Trends and styles in visual masking

The masked prime task was used to investigate low-level inhibitory motor control processes in two groups of children (7-8 years and 11-12 years) and in older adolescents/young adults (16-23 years). Masked prime stimuli, presented below the level of conscious awareness, systematically affected reaction times (RTs) to subsequent supraliminal target stimuli: RTs were longer when prime and target were mapped to the same response than when they were mapped to different responses. This negative compatibility effect did not differ significantly between age groups, consistent with the hypothesis that the underlying low-level inhibition processes are already fully developed in children as young as seven years of age. In contrast, performance differences between response repetition and response alternation trials were significantly larger in children, consistent with the hypothesis that higher-level control processes are less effective in children. Results provide converging evidence that whereas the latter processes are mediated by late-maturing (prefrontal cortical) areas, the former processes are mediated by earlier-maturing (possibly subcortical) structures.

Early components of visual evoked potentials (VEP) in EEG seem to be unaffected by target visibility in visual masking studies. Bridgeman's reanalysis of Jeffreys and Musselwhite's (1986) data suggests that a later visual component in the VEP, around 250 ms reflects the perceptual effect of masking. We challenge this view on the ground that temporal interactions between targets and masks unrelated to stimulus visibility could account for Bridgeman's observation of a U-shaped time course in VEP amplitudes for this later component. In an MEG experiment of metacontrast masking with variable stimulus onset asynchrony, we introduce a proper control, a pseudo mask. In contrast to an effective mask, the pseudomask should produce neither behavioral masking nor amplitude modulations of late VEPs. Our results show that effective masks produced a strong U-shaped perceptual effect of target visibility while performance remained virtually perfect when a pseudomask was used. The visual components around 250 ms after target onset did not show a distinction between mask and pseudomask conditions. The results indicate that these visual evoked potentials do not reveal neurophysiological correlates of stimulus visibility but rather reflect dynamic interactions between superimposed potentials elicited by stimuli in close temporal proximity. However, we observed a postperceptual component around 340 ms after target onset, located over temporal-parietal cortex, which shows a clear effect of visibility. Based on P300 ERP literature, this finding could indicate that working memory related processes contribute to metacontrast masking.

When objects denoted by target words are classified as belonging to a certain category (e.g., to be either small or large) responding is faster when the target word is preceded by a masked prime word belonging to the same rather than a different category. Recently, there has been some controversy on whether such masked priming effects are confined to primes that are practised as targets as well, or whether they transfer to other novel prime words. We report data which show that the transfer of unconscious priming to unpractised stimuli depends on the size of the target set. Priming does transfer to novel (unpractised) primes with a large target set (40 different target words), whereas no transfer to novel primes occurs with a small target set (4 different target words). We conclude that the size and structure of the target set crucially determine the way participants handle a task and thus, determine how unconscious stimuli are processed.

The priming of motor responses can be induced by preceding visual stimuli that have been made invisible by metacontrast masking ('primes'). According to the concept of direct parameter specification (DPS; Neumann, 1990), strong similarity between prime and target results in the processing operations that are to be applied to the target being also induced by the prime. As targets have to be attended to, this also implies that attention is captured by the location of a prime, thereby facilitating motor priming effects. This hypothetical effect may be viewed as a form of top-down attentional capture. In some subliminal priming experiments (e.g. Jaśkowski, Skalska, & Verleger, 2003), however, attentional capture may have been unrelated to target identity, as stimuli with unique features (singletons) are known to induce bottom-up attentional capture. Three experiments were performed that largely confirmed the view that the results of these earlier experiments were due to top-down attentional capture, in line with DPS. However, the priming effect was also evoked by a singleton irrelevant to the participants' task, although this effect was weaker than in case of strong similarity between prime and target. Priming effects remained when singletons were absent from one side of the visual field, suggesting that the presence of singletons is not a requirement for the observation of motor priming effects.

In the current study, we tested whether a masked and, thus, invisible singleton-cue captures attention in a stimulus-driven manner or in a top-down contingent manner. The manual RT (Reaction Time) capture effect with the invisible singleton-cue decreased substantially when a match between the singleton-cue and the top-down controlled set of searched-for target features was also decreased. By contrast, with the PCN (Posterior Contralateral Negativity), an electrophysiological measure of the capture of visuospatial attention by the invisible singleton-cue, no significant decrement was observed. Taken together, the results support the assumption that an invisible singleton-cue can capture attention in a stimulus-driven manner, and that different delays in the deallocation of attention (i.e., attention is deallocated more efficiently from acue that does not match the top-down controlled set than from a cue that does match the same set) account for the weaker manual RT capture effect with a set-nonmatching invisible singleton-cue.

Feature integration is one of the most fundamental problems in neuroscience. In a recent contribution, we showed that a trailing grating can diminish the masking effects one vernier exerts on another, preceding vernier. Here, we show that this temporal unmasking depends on neural spatial interactions related to the trailing grating. Hence, our paradigm allows us to study the spatio-temporal interactions underlying feature integration.

The phenomena of illusory line motion and perceptual latency priming are both assumed to reflect a facilitation of perceptual latency.The explanation of illusory line motion presupposes that attention is distributed in a gradient fashion whereas this is not a necessary part of the explanation of perceptual latency priming. Two experiments test whether an attentional gradient is present in perceptual latency priming. Evidence for a gradient was found within 2.5° of visual angle around the attended location, but not at a distance of 5° and more.