Distractor Stimuli Research Articles

Capacity limits during perceptual encoding

When a unique stimulus is embedded in an otherwise homogenous display, it is thought to "pop-out" due to its relative increase in salience. We investigated whether the visual system has the ability to equally prioritize multiple salient pop-out items competing for awareness or whether this early stage of visual processing is constrained by capacity limits. We used signal detection (d') methods to determine if sensitivity to a salient pop-out item decreases as function of the number of total salient items present in the visual display. Participants engaged in a signal detection task where they had to report the presence or absence of a simple display change that involved either a pop-out or a non-salient distractor stimulus. Results across four experiments consistently showed that sensitivity to changes involving pop-out stimuli was significantly reduced after more than one of these items was present in the visual array. Results are discussed in terms of neural models of visual encoding and other known capacity limits during visual processing.

Now You'll Feel It, Now You Won't: EEG Rhythms Predict the Effectiveness of Perceptual Masking

Do ongoing brain states determine conscious perception of an upcoming stimulus? Using the high temporal resolution of EEG, we investigated the relationship between prestimulus neuronal oscillations and the perceptibility of two competing somatosensory stimuli embedded in a backward masking paradigm. We identified two prestimulus EEG signatures predictive for a suprathreshold yet weak target stimulus to become perceptually resistant against masking by a stronger distractor stimulus: (i) over left frontal cortex a desynchronization of the regional beta rhythm (approximately 20 Hz) 500 msec prior to a perceived target, and (ii) a subsequent additional attenuation of both mu (approximately 10 Hz) and beta "idling" rhythms over those pericentral sensorimotor cortices which are going to process the upcoming target stimulus. Furthermore, across subjects the probability for target perception strongly correlates with the individual absolute level of pre-target amplitudes in these frequency bands and locations. These signatures significantly differed from the EEG characteristics preceding detected and undetected single stimuli. We suggest that the early activation of left frontal areas involved in top-down attentional control is critical for preventing backward masking and leads the preparation of primary sensory cortices: The ensuing prestimulus suppression of sensory idling rhythms warrants an intensified poststimulus processing, and thus, effectively promotes conscious perception of suprathreshold target stimuli embedded into an ecologically relevant condition featuring competing environmental stimuli.

Low-Frequency Repetitive Transcranial Magnetic Stimulation (rTMS) Affects Event-Related Potential Measures of Novelty Processing in Autism

In our previous study on individuals with autism spectrum disorder (ASD) (Sokhadze et al., Appl Psychophysiol Biofeedback 34:37-51, 2009a) we reported abnormalities in the attention-orienting frontal event-related potentials (ERP) and the sustained-attention centro-parietal ERPs in a visual oddball experiment. These results suggest that individuals with autism over-process information needed for the successful differentiation of target and novel stimuli. In the present study we examine the effects of low-frequency, repetitive Transcranial Magnetic Stimulation (rTMS) on novelty processing as well as behavior and social functioning in 13 individuals with ASD. Our hypothesis was that low-frequency rTMS application to dorsolateral prefrontal cortex (DLFPC) would result in an alteration of the cortical excitatory/inhibitory balance through the activation of inhibitory GABAergic double bouquet interneurons. We expected to find post-TMS differences in amplitude and latency of early and late ERP components. The results of our current study validate the use of low-frequency rTMS as a modulatory tool that altered the disrupted ratio of cortical excitation to inhibition in autism. After rTMS the parieto-occipital P50 amplitude decreased to novel distracters but not to targets; also the amplitude and latency to targets increased for the frontal P50 while decreasing to non-target stimuli. Low-frequency rTMS minimized early cortical responses to irrelevant stimuli and increased responses to relevant stimuli. Improved selectivity in early cortical responses lead to better stimulus differentiation at later-stage responses as was made evident by our P3b and P3a component findings. These results indicate a significant change in early, middle-latency and late ERP components at the frontal, centro-parietal, and parieto-occipital regions of interest in response to target and distracter stimuli as a result of rTMS treatment. Overall, our preliminary results show that rTMS may prove to be an important research tool or treatment modality in addressing the stimulus hypersensitivity characteristic of autism spectrum disorders.

Bilateral impairment of concurrent saccade programming in hemispatial neglect

When healthy observers make a saccade that is erroneously directed toward a distracter stimulus, they often produce a corrective saccade within 100ms after the end of the primary saccade. Such short inter-saccadic intervals indicate that programming of the secondary saccade has been initiated prior to the execution of the primary saccade and hence that the two saccades have been programmed concurrently. Here we show that concurrent saccade programming is bilaterally impaired in left spatial neglect, a strongly lateralized disorder of visual attention resulting from extensive right cerebral damage. Neglect patients were asked to make saccades to targets presented left or right of fixation while disregarding a distracter presented in the opposite hemifield. We examined those experimental trials on which participants first made a saccade to the distracter, followed by a secondary (corrective) saccade to the target. Compared to healthy and right-hemisphere damaged control participants the proportion of secondary saccades directing gaze to the target instead of bringing it even closer to the distracter was bilaterally reduced in neglect patients. In addition, the characteristic reduction of secondary saccade latency observed in both control groups was absent in neglect patients, whether the secondary saccade was directed to the left or right hemifield. This pattern is consistent with a severe, bilateral impairment of concurrent saccade programming in left spatial neglect.

When less is more: TPJ and default network deactivation during encoding predicts working memory performance

Previous work has shown that temporo-parietal junction (TPJ), part of a ventral attention network for stimulus-driven reorienting, deactivates during effortful cognitive engagement, along with the default mode network (DMN). TPJ deactivation has been reported both during working memory (WM) and rapid visual search, ostensibly to prevent reorienting to irrelevant objects. We tested whether the magnitude of this deactivation during WM encoding is predictive of subsequent WM performance. Using slow event-related fMRI and a delayed WM task in which distracter stimuli were presented during the maintenance phase, we found that greater TPJ and DMN deactivation during the encoding phase predicted better WM performance. TPJ and DMN, however, also showed several functional dissociations: (1) TPJ exhibited a different task-evoked pattern than DMN, responding to distracters sharing task-relevant features, but not to other types of distracters; and (2) TPJ showed strong functional connectivity with the DMN at encoding but not during distracter presentation. These results provide further evidence for the functional importance of TPJ suppression and indicate that TPJ and DMN deactivation is especially critical during WM trace formation. In addition, the functional connectivity results suggest that TPJ, while not part of the DMN during the resting state, may flexibly “couple” with this network depending on task demands.

Reduced brain responses to novel sounds in depression: P3 findings in a novelty oddball task

There have been conflicting findings as to whether the P3 brain potential to targets in oddball tasks is reduced in depressed patients. The P3 to novel distracter stimuli in a three-stimulus oddball task has a more frontocentral topography than P3 to targets and is associated with different cognitive operations and neural generators. The novelty P3 potential was predicted to be reduced in depressed patients. EEG was recorded from 30 scalp electrodes (nose reference) in 20 unmedicated depressed patients and 20 matched healthy controls during a novelty oddball task with three stimuli: infrequent target tones (12%), frequent standard tones (76%) and nontarget novel stimuli, e.g., animal or environment sounds (12%). Novel stimuli evoked a P3 potential with shorter peak latency and more frontocentral topography than the parietal-maximum P3b to target stimuli. The novelty P3 was markedly reduced in depressed patients compared to controls. Although there was a trend for patients to also have smaller parietal P3b to targets, this group difference was not statistically significant. Nor was there a group difference in the earlier N1 or N2 potentials. The novelty P3 reduction in depressed patients is indicative of a deficit in orienting of attention and evaluation of novel environmental sounds.

How do amplitude spectra influence rapid animal detection?

Amplitude spectra might provide information for natural scene classification. Amplitude does play a role in animal detection because accuracy suffers when amplitude is normalized. However, this effect could be due to an interaction between phase and amplitude, rather than to a loss of amplitude-only information. We used an amplitude-swapping paradigm to establish that animal detection is partly based on an interaction between phase and amplitude. A difference in false alarms for two subsets of our distractor stimuli suggests that the classification of scene environment (man-made versus natural) may also be based on an interaction between phase and amplitude. Examples of interaction between amplitude and phase are discussed.

Experience-dependent changes in the topography of visual crowding

The present work examined discrimination accuracy for targets that were presented either alone in the visual field (clean displays) or embedded within a dense array of letter distractors (crowded displays). The strength of visual crowding varied strongly across the four quadrants of the visual field. Furthermore, this spatial bias in crowding was strongly influenced by the observers' prior experience with specific distractor stimuli. Observers who were monolingual readers of English experienced amplified crowding in the upper-left quadrant, while subjects with primary reading skills in Korean, Chinese, or Japanese tended towards worse target discrimination in the lower visual field. This interaction with language experience was eliminated when non-alphanumeric stimuli were employed as distractors, suggesting that prior reading experience induced a stimulus-specific change in the topography of visual crowding from English letters.

Dilution of compatibility effects in Simon-type tasks depends on categorical similarity between distractors and diluters

The presence of a distracting stimulus during performance of the Stroop color-naming task leads to dilution of the Stroop effect. Because the automatic activation of word meaning may interfere with the task-relevant stimulus feature (text color; stimulus-stimulus [S-S] interference) and the response (saying the text color; stimulus-response [S-R] interference), it is unclear which of these types of interference is diluted. We introduce a new dilution paradigm using word- and arrow-based Simon tasks, in which only S-R interference is present. Participants made a left or right response to a central color target. A task-irrelevant location-word (Experiment 1) or arrow (Experiment 2) distractor adjacent to the target produced S-R compatibility effects. An additional neutral word or symbol series (diluter) was sometimes presented on the opposite side of the target from the distractor. The compatibility effect was smaller when the distractor and diluter category domains matched than when they mismatched. This result provides evidence that S-R compatibility effects are susceptible to the presence of diluters that are categorically similar to the distractors.

Short article: Distractor interference in focused attention tasks is not mediated by attention capture

Distractor stimuli possessing information that is relevant for a task (henceforth, task-relevant distractors) often interfere with task performance. The interference by task-relevant distractors is observed even when distractors are positioned outside the main attentional focus. We investigated whether such interference is due to an attention capture by the distractors. Participants responded to a target colour while ignoring word distractors positioned within (Experiment 1) or outside (Experiments 2 and 3) the attentional focus. The words carried task-relevant information in their colour and personally significant information in their content. Because personally significant information affects performance only when positioned in an attended region, it was used as a marker for the locus of the attentional focus. As expected, when distractors were attended, both task-relevant and personally significant information affected performance. However, when distractors were unattended, only task-relevant information caused interference, suggesting that attention did not shift to the distractors' location. We discuss possible accounts for interference effects in focused-attention tasks.

Temporal dynamics of saccadic distraction

The saccadic distractor effect, in which irrelevant stimuli delay saccades to target stimuli, is a popular tool for investigating saccadic competition. Here, we outline the main components of a competition framework to account for the temporal dynamics of the distractor effect, inspired by race models of saccade generation. We first test a key prediction of this framework: the degree of interference should depend upon the degree of temporal overlap of target and distractor signals in the competition stage, which will vary systematically with the relative processing speeds of the competing visual signals. In agreement with this, we found that, when varying the contrast of distractor stimuli, the presentation delay between target and distractor that maximizes interference varies systematically for different target-distractor pairs, correlated with the difference in saccadic latency for the pair. Second, our data illustrate a crucial methodological point: when comparing the effect of different distractors, measuring at only one time-point (e.g. simultaneity, as most studies have done) can produce misleading and contradictory results. Thus, it is essential to take the temporal dynamics of the system into account. Lastly, the framework predicts that the optimal delay for the latency distractor effect is different from that maximizing error rate, and our data confirms this.

Learning to attend: Effects of practice on information selection

Though practice can lead to improved performance in many domains, it is currently unknown how practice affects the deployment of selective attention to filter distracting information. We conducted a series of experiments to address this issue by examining how performance on a task changed after repeated exposure to distractors. Distraction initially slowed response time during task performance, an effect that diminished with repeated exposure to the distractors. When the distractors were consistent in appearance, the practice effect developed quickly but was stimulus-specific. When the distractors were more variable in appearance, the practice effect developed slowly but transferred more readily to other conditions. These data indicate that practice with overcoming distraction leads to improvements in information filtering mechanisms that generalize beyond the training regimen when variable distractor stimuli are experienced.

Reflexive social attention is mapped according to effector-specific reference systems

Studies exploring reflexive joint attention report that attention is more powerfully captured by interfering social signals (such as others' gaze or hand orientation) than by non-biological directional cues (such as an arrow). However, what remains unknown is whether these effects are mapped in purely spatial or in body-part specific reference frames. Changes of a central, black fixation point into blue or orange were the imperative instruction signal for the experimental subjects to make a leftward or a rightward movement (saccades in Study 1 and hand pointing in Study 2) while ignoring distracting stimuli (leftward or rightward oriented gaze, hand pointing or arrow). Gaze and pointing hand distracters that were directionally incongruent with the instruction cue impaired the goal-driven saccadic and pointing performance, respectively. This pattern of results indicates that reflexive social attention is mapped not only in spatial but also in body-part specific reference frames.

Exploring BOLD Changes during Spatial Attention in Non-Stimulated Visual Cortex

Blood oxygen level-dependent (BOLD) responses were measured in parts of primary visual cortex that represented unstimulated visual field regions at different distances from a stimulated central target location. The composition of the visual scene varied by the presence or absence of additional peripheral distracter stimuli. Bottom-up effects were assessed by comparing peripheral activity during central stimulation vs. no stimulation. Top-down effects were assessed by comparing active vs. passive conditions. In passive conditions subjects simply watched the central letter stimuli and in active conditions they had to report occurrence of pre-defined targets in a rapid serial letter stream. Onset of the central letter stream enhanced activity in V1 representations of the stimulated region. Within representations of the periphery activation decreased and finally turned into deactivation with increasing distance from the stimulated location. This pattern was most pronounced in the active conditions and during the presence of peripheral stimuli. Active search for a target did not lead to additional enhancement at areas representing the attentional focus but to a stronger deactivation in the vicinity. Suppressed neuronal activity was also found in the non distracter condition suggesting a top-down attention driven effect. Our observations suggest that BOLD signal decreases in primary visual cortex are modulated by bottom-up sensory-driven factors such as the presence of distracters in the visual field as well as by top-down attentional processes.

The role(s) of capacity limitations in speech perception.

This paper reviews some current research exploring the role that capacity limitations play in recognizing speech, particularly in the presence of competing speech. Load theory [Lavie, Trends Cogn Sci 92 (2005)] suggests that perceptual capacity limits the number of objects (or features) that can be processed simultaneously, while cognitive capacity limits rejection of irrelevant information. Functional imaging studies show increased cognitive (working memory) load corresponding to failure of inhibition of (increased activity in) brain regions associated with processing distractor stimuli [de Fockert et al., Science 291 (2001)], whereas increased perceptual load results in decreased activity in distractor-related areas [Rees et al., Science 278 (1997)]. However, all of these studies were carried out in the visual modality, and it is unclear whether the same predictions were obtained in audition or speech perception. A series of experiments in which perceptual and working memory load were varied as listeners identified words produced by one talker while ignoring another provides qualified support for the predictions of load theory and lays the groundwork for subsequent studies of the effects of hearing impairment, cognitive deficits, and perceptual and cognitive training on the perception of speech in competing speech. [Work supported by NIDCD R03DC006811.]

Spatial negative priming in bilingualism

Balanced bilinguals have been shown to have an enhanced ability to inhibit distracting information. In this study, we investigated the hypothesis that the bilinguals' efficiency in inhibitory control can be advantageous in some conditions, but disadvantageous in others-for example, negative priming conditions, in which previously irrelevant information becomes relevant. Data collected in a target-stimulus locating task from 29 early bilingual adults and 29 age-matched monolinguals showed that the bilinguals' greater inhibition of irrelevant spatial information (i.e., the position of a distractor stimulus) resulted in a smaller effect of the distractor presence (i.e., a smaller difference in error rates in trials with and without distractors) and a larger negative priming effect (i.e., a larger difference between the error rates shown in trials wherein the target position corresponded to the position of the previous-trial distractor and trials wherein the target was presented in a previously vacant position). These findings support the hypothesis of specific nonlinguistic cognitive effects of bilingualism on inhibitory control functions, which are not necessarily reflected in cognitive advantages.

Executive and motivational inhibition: Associations with self-report measures related to inhibition

Inhibition involves the withholding or suppressing of attention or responses to irrelevant or distracting stimuli. We examined the relationship between five experimental tasks of inhibition, represented by two measures of executive, intentional control inhibition and three measures of motivational inhibition characterized by bottom-up interruption of affective and reward/punishment sensitive mechanisms. Associations between these experimental tasks with three self-report measures related to inhibition were also examined. Correlational analyses indicated a small but significant association between the measures in the executive domain (stop task and Stroop task), but a lack of associations between the measures in the motivational domain (emotional Stroop task, a card playing task involving rewards and punishments, and a gambling task). Both measures of executive and motivational inhibition entered as significant predictors on the self-report measures related to inhibition in simultaneous regression analyses, but not consistently in the expected direction. The results suggest that inhibition is not a unitary construct, and demonstrate an association between experimental measures of inhibition and self-report measures related to inhibition.

Neural Correlates of Impaired Cognitive-Behavioral Flexibility in Anorexia Nervosa

Impaired cognitive-behavioral flexibility is regarded as a trait marker in anorexia nervosa patients. The authors sought to investigate the neural correlates of this deficit in executive functioning in anorexia nervosa. Fifteen women with anorexia nervosa and 15 age-matched healthy comparison women underwent event-related functional MRI while performing a target-detection task. The task distinguished between shifts in behavioral response and shifts in cognitive set. It involved infrequent target and non-target distractor stimuli embedded in a sequence of prepotent standard stimuli. Relative to comparison subjects, anorexia nervosa patients showed a significantly higher error rate in behavioral response shifting, independent of whether those runs also involved cognitive set shifting. During behavioral response shifting, patients showed reduced activation in the left and right thalamus, ventral striatum, anterior cingulate cortex, sensorimotor brain regions, and cerebellum that differed significantly from the comparison group but showed dominant activation in frontal and parietal brain regions. These differential activations in patients and comparison subjects were specific to shifts in behavioral response: except for thalamic activation, they were not observed in response to non-target distractor trials that required no alteration in behavioral response. Impaired behavioral response shifting in anorexia nervosa seems to be associated with hypoactivation in the ventral anterior cingulate-striato-thalamic loop that is involved in motivation-related behavior. In contrast, anorexia nervosa patients showed predominant activation of frontoparietal networks that is indicative of effortful and supervisory cognitive control during task performance.

Event-related Potential Study of Novelty Processing Abnormalities in Autism

To better understand visual processing abnormalities in autism we studied the attention orienting related frontal event potentials (ERP) and the sustained attention related centro-parietal ERPs in a three stimulus oddball experiment. The three stimulus oddball paradigm was aimed to test the hypothesis that individuals with autism abnormally orient their attention to novel distracters as compared to controls. A dense-array 128 channel EGI electroencephalographic (EEG) system was used on 11 high-functioning children and young adults with autism spectrum disorder (ASD) and 11 age-matched, typically developing control subjects. Patients with ASD showed slower reaction times but did not differ in response accuracy. At the anterior (frontal) topography the ASD group showed significantly higher amplitudes and longer latencies of early ERP components (e.g., P100, N100) to novel distracter stimuli in both hemispheres. The ASD group also showed prolonged latencies of late ERP components (e.g., P2a, N200, P3a) to novel distracter stimuli in both hemispheres. However, differences were more profound in the right hemisphere for both early and late ERP components. Our results indicate augmented and prolonged early frontal potentials and a delayed P3a component to novel stimuli, which suggest low selectivity in pre-processing and later-stage under-activation of integrative regions in the prefrontal cortices. Also, at the posterior (centro-parietal) topography the ASD group showed significantly prolonged N100 latencies and reduced amplitudes of the N2b component to target stimuli. In addition, the latency of the P3b component was prolonged to novel distracters in the ASD group. In general, the autistic group showed prolonged latencies to novel stimuli especially in the right hemisphere. These results suggest that individuals with autism over-process information needed for the successful differentiation of target and novel stimuli. We propose the potential application of ERP evaluations in a novelty task as outcome measurements in the biobehavioral treatment (e.g., EEG biofeedback, TMS) of autism.

Difficulty of Discrimination Modulates Attentional Capture by Regulating Attentional Focus

Attentional capture for distractors is enhanced by increasing the difficulty of discrimination between the standard and the target in the three-stimulus oddball paradigm. In this study, we investigated the cognitive mechanism of this modulation of attentional capture. Event-related brain potentials were recorded from participants while they performed a visual three-stimulus oddball paradigm (frequent standard, rare target, and rare distractor). The discrimination difficulty between standard and target was manipulated in the central location. Distractor stimuli were presented in the central or surrounding locations. The P3a component was elicited by distractor stimuli and was used as a measure of attentional capture. The results revealed that discrimination difficulty had opposite effects on the P3a response between central and surrounding locations. With an increase in the difficulty of discrimination, the P3a response was enhanced when distractor stimuli were presented in the central location. In contrast, the P3a response was reduced when distractor stimuli were presented in a surrounding location. This finding suggests that spatial attention was focused by the difficulty of discrimination, and deviant processing was increased within its focus but decreased outside its focus. Therefore, attentional capture for deviant distractors is modulated by top-down controlled attentional focus.