how to manipulate and measure visual consciousness
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Curator: Dr. Naotsugu Tsuchiya, California Institute of Technology, Pasadena, CA, USA
Curator: Dr. Christof Koch, Division of Biology, Caltech, Pasadena, CA
Contents |
How to manipulate and measure visual consciousness
Visual consciousness can be manipulated using a multitude of illusions, such as backward masking, the standing wave of invisibility, visual crowding, bistable figures, binocular rivalry, flash suppression, continuous flash suppression (Tsuchiya and Koch, 2005; Tsuchiya et al., 2006), motion-induced blindness and attentional blink (for a review see Kim and Blake, 2005). These techniques control the visibility of an object or a part thereof in both space and time. Yet how is visibility assayed? More generally, how can the degree of consciousness be probed?
Subjective measures
The most lenient criterion is to accept what subjects subsequently report verbally; e.g., “I never saw the face.” Though widely used (such as when obtaining reports immediately after an fMRI session), this method is unsatisfactory because unattended items or task-irrelevant (implicit) features of stimuli may be inaccessible in subsequent recognition or recall tasks (Sperling, 1960; Wolfe, 1999; Landman et al., 2003). A somewhat more stringent criterion for non-conscious processing is to ask subjects about their experience directly at the time the stimulus is processed. When subjects deny seeing a stimulus, the implication is that it was processed at a subjectively non-conscious level. Although many studies involving non-conscious states adopt this convention, the definition suffers from the possibility of criterion shifts: for the same subjective experience of visibility, some subjects may deny seeing a stimulus while others may report seeing it, because their criterion of what to count as “seen” differs (Kunimoto et al., 2001).
Objective measures
The strictest procedure is to demonstrate null sensitivity using an appropriate overt behavioral measure, that is, d’ = 0. For example, subjects can be given two alternative temporal intervals (or locations), each of which contains the stimulus equally often. If they are at chance in detecting/discriminating one from the other, they are (objectively) unaware of the stimulus (our use of “subjective” and “objective” here refers to the method used, not to the nature of the conscious experience, which is of course always subjective in terms of its phenomenology). Note that above-chance behavioral discrimination performance does not necessarily demonstrate conscious awareness, since patients with blindsight exhibit precisely such performance.
Objectively measuring subjectivity
However, such an objective definition does not directly reflect phenomenal experience, which is the central issue. By applying the objective measure of signal discriminability to one’s own judgment of whether the stimulus is seen or not, one can objectively measure subjectivity. That is, one can consider the discriminability (d’ or area under the ROC curve (A')) of one’s own experience. For this method, subjects first make a detection/discrimination judgment, then rate the confidence in their decision, where confidence is presumably based on whether their phenomenal experience was strongly, moderately, weakly or perhaps not at all in support of their decision. Defining ‘hit’ as proportion of high confidence ratings given that the decision was correct - p(high confidence | correct) - and ‘false alarm’ as the proportion of high confidence ratings given that the decision was incorrect - p(high confidence | incorrect) - one can calculate the discriminability (d’ or area under the curve) of those instances where phenomenal experience informed the decision from those where it did not. In signal detection theory, this is called Type 2 analysis (Galvin et al., 2003), and it has been applied to evaluation of above chance behavior in non-conscious perception (Kolb and Braun, 1995; Kunimoto et al., 2001).
Post-decision wagering
However, reflecting upon one’s own judgment may require substantial internal focus and such an act itself can modify conscious experience significantly (Maia and McClelland, 2004). With a recently proposed new method, post-decision wagering, this contamination due to introspection can be minimized (Kunimoto et al., 2001; Persaud et al., 2007). Following each response, subjects wager on their performance, betting either high or low. If the subject is confident that she saw the stimulus, reward maximization would presume that she would wager a higher amount than when she is unaware of the stimulus and is guessing.
Here, subjects’ awareness is gauged by the discriminability of their own judgment. This method proves to be easy and intuitive for subjects to use and very effective in reflecting one’s subjective aspects of consciousness while minimizing interference to the quality of the experience. Persaud and colleagues (Persaud et al., 2007) observed above-chance behaviors in blindsight patients, and in implicit learning and Iowa gambling tasks with normal subjects while demonstrating, using post-decision wagering, that there was little or no conscious access to the information that informed those behaviors.
References
- Galvin SJ, Podd JV, Drga V, Whitmore J (2003) Type 2 tasks in the theory of signal detectability: discrimination between correct and incorrect decisions. Psychon Bull Rev 10:843-876.
- Kim CY, Blake R (2005) Psychophysical magic: rendering the visible 'invisible'. Trends Cogn Sci 9:381-388.
- Kolb FC, Braun J (1995) Blindsight in normal observers. Nature 377:336-338.
- Kunimoto C, Miller J, Pashler H (2001) Confidence and accuracy of near-threshold discrimination responses. Conscious Cogn 10:294-340.
- Landman R, Spekreijse H, Lamme VA (2003) Large capacity storage of integrated objects before change blindness. Vision Res 43:149-164.
- Maia TV, McClelland JL (2004) A reexamination of the evidence for the somatic marker hypothesis: what participants really know in the Iowa gambling task. Proc Natl Acad Sci U S A 101:16075-16080.
- Persaud N, McLeod P, Cowey A (2007) Post-decision wagering objectively measures awareness. Nat Neurosci 10:257-261.
- Sperling G (1960) The information available in brief visual presentations. Psychological Monographs 74:1-29.
- Tsuchiya N, Koch C (2005) Continuous flash suppression reduces negative afterimages. Nat Neurosci 8:1096-1101.
- Tsuchiya N, Koch C, Gilroy LA, Blake R (2006) Depth of interocular suppression associated with continuous flash suppression, flash suppression, and binocular rivalry. J Vis 6:1068-1078.
- Wolfe JM (1999) Inattentional amnesia. In: Fleeting memories (Coltheart V, ed), pp 71-94. Cambridge, MA: MIT Press.
See also
Signal Detection Theory, ROC curve, Post-decision wagering
