On the relation between identifying and locating masked targets in visual search

Three experiments were designed to determine whether naming is contingent on locating in a visual search task. Subjects were required to identify a masked target whose location was known (I|L) or unknown (I) and to locate a masked target whose identity was known (L|I) or unknown (L). The location-contingent hypothesis predicts a relationship among the tasks such that P(L) P(I|L) = P(I), since P(I) and P(L) P(I|L) both estimate the joint probability of identifying and locating the target (i.e. P(IλL)). This relationship held in Experiment I where targets were presented alone, and in Experiment II where targets were presented with dots as noise elements, but not in Experiment III where Xs were noise elements. The results are discussed in terms of the generality of the location-contingent hypothesis.     

Incidental visual memory for targets and distractors in visual search

We explored incidental retention of visual details of encountered objects during search. Participants searched for conjunction targets in 32 arrays of 12 pictures of real-world objects and then performed a token discrimination task that examined their memory for visual details of the targets and distractors from the search task. The results indicate that even though participants had not been instructed to memorize the objects, the visual details of search targets and distractor objects related to the targets were retained after the search. Distractor objects unrelated to the search target were remembered more poorly. Eye-movement measures indicated that the objects that were remembered were looked at more frequently during search than those that were not remembered. These results provide support that detailed visual information is included incidentally in the visual representation of an object after the object is no longer in view.     

Intra- and cross-dimensional visual search for single-feature targets

Cross-dimensional visual search for single-feature targets that differed from the distractors along two dimensions (color and orientation) was compared with intradimensional search for targets that differed from the distractors along a single dimension (either orientation or color). The design of the first three experiments differed from those of previous studies in that participants were required to respond differently to each of the targets. Experiments 1-3 were similar except that in Experiment 1, the distractors were homogeneous; in Experiment 2, two types of distractors were used in equal proportions; and in Experiment 3, two types of distractors were used but one of the distractors was a singleton. The findings, contrary to those of previous studies, revealed that cross-dimensional search is at least as efficient and for some targets even more efficient than intradimensional search. These results suggest that the details of stimulus-to-response mapping are essential in comparing intra- and cross-dimensional tasks. Experiment 4 used a priming design and did not support an explanation based on grouping processes. We outline an explanation for all the findings based on a recent cross-dimensional response selection model by Cohen and Shoup (1997).     

Asymmetry in visual search for targets defined by differences in movement speed.

Perception of motion speed was investigated with the visual search paradigm, using human Ss. When searching for a fast target among slow distractors, reaction time was minimally affected as the number of distractors was increased. In contrast, reaction time to detect a slow target among fast distractors was slow and linearly related to the number of distractors. The effect cannot be attributed to differences in temporal frequency, discriminability, or one type of representation that might result from spatiotemporal filtering. An alternative hypothesis that can account for the asymmetry is that speed detectors operate as high-pass filters in the velocity domain. This hypothesis is in agreement with results obtained in psychophysical studies on motion adaptation as well as data from single-cell recordings in nonhuman species.     

False feedback increases detection of low-prevalence targets in visual search

Many critical search tasks, such as airport and medical screening, involve searching for targets that are rarely present. These low-prevalence targets are associated with extremely high miss rates Wolfe, Horowitz, & Kenner (Nature, 435, 439?C440, 2005). The inflated miss rates are caused by a criterion shift, likely due to observers attempting to equate the numbers of misses and false alarms. This equalizing strategy results in a neutral criterion at 50?% target prevalence, but leads to a higher proportion of misses for low-prevalence targets. In the present study, we manipulated participants?? perceived number of misses through explicit false feedback. As predicted, the participants in the false-feedback condition committed a higher number of false alarms due to a shifted criterion. Importantly, the participants in this condition were also more successful in detecting targets. These results highlight the importance of perceived prevalence in target search tasks.     

The masked onset priming effect in picture naming

Reading aloud is faster when targets (e.g., PAIR) are preceded by visually masked primes sharing just the onset (e.g., pole) compared to all different primes (e.g., take). This effect is known as the masked onset priming effect (MOPE). One crucial feature of this effect is its presumed non-lexical basis. This aspect of the MOPE is tested in the current study. Dutch participants named pictures having bisyllabic names, which were preceded by visually masked primes. Picture naming was facilitated by first-segment but not last-segment primes, and by first-syllable as well as last-syllable primes. Whole-word primes with first or last segment overlap slowed down picture naming latencies significantly. The first-segment priming effect (i.e., MOPE) cannot be accounted for by non-lexical response competition since pictures cannot be named via the non-lexical route. Instead, the effects obtained in this study can be accommodated by a speech-planning account of the MOPE.     

The left-to-right nature of the masked onset priming effect in naming

Two experiments were performed to investigate the nature of the masked onset priming effect in naming, that is, the facilitation in naming latency that is observed when a target shares the initial grapheme/phoneme with a masked prime. Experiment 1 showed that the effect is not due to position-independent letter priming, since the naming of nonword targets preceded by masked primes was facilitated only if the prime shared the initial letter with the target (e.g., suf-SIB) and not if the prime shared the final letter (e.g., mub-SIB). Experiment 2 showed that the effect reflects the sharing of onsets rather than the initial letter, since facilitation due to an overlap of the initial letter was observed only for the simple onset target (e.g., penny-PASTE) for which the letter corresponded to the onset, and not for complex onset targets (e.g., bingo-BLISS). It is argued that the serial nature of the masked onset priming effect is best interpreted as the planning of articulation, rather than as the computation of phonology from orthography.     

A study of masked form priming in picture and word naming

We report a series of picture- and word-naming experiments in which the masked priming paradigm with prime exposures brief enough to prevent prime identification were used. Experiment 1 demonstrates that the prior presentation of the same word prime facilitates both picture and word naming independently of target frequency. In Experiments 2 and 3, primes that were pseudohomophones of picture targets produced facilitatory effects compared with orthographic controls, but these orthographically similar nonword primes did not facilitate picture naming compared with unrelated controls. On the other hand, word targets were primarily facilitated by orthographic prime— target overlap. This marked dissociation in the priming effects obtained with picture and word targets is discussed in relation to different explanations of masked form priming effects in visual word recognition and current models of picture and word naming.     

The effects of the binocular disparity differences between targets and maskers on visual search

A visual search for targets is facilitated when the target objects are on a different depth plane than other masking objects cluttering the scene. The ability of observers to determine whether one of four letters presented stereoscopically at four symmetrically located positions on the fixation plane differed from the other three was assessed when the target letters were masked by other randomly positioned and oriented letters appearing on the same depth plane as the target letters, or in front, or behind it. Three additional control maskers, derived from the letter maskers, were also presented on the same three depth planes: (1) random-phase maskers (same spectral amplitude composition as the letter masker but with the phase spectrum randomized); (2) random-pixel maskers (the locations of the letter maskers’ pixel amplitudes were randomized); (3) letter-fragment maskers (the same letters as in the letter masker but broken up into fragments). Performance improved with target duration when the target-letter plane was in front of the letter-masker plane, but not when the target letters were on the same plane as the masker, or behind it. A comparison of the results for the four different kinds of maskers indicated that maskers consisting of recognizable objects (letters or letter fragments) interfere more with search and comparison judgments than do visual noise maskers having the same spatial frequency profile and contrast. In addition, performance was poorer for letter maskers than for letter-masker fragments, suggesting that the letter maskers interfered more with performance than the letter-fragment maskers because of the lexical activity they elicit.     

Automatization and training in visual search.

In several search tasks, the amount of practice on particular combinations of targets and distractors was equated in varied-mapping (VM) and consistent-mapping (CM) conditions. The results indicate the importance of distinguishing between memory and visual search tasks, and implicate a number of factors that play important roles in visual search and its learning. Visual search was studied in Experiment 1. VM and CM performance were almost equal, and slope reductions occurred during practice for both, suggesting the learning of efficient attentive search based on features, and no important role for automatic attention attraction. However, positive transfer effects occurred when previous CM targets were re-paired with previous CM distractors, even though these targets and distractors had not been trained together. Also, the introduction of a demanding simultaneous task produced advantages of CM over VM. These latter two results demonstrated the operation of automatic attention attraction. Visual search was further studied in Experiment 2, using novel characters for which feature overlap and similarity were controlled. The design and many of the findings paralleled Experiment 1. In addition, enormous search improvement was seen over 35 sessions of training, suggesting the operation of perceptual unitization for the novel characters. Experiment 3 showed a large, persistent advantage for CM over VM performance in memory search, even when practice on particular combinations of targets and distractors was equated in the two training conditions. A multifactor theory of automatization and attention is put forth to account for these findings and others in the literature.     

Beyond the search surface: visual search and attentional engagement.

Treisman (1991) described a series of visual search studies testing feature integration theory against an alternative (Duncan & Humphreys, 1989) in which feature and conjunction search are basically similar. Here the latter account is noted to have 2 distinct levels: (a) a summary of search findings in terms of stimulus similarities, and (b) a theory of how visual attention is brought to bear on relevant objects. Working at the 1st level, Treisman found that even when similarities were calibrated and controlled, conjunction search was much harder than feature search. The theory, however, can only really be tested at the 2nd level, because the 1st is an approximation. An account of the findings is developed at the 2nd level, based on the 2 processes of input-template matching and spreading suppression. New data show that, when both of these factors are controlled, feature and conjunction search are equally difficult. Possibilities for unification of the alternative views are considered.     

Attention in visual search: multiple search classes.

Data from visual-search tasks are typically interpreted to mean that searching for targets defined by feature differences does not require attention and thus can be performed in parallel, whereas searching for other targets requires serial allocation of attention. The question addressed here was whether a parallel-serial dichotomy would be obtained if data were collected using a variety of targets representing each of several kinds of defining features. Data analyses included several computations in addition to search rate: (1) target-absent to target-present slope ratios; (2) two separate data transformations to control for errors; (3) minimum reaction time; and (4) slopes of standard deviation as a function of set size. Some targets showed strongly parallel or strongly serial search, but there was evidence for several intermediate search classes. Sometimes, for a given target-distractor pair, the results depended strongly on which character was the target and which was the distractor. Implications from theories of visual search are discussed.     

Influence of single and multiple onset distractors on visual search for singleton targets

In three experiments, we examined attentional and oculomotor capture by single and multiple abrupt onsets in a singleton search paradigm. Subjects were instructed to move their eyes as quickly as possible to a color singleton target and to identify a small letter located inside of it. In Experiment 1, task-irrelevant sudden onsets appeared simultaneously on half the trials with the presentation of the color singleton target. Response times (RTs) were longer when onsets appeared in the display regardless of the number of onsets. Eye-scan strategies were also disrupted by the appearance of the onset distractors, although the proportion of trials on which the eyes were directed to the onsets was the same regardless of the number of onsets. In Experiment 2, we manipulated the time of presentation of two task-irrelevant onsets in order to further examine whether multiple onsets would be attended and fixated prior to attending a color singleton target. Again, subjects made a saccade to a task-irrelevant onset on a substantial proportion of trials prior to fixating the target. However, saccades to the second onset were rare. Experiment 3 served as a replication of Experiment 1 but without the requirement for subjects to move their eyes to detect and identify the singleton target. The RT results were consistent with those in Experiment 1; dual onsets had no larger an effect on response speed than single onset distractors. These data are discussed in terms of the interaction between top-down and bottom-up control of attention and the eyes.     

Influence of single and multiple onset distractors on visual search for singleton targets

In three experiments, we examined attentional and oculomotor capture by single and multiple abrupt onsets in a singleton search paradigm. Subjects were instructed to move their eyes as quickly as possible to a color singleton target and to identify a small letter located inside of it. In Experiment 1, taskirrelevant sudden onsets appeared simultaneously on half the trials with the presentation of the color singleton target. Response times (RTs) were longer when onsets appeared in the display regardless of the number of onsets. Eye-scan strategies were also disrupted by the appearance of the onset distractors, although the proportion of trials on which the eyes were directed to the onsets was the same regardless of the number of onsets. In Experiment 2, we manipulated the time of presentation of two taskirrelevant onsets in order to further examine whether multiple onsets would be attended and fixated prior to attending a color singleton target. Again, subjects made a saccade to a task-irrelevant onset on a substantial proportion of trials prior to fixating the target. However, saccades to the second onset were rare. Experiment 3 served as a replication of Experiment 1 but without the requirement for subjects to move their eyes to detect and identify the singleton target. The RT results were consistent with those in Experiment 1; dual onsets had no larger an effect on response speed than single onset distractors. These data are discussed in terms of the interaction between top-down and bottom-up control of attention and the eyes.