Subliminal Perception

Zoltán Dienes Conscious and unconscious mental processes

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

Noise mask:

Each pixel can be randomly on or off

MXgFhhY

Pattern mask:

Random collection of the parts of the stimulus to be masked – e.g. letters or letter parts for word stimuli

Monoptic: mask and word presented to one eye – the 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 the effect of noise masking

Marcel (1983) Experiment Four

N=12 subjects , three masking conditions

RT (msec) Unassoc’d Associated Facilitation

Unmasked control

590 528 +62

Pattern masked (dichoptic)

597 541 +56

Noise-masked (monoptic)

586 582 +4

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 had been presented.

(Now even if subjects believed they saw nothing, have to guess something.)

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 are performing at chance

Objective threshold = the SOA at which subjects really are performing at chance.

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.

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.

Process dissociation applied to unconscious perception

 

Jacoby, Toth, Lindsay, & Debner (1992)

Procedure:

 

time event

1000 ms ' +'

500 ms 'glove'

50 or 500 ms 'patch' or 'staff'

500 ms 'flare'

500 ms

response pat--

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 threshold – you exclude a word if you think you saw it.

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

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

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 longer – horizontal 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.

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” or “green”

(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

THRESH

suprasub

Mea

n R

T

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 green ink).

Unconscious knowledge cannot combine with just any other possibly relevant belief or desire to plan action. 

=> Only conscious knowledge is inferentially promiscuous.

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)

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 conscious experience?

FFA = fusiform face Area; PPA = parahippocampal place area

Leopold & Logothetis, 1996

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 and planning.

Strategy 2: compare brain activity between conscious and unconscious perception of the same stimulus

Dehaene et al 2001

fMRI:

Both conscious and unconscious words activated left fusiform area (to some degree); conscious words led to more extensive brain activity, including the prefrontal cortex. (Experiment 2 showed words were processed to the level of identity.)

Allows two interpretations:

1. Conscious awareness depends simply on information being widely broadcast in the brain (Baars, Dennett, Dehaene) (But would using a purely subjective rather than obj threshold lead to more activation?)

2. Conscious awareness depends on information reaching a certain area, e.g. prefrontal cortex (the HOT box?)