Keeping an Eye on the Periphery:How Eccentricity affects Visual Selection

After reading the work of this thesis I hope you are convinced that eccentricity is a major factor that cannot be ignored when it comes to understanding visual selection. More specifically, in Chapter 2 we showed that while the proportion of selecting targets and salient items does not change with eccentricity, the dynamics of saliency- and relevance-driven selection do change. That is, the effect of saliency was protracted at a further eccentricity, while the effect of relevance was delayed. This discrepancy between overall selection and performance over time can be explained by the difference in saccade latencies between conditions. That is, as the saccade latency distribution shifted in time with increasing eccentricity, so did the effects of saliency and relevance. In Chapter 3, consistent with earlier work, we challenged existing models of visual selection, showing that the time at which a saccade is initiated greatly influences whether it will be saliency- or relevance-driven. That is, short latency eye movements are more likely to be saliency driven while later eye movements are more likely to be relevance driven. Importantly, we showed for the first time that this separation in time leads to a brief period in between saliency-driven and relevance-driven selection in which the eyes appear to be in ‘limbo’. That is, selection appears to operate randomly, irrespective of saliency and relevance. By fitting different models on the data, we showed that the dynamics of saliency- and relevance-based selection are best described as two independent processes that do not influence each other. We propose an alternative view on the classic priority map model, in which saliency effects are actually a byproduct of a difference in processing speed between different items. That is, on the priority map, salient items are available for selection earlier than non-salient items as they are processed more quickly and elicit therefore more activation at an earlier point in time. After a while, this difference in activation disappears because then non-salient items are processed as well, resulting in a period of non-selectivity. After this, the influence of behavioral relevance takes effect, and activity for the relevant item increases. In Chapter 4 we showed that subjects are more likely to select items that are presented close to fixation than items presented further away. This central selection bias was larger than would be expected based on low-level sensory differences between eccentricities suggesting an important role for attentional competition. In Chapter 5 we were able to determine, for the first time, the time course that the effect of eccentricity follows. Here we showed that eccentricity mainly influences those saccades that are initiated early. That is, eccentricity operates in a similar time window as saliency. As a consequence, the effects of saliency were diminished as the eccentricity difference between the two items grew, but those of relevance were unaffected. In Chapter 6 we showed that attentional capture by salient distractors is modulated by the bias that is described in Chapter 4. That is, even though we saw no effect of eccentricity on attentional capture in overall manual RTs, using eye movement data we showed that participants are more likely to select an item closer to fixation than an item presented further away. Crucially, on those trials in which an eye movement was made towards the distractor reaction times increased with distractor eccentricity while the likelihood of making an eye movement to the distractor in the first place decreases with increasing distractor eccentricity. As these effects go in opposite directions, overall RT showed no effect of distractor eccentricity when all trials were combined together

Expecting the unexpected : Temporal expectation increases the flash-grab effect

Acknowledgments EvH, TB, KC, and HH were supported by the Australian Government through the Australian Research Council's Discovery Projects funding scheme (project DP180102268). PC was supported by grants from Dartmouth College and from Natural Sciences and Engineering Research Council Canada.Peer reviewedPublisher PD

Detection of object displacement during a saccade is prioritized by the oculomotor system

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The dynamics of saliency-driven and goal-driven visual selection as a function of eccentricity

Both saliency and goal information are important factors in driving visual selection. Saliency-driven selection occurs primarily in early responses, whereas goal-driven selection happens predominantly in later responses. Here, we investigated how eccentricity affects the time courses of saliency-driven and goal-driven visual selection. In three experiments, we asked people to make a speeded eye movement toward a predefined target singleton which was simultaneously presented with a non-target singleton in a background of multiple homogeneously oriented other items. The target singleton could be either more or less salient than the non-target singleton. Both singletons were presented at one of three eccentricities (i.e., near, middle, or far). The results showed that, even though eccentricity had only little effect on overall selection performance, the underlying time courses of saliency-driven and goal-driven selection altered such that saliency effects became protracted and relevance effects became delayed for far eccentricity conditions. The protracted saliency effect was shown to be modulated by expectations as induced by the preceding trial. The results demonstrate the importance of incorporating both time and eccentricity as factors in models of visual selection

The eyes prefer targets nearby fixation: Quantifying eccentricity-dependent attentional biases in oculomotor selection

An important function of peripheral vision is to provide the target of the next eye movement. Here we investigate the extent to which the eyes are biased to select a target closer to fixation over one further away. Participants were presented with displays containing two identical singleton targets and were asked to move their eyes to either one of them. The targets could be presented at three different eccentricities relative to central fixation. In one condition both singletons were presented at the same eccentricity, providing an estimate of the speed of selection at each of the eccentricities. The saccadic latency distributions from this same-eccentricity condition were then used to predict the selection bias when both targets were presented at different eccentricities. The results show that when targets are presented at different eccentricities, participants are biased to select the item closest to fixation. This eccentricity-based bias was considerably stronger than predicted on the basis of saccadic latency distributions in the same-eccentricity condition. This rules out speed of processing per se as a sole explanation for such a bias. Instead, the results are consistent with attentional competition being weighted in favour of items close to fixation

Motion Extrapolation for Eye Movements Predicts Perceived Motion-Induced Position Shifts

Transmission delays in the nervous system pose challenges for the accurate localization of moving objects as the brain must rely on outdated information to determine their position in space. Acting effectively in the present requires that the brain compensates not only for the time lost in the transmission and processing of sensory information, but also for the expected time that will be spent preparing and executing motor programs. Failure to account for these delays will result in the mislocalization and mistargeting of moving objects. In the visuomotor system, where sensory and motor processes are tightly coupled, this predicts that the perceived position of an object should be related to the latency of saccadic eye movements aimed at it. Here we use the flash-grab effect, a mislocalization of briefly flashed stimuli in the direction of a reversing moving background, to induce shifts of perceived visual position in human observers (male and female). We find a linear relationship between saccade latency and perceived position shift, challenging the classic dissociation between “vision for action” and “vision for perception” for tasks of this kind and showing that oculomotor position representations are either shared with or tightly coupled to perceptual position representations. Altogether, we show that the visual system uses both the spatial and temporal characteristics of an upcoming saccade to localize visual objects for both action and perception.</p

The effects of eccentricity on attentional capture

Visual attention may be captured by an irrelevant yet salient distractor, thereby slowing search for a relevant target. This phenomenon has been widely studied using the additional singleton paradigm in which search items are typically all presented at one and the same eccentricity. Yet, differences in eccentricity may well bias the competition between target and distractor. Here we investigate how attentional capture is affected by the relative eccentricities of a target and a distractor. Participants searched for a shape-defined target in a grid of homogeneous nontargets of the same color. On 75% of trials, one of the nontarget items was replaced by a salient color-defined distractor. Crucially, target and distractor eccentricities were independently manipulated across three levels of eccentricity (i.e., near, middle, and far). Replicating previous work, we show that the presence of a distractor slows down search. Interestingly, capture as measured by manual reaction times was not affected by target and distractor eccentricity, whereas capture as measured by the eyes was: items close to fixation were more likely to be selected than items presented further away. Furthermore, the effects of target and distractor eccentricity were largely additive, suggesting that the competition between saliency- and relevance-driven selection was modulated by an independent eccentricity-based spatial component. Implications of the dissociation between manual and oculomotor responses are also discussed.</p