Automatic adjustment of visuomotor readiness

Participants initiated a reaching movement to a single target more rapidly than to an odd-color target among distractors when the two trial types were presented in separate blocks, reflecting differentiated states of sensorimotor readiness for a relatively easy (single target) versus harder (odd-color target) tasks. This pattern was eliminated when the two trial types were randomly mixed. Latencies for the easy single trials increased, and those for the harder odd-color trials decreased, showing homogenization. The faster movement initiation in the odd-color target task was accompanied by curved trajectories, directed toward a distractor initially but corrected in midflight. Two possible hypotheses could account for this differentiated adjustment in visuomotor readiness: (1) explicit knowledge of upcoming trial types and (2) implicit leaning derived from history of the very recent past, that is, repetition of the same type of trials. To distinguish between these two accounts, we included a third condition where the trial types were predictably alternated. Contrary to the explicit knowledge hypothesis, this also led to homogenization of initiation latencies, and curved trajectories. We conclude that visuomotor readiness is automatically adjusted by the recent experience of trial difficulty.     

Unconfounding the direction of motion in depth, time to passage and rotation rate of an approaching object

Observers were presented with a set of 216 simulated approaching textured baseballs in random order. In Experiment 1 each had a different combination of time to passage (TTP), direction of motion in depth (dMID) in the vertical plane and total change in angular size (Δθ). In Experiments 2 and 3 each had a different combination of TTP, dMID and rate of ball rotation (RR). When required to discriminate TTP and dMID in separate experimental blocks for a non-rotating baseball (Experiment 1), observers could not discriminate dMID independently of variations in TTP but instead showed a bias towards perceiving objects approaching on a trajectory close to the nose as having a shorter TTP than objects approaching on a trajectory that would miss the face. When required to discriminate TTP, dMID and RR in separate experimental blocks (Experiment 2), TTP judgments were again influenced by dMID but could be made independently of RR. Judgments of the relative dMID were affected by variations in RR and rotation direction: for simulated overspin the (i.e., the top of the ball spins towards the observer) perceived ball trajectory was biased towards the ground whereas for simulated underspin the perceived ball trajectory was biased towards the sky. RR could be discriminated independently of both TTP and dMID. When required to make all three of these judgments simultaneously on each trial (Experiment 3) discrimination thresholds were not appreciably different from those found in Experiment 2. We conclude that TTP, dMID and RR can be estimated in parallel but not completely independently within the human visual system.     

Expectancies modulate attentional capture by salient color singletons

In singleton feature search for a form-defined target, the presentation of a task-irrelevant, but salient singleton color distractor is known to interfere with target detection [Theeuwes, J. (1991). Cross-dimensional perceptual selectivity. Perception & Psychophysics, 50, 184–193; Theeuwes, J. (1992). Perceptual selectivity for color and form. Perception & Psychophysics, 51, 599–606]. The present study was designed to re-examine this effect, by presenting observers with a singleton form target (on each trial) that could be accompanied by a salient) singleton color distractor, with the proportion of distractor to no-distractor trials systematically varying across blocks of trials. In addition to RTs, eye movements were recorded in order to examine the mechanisms underlying the distractor interference effect. The results showed that singleton distractors did interfere with target detection only when they were presented on a relatively small (but not on a large) proportion of trials. Overall, the findings suggest that cross-dimensional interference is a covert attention effect, arising from the competition of the target with the distractor for attentional selection [Kumada, T., & Humphreys, G. W. (2002). Cross-dimensional interference and cross-trial inhibition. Perception & Psychophysics, 64, 493–503], with the strength of the competition being modulated by observers’ (top-down) incentive to suppress the distractor dimension.     

Higher set sizes in pop-out search displays do not eliminate priming or enhance target selection

Previous research shows that salient stimuli do not pop out solely in virtue of their feature contrast. Rather, visual selection of a pop-out target is strongly modulated by feature priming: Repeating the target feature (e.g., red) across trials primes attention shifts to the target but delays target selection when the target feature changes (e.g., from red to green). However, it has been argued that priming modulated target selection only because the stimuli were too sparsely packed, suggesting that pop-out is still mostly determined by the target’s saliency (i.e., local feature contrast). Here, we tested these different views by measuring the observer’s eye movements in search for a colour target (Exp. 1) or size target (Exp. 2), when the target was similar versus dissimilar to the target, and when the displays contained 6 or 12 search items. The results showed that making the target less similar to the nontargets indeed eliminated priming effects in search for colour, but not in search for size. Moreover, increasing the set size neither increased search efficiency nor eliminated feature priming effects. Taken together, the results indicated that priming can still modulate target selection even in search for salient targets.     

Is contrast just another feature for visual selective attention?

The biased-competition theory of attention [Annual Review of Neuroscience 18 (1995) 193] suggests that attention and stimulus contrast trade off, and implies that high-contrast stimuli should be easy to attend to and hard to ignore. To test this, observers searched displays for a target digit. Observers were well able to exclude high-contrast distractors when attempting to search only among low-contrast stimuli (Experiment 1). In Experiments 2 and 3, location determined which stimuli were relevant. When contrast of relevant and irrelevant stimuli was uncertain (due to contrast varying between trials, Experiment 2), increasing the contrast of distractors impaired performance. However, when contrast was certain (due to blocking of trials, Experiment 3) and targets were of low contrast, high contrast distractors produced less interference than low contrast distractors. The ability of subjects to attend selectively to low vs. high contrast items in Experiments 1 and 3 suggests that selectivity for stimulus contrast might be similar to other types of feature selectivity (e.g., color and location). Such findings are inconsistent with the biased competition theory regarding the interplay of contrast and attention. However, results from Experiment 2 suggest that, when target contrast varies, the default tendency is to attend to high-contrast items.     

The inhibitory effect of a recent distracter

A series of experiments were conducted to examine the inhibitory effect of a visual distracter on saccadic eye movements. Participants were presented with a sequence of two critical displays. In one display a red target was presented together with a green distracter. This was followed by a display with a new red target presented in isolation at one of three locations with respect to the previous display. The lone target was presented either at the location of the recent target, the location of the recent distracter, or a new location. Participants were instructed to fixate the target in both displays and to ignore the green distracter. Experiment 1 revealed a significant increase in saccadic reaction times (SRTs) when the target was presented at the location of the recent distracter. Experiment 2 revealed that SRTs increased only in the conditions where the new target was presented at the location of the recent distracter, irrespective of its colour. Experiment 3 found that the inhibitory effect lasted for at least 2 s. In Experiment 4 the inhibitory effect was abolished when a lone distracter (i.e., anti-target) was presented without a target. Experiments 5 and 6 revealed that inhibition at the location of the recent target ('inhibition-of-return') also emerged with a shorter inter-display interval and when the distracter was removed from the recent display. These results distinguished between inhibition of a recent distracter and 'inhibition-of-return' and are consistent with models of competitive interactions which generate inhibitory effects on the spatial representation of a distracter.     

Spatiotemporal relationships in a dynamic scene: stereomotion induction and suppression

We establish the existence of purely stereoscopic motion induction, i.e., perceived depth motion induced into a fixed-disparity target by disparity changes in a surround region. The stimuli were dynamic autostereograms consisting of a target and a surround, both consisting of horizontal lines of discs. We explored the stereomotion induction process by (i) direct estimation of the perceived distance moved, (ii) a cancellation technique with compensatory target motion, and (iii) extension of the compensatory motion into the zone beyond the null point. Adding compensatory stereomotion to the target reduced the induced motion experience to a null point. Beyond the cancellation point, two surprising results were obtained; perceived motion in the target increased, while the surround stereomotion perception was almost suppressed over a wide range of disparity changes (reciprocal stereomotion suppression). A model of the target/surround interactions was developed in the context of dynamic organization principles operating in stereomotion perception and misperception.     

Convex hull test of the linear separability hypothesis in visual search.

Visual search for a colour target in distractors of two other colours is dramatically affected by the configuration of the colours in CIE (x, y) space. To a first approximation, search is difficult when a target's chromaticity falls directly between (i.e. is not linearly separable from) two distractor chromaticities, otherwise search is easy (D'Zmura [1991, Vision Research, 31, 951-966]; Bauer, Jolicoeur, & Cowan [1996a, Vision Research, 36, 1439-1466]; Bauer, Jolicoeur, & Cowan [1996b, Perception, 25, 1282-1294]). In this paper, we demonstrate that the linear separability effect transcends the two distractor case. Placing a target colour inside the convex hull defined by a set of distractors hindered search performance compared with a target placed outside the convex hull. This is true whether the target was linearly separable in chromaticity only (Experiments 1 and 2), or in a combination of luminance and chromaticity (Experiments 3 and 4).     

The relationship between the subjective and objective aspects of visual filling-in

We explored the relationship between filling-in processes and the known increase in detection sensitivity observed for targets presented between collinear flankers. Filling-in was probed using a Yes/No detection task by measuring the false-positive reports (false-alarm, FA) and hit rate (Hit) for a low-contrast Gabor target with different target-flankers distances. Observers increased the number of reports on the presence of a target (FA and Hit) when the flankers' distance was within the known range of facilitatory lateral interactions. This bias in reporting was reduced with blocked stimulation, when the target-flanker distance was kept fixed across trials. When different distances were mixed by trials the bias followed the pattern of lateral interactions across distance. The effect was maximal when flankers and targets were aligned. These false perceptions are most likely the result of a filling-in process by lateral excitation that produces illusory contours.     

Perceptual limits of common fate

We studied the perception of a coherently moving group of collinearly arranged dots ("target dots") that traveled orthogonally to their linear orientation within a background of noise dots moving in random yet straight directions at constant speed ("random-direction noise"). Using a 2-interval forced-choice task we obtained coherence thresholds equal to a signal-to-noise ratio of 1-2%. These thresholds are lower than the 4-10% reported in the literature suggesting that the collinear arrangement of the target dots, in addition to movement, provided form information. Weber's Law was found to hold 4-7 target dots. Overall, sensitivity was constant for a broad range of dot speeds up to at least 6.5 deg/s. Lifetime required for optimal perception was 430 ms, far shorter than the threshold duration of 1 s reported for randomly distributed (i.e., nonaligned) target dots [Vis. Res. 41 (2001) 1891]. Angular deviations from parallel between adjacent motion trajectories were tolerated up to 27 deg for divergence and up to 19 deg for convergence. Diverging motion was detected earlier (after 600-800 ms) than converging motion (>1 s). Forced-choice discrimination yielded a higher proportion of correct responses than the actual (i.e., conscious) perception of the coherently moving group of dots. Our results are consistent with findings from neurophysiological recordings and neuroimaging of motion-sensitive neurons in areas V1 and MT showing broad tuning curves for speed and direction of a moving visual stimulus.     

A search order lost effect: Ignoring a singleton distractor affects visual search efficiency

Four experiments investigated after-effects of attentional capture by a target-feature singleton distractor. In Experiments 1 and 3, participants searched for an orientation singleton target in a visual display and responded to a reported-attribute in the target (a compound search task). On some trials, a singleton distractor with the same orientation as, but a different color from, the target occurred. In the singleton distractor-absent trials reaction times for targets were unchanged irrespective of the number of nontargets. However, on singleton distractor-present trials, target reaction times increased with number of displayed nontargets. Ignoring target-feature singleton distractors induced inefficient visual searches slowed target search, suggesting that targets were searched serially in the presence of a singleton distractor induces inefficient serial search. This result implies that the search order, corresponding to relative item salience, is lost following attentional capture by a singleton distractor. Subsequent experiments explored conditions that might elicit the search order lost effect. It did not occur when task-irrelevant singleton distractors occurred in a compound search task (Experiment 2) or when target-feature (Experiment 4) singleton distractors occurred in a simple target detection task. Together, results suggest that the search order lost effect is mediated by dynamic computations involving saliency and feature maps. An explanation of this effect is proposed.     

The modulation of perceptual selection by working memory is dependent on the focus of spatial attention

Recent research suggests that visual selection can be automatically biased to those stimuli matching the contents of working memory (WM). However, a complete functional account of the interplay between WM and attention remains to be established. In particular, the boundary conditions of the WM effect on selection are unclear. Here, the authors investigate the influence of the focus of spatial attention (i.e., diffused vs. focused) by assessing the effect of spatial precues on attentional capture by WM. Experiments 1 and 2 showed that relative to a neutral condition without memory-matching stimuli, the presence of a memory distractor can trigger attentional capture despite being entirely irrelevant for the attention task but this happened only when the item was actively maintained in WM and not when it was merely repeated. Experiments 3a, 3b and 3c showed that attentional capture by WM can be modulated by endogenous spatial pre-cueing of the incoming target of selection. The authors conclude that WM-driven capture of visual selection is dependent on the focus of spatial attention.     

Locations of serial reach targets are coded in multiple reference frames

Previous work from our lab, and elsewhere, has demonstrated that remembered target locations are stored and updated in an eye-fixed reference frame. That is, reach errors systematically vary as a function of gaze direction relative to a remembered target location, not only when the target is viewed in the periphery (Bock, 1986, known as the retinal magnification effect), but also when the target has been foveated, and the eyes subsequently move after the target has disappeared but prior to reaching (e.g., [Henriques et al., 1998] , [Sorrento and Henriques, 2008] and [Thompson and Henriques, 2008] ). These gaze-dependent errors, following intervening eye movements, cannot be explained by representations whose frame is fixed to the head, body or even the world. However, it is unknown whether targets presented sequentially would all be coded relative to gaze (i.e., egocentrically/absolutely), or if they would be coded relative to the previous target (i.e., allocentrically/relatively). It might be expected that the reaching movements to two targets separated by 5° would differ by that distance. But, if gaze were to shift between the first and second reaches, would the movement amplitude between the targets differ? If the target locations are coded allocentrically (i.e., the location of the second target coded relative to the first) then the movement amplitude should be about 5°. But, if the second target is coded egocentrically (i.e., relative to current gaze direction), then the reaches to this target and the distances between the subsequent movements should vary systematically with gaze as described above. We found that requiring an intervening saccade to the opposite side of 2 briefly presented targets between reaches to them resulted in a pattern of reaching error that systematically varied as a function of the distance between current gaze and target, and led to a systematic change in the distance between the sequential reach endpoints as predicted by an egocentric frame anchored to the eye. However, the amount of change in this distance was smaller than predicted by a pure eye-fixed representation, suggesting that relative positions of the targets or allocentric coding was also used in sequential reach planning. The spatial coding and updating of sequential reach target locations seems to rely on a combined weighting of multiple reference frames, with one of them centered on the eye.     

The watercolor effect: a new principle of grouping and figure-ground organization

The watercolor effect is perceived when a dark (e.g., purple) contour is flanked by a lighter chromatic contour (e.g., orange). Under these conditions, the lighter color will assimilate over the entire enclosed area. This filling-in determines figure-ground organization when it is pitted against the classical Gestalt factors of proximity, good continuation, closure, symmetry, convexity, as well as amodal completion, and past experience. When it is combined with a given Gestalt factor, the resulting effect on figure-ground organization is stronger than for each factor alone. When the watercolor effect is induced by a dark red edge instead of an orange edge, its figural strength is reduced, but still stronger than without it. Finally, when a uniform surface is filled physically using the color of the orange fringe, figure-ground organization is not different from that for the purple contour only. These findings show that the watercolor effect induced by the edge could be an independent factor, different from the classical Gestalt factors of figure-ground organization.     

Target uncertainty does not lead to greater singleton distractor interference when target shapes are not interchangeable with nontarget shapes

Researchers have investigated whether attentional capture during visual search is driven by top-down processes, i.e. experimental goals and directives, or by bottom-up processes, i.e. the properties of the items within a search display. Some research has demonstrated that subjects cannot avoid attending to a task-irrelevant salient item, such as a singleton distractor, even when the identity of the target item is known. Research has also shown that repeating the target feature across successive search displays will prime the visual pop out effect for a unique target (priming of pop out). However, other research has shown that subjects can strategically guide their attention and may locate a target based on its uniqueness (a singleton search mode) or based on knowing and searching for the target feature (a feature search mode). When using the feature search mode subjects are attuned to the specific target feature and are therefore less susceptible to singleton distractor interference than when using the singleton search mode. Recent research has compared singleton distractor interference for targets that are variable and uncertain to targets that are constant and certain across search displays. When the target is constant subjects can use a feature search mode and should theoretically demonstrate less singleton distractor interference than when targets are variable and they must use a singleton search mode. Indeed, variable targets have historically demonstrated greater singleton distractor interference than constant targets, even when the target feature has been repeated. However, the current experiments found that singleton distractor interference was no greater for variable targets than for constant targets when targets and nontargets did not share shapes across search displays.     

How fast can you change your mind? The speed of top-down guidance in visual search

Most laboratory visual search tasks involve many searches for the same target, while in the real world we typically change our target with each search (e.g. find the coffee cup, then the sugar). How quickly can the visual system be reconfigured to search for a new target? Here observers searched for targets specified by cues presented at different SOAs relative to the search stimulus. Search for different targets on each trial was compared to search for the same target over a block of trials. Experiments 1 and 2 showed that an exact picture cue acts within 200 ms to make varied target conjunction search as fast and efficient as blocked conjunction search. Word cues were slower and never as effective. Experiment 3 replicated this result with a task that required top-down information about target identity. Experiment 4 showed that the effects of an exact picture cue were not mandatory. Experiments 5 and 6 used pictures of real objects to cue targets by category level.     

Visually and memory-guided grasping: aperture shaping exhibits a time-dependent scaling to Weber's law

The ‘just noticeable difference’ (JND) represents the minimum amount by which a stimulus must change to produce a noticeable variation in one’s perceptual experience and is related to initial stimulus magnitude (i.e., Weber’s law). The goal of the present study was to determine whether aperture shaping for visually derived and memory-guided grasping elicit a temporally dependent or temporally independent adherence to Weber’s law. Participants were instructed to grasp differently sized objects (20, 30, 40, 50 and 60 mm) in conditions wherein vision of the grasping environment was available throughout the response (i.e., closed-loop), when occluded at movement onset (i.e., open-loop), and when occluded for a brief (i.e., 0 ms) or longer (i.e., 2000 ms) delay in advance of movement onset. Within-participant standard deviations of grip aperture (i.e., the JNDs) computed at decile increments of normalized grasping time were used to determine participant’s sensitivity to detecting changes in object size. Results showed that JNDs increased linearly with increasing object size from 10% to 40% of grasping time; that is, the trial-to-trial stability (i.e., visuomotor certainty) of grip aperture (i.e., the comparator) decreased with increasing object size (i.e., the initial stimulus). However, a null JND/object size scaling was observed during the middle and late stages of the response (i.e., >50% of grasping time). Most notably, the temporal relationship between JNDs and object size scaling was similar across the different visual conditions used here. Thus, our results provide evidence that aperture shaping elicits a time-dependent early, but not late, adherence to the psychophysical principles of Weber’s law.     

Gaze-induced joint attention persists under high perceptual load and does not depend on awareness

The automaticity of gaze-induced joint attention is well known in relatively easy cognitive tasks; but its role in harder tasks had never been examined. This encouraged us to study automaticity in hard tasks, tasks presenting the subjects with high perceptual loads. The Rapid Serial Visual Presentation (RSVP) paradigm was used to present participants with two streams of bilaterally displayed digit-flows while they fixated at the center of a synthetic representation of a human face. The face was presented both above (Experiments 1 and 2) and below (Experiment 3) the face’s visual threshold (henceforth called “supraliminal” and “subliminal”, respectively). Interocular suppression was used to make the face stimulus invisible (subliminal). In the critical trials of all three experiments, the gaze direction shown on the face was randomly diverted to either the left or to the right. This directed the participant’s gaze either towards or away from the location of a target in the RSVP. The perceptual load was always relatively high. It was either set (Experiments 1 and 3) or manipulated (Experiment 2) during the experiment. In all three experiments, an appreciably higher and significant level of target detection was found when an uninformative gaze-cue was congruent with the location of the target. This result, which had only been reported with relatively easy tasks previously, is called the “gaze-cueing effect”. Our novel findings include showing that: (i) the attentional effect of gaze persists under high perceptual loads, and (ii) awareness of the gaze stimuli is not required to obtain the gaze-cueing effect. They also serve to validate prior support for an important role of automaticity in gaze-induced joint attention.     

Comparing crowding in human and ideal observers

A visual target is more difficult to recognize when it is surrounded by other, similar objects. This breakdown in object recognition is known as crowding. Despite a long history of experimental work, computational models of crowding are still sparse. Specifically, few studies have examined crowding using an ideal-observer approach. Here, we compare crowding in ideal observers with crowding in humans. We derived an ideal-observer model for target identification under conditions of position and identity uncertainty. Simulations showed that this model reproduces the hallmark of crowding, namely a critical spacing that scales with viewing eccentricity. To examine how well the model fits quantitatively to human data, we performed three experiments. In Experiments 1 and 2, we measured observers' perceptual uncertainty about stimulus positions and identities, respectively, for a target in isolation. In Experiment 3, observers identified a target that was flanked by two distractors. We found that about half of the errors in Experiment 3 could be accounted for by the perceptual uncertainty measured in Experiments 1 and 2. The remainder of the errors could be accounted for by assuming that uncertainty (i.e., the width of internal noise distribution) about stimulus positions and identities depends on flanker proximity. Our results provide a mathematical restatement of the crowding problem and support the hypothesis that crowding behavior is a sign of optimality rather than a perceptual defect.     

Effects of the duration of early strabismus on the binocular responses of neurons in the monkey visual cortex (V1)

PURPOSE: To determine how the duration of early strabismus influences the severity of loss of disparity sensitivity in V1 neurons and the effects of extensive poststrabismus visual experience on the maintenance of functional binocular connections. METHODS: Concomitant strabismus was optically simulated in 10 rhesus monkeys using a prism-rearing procedure. The onset of strabismus was kept constant at 4 weeks of age and the duration was maintained for 2, 4, or 8 weeks. In one group of monkeys (infants), the neurophysiological experiments were conducted immediately after the period of rearing with prisms. In another group (adults), after the termination of the prism-rearing regimen at either 8 or 12 weeks of age, the monkeys were kept in a normal housing environment until maturity and behavioral testing was conducted before the recording experiments to determine the animal's monocular and binocular visual capacities. To assess the effects of the period of early strabismus on binocular interactions in V1, extracellular single-unit recording methods were used in anesthetized and paralyzed monkeys, and dichoptic sine-wave gratings were used as stimuli. RESULTS: In all strabismic monkeys, the sensitivity of V1 units to interocular spatial phase disparity (disparity sensitivity) was significantly reduced, and the prevalence of binocular suppression was higher than that found in age-matched control animals. Although 8 weeks of strabismus resulted in a slightly larger loss of disparity sensitivity, the overall effects of the duration of strabismus were surprisingly small in infant strabismic monkeys. After poststrabismus visual experience, a small but significantly higher degree of disparity sensitivity was noted in V1 if prism-rearing was terminated after 4 weeks of strabismus (i.e., at 8 weeks of age), but not after 8 weeks of strabismus (i.e., at 12 weeks of age). CONCLUSIONS: A brief period (2 weeks) of misalignment after the emergence of stereopsis is sufficient to drastically reduce the functional binocular connections in V1, and longer periods of strabismus result in little additional loss in disparity sensitivity. Clinically, these results suggest that taking corrective measures for infantile esotropes before the known onset age for stereopsis may be important for maintaining better binocular sensory function and better interocular alignment at later stages of development.