Removal of reinforcement improves instrumental performance in humans by decreasing a general action bias rather than unmasking learnt associations.

Performance during instrumental learning is commonly believed to reflect the knowledge that has been acquired up to that point. However, recent work in rodents found that instrumental performance was enhanced during periods when reinforcement was withheld, relative to periods when reinforcement was provided. This suggests that reinforcement may mask acquired knowledge and lead to impaired performance. In the present study, we investigated whether such a beneficial effect of removing reinforcement translates to humans. Specifically, we tested whether performance during learning was improved during non-reinforced relative to reinforced task periods using signal detection theory and a computational modelling approach. To this end, 60 healthy volunteers performed a novel visual go/no-go learning task with deterministic reinforcement. To probe acquired knowledge in the absence of reinforcement, we interspersed blocks without feedback. In these non-reinforced task blocks, we found an increased d', indicative of enhanced instrumental performance. However, computational modelling showed that this improvement in performance was not due to an increased sensitivity of decision making to learnt values, but to a more cautious mode of responding, as evidenced by a reduction of a general response bias. Together with an initial tendency to act, this is sufficient to drive differential changes in hit and false alarm rates that jointly lead to an increased d'. To conclude, the improved instrumental performance in the absence of reinforcement observed in studies using asymmetrically reinforced go/no-go tasks may reflect a change in response bias rather than unmasking latent knowledge.

Frontal cortex differentiates between free and imposed target selection in multiple-target search.

Cognitive control can involve proactive (preparatory) and reactive (corrective) mechanisms. Using a gaze-contingent eye tracking paradigm combined with fMRI, we investigated the involvement of these different modes of control and their underlying neural networks, when switching between different targets in multiple-target search. Participants simultaneously searched for two possible targets presented among distractors, and selected one of them. In one condition, only one of the targets was available in each display, so that the choice was imposed, and reactive control would be required. In the other condition, both targets were present, giving observers free choice over target selection, and allowing for proactive control. Switch costs emerged only when targets were imposed and not when target selection was free. We found differential levels of activity in the frontoparietal control network depending on whether target switches were free or imposed. Furthermore, we observed core regions of the default mode network to be active during target repetitions, indicating reduced control on these trials. Free and imposed switches jointly activated parietal and posterior frontal cortices, while free switches additionally activated anterior frontal cortices. These findings highlight unique contributions of proactive and reactive control during visual search.

Humans can efficiently look for but not select multiple visual objects.

The human brain recurrently prioritizes task-relevant over task-irrelevant visual information. A central question is whether multiple objects can be prioritized simultaneously. To answer this, we let observers search for two colored targets among distractors. Crucially, we independently varied the number of target colors that observers anticipated, and the number of target colors actually used to distinguish the targets in the display. This enabled us to dissociate the preparation of selection mechanisms from the actual engagement of such mechanisms. Multivariate classification of electroencephalographic activity allowed us to track selection of each target separately across time. The results revealed only small neural and behavioral costs associated with preparing for selecting two objects, but substantial costs when engaging in selection. Further analyses suggest this cost is the consequence of neural competition resulting in limited parallel processing, rather than a serial bottleneck. The findings bridge diverging theoretical perspectives on capacity limitations of feature-based attention.

Anticipatory Distractor Suppression Elicited by Statistical Regularities in Visual Search.

Salient yet irrelevant objects often capture our attention and interfere with our daily tasks. Distraction by salient objects can be reduced by suppressing the location where they are likely to appear. The question we addressed here was whether suppression of frequent distractor locations is already implemented beforehand, in anticipation of the stimulus. Using EEG, we recorded cortical activity of human participants searching for a target while ignoring a salient distractor. The distractor was presented more often at one location than at any other location. We found reduced capture for distractors at frequent locations, indicating that participants learned to avoid distraction. Critically, we found evidence for "proactive suppression" as already "prior to display onset," there was enhanced power in parieto-occipital alpha oscillations contralateral to the frequent distractor location-a signal known to occur in anticipation of irrelevant information. Locked to display onset, ERP analysis showed a distractor suppression-related distractor positivity (PD) component for this location. Importantly, this PD was found regardless of whether distracting information was presented at the frequent location. In addition, there was an early PD component representing an early attentional index of the frequent distractor location. Our results show anticipatory (proactive) suppression of frequent distractor locations in visual search already starting prior to display onset.

Beta and Theta Oscillations Differentially Support Free Versus Forced Control over Multiple-Target Search.

Many important situations require human observers to simultaneously search for more than one object. Despite a long history of research into visual search, the behavioral and neural mechanisms associated with multiple-target search are poorly understood. Here we test the novel theory that the efficiency of looking for multiple targets critically depends on the mode of cognitive control the environment affords to the observer. We used an innovative combination of electroencephalogram (EEG) and eye tracking while participants searched for two targets, within two different contexts: either both targets were present in the search display and observers were free to prioritize either one of them, thus enabling proactive control over selection; or only one of the two targets would be present in each search display, which requires reactive control to reconfigure selection when the wrong target has been prioritized. During proactive control, both univariate and multivariate signals of beta-band (15-35 Hz) power suppression before display onset predicted switches between target selections. This signal originated over midfrontal and sensorimotor regions and has previously been associated with endogenous state changes. In contrast, imposed target selections requiring reactive control elicited prefrontal power enhancements in the delta/theta band (2-8 Hz), but only after display onset. This signal predicted individual differences in associated oculomotor switch costs, reflecting reactive reconfiguration of target selection. The results provide compelling evidence that multiple target representations are differentially prioritized during visual search, and for the first time reveal distinct neural mechanisms underlying proactive and reactive control over multiple-target search.SIGNIFICANCE STATEMENT Searching for more than one object in complex visual scenes can be detrimental for search performance. Although perhaps annoying in daily life, this can have severe consequences in professional settings such as medical and security screening. Previous research has not yet resolved whether multiple-target search involves changing priorities in what people attend to, and how such changes are controlled. We approached these questions by concurrently measuring cortical activity and eye movements using EEG and eye tracking while observers searched for multiple possible targets. Our findings provide the first unequivocal support for the existence of two modes of control during multiple-target search, which are expressed in qualitatively distinct time-frequency signatures of the EEG both before and after visual selection.

Lack of free choice reveals the cost of multiple-target search within and across feature dimensions.

Having to look for multiple targets typically results in switch costs. However, using a gaze-contingent eyetracking paradigm with multiple color-defined targets, we have recently shown that the emergence of switch costs depends on whether observers can choose a target or a target is being imposed upon them. Here, using a similar paradigm, we tested whether these findings generalize to the situation in which targets are specified across different feature dimensions. We instructed participants to simultaneously search for, and then fixate, either of two possible targets presented among distractors. The targets were defined as either two colors, two shapes, or one color and one shape. In one condition, only one of the two targets was available in each display, so that the choice was imposed. In the other condition, both targets would be present in each display, which gave observers free choice over what to search for. Consistent with our earlier findings, switch costs emerged when targets were imposed, whereas no switch costs emerged when target selection was free, irrespective of the dimension in which the targets were defined. The results are consistent with the operation of different modes of control in multiple-target search, with switch costs emerging whenever reactive control is required and being reduced or absent when displays allow for proactive control.

Lack of Free Choice Reveals the Cost of Having to Search for More Than One Object.

It is debated whether people can actively search for more than one object or whether this results in switch costs. Using a gaze-contingent eye-tracking paradigm, we revealed a crucial role for cognitive control in multiple-target search. We instructed participants to simultaneously search for two target objects presented among distractors. In one condition, both targets were available, which gave the observer free choice of what to search for and allowed for proactive control. In the other condition, only one of the two targets was available, so that the choice was imposed, and a reactive mechanism would be required. No switch costs emerged when target choice was free, but switch costs emerged reliably when targets were imposed. Bridging contradictory findings, the results are consistent with models of visual selection in which only one attentional template actively drives selection and in which the efficiency of switching targets depends on the type of cognitive control allowed for by the environment.

It depends on when you look at it: Salience influences eye movements in natural scene viewing and search early in time.

It is generally accepted that salience affects eye movements in simple artificially created search displays. However, no such consensus exists for eye movements in natural scenes, with several reports arguing that it is mostly high-level cognitive factors that control oculomotor behavior in natural scenes. Here, we manipulate the salience distribution across images by decreasing or increasing the contrast in a gradient across the image. We recorded eye movements in an encoding task (Experiment 1) and a visual search task (Experiment 2) and analyzed the relationship between the latency of fixations and subsequent saccade targeting throughout scene viewing. We find that short-latency first saccades are more likely to land on a region of the image with high salience than long-latency and subsequent saccades in both the encoding and visual search tasks. This implies that salience indeed influences oculomotor behavior in natural scenes, albeit on a different timescale than previously reported. We discuss our findings in relation to current theories of saccade control in natural scenes.