Unconscious Cognition 2-Going Beyond Zero Awareness

In the first part "Unconscious Cognition 1- Simple Dissociation", we established two sets of assumptions for the zero awareness criterion. The exhaustiveness assumption where the direct measure D is a strictly monotonic function of conscious information c and a weakly monotonic function of unconscious information u and the indirect measure I is a weakly monotonic function of both c and u. Under these conditions, D(c,u)=0 necessitates c=0 and I(c,u)>0 implies I(0,u)>0 which in turn implies u>0. In such a case, I is a perfect measure of unconscious processing. The second assumption was the exclusiveness assumption, where I  is an exclusive weakly monotonic function of u alone, and is unaffected by c. And so, the exclusiveness assumption abolishes the need for a direct measure altogether 

An interesting way to circumvent these  assumptions is to let awareness vary over experimental conditions. It may then be possible to establish a double dissociation, which consists of finding an experimental manipulation that changes D and I in opposite directions (see. Fig 2a). In particular, any pair of experimental conditions that leads  to opposite ordering of data points in direct and indirect measures  gives evidence for double dissociation. One example could be a priming experiment with two (or more) masking conditions where the priming effect (indirect measure) increases while prime identification (direct measure) performance decreases over experimental conditions. It is obvious that two measures of information going in opposite directions cannot be monotonically driven by a single information source (see Schmidt and Vorberg (2006) for a formal proof). 

Double dissociations have surprising features. Firstly, they require D to be non-constant. In order to obtain a double dissociation, variations in awareness over experimental conditions must occur so that there is a non-zero awareness of the prime under atleast some conditions. Also, the assumptions are less stringent. The only assumption we make here is that both D and I are weakly monotonic in c (see Fig 2b). One can even drop the weak monotonicity assumption on u, allowing for c and u to produce arbitrary interactive effects on D and I for instance, c and u could be mutually inhibitory).

An example of Simple Dissociation

There are numerous experiments demonstrating simple dissociation. In one such experiment (Vorberg, Mattler, Heinecke, Schmidt and Schwarzbach (2003, 2004)), participants were asked to make speeded keypress responses to the direction of an arrow-shaped masking stimulus that was preceded by an arrow-shaped prime. The mask has a dual purpose. It acts as the target of the response and at the same time, it reduces the visibility of the prime by metacontrast masking (a form of visual backward masking). As the stimulus onset asynchrony (SOA) between prime and mask increased, the priming effect (indirect measure) also increased with primes pointing in the same direction as the mask shortening the response times while primes pointing in the opposite direction lengthening them. strikingly, this priming response was independent of visual awareness of the prime.  This was determined by using stimulus conditions that produced different time-courses of metacontrast masking. Participants were unable to perform better than chance when asked to point the direction of the prime.

 An example of Double Dissociation

In a second experiment by the same group, all four pairings of short-duration (14 ms) and long-duration (42 ms) primes and masks were compared, yielding very different kinds of masking functions. When 14 ms primes were combined with 42 ms masks (14:42), the prime identification performance was low and increased only slightly with SOA. When mask duration was reduced to 14 ms (14:14), performance was better. When a 42 ms prime was paired with a 14 ms mask (42:14), performance was nearly perfect. But the 42:14 condition  yielded an effect called type-B masking where prime identification performance markedly decreases with prime-mask SOA, then increases again, while the priming effect only increases monotonically all throughout producing a strong double dissociation.

Reference

Schmidt, T. (2007). Measuring unconscious cognition: Beyond the zero-awareness criterion Advances in Cognitive Psychology, 3 (1), 275-287 DOI: 10.2478/v10053-008-0030-3

Unconscious Cognition 1-Simple Dissociation

Attempts to demonstrate unconscious processing of visual stimuli are very old and riddled in controversies, but the controversy does not so much concern the existence of unconscious processing (most researchers seem to be convinced of this), but rather the question of how to demonstrate unconscious processing in a given experiment. If one needs to demonstrate it, one has to make sure that a critical stimulus was completely outside of awareness (the so called zero-awareness criterion). Schmidt (2007) proposes two lines of attack for establishing unconscious processing beyond the zero-awareness criterion. The paper deals with different types of dissociation between measures of awareness and measures of processing.

Simple Dissociations

To demonstrate that a critical stimulus was processed unconsciously, one usually produces some dissociation between different behavioral measures of performance. This is done by comparing two measures obtained from different tasks. One measure (called the direct measure, D) signals the observer's awareness of a critical stimulus. For example, consider a visual perception experiment, where the task is to detect the offset of a vernier (a vernier is simply a set of two vertical lines, one below the other, where the lower line can be offset either to the right or left of the upper line). However, before the vernier is shown (let's say for 25 msec), a prime is flashed for a short period of time (say 15 msec). This prime could be, for example, arrows pointing to the right or left. A forced-choice prime discrimination task ("Was the arrow to the right or left" ) would measure if the observer was aware of the stimulus, and hence would comprise the direct measure. The second measure (called the indirect measure, I) would indicate that the primes themselves are not consciously detected, but they are involved in a priming effect, and hence affect the reaction times of responding to whether the target vernier is offset to the right or left. For example, if the prime and vernier are both congruent (i.e. the arrow points to the right and the vernier is also offset to the right) and if the subject cannot consciously detect the direction of the prime but the reaction times during the congruent cases are always shorter than the reaction times during the incongruent cases (prime pointing to the right but vernier offset to the left), then this would comprise the indirect measure.

Schmidt and Vorberg (2006) examined the assumptions required by the zero-awareness criterion and other approaches. They start by assuming that the direct and indirect measures may depend on two sources of information labeled conscious (c)and unconscious (u). In other words, D = D(c,u) and I = I(c,u), where information is non-negative. The dependency is weakly monotonic, which means that if any  type of information increases, the measures can only increase or remain constant. An additional constraint is that if D and I are to be modeled as functions of c and u, then both D and I must be functions of the same underlying conscious and unconscious information. Thus the direct and indirect tasks must be designed to use identical stimuli, identical responses and identical stimulus-response mappings. Schmidt gives an example of this mismatch in the study by Dehaene et. al (1998), where the indirect task was to determine as quickly as possible if a target digit was numerically larger or smaller than 5. The target was preceded by a masked prime digit. The response times were much shorter if the prime was consistent with the target (i.e. both were less than 5 or greater than 5) than when the two were inconsistent (one of them was larger than 5 and the other was smaller). The optimal direct task in this experiment would have been to ask the subject "Was the prime greater than or lesser than 5"  because then, both measures would've been tapping into the same source of information (conscious or unconscious). Instead, Dehaene et. al chose two different direct tasks where the subject was asked to detect primes against an empty background, and the second where the subject was asked to discriminate primes from random strings of letter, none of which addressed the critical question of whether the prime was larger or smaller than 5.

Given that D-I mismatch can be avoided, how can the null model of only conscious processing be disproved. The traditional way is the zero-awareness criterion, which produces what is called a simple dissociation of direct and indirect measure (zero D in presence of non-zero I, see Fig. 1a). In other words, I(c,u)> 0, implies that either c > 0, or u > 0, or c and u > 0. But, does, D(c,u) = 0, imply c = 0  and hence I(c,u) > 0 only because u > 0 ? No, because, D is a weakly monotonic function of c. It is possible that under this assumption, c did change but D was not sensitive enough to pick up this change. To resolve this problem, one must make the stronger assumption (see Fig. 1b) that D is a strictly monotonic function of c, which means that D can detect any changes in c, no matter how small. Hence, D(c,u) = 0 implies c = 0, irrespective of whether u is zero or not, since D is weakly monotonic on u anyway. And now, if  I(c,u) > o, then, I(0,u) > 0 which finally implies u > 0.

The exhaustiveness assumption is important. In it's absence, one can always argue that it is only conscious processing c, that is occurring and while I is sensitive enough to detect it (I > 0), D is not (D=0). Finally, there is an alternative set of assumptions that abolishes the need for an direct measure altogether. This is when the indirect measure can be assumed to be an exclusive monotonic function of u and is unaffected by c.  In this case, I(c,u) = I(u) > 0 implies u > 0 directly (see Fig. 1c)  

In the next post, we shall look at double dissociation and beyond

Reference

Schmidt, T. (2007). Measuring unconscious cognition: Beyond the zero-awareness criterion Advances in Cognitive Psychology, 3 (1), 275-287 DOI: 10.2478/v10053-008-0030-3

On the Integration of Tactile, Proprioceptive and Visual Signals by the Brain

The integration of different sensory inputs in the brain is crucial not only for taking appropriate motor actions but also for body perception and awareness of the bodily self. Integration occurs in higher areas in the brain usually in areas belonging to the parietal lobe

1) Integrating Vision and Proprioception in Area 5 ( Graziano et. al (2000))

Brodmann Area 5 (or Area 5) is part of the parietal cortex in humans, and in monkeys, is a subdivision of the parietal lobe, occupying primarily the superior parietal lobule. Graziano et. al (2000) studied single neuron responses from Area 5 of monkeys. In their experiments, the arm contralateral to the recording site was outstretched. The real arm of the monkey was covered, and instead a realistic fake hand was kept in view.  The experimental design was 2 x 2

2 x 2 experimental design. Cross indicates fixation spot. The gray arm is the

 fake arm. The real arm is covered in the experiment (Graziano et. al (2000))

Single neuron recordings indicated that the firing rate of individual neurons depends not only on the position of the real arm, but also on the position of the fake arm. The neuron is significantly affected by the  position of the real arm, firing more when he real arm is to the left. Additionally, the firing rate further increases when the fake arm is also to the left. 

Single neuron recording from Area 5.  The firing rate is maximum when 

the real and the fake arms are in congruent position (Graziano et. al (2000))

2) Integrating Vision and Touch in Ventral Intraparietal Area (Duhamel et. al (1998))

The Ventral Intraparietal Area (or VIP) is a discrete area in the depths of the intraparietal sulcus. Duhamel et. al carried out single neuron recordings in the VIP. They found the neurons to possess a bimodal receptive field. Not only did they respond to a combination of visual and tactile stimuli, but the bimodal receptive fields were arranged in an orderly manner.

Bimodal receptive fields of VIP neurons (Duhamel et. al (1998))

Small central visual receptive fields  were associated with small tactile receptive fields on the muzzle, whereas large peripheral receptive fields were associated with  large tactile receptive fields on the side of the head or body. The neurons also demonstrated direction selectivity, in the sense that a visual or tactile stimulus moving in one direction was preferred over the other, and this preferred direction of visual and tactile stimuli coincided in the majority of cells. 

Direction selectivity of VIP neurons (Duhamel et. al (1998))

References

Graziano, M. (2000). Coding the Location of the Arm by Sight Science, 290 (5497), 1782-1786 DOI: 10.1126/science.290.5497.1782

Duhamel JR, Colby CL, & Goldberg ME (1998). Ventral intraparietal area of the macaque: congruent visual and somatic response properties. Journal of neurophysiology, 79 (1), 126-36 PMID: 9425183