Illusory conjunctions and the cerebral hemispheres

Two visual half-field experiments tested Moscovitch’s (1979) proposition that cerebral asymmetry does not concern the earliest perceptual stages but only later processing. Subjects were briefly shown displays that included one (Experiment 1) or two (Experiment 2) types of forms differing in size and which, according to previous evidence, might lead to opposite laterality effects. Laterality effects were assessed for correct detections and for illusory conjunctions, both in terms of raw detection scores and in terms of perceptual discriminability (dr scores). In Experiment 1, displays included either rectangles or triangles. In the first case, the target was a cross; in the second case, it was a Star of David. A hemifield x size interaction was observed both on correct detections and on associated discriminability. Yet, no such interaction was obtained for illusory conjunctions or for associated d’ scores. In Experiment 2, the two types of forms were presented simultaneously, with the small ones either inside or outside the large ones. No laterality effects were observed. Some implications of these data for both hemispheric asymmetry and feature integration issues are discussed. The results suggest that early preattentive processes of feature extraction are not lateralized, whereas some integrative mechanisms, such as Treisman’s (1988) focal attention, may operate differently in the two hemispheres.     

Vision outside the focus of attention

We investigated the relationship between focal attention and a feature-gradient detection that is performed in a parallel manner. We found that a feature gradient can be detected without measurable impairment of performance even while a concurrent form-recognition task is carried out. In spite of the fact that the form-recognition task engages focal attention and thus removes attentive resources from the vicinity of the feature gradient. This outcome suggests strongly that certain perceptions concerning salient boundaries and singularities in a visual scene can be accomplished without the aid of resource-limited processes, such as focal attention, and, by implication, that there may exist two distinct perceptual faculties (one attentive, the other not) that are able to bring complementary kinds of visual information simultaneously to our awareness.     

Aging and working memory inside and outside the focus of attention: dissociations of availability and accessibility

Two experiments used the N-Back task to test for age differences in working memory inside and outside the focus of attention. Manipulations of the difficulty of item-context binding (Experiment 1) and of stimulus feature binding (Experiment 2) were used to create conditions that varied in their demand on working memory, with the expectation that greater demand might increase age differences in focus-switching costs and the search rate outside the focus of attention. Results showed, however, that although age differences were evident in measures of overall speed and accuracy, and the manipulations significantly affected response times and accuracy in the expected direction, the experimental manipulations had no impact on age differences. Findings instead pointed to age-related reductions in accuracy but not speed of focus-switching and search outside the focus of attention. Thus, age-related deficits appear to involve the availability of representations in working memory, but not their accessibility.     

Illusory conjunctions in the perception of objects

In perceiving objects we may synthesize conjunctions of separable features by directing attention serially to each item in turn (A. Treisman and G. Gelade, Cognitive Psychology, 1980, 12, 97–136). This feature-integration theory predicts that when attention is diverted or overloaded, features may be wrongly recombined, giving rise to “illusory conjunctions.” The present paper confirms that illusory conjunctions are frequently experienced among unattended stimuli varying in color and shape, and that they occur also with size and solidity (outlined versus filled-in shapes). They are shown both in verbal recall and in simultaneous and successive matching tasks, making it unlikely that they depend on verbal labeling or on memory failure. They occur as often between stimuli differing on many features as between more similar stimuli, and spatial separation has little effect on their frequency. Each feature seems to be coded as an independent entity and to migrate, when attention is diverted, with few constraints from the other features of its source or destination.     

Illusory conjunctions in the perception of Chinese characters

A Chinese compound character consists of a radical component and a stem component. When compound characters were presented briefly, Ss often reported seeing illusory recombinations of radicals and stems. A series of 5 experiments suggested that the probability of seeing illusory characters is not under the direct influence of lexicality, pronounceability, or character frequency, but depends on 2 factors: (1) familiarity defined in terms of unit frequency, i.e., the frequency of occurrence of a unit either by itself or as part of a larger unit, and (2) the context-dependent perceptual distinctiveness of the components of a given character. It is suggested that the seemingly unreliable lexicality effect obtained in English studies may be reduced to a familiarity effect, and that what McClelland and Mozer (1986) referred to as the surround-similarity effect may be better characterized as an effect of perceptual distinctiveness.     

Revisiting the global-dominance phenomenon outside the focus of attention

Previous research has demonstrated global dominance for attended and unattended stimuli. In this paper, this phenomenon is shown to be restricted to small compound stimuli. As a first step, local dominance was obtained with large (8 deg in height) attended stimuli when a single stimulus was displayed. Next, dominance in attended and unattended stimuli was investigated by displaying two large compound stimuli, one surrounded by a square (attended compound stimulus), the other one enclosed in a circle (unattended compound stimulus). The way attention was directed to the attended stimulus was varied. No dominance was observed when subjects were instructed to process the stimulus appearing in the square (experiment 2). However, when a rapid-onset cue pre-directed attention to the attended stimulus, local dominance emerged for attended, but not for unattended stimuli (experiment 3). This latter result was obtained whether or not subjects were more experienced at local than global processing (experiment 4). The implications of the results for the locus of processing dominance are discussed.     

Illusory conjunctions reflect the time course of the attentional blink

Illusory conjunctions in the time domain are binding errors for features from stimuli presented sequentially but in the same spatial position. A similar experimental paradigm is employed for the attentional blink (AB), an impairment of performance for the second of two targets when it is presented 200–500 msec after the first target. The analysis of errors along the time course of the AB allows the testing of models of illusory conjunctions. In an experiment, observers identified one (control condition) or two (experimental condition) letters in a specified color, so that illusory conjunctions in each response could be linked to specific positions in the series. Two items in the target colors (red and white, embedded in distractors of different colors) were employed in four conditions defined according to whether both targets were in the same or different colors. Besides the U-shaped function for hits, the errors were analyzed by calculating several response parameters reflecting characteristics such as the average position of the responses or the attentional suppression during the blink. The several error parameters cluster in two time courses, as would be expected from prevailing models of the AB. Furthermore, the results match the predictions from Botella, Barriopedro, and Suero’s (Journal of Experimental Psychology: Human Perception and Performance, 27, 1452–1467, 2001) model for illusory conjunctions.     

Processing of threat-related information outside the focus of visual attention

This study investigates whether threat-related words are especially likely to be perceived in unattended locations of the visual field. Threat-related, positive, and neutral words were presented at fixation as probes in a lexical decision task. The probe word was preceded by 2 simultaneous prime words (1 foveal, i.e., at fixation; 1 parafoveal, i.e., 2.2 deg. of visual angle from fixation), which were presented for 150 ms, one of which was either identical or unrelated to the probe. Results showed significant facilitation in lexical response times only for the probe threat words when primed parafoveally by an identical word presented in the right visual field. We conclude that threat-related words have privileged access to processing outside the focus of attention. This reveals a cognitive bias in the preferential, parallel processing of information that is important for adaptation.     

Dissociations of personally significant and task-relevant distractors inside and outside the focus of attention: a combined behavioral and psychophysiological study

Studies of attentional capture by personally significant stimuli have reached inconsistent results, possibly because of improper control of the participants' attention. In the present study, the authors controlled visual attention by using a Stroop-like task. Participants responded to a central color and ignored a word presented either centrally (i.e., at the focus of attention) or peripherally (i.e., outside the focus of attention). Central words led to slower reaction times and larger orienting responses for significant items than for neutral items. These effects largely disappeared when the words appeared in a peripheral location. The peripheral words interfered with performance when they were relevant to task demands. These results indicate that there is a fundamental difference between task-relevant words and personally significant words: The former capture attention even when presented peripherally, whereas the latter do not.     

Illusory conjunctions of pitch and duration in unfamiliar tone sequences

In 3 experiments, the authors examined short-term memory for pitch and duration in unfamiliar tone sequences. Participants were presented a target sequence consisting of 2 tones (Experiment 1) or 7 tones (Experiments 2 and 3) and then a probe tone. Participants indicated whether the probe tone matched 1 of the target tones in both pitch and duration. Error rates were relatively low if the probe tone matched 1 of the target tones or if it differed from target tones in pitch, duration, or both. Error rates were remarkably high, however, if the probe tone combined the pitch of 1 target tone with the duration of a different target tone. The results suggest that illusory conjunctions of these dimensions frequently occur. A mathematical model is presented that accounts for the relative contribution of pitch errors, duration errors, and illusory conjunctions of pitch and duration.     

Processing of distractors inside and outside the attentional focus in a priming procedure

A priming procedure was used to study the processing of distractors located either inside (between the location of two targets) or outside (peripherally to the locations of the targets) the focus of attention. The stimuli were five-letter arrays, and participants had to decide whether two marked target letters were the same or different. In Experiments 1 and 2, positive priming was obtained both when targets and in-distractors in primes repeated as targets in probes; negative priming was found when out-distractor primes repeated as targets in probes. In Experiment 3, we also manipulated the match in letter case from primes to probes. In-distractors produced reliable positive priming, irrespective of whether the letters matched in case. In contrast out-distractors produced negative priming but only when the letters had the same case in primes and probes. These results are attributed to a spatial attention process operating (in this case) on low-level visual features, and an object-based selection process that enables more abstract information to be processed for selected stimuli.     

The representation of simple ensemble visual features outside the focus of attention

The representation of visual information inside the focus of attention is more precise than the representation of information outside the focus of attention. We found that the visual system can compensate for the cost of withdrawing attention by pooling noisy local features and computing summary statistics. The location of an individual object is a local feature, whereas the center of mass of several objects (centroid) is a summary feature representing the mean object location. Three experiments showed that withdrawing attention degraded the representation of individual positions more than the representation of the centroid. It appears that information outside the focus of attention can be represented at an abstract level that lacks local detail, but nevertheless carries a precise statistical summary of the scene. The term ensemble features refers to a broad class of statistical summary features that we propose collectively make up the representation of information outside the focus of attention.     

Detecting event-based prospective memory cues occurring within and outside the focus of attention

Event-based prospective memory cues are environmental stimuli that are associated with a previously established intention to perform an activity. Such cues traditionally have been placed in materials that receive focal attention during an ongoing activity. This article reports a direct comparison of event-based cues that occurred either within the focus of attention or at the periphery of such attention. When the cue occurred outside focal attention, manipulating that cue changed event-based prospective memory. The identical manipulation had no effect on event-based responding if the cue occurred within focal attention. These results suggest that cue characteristics can compensate for attention being directed away from an aspect of an ongoing task that contains event-based prospective memory.     

Illusory conjunctions in the time domain and the resulting time-course of the attentional blink

Illusory conjunctions in the time domain are errors made in binding stimulus features presented In the same spatial position in Rapid Serial Visual Presentation (RSVP) conditions. Botella, Barriopedro, and Suero (2001) devised a model to explain how the distribution of responses originating from stimuli around the target in the series is generated. They proposed two routes consisting of two sequential attempts to make a response. The second attempt (sophisticated guessing) is only employed if the first one (focal attention) fails in producing an integrated perception. This general outline enables specific predictions to be made and tested related to the efficiency of focal attention in generating responses in the first attempt. Participants had to report the single letter in an RSVP stream of letters that was presented in a previously specified color (first target, T1) and then report whether an X (second target, T2) was or was not presented. Performance on T2 showed the typical U-shaped function across the T1-T2 lag that reflects the attentional blink phenomenon. However, as was predicted by Botella, Barriopedro, and Suero's model, the time-course of the interference was shorter for trials with a correct response to T1 than for trials with a T1 error. Furthermore, longer time-courses of interference associated with pre-target and post-target errors to the first target were indistinguishable.     

The role of attention in illusory conjunctions

In five experiments, we investigated the effects of attention on illusory conjunctions formed between features of unrelated objects. The first three experiments used a weak manipulation of attention and found that illusory conjunctions formed either among features receiving high attentional priority or among features receiving low attentional priority were not more frequent than were conjunctions formed between mixed features of different attentional priority. The last two experiments used a strong manipulation of attention and failed to reveal any evidence of true illusory conjunctions. The results are inconsistent with the feature-integration theory, which predicts that when attention is focused on a subset of items, illusory conjunctions ought to occur within and outside of the attended subset, but not between the attended and unattended items.     

Illusory conjunctions of forms, objects, and scenes during rapid serial visual search

"Temporal migration" describes a situation in which subjects viewing rapidly presented stimuli (e.g., 9-20 items/s) confidently report a target element as having been presented in the same display as a previous or following stimulus in the sequence. Four experiments tested a short-term buffer model of this phenomenon. Experiments 1 and 4 tested the hypothesis that subjects' errors are due to the demands of the verbal report procedure rather than to perceptual integration. In Experiment 1, 12 color objects were presented at a rate of 9/s. Prior to each sequence, an object was named and subjects responded "yes" or "no" to indicate whether the target element (a black frame) occurred with that object. Consistent with the perceptual hypothesis, the yes/no procedure yielded the same results as the verbal report procedure. Experiment 2 tested the hypothesis that the direction of migration depends on "frame" detection time. Results showed that reaction time to frame detection was significantly faster in trials in which subjects reported the frame on a preceding rather than a following picture. Experiments 3 and 4 used the standard naming procedure and the yes/no procedure to test temporal migration using more complex, interrelated stimuli (objects and scenes). Implications for the use of the temporal migration effect to study visual integration within eye fixations are discussed.     

Illusory conjunctions are alive and well: a reply to Donk (1999).

When presented with a red T and a green O, observers occasionally make conjunction responses and indicate that they saw a green T. These errors have been interpreted as reflecting separable processing stages of feature detection and integration with the illusory conjunctions arising from a failure at the integration stage. Recently, M. Donk (1999) asserted that the phenomenon of illusory conjunctions is an artifact. Conjunction reports are actually the result of confusing a nontarget item (O in the example above) for a target item (the letter T) and (correctly) reporting the color associated with the (incorrectly) selected target. The authors demonstrate that although target-nontarget confusion errors are a potential source of conjunction reports, there is a plethora of findings that cannot be accounted for by this confusion model. A review of the literature indicates that in many studies, illusory conjunctions do result from a failure to properly integrate features.     

Processing of emotional visual scenes outside the focus of spatial attention: The role of eccentricity

Pairs of emotional-neutral pictures were presented parafoveally or peripherally (i.e., 2.2° or 5.2° away from fixation) as primes for 150 ms, followed by a recognition probe. The probe was either identical or related in semantic content to one of the primes. Physical features (colour, size, and spatial orientation) were always different for the primes and the probe. Results indicated that, for parafoveal displays, hit rate and sensitivity (A′) were higher for the identical emotional probes than for the identical neutral probes; in contrast, for peripheral displays there were more false alarms for the related emotional probes than for the related neutral probes. It is concluded that processing of specific semantic content is more likely to be obtained from emotional than from neutral scenes in parafoveal vision, whereas an overall, vague impression is extracted in peripheral vision.     

Illusory conjunctions die hard: a reply to Prinzmetal, Diedrichsen, and Ivry (2001).

M. Donk (1999) showed that various data patterns that have been considered as evidence for the existence of illusory conjunctions may be due to errors of target-nontarget confusion, an account that challenges the mere existence of illusory conjunction. In a reply, W. Prinzmetal, J. Diedrichsen, and R. B. Ivry (2001) argued against this conclusion, claiming that some earlier findings can be explained only when one assumes that illusory conjunctions exist. The current article shows that Prinzmetal et al.'s claims cannot refute any of Donk's earlier conclusions, suggesting indeed that one can only conclude that "illusory conjunctions are an illusion."     

Exploring students' prospective memory inside and outside the lab

This study compared activity-based intentions with event- and time-based intentions. Forty-five participants completed a laboratory session that tested activity-, event-, and time-based prospective memory. Participants were then asked to keep a record of their planned academic and work-related activities for 1 week. Results revealed a main effect of context such that participants were better at performing intentions in the naturalistic setting than in the laboratory. A strong trend of intention type showed that event-based tasks were more likely to be performed than activity- and time-based tasks. An interaction of context and intention type suggested that participants were significantly better at performing time-based tasks in the naturalistic setting than in the lab. Nearly half of the participants reported using simple memory aids such as alarm clocks, parent or friend reminders, or other external cues. Strikingly, very few reported using electronic devices to aid their memory for future intentions.