On the interaction between perceptual and response selection: neuropsychological evidence

We examined the relations between selection for perception and selection for action in a patient FK, with bilateral damage to his temporal and medial frontal cortices. The task required a simple grasp response to a common object (a cup) in the presence of a distractor (another cup). The target was cued by colour or location, and FK made manual responses. We examined the effects on performance of cued and uncued dimensions of both the target and the distractor. FK was impaired at perceptually selecting the target when cued by colour, when the target colour but not its location changed on successive trials. The effect was sensitive to the relative orientations of targets and distractors, indicating an effect of action selection on perceptual selection, when perceptual selection was weakly instantiated. The dimension-specific carry-over effect on reaching was enhanced when there was a temporal delay between a cue and the response, and it disappeared when there was a between-trial delay. The results indicate that perceptual and action selection systems interact to determine the efficiency with which actions are selected to particular objects.     

An ERP study of preparatory and inhibitory mechanisms in a cued saccade task

The present study uses event-related potentials (ERPs) to investigate the neurophysiological correlates of the mechanisms involved in selection of locations for saccades. Participants performed a task in which both target and distractor locations were cued on a trial-by-trial basis. Participants were instructed to make an eye movement to the cued target location and to ignore elements at the distractor location. This experimental set-up allowed the investigation of the mechanisms involved in the top-down preparation and inhibition of locations for an eye movement. When comparing responses to leftward and rightward pointing cues, we observed an early directing attention negativity (EDAN) and an anterior directing attention negativity (ADAN) effect in the cue-target interval. These effects were similar to those observed in studies investigating attentional allocation, suggesting a close link between shifts of spatial attention and the preparation of eye movements. These components were followed by a late widespread contralateral negativity (LDAN) that was assumed to reflect both the oculomotor programming of the upcoming eye movement as well as attentional orienting. Furthermore, a new component was observed related to top-down inhibition of the distractor location. In response to the distractor cue, an early positivity above the right hemisphere (RLIP) was revealed. Finally, no modulations of early target-evoked ERP components were observed, suggesting that these components are unaffected when no further processing is required at the cued location.     

A kinematic analysis of distractor interference effects during visually guided action in spatial neglect.

Patients with left spatial neglect following right hemisphere damage may show anomalies in ipsilesional-limb movements directed to targets on their affected side, in addition to their characteristic perceptual deficits. In this study we examined the extent to which visually guided movements made by neglect patients are susceptible to interference from concurrent visual distractors on the contralesional or ipsilesional side of a designated target. Eleven right hemisphere patients with visual neglect, plus 11 matched healthy controls, performed a double-step movement task upon a digitizing tablet, using their ipsilesional hand to respond. On each double-step trial the first component of the movement was cued to a common central target, whereas the second component was cued unpredictably to a target on either the contralesional or ipsilesional side. On separate trials lateral targets either appeared alone or together with a concurrent distractor in an homologous location in the opposite hemispace. In addition to being significantly slower and more error prone than controls, neglect patients also exhibited a number of interference effects from ipsilesional distractors. They often failed to move to left targets in the presence of a right-sided distractor, or else they moved to the distractor itself rather than to a contralesional target. The initial accelerative phase of their movements to contralesional targets tended to be interrupted prematurely, and they spent significantly more time in the terminal guidance phase of movements to contralesional targets in the presence of an ipsilesional distractor. In contrast, contralesional distractors had little effect on patients' movements to ipsilesional targets. We conclude that right hemisphere damage induces a competitive bias that favors actions to ipsilesional targets. This bias affects multiple stages of processing within the visuomotor system, from initial programming through to the final stages of terminal guidance.     

A perceptual level mechanism of the inhibition of return in oculomotor planning

A motor response to a visual target presented at a precued spatial location is facilitated if the target is presented shortly after the cue and inhibited when the cue target onset asynchrony approaches a few hundred milliseconds. The latter effect is termed inhibition of return (IOR). It is suggested that IOR provides an important strategy for effective search in our visual environment. Despite studies demonstrating IOR in a number of behavioral tasks, its neural mechanism has remained elusive. As a fundamental step toward understanding these mechanisms, the current study examines whether IOR mainly involves a perceptual or a motor process. We conducted a series of experiments, in which the target instructed saccades to the cued or to a different location. In each experiment, we observed a similar pattern of IOR when the target followed the cue, but not when the saccade was directed to the cued location. In another two experiments, we demonstrated that the magnitude and temporal profile of IOR varied depending on whether an eye movement or a manual response was involved. Overall, the present study suggests that IOR results predominantly from a perceptual level mechanism, with its magnitude and time course modulated by the activation of specific motor effectors. We discuss the implications of these results for attention gating of perceptual inputs and for mechanisms of visuomotor control.     

Synchronising horizontal arm movement with transparent motion

The relationship between perception and motor performance was studied in a situation that required perceptual processing of a complex motion stimulus in which a target signal had to be segmented, selected, and tracked. Participants were asked to move their arm in synchrony with one surface of a transparent motion display in which two surfaces moved horizontally back-and-forth over each other. The quality of tracking performance was measured as a function of bottom-up and top-down perceptual cues and their interplay. Target signal strength was manipulated by lowering the relative amount of signal dots constituting the target, i.e., the coherence level (100%-50%-30%-10%; the distractor surface was always 100% coherent). A colour cue that distinguished the target from the distractor surface was either available or absent. In the presence of a colour cue, participants experienced little or no difficulties at coherence levels of 50%-100% but when surface formation was complicated by lowering the coherence level, synchronisation consistency decreased. This corresponds with continuous attempts, successful and unsuccessful, to correct inaccurate synchronisation. In the absence of a colour cue, difficulties were frequently observed in all coherence conditions, but they differed depending on the coherence level. Overall, these results suggest that colour can serve as a strong top-down cue for proper target selection and tracking, provided that bottom-up motion signals are sufficiently strong.     

Saccade execution suppresses discrimination at distractor locations rather than enhancing the saccade goal location

As we have limited processing abilities with respect to the plethora of visual information entering our brain, spatial selection mechanisms are crucial. These mechanisms result in both enhancing processing at a location of interest and in suppressing processing at other locations; together, they enable successful further processing of locations of interest. It has been suggested that saccade planning modulates these spatial selection mechanisms; however, the precise influence of saccades on the distribution of spatial resources underlying selection remains unclear. To this end, we compared discrimination performance at different locations (six) within a work space during different saccade tasks. We used visual discrimination performance as a behavioral measure of enhancement and suppression at the different locations. A total of 14 participants performed a dual discrimination/saccade countermanding task, which allowed us to specifically isolate the consequences of saccade execution. When a saccade was executed, discrimination performance at the cued location was never better than when fixation was maintained, suggesting that saccade execution did not enhance processing at a location more than knowing the likelihood of its appearance. However, discrimination was consistently lower at distractor (uncued) locations in all cases where a saccade was executed compared with when fixation was maintained. Based on these results, we suggest that saccade execution specifically suppresses distractor locations, whereas attention shifts (with or without an accompanying saccade) are involved in enhancing perceptual processing at the goal location.     

Predictive distractor context facilitates attentional selection of high,but not intermediate and low,salience targets

It is well established that we can focally attend to a specific region in visual space without shifting our eyes, so as to extract action‐relevant sensory information from covertly attended locations. The underlying mechanisms that determine how fast we engage our attentional spotlight in visual‐search scenarios, however, remain controversial. One dominant view advocated by perceptual decision‐making models holds that the times taken for focal‐attentional selection are mediated by an internal template that biases perceptual coding and selection decisions exclusively through target‐defining feature coding. This notion directly predicts that search times remain unaffected whether or not participants can anticipate the upcoming distractor context. Here we tested this hypothesis by employing an illusory‐figure localization task that required participants to search for an invariant target amongst a variable distractor context, which gradually changed—either randomly or predictably—as a function of distractor‐target similarity. We observed a graded decrease in internal focal‐attentional selection times—correlated with external behavioral latencies—for distractor contexts of higher relative to lower similarity to the target. Critically, for low but not intermediate and high distractor‐target similarity, these context‐driven effects were cortically and behaviorally amplified when participants could reliably predict the type of distractors. This interactive pattern demonstrates that search guidance signals can integrate information about distractor, in addition to target, identities to optimize distractor‐target competition for focal‐attentional selection. Hum Brain Mapp 36:935–944, 2015. © 2014 Wiley Periodicals, Inc.     

Activity of prefrontal neurons during location and color delayed matching tasks.

Many cells in prefrontal cortex show enhanced activity prior to movement onset in delayed or memory-guided saccade tasks. This activity is a possible neural correlate of spatial attention and working memory. The goal of this study was to determine whether delay activity is evoked when non-spatial cues such as color are used to guide saccades. Monkeys were trained on a saccade target selection task in which they were cued for either the location or color of the rewarded target. When the location of the target was specified explicitly, many cells showed visual responses and delay activity that were spatially selective. Color selective visual responses or delay activity were both rare and weak. However, for many cells, spatially selective delay activity could be evoked when color was used to specify the location of the target. These results indicate that color is capable of eliciting spatially selective activity from cells that have no overt color selectivity.     

Orienting attention in time activates left intraparietal sulcus for both perceptual and motor task goals

Attention can be directed not only toward a location in space but also to a moment in time ("temporal orienting"). Temporally informative cues allow subjects to predict when an imminent event will occur, thereby speeding responses to that event. In contrast to spatial orienting, temporal orienting preferentially activates left inferior parietal cortex. Yet, left parietal cortex is also implicated in selective motor attention, suggesting its activation during temporal orienting could merely reflect incidental engagement of preparatory motor processes. Using fMRI, we therefore examined whether temporal orienting would still activate left parietal cortex when the cued target required a difficult perceptual discrimination rather than a speeded motor response. Behaviorally, temporal orienting improved accuracy of target identification as well as speed of target detection, demonstrating the general utility of temporal cues. Crucially, temporal orienting selectively activated left inferior parietal cortex for both motor and perceptual versions of the task. Moreover, conjunction analysis formally revealed a region deep in left intraparietal sulcus (IPS) as common to both tasks, thereby identifying it as a core neural substrate for temporal orienting. Despite the context-independent nature of left IPS activation, complementary psychophysiological interaction analysis revealed how the functional connectivity of left IPS changed as a function of task context. Specifically, left IPS activity covaried with premotor activity during motor temporal orienting but with visual extrastriate activity during perceptual temporal orienting, thereby revealing a cooperative network that comprises both temporal orienting and task-specific processing nodes.     

A psychophysical study of visual extinction: ipsilesional distractor interference with contralesional orientation thresholds in visual hemineglect patients

Visual extinction was investigated in left (n=15) and right (n=25) brain-damaged patients with or without visual neglect, and in normal control subjects (n=14), using a psychophysical paradigm. Orientation discrimination thresholds were determined for both left and right hemifield gratings presented either in isolation or simultaneously with a contralateral distractor grating. To minimize the influence of possible sensory-perceptual deficits, the luminances of both target and distractor gratings were chosen to be 20 times the luminances necessary to discriminate between horizontal and vertical grating orientations. The location of the target grating was always cued, making the distractor grating task irrelevant. Even after equalizing the visibility of left and right hemifield stimuli, neglect patients still displayed an increased interference effect from an ipsilesional distractor (and no interference from a contralesional distractor). Left or right brain-damaged controls did not show this asymmetric interference of irrelevant distractors, even the patients who demonstrated extinction on standard extinction testing. This suggests that visual extinction is a critical component of the visual neglect syndrome and that it involves an attentional deficit.     

Effects of temporal context and temporal expectancy on neural activity in inferior temporal cortex

Timing is critical. The same event can mean different things at different times and some events are more likely to occur at one time than another. We used a cued visual classification task to evaluate how changes in temporal context affect neural responses in inferior temporal cortex, an extrastriate visual area known to be involved in object processing. On each trial a first image cued a temporal delay before a second target image appeared. The animal's task was to classify the second image by pressing one of two buttons previously associated with that target. All images were used as both cues and targets. Whether an image cued a delay time or signaled a button press depended entirely upon whether it was the first or second picture in a trial. This paradigm allowed us to compare inferior temporal cortex neural activity to the same image subdivided by temporal context and expectation. Neuronal spiking was more robust and visually evoked local field potentials (LFP's) larger for target presentations than for cue presentations. On invalidly cued trials, when targets appeared unexpectedly early, the magnitude of the evoked LFP was reduced and delayed and neuronal spiking was attenuated. Spike field coherence increased in the beta-gamma frequency range for expected targets. In conclusion, different neural responses in higher order ventral visual cortex may occur for the same visual image based on manipulations of temporal attention.     

Expectation-based attentional modulation of visual extinction in spatial neglect

Visual extinction, the failure of patients with unilateral focal brain damage to report the contralesional of two simultaneously presented stimuli, may be modulated by characteristics of the display such as similarity, collinearity, or connectedness. Since these factors affect the perceptual configuration of stimuli, the modulation of extinction is believed to reflect low-level perceptual grouping. In the present study, patient AG did not show any modulation of contralesional detection when the ipsilesional and contralesional stimulus grouped by colour, by form, or both (Experiment 1). In contrast, identification of the contralesional stimulus was facilitated when the stimuli grouped (Experiment 2), suggesting a modulation of extinction by specific task demands. Experiment 3 used a cueing procedure to demonstrate modulation of extinction by expectation biases. Prior to stimulus presentation, AG was cued to attend to a particular feature (e.g. colour). After stimulus exposure he was prompted to identify the expected feature on valid trials and the unexpected feature on invalid trials. AG showed a significant validity effect for contralesional stimuli i.e. he identified the expected feature (e.g. colour) significantly better than the unexpected feature (e.g. form). These results suggest that competition for selection between visual stimuli may not only be influenced by perceptual characteristics of the display, but also by high-level factors such as the response criterion or expectation biases.     

Kanizsa subjective figures capture visual spatial attention: evidence from electrophysiological and behavioral data

Figural binding and attention are two important processes that help to perceive the outside world. Binding is necessary to link together the different features of single objects which are represented in a distributed fashion in the brain. Attention serves to focus onto a small subset of incoming information. It is still not clear how exactly these two mechanisms operate and interact. We performed two experiments employing illusory Kanizsa figures (KFs) to investigate the temporal order of figural binding and spatial attention. In a visual search task, subjects had to detect the presence of a KF among distractor stimuli. We found only a slight increase of reaction times when increasing the number of distractors, indicating that KFs popped out and drew the perceiver's attention. In a further event-related potential (ERP) study, we used displays of the search task as non-informative cue for a subsequent target choice-reaction task. Enhanced contralateral negative amplitudes (starting at about 230 ms) over ventral occipital areas were found for cue displays which included a KF. For target stimuli, faster reaction times and enhanced ipsilateral N1 amplitudes over occipito-parietal areas were observed for validly (target presentation inside a KF) as compared to invalidly cued targets (target presentation outside a KF). Furthermore, enhanced contralateral N1 amplitudes were found for invalidly cued targets. It might be that interactions between perceptual closure processing of the ventral pathway and spatial target processing of the dorsal pathway contributed to the present result. We conclude that KFs automatically capture spatial attention when used as visual cues.     

The role of perceptual load in neglect: rejection of ipsilesional distractors is facilitated with higher central load

Neglect is known to produce a bias towards the ipsilesional side. Here we examined whether this bias is automatic or can be modulated by manipulating perceptual load in a relevant task [e.g., Lavie, N. (1995). Perceptual load as a necessary condition for selective attention. Journal of Experimental Psychology: Human Perception and Performance, 21, 451-468]. Three patients with left neglect and three healthy controls made speeded choice responses to a target letter in the center of the display while attempting to ignore an irrelevant distractor presented on left or right. Perceptual load was manipulated by inducing a search for the target that appeared with another central stimulus, which was either a blob (low load) or a nontarget letter (higher load). Response competition effects from ipsilesional distractors were significantly reduced by higher load. The same increase of load, however, did not decrease distractor effects in the control group, as expected [e.g., Lavie, N., & Cox, S. (1997). On the efficiency of attentional selection: Efficient visual search results in inefficient rejection of distraction. Psychological Science, 8, 395-398]. These results demonstrate that ipsilesional bias in neglect is not fully automated and emphasize an additional restriction of perceptual capacity. Moreover, they supported our prediction that reduced perceptual capacity in neglect can lead to improved distractor rejection with just small increases in perceptual load.     

Repetition of distractor sets improves visual search performance in hemispatial neglect

Priming from repeated distractor sets, or search context, in conjunctive visual search was examined in four patients with hemispatial neglect. In the first experiment overall context was either changed or repeated while the target was always the same to control for any modulatory effect of target priming. Considerable priming was seen from repeated context. In the second experiment the context was either repeated on the left side, on the right side, on both sides, or the context was new. Priming from repeated context was found to arise from the left visual field, as well as the right visual field, as well as when overall context was repeated. Brief masked displays were used in experiment 3, the results again showing strong priming from repeated overall context. The results of the three experiments suggest that visual grouping, or perceptual organization, of distractor sets is relatively intact in the affected hemifield of parietal neglect patients. Furthermore, repetition of context may even temporarily ameliorate neglect symptoms in search. These findings are consistent with claims that grouping is distinct from attentional processing and that it operates at lower levels of the perceptual hierarchy.     

Facilitation and inhibition arising from the exogenous orienting of covert attention depends on the temporal properties of spatial cues and targets.

On the covert orienting of visual attention task (COVAT), responses to targets appearing at the location indicated by a non-predictive spatial cue are faster than responses to targets appearing at uncued locations when stimulus onset asynchrony (SOA) is less than approximately 200 ms. For longer SOAs, this pattern reverses and RTs to targets appearing at uncued locations become faster than RTs to targets appearing at the cued location. This facilitation followed by inhibition has been termed the biphasic effect of non-predictive peripheral spatial cues. Currently, there is debate about whether these two processes are independent. This issue was addressed in a series of experiments where the temporal overlap between the peripheral cue and target was manipulated at both short and long SOAs. Results showed that facilitation was present only when the SOA was short and there was temporal overlap between cue and target. Conversely, inhibition occurred only when the SOA was long and there was no temporal overlap between cue and target. The biphasic effect, with an early facilitation followed by a later inhibition, occurred only when the cue duration was fixed such that there was temporal overlap between the cue and target at short but not long SOAs. In a final experiment, the duration of targets the temporal overlap between cue and target and the SOA were manipulated factorially. The results showed that facilitation occurred only when the SOA was short, there was temporal overlap between cue and target and the target remained visible until the subject responded. These results suggest that the facilitation and inhibition found on COVATs which use non-informative peripheral cues are independent processes and their presence and magnitude is related to the temporal properties of cues and targets.     

Electrophysiological evidence for attentional guidance by the contents of working memory

The deployment of visual attention can be strongly modulated by stimuli matching the contents of working memory (WM), even when WM contents are detrimental to performance and salient bottom-up cues define the critical target [D. Soto et al. (2006) Vision Research, 46, 1010–1018]. Here we investigated the electrophysiological correlates of this early guidance of attention by WM in humans. Observers were presented with a prime to either identify or hold in memory. Subsequently, they had to search for a target line amongst different distractor lines. Each line was embedded within one of four objects and one of the distractor objects could match the stimulus held in WM. Behavioural data showed that performance was more strongly affected by the prime when it was held in memory than when it was merely identified. An electrophysiological measure of the efficiency of target selection (the N2pc) was also affected by the match between the item in WM and the location of the target in the search task. The N2pc was enhanced when the target fell in the same visual field as the re-presented (invalid) prime, compared with when the prime did not reappear in the search display (on neutral trials) and when the prime was contralateral to the target. Merely identifying the prime produced no effect on the N2pc component. The evidence suggests that WM modulates competitive interactions between the items in the visual field to determine the efficiency of target selection.     

Neural mechanisms of attentional modulation of perceptual grouping by collinearity

Psychophysical research showed that detection of an oriented visual target is facilitated when the target is grouped with collinear visual flankers. However, this collinear grouping effect is evident only when the flankers are attended. This study examined neural mechanisms underlying the interaction between attention and grouping by collinearity. Event-related potentials were recorded from study participants who judged whether oriented Gabor patches (i.e. visual elements consisting of a sinusoidal contrast modulation convolved with a Gaussian function) along the cued orientation were collinear or orthogonal. Event-related potentials showed an enhanced negativity over the posterior occipital cortex at 48-72 ms when collinear patches were congruent rather than incongruent with the cued orientation. A negative shift between 260 and 380 ms was observed over the occipital-parietal areas in the congruent rather than incongruent conditions. The long-latency effect, however, was evident only when the collinear patches were allocated along 45 degrees . The event-related potential results suggest that the interaction between attention and collinear grouping may take place as early as in the primary visual cortex and is independent of global orientations of perceptual groups.     

Target selection for pursuit and saccadic eye movements in humans

Eye movements were recorded from three subjects as they initiated tracking of a small circle ("target") moving leftward or rightward, above or below the horizontal meridian, either alone or in the presence of a small square ("distractor") moving leftward or rightward on the other side of the horizontal meridian. At the start of each trial, subjects were provided with either a "form" cue (always centrally positioned and having the circular shape and color of the upcoming moving target) or a "location" cue (a small white square positioned where the upcoming target would appear). The latency of pursuit increased in the presence of an oppositely moving distractor when subjects were provided the form cues but not when they were provided the location cues. The latency of saccades showed similar, but smaller, increases when subjects were given the form cues. On many trials with the form cues, pursuit started in the direction of the distractor and then reversed to follow the target. On these trials, the initial saccade often, but not always, also followed the distractor. These results indicate that the mechanisms of target selection for pursuit and saccades are tightly coordinated but not strictly yoked. The shared effects of the distractor on the latencies of pursuit and saccades probably reflect the common role of visual attention in filtering the inputs that guide these two types of eye movements. The differences in the details of the effects on pursuit and saccades suggest that the neural mechanisms that trigger these two movements can be independently regulated.     

Presentation modality influences behavioral measures of alerting, orienting, and executive control.

The Attention Network Test (ANT) uses visual stimuli to separately assess the attentional skills of alerting (improved performance following a warning cue), spatial orienting (an additional benefit when the warning cue also cues target location), and executive control (impaired performance when a target stimulus contains conflicting information). This study contrasted performance on auditory and visual versions of the ANT to determine whether the measures it obtains are influenced by presentation modality. Forty healthy volunteers completed both auditory and visual tests. Reaction-time measures of executive control were of a similar magnitude and significantly correlated, suggesting that executive control might be a supramodal resource. Measures of alerting were also comparable across tasks. In contrast, spatial-orienting benefits were obtained only in the visual task. Auditory spatial cues did not improve response times to auditory targets presented at the cued location. The different spatial-orienting measures could reflect either separate orienting resources for each perceptual modality, or an interaction between a supramodal orienting resource and modality-specific perceptual processing.