The increasing use of ERP methodologies has also helped to provide more detailed insights into the mechanisms and time course of feature-guided attentional control processes in different types of visual search tasks. Kiss, Jolicœur, Dell’Acqua and Eimer (2008) investigated whether the current task set can bias attention towards particular feature dimensions. In this task, the visual search display was composed of homogeneous green circle distractors and either a colour singleton (e.g. red circle) or a shape singleton (e.g. green diamond). In separate blocks, participants were tasked with either reporting the location of a specific colour singleton (e.g. a red circle in the Colour Task) or a specific shape singleton (e.g. a green diamond in the Shape Task) relative to the central fixation cross. When the target was absent in the display, which occurred on 67% of trials, the relevant singleton was replaced with either a nontarget colour singleton (e.g. a blue circle) or a nontarget shape singleton (e.g. a green diamond) depending on the task. Relevant-dimension nontarget singletons triggered larger N2pcs than irrelevant-dimension nontarget singletons, despite being physically identical between tasks. Furthermore, Kiss et al. (2009) showed that only relevant-dimension nontarget singletons elicited an N2pc when appearing in task-irrelevant locations (experiment 2) and that there was a significant N2pc towards the
relevant-dimension nontarget singleton when displayed opposite an irrelevant-relevant-dimension nontarget singleton (experiment 3). Together, these experiments support the notion that feature-guided attention involves top-down processes that can bias visual processing in favour of specific currently task-relevant feature dimensions, such as colour or shape.
Other work by Kiss, Grubert and Eimer (2013) investigated the nature of the
representation of conjunction targets. Do such “target templates” represent target objects in a fully integrated fashion, or do they represent each target feature separately and
independently? In their study, participants were tasked with reporting the orientation (vertical or horizontal) of specific target singleton bars per block defined by both a colour (e.g. red or blue) and a size (e.g. small or large) among grey medium-sized distractors. Before the presentation of the target display, a task-irrelevant and spatially uninformative cue display consisting of a singleton appeared which either matched both, one, or none of the target features. The researchers reported cueing effects for fully matching targets but not for
partially matching or non-matching nontargets. These results suggest that only cues that fully match the target were able to capture attention, indicating that the representation of the target features is integrated rather than independent. However, Kiss et al. (2013) also found that N2pc components were triggered by the cues when they were both fully matching and partially matching the target, indicating that – in contrast to what was suggested by the behavioural spatial cueing effects – both types of cues were able to attract attention. This apparent contradiction was resolved by assuming that feature-based attention for items defined by a conjunction of target features works in two separable stages. Firstly, attention selects all objects in the visual field which match at least one of the target-defining features.
Secondly, attention quickly disengages from objects which do not fully correspond to the target definition (i.e. partial nontargets). In a second experiment, Kiss et al. (2013) presented participants with displays composed of singletons which were, with equiprobability, small blue, large blue, small red, and large red. In this task, participants had to remember two target features (e.g. red and small) and asked to respond if the singleton had one or both of these features. Under these conditions, unlike in Experiment 1, there was an equal RT cueing effect for cues which matched either one or both of the target-defining features. Kiss et al. (2013) concluded that the task instructions marking singletons matching only one of the target features as response-relevant bypassed the disengagement stage of the 2-step process.
Another important question in visual search is whether attention can be guided simultaneously by two features from the same dimension (e.g., two colours). According to the Guided Search model (Wolfe, 2007), this is not possible: Only one feature per dimension can be involved in the guidance of attention to visual search targets, as reflected by
substantial performance cost when targets are defined by a combination of features from the same dimension (e.g., two colours). However, not all researchers agree. For example, Carrasco, Ponte, Rechea, and Sampedro (1998) showed that homogeneously coloured distractors in a search for a conjunction colour target can be rapidly rejected, resulting in a highly efficient search. To further examine the relative efficiency of searching for colour-colour conjunction targets, Berggren and Eimer (2016a) employed N2pc components.
Participants searched for a target object defined by a particular configuration of colours (e.g.
red above blue). They were required to respond, via button press, whether the target item was present or absent in the displays. In non-competition trials, targets or partially matching distractors, which matched one of the two target colours, were displayed amongst three similar dual-colour distractors. In competition trials, both a target and partially matching distractor were presented such that one item was on the vertical meridian and the other on the horizontal meridian. These displays had two non-matching distractors.
Results showed that N2pc components were elicited by both target objects and partially matching objects at the same onset latency. Furthermore, the sum of the N2pcs to the two partially matching objects was of equal amplitude to N2pcs to targets up to 250ms post-display. From this point onwards, the target N2pc was enhanced relative to the sum of the partially matching distractors. This suggests that attention was initially allocated in parallel and independently to target-matching features, regardless of whether or not the other target features were present. However, at about 250 ms post-stimulus, the presence of both target colours in the same object is registered and starts to affect the allocation of attention.
These findings suggest that, at least for colour, same-dimension conjunction targets can be found remarkably efficiently, and thus challenges one of the central assumptions of Guided Search (Wolfe, 2007).
Finally, another question is whether feature-based attentional guidance processes can be restricted to particular locations, thus effectively combining feature-based and space-based attentional control. To test this, recent N2pc work has investigated whether attentional
selection can be guided by representations of target features at particular locations. Berggren, Jenkins, McCants and Eimer (2017) presented participants with visual search displays
consisting of four uniquely coloured bars located diagonally from the central fixation at either vertical or horizontal orientation. Different colour targets were defined for opposing lateral visual fields (e.g. red in the left field, blue in the right field), so that the task set contained two colour/location combinations. Each target display was preceded by a noninformative cue containing target-coloured objects at locations that were either task-relevant or irrelevant for this particular colour (target-matching versus mismatching locations). In two experiments, N2pcs to location-matching and mismatching colour cues were identical. This suggests that feature-based attention cannot operate selectively for specific areas of the visual field, suggesting that feature-based and space-based attentional control operate largely
independently and cannot be integrated. Feature-based attention appears to work in a spatially global manner, irrespective of whether particular locations are relevant or irrelevant for specific features. This hypothesis will be further tested in the experiments reported in Chapter 4 of this thesis.