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.
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