Search

When two verniers are presented in rapid succession at the same location, feature fusion occurs. Instead of perceiving two separate verniers, participants typically report perceiving one fused vernier, whose offset is a combination of the two previous verniers, with the later one slightly dominating. Here, we examined the effects of sustained attention-the voluntary component of spatial attention-on feature fusion. One way to manipulate sustained attention is via the degree of certainty regarding the stimulus location. In the attended condition, the stimulus appeared always in the same location, and in the unattended condition it could appear in one of two possible locations. Participants had to report the offset of the fused vernier. Experiments 1 and 2 measured attentional effects on feature fusion with and without eye-tracking. In both experiments, we found a higher rate of reports corresponding to the offset of the second vernier with focused attention than without focused attention, suggesting that attention strengthened the final percept emerging from the fusion operation. In Experiment 3, we manipulated the stimulus duration to encourage a final fused percept that is dominated by either the first or second vernier. We found that attention strengthened the already dominant percept, regardless of whether it corresponded to the offset of the first or second vernier. These results are consistent with an attentional mechanism of signal enhancement at the encoding stage.

Recently, we demonstrated temporal crowding with normal observers: Target identification was impaired when it was surrounded by other stimuli in time, even when the interstimuli intervals (ISIs) were relatively long. Here, we examined whether temporal and spatial uncertainties play a critical role in the emergence of temporal crowding. We presented a sequence of three letters to the same peripheral location, right or left of fixation, separated by varying ISI (106-459 ms). One of these letters was the target, and the observers indicated its orientation. To eliminate temporal uncertainty, the position of the target within the sequence was fixed for an entire block (Experiment 1). To eliminate spatial uncertainty, we employed spatial attentional precues that indicated the letters' location. The precue was either auditory (Experiment 2) or visual (Experiment 3). We found temporal crowding to result in worse performance with shorter ISIs, even when there was no temporal or spatial uncertainty. Unlike the auditory cue, the visual cue affected performance. Specifically, when there was uncertainty regarding the target location (i.e., when the target appeared in the first display), precueing the target location improved overall performance and reduced the ISI effect, although it was not completely eliminated. These results suggest that temporal and spatial uncertainties are not necessary for the emergence of temporal crowding and that spatial attention can reduce temporal crowding.

Spatial crowding refers to impaired target identification when the target is surrounded by other stimuli in space temporal crowding refers to impaired target identification when the target is surrounded by other stimuli in time previously, when spatial and temporal crowding were measured in the fovea they were interrelated with amblyopic observers but almost absent with normal observers bonneh, sagi, & polat, 2007. In the current study we examined whether reliable temporal crowding can be found for normal observers with peripheral presentation 9° of eccentricity, and whether similar relations between temporal and spatial crowding will emerge to that end, we presented a sequence of three displays separated by a varying interstimulus interval (ISI). Each display included either one letter : experiments 1a ,: 1b ,: 1c or three letters separated by a varying interletter spacing: Experiments 2a ,: 2b). One of these displays included an oriented T. Observers indicated the T's orientation. As expected, we found spatial crowding: accuracy improved as the interletter spacing increased. Critically, we also found temporal crowding: in all experiments accuracy increased as the ISI increased, even when only stimulus-onset asynchronies (SOAs) larger than 150 ms were included, ensuring this effect does not reflect mere ordinary masking. Thus, with peripheral presentation, temporal crowding also emerged for normal observers. However, only a weak interaction between temporal and spatial crowding was found., (© 2015 ARVO.)

Previous studies have shown crowding alleviation when target and flankers similarity is reduced. However, in the case of contrast dissimilarity, the findings were inconsistent. This study examined the effect of stimulus contrast, particularly contrast dissimilarity, on both overall performance under crowded conditions and the critical distance-the spatial extent of crowding. To this end, we measured orientation identification of a rotated T presented with and without flankers. Target contrast was either the same as the flankers or different: higher in Experiment 1 and lower in Experiment 2. Experiment 3 investigated the hypothesis that higher target contrast reduces crowding through attraction of attention to the salient target. Thus, this experiment included orthogonal manipulations of transient attention, via attentional precues, and contrast. The results show reduced crowding effects-better performance and smaller critical distance-when target contrast was higher than its flankers and increased crowding effects when target contrast was lower. In addition, the effects of attention did not interact with those of contrast, suggesting that the effect of high target contrast is not solely due to attraction of attention. Our results suggest that contrast dissimilarity effects reflect a differential contribution of the target and flankers to the faulty integration process underlying crowding., (© 2014 ARVO.)

The identification of a peripheral target surrounded by flankers is often harder than the identification of an identical isolated target. This study examined whether this crowding phenomenon, and particularly its spatial extent, is affected by the allocation of spatial attention to the target location. We measured orientation identification of a rotated T with and without flankers. The distance between the target and the flankers and their eccentricity varied systematically. We manipulated attention via peripheral precues: in the cued condition, a dot indicated the target location prior to its onset. On the neutral condition, a central disk conveyed no information regarding the target location (Experiments 1-2), and on the invalid condition (Experiment 3), an invalid cue attracted attention to a nontarget location. We found, across all experiments, at all eccentricities, a significant attentional enhancement of identification accuracy. Most importantly, we found a significant attentional reduction of the critical distance (i.e., the target-flankers distance at which the flankers no longer interfere with target identification). These attentional effects were found regardless of the presence or absence of a backward mask and whether the attentional cue was informative or not. These findings suggest that attention reduces the spatial extent of crowding.

The effect of distractor homogeneity and target-distractor similarity on visual search was previously explored under two models designed for computer vision. We extend these models here to account for internal noise and to evaluate their ability to predict human performance. In four experiments, observers searched for a horizontal target among distractors of different orientation (orientation search; Experiments 1 and 2) or a gray target among distractors of different color (color search; Experiments 3 and 4). Distractor homogeneity and target-distractor similarity were systematically manipulated. We then tested our models' ability to predict the search performance of human observers. Our models' predictions were closer to human performance than those of other prominent quantitative models.

Visual attention can increase spatial resolution even when it leads to a decrease in performance. Whether this effect is mediated by reduction of external noise or by signal enhancement is an unsettled question. Although we previously demonstrated that attention can improve speed and accuracy in an acuity task, those experiments made use of a local postmask, which could be considered a source of external noise. In this work, a peripheral cue improved observers' abilities to indicate which side of a Landolt-square target had a gap whether or not a local postmask was used and with both central- and spread-neutral cues. In addition, we documented the presence of visual field inhomogeneities in a resolution task. Given that these experiments presented the target alone with no external noise added (i.e., without distracters or masks), our results indicate that transient attention enhanced the quality of the stimulus representation. Furthermore, because performance in the Landolt-square task indexes resolution, this attentional benefit indicates that transient attention can produce signal enhancement through finer spatial resolution.