spot

25. Visual Transduction: From Light Waves
to Nerve Impulses
• Photopigment – protein molecules in the rods &
cones that help them translate light waves into
nerve impulse
• Absorption of light by the photopigment 
chemical reaction  Δ in the rate of
neurotransmitter release  signal passed on to
the bipolar cells  ganglion cells  optic nerve
(bundle of ganglion cells’ axons)  visual relay
station (thalamus)  visual cortex (cerebrum)

26. Brightness & Dark Adaptation
• Rods & cones sensitivity in low illumination
– Rods: sensitive throughout the spectrum except
for red
– Cones: sensitive in the greenish-yellow range
– Change of fire engines from red to greenish-yellow
– Rods are not always ready to fulfill its function

27. Brightness & Dark Adaptation
• Dark adaptation – the progressive
improvement in brightness sensitivity that
occurs over time under conditions of low
illumination
– Photopigments of rods vs photopigments of cones
– Used for enhancing the night vision of pilot during
the WW II

28. Color Vision
• Young-Helmholtz trichromatic theory –
assumes that there are three types of color
receptors in the retina
– Cones sensitive to blue, green and/or red (if all are
activated  white light)
– Criticisms: perception of yellow & color
afterimage
Retinal receptor
Brain

29. Kinds of color mixture

30. Color Vision
• Hering’s opponent-process theory – proposes
that each of the three cone types respond to
two different wavelengths
– Blue-yellow cones, red-green cones & black-white
cones

31. Opponent Process: Afterimage

32. Color Vision
• Dual-process theory – combines trichromatic
& opponent-process theories to account for
the color transduction process
– Trichromatic theory: cones contain one of the 3
different photopigments sensitive to blue, green &
red
– Ganglion cells, neurons in visual relay station &
the visual cortex are responsible for opponent-
process, not the cones

33. Opponent-process Mechanisms

34. Color-Deficient Vision
• Trichromats
• Dichromats – color
blind in only one of
the systems (blue-
yellow or red-
green)
• Monochromats –
sensitive only to the
black-white color
system (total color-
blindness)

35. Audition
• Sound wave – a form of mechanical energy &
the stimulus for the sense of hearing
• Sound – pressure waves in air, water or some
other conducting medium  vibration 
successive waves of compression & expansion
among air molecules

36. Characteristics of Sound Waves
• Frequency – the number of sound waves, or
cycles, per second; hertz (Hz) – the technical
measure of cycle per second; related to perceived
pitch
• Amplitude – the vertical size of the sound waves,
amount of compression & expansion of the
molecules in the conducting medium; decibels
(dB) – a measure of the physical pressures that
occur at the eardrum; related to perceived
loudness

37. Auditory Transduction: From Pressure
Waves to Nerve Impulse

38. The Cochlea

39. Coding of Pitch & Loudness
• Loudness is coded in terms of both the rate of
firing in the axon (auditory nerve) & the
specific hair cells that are sending messages

40. Coding of Pitch & Loudness
• Coding of wave frequency (pitch)
– Frequency theory of pitch perception – assumes
that nerve impulses sent to the brain match the
frequency of the sound wave
– Place theory of pitch perception – suggests that
the specific point in the cochlea where the fluid
wave peaks & most strongly bends the hair cells
serves as a frequency coding cue

41. Sound Localization
• Humans’ binaural (two-eared) ability to
localize sounds

42. Hearing Loss
• 2 major types of hearing loss (U.S.)
– Conduction deafness – involves problems w/ the
mechanical system that transmits sound waves to
the cochlea
– Nerve deafness – caused by damage receptors
within the inner ear or damage to the auditory
nerve itself

43. Taste & Smell: The Chemical Senses
• Gustation – the sense of taste
• Olfaction – the sense of smell
– Their receptors are sensitive to chemical
molecules rather than to some form of energy

44. Gustation: The Sense of Taste
Taste buds – chemical receptors
concentrated along the tip, edges & back
surface of the tongue

45. Olfaction: The Sense of Smell
Pheromones – chemical
signals found in natural
body scents
Menstrual synchrony –
the tendency of women
who live together or are
close friends to become
more similar in their
menstrual cycle

46. The Skin & Body Senses

47. The Tactile Senses
• Four tactile sensations: pressure (touch), pain,
warmth & cold
• Mechanoreceptors
– Merkel receptors – sensing fine details
– Meissner corpuscle – controlling handgrip
– Ruffini cylinder – perceiving stretching of the skin
– Pacinian corpuscle – sensing rapid vibration & fine
texture

48. The Body Senses
• Kinesthesis – provides us with feedback about
our muscles’ and joints’ positions &
movements
– Receptors: nerve endings in the muscles, tendons
& joints
• Vestibular sense – sense of body orientation
& equilibrium