Perceptual organization and focused attention: The role of objects and proximity in visual processing

The influence of the Gestalt grouping principles of similarity, closure, and proximity on the size of the response-compatibility effect was examined in a focused attention task. In three studies, subjects responded to a centrally located target and attempted to ignore adjacent distractors. The distractors, which served as targets on other trials, could be compatible, incompatible, or neutral with respect to the response of the target. In addition, the distractors and the target could be embedded in the same object, presented in the same color, presented on different objects, or presented in different colors. The typical response-compatibility effect (B. A. Eriksen & C. W. Eriksen, 1974) was found when the target and distractors were embedded in the same object or presented in the same color. Performance was poorer when the target was surrounded by response-incompatible distractors than when it was surrounded by response-compatible distractors. However, the response-compatibility effect was eliminated-w-hen. the target and distractors were embedded in different objects, even when the distance between the items was less than.25° of visual angle. Furthermore, the response-compatibility effect was of intermediate size when the distractors were not grouped strongly with the target or with neutral flankers. The results are discussed in terms of space- and object-based models of visual attention.     

Perceptual organization and focused attention: the role of objects and proximity in visual processing.

The influence of the Gestalt grouping principles of similarity, closure, and proximity on the size of the response-compatibility effect was examined in a focused attention task. In three studies, subjects responded to a centrally located target and attempted to ignore adjacent distractors. The distractors, which served as targets on other trials, could be compatible, incompatible, or neutral with respect to the response of the target. In addition, the distractors and the target could be embedded in the same object, presented in the same color, presented on different objects, or presented in different colors. The typical response-compatibility effect (B. A. Eriksen & C. W. Eriksen, 1974) was found when the target and distractors were embedded in the same object or presented in the same color. Performance was poorer when the target was surrounded by response-incompatible distractors than when it was surrounded by response-compatible distractors. However, the response-compatibility effect was eliminated when the target and distractors were embedded in different objects, even when the distance between the items was less than .25 degrees of visual angle. Furthermore, the response-compatibility effect was of intermediate size when the distractors were not grouped strongly with the target or with neutral flankers. The results are discussed in terms of space- and object-based models of visual attention.     

Movement and focused attention: A failure to replicate

Our original goal was to explore the nature of the grouping-by-movement phenomenon reported by Driver and Baylis (1989). In their studies, distractors that moved in common with a centrally located target had a larger influence on focused-attention performance than did more proximate but stationary distractors. These results seemed particularly important since they suggested, contrary to the predictions of space-based models of attention, that attention could be allocated to noncontiguous regions of the visual field. Their results also suggested mandatory processing of stimuli with common motion. Unfortunately, we were unable to replicate this grouping-by-movement effect. In the conditions of Experiment 1 in which we replicated Driver and Baylis’s methodology, stationary distractors produced a larger response-compatibility effect than did the more distant distractors that moved in common with the target. In Experiment 2, we redundantly coded the centrally located target and the far distractors with common movement and color. However, the results were identical to those obtained in Experiment J. The stationary near distractors that appeared in a different color from the target and the far distractors produced the largest response-compatibility effect. In a final experiment, we attempted to compensate for the reduced acuity of the moving distractors by adjusting their size by a cortical magnification factor. However, even with this manipulation, we found a larger response-compatibility effect for the stationary near distractors than for the moving distant distractors. Our results suggest that subjects are capable of selectively processing a target item that moves in common with distractors.     

Movement and focused attention: a failure to replicate.

Our original goal was to explore the nature of the grouping-by-movement phenomenon reported by Driver and Baylis (1989). In their studies, distractors that moved in common with a centrally located target had a larger influence on focused-attention performance than did more proximate but stationary distractors. These results seemed particularly important since they suggested, contrary to the predictions of space-based models of attention, that attention could be allocated to noncontiguous regions of the visual field. Their results also suggested mandatory processing of stimuli with common motion. Unfortunately, we were unable to replicate this grouping-by-movement effect. In the conditions of Experiment 1 in which we replicated Driver and Baylis's methodology, stationary distractors produced a larger response-compatibility effect than did the more distant distractors that moved in common with the target. In Experiment 2, we redundantly coded the centrally located target and the far distractors with common movement and color. However, the results were identical to those obtained in Experiment 1. The stationary near distractors that appeared in a different color from the target and the far distractors produced the largest response-compatibility effect. In a final experiment, we attempted to compensate for the reduced acuity of the moving distractors by adjusting their size by a cortical magnification factor. However, even with this manipulation, we found a larger response-compatibility effect for the stationary near distractors than for the moving distant distractors. Our results suggest that subjects are capable of selectively processing a target item that moves in common with distractors.     

Visual selective attention with virtual barriers

Previous studies have shown that interference effects in the flanker task are reduced when physical barriers (e.g., hands) are placed around rather than below a target flanked by distractors. One explanation of this finding is the referential coding hypothesis, whereby the barriers serve as reference objects for allocating attention. In five experiments, the generality of the referential coding hypothesis was tested by investigating whether interference effects are modulated by the placement of virtual barriers (e.g., parentheses). Modulation of flanker interference was found only when target and distractors differed in size and the virtual barriers were beveled wood-grain objects. Under these conditions and those of previous studies, the author conjectures that an impression of depth was produced when the barriers were around the target, such that the target was perceived to be on a different depth plane than the distractors. Perception of depth in the stimulus display might have led to referential coding of the stimuli in three-dimensional (3-D) space, influencing the allocation of attention beyond the horizontal and vertical dimensions. This 3-D referential coding hypothesis is consistent with research on selective attention in 3-D space that shows flanker interference is reduced when target and distractors are separated in depth.     

Orientation asymmetry in the flanker task

Experiments 1 and 2 of this study show that when the target is either a vertical or a horizontal line, diagonal-line flankers tilted 45° either to the right or to the left have the same effect as do incongruent flankers. When the target is a diagonal line tilted either to the right or to the left, vertical- or horizontal-line flankers do not have the same effect as do incongruent flankers. Experiment 3 demonstrates that this asymmetry is not caused by the temporal-dynamic aspects of the processing. Together, these experiments suggest that there is an asymmetrical relation between diagonal lines and either vertical or horizontal lines otttside of the central focus of attention. Experiment 4 shows that despite this asymmetry in the flanker task, visual search for a vertical- or a horizontal-line target among diagonal-line distractors is not affected by the number of distractors. Possible explanations of this phenomenon are discussed.     

Effects of precuing horizontal and vertical dimensions on right—left prevalence

When stimuli and responses can be coded along horizontal and vertical dimensions simultaneously, a right-left prevalence effect is often obtained for which the advantage for a compatible mapping is larger on the horizontal dimension than on the vertical dimension. The present study investigated the role of preparatory processes in this right-left prevalence effect using a method in which the relevant dimension was cued at short and long intervals prior to presentation of the target stimulus. In three experiments, the right-left prevalence effect did not vary significantly in magnitude as a function of cue-target interval, suggesting that the effect is due primarily to relative salience of the horizontal and vertical codes, as determined by the task structure, and not to a greater ease of attending to the horizontal dimension.     

Illusions of brightness, length, and now size: comparable effects derived from the Craik-O'Brien-Cornsweet distributions

Analogous illusions of brightness, line length, and object size occur when stimulus distributions follow the Mach function. Similarly, analogous illusions of brightness and length are produced by stimulus distributions that follow the Craik-O'Brien-Cornsweet function. In Exp. 1, object size is studied using Craik-O'Brien-Cornsweet stimuli composed of 12 clip-art representations of candles, swords, forks, screwdrivers, or pens. Despite their equal dimensions, the three objects next to the taller gradient were judged larger than the three adjacent to the shorter gradient. In Exp. 2, variations of the sword and screwdriver figures having three separations of their constituent objects were used. The illusion produced in Exp. 1 was replicated and, for the figures composed of screwdrivers, increasing separation reduced the hypothesized mis-estimations. The relationship of these Craik-O'Brien-Cornsweet size illusions to their brightness and length counterparts mirrors the relationships previously reported for Mach stimuli; moreover, these findings converge to suggest that the visual system registers size of objects with a frequency code and that illusions of size appear when interactions among neurons disrupt this code.     

The role of attention in the occurrence of the affordance effect

It has been demonstrated that visual objects activate responses spatially corresponding to the orientation (left or right) of their graspable parts. To investigate the role of attention orienting in the generation of this effect, which we will refer to as affordance effect, we ran three experiments in which the target stimulus could either correspond or not with a dynamic event capturing attention. Participants were required to press a left or right key according to the vertical orientation (upward or inverted) of objects presented with their handles oriented to the right or to the left. In Experiments 1 and 2, the objects were located above or below fixation, while in Experiment 3, to assess the contemporary presence of the affordance and Simon effects, the objects were located to the left or right of fixation. The results showed that while the affordance effect, when evident, was always relative to the target object, irrespective of its attentional capturing properties, the Simon effect occurred relative to the event capturing attention. These findings suggest that automatic and controlled processes of visual attention may play a differential role in the occurrence of the two effects.     

Interactions between search mechanisms in conjunction search.

We report the results of a technique designed to measure interactions between different visual search processes. We interrupted pop-out search before it produced a detection response, by adding extra distractors to the display so that a target initially defined by a single feature difference (e.g., a yellow horizontal line among yellow vertical lines) could then only be found on the basis of the conjunction of two features (a yellow horizontal line among yellow vertical lines and pink horizontal lines; difficult search). This technique has been used to measure the duration of the perceptual components of pop-out search, independent of over-all response time, for targets presented among different sets of distractors. In addition, when pop-out failed because it was interrupted, past work has shown that it nevertheless provided useful information to the processes responsible for difficult search. That is, partial pop-out assisted difficult search, when extra distractors made search difficult because the target was between the two types of distractors in the relevant feature space (Olds, Cowan, & Jolicoeur, 2000a,b,c). The present results demonstrate that partial pop-out also assists difficult search when difficult search is a conjunction search, and therefore these interactions may occur at a stage where information from different feature dimensions is combined.     

On the preattentive accessibility of stereoscopic disparity: Evidence from visual search

We examined the generality of the claim that stereoscopic disparity is detectable in parallel across the visual field. Using a search paradigm with random-dot stereograms, we varied the relative disparity of target and distractor items. When both target and distractors had crossed disparities, both search functions (i.e., target in front of distractors and target behind distractors) were linear with positive slopes. When both target and distractors had uncrossed disparities, the pattern of results depended upon whether the target was in front of or behind the distractors—specifically, when the target was in front of the distractors, search functions were similar to those seen for “crossed” search, but when the target was behind the distractors, a nonlinear search function was found. Finally, when the target and distractors straddled the plane of fixation, a nonlinear search function was found when the target was in front of the distractors; however, when the target was behind the distractors, a linear search function with a large positive slope was found. We show that the nonlinear search functions are consistent with the effects of an intervening global surface percept. We also show that the size of the stimulus display may be a factor in some relative depth cases. Additionally, we replicate Steinman’s (1987) finding that search is parallel when the distractors are located at the plane of fixation and the target disparity is crossed, eliminating monocular and spatial cues to target presence that may have been present in his original study. In a final control experiment, we showed that reaction times did not increase with set size when observers performed another kind of perceptual task on similar random-dot stereogram displays. This eliminates the possibility that some of the results obtained here can be explained by increases in the difficulty of perceiving/fusing the stimuli when the number of distractors is increased.     

Segregation by color and stereoscopic depth in three-dimensional visual space

Two experiments were conducted to investigate how color and stereoscopic depth information are used to segregate objects for visual search in three-dimensional (3-D) visual space. Eight observers were asked to indicate the alphanumeric category (letter or digit) of the target which had its unique color and unique depth plane. In Experiment 1, distractors sharing a common depth plane or a common color appeared in spatial contiguity in thexy plane. The results suggest that visual search for the target involves examination of kernels formed by homogeneous items sharing the same color and depth. In Experiment 2, thexy contiguity of distractors sharing a common color or a common depth plane was varied. The results showed that when target-distractor distinction becomes more difficult on one dimension, the other dimension becomes more important in performing visual search, as indicated by a larger effect on search time. This suggests that observers can make optimal use of the information available. Finally, color had a larger effect on search time than did stereoscopic depth. Overall, the results support models of visual processing which maintain that perceptual segregation and selective attention are determined by similarity among objects in 3-D visual space on both spatial and nonspatial stimulus dimensions.     

Indications for two attentional gradients in endogenous visual-spatial attention

Four experiments were designed to investigate the meridian effect on attentional orienting (characterized by a larger cost when the cue and target fall in different visual fields relative to when they fall in the same fields) when employing an endogenous cue. In Experiment 1 a circular display of eight boxes was employed and no meridian effect was found. In Experiment 2 three displays were compared: Semicircular, vertical, and horizontal. No meridian effect was found in the semicircular display. However, in the two linear displays (vertical, horizontal) there was a significant meridian effect. Experiments 3 and 4 were designed to investigate an attentional gradient hypothesis. In all experiments a distance effect was found. Nevertheless, the distance effect was not directly related to the actual distance between the target positions. The results suggest the existence of two attentional gradients that develop simultaneously in the visual field: an excitation gradient centred at the cued location (the strongest excitation) and an inhibition gradient, centred at the fixation point (the strongest inhibition).     

Parallel and serial search in haptics

We propose a model that distinguishes between parallel and serial search in haptics. To test this model, participants performed three haptic search experiments in which a target and distractors were presented to their fingertips. The participants indicated a target's presence by lifting the corresponding finger, or its absence by lifting all fingers. In one experiment, the target was a cross and the distractors were circles. In another, the target was a vertical line and the distractors were horizontal lines. In both cases, we found a serial search pattern. In a final experiment, the target was a horizontal line and the distractors were surfaces without any contours. In this case, we found a parallel search pattern. We conclude that the model can describe our data very well.     

Event-related brain potentials and cognitive processes related to perceptual—motor information transmission

In the present study, event-related potentials (ERPs) were recorded to investigate cognitive processes related to the partial transmission of information from stimulus recognition to response preparation. Participants classified two-dimensional visual stimuli with dimensions size and form. One feature combination was designated as the go-target, whereas the other three feature combinations served as no-go distractors. Size discriminability was manipulated across three experimental conditions. N2c and P3a amplitudes were enhanced in response to those distractors that shared the feature from the faster dimension with the target. Moreover, N2c and P3a amplitudes showed a crossover effect: Size distractors evoked more pronounced ERPs under high size discriminability, but form distractors elicited enhanced ERPs under low size discriminability. These results suggest that partial perceptual—motor transmission of information is accompanied by acts of cognitive control and by shifts of attention between the sources of conflicting information. Selection negativity findings imply adaptive allocation of visual feature-based attention across the two stimulus dimensions.     

Action matters: The role of action plans and object affordances in selection for action

In a series of three experiments requiring selection of real objects for action, we investigated whether characteristics of the planned action and/or the “affordances” of target and distractor objects affected interference caused by distractors. In all ofthe experiments, the target object was selectedon the basis of colour and was presented alone or with a distractor object. We examined the effect of type of response (button press, grasping, or pointing), object affordances (compatibility with the acting hand, affordances for grasping or pointing), and target/distractor positions (left or right) on distractor interference (reaction time differences between trials with and without distractors). Different patterns of distractor interference were associated with different motor responses. In the button-press conditions of each experiment, distractor interference was largely determined by perceptual salience (e.g., proximity to initial visual fixation). In contrast, in tasks requiring action upon the objects in the array, distractors with handles caused greater interference than those without handles, irrespective of whether the intended action was pointing or grasping. Additionally, handled distractors were relatively more salient when their affordances for grasping were strong (handle direction compatible with the acting hand) than when affordances were weak. These data suggest that attentional highlighting of specific target and distractor features is a function of intended actions.     

Discriminability and dimensionality effects invisual search for featural conjunctions: A functional pop-out

Treisman and Gelade’s (1980) feature-integration theory of attention states that a scene must be serially scanned before the objects in it can be accurately perceived. Is serial scanning compatible with the speed observed in the perception of real-world scenes? Most real scenes consist of many more dimensions (color, size, shape, depth, etc.) than those generally found in search paradigms. Furthermore, real objects differ from each other along many of these dimensions. The present experiment assessed the influence of the total number of dimensions and target/ distractor discriminability (the number of dimensions that suffice to separate a target from distractors) on search times for a conjunction of features. Search was always found to be serial. However, for the most discriminate targets, search rate was so fast that search times were in the same range as pop-out detection times. Apparently, greater discriminability enables subjects to direct attention at a faster rate and at only a fraction of the items in a scene.     

Attentional enhancement during multiple-object tracking

What is the role of attention in multiple-object tracking? Does attention enhance target representations, suppress distractor representations, or both? It is difficult to ask this question in a purely behavioral paradigm without altering the very attentional allocation one is trying to measure. In the present study, we used event-related potentials to examine the early visual evoked responses to task-irrelevant probes without requiring an additional detection task. Subjects tracked two targets among four moving distractors and four stationary distractors. Brief probes were flashed on targets, moving distractors, stationary distractors, or empty space. We obtained a significant enhancement of the visually evoked P1 and N1 components (～100–150 msec) for probes on targets, relative to distractors. Furthermore, good trackers showed larger differences between target and distractor probes than did poor trackers. These results provide evidence of early attentional enhancement of tracked target items and also provide a novel approach to measuring attentional allocation during tracking.     

Coactivation, pop-out, and symmetry affect line discrimination

17 college students discriminated between vertical and horizontal target lines flanked by vertical and horizontal distractors. Large displays with 3.0-cm lines and small displays half this size were used. The distractors were positioned above and below the target or to the left and right of it, forming vertical and horizontal global orientations. The number of flankers sharing the target's orientation was varied. Vertical targets and small display sizes were processed more quickly. Speed of responding did not differ for the two global orientations. There was no advantage for target lines consistent with global orientation. The results were replicated in Exp. 2 with diagonal flankers. This supports an explanation using stimulus compatibility, since diagonal lines do not correspond to either of the response options and cannot induce a tendency to respond more to one target than another. Displays with multiple symmetry were responded to faster than those with a single symmetry which were faster than asymmetrical displays, supporting the idea that symmetrical axes parallel and perpendicular to the target orientation provide a facilitative frame of reference. Coactivation from same-oriented flankers, pop-out from differences in orientation, and symmetry all influenced responding in these patterns.