Subliminal Perception 2008

7/28/2019 Subliminal Perception 2008

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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?

Subliminal Perception 2008

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