Unconscious processing revealed by visual masking

1. Unconscious processing revealed by visual masking Chapter 8 Sebastian Dieguez Laboratory of cognitive neuroscience

2. Table • Introductory words • Target recovery (disinhibition) reveals the unconscious mechanism of metacontrast • 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

3. Introduction and quick summary of everything until now • What is masking, when it (may) occur (criterion content, task parameters) • What is masking for? Priming, microgenesis of visual consciousness, attention, perception without awareness (« subliminal » perception). • Controversial? On what grounds? • Methodological • Philosophical • Ethical • Phenomenology • Physiology, models: dual process? M and P channels. Early and late components (feed-forward: stimulus-dependent; re-entrant: perception-dependent).

4. The unconscious mechanism of metacontrast suppression revealed bytarget recovery A secondary mask (M2) yields interesting effects on the visibility of both the primary mask and the target. Breitmeyer et al. (1981): « this study not only bears significantly on our understanding of consciousness and its neural correlates but also has important theoretical implications. » – M1 – M2 T Task: judge the apparent contrast of either T or M1 (in separate blocks) by picking one of 10 square patches ranging from black to white in a greyscale.

5. (Recovery) Log change in contrast visibility (Masking) Results Introducing M2 produces a double dissociation: At negative SOAs, M1’s visibility is unaffected, but it progressly loses its suppressive powers. At positive SOAs, M1 visibility is suppressed, but still it keeps its capacity to mask T. Hence, a stimuli visibility and masking effectiveness are different things. M1-M2 onset asynchrony (ms)

6. So what? • It seems that the mask does not « kill » the target after all. Or is it « resuscitated »? Moreover, a mask need not be visible to be able to mask. • This double dissociation between visibility and masking effectiveness is taken as a « strong indication for the existence of two separate neural mechanisms or processes ». • M1 gives rise to two processes: • « S »: leads to masking of the target, can be suppressed by M2, does not contribute to « the qualia-rich contour and contrast visibility of M1 » and « does not register in visual awareness » • « V »: contributes to the qualia-rich awareness of M1 and is not involved in masking. • This is immediately related to: • the M and P pathways • Crick and Koch (2003) • Milner and Goodale (1995) • Blindsight • « pure phi » sensations in apparent motion. • Evidence of convergence of M and P activities in normal subjects. Some processes in the brain lead to awareness, others don’t.

7. Unconscious processing ofobject location • Remember (chap 2): masking effectiveness is all about “criterion content” and the specifics of the experimental task. • RTs for a simple detection task are not affected by changes in SOA in metaconstrast studies (Fehrer et al, 1962). This is said to be a “counterintuitive finding relating simple reaction time detection to lack of subjective awareness”. • Even choice RTs, involving the location of the target, do not vary with SOA. • 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: • « 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 ».

8. Again: “that the target cannot be seen at optimal metacontrast SOAs implies that the detection of location 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 of processing” “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-rich identity information is optimally suppressed » So, what exactly is unconscious here? Why should something be unconsciously processed just because it is residual? Why should identity be qualia-rich and « a transient blip » qualia poor?

9. From Klotz & Wolf, 1995 Unconscious priming byForm Object identity processing involves the ventral pathway, which is associated with “perception or conscious vision”. But still, “unconscious form and color processing may occur at several levels in this pathway”. Task: depress as quickly as possible the key corresponding to the mask presented (which could be either congruent or incongruent with the target). Results: congruent pairings yielded faster RTs. Controls: signal-detection revealed that the targets were not visible at all. Conclusion: the priming effect of the target (facilitative or interfering) occurred at an unconscious level of processing.

10. Unconscious priming byColor Schmidt (2000, 2002): choice RTs for mask color are facilitated when the priming target is color-congruent. 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 was perceptually more confusable with the blue than the green target ». • Hence, the priming by color is wavelenght (stimulus) dependent, not color (percept) dependent. Indeed, the white target was generated with red, blue and green phosphors of the monitor, and « the green phosphor contributed the largest luminance component of the white target.

11. Applying the target recovery paradigm to priming studies • What if an additional mask (M2) is presented before the target (prime) and the primary mask (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 the target prime reduces the color priming effect of the target on the subsequently presented metacontrast mask. » • unconscious priming occurs at early wavelenth-dependent levels of processing. • Again, surface and contour are processed differently (Chap. 2). But both can be primed: are there different kinds of « unconscious processes »?

12. Unconscious processing andattention • Unconscious priming effects of numbers and emotional faces require attention. • The blindsight phenomenon is facilitated if the patient’s attention is directed to the blind field. • Masked stimuli like arrows can prime the deployment of attention to a specific location. • Therefore, attention and consciousness are different things. • Attention seems needed for A-consciousness, but this is not obvious for P-consciousness (Block, 1995).

13. Target visibility (% correct) Masking visual targets withTMS • Corthout et al. (1999): Identification of a target letter with varying SOAs of TMS pulse. • Two masking maxima: • early feedforward activation. • re-entrant feedback (100ms).

14. A comparison between visual masking and TMS masking • Breitmeyer et al (2004) compared the results of a typical para- and metacontrast masking study with the Corthout et al (1999) study. • they normalized target visibility for both studies • they aligned the visual masking SOA of 0ms with the TMS SOA of 60ms. • the two masking functions “agree to a surprising extent, especially regarding the SOA at which masking maxima occur”. TMS works for masking studies in a similar (type B) fashion as metacontrast masking.

15. Stimulus visibility (% correct) Annulus-TMS SOA (ms) TMS and the disinhibition or recovery of visually masked targets Ro et al. (2003): an optimal metacontrast SOA of 42ms with an additional TMS pulse at varying SOAs. • Results • As TMS suppresses the visibility of the mask, the target’s visibility increases (recovery). • TMS masking function is too coarse, it disrupts both M and P activity, thus the mask’s visibility AND its masking effectiveness are destroyed. • Classic metacontrast is more specific than TMS, hence it is more useful when applied to the “study of unconscious and conscious processes in vision”.

16. 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), the other 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 recover inhibited stimuli.

17. So, what did we learn about « unconscious processes »? • actually, not much. • the notion of « unconscious process » is never defined, so it allows the discussion of lots of things that hardly seem related. • There is much confusion about different kinds of approaches to consciousness: • 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.

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