There are now extensive behavioral and neuropsychological evidence to indicate that semantic information of a word can be activated without conscious awareness. However, semantic activation alone may not be sufficient for observing semantic priming effects in masked lexical decision task. In the following study, two tasks were used: lexical decision and semantic categorization. Conscious awareness of the prime was systematically manipulated by varying the duration of the prime and by varying the placement of the mask in the prime-target presentation sequence. Priming effects were observed in the semantic categorization task at prime durations of 42 milliseconds but no semantic priming was observed for the same prime duration in the lexical decision task. However, semantic priming effects began to emerge in lexical decision at the longer prime durations (55 & 69 ms) and under the least effective prime-mask presentation sequences. It is proposed that semantic activation alone is not sufficient for semantic priming effects in the lexical decision task but that central executive involvement is necessary, if only at the lowest level, for facilitatory effects to be observed. Furthermore, no such central executive involvement appears to be required for the semantic categorization task. The priming effects obtained in this task is interpreted in terms of a "decision priming" effect.

In the response priming paradigm, a small briefly presented visual stimulus (i.e., prime) is followed by a larger visible stimulus (i.e., mask) that renders the prime invisible and specifies the target location. According to the rapid-chase theory, the initial portion of the movement is dictated by the prime (initiation criterion) while the later portion is dictated by the mask (takeover criterion) and the prime is initially processed independently from the mask (independence criterion). The purpose of the first experiment was to determine if the processing of the prime and mask fit the predictions of the rapid-chase theory when the prime and mask are presented during an ongoing movement. The second experiment was designed to examine the impact of the prime when it is presented at various times throughout the execution of the movement. Participants initiated rapid pointing movements to a center target. On 1/3 of the trials, participants had to correct their movements to the left (or right) target in response to a left-pointing (or right-pointing) mask arrow, which was preceded by a neutral, left-, or right-pointing prime arrow. In Experiment 1, the prime was presented at movement onset and the mask randomly appeared 33, 50, or 67 ms after prime onset. In Experiment 2, the prime followed movement onset with a delay of 17, 33, or 50 ms and the mask was presented 50 ms after prime onset. In both experiments, participants first modified their movements in the direction indicated by the prime before completing their movements to the correct target in the majority of trials; thus, supporting the initiation and takeover criteria. However, the spatial priming effects did not follow the time course predicted by the independence criterion. Overall, the rapid-chase theory does not seem to apply to movement execution, but the prime is still able to influence the movement despite being presented later in the pointing trajectory.

Stimuli that cannot be perceived (i.e., that are subliminal) can still elicit neural responses in an observer, but can such stimuli influence behavior and higher-order cognition? Empirical evidence for such effects has periodically been accepted and rejected over the last six decades. Today, many psychologists seem to consider such effects well-established and recent studies have extended the power of subliminal processing to new limits. In this thesis, I examine whether this shift in zeitgeist is matched by a shift in evidential strength for the phenomenon. This thesis consists of three empirical studies involving more than 250 participants, a simulation study, and a quantitative review. The conclusion based on these efforts is that several methodological, statistical, and theoretical issues remain in studies of subliminal processing. These issues mean that claimed subliminal effects might be caused by occasional or weak percepts (given the experimenters’ own definitions of perception) and that it is still unclear what evidence there is for the cognitive processing of subliminal stimuli. New data are presented suggesting that even in conditions traditionally claimed as “subliminal”, occasional or weak percepts may in fact influence cognitive processing more strongly than do the physical stimuli, possibly leading to reversed priming effects. I also summarize and provide methodological, statistical, and theoretical recommendations that could benefit future research aspiring to provide solid evidence for subliminal cognitive processing. / <p>At the time of the doctoral defense, the following papers were unpublished and had a status as follows: Paper 1: Manuscript. Paper 4: Manuscript.</p><p> </p>

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