Alphanumeric category effects: Evidence against the sufficiency of a localization explanation

The Hatfield Polytechnic, Hatfield, Herts ALIO 9AB, England The experiment utilized a serial choice reaction time (RT) paradigm in which only one alphanumeric stimulus was presented per trial, and the target set consisted of a single identified item. The categorical relationship between the target and nontarget items was varied as a property of blocks of trials. Target and nontarget RTs were smaller when the specified target item (e.g., the number 6) was categorically distinct from the nontargets (e.g., letters) than when it was from the same category (e.g., digits). The processing of catch-trial stimuli (items from the alternate category to the nontargets) and homographie category-ambiguous items was inhibited only in the former, between-category, condition. The results are contrasted with those obtained in visual search tasks. They suggest that a “locational-cue” explanation of alphanumeric category effects is inadequate.     

Depth of nontarget processing in an attention task

Two studies examined the effect of the sensory discriminability of targets from nontargets on depth of nontarget processing. Subjects shadowed target words that were binaurally presented with coincident nontarget words. Targets and nontargets were spoken in the same male voice under low sensory discriminability and in male and female voices, respectively under high sensory discriminability. Across the two studies, depth of nontarget processing was assessed in three ways: extent to which shadowing accuracy was disrupted by a semantic overlap between targets and nontargets, expenditure of capacity (reaction time to subsidiary light signals), and nontarget recall. All three possible measures of depth of nontarget processing decreased as sensory discriminability increased. The data support the assumption of multiple-loci theories of attention that nontargets can be perceptually inhibited; they contraindicate the assumption of late-selection theories that perceptual processing is automatic and irrepressible.     

Representation of linear orders

Two binary classification tasks were used to explore the associative structure of linear orders. In Experiment 1, college students classified English letters as targets or nontargets, the targets being consecutive letters of the alphabet. The time to reject nontargets was a decreasing function of the distance from the target set, suggesting response interference mediated by automatic associations from the target to the nontarget letters. The way in which this interference effect depended on the placement of the boundaries between the target and nontarget sets revealed the relative strengths of individual interletter associations. In Experiment 2, students were assigned novel linear orders composed of letterlike symbols and asked to classify pairs of symbols as being adjacent or nonadjacent in the assigned sequence. Reaction time was found to be a joint function of the distance between any pair of symbols and the relative positions of those symbols within the sequence. The effects of both distance and position decreased systematically over 6 days of practice with a particular order, beginning at a level typical of unfamiliar orders and converging on a level characteristic of familiar orders such as letters and digits. These results provide an empirical unification of two previously disparate sets of findings in the literature on linear orders, those concerning familiar and unfamiliar orders, and the systematic transition between the two patterns of results suggests the gradual integration of a new associative structure.     

Category effects in visual search: A failure to replicate the “oh-zero” phenomenon

In the experiment of Jonides and Gleitman (1972), subjects searched displays of digits or letters for single, specified digit or letter targets. The slope of the function relating reaction time to display size was positive (mean=25 msec/item) if target and nontargets belonged to the same alphanumeric category (within-category search), but zero if target and nontargets belonged to different categories (between-category search). This held even for the target O, whose categorical relationship to nontargets was determined entirely by the name it was given. In the present paper, two attempted replications are reported, one as close as practically possible. For the unambiguous targets A, Z, 2, and 4, slopes were greater in within-category search than in between-category search, but positive and very variable in both cases. For the ambiguous target O, slopes were identical in within-category and between-category search, and again positive. The results suggest that with single, specified targets, differences between within-category and between-category search may be due entirely to variation in the average physical resemblance between target and nontargets. In line with previous findings, they show that one cannot characterize within-category search as generally “serial” and between-category search as generally “parallel.”     

Processing of repeated letters in search and matching tasks

Several published experiments have used a variant of the matching or “same”-“different” paradigm in which the subject indicates whether or not all of the items in a visual array are the same. Contrary to the results of most paradigms in which subjects process multiple items, reaction time (RT) in this matching task is independent of the number of items in the display (N). One possible explanation of this independence holds that subjects respond to some overall property of the display, such as its general symmetry, and not to the individual items. I tested this hypothesis with a visual search task in which subjects searched a circular display of N=2, 4, or 6 letters for a specified target letter under three stimulus conditions. In the varied condition, the nontarget letters were all different from each other. In the repeated condition, the nontargets were repetitions of the same letter, but a single discrepant letter was always present in the display. The uniform condition also used repeated nontargets, but the negative (target-absent) displays contained N repetitions of a single nontarget item. I found that RT increased at a rate of 27 msec/item for the varied condition, but at only 13 and 8 msec/item in the repeated and uniform conditions, respectively. The slopes in the latter conditions were significantly lower than the slope for the varied condition, and they also differed significantly from each other. Hence, there was some advantage attributable to properties of the overall patterns associated with positive and negative responses, suggesting that at least some responses were based on these properties. There was a much larger advantage, however, attributable to the use of repeated nontargets, regardless of whether the overall patterns differed. This latter observation implies that the flat RT functions observed in matching studies must be interpreted primarily in terms of responses to individual items rather than to the pattern as a whole.     

Boundary conditions on parallel processing in human vision

A new theory of visual search is tested experimentally with simple colour patches. The essential element of this new theory is that, whatever the search materials, efficiency increases continuously with (i) decreasing similarity between targets and nontargets, and (ii) increasing similarity between one nontarget and another. Control of 'attention' (access to visual short-term memory) is seen as a competitive interaction between display elements, and the theory shows how stimulus similarities influence the outcome of this competition. One alternative view is that parallel visual processes are limited to local mismatch detection. Search is parallel if the target forms a break in an otherwise homogeneous field, but is serial when absolute stimulus identification is required. It is shown, however, that even colour identification can be parallel, providing targets and nontargets are sufficiently dissimilar. A second alternative view is that search for simple features is parallel whereas search for conjunctions is serial. Conjunction search, however, has a characteristic similarity structure: different kinds of nontarget each share one relevant attribute with the target, but none with one another. When this structure is mimicked in search for colour patches, correspondingly poor performance is obtained.     

Perceptual selectivity is task dependent: The pop-out effect poops out

An attempt was made to determine why evidence for perceptual selectivity based on conceptual category (e.g., digits vs. letters) has been found in some experiments but not in others. Experiments using the partial-report paradigm find no partial-report superiority when the report is cued by category, whereas, in recent visual search studies, evidence for perceptual selectivity has been obtained for arrays containing a single item that was categorically different from the other items (e.g., a digit among letters). Using a search task, Experiment 1 investigated the possibility that the number of categorically different items in the arrays could be a determinant of selectivity. One, two, or three digits and a variable number of letters were presented on each trial, and subjects determined if a particular digit was present. No evidence of selectivity was obtained, even for the one-digit condition. Experiment 2 verified this result, and Experiment 3 extended the failure of selectivity to a search task in which the possible targets differed in color from the distractor items. In Experiment 4, subjects counted the number of digits or red letters in arrays in which black letters were the distractor items. The counting task was used to eliminate the requirement in our previous tasks that the subjects search forspecific items. Evidence was obtained in the counting task for selectivity based on the color difference but not on the categorical difference. The color stimuli used in the counting task were essentially the same as those that did not yield any evidence of selectivity in the search task. The results suggest that task demands are an important determinant of whether or not perceptual selectivity will occur.     

Early and late selection in partial report: Evidence from degraded displays

In studies of iconic memory using the bar-probe task, subjects see a brief display of target letters and are probed by an arrow to report one of them. According to the classic early-selection account, subjects use the probe to select material for perceptual analysis from a precategorical (iconic) memory, but according to late-selection theories, subjects first identify the letters and then use the probe to select one letter for report from the set of categorized items. Pashler (1984) based his test for the locus of selection on a manipulation of display quality in previewed displays. He presented a target for 200 msec and then added a probe, together with the target, for an additional 150 msec. Reducing the target’s stimulus quality increased response latency. If the subjects identified the characters before the probe appeared and then selected an item for report, the clarity of the original array should not have affected response latency. Hence, Pashler concluded that his subjects used the probe to select from a precategorical store (early selection). Pashler’s experiment did not force subjects to rely on memory of the target; hence, although his experiment documented a situation in which subjects used early selection, it did not rule out late selection in studies of information persistence. We replicated Pashler’s findings and, using his logic, showed that when subjects are forced to rely on memory of the target, they select from a categorized store.     

Perceptual processing of nontargets in an attention task

The capability of nontargets to qualitatively influence the semantic processing of coincident targets was investigated in three experiments. Subjects were aurally presented a series of word pairs and attempted to detect homonymic instances of a predesignated category (e.g., animals). The nontarget with which a target (e.g., ANT) was paired was appropriate (e.g., CRAWLING), inappropriate (e.g., UNCLE), or neutral (e.g., STRAW). Experiments 1 and 2 established that detection of targets can be facilitated by appropriate nontargets and inhibited by inappropriate ones. Thus, nontargets can influence the way in which targets are semantically represented. Experiment 3 showed that this effect is eliminated when subjects are precued as to the ear of entry of targets. Thus, precuing appears to curtail the perceptual processing of nontargets. The data run counter to theories that claim that focused attention does not entail the perceptual suppression of nontargets.     

Influences of target and nontarget shapes on target identification

The orientation of a nontarget in a location-cuing paradigm has been shown to affect accuracy of identification of target orientation when modified plus-signs (Ts) are the stimuli. In the current study, similar effects were found with Landolt C (C) stimuli. Both shapes of targets were identified by orientation. Moreover, targets were affected by nontargets whether both target and nontarget were the same shape or if they were different shapes. A single nontarget with an orientation that matched that of the target improved accuracy of identification of target orientation, whereas a nontarget with an orientation that did not match that of the target impaired accuracy of identification of target orientation, even though the nontargets appeared in locations that were never cued and could never contain targets. The data are consistent with either the Variable and Permeable Filters metaphor or a response competition account. Received: 1 July 1998 / Accepted: 22 October 1998     

Early and late selection in partial report: evidence from degraded displays.

In studies of iconic memory using the bar-probe task, subjects see a brief display of target letters and are probed by an arrow to report one of them. According to the classic early-selection account, subjects use the probe to select material for perceptual analysis from a precategorical (iconic) memory, but according to late-selection theories, subjects first identify the letters and then use the probe to select one letter for report from the set of categorized items. Pashler (1984) based his test for the locus of selection on a manipulation of display quality in previewed displays. He presented a target for 200 msec and then added a probe, together with the target, for an additional 150 msec. Reducing the target's stimulus quality increased response latency. If the subjects identified the characters before the probe appeared and then selected an item for report, the clarity of the original array should not have affected response latency. Hence, Pashler concluded that his subjects used the probe to select from a precategorical store (early selection). Pashler's experiment did not force subjects to rely on memory of the target; hence, although his experiment documented a situation in which subjects used early selection, it did not rule out late selection in studies of information persistence. We replicated Pashler's findings and, using his logic, showed that when subjects are forced to rely on memory of the target, they select from a categorized store.     

The associated non-targets effect: evidence of nontarget processing at and beyond the level of letter recognition

In four visual search tasks participants were asked to make a target response if either of two targets was present and to make a nontarget response if neither target was present. Some target-absent displays included only nontarget stimuli or features that never occurred in the same displays as targets, whereas other target-absent displays included nontarget stimuli or features that did sometimes occur with targets. Nontarget responses were reliably faster in the former case than in the latter. This “associated nontargets effect” indicates that nontargets are not simply classified as nontargets but in addition are discriminated from one another. Current visual search models may underestimate the degree to which nontargets are processed during search.     

Accuracy of target detection in new-object and old-object displays

In 4 experiments, the authors investigated accuracy of detecting a target among nontargets. In some experiments, the target was a second-order square of stationary lines on a background of downward-moving lines, and nontargets were second-order squares of upward-moving lines. In other experiments, target and nontarget squares and background were shades of gray. The principal comparison was between "new" and "old" object displays. In new-object displays, search items appeared abruptly and one might be a target. In old-object displays, search items appeared abruptly, and after a delay one might become a target. Search displays in both conditions terminated shortly after target onset. Except when target onset was associated with the sole luminance change in a display, targets were much better detected in new- than in old-object displays. It is suggested that object onsets elicit a brief stimulus-driven enhancement of attention to the new objects.     

Classification without identification in visual search

Six subjects scanned displays of random consonants for a single target which was (a) another consonant; (b) a given number; or (c) any number. A second group of six subjects took part in three comparable conditions with number displays, and letters or numbers as targets. Scanning time for a number in a letter display or a letter in a number display was more rapid than scanning for a target drawn from the same set as the background. Several unpractised subjects, and all the subjects who practised the task, were able to scan as fast through letters for “any number” as for a specific number, or conversely through digits. The finding of different scanning rates for two precisely physically specified targets, depending on which class they were drawn from, runs counter to an explanation of high-speed scanning in terms of the operation of visual feature analysers. It is suggested that familiar categorization responses may be immediate and may provide the basis for the discrimination of relevant from irrelevant items in rapid visual scanning.     

The associated nontargets effect: Evidence of nontarget processing at and beyond the level of letter recognition

In four visual search tasks participants were asked to make a target response if either of two targets was present and to make a nontarget response if neither target was present. Some target-absent displays included only nontarget stimuli or features that never occurred in the same displays as targets, whereas other target-absent displays included nontarget stimuli or features that did sometimes occur with targets. Nontarget responses were reliably faster in the former case than in the latter. This “associated nontargets effect” indicates that nontargets are not simply classified as nontargets but in addition are discriminated from one another. Current visual search models may underestimate the degree to which nontargets are processed during search.     

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.     

The equivalence of target and nontarget processing in visual search

A comparison of a forced-choice visual search task with an item recognition task did not support Neisser’s (1967) hypothesis of a preattentive stage that processes targets and nontargets differentially. In the forced-choice condition, Ss indicated which of two items in a visual display was a target; in item recognition, Ss determined whether or not the single item in the visual display was a target. The size of the memorized set of possible targets was varied from one to six items for both tasks. Latencies increased linearly with memory set size in both conditions; the slopes for forced choice and item recognition were 41.8 and 27.9 msec per item, respectively. The ratio of 1.38 between the two slopes was well fit by Sternberg’s (1967) item recognition model, which predicts a ratio of 1.50.     

Serial,parallel, or holistic identification of single words in the two visual fields?

Subjects made timed manual responses in judging whether laterally presented four-letter words were identical to targets. In Experiment 1, nontargets differed by a single letter from targets. A right-field superiority occurred only for targets (which were detected fastest of all) and for nontargets where a letter changed at Position 2 or Position 3. Changes at initial and final positions were detected faster than the two middle positions, and there were no significant field differences. In Experiment 2, ascenders and descenders were controlled and changes were made in nontargets at all four letter positions, at Positions 1 and 4, at Positions 2 and 3, or at 2 alone. Response times for nontargets varied inversely with the number of differing letters, regardless of position. Significant field differences again only appeared for changes in the two middle positions. Letters at the beginning and end of a word seem to be processed faster than and differently from those within, where field differences are strongest. Vowel-consonant differences probably do not account for these effects, which are more compatible with some form of parallel, rather than either serial or holistic, processing.     

多目标追踪任务中不同运动方式非目标的抑制机制

采用多目标追踪范式结合点探测技术的方法, 考察视觉系统对不同运动方式(静止和规则运动)非目标的抑制机制。实验一将部分非目标设置为静止, 考察静止非目标的抑制机制; 实验二将部分非目标设置为规则运动, 以提高任务难度, 考察规则运动非目标的抑制机制。结果发现: (1) 实验一中静止非目标受到抑制, 抑制量与运动非目标没有差异; (2) 实验二中规则运动非目标受到抑制, 抑制量显著小于随机运动非目标; (3)综合两实验发现, 随着任务难度的提高, 只有随机运动非目标的抑制量显著增加, 而静止非目标与规则运动非目标的抑制量差异不显著。最后, 就视觉系统对不同运动方式非目标的抑制机制进行了讨论。     

The costs of target prioritization and the external requirements for using a recall-to-reject strategy in memory exclusion tasks: a meta-analysis

In event-related potential (ERP) studies, the left-parietal old/new effect is commonly considered as a neural correlate of recollection. In memory exclusion tasks, the effect is usually observed when the targeted information is identified, but it is not necessarily present when studied items are rejected as nontargets. Interestingly, both the presence and the absence of such old/new effects to nontargets have been regarded as indicator for strategic retrieval. We reviewed previous ERP studies using memory exclusion tasks to analyze the reaction time (RT) pattern in such studies, as well as the influence of task difficulty on the occurrence of nontarget retrieval. We identified 44 test conditions, reported in 24 studies, and subjected the behavioral data to a meta-analysis. The RTs to correctly rejected new items were shorter than the RTs to hits, in particular in studies that required differentiating conceptual or perceptual information at test. When the retrieval of target information was prioritized, RTs to nontargets were delayed relative to targets. Without such prioritization, no such RT differences were observed. For test conditions with nontarget retrieval, the retrieval accuracy was poorer compared with test conditions without such retrieval. The findings support previous studies that claimed that nontarget retrieval becomes more likely when target retrieval is difficult, but the strong overlap in task difficulty between conditions with and without nontarget retrieval indicates that other, partly yet to-be-identified factors contribute to the occurrence of nontarget retrieval as well.