Manual pointing to remembered targets...but also in a remembered visual context

In this paper we investigated whether visual background information available during target presentation influences manual pointing to remembered targets. Younger and older participants manually pointed with their unseen hands to remembered or visible targets that were presented or not over a structured visible background. The results indicated that a structured visual background biased movement planning processes, but did not influence motor control processes, regardless of the fact that target location and the background were visible or remembered. How one uses visual background information for movement planning is not modified by aging.     

Visual imagery in backward masking

Observers viewed briefly presented target dot patterns, either at low contrast without a mask (no mask, or NM) or at high contrast and followed by a long-lasting patterned mask (backward masking, or BM). Experiment 1 demonstrated independent processing of NM target dots but limited capacity processing of BM target dots. Experiments 2 and 3 showed that visual images may radically change sensitivity (d′) in BM but not in NM. Results suggest that d′ is reduced if the image suppresses dots relevant for the detection task, but that d′ is raised if the image suppresses dots that compete for processing with those the observer must detect.     

Effects of letter-names on visual search

When a subject searches through a list of letters of mixed case in a letter cancellation task search is slowed if the background items include the target letter in the other case. This effect is largest when target and confusing background have visual analogue similarity (e.g., cC) but still obtains when no special visual similarity exists (e.g., aA). In searches for two targets, one of either case, search is facilitated when the targets are both cases of the same letter.     

Visual marking beside the mark: prioritizing selection by abrupt onsets

In a standard visual marking experiment, observers are presented with a display containing one set of elements (old elements) followed after a certain time interval by a second set of elements (new elements). The task of observers is to search for a target among the new elements. Typically, the time to find the target depends only on the number of new elements in the display and not on the number of old elements, showing that observers search only among the new elements. This effect of prioritizing new elements over old elements is explained in terms of top-down inhibition of old objects-that is, visual marking (Watson & Humphreys, 1997). The present study addressed whether this prioritizing is in fact mediated by top-down inhibition of old objects, as suggested by Watson and Humphreys (1997), or whether it is mediated by the abrupt onsets of the newly presented elements (Yantis & Jonides, 1984). In three experiments, the presentations of the old and new elements were or were not accompanied by a luminance change. The results showed that if new elements were equiluminant with the background, no visual marking occurred, suggesting that new elements must have a luminance onset in order to be prioritized over old elements. Implications for current theories on visual selection are discussed.     

Visual marking beside the mark: Prioritizing selection by abrupt onsets

In a standard visual marking experiment, observers are presented with a display containing one set of elements (old elements) followed after a certain time interval by a second set of elements (new elements). The task of observers is to search for a target among the new elements. Typically, the time to find the target depends only on the number of new elements in the display and not on the number of old elements, showing that observers search only among the new elements. This effect of prioritizing new elements over old elements is explained in terms of top-down inhibition of old objects—that is, visual marking (Watson & Humphreys, 1997). The present study addressed whether this prioritizing is in fact mediated by top-down inhibition of old objects, as suggested by Watson and Humphreys (1997), or whether it is mediated by the abrupt onsets of the newly presented elements (Yantis & Jonides, 1984). In three experiments, the presentations of the old and new elements were or were not accompanied by a luminance change. The results showed that if new elements were equiluminant with the background, no visual marking occurred, suggesting that new elements must have a luminance onset in order to be prioritized over old elements. Implications for current theories on visual selection are discussed.     

Visual storage or visual masking? an analysis of the “retroactive contour enhancement” effect

Standing and Dodwell (1972) reported that a contoured target stimulus which is only poorly identified when exposed briefly against a steady background field can be identified accurately if the field is terminated shortly after target offset. This observation was confirmed and, in addition, it was shown that target identification is enhanced when the field is initiated shortly before target onset. Furthermore, it was demonstrated that a continuous background field is not essential for either effect. It was argued that these “retroactive” and “proactive” enhancements of target identification were due to a complex interaction among forward, backward, and simultaneous masking.     

Contextual cuing by global features

In visual search tasks, attention can be guided to a target item--appearing amidst distractors--on the basis of simple features (e.g., finding the red letter among green). Chun and Jiang's (1998) contextual cuing effect shows that reaction times (RTs) are also speeded if the spatial configuration of items in a scene is repeated over time. In the present studies, we ask whether global properties of the scene can speed search (e.g., if the display is mostly red, then the target is at location X). In Experiment 1A, the overall background color of the display predicted the target location, and the predictive color could appear 0, 400, or 800 msec in advance of the search array. Mean RTs were faster in predictive than in nonpredictive conditions. However, there was little improvement in search slopes. The global color cue did not improve search efficiency. Experiments 1B-1F replicated this effect using different predictive properties (e.g., background orientation-texture and stimulus color). The results showed a strong RT effect of predictive background, but (at best) only a weak improvement in search efficiency. A strong improvement in efficiency was found, however, when the informative background was presented 1,500 msec prior to the onset of the search stimuli and when observers were given explicit instructions to use the cue (Experiment 2).     

Negative priming depends on prime-probe similarity: Evidence for episodic retrieval

It is well established that requiring a person to respond to a recently ignored object in a visual selection task leads to slower responding (i.e., negative priming). In the present experiment, subjects identified target letters flanked by incompatible distractor letters on prime and probe displays. Prime display distractors appeared as the target letter on one third of subsequent probe displays. We manipulated stimulus strength by means of intensity contrast between letter displays and their background. Displays were presented with either high contrast (white against a black background) or low contrast (dark gray against a black background). The important finding was that negative priming was maximal when prime and probe displays shared the same intensity contrast. These results suggest that greater similarity between prime and probe displays results in improved retrieval of prime display information. The results provide strong support for an episodic retrieval account of negative priming.     

Saccadic suppression of displacement: effects of illumination and background manipulation

In contrast to other functions which are suppressed during saccades, saccadic suppression of displacement (SSD--a decrease in sensitivity to visual displacements during saccades) has often been considered to be due to efferent processes rather than to visual masking. The aim of this study was to explicitly assess the importance of visual conditions in SSD. In two experiments, a small computer-generated target made random horizontal jumps. An infrared eye tracker was used to detect the saccade toward the new position, triggering a smaller centripetal displacement of the target. Subjects reported awareness of these intrasaccadic displacements by pressing a key. In the first experiment, the task was performed in both a well-lit environment and in darkness. In the second experiment these conditions were replicated and additional factors such as the contrast of the background and the effect of moving the target spot alone or the target plus the entire background were investigated. Unlike other forms of saccadic suppression, SSD was stronger in the dark, although subjects also had a greater bias to report detections in that condition. Other background manipulations had no effect. The effect of ambient lighting on SSD is small and subtle. Effects of other background manipulations may be overridden by the focusing of attention on a small moving target.     

An examination of the effects of axis foreshortening, monocular depth cues, and visual field on object identification

Four experiments are reported on the identification of line drawings of common objects. In each experiment, performance on “unconventional” views of the objects, in which the major axis of the object was foreshortened, was compared to performance on more “conventional” views without appreciable foreshortening. In each experiment, except Experiment 2, where performance on the two views was experimentally equated, the foreshortened views were more difficult to identify than were the conventional views. Experiments 1 and 2 showed that if the foreshortened views were presented on a background with strong monocular depth cues, object identification was improved. This result suggests that part of the difficulty in identifying objects depicted from such a view stems from an improper depth interpretation of the object depictions. Experiments 3 and 4 examined visual field differences in the identification of the two types of object view. Results reported in the neuropsychological literature have shown that people with right-hemisphere damage have particular difficulty with the identification of unconventional views of objects that foreshorten major axes. Accordingly, it was expected that there would be a left visual field advantage for the foreshortened views. Neither experiment yielded any visual field effects consistent with this expectation. Possible reasons for the lack of a field effect are discussed.     

Surprise-induced blindness: a stimulus-driven attentional limit to conscious perception

The cost of attending to a visual event can be the failure to consciously detect other events. This processing limitation is well illustrated by the attentional blink paradigm, in which searching for and attending to a target presented in a rapid serial visual presentation stream of distractors can impair one's ability to detect a second target presented soon thereafter. The attentional blink critically depends on 'top-down' attentional settings, for it does not occur if participants are asked to ignore the first target. Here we show that 'bottom-up' attention can also lead to a profound but ephemeral deficit in conscious perception: Presentation of a novel, unexpected, and task-irrelevant stimulus virtually abolishes conscious detection of a target presented within half a second after the 'Surprise' stimulus, but only for its earliest occurrences (generally 1 to 2 presentations). This powerful but short-lived deficit contrasts with a milder but more enduring form of attentional capture that accompanies singleton presentations in rapid serial visual presentations. We conclude that the capture of stimulus-driven attention alone can limit explicit perception.     

Achromatic color matching as a function of apparent target orientation,target and background luminance,and lightness or brightness instructions

In two experiments, achromatic color matches for a fixed target, under constant illumination, were compared under conditions where the target appeared perpendicular to illumination direction and coplanar with the background (monocular viewing) and where the target appeared nonperpendicular to illumination direction and noncoplanar with the background (binocular viewing). Contrary to the coplanar ratio hypothesis, which predicts a “lightening” of the target seen coplanar with a darker background, a general “darkening” of the target occurred for both white (N9.5/) and black (N3/) backgrounds and for both dark (N5.5/) and light (N6.5/) targets. This darkening effect was greatest for the darker target and black background, and approximately equal for other combinations of target value and background. The direction of the darkening effect is consistent with the albedo hypothesis, which assumes an inferential correction for changes in conditions of illumination. However, variation in the magnitude of the darkening effect is problematical, and cannot be easily explained by any existing theory. In both experiments, instructions to judge “lightness” or “brightness” failed to produce any substantial differences in performance, although postexperimental questioning suggested that subjects had a verbal understanding of these concepts. Apparently, under reduction conditions, subjects lack cues to illumination and make only lightness matches, regardless of instructions.     

Changes in infants' ability to switch visual attention in the first three months of life.

The abilities of 1-month-old and 3-month-old infants to shift their gaze from a central target to a peripheral target were compared in four experiments. In experiment 1 targets matched in mean luminance to the background were presented to infants in the periphery at varying levels of contrast. The contrast thresholds for target detection were found to be significantly different for 1-month-olds compared with 3-month-olds. With targets set close to these contrast thresholds, correct refixations and the latency for shifting attention were examined in experiment 2. Two conditions were used: a peripheral target was presented against a homogeneous background (noncompetition); and in the second condition, the patterned target appeared at one of two lighter peripheral windows set against a darker background (competition). Although there was no difference between the two age groups in the latency for shifting visual attention, 1-month-olds were found to make more directional errors in the competition condition. The competition effect of two potential targets on latencies was examined in experiment 3. In the competition condition, two identical peripheral patterned targets were presented to the infants. The 3-month-olds refixated more quickly to one of the double targets in the competition condition than to a single peripheral target, whereas 1-month-olds were slowed down by a double target display. Finally, in experiment 4 the ability of the infants to process and disengage from a central stimulus and to refixate towards a similar peripheral target was examined. This type of competition disrupted both the direction of the first eye movement and the latency to shift attention in both age groups. However, the effect was significantly greater for the 1-month-olds. Taken together, the results of these experiments demonstrate the greater disruption of fixation-shift behaviour in 1-month-olds compared with 3-month-olds when competing visual stimuli are used. This developmental change is explained in terms of maturation of executive cortical orienting systems over the first months of life.     

Icon flickering, flicker rate, and color combinations of an icon's symbol/background in visual search performance

The effects of color combinations of an icon's symbol/background and components of flicker and flicker rate on visual search performance on a liquid crystal display screen were investigated with 39 subjects who searched for a target icon in a circular stimulus array (diameter = 20 cm) including one target and 19 distractors. Analysis showed that the icon's symbol/background color significantly affected search time. The search times for icons with black/red and white/blue were significantly shorter than for white/yellow, black/yellow, and black/blue. Flickering of different components of the icon significantly affected the search time. Search time for an icon's border flickering was shorter than for an icon symbol flickering; search for flicker rates of 3 and 5 Hz was shorter than that for 1 Hz. For icon's symbol/background color combinations, search error rate for black/blue was greater than for black/red and white/blue combinations, and the error rate for an icon's border flickering was lower than for an icon's symbol flickering. Interactions affected search time and error rate. Results are applicable to design of graphic user interfaces.     

Driving attention with the top down: the relative contribution of target templates to the linear separability effect in the size dimension

Bauer, Jolicoeur, and Cowan (1996a, 1996b, 1998) have shown that visual search for a target among distractors is apparently serial if the target is nonlinearly separable from the distractors in a particular feature space (e.g., color or size). In contrast, if the target is linearly separable from the distractors, search is relatively easy and seemingly spatially parallel. We examined the contribution of top-down knowledge of the target to the linear separability effect on search. Two visual search experiments were conducted using small, medium, or large circles as targets. In the first experiment, participants could use knowledge of the target to guide search, whereas, in the second, the target was unknown on each trial. Search for a medium (nonlinearly separable) target among small or large distractors benefited least from knowledge of the target as compared with search for a small or large target. Thus, the linear separability effect can be determined in part by use of top-down knowledge to facilitate the detection of targets at the ends of a continuum defining the stimuli.     

Driving attention with the top down: The relative contribution of target templates to the linear separability effect in the size dimension

Bauer, Jolicoeur, and Cowan (1996a, 1996b, 1998) have shown that visual search for a target among distractors is apparently serial if the target is nonlinearly separable from the distractors in a particular feature space (e.g., color or size). In contrast, if the target is linearly separable from the distractors, search is relatively easy and seemingly spatially parallel. We examined the contribution of top-down knowledge of the target to the linear separability effect on search. Two visual search experiments were conducted using small, medium, or large circles as targets. In the first experiment, participants could use knowledge of the target to guide search, whereas, in the second, the target was unknown on each trial. Search for a medium (nonlinearly separable) target among small or large distractors benefited least from knowledge of the target as compared with search for a small or large target. Thus, the linear separability effect can be determined in part by use of top-down knowledge to facilitate the detection of targets at the ends of a continuum defining the stimuli.     

Task- and location-switching effects on visual attention

In two experiments, we examined the effects of task and location switching on the accuracy of reporting target characters in an attentional blink (AB) paradigm. Single-character streams were presented at a rate of 100 msec per character in Experiment 1, and successive pairs of characters on either side of fixation were presented in Experiment 2. On each trial, two targets appeared that were either white letters or black digits embedded in a stream of black letter distractors, and they were separated by between zero and five items in the stream (lags 1-6). Experiment 1 showed that report of the first target was least accurate if it immediately preceded the second target and if the two targets were either both letters or both digits (task repetition cost). Report of the second target was least accurate if one or two distractors intervened between the two targets (the U-shaped AB lag effect) and if one target was a letter and the other a digit (task switch cost). Experiment 2 added location uncertainty as a factor and showed similar effects as Experiment 1, with one exception. Lag 1 sparing (the preserved accuracy in reporting the second of two targets if the second immediately follows the first) was completely eliminated when the task required attention switching across locations. Two-way additive effects were found between task switching and location switching in the AB paradigm. These results suggests separate loci for their attentional effects. It is likely that the AB deficit is due mainly to central memory limitations, whereas location-switching costs occur at early visual levels. Task-switching costs occur at an intermediate visual level, since the present task switch involved encoding differences without changes in stimulus-response mapping rules (i.e., the task was character identification for both letters and digits).     

The role of the adjacency between background cues and objects in visual localization during ocular pursuit

Subjects used eye movements to pursue a light target that moved from left to right with a velocity of 15 deg s-1. The stimulus was a sudden five-fold decrease in target intensity during the movement. The subject's task was to localize the stimulus relative to either a single stationary background point or the midpoint between two points (28 deg apart) placed 0.5 deg above the target path. The stimulus was usually mislocated in the direction of eye movement; the mislocation was affected by the spatial adjacency between background and stimulus. When an auditory, rather than a visual, stimulus was presented during tracking, target position at the time of stimulus presentation was visually mislocated in the direction opposite to that of eye movement. The effect of adjacency between background and target remained the same. The involvement of processes of subject-relative and object-relative visual perception is discussed.     

Visual assessment of camouflaged targets with different background similarities

The study investigated the effectiveness of different camouflage designs using a computational image quality index. Camouflaged human targets were presented on a natural landscape and the targets were designed to be similar to the landscape background with different levels of background similarity as estimated by the image index. The targets were presented in front of the observer (central 0 degrees) or at different angles in the left (-7 degrees, -14 degrees, -21 degrees) or right (+7 degrees, +14 degrees, +21 degrees) visual fields. The observer had to detect the target using peripheral vision if the target appeared in the left and right visual fields. The camouflage effectiveness was assessed by detection hit rates, detection times, and subjective ratings on detection confidence and task difficulty. The study showed that the psychophysical measures correlated well with the image similarity index, suggesting a potentially more efficient camouflage effectiveness assessment tool if the relationship between the psychophysical results and the index can be quantified in the future.     

The equivalent background of bleaching

Stiles and Crawford proposed that a retinal region bleached by preexposure to intense light behaves as if it were illuminated by some steady veiling or background luminance. We test this notion by comparing the afterimage of a bleaching light with a steady (and retinally stabilized) light of adjustable intensity, in the manner of Barlow and Sparrock. With their matching procedure, and also with a new procedure, we find as they did that during the rod phase of recovery the afterimage does look like a stabilized field of an intensity which, presented as a background, brings visual sensitivity to the same level. It is as if the two conditions produce equal signals at some stage of the visual pathway. Liked Barlow and Sparrock we observe a rod-cone break in the afterimage matches. However, we argue that the appearance of the rod-cone break presents a paradox and we show a way to resolve it.