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Subliminal Perception
Zoltn Dienes Conscious and unconscious mental processes
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time
Forward
mask
Backward
mask
ISI
SOA
ISI = inter-stimulus interval
SOA = stimulus onset asynchrony
Subliminal perception = perceiving without being aware of perceiving
Conscious perception can be blocked with masking
Masked
stimulus
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Noise mask:
Each pixel can
be randomly
on or off
MXgFhhY
Pattern mask:
Random
collection of
the parts of thestimulus to be
maskede.g.
letters or letter
parts for wordstimuli
Monoptic: mask and word presented to one eyethe same eye
Dichoptic: mask and word presented to different eyes
Noise mask must be
presented to same eye as to-
be-masked stimulus
Pattern mask can be presented
dichoptically
=> The effect of a pattern
mask on visual processing
occurs more centrally than theeffect of noise masking
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Marcel (1983) Experiment Four
N=12 subjects , three masking conditions
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RT (msec) Unassocd Associated Facilitation
Unmasked
control
590 528 +62
Patternmasked
(dichoptic)
597 541 +56
Noise-
masked(monoptic)
586 582 +4
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Cheesman & Merikle (1984, 1986)In the Marcel experiments, if subjects not confident they definitely saw
something, they may say no on every trial.
=> discrimination performance = 50%
But if subjects are forced to respond yes 50% of time, maybe they
would be above chance?
Maybe Marcel had not found the objective threshold for discrimination?
Cheesman & Merikle presented subjects with one of four colour words
(red,green, etc). On each trial, subjects had to say which word hadbeen presented.
(Now even if subjects believed they saw nothing, have to guess
something.)
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After each block of 40 trials, the SOA was reduced until subjects were
performing at chance.
After each block of trials, subjects estimated how accurate they had been.
- If they felt they had no information whatsoever, they were just
guessing purely randomly, subjects gave an estimate of 25% correct
(=chance expectation).
- If they were certain, they would give an estimate of 100%.
- Subjects could give any value between 25% and 100%.
Subjective threshold = the SOA at which subjects believe they areperforming at chance
Objective threshold = the SOA at which subjects really are performing
at chance.
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Results:
1) The subjective threshold is reached at a higher SOA
than the objective threshold.=> the two thresholds are different
2) Stroop priming is obtained for colour words presented
below the subjective threshold, but NOT below the objective
threshold.
Conclusion: BOTH results indicate unconscious processing below
a subjective threshold.
But no evidence for unconscious processing below an objective
threshold.
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Research since then has confirmed it is easy to get unconscious
perception below a subjective threshold, but difficult below an
objective threshold (for semantic processing)
Greenwald (1992) found that 93% of the cognitive
psychologists he surveyed regarded subliminal perception as
having been demonstrated below a subjective threshold.
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Process dissociation applied to unconscious perception
Jacoby, Toth, Lindsay, & Debner (1992)Procedure:
time event1000 ms ' +'
500 ms 'glove'
50 or 500 ms 'patch' or 'staff'
500 ms 'flare'
500 ms
response pat--
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Exclusion test: Complete the stem with a word that comes to mind but
not any you just saw displayed.
Inclusion test: Complete the stem with one of the words flashed or, if
unable to do so, with the first word that comes to mind.
Another way of testing for the subjective thresholdyou exclude a word
ifyou think you saw it.
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Results:
500ms 50ms Baseline
Exclusion .10 .50 .36
Inclusion .96 .63 .38
For 50 ms words:
Any difference between inclusion and exclusion must be based on
conscious perception:
Prob. of conscious perception = .13
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Results:
500ms 50ms Baseline
Exclusion .10 .50 .36
Inclusion .96 .63 .38
Conscious perception would make exclusion performance BELOW
baseline
At 500ms, exclusion is indeed below baseline
At 50ms, exclusion is ABOVE baseline, so there must have been
unconscious perception
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Inattentional blindness: Information can be below a subjective
threshold when attention is focussed on another object
Mack and Rock 1998
Which arm of the cross is longerhorizontal or vertical?
200
ms
1500
ms
500 ms
60% subjects reported not seeing the word on the third trial
Of these blind subjects, 36% completed a word stem with the
presented word (4% for control). Recognition also above chance.
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Is the subjective threshold just a curiosity or theoretically
interesting?
Need to establish qualitative differences between knowledge above and
below the subjective threshold.Merikle & Joordens (1997)Only two words used: Red/green.
Red orgreen
(variable SOA)
backward mask
&&&&& (in red or green ink)
Task: Name colour of ink. 75% of trials, prime and target incongruent
(e.g. red followed by green ink) ; 25% of trials congruent
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THRESH
suprasub
400
390
380
370
TRIAL
cong
incong
A conscious belief (e.g. I am seeing the word RED) can be combined
with any other belief or desire to which it may be relevant in order to
produce further beliefs or actions (in this case, to be prepared for greenink).
Unconscious knowledge cannot combine with just any other possibly
relevant belief or desire to plan action.
=> Only conscious knowledge is inferentially promiscuous.
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Semantic processing under the objective threshold?
Naccache et al 2005
Patients undergoing brain surgery categorised masked and unmaskedwords as threatening or neutral
The three patients at chance in categorising the masked words
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Electric field potentials recorded in the amygdala to threatening
(red) and neutral (green) words. Blue line = significant difference
Both conscious and subliminal words lead to different amygdala
activations in all three patients
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Neural correlates of consciousness
What is the difference between conscious and unconscious perception
at the level of wetware?
What process in the brain is correlated with conscious awareness?
What brain structure or process corresponds to having higher order
thoughts?
Strategies using brain imaging, e.g. fMRI:
1. Compare conscious perception with no conscious perception when
external stimulus is the same (binocular rivalry)
2. Compare conscious and unconscious perception of the same
stimulus (conscious vs subliminal perception)
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strategy 1. Binocular rivalry
One eye presented with e.g. face, the other with e.g. a house
Both stimuli constantly presented
but one only sees one thing at a time - conscious percept alternates
every few seconds
What brain activity correlates with the alternating consciousexperience?
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FFA = fusiform face Area; PPA = parahippocampal place area
Leopold & Logothetis, 1996
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Activity in V1 not related to conscious perception.
Lumer & Rees (1999):
Conscious perception not just related to activity in specific visual
areas like fusiform
also to prefrontal cortex which is involved in working memory andplanning.
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Strategy 2: compare brain activity between conscious and unconscious
perception of the same stimulus
Dehaene et al 2001
Both conscious and unconscious words activated left fusiform area
(to some degree);
conscious words led to more extensive brain activity, including the
prefrontal cortex.
fMRI words either at objective threshold or clearly visible:
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Allows two interpretations:
1. Conscious awareness depends simply on information being widely
broadcast in the brain (Baars, Dennett, Dehaene)
2. Conscious awareness depends on information reaching a certain area,
e.g. prefrontal cortex (the HOT box?)
L d P i h 2006
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Lau and Passingham 2006:
Need to equate objective performance between conscious and
unconscious and only subjective experience differ
- otherwise performance confounded with consciousness
Two conditions: Visual discrimination task with same level of
objective performance but different probability of thinking one saw
the stimulus
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Activation difference between conscious and unconscious now very
specific: left Mid dorsolateral prefrontal cortex
HOT box responsible for creating accurate higher order thoughts?