Redundant combination of target features from separable dimensions can expedite visual search. The dimension-weighting account explains these “redundancy gains” by assuming that the attention-guiding priority map integrates the feature-contrast signals generated by targets within the respective dimensions. The present study investigated whether this hierarchical architecture is sufficient to explain the gains accruing from redundant targets defined by features in different modalities, or whether an additional level of modality-specific priority coding is necessary, as postulated by the modality-weighting account (MWA). To address this, we had observers perform a visuo-tactile search task in which targets popped out by a visual feature (color or shape) or a tactile feature (vibro-tactile frequency) as well as any combination of these features. The RT gains turned out larger for visuo-tactile versus visual redundant targets, as predicted by the MWA. In addition, we analyzed two lateralized event-related EEG components: the posterior (PCN) and central (CCN) contralateral negativities, which are associated with visual and tactile attentional selection, respectively. The CCN proved to be a stable somatosensory component, unaffected by cross-modal redundancies. In contrast, the PCN was sensitive to cross-modal redundancies, evidenced by earlier onsets and higher amplitudes, which could not be explained by linear superposition of the earlier CCN onto the later PCN. Moreover, linear mixed-effect modeling of the PCN amplitude and timing parameters accounted for approximately 25% of the behavioral RT variance. Together, these behavioral and PCN effects support the hierarchy of priority-signal computation assumed by the MWA.

Using a combination of behavioral and EEG measures in a tactile odd-one-out search task with collocated visual items, we investigated the mechanisms underlying facilitation of search by repeated (vs. nonrepeated) spatial distractor–target configurations (“contextual cueing”) when either the tactile (same-modality) or the visual array (different-modality) context was predictive of the location of the tactile singleton target. Importantly, in both conditions, the stimulation was multisensory, consisting of tactile plus visual items, although the target was singled out in the tactile modality and so the visual items were task-irrelevant. We found that when the predictive context was tactile, facilitation of search RTs by repeated configurations was accompanied by, and correlated with, enhanced lateralized ERP markers of pre-attentive (N1, N2) and, respectively focal-attentional processing (contralateral delay activity) not only over central (“somatosensory”), but also posterior (“visual”) electrode sites, although the ERP effects were less marked over visual cortex. A similar pattern—of facilitated RTs and enhanced lateralized (N2 and contralateral delay activity) ERP components—was found when the predictive context was visual, although the ERP effects were less marked over somatosensory cortex. These findings indicate that both somatosensory and visual cortical regions contribute to the more efficient processing of the tactile target in repeated stimulus arrays, although their involvement is differentially weighted depending on the sensory modality that contains the predictive information.

This study investigates the causal contribution of the left frontopolar cortex (FPC) to the processing of violated expectations from learned target–distractor spatial contingencies during visual search. The experiment consisted of two phases: learning and test. Participants searched for targets presented either among repeated or nonrepeated target–distractor configurations. Prior research showed that repeated encounters of identically arranged displays lead to memory about these arrays, which then can come to guide search (contextual cueing effect). The crucial manipulation was a change of the target location, in a nevertheless constant distractor layout, at the transition from learning to test. In addition to this change, we applied repetitive transcranial magnetic stimulation (rTMS) over the left lateral FPC, over a posterior control site, or no rTMS at all (baseline; between-group manipulation) to see how FPC rTMS influences the ability of observers to adapt context-based memories acquired in the training phase. The learning phase showed expedited search in repeated relative to nonrepeated displays, with this context-based facilitation being comparable across all experimental groups. For the test phase, the recovery of cueing was critically dependent on the stimulation site: Although there was evidence of context adaptation toward the end of the experiment in the occipital and no-rTMS conditions, observers with FPC rTMS showed no evidence of relearning at all after target location changes. This finding shows that FPC plays an important role in the regulation of prediction errors in statistical context learning, thus contributing to an update of the spatial target–distractor contingencies after target position changes in learned spatial arrays.

Selective attention controls the distribution of our visual system's limited processing resources to stimuli in the visual field. Two independent parameters of visual selection can be quantified by modeling an individual's performance in a partial-report task based on the computational theory of visual attention (TVA): (i) top–down control α, the relative attentional weighting of relevant over irrelevant stimuli, and (ii) spatial bias w λ , the relative attentional weighting of stimuli in the left versus right hemifield. In this study, we found that visual event-related electroencephalographic lateralizations marked interindividual differences in these two functions. First, individuals with better top–down control showed higher amplitudes of the posterior contralateral negativity than individuals with poorer top–down control. Second, differences in spatial bias were reflected in asymmetries in earlier visual event-related lateralizations depending on the hemifield position of targets; specifically, individuals showed a positivity contralateral to targets presented in their prioritized hemifield and a negativity contralateral to targets presented in their nonprioritized hemifield. Thus, our findings demonstrate that two functionally different aspects of attentional weighting quantified in the respective TVA parameters are reflected in two different neurophysiological measures: The observer-dependent spatial bias influences selection by a bottom–up processing advantage of stimuli appearing in the prioritized hemifield. By contrast, task-related target selection governed by top–down control involves active enhancement of target, and/or suppression of distractor, processing. These results confirm basic assumptions of the TVA framework, complement the functional interpretation of event-related lateralization components in selective attention studies, and are of relevance for the development of neurocognitive attentional assessment procedures.

The redundant-signals effect (RSE) refers to a speed-up of RT when the response is triggered by two, rather than just one, response-relevant target elements. Although there is agreement that in the visual modality RSEs observed with dimensionally redundant signals originating from the same location are generated by coactive processing architectures, there has been a debate as to the exact stage(s)—preattentive versus postselective—of processing at which coactivation arises. To determine the origin(s) of redundancy gains in visual pop-out search, the present study combined mental chronometry with electrophysiological markers that reflect purely preattentive perceptual (posterior-contralateral negativity [PCN]), preattentive and postselective perceptual plus response selection-related (stimulus-locked lateralized readiness potential [LRP]), or purely response production-related processes (response-locked LRP). As expected, there was an RSE on target detection RTs, with evidence for coactivation. At the electrophysiological level, this pattern was mirrored by an RSE in PCN latencies, whereas stimulus-locked LRP latencies showed no RSE over and above the PCN effect. Also, there was no RSE on the response-locked LRPs. This pattern demonstrates a major contribution of preattentive perceptual processing stages to the RSE in visual pop-out search, consistent with parallel-coactive coding of target signals in multiple visual dimensions [Müller, H. J., Heller, D., & Ziegler, J. Visual search for singleton feature targets within and across feature dimensions. Perception & Psychophysics, 57, 1–17, 1995].

Endogenous control of visual search can influence search guidance at the level of a supradimensional topographic saliency map [Wolfe, J. M. Guided Search 2.0: A revised model of visual search. Psychonomic Bulletin & Review, 1, 202–238, 1994], and modulate nonspatial mechanisms coding saliency in dimension-specific input modules [Müller, H. J., Reimann, B., & Krummenacher, J. Visual search for singleton feature targets across dimensions: Stimulus- and expectancy-driven effects in dimensional weighting. Journal of Experimental Psychology: Human Perception and Performance, 29, 1021–1035, 2003]. The current experiment used fMRI to dissociate these mechanisms in a singleton feature search task in which the likely target dimension (color or orientation) was semantically precued and target saliency in each dimension was varied parametrically. BOLD signal increases associated with increased demands for top–down guidance were observed within the fronto-parietal attention network and in the right anterior middle frontal gyrus. Decreasing requirements for top–down control led to BOLD signal increases in medial anterior prefrontal cortex, consistent with a gating mechanism in favor of stimulus-related processing [Burgess, P. W., Dumontheil, I., & Gilbert, S. J. The gateway hypothesis of rostral prefrontal cortex (area 10) function. Trends in Cognitive Sciences, 11, 290–298, 2007]. Another network of brain areas consisting of left lateral fronto-polar cortex, the left supramarginal gyrus, and the cerebellum, as well as a bilateral network consisting of the posterior orbital gyrus, the inferior frontal gyrus, and the pre-SMA were associated with top–down dimensional (re-) orienting. These data argue in favor of distinct endogenous control systems for visuospatial and dimension-based attentional processing. Finally, cue validity modulated saliency processing in the left temporo-parietal junction (TPJ), pointing to a crucial role of the left TPJ in integrating an endogenous dimension-based attention set with bottom–up saliency signals.

Processing of a given target is facilitated when it is defined within the same (e.g., visual–visual), compared to a different (e.g., tactile–visual), perceptual modality as on the previous trial [Spence, C., Nicholls, M., & Driver, J. The cost of expecting events in the wrong sensory modality. Perception & Psychophysics, 63, 330–336, 2001]. The present study was designed to identify electrocortical (EEG) correlates underlying this “modality shift effect.” Participants had to discriminate (via foot pedal responses) the modality of the target stimulus, visual versus tactile (Experiment 1), or respond based on the target-defining features (Experiment 2). Thus, modality changes were associated with response changes in Experiment 1, but dissociated in Experiment 2. Both experiments confirmed previous behavioral findings with slower discrimination times for modality change, relative to repetition, trials. Independently of the target-defining modality, spatial stimulus characteristics, and the motor response, this effect was mirrored by enhanced amplitudes of the anterior N1 component. These findings are explained in terms of a generalized “modality-weighting” account, which extends the “dimension-weighting” account proposed by Found and Müller [Searching for unknown feature targets on more than one dimension: Investigating a “dimension-weighting” account. Perception & Psychophysics, 58, 88–101, 1996] for the visual modality. On this account, the anterior N1 enhancement is assumed to reflect the detection of a modality change and initiation of the readjustment of attentional weight-setting from the old to the new target-defining modality in order to optimize target detection.

Illusory figure completion demonstrates the ability of the visual system to integrate information across gaps. Mechanisms that underlie figural emergence support the interpolation of contours and the filling-in of form information [Grossberg, S., & Mingolla, E. Neural dynamics of form perception: Boundary completion, illusory figures and neon colour spreading. Psychological Review, 92, 173–211, 1985]. Although both processes contribute to figure formation, visual search for an illusory target configuration has been shown to be susceptible to interfering form, but not contour, information [Conci, M., Müller, H. J., & Elliott, M. A. The contrasting impact of global and local object attributes on Kanizsa figure detection. Submitted]. Here, the physiological basis of form interference was investigated by recording event-related potentials elicited from contour- and surface-based distracter interactions with detection of a target Kanizsa figure. The results replicated the finding of form interference and revealed selection of the target and successful suppression of the irrelevant distracter to be reflected by amplitude differences in the N2pc component (240–340 msec). In conclusion, the observed component variations reflect processes of target selection on the basis of integrated form information resulting from figural completion processes.