Diversion of Attention Leads to Conflict between Concurrently Attended Stimuli, Not Delayed Orienting to the Object of Interest

The control processes that guide attention to a visual-search target can result in the selection of an irrelevant object with similar features (a distractor). Once attention is captured by such a distractor, search for a subsequent target is momentarily impaired if the two stimuli appear at different locations. The textbook explanation for this impairment is based on the notion of an indivisible focus of attention that moves to the distractor, illuminates a nontarget that subsequently appears at that location, and then moves to the target once the nontarget is rejected. Here, we show that such delayed orienting to the target does not underlie the behavioral cost of distraction. Observers identified a color-defined target appearing within the second of two stimulus arrays. The first array contained irrelevant items, including one that shared the target's color. ERPs were examined to test two predictions stemming from the textbook serial-orienting hypothesis. Namely, when the target and distractor appear at different locations, (1) the target should elicit delayed selection activity relative to same-location trials, and (2) the nontarget search item appearing at the distractor location should elicit selection activity that precedes selection activity tied to the target. Here, the posterior contralateral N2 component was used to track selection of each of these search-array items and the previous distractor. The results supported neither prediction above, thereby disconfirming the serial-orienting hypothesis. Overall, the results show that the behavioral costs of distraction are caused by perceptual and postperceptual competition between concurrently attended target and nontarget stimuli.

Pre-trial Gaze Stability Predicts Momentary Slips of Attention

Our ability to maintain focus on a task waxes and wanes. Recent research suggests that eye-tracking may be a useful tool to capture the momentary slips of attention. In this paper, we show that pre-trial gaze stability predicted momentary slips of attention on the upcoming trial. In two visual search tasks, we asked participants to stabilize their gaze on a fixation cross before the search array appeared. We recorded participants’ manual responses and eye movements as they searched for the target. We also occasionally presented thought probes to examine whether participants were mind-wandering on a subset of trials. Results from the two tasks revealed a converging pattern: lower pre-trial gaze stability predicted worse performance in the upcoming trial. Specifically, participants had longer response times, were more influenced by distractors, and inspected the target for a longer duration. Participants also reported greater mind-wandering at the end of the trial if they had low pre-trial gaze stability. Overall, these findings suggest that pre-trial gaze stability is a simple and objective measure that can predict moments of inattention, which may be used to proactively curb its negative effects before the actual task starts.

Reward Does Not Modulate the Preview Benefit in Visual Search

Preview benefit refers to faster search for a target when a subset of distractors is seen prior to the search display. We investigated whether reward modulates this effect. Participants identified a target among non-targets on each trial. On “preview” trials, placeholders occupied half the search array positions prior to the onset of the full array. On “non-preview” trials, no placeholders preceded the full search array. On preview trials, the target could appear at either a placeholder position (old-target-location condition) or a position where no placeholder had been (new-target-location condition). Critically, the color of the stimulus array indicated whether participants would earn reward for a correct response. We found a typical preview benefit, but no evidence that reward modulated this effect, despite a manipulation check showing that stimuli in the reward-signaling color tended to capture attention on catch trials. The results suggest that reward learning does not modulate the preview benefit.

Visual Search May Not Require Target Representation in Working Memory or Long-Term Memory

We spend a lot of time searching for things. If we know what we are looking for in advance, a memory representation of the target will be created to guide search. But if the identity of the search target is revealed simultaneously with the presentation of the search array, is a similar memory representation formed? In the present study, 96 observers determined whether a central target was present in a peripheral search array. The results revealed that as long as the central target remained available for inspection (even if only in iconic memory), observers reinspected it after each distractor was checked, apparently forgoing consolidation of the target into working memory. The present findings challenged the assumption that evaluating items in a search array must involve comparison with a template in working memory.

Human visual search behaviour is far from ideal

Evolutionary pressures have made foraging behaviours highly efficient in many species. Eye movements during search present a useful instance of foraging behaviour in humans. We tested the efficiency of eye movements during search using homogeneous and heterogeneous arrays of line segments. The search target is visible in the periphery on the homogeneous array, but requires central vision to be detected on the heterogeneous array. For a compound search array that is heterogeneous on one side and homogeneous on the other, eye movements should be directed only to the heterogeneous side. Instead, participants made many fixations on the homogeneous side. By comparing search of compound arrays to an estimate of search performance based on uniform arrays, we isolate two contributions to search inefficiency. First, participants make superfluous fixations, sacrificing speed for a perceived (but not actual) gain in response certainty. Second, participants fixate the homogeneous side even more frequently than predicted by inefficient search of uniform arrays, suggesting they also fail to direct fixations to locations that yield the most new information.

It's all about the transient: Intra-saccadic onset stimuli do not capture attention

An abrupt onset stimulus was presented while the participants' eyes were in motion. Because of saccadic suppression, participants did not perceive the visual transient that normally accompanies the sudden appearance of a stimulus. In contrast to the typical finding that the presentation of an abrupt onset captures attention and interferes with the participants' responses, we found that an intra-saccadic abrupt onset does not capture attention: It has no effect beyond that of increasing the set-size of the search array by one item. This finding favours the local transient account of attentional capture over the novel object hypothesis.

TMS of the right angular gyrus modulates priming of pop-out in visual search: combined TMS-ERP evidence

During priming of pop-out, performance at discriminating a pop-out feature target in visual search is affected by whether the target on the previous trial was defined by the same feature as on the upcoming trial. Recent studies suggest that priming of pop-out relies on attentional processes. With the use of simultaneous, combined transcranial magnetic stimulation and event-related potential recording (TMS-ERP), we tested for any critical role of the right angular gyrus (rANG) and left and right frontal eye fields (FEFs)—key attentional sites—in modulating both performance and the ERPs evoked by such visual events. Intertrial TMS trains were applied while participants discriminated the orientation of a color pop-out element in a visual search array. rANG TMS disrupted priming of pop-out, reducing reaction time costs on switch trials and speeding responses when the color of the pop-out target switched. rANG TMS caused a negativity in the ERP elicited in response to the visual stimulus array, starting 210 ms after stimulus onset. Both behavioral and ERP effects were apparent only after rANG TMS, on switch trials, and when the target in the visual search array was presented in the left visual field, with no effects after left or right FEF TMS. These results provide evidence for an attentional reorienting mechanism, which originates in the rANG and is modulated by the implicit memory of the previous trial. The rANG plays a causal role on switch trials during priming of pop-out by interacting with visual processing, particularly in the ipsilateral hemisphere representing the contralateral hemifield.

Roles of Narrow- and Broad-Spiking Dorsal Premotor Area Neurons in Reach Target Selection and Movement Production

Most visual scenes are complex and crowded, with several different objects competing for attention and action. Thus a complete understanding of the production of goal-directed actions must incorporate the higher-level process of target selection. To examine the neural substrates of target selection for visually guided reaching, we recorded the activity of isolated neurons in the dorsal premotor area (PMd) of monkeys performing a reaction-time visual search task. In this task, monkeys reached to an odd-colored target presented with three distractors. We found that PMd neurons typically discriminate the target before movement onset, ∼150–200 ms after the appearance of the search array. In one subset of neurons, discrimination occurred at a consistent time after search array onset regardless of when the reaching movement occurred, suggesting that these neurons are involved in target selection. In a second group of neurons, discrimination time depended on reach reaction time, consistent with involvement in movement production but not in target selection. To look for physiological corroboration of these two functionally defined groups, we analyzed the extracellular spike waveforms of recorded neurons. This analysis showed a population of neurons with narrow action potentials that carried signals related to target selection. A second population with broader action potentials was more heterogeneous, with some neurons showing activity related to target selection and others showing only movement production activity. These results suggest that PMd contains signals related to target selection and movement execution and that different signals are carried by distinct neural subpopulations.