Visibility, type of masking, prime duration and task as determinants of effects of masked stimuli

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Recent theories state that consciousness is related to the integration and accessibility of information. From this it follows that mental functions exist which do not operate at all or at least operate in an impaired way when consciousness is impaired. More specifically, processing of simple (non-integrated) stimulus features should be independent from consciousness whereas semantic processing of complex (integrated) features should increase with an increasing consciousness of the stimuli. A series of empirical reports support this view. Thus, early perceptual or motor priming effects do not depend on the prime visibility whereas semantic priming effects seem to depend on prime visibility. However, in many studies the manipulation of prime visibility has been confounded with the duration of the primes because trials with short and long prime durations have been compared as unconscious and conscious conditions, respectively. Additionally, semantic studies most often used pattern masking which might produce an artificial correlation between priming effects and prime visibility. Independence of motor priming effects and visibility has been reported frequently from studies that used metacontrast masking. And indeed, our pilot studies show that semantic and motor priming effects depend on prime visibility when pattern masking is used, but not with metacontrast masking. Beyond this, the relation between priming effects and prime visibility could also depend on the processing of the primes which is needed to generate priming effects. The planned project comprises 13 experiments which systematically compare the effects of three different masking procedures (pattern masking, metacontrast, and crowding) when the same stimuli are used in different tasks. Within each experiment, prime visibility is varied by masks of a specific type but with different masking strength and the relationship between priming effects and visibility is recorded. If the processing of the primes in a certain task depends on consciousness of the primes the priming effect should also depend on prime visibility. To grasp also discontinuous relationships, the time that is given to the prime to generate its effects is varied. Functional magnetic resonance imaging is used to examine whether the prime induced cortical activation at later levels of processing in the visual hierarchy is systematically determined by the type of masking procedure used. In this way the expansion of the effects of masked stimuli in the brain and the loci of masking can be directly investigated. The studies of this project contribute to confine the search for the function of consciousness and to improve our understanding of the mechanisms that produce priming effects and the different types of masking effects.