The time course of feature integration in plaid patterns revealed by meta- and paracontrast masking.

When a plaid object is presented, the visual system decomposes it into its constituting orientation primitives and integrates them at later processing stages. The present study reveals the time course of this process by applying meta- and paracontrast masking to both simple oriented and plaid gratings. With various stimulus onset asynchronies (SOA) between target gratings and surrounding mask annuli, subjects were asked to identify whether targets were simple gratings collinear to the masks, orthogonal to the masks, plaid, or whether no target was presented. The resulting time courses for each type of stimulus confusion showed that metacontrast peaked when orientation primitives had already begun to be integrated into one object, indicated by a dominance of "no target" responses given to plaid stimuli at SOAs around 70 ms. At SOAs around 10 to 30 ms masking also had a significant impact but acted on separable components, indicated by a dominance of "orthogonal" responses given plaid stimuli. Probability summation of "no target" responses given simple gratings revealed that only at shorter SOAs performance for plaid stimuli could be predicted assuming independent features but not at SOAs of at 50-70 ms. We discuss in how far these results could also be explained by the dynamics of cross-orientation suppression (COS) and how they might relate to the process of feature integration in plaids.

Evidence for rapid prefrontal emotional evaluation from visual evoked responses to conditioned gratings.

The human brain's ability to rapidly identify emotional stimuli is subject of ongoing debate. The 'standard hypothesis' postulates a fast but coarse screening of the stimulus valence in subcortical regions, the amygdala in particular, followed by a precise, cortically driven analysis. Recent electrophysiological studies reported differential effects of conditioned faces in prefrontal regions as early as 60-80 ms after target onset, suggesting considerably faster cortical processing than traditionally assumed. Evidence for rapid prefrontal evaluation was provided specifically for complex and evolutionarily significant stimuli, i.e. faces. Here we used simple gratings in a conditioning paradigm, testing the generalization of these results. Event-related potentials and source reconstruction identified rapid (60-80 ms) enhanced processing of affectively conditioned gratings in occipital as well as prefrontal areas. Our results support the assumption of a general fast feed-forward sweep of information, partially activating an interconnected network of affective processing encompassing sensory, subcortical and prefrontal cortex regions.

The effects of spatial and temporal cueing on metacontrast masking.

We studied the effects of selective attention on metacontrast masking with 3 different cueing experiments. Experiments 1 and 2 compared central symbolic and peripheral spatial cues. For symbolic cues, we observed small attentional costs, that is, reduced visibility when the target appeared at an unexpected location, and attentional costs as well as benefits for peripheral cues. All these effects occurred exclusively at the late, ascending branch of the U-shaped metacontrast masking function, although the possibility exists that cueing effects at the early branch were obscured by a ceiling effect due to almost perfect visibility at short stimulus onset asynchronies (SOAs). In Experiment 3, we presented temporal cues that indicated when the target was likely to appear, not where. Here, we also observed cueing effects in the form of higher visibility when the target appeared at the expected point in time compared to when it appeared too early. However, these effects were not restricted to the late branch of the masking function, but enhanced visibility over the complete range of the masking function. Given these results we discuss a common effect for different types of spatial selective attention on metacontrast masking involving neural subsystems that are different from those involved in temporal attention.