Experimental Psychology
PSY 433
Chapter 7
Perception (Cont.)
DVs in Perception Experiments
 Verbal descriptions of experience.


Imprecise.
Not immediately verifiable.
 Reaction times.
 Reports that can be verified:

What did you see?
 Confidence ratings – how sure are you that
you are correct?
IVs in Perception Experiments
 Physical characteristics of stimuli:


Visual: size, shape, background, perspective.
Auditory: pitch, intensity, waveform (timbre),
complexity, relation of sounds to each other.
 Time course – brief presentation.
 Sensory degradation or deprivation.
Control Variables
 Physical aspects of the stimuli that are not
being investigated (manipulated).
 Emotional and motivational aspects of the
task.

Hungry people see food-related objects.
 Decision aspects of the task.
Verbal Reports
 Since we cannot know the phenomenological
experience of a person viewing a stimulus,
what do verbal reports mean?
 A verbal report is meaningful if there is a
relationship between it and the characteristics
of the preceding perceptual event.


Most people respond in similar ways to the
same stimuli.
Because a relationship exists, we can infer the
phenomenology from the verbal report given.
Redefining Perception
 If perception is the interpretation of sensation,
how can phenomena such as “blindsight”
occur?
 Blindsight – visual capacity in a blind spot
while there is no awareness of perception.


Someone’s performance shows they can “see”
but they report not seeing anything.
D.B.’s scotoma (blind spot) was related to
brain injury to the system that identifies
objects.
Can You Learn in Your Sleep?
 How about weight-loss during sleep?
 Are teenagers being influenced by satanic
verses hidden in rock & roll songs?

http://www.umich.edu/~onebook/pages/frames/legalF.html
 Do “subliminal” pictures of hot buttered
popcorn increase the likelihood that people
will eat popcorn in a movie theater?
Perception Without Awareness?
 Marcel (1983) used a primed Stroop task
presented using a tachistoscope.
 A prime word (a color word or a neutral word)
was presented, followed by a mask at:


400 ms (aware condition)
A shorter interval allowing only 60% detection
of the prime (unaware condition)
 A color patch followed the mask.
 Prime and patch were either congruent,
incongruent, or neutral.
Marcel (1983) Results
 Regardless of whether subjects were
aware or unaware of the prime, compared
to control:


Faster responses on congruent trials
Slower responses on incongruent trials
 Conclusion: meaning CAN be perceived
without awareness
 Conclusion: the prime’s meaning is
processed mentally, despite subjects’
verbal reports that they never saw the
prime.
What is a Threshold?
 Threshold -- an intensity value above which a
person always perceives, and below which a
person does not perceive.

Think of a door – the higher the threshold, the
more difficult it is to enter a room
 Threshold and sensitivity are inversely
related.


Higher threshold means less detection.
Lower threshold means more detection.
Cheesman and Merikle (1984)
 Questioned Marcel’s results – different
thresholds may be used by different cognitive
processes:


Detection threshold
Verbal report threshold
 The threshold for conscious awareness of
words may be higher than for responding
differentially to their meanings.
 Marcel’s mask may have interfered with one
threshold (consciousness) but not the other.
Cheesman & Merikle’s Method
 The prime-mask interval was set by forced-
choice detection of the 4 color word primes
at 25%, 55%, and 90%
 Subjects said they couldn’t see prime in all
three conditions (25%, 55%, 90%).
 Otherwise, their method was similar to
Marcel’s.
Verbally unaware
V. Aware
Two Threshold Theory
 Based on their findings, Cheesman & Merikle
suggest that there exist two kinds of
thresholds:


Objective threshold – where discriminative
responding is at chance.
Subjective threshold – where responding is
above chance but subjects report being
unaware of the stimuli.
 This defines the three regions.
Cheesman & Merikle (1986)
 Does unconscious processing differ from
conscious processing? (Yes, it does)

Varied % of congruent trials (33 vs. 66)
 Previous results: For conscious processing,
the more congruent trials, the greater the
difference between congruent/incongruent
RTs
 Subjects are biased to respond “congruent”
because they know it’s the most frequent trial
 Does this occur when they don’t know?