Unconscious Processing and Masking: Metacontrast and TMS Studies, Slides of Ophthalmology

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Unconscious processing

revealed by visual masking

Table • Introductory words• Target recovery (disinhibition) reveals the unconscious mechanism ofmetacontrast• Object location can be unconsciously processed• What about object identity? Form and color processing in priming studies.• Attention and consciousness• Comparing TMS and metacontrast masking.• Summary and conclusions

The unconscious mechanism of metacontrast suppressionrevealed by

target recovery

T

Task: judge the apparent contrast of either T or M1 (in separate blocks) bypicking one of 10 square patches ranging from black to white in a greyscale.

A secondary mask (M2) yields interesting effects on the visibility of both the primarymask and the target.

• M1 – M

Breitmeyer et al. (1981):« this study not only bears significantly on our understanding of consciousness and itsneural correlates but also has important theoretical implications. »

Log change in contrast visibility

(Recovery) (Masking)

M1-M2 onset asynchrony (ms)

Results

Introducing M2 produces adouble dissociation:At negative SOAs, M1’svisibility is unaffected, but itprogressly loses itssuppressive powers.At positive SOAs, M1visibility is suppressed, butstill it keeps its capacity tomask T.Hence, a stimuli visibilityand masking effectivenessare different things.

Unconscious processing of object location Yet, when subjects have to judge the

identity

of the target, RTs vary with SOA in a

type B function.How is this possible? Because of the special

phenomenology

of visual masking:

Remember (chap 2): masking effectiveness is all about “criterion content” andthe specifics of the experimental task.• RTs for a simple • « a sensation of objectless « explosive » or « split » apparent motion »• « a paradoxical reversal of the target’s perceived contrast »• « a non-moving transient « blip » in the target area ».

detection

task are not affected by changes in SOA in

metaconstrast studies (Fehrer et al, 1962). This is said to be a “counterintuitivefinding relating simple reaction time detection to lack of subjective awareness”.• Even

choice

RTs, involving the

location

of the target, do not vary with SOA.

Again: “that the target cannot be seen at optimal metacontrast SOAs implies that the detection oflocation can rely on unconscious processes”“these unconscious processes correspond to activity in the transient [dorsal] M pathway”“[this is] the most likely cortical stream supporting the “zombie” or unconscious mode ofprocessing”“these results [indicate] a dissociation between motor response and conscious percept” However

« such qualia-suppressed target location

representations

[show] that some

residual target

information

is immune to metacontrast masking »

« observers are able to detect, on the basis of

residual mask-immune information

, the mere

presence or location of the target at metacontrast SOAs at which perception of its qualia-richidentity information is optimally suppressed »So, what exactly is unconscious here? Why should something be unconsciouslyprocessed just because it is

residual

? Why should identity be qualia-rich and « a

transient blip » qualia poor?

Unconscious priming by Color

Breitmeyer et al. (2004,

Psych Sci

): same study but with

desaturated colors and a white « neutral prime ».

Results • there is a clear congruency/incongruency effect.• the white disk acted more like a green than a blue one,as shown by an «

incongruency effect

that was

significantly larger when the following mask was blue

than when it was green ».

• Additional controls showed that « the white target wasperceptually more confusable with the blue than the greentarget ».• Hence, the priming by color is wavelenght (stimulus)dependent, not color (percept) dependent. Indeed, thewhite target was generated with red, blue and greenphosphors of the monitor, and « the green phosphorcontributed the largest luminance component of the whitetarget. Schmidt (2000, 2002): choice RTs for mask color are facilitated when the primingtarget is color-congruent.

Applying the target recovery paradigm to priming studies What if an additional mask (M2) is presented before the target (prime) and the primarymask (T-M1)? Form: Breitmeyer et al (2004,

Consc & Cogn

): the priming effect on choice RTs for form

remained the same than without M2. 

earliest cortical response levels from V1 ruled-out as the sites were unconscious form

priming occurs. Color: Contrary to form, the paracontrast mask has an effect on priming.Bretimeyer et al. (2004): « a mask presented at optimal paracontrast SOAs preceding thetarget prime reduces the color priming effect of the target on the subsequently presentedmetacontrast mask. » 

unconscious priming occurs at early wavelenth-dependent levels of processing.

Again, surface and contour are processed differently (Chap. 2). But bothcan be primed: are there different kinds of « unconscious processes »?

Masking visual targets with TMS

Corthout et al. (1999):Identification of a target letterwith varying SOAs of TMSpulse.Two masking maxima:• early feedforward activation.• re-entrant feedback (100ms).

Target visibility (% correct)

TMS works for masking studies in a similar (type B) fashion asmetacontrast masking.

A comparison between visual masking and TMS masking

Breitmeyer et al (2004) compared theresults of a typical para- and metacontrastmasking study with the Corthout et al(1999) study.• they normalized target visibility for bothstudies• they aligned the visual masking SOA of0ms with the TMS SOA of 60ms.• the two masking functions “agree to asurprising extent, especially regarding theSOA at which masking maxima occur”.

Summary and comments

Double dissociation between masking effectiveness and mask visibility

Subpersonal processes are operating in the brain. • Object detection and location is immune to masking Something that is less “rich” than something fully conscious • Form and color have a priming effect on a subsequent mask Different levels of the temporal flow of information in the visual cortex. • Attention and consciousness are not the same because attention has effects on,and is modulated by « unconscious processes » There are two kinds of consciousness, one requires attention (A-consc), theother we don’t know what it is (P-consc). • TMS pulses have similarities and differences with metacontrast masking A « new » technique to suppress stimuli from awareness and to recoverinhibited stimuli.

• actually, not much.• the notion of « unconscious process » is never defined, so it allows thediscussion of lots of things that hardly seem related.• There is much confusion about different kinds of approaches toconsciousness:
  • processes and their outcome• early and late components in V1,• different processing pathways,• richness of qualia,• blindsight studies,• philosophical theory (Block),• methodological issues.
    • This chapter will be disappointing for a whole range of people.

So, what did we learn about « unconscious processes »?