The development of letter processing efficiency

The development of efficiency in letter processing skills was studied using a letter search task. In two experiments, subjects searched for a target letter displayed with items varying in their visual featural or conceptual categorical similarity to the target. Accuracy and reaction time of search were evaluated for evidence of the visual search “category effect.” In order to determine if subjects could efficiently use knowledge of stimulus differences to facilitate search, conditions tested search time as a function of the amount of information to be processed both within the visual display and in short-term memory. In the two experiments, subjects of ages 6 years through adulthood showed the category effect; however, efficiency of letter processing was found to be related to the amount of information that had to be processed in memory. While there were drastic changes in search speed with increasing grade level, patterns of processing were consistent, leading to the conclusion that the knowledge required to process the letter information accurately is acquired very early. Results were discussed in terms of the distinctions among accuracy, automaticity, and efficiency of skill development and the relationship of these to general reading and intellectual development.     

Is evidence for late selection due to automatic or attentional processing of stimulus identities?

This study determined whether evidence for late selection is due to attention processing or to processing by an automatic system that is separate from attention (two systems framework; Eriksen, Webb, & Fournier, 1990). The task was a two-choice discrimination of a target that appeared in one of two sequentially cued locations in an eight-letter visual display. Attention was directed to the first cued location (cue 1), and whether identification processing occurred at a different location before the second cue (cue 2) directed attention there was determined. Cue validity varied across two experiments, and critical trials were those in which the target appeared at cue 2. For these trials, the target was preceded by a letter (either identical, neutral, or incompatible) that changed to the target at various time intervals following cue 2. Automatic identification was assumed if the incompatible letter interfered with response to the target when it appeared only before cue 2 onset and independent of cue validity. The incompatible letter appearing only before cue 2 onset interfered with the target when the target occurred equally often at cue 1 and cue 2, but not when the target occurred at cue 1 70% and at cue 2 30% of the time. This disconfirms the two systems framework and suggests that attention is required for spatial form processing and response competition.     

Attentional selection during preparation of eye movements

Two experiments were conducted to examine the relationship between visual attention and saccade programming. Participants had to saccade to a letter string and detect a letter change presented briefly before the saccade onset. Hit probability (i.e., correct detection of a letter change in different positions) was taken as a measure of visual attention focus. The first experiment shows that hit probability depends on the actual landing position. These findings argue for a spatial coupling between saccade programming and the orienting of attention. Also, an unfamiliar letter cluster at the beginning captures attention and prevails over the influence of the saccade in preparation. Experiment 2, in which the letter change occurred at different times during the saccade latency, shows that attention shifts and focuses on the saccade target at the expense of the other parts of the stimulus when the motor program is ready to be executed. The theoretical implications of these results are discussed.     

Age similarities in the inertial properties of attention.

Adult age differences in the mode of allocation of visual attention were investigated, using a visual search task with a circular display containing one target letter and seven distractor letters. In two experiments, a total of 56 younger adults (M = 20 years) and 56 older adults (M = 66 years) searched for a target appearing with equal probability at one of two cued locations. The first cue appeared 115 msec before display onset, and the second cue appeared with display onset; distance between the two cued locations was varied. Target identification performance indicated that attention was inertial, in that reaction time for second-cued targets was related either to the area of the portion of the visual field containing possible target locations or to the mean path length of a serial self-terminating search. There were no age-related decrements in the allocation of visual attention.     

Age similarities in the inertial properties of attention

Adult age differences in the mode of allocation of visual attention were investigated, using a visual search task with a circular display containing one target letter and seven distractor letters. In two experiments, a total of 56 younger adults (M=20 years) and 56 older adults (M=66 years) searched for a target appearing with equal probability at one of two cued locations. The first cue appeared 115 msec before display onset, and the second cue appeared with display onset; distance between the two cued locations was varied. Target identification performance indicated that attention was inertial, in that reaction time for second-cued targets was related either to the area of the portion of the visual field containing possible target locations or to the mean path length of a serial self-tenminating search. There were no age-related decrements in the allocation of visual attention.     

Visual search by children: the effect of background variation and the use of visual cues

A visual search task was used to assess developmental changes in children's selective attention to specified portions of a visual display. Seven-, nine-, and twelve-year-olds searched for a target letter in matrices of letters, each of which was centered in a form. On each matrix the forms were uniform or they varied in color, shape or both color and shape. The children searched with either no cues or with color or shape cues that could be used to restrict and speed their search. In all conditions search speed increased with age. Comparisons among conditions revealed three different age trends. With no cues children of all ages were slowed comparably by variation in background forms. With color cues all children increased their search speeds relative to no-cue speeds, suggesting selective fixation, but the 12-year-olds benefited most from the cues. With shape cues the search speed of 9- and 12-year-olds was slowed while that of 7-year-olds was either unchanged or was slowed only slightly. These different trends caution against overly general statements of changes with age in selective attention, and highlight the need to consider both particular task requirements and the processes used by subjects of different ages in tasks requiring selective attention.     

Age and the selectivity of visual information processing

Two experiments were conducted to assess age differences in the selectivity of visual information processing. Selectivity was measured by the amount of interference caused by nontarget letters when subjects detected a target letter in a visual display. In both experiments, young and elderly groups participated in search and nonsearch conditions; in the search condition targets appeared anywhere in the display, whereas in the nonsearch condition targets were confined to the center position of the display. In the first experiment, subjects were assigned to either condition for two sessions of testing, and in the second experiment each subject participated in both conditions. In both experiments nontargets produced larger interference effects for old compared to young adults in the search condition but not in the nonsearch condition. The obtained pattern of age effects could not be explained by age-related reductions in parafoveal acuity. The findings indicate that the magnitude of divided-attention deficit increases with age, whereas focused-attention deficits are unaffected by aging.     

Components of the location probability effect in visual search tasks

In visual search tasks, targets are detected more rapidly when they appear in locations that commonly contain a target than when they appear in locations that rarely contain a target. Five experiments were conducted to investigate two specific properties of this location probability effect: its dependence on spatial location versus relative position and its dependence on or independence of target identity. In Experiment 1 spatial location of a stimulus row was varied to determine whether high location probability facilitates target detection in a particular location in visual space or a particular relative position within the row. Both were facilitated to approximately the same extent. In Experiment 2 an inducing target occurred with high probability in one of four display locations, and a test target occurred with equal probability in all four locations. Both targets were found more quickly in the high-probability location than in the other locations, but the advantage associated with targets in the high-probability location was larger for the inducing target than for the test target. In Experiments 3-5 the correspondence between the components observed in Experiments 1 and 2 was examined. The overall pattern of results was compatible with a model in which the location probability effect is produced partly by an attentional spotlight, which facilitates processing of any stimulus appearing in a particular location in visual space, and partly by a network of position-specific letter detectors, which facilitates detection of a particular letter in a particular relative position within a display. Models with flexible scanning strategies were also considered.     

Looking for two targets at the same time: One search or two?

A considerable amount of evidence suggests that, under conditions of high discriminability, subjects are able to process multiple elements in a visual display simultaneously when searching for a single target among distractors. Relatively little emphasis, however, has been placed on the question of whether subjects can search for and detect multiple targets simultaneously. This latter question is the focus of the present report. In two experiments, we compare performance in single-target and multiple-target detection tasks in order to investigate whether or not multiple targets can be detected simultaneously. In Experiment 1, subjects searched for one or two targets that were defined by color. In Experiment 2, subjects searched for a color and/or a letter target. When the two targets were presented in the same location (e.g., a red X when Target 1 was red and Target 2 was an X), they seemed to be detected simultaneously. Implications for object-based processing of visual information are discussed.     

The control of attention by abrupt visual onsets and offsets

A letter can be presented visually either by the abrupt appearance of lines that make up the letter (onset transient) or by the abrupt disappearance of extra lines from a form in which the letter is embedded (offset transient). Recent evidence from visual-search tasks has suggested that onset transients have absolute priority over offset transients with respect to the allocation of visual attention. Specifically, these studies have found that a single onset-transient target letter pops out of a background of offset-transient distractor letters (i.e., time to detect the target is independent of the number of distractors), which indicates that attention is automatically directed to the location of an onset-transient stimulus even when there are competing offset transients (Yantis & Jonides, 1984). Because of the way the offset letters were created, however, the total display change (number of offset line-segments plus number of onset line-segments) was greater for onset than for offset letters. Thus, onset targets might have popped out because they produced greater overall display changes rather than because they were the only letters with onset transients. In the present study, a figure that included more offset-transient line segments in the offset-transient letters was used. Under these conditions, onset-transient targets did not pop out of a background of offset-transient distractors. It is suggested that visual attention may be influenced by total display change and, therefore, that onset transients are not necessarily sufficient to control attention when there are many competing offset transients.     

Types and tokens in visual letter perception

Five experiments demonstrate that in briefly presented displays, subjects have difficulty distinguishing repeated instances of a letter or digit (multiple tokens of the same type). When subjects were asked to estimate the numerosity of a display, reports were lower for displays containing repeated letters, for example, DDDD, than for displays containing distinct letters, for example, NRVT. This homogeneity effect depends on the common visual form of adjacent letters. A distinct homogeneity effect, one that depends on the repetition of abstract letter identities, was also found: When subjects were asked to report the number of As and Es in a display, performance was poorer on displays containing two instances of a target letter, one appearing in uppercase and the other in lowercase, than on displays containing one of each target letter. This effect must be due to the repetition of identities, because visual form is not repeated in these mixed-case displays. Further experiments showed that this effect was not influenced by the context surrounding the target letters, and that it can be tied to limitations in attentional processing. The results are interpreted in terms of a model in which parallel encoding processes are capable of automatically analyzing information from several regions of the visual field simultaneously, but fail to accurately encode location information. The resulting representation is thus insufficient to distinguish one token from another because two tokens of a given type differ only in location. However, with serial attentional processing multiple tokens can be kept distinct, pointing to yet another limit on the ability to process visual information in parallel.     

Finding a new target in an old display: Evidence for a memory recency effect in visual search

Visual search often involves searching the same environment, consecutively, for a number of different targets. Here we investigate the extent to which search benefits from such previous exposure. In the experiment participants searched the same display consecutively for two different targets. Manual responses were faster in the second search than the first search regardless of whether a target was present or absent in the second search. Eye movement recordings demonstrated that the time necessary to find a target letter in the second search depended on when that letter was last fixated in the previous search. This fixation recency effect lasted for about four fixations. In addition, when a target was absent during the second search, participants were less likely to refixate a distractor if it had been recently fixated in the previous search and refixations tended to also occur later on in the search. These results provide evidence for a limited capacity short-term memory store in this kind of visual search.     

Implicit learning of ignored visual context

Humans process a visual display more efficiently when they encounter it for a second time, showing learning of the display. This study tests whether implicit learning of complex visual contexts depends on attention. Subjects searched for a white target among black and white distractors. When the locations of the target and the attended set (white distractors) were repeated, search speed was enhanced, but when the locations of the target and the ignored set (black distractors) were repeated, search speed was unaffected. This suggests that the expression of learning depends on attention. However, during the transfer test, when the previously ignored set now was attended, it immediately facilitated performance. In contrast, when the previously attended set now was ignored, it no longer enhanced search speed. We conclude that the expression of visual implicit learning depends on attention but that latent learning of repeated information does not.     

Temporal and spatial characteristics of selective encoding from visual displays

The time required for Ss to voice a target letter in a visual display was studied as a function of the spatial proximity of two kinds of noise elements (letters or disks) to the target and as a function of when the noise elements were presented following the onset of the target letter. The results were not consistent with a focusing model of attention or selective encoding. Instead, there appears to be a small area in the visual field (about 1 deg of angle) in which all stimuli are processed in detail.     

Processing redundant signals: coactivation, divided attention, or what?

The evidence for and against a redundancy gain in reaction time (RT) when the target is repeated in the visual display is reviewed. We consider the relevance of redundancy gains under these circumstances to the question of whether attention can be simultaneously directed to separate locations in the visual field. In the present experiments, two capital letters were the target stimuli in a two-alternative forced-choice RT paradigm. In addition to the usual conditions of single-target trials, trials on which the target is repeated in the display, and trials on which the target occurs with a noise letter, we introduced the innovation of a condition in which both targets occur in the display. In our two experiments, RT was fastest with single-target displays and slowest with displays containing a target and a noise letter. There was no significant difference in RT to displays in which the target was repeated and displays in which both targets were presented. Both conditions showed a redundancy gain when compared with displays containing a target and a noise letter. The lack of response competition in the both-targets condition and the overall pattern of the results were well explained by a unitary attentional focus that serially processed the letters in the display. Analyses of minima and maxima RTs were consistent with this interpretation.     

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.     

Effects of noise letters on decisions: Discrete or continuous flow of information?

Four forced-choice letter-detection experiments examined the effect on detection latency of noise letters that were visually similar to target letters. A single target letter was present in each display. Noise letters were similar to the target letter present in the display (the signal), to a different target letter assigned to the same response as the signal, or to a target letter assigned to the opposite response from the signal. Noise letters were present in either relevant or irrelevant display positions, and two quite different stimulus sets were used. The experiments were designed to test a prediction of models in which information about noise letters is transmitted continuously from the recognition to the decision process. These models predict that responses should be faster when noise letters are visually similar to a target assigned to the same response as the signal than when noise letters are similar to a target assigned to the opposite response. Statistically reliable effects of the type predicted by continuous models were obtained when noise letters appeared in relevant display positions, but not when they appeared in irrelevant positions.     

Global-local orientation congruency effects in visual search

In a visual search task, subjects had to decide which of 2 possible target letters was presented among 12 distractor letters. The 13 characters were arranged to form a global Navon-type letter. The global letter and the local letters (target and distractors) were independently presented in four different viewer-related orientations. When the global letter and the target were frequently congruently oriented, the response times increased with growing orientation disparity between them. This global-target congruency effect was independent from target identity (Experiment 1), and it diminished when global and target orientations were not correlated (Experiment 2). The results indicate that the orientation of the global letter can be deliberately used in order to facilitate the processing of congruently oriented local targets. The alignment of a spatial frame of reference is discussed as the most probable process underlying this facilitation.     

Recognition and attention guidance during contextual cueing in real-world scenes: evidence from eye movements

When confronted with a previously encountered scene, what information is used to guide search to a known target? We contrasted the role of a scene's basic-level category membership with its specific arrangement of visual properties. Observers were repeatedly shown photographs of scenes that contained consistently but arbitrarily located targets, allowing target positions to be associated with scene content. Learned scenes were then unexpectedly mirror reversed, spatially translating visual features as well as the target across the display while preserving the scene's identity and concept. Mirror reversals produced a cost as the eyes initially moved toward the position in the display in which the target had previously appeared. The cost was not complete, however; when initial search failed, the eyes were quickly directed to the target's new position. These results suggest that in real-world scenes, shifts of attention are initially based on scene identity, and subsequent shifts are guided by more detailed information regarding scene and object layout.     

How much processing do nonattended stimuli receive? Apparently very little, but...

The early versus late selection issue in attention models was examined by means of a new methodology. Through cues or precues, attention was directed to one location of a multistimulus visual display and, while attention was so engaged, the identity of a stimulus located at a different position in the display was changed. By varying the time after display onset before the stimulus was changed, we controlled the preview time that the original stimulus was represented on the retina. Then, using a marker cue, we directed the subject's attention to the location of the changed stimulus. The subject's response was a timed discrimination between two possible target letters. The data of main interest was the effect of preview time upon the subject's latency in identifying the new target that appeared in the changed location. We found that the preview time of the original stimulus, before RT was affected to the new target, depended upon whether the original stimulus was a neutral (noise) letter or whether it was the alternative target. When the original stimulus was a noise letter, RTs to the new target were just as fast as those obtained in the control condition in which the target was present throughout the preview interval and did not change its identity. Significant effects upon RT were obtained at preview times of 83 msec when the original stimulus was one of the targets that changed to the alternative target. Preview times also varied as a function of precuing. Preview times were correspondingly shortened when the first cue occurred 50 msec before display onset, thus providing an extra 50 msec for attention to be directed to the first display location. The results were interpreted in terms of two separate information-processing systems in the human: an automatic system and an attentional system. Even though a stimulus may have been automatically processed, when the attention system is directed to that stimulus, processing starts at the beginning again.