Stimulus Display Research Articles

The timecourse of space- and object-based attentional prioritization with varying degrees of certainty

The relative contributions of objects (i.e., object-based) and underlying spatial (i.e., space-based representations) to attentional prioritization and selection remain unclear. In most experimental circumstances, the two representations overlap thus their respective contributions cannot be evaluated. Here, a dynamic version of the two-rectangle paradigm allowed for a successful de-coupling of spatial and object representations. Space-based (cued spatial location), cued end of the object, and object-based (locations within the cued object) effects were sampled at several timepoints following the cue with high or low certainty as to target location. In the high uncertainty condition spatial benefits prevailed throughout most of the timecourse, as evidenced by facilitatory and inhibitory effects. Additionally, the cued end of the object, rather than a whole object, received the attentional benefit. When target location was predictable (low uncertainty manipulation), only probabilities guided selection (i.e., evidence by a benefit for the statistically biased location). These results suggest that with high spatial uncertainty, all available information present within the stimulus display is used for the purposes of attentional selection (e.g., spatial locations, cued end of the object) albeit to varying degrees and at different time points. However, as certainty increases, only spatial certainty guides selection (i.e., object ends and whole objects are filtered out). Taken together, these results further elucidate the contributing role of space- and object-representations to attentional guidance.

Global interference during early visual processing: ERP evidence from a rapid global/local selective task

Visual perception depends on the integration of local elements of a visual scene into a global frame. Evidence from behavioral studies shows that (1) the detection of the global frame is faster than the detection of the local parts, a phenomenon called the global advantage, and that (2) an interference of the global shape is also present during local processing. Together, these effects are called the global precedence effect (GPE). Even if the global advantage appears to impact neural processing as early as the first 100 ms post-stimulus, previous studies failed to find a global interference effect before 200 ms post-stimulus. Using for the first time a rapid display of letter component stimuli during a global/local selective task in which conditions with perceptual conflict, congruent and incongruent conditions were considered, the present event-related potential (ERP) study shows a global interference effect occurring as early as the time range of the N1 component. In particular, only congruent stimuli elicited similar N1 amplitude during the global and local tasks, whereas an increased of the N1 amplitude during the global task was observed (as compared to the local task) for both stimuli with perceptual conflict and incongruent stimuli. This finding corroborates the recent neural models of human visual perception.

Dissociation of first- and second-order motion systems by perceptual learning

Previous studies investigating transfer of perceptual learning between luminance-defined (LD) motion and texture-contrast-defined (CD) motion tasks have found little or no transfer from LD to CD motion tasks but nearly perfect transfer from CD to LD motion tasks. Here, we introduce a paradigm that yields a clean double dissociation: LD training yields no transfer to the CD task, but more interestingly, CD training yields no transfer to the LD task. Participants were trained in two variants of a global motion task. In one (LD) variant, motion was defined by tokens that differed from the background in mean luminance. In the other (CD) variant, motion was defined by tokens that had mean luminance equal to the background but differed from the background in texture contrast. The task was to judge whether the signal tokens were moving to the right or to the left. Task difficulty was varied by manipulating the proportion of tokens that moved coherently across the four frames of the stimulus display. Performance in each of the LD and CD variants of the task was measured as training proceeded. In each task, training produced substantial improvement in performance in the trained task; however, in neither case did this improvement show any significant transfer to the nontrained task.

Evidence for mental subdivision of space by infants: 3- to 4-month-olds spontaneously bisect a small-scale area into left and right categories

Young infants have been shown to represent the left versus right spatial category relations of a target object and a vertical referent bar. In the present study, we examined whether infants would represent left versus right when the vertical bar was removed from the stimulus display. In Experiment 1, 3- to 4-month-olds who had been familiarized with stimuli depicting a diamond appearing in different locations to the left or right of the vertical midline displayed a mean novel category preference for a stimulus depicting the diamond on the opposite side of the midline. In Experiment 2, another group of 3- to 4-month-olds discriminated the within-category position changes of the diamond as it appeared to the left or right of the vertical midline. The results indicate that young infants can mentally bisect small-scale space into left versus right categories.

Attentional Focus and the Dynamics of Dual Identity Integration

Do situational cues to individuals’ social identities shift the way they look at objects? Do such shifts hinge on the structure of individuals’ self-concept? We hypothesized individuals with integrated identities would exhibit attentional biases congruent with identity cues (assimilative response), whereas those with nonintegrated identities would exhibit attentional biases incongruent with identity cues (contrastive response). Dual identity participants (Asian Americans, Study 1; female lawyers, Study 2) were exposed to identity primes and then asked to focus on central, focal objects in a stimulus display. Among participants with high identity integration, American (Study 1) or lawyer priming (Study 2) shifted attention toward focal objects (assimilative response). Among participants with low identity integration, Asian (Study 1) or female priming (Study 2) shifted attention toward focal objects (contrastive response). Dual identity integration moderates responses to identity cues in attentional focus. Implications for identity structure, object perception, and task performance are discussed.

Simultaneous Brightness and Apparent Depth from True Colors on Grey: Chevreul Revisited

We show that true colors as defined by Chevreul (1839) produce unsuspected simultaneous brightness induction effects on their immediate grey backgrounds when these are placed on a darker (black) general background surrounding two spatially separated configurations. Assimilation and apparent contrast may occur in one and the same stimulus display. We examined the possible link between these effects and the perceived depth of the color patterns which induce them as a function of their luminance contrast. Patterns of square-shaped inducers of a single color (red, green, blue, yellow, or grey) were placed on background fields of a lighter and a darker grey, presented on a darker screen. Inducers were always darker on one side of the display and brighter on the other in a given trial. The intensity of the grey backgrounds varied between trials only. This permitted generating four inducer luminance contrasts, presented in random order, for each color. Background fields were either spatially separated or consisted of a single grey field on the black screen. Experiments were run under three environmental conditions: dark-adaptation, daylight, and rod-saturation after exposure to bright light. In a first task, we measured probabilities of contrast, assimilation, and no effect in a three-alternative forced-choice procedure (background appears brighter on the 'left', on the 'right' or the 'same'). Visual adaptation and inducer contrast had no significant influence on the induction effects produced by colored inducers. Achromatic inducers produced significantly stronger contrast effects after dark-adaptation, and significantly stronger assimilation in daylight conditions. Grouping two backgrounds into a single one was found to significantly decrease probabilities of apparent contrast. Under the same conditions, we measured probabilities of the inducers to be perceived as nearer to the observer (inducers appear nearer on 'left', on 'right' or the 'same'). These, as predicted by Chevreul's law of contrast, were determined by the luminance contrast of the inducers only, with significantly higher probabilities of brighter inducers to be seen as nearer, and a marked asymmetry between effects produced by inducers of opposite sign. Implications of these findings for theories which attempt to link simultaneous induction effects to the relative depth of object surfaces in the visual field are discussed.

The integration of disparity, shading and motion parallax cues for depth perception in humans and monkeys

A visual stimulus display was created that enabled us to examine how effectively the three depth cues of disparity, motion parallax and shading can be integrated in humans and monkeys. The display was designed to allow us to present these three depth cues separately and in various combinations. Depth was processed most effectively and most rapidly when all three cues were presented together indicating that these separate cues are integrated at yet unknown sites in the brain. Testing in humans and monkeys yielded similar results suggesting that monkeys are a good animal model for the study of the underlying neural mechanisms of depth perception.

The influence of phonetic dimensions on aphasic speech perception

Individuals with aphasia have more problems detecting small differences between speech sounds than larger ones. This paper reports how phonemic processing is impaired and how this is influenced by speechreading. A non-word discrimination task was carried out with ‘audiovisual’, ‘auditory only’ and ‘visual only’ stimulus display. Subjects had to decide whether two presented stimuli were the same. Six aphasic subjects with speech sound processing difficulties and 14 non-brain-damaged control subjects participated in this study. It was found that the aphasic subjects have difficulties in discriminating pairs of non-words, which are more profound for small differences. Differences in ‘voicing’ were least often detected and therefore seem most difficult to perceive. This implies different processing of the phonetic dimensions in speech sound perception. Performance improved when speechreading was possible. As this improvement is not based on differences in place of articulation only, theories of audiovisual processing need to be revised.

Working memory, perceptual priming, and the perception of hierarchical forms: Opposite effects of priming and working memory without memory refreshing

Previous research has shown that stimuli held in working memory (WM) can influence spatial attention. Using Navon stimuli, we explored whether and how items in WM affect the perception of visual targets at local and global levels in compound letters. Participants looked for a target letter presented at a local or global level while holding a regular block letter as a memory item. An effect of holding the target's identity in WM was found. When memory items and targets were the same, performance was better than in a neutral condition when the memory item did not appear in the hierarchical letter (a benefit from valid cuing). When the memory item matched the distractor in the hierarchical stimulus, performance was worse than in the neutral baseline (a cost on invalid trials). These effects were greatest when the WM cue matched the global level of the hierarchical stimulus, suggesting that WM biases attention to the global level of form. Interestingly, in a no-memory priming condition, target perception was faster in the invalid condition than in the neutral baseline, reversing the effect in the WM condition. A further control experiment ruled out the effects of WM being due to participants' refreshing their memory from the hierarchical stimulus display. The data show that information in WM biases the selection of hierarchical forms, whereas priming does not. Priming alters the perceptual processing of repeated stimuli without biasing attention.

Evidence for top-down control of eye movements during visual decision making

Participants' eye movements were monitored while they viewed displays containing 6 exemplars from one of several categories of everyday items (belts, sunglasses, shirts, shoes), with a column of 3 items presented on the left and another column of 3 items presented on the right side of the display. Participants were either required to choose which of the two sets of 3 items was the most expensive (2-AFC) or which of the 6 items was the most expensive (6-AFC). Importantly, the stimulus display, and the relevant stimulus dimension, were held constant across conditions. Consistent with the hypothesis of top-down control of eye movements during visual decision making, we documented greater selectivity in the processing of stimulus information in the 6-AFC than the 2-AFC decision. In addition, strong spatial biases in looking behavior were demonstrated, but these biases were largely insensitive to the instructional manipulation, and did not substantially influence participants' choices.

Robustness of the retinotopic attentional trace after eye movements

With each eye movement, the image received by the visual system changes drastically. To maintain stable spatiotopic (world-centered) representations, the relevant retinotopic (eye-centered) coordinates must be continually updated. Although updating or remapping of visual scene representations can occur very rapidly, J. D. Golomb, M. M. Chun, and J. A. Mazer (2008) demonstrated that representations of sustained attention update more slowly than the remapping literature would predict; attentional benefits at previously attended retinotopic locations linger after completion of the saccade, even when this location is no longer behaviorally relevant. The present study explores the robustness of this "retinotopic attentional trace." We report significant retinotopic facilitation despite attempts to eliminate or reduce it by enhancing spatiotopic reference frames with permanent visual cues in the stimulus display and by introducing a different task where the attended location is the saccade target itself. Our results support and extend our earlier model of native retinotopically organized salience maps that must be dynamically updated to reflect the task-relevant spatiotopic location with each saccade. Consistent with the idea that attentional facilitation arises from persistent, recurrent neural activity, it takes measurable time for this facilitation to decay, leaving behind a retinotopic attentional trace after the saccade has been executed, regardless of conflicting task demands.

Top-Down Modulation of Motor Priming by Belief About Animacy

There is recent evidence that we directly map observed actions of other agents onto our own motor repertoire, referred to as direct matching (Iacoboni et al., 1999). This was shown when we are actively engaged in joint action with others' (Sebanz et al. 2003) and also when observing irrelevant movements while executing congruent or incongruent movements (Brass et al., 2000). However, an open question is whether direct matching in human beings is limited to the perception of intentional agents. Recent research provides contradictory evidence with respect to the question whether the direct matching system has a biological bias possibly emerging from perceptual differences of the stimulus display. In this study all participants performed a motor priming task observing the identical animation showing finger lifting movements of a hand in a leather glove. Before running the experiment we presented either a human hand or a wooden analog hand wearing the leather glove. We found a motor priming effect for both human and wooden hands. However, motor priming was larger when participants believed that they interacted with a human hand than when they believed to interact with a wooden hand. The stronger motor priming effect for the biological agent suggests that the "direct matching system" is tuned to represent actions of animate agents.

The reduction of the effect of the Müller-Lyer illusion saccade amplitude by classic adaptation.

The effect of Müller-Lyer stimuli on saccade amplitude varies across studies. One methodological difference between studies is stimulus display time; studies with long stimulus display times tend to report smaller effects than studies with short display times. Is it possible that long display times might provide conditions in which saccade adaption takes place? Five adult subjects were exposed to runs of the same illusion-inducing Müller-Lyer stimulus, presented for 1 s, interspersed with probe trials in which a point target was presented for 200 ms. While saccade amplitude was consistently larger with ‘in-configurations’ than with ‘out-configurations’ at the beginning of runs, amplitude declined over runs with the in-configuration. On average, it was constant in out-configuration runs. The net effect was a decline in the apparent effect size (in-amp − out-amp / out-amp) of the Müller-Lyer stimulus. Probe trial saccade amplitude increased in ‘out’ runs and decreased in ‘in’ runs. These effects were not present in control experiments, in which stimulus display time was 200 ms. One explanation for this pattern of results is that long stimulus presentation times allow for the generation of retinal error signals. This in turn leads to saccade adaptation, causing an underestimation of the effect of this type of stimulus on saccade amplitude.

Psychophysical properties of two-stroke apparent motion

In two-stroke apparent movement, repeated presentation of a two-frame pattern displacement followed by a brief inter-stimulus interval (ISI) can create an impression of continuous forward motion (G. Mather, 2006). Does the ISI in two-stroke motion just break the connection between adjacent frames, switching off the motion signal they normally generate, or does it actually generate a reversed motion signal? Reversed apparent motion in two-frame stimuli separated by a brief ISI has been reported in several previous papers (ISI reversal), which found that the effect is optimal at short, mean-luminance ISIs, and is abolished at scotopic luminances. A series of five experiments compared two-stroke apparent motion with ISI reversal using the same stimulus display. The two effects show the same dependence on ISI duration and luminance and are both abolished at low mean luminance. Results therefore support the conclusion that the ISI in two-stroke apparent motion does contribute a reversed motion signal and constrain theoretical explanations of two-stroke apparent motion.

Eye movements during list execution reveal no planning in monkeys (Macaca fascicularis).

When trained on a serial-order task to respond to 5 simultaneously displayed stimuli, monkeys (Macaca fascicularis) show an elevated latency to respond to the first item (A) or the first 2 items (A and B) followed by short and uniform latencies to respond to the remaining items. This pattern has been interpreted as evidence of planning, with the long initial latency reflecting the animal planning their responses, and the subsequent short and uniform latencies reflecting the execution of the planned responses. An analysis of eye movements, however, revealed no evidence of planning. The elevated latency to item A is likely an artifact of the method of stimulus display.

On the immunity of perceptual implicit memory to manipulations of attention

In four experiments, we examined the effect of manipulating study phase attention in a Stroop task on the extent of repetition priming in the lexical decision task (LDT). Experiment 1 replicated the immunity of the LDT to division of attention reported by Szymanski and MacLeod (1996), using a standard Stroop configuration. Response times to previously encountered words were identical regardless of whether the participants were required to read the words or name the color in which they were presented. Experiment 2 demonstrated that implementing the Stroop manipulation across separate visual objects reduced but did not eliminate priming of unattended words, provided the words remained in the attended region of the stimulus display. When this constraint was removed in Experiment 3, priming of unattended words disappeared. Experiment 4 demonstrated statistically equivalent priming for attended and unattended words when the Stroop manipulation remained in the same visual object but attention was directed to a single letter of the word. In all four experiments, the Stroop manipulation had a clear effect on recognition. These results qualify claims that the LDT might be immune to manipulations of study phase attention and suggest that the LDT has a lower threshold level of attention at encoding than do other standard implicit tests of memory.

Dissociation of the N2pc and sustained posterior contralateral negativity in a choice response task

The N2pc, a greater negativity at posterior electrodes on the side contralateral to an attended visual stimulus, usually between 180 and 280 ms, is thought to reflect the moment-to-moment deployment of visual–spatial attention. In tasks that require the retention of information in visual short-term memory, there is also a sustained posterior contralateral negativity (SPCN) that often begins about 300–400 ms after stimulus onset and that persists for the duration of the retention interval. A positive-going deflection at around 300 ms often separates the N2pc and the SPCN. An SPCN is also observed in tasks that are not defined as memory tasks, but that presumably engage visual short-term memory as an intermediate processing buffer (e.g., in order to make a choice response to a briefly-presented visual stimulus). The SPCN in memory tasks has been shown to increase in amplitude as the memory load is increased. We used this property of the SPCN to verify that the SPCN observed during the performance of a choice task with a response following each stimulus display is related to the SPCN observed in tasks that are structured as memory tasks. Using identical physical stimuli, we manipulated the hypothesized memory load across trial blocks by instructions either to encode only one stimulus or two stimuli. We observed an increase of the amplitude of the SPCN as memory load increased, with no concomitant increase in the amplitude of the N2pc that immediately preceded it. The results provide a clear dissociation between the N2pc (spatial attention, not affected by memory load) and the SPCN (visual short-term memory, sharply sensitive to memory load).

A NOVEL METHOD FOR TEACHING THE FIRST INSTANCES OF SIMPLE DISCRIMINATION TO NONVERBAL CHILDREN WITH AUTISM IN A LABORATORY ENVIRONMENT.

A novel method for initiating discrimination training with nonverbal children combines a delayed S+ procedure that requires children to refrain from responding to either of 2 physically different choice stimuli until a prompt stimulus is added onto 1 of the choices, and a delayed prompting procedure that presents the same 2-choice stimulus display, but stimuli are initially added onto both choices. After a short delay, the added stimulus on the S- is removed, and the choice of the S+ is thus prompted. If the children learn to observe and respond to the defining features of the S+ choice stimulus, then they may respond to the S+ prior to the added-stimulus removal. Implementation was successful with 8 nonverbal children who had not previously exhibited simple simultaneous discrimination, suggesting a useful methodology for initiating discrimination training with populations for whom verbal instruction is ineffective.

Learning strategies in matching to sample: If-then and configural learning by pigeons

Pigeons learned a matching-to-sample task with a split training-set design in which half of the stimulus displays were untrained and tested following acquisition. Transfer to the untrained displays along with no novel-stimulus transfer indicated that these pigeons learned the task (partially) via if-then rules. Comparisons to other performance measures indicated that they also partially learned the task via configural learning (learning the gestalt of the whole stimulus display). Differences in the FR-sample requirement (1 vs. 20) had no systematic effect on the type of learning or level of learning obtained. Differences from a previous study [Wright, A.A., 1997. Concept learning and learning strategies. Psychol. Sci. 8, 119-123] are discussed, including the effect of displaying the stimuli vertically (traditional display orientation) or horizontally from the floor.

Altered vision near the hands

The present study explored the manner in which hand position may affect visual processing. We studied three classic visual attention tasks (visual search, inhibition of return, and attentional blink) during which the participants held their hands either near the stimulus display, or far from the display. Remarkably, the hands altered visual processing: people shifted their attention between items more slowly when their hands were near the display. The same results were observed for both visible and invisible hands. This enhancement in vision for objects near the hands reveals a mechanism that could facilitate the detailed evaluation of objects for potential manipulation, or the assessment of potentially dangerous objects for a defensive response.