Visual extinction and prior entry: impaired perception of temporal order with intact motion perception after unilateral parietal damage

The neuroanatomy of visual extinction following right hemisphere brain damage: Insights from multivariate and Bayesian lesion analyses in acute stroke

Multi-target attention, that is, the ability to attend and respond to multiple visual targets presented simultaneously on the horizontal meridian across both visual fields, is essential for everyday real-world behaviour. Given the close link between the neuropsychological deficit of extinction and attentional limits in healthy subjects, investigating the anatomy that underlies extinction is uniquely capable of providing important insights concerning the anatomy critical for normal multi-target attention. Previous studies into the brain areas critical for multi-target attention and its failure in extinction patients have, however, produced heterogeneous results. In the current study, we used multivariate and Bayesian lesion analysis approaches to investigate the anatomical substrate of visual extinction in a large sample of 108 acute right hemisphere stroke patients. The use of acute stroke patient data and multivariate/Bayesian lesion analysis approaches allowed us to address limitations associated with previous studies and so obtain a more complete picture of the functional network associated with visual extinction. Our results demonstrate that the right temporo-parietal junction (TPJ) is critically associated with visual extinction. The Bayesian lesion analysis additionally implicated the right intraparietal sulcus (IPS), in line with the results of studies in neurologically healthy participants that highlighted the IPS as the area critical for multi-target attention. Our findings resolve the seemingly conflicting previous findings, and emphasise the urgent need for further research to clarify the precise cognitive role of the right TPJ in multi-target attention and its failure in extinction patients.

TVA in the wild: Applying the theory of visual attention to game-like and less controlled experiments

As a formal theory, Bundesen’s theory of visual attention (TVA) enables the estimation of several theoretically meaningful parameters involved in attentional selection and visual encoding. As of yet, TVA has almost exclusively been used in restricted empirical scenarios such as whole and partial report and with strictly controlled stimulus material. We present a series of experiments in which we test whether the advantages of TVA can be exploited in more realistic scenarios with varying degree of stimulus control. This includes brief experimental sessions conducted on different mobile devices, computer games, and a driving simulator. Overall, six experiments demonstrate that the TVA parameters for processing capacity and attentional weight can be measured with sufficient precision in less controlled scenarios and that the results do not deviate strongly from typical laboratory results, although some systematic differences were found.

Dissociable effects of attention vs working memory training on cognitive performance and everyday functioning following fronto-parietal strokes

Difficulties with attention are common following stroke, particularly in patients with frontal and parietal damage, and are associated with poor outcome. Home-based online cognitive training may have the potential to provide an efficient and effective way to improve attentional functions in such patients. Little work has been carried out to assess the efficacy of this approach in stroke patients, and the lack of studies with active control conditions and rigorous evaluations of cognitive functioning pre and post-training means understanding is limited as to whether and how such interventions may be effective. Here, in a feasibility pilot study, we compare the effects of 20 days of cognitive training using either novel Selective Attention Training (SAT) or commercial Working Memory Training (WMT) programme, versus a waitlist control on a range of attentional and working memory tasks. We demonstrate separable effects of each training condition, with SAT leading to improvements in spatial and non-spatial aspects of attention and WMT leading to improvements on closely related working memory tasks. In addition, both training groups reported improvements in everyday functioning, which were associated with improvements in attention, suggesting that improving attention may be of particular importance in maximising functional improvements in this patient group.

Remote, brain region-specific control of choice behavior with ultrasonic waves

The ability to modulate neural activity in specific brain circuits remotely and systematically could revolutionize studies of brain function and treatments of brain disorders. Sound waves of high frequencies (ultrasound) have shown promise in this respect, combining the ability to modulate neuronal activity with sharp spatial focus. Here, we show that the approach can have potent effects on choice behavior. Brief, low-intensity ultrasound pulses delivered noninvasively into specific brain regions of macaque monkeys influenced their decisions regarding which target to choose. The effects were substantial, leading to around a 2:1 bias in choices compared to the default balanced proportion. The effect presence and polarity was controlled by the specific target region. These results represent a critical step towards the ability to influence choice behavior noninvasively, enabling systematic investigations and treatments of brain circuits underlying disorders of choice.

Neural correlates of tactile simultaneity judgement: a functional magnetic resonance imaging study

Simultaneity judgement (SJ) is a temporal discrimination task in which the targets span an ultimately short time range (zero or not). Psychophysical studies suggest that SJ is adequate to probe the perceptual components of human time processing in pure form. Thus far, time-relevant neural correlates for tactile SJ are unclear. We performed functional magnetic resonance imaging (fMRI) to investigate the neural correlates of tactile SJ using tactile number judgement as a time-irrelevant control task. As our main result, we demonstrated that the right inferior parietal lobule (IPL) is an SJ-specific region. The right IPL was detected by both parametric and non-parametric statistical analyses, and its activation intensity fulfilled a strict statistical criterion. In addition, we observed that some left-dominant regions (e.g., the striatum) were specifically activated by successive stimuli during SJ. Meanwhile, no region was specifically activated by simultaneous stimuli during SJ. Accordingly, we infer that the neural process for tactile SJ is as follows: the striatum estimates the time interval between tactile stimuli; based on this interval, the right IPL discriminates the successiveness or simultaneity of the stimuli. Moreover, taking detailed behavioural results into account, we further discuss possible concurrent or alternative mechanisms that can explain the fMRI results.

Trunk rotation and handedness modulate cortical activation in neglect-associated regions during temporal order judgments

The rotation of the trunk around its vertical midline could be shown to bias visuospatial temporal judgments towards targets in the hemifield ipsilateral to the trunk orientation and to improve visuospatial performance in patients with visual neglect. However, the underlying brain mechanisms are not well understood. Therefore, the goal of the present study was to investigate the neural effects associated with egocentric midplane shifts under consideration of individual handedness. We employed a visuospatial temporal order judgment (TOJ) task in healthy right- and left-handed subjects while their trunk rotation was varied. Participants responded by a saccade towards the stimulus perceived first out of two stimuli presented with different stimulus onset asynchronies (SOA). Apart from gaze behavior, BOLD-fMRI responses were measured using functional magnetic resonance imaging (fMRI). Based on findings from spatial neglect research, analyses of fMRI-BOLD responses were focused on a bilateral fronto-temporo-parietal network comprising Brodmann areas 22, 39, 40, and 44, as well as the basal ganglia core nuclei (caudate, putamen, pallidum).

We observed an acceleration of saccadic speed towards stimuli ipsilateral to the trunk orientation modulated by individual handedness. Left-handed participants showed the strongest behavioral and neural effects, suggesting greater susceptibility to manipulations of trunk orientation. With respect to the dominant hand, a rotation around the vertical trunk midline modulated the activation of an ipsilateral network comprising fronto-temporo-parietal regions and the putamen with the strongest effects for saccades towards the hemifield opposite to the dominant hand. Within the investigated network, the temporo-parietal junction (TPJ) appears to serve as a region integrating sensory, motor, and trunk position information.

Our results are discussed in the context of gain modulatory and laterality effects.

We examined the effect of trunk rotation on brain responses in neglect-associated areas.•

Trunk-related BOLD-fMRI activation patterns depend on handedness.

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They were modulated most during trunk rotation contralateral to the dominant hand.

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Trunk rotation and saccade direction show interaction effects at TPJ.

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TPJ serves as a region integrating sensory, motor, and trunk position information.

Recovery of contralesional saccade choice and reaction time deficits after a unilateral endothelin-1-induced lesion in the macaque caudal prefrontal cortex

The caudal primate prefrontal cortex (PFC) is involved in target selection and visually guided saccades through both covert attention and overt orienting eye movements. Unilateral damage to the caudal PFC often leads to decreased awareness of a contralesional target alone, referred to as “neglect,” or when it is presented simultaneously with an ipsilesional target, referred to as “extinction.” In the current study, we examined whether deficits in contralesional target selection were due to contralesional oculomotor deficits, such as slower reaction times. We experimentally induced a focal ischemic lesion in the right caudal PFC of 4 male macaque monkeys using the vasoconstrictor endothelin-1 and measured saccade choice and reaction times on double-stimulus free-choice tasks and single-stimulus trials before and after the lesion. We found that 1) endothelin-1-induced lesions in the caudal PFC produced contralesional target selection deficits that varied in severity and duration based on lesion volume and location; 2) contralesional neglect-like deficits were transient and recovered by week 4 postlesion; 3) contralesional extinction-like deficits were longer lasting and recovered by weeks 8–16 postlesion; 4) contralesional reaction time returned to baseline well before the contralesional choice deficit had recovered; and 5) neither the mean reaction times nor the reaction time distributions could account for the degree of contralesional extinction on the free-choice task throughout recovery. These findings demonstrate that the saccade choice bias observed after a right caudal PFC lesion is not exclusively due to contralesional motor deficits, but instead reflects a combination of impaired motor and attentional processing.

NEW & NOTEWORTHY Unilateral damage to the caudal prefrontal cortex in macaque monkeys results in impaired contralesional target selection during the simultaneous presentation of an ipsilesional target. We show that the recovery of contralesional target selection cannot be explained by the recovery of prolonged contralesional saccadic reaction times alone. This indicates that an impairment in contralesional attentional processing contributes to the magnitude of the saccade choice bias in the weeks following a unilateral caudal prefrontal cortex lesion.

Low-Frequency Repetitive Transcranial Magnetic Stimulation to Right Parietal Cortex Disrupts Perception of Briefly Presented Stimuli

Right parietal cortex has recently been linked to the temporal resolution of attention. We therefore sought to investigate whether disruption to right parietal cortex would affect attention to visual stimuli presented for brief durations. Participants performed a visual discrimination task before and after 10 minutes repetitive transcranial magnetic stimulation (1 Hz) to right or central parietal cortex as well as 20 minutes after the second block of trials. Participants reported the spatial frequency of a masked Gabor patch presented for a brief duration of 60, 120, or 240 ms. We calculated error magnitudes by comparing accuracy to a guessing model. We then compared error magnitudes to blocks with no stimulation, producing a measure of baselined performance. Baselined performance was poorer at longer stimulus durations after right parietal than central parietal stimulation, suggesting that right parietal cortex is involved in attention to briefly presented stimuli, particularly in situations where rapid accumulation of visual evidence is needed.

Biased temporal order judgments in chronic neglect influenced by trunk position

Numerous studies have reported that temporal order perception is biased in neurological patients with extinction and neglect. These individuals tend to perceive two objectively simultaneous stimuli as occurring asynchronously, with the ipsilesional item being perceived as appearing prior to the contralesional item. Likewise, they report that two stimuli occurred simultaneously in situations where the contralesional item is presented substantially prior to the ipsilesional item. Therefore, they exhibit a biased point of subjective simultaneity (PSS). Here we demonstrate that the magnitude of this effect is modulated by the relative position of the stimuli with respect to the patient's trunk. This effect was only observed in patients who still exhibited neglect symptoms, and neither the pathological bias nor substantial modulation were observed in individuals who had recovered from neglect, those who never had neglect or neurologically healthy controls. Crucially, our design kept the retinal and head-centered coordinates of these stimuli constant, providing a pure measure for the influence of egocentric trunk position. This finding emphasizes the interaction of egocentric spatial position on the temporal symptoms observed in these individuals.

The Pivotal Role of the Right Parietal Lobe in Temporal Attention

The visual system is extremely efficient at detecting events across time even at very fast presentation rates; however, discriminating the identity of those events is much slower and requires attention over time, a mechanism with a much coarser resolution [Cavanagh, P., Battelli, L., & Holcombe, A. O. Dynamic attention. In A. C. Nobre & S. Kastner (Eds.), The Oxford handbook of attention (pp. 652-675). Oxford: Oxford University Press, 2013]. Patients affected by right parietal lesion, including the TPJ, are severely impaired in discriminating events across time in both visual fields [Battelli, L., Cavanagh, P., & Thornton, I. M. Perception of biological motion in parietal patients. Neuropsychologia, 41, 1808-1816, 2003]. One way to test this ability is to use a simultaneity judgment task, whereby participants are asked to indicate whether two events occurred simultaneously or not. We psychophysically varied the frequency rate of four flickering disks, and on most of the trials, one disk (either in the left or right visual field) was flickering out-of-phase relative to the others. We asked participants to report whether two left-or-right-presented disks were simultaneous or not. We tested a total of 23 right and left parietal lesion patients in Experiment 1, and only right parietal patients showed impairment in both visual fields while their low-level visual functions were normal. Importantly, to causally link the right TPJ to the relative timing processing, we ran a TMS experiment on healthy participants. Participants underwent three stimulation sessions and performed the same simultaneity judgment task before and after 20 min of low-frequency inhibitory TMS over right TPJ, left TPJ, or early visual area as a control. rTMS over the right TPJ caused a bilateral impairment in the simultaneity judgment task, whereas rTMS over left TPJ or over early visual area did not affect performance. Altogether, our results directly link the right TPJ to the processing of relative time.

Animal Models of Cerebral Neglect and Its Cancellation

The purpose of this perspective is twofold: 1) to alert and inform the neurospychology and neurology communities on how animal models can improve our understanding of spatial neglect in humans, and 2) to serve as a guide to rehabilitation strategies. Spatial neglect is a neurological syndrome that is inextricably linked to the ability to overtly or covertly reorient attention to new loci. Literature describing variants of neglect leads to the perception of lesion-induced neglect as a uniquely human syndrome for which there are limited treatment options. To the contrary, neglect has been reversed in laboratory animals, and results show that adequate neural representations and motor mechanisms for reversal are present despite damaged or deactivated cerebral cortex. These results and conclusions provoke thought on strategies that can be employed on humans to cancel neglect, and they suggest that long-term amelioration of neglect can be induced by training of specific bypass circuits.

Dissociating the neural correlates of tactile temporal order and simultaneity judgements

Perceiving temporal relationships between sensory events is a key process for recognising dynamic environments. Temporal order judgement (TOJ) and simultaneity judgement (SJ) are used for probing this perceptual process. TOJ and SJ exhibit identical psychometric parameters. However, there is accumulating psychophysical evidence that distinguishes TOJ from SJ. Some studies have proposed that the perceptual processes for SJ (e.g., detecting successive/simultaneity) are also included in TOJ, whereas TOJ requires more processes (e.g., determination of the temporal order). Other studies have proposed two independent processes for TOJ and SJ. To identify differences in the neural activity associated with TOJ versus SJ, we performed functional magnetic resonance imaging of participants during TOJ and SJ with identical tactile stimuli. TOJ-specific activity was observed in multiple regions (e.g., left ventral and bilateral dorsal premotor cortices and left posterior parietal cortex) that overlap the general temporal prediction network for perception and motor systems. SJ-specific activation was observed only in the posterior insular cortex. Our results suggest that TOJ requires more processes than SJ and that both TOJ and SJ implement specific process components. The neural differences between TOJ and SJ thus combine features described in previous psychophysical hypotheses that proposed different mechanisms.

The whole is faster than its parts: evidence for temporally independent attention to distinct spatial locations

Behavioral and electrophysiological evidence suggests that visual attention operates in parallel at distinct spatial locations and samples the environment in periodic episodes. This combination of spatial and temporal characteristics raises the question of whether attention samples locations in a phase-locked or temporally independent manner. If attentional sampling rates were phase locked, attention would be limited by a global sampling rate. However, if attentional sampling rates were temporally independent, they could operate additively to sample higher rates of information. We tested these predictions by requiring participants to identify targets in 2 or 4 rapid serial visual presentation (RSVP) streams, synchronized or asynchronized to manipulate the rate of new information globally (across streams). Identification accuracy exhibited little or no change when the global rate of new information doubled from 7.5 to 15 Hz (Experiment 1) or quadrupled to 30 Hz (Experiment 2). This relatively stable identification accuracy occurred even though participants reliably discriminated 7.5 Hz synchronous displays from displays globally asynchronized at 15 and 30 Hz (Metamer Control Experiment). Identification accuracy in the left visual field also significantly exceeded that in the right visual field. Overall, our results are consistent with temporally independent attention across distinct spatial locations and support previous reports of a right parietal "when" pathway specialized for temporal attention.

Trunk rotation affects temporal order judgments with direct saccades: Influence of handedness

Manipulation of the trunk midline has been shown to improve visuospatial performance in patients with unilateral visual neglect. The goal of the present study was to disentangle motor and perceptual components of egocentric midline manipulations and to investigate the contribution of individual hand preference. Two versions of visual temporal order judgment (TOJ) tasks were tested in healthy right- and left-handed subjects while trunk rotation was varied. In the congruent version, subjects were required to execute a saccade to the first of two horizontal stimuli presented with different stimulus onset asynchronies (SOA). In the incongruent version, subjects were required to perform a vertical saccade to a pre-learned color target, thereby dissociating motor response from the perceptual stimulus location. The main findings of this study are a trunk rotation and response direction specific impact on temporal judgments in form of a prior entry bias for right hemifield stimuli during rightward trunk rotation, but only in the congruent task. This trunk rotation-induced spatial bias was most pronounced in left-handed participants but had the same sign in the right-handed group. Results suggest that egocentric midline shifts in healthy subjects induce a spatially-specific motor, but not a perceptual, bias and underline the importance of taking individual differences in functional laterality such as handedness and mode of perceptual report into account when evaluating effects of trunk rotation in either healthy subjects or neurological patients.

Effects of broken affordance on visual extinction

Previous studies have shown that visual extinction can be reduced if two objects are positioned to “afford” an action. Here we tested if this affordance effect was disrupted by “breaking” the affordance, i.e., if one of the objects actively used in the action had a broken handle. We assessed the effects of broken affordance on recovery from extinction in eight patients with right hemisphere lesions and left-sided extinction. Patients viewed object pairs that were or were not commonly used together and that were positioned for left- or right-hand actions. In the unrelated pair conditions, either two tools or two objects were presented. In line with previous research (e.g., Riddoch et al., 2006), extinction was reduced when action-related object pairs and when unrelated tool pairs were presented compared to unrelated object pairs. There was no significant difference in recovery rate between action-related (object-tool) and unrelated tool pairs. In addition, performance with action-related objects decreased when the tool appeared on the ipsilesional side compared to when it was on the contralesional side, but only when the tool handle was intact. There were minimal effects of breaking the handle of an object rather than a tool, and there was no effect of breaking the handle on either tools or objects on single item trials. The data suggest that breaking the handle of a tool lessens the degree to which it captures attention, with this attentional capture being strongest when the tool appears on the ipsilesional side. The capture of attention by the ipsilesional item then reduces the chance of detecting the contralesional stimulus. This attentional capture effect is mediated by the affordance to the intact tool.

Neural Mechanisms of Temporal Resolution of Attention

The dynamic nature of the world requires that our visual representations are continuously updated. These representations are more precise if there is a narrow time window over which information is averaged. We assess the neural processes of visual updating by testing patients with lesions including inferior parietal cortex, control patients and healthy adults on a continuous visual monitoring task. In Experiment 1, observers kept track of the changing spatial period of a luminance grating and identified the final spatial period after the stimulus disappeared. Healthy older adults and neurological controls were able to perform better than simulated guesses, but only 3 of 11 patients with damage including parietal cortex were able to reach performance that differed from simulated guesses. The effects were unrelated to lesion size. Poor performance on this task is consistent with an inability to selectively attend to the final moment at which the stimulus was seen. To investigate the temporal limits of attention, we varied the rate of stimulus change in Experiment 2. Performance remained poor for some patients even with slow 2.5 Hz change rates. The performance of 4 patients with parietal damage displayed poor temporal precision, namely recovery of performance with slower rates of change.

Motor role of parietal cortex in a monkey model of hemispatial neglect

Parietal cortex is central to spatial cognition. Lesions of parietal cortex often lead to hemispatial neglect, an impairment of choices of targets in space. It has been unclear whether parietal cortex implements target choice at the general cognitive level, or whether parietal cortex subserves the choice of targets of particular actions. To address this question, monkeys engaged in choice tasks in two distinct action contexts--eye movements and arm movements. We placed focused reversible lesions into specific parietal circuits using the GABAA receptor agonist muscimol and validated the lesion placement using MRI. We found that lesions on the lateral bank of the intraparietal sulcus [lateral intraparietal area (LIP)] specifically biased choices made using eye movements, whereas lesions on the medial bank of the intraparietal sulcus [parietal reach region (PRR)] specifically biased choices made using arm movements. This double dissociation suggests that target choice is implemented in dedicated parietal circuits in the context of specific actions. This finding emphasizes a motor role of parietal cortex in spatial choice making and contributes to our understanding of hemispatial neglect.

Relative timing: from behaviour to neurons

Processing of temporal information is critical to behaviour. Here, we review the phenomenology and mechanism of relative timing, ordinal comparisons between the timing of occurrence of events. Relative timing can be an implicit component of particular brain computations or can be an explicit, conscious judgement. Psychophysical measurements of explicit relative timing have revealed clues about the interaction of sensory signals in the brain as well as in the influence of internal states, such as attention, on those interactions. Evidence from human neurophysiological and functional imaging studies, neuropsychological examination in brain-lesioned patients, and temporary disruptive interventions such as transcranial magnetic stimulation (TMS), point to a role of the parietal cortex in relative timing. Relative timing has traditionally been modelled as a 'race' between competing neural signals. We propose an updated race process based on the integration of sensory evidence towards a decision threshold rather than simple signal propagation. The model suggests a general approach for identifying brain regions involved in relative timing, based on looking for trial-by-trial correlations between neural activity and temporal order judgements (TOJs). Finally, we show how the paradigm can be used to reveal signals related to TOJs in parietal cortex of monkeys trained in a TOJ task.

Temporal-order judgment of audiovisual events involves network activity between parietal and prefrontal cortices

Our perception of the temporal order of everyday external events depends on the integrated sensory information in the brain. Our understanding of the brain mechanism for temporal-order judgment (TOJ) of unisensory events, particularly in the visual domain, is advanced. In case of multisensory events, however, there are unanswered questions. Here, by using physically synchronous and asynchronous auditory-visual events in functional magnetic resonance imaging (fMRI) experiments, we identified the brain network that is associated with the perception of the temporal order of multisensory events. The activation in the right temporo-parietal junction was modulated by the perception of asynchronous audiovisual events. During this perception of temporal order, the right dorsolateral prefrontal cortex coordinated activity with the right temporo-parietal and the left inferior parietal cortices. These results suggest that the TOJ in the multisensory domain underlies a network activity between parietal and prefrontal cortices unlike the regional activity in the right temporo-parietal junction in the unisensory visual domain.

Further evidence of auditory extinction in aphasia

PURPOSE

Preliminary research (Shisler, 2005) suggests that auditory extinction in individuals with aphasia (IWA) may be connected to binding and attention. In this study, the authors expanded on previous findings on auditory extinction to determine the source of extinction deficits in IWA.

METHOD

Seventeen IWA (M(age) = 53.19 years) and 17 neurologically intact controls (M(age) = 55.18 years) participated. Auditory stimuli were spoken letters presented in a free-field listening environment. Stimuli were presented in single-stimulus stimulation (SSS) or double-simultaneous stimulation (DSS) trials across 5 conditions designed to determine whether extinction is related to binding, inefficient attention resource allocation, or overall deficits in attention. All participants completed all experimental conditions.

RESULTS

Significant extinction was demonstrated only by IWA when sounds were different, providing further evidence of auditory extinction. However, binding requirements did not appear to influence the IWA's performance.

CONCLUSIONS

Results indicate that, for IWA, auditory extinction may not be attributed to a binding deficit or inefficient attention resource allocation because of equivalent performance across all 5 conditions. Rather, overall attentional resources may be influential. Future research in aphasia should explore the effect of the stimulus presentation in addition to the continued study of attention treatment.

Attention and the speed of information processing: posterior entry for unattended stimuli instead of prior entry for attended stimuli

Why are nearly simultaneous stimuli frequently perceived in reversed order? The origin of errors in temporal judgments is a question older than experimental psychology itself. One of the earliest suspects is attention. According to the concept of prior entry, attention accelerates attended stimuli; thus they have "prior entry" to perceptive processing stages, including consciousness. Although latency advantages for attended stimuli have been revealed in psychophysical studies many times, these measures (e.g. temporal order judgments, simultaneity judgments) cannot test the prior-entry hypothesis completely. Since they assess latency differences between an attended and an unattended stimulus, they cannot distinguish between faster processing of attended stimuli and slower processing of unattended stimuli. Therefore, we present a novel paradigm providing separate estimates for processing advantages respectively disadvantages of attended and unattended stimuli. We found that deceleration of unattended stimuli contributes more strongly to the prior-entry illusion than acceleration of attended stimuli. Thus, in the temporal domain, attention fulfills its selective function primarily by deceleration of unattended stimuli. That means it is actually posterior entry, not prior entry which accounts for the largest part of the effect.

The contribution of stimulus-driven and goal-driven mechanisms to feature-based selection in patients with spatial attention deficits

When people search a display for a target defined by a unique feature, fast saccades are predominantly stimulus-driven whereas slower saccades are primarily goal-driven. Here we use this dissociative pattern to assess whether feature-based selection in patients with lateralized spatial attention deficits is impaired in stimulus-driven processing, goal-driven processing, or both. A group of patients suffering from extinction or neglect after parietal damage, and a group of healthy, age-matched controls, were instructed to make a saccade to a uniquely oriented target line which was presented simultaneously with a differently oriented distractor line. We systematically varied the salience of the target and distractor by changing the orientation of background elements, and used a time-based model to extract stimulus-driven (salience) and goal-driven (target set) components of selection. The results show that the patients exhibited reduced stimulus-driven processing only in the contralesional hemifield, while goal-driven processing was reduced across both hemifields.

Visual search in spatial neglect studied with a preview paradigm

Impaired visual search is a hallmark of spatial neglect. When searching for an unique feature (e.g., color) neglect patients often show only slight visual field asymmetries. In contrast, when the target is defined by a combination of features (e.g., color and form) they exhibit a severe deficit of contralesional search. This finding suggests a selective impairment of the serial deployment of spatial attention. Here, we examined this deficit with a preview paradigm. Neglect patients searched for a target defined by the conjunction of shape and color, presented together with varying numbers of distracters. The presentation time was varied such that on some trials participants previewed the target together with same-shape/different-color distracters, for 300 or 600 ms prior to the appearance of additional different-shape/same-color distracters. On the remaining trials the target and all distracters were shown simultaneously. Healthy participants exhibited a serial search strategy only when all items were presented simultaneously, whereas in both preview conditions a pop-out effect was observed. Neglect patients showed a similar pattern when the target was presented in the right hemifield. In contrast, when searching for a target in the left hemifield they showed serial search in the no-preview condition, as well as with a preview of 300 ms, and partly even at 600 ms. A control experiment suggested that the failure to fully benefit from item preview was probably independent of accurate perception of time. Our results, when viewed in the context of existing literature, lead us to conclude that the visual search deficit in neglect reflects two additive factors: a biased representation of attentional priority in favor of ipsilesional information and exaggerated capture of attention by ipsilesional abrupt onsets.

Neglect: a multisensory deficit?

Neglect is a neurological syndrome characterised by a lack of conscious perception of events localised in the contralesional side of space. Here, we consider the possible multisensory nature of this disorder, critically reviewing the literature devoted to multisensory manifestations and processing in neglect. Although its most striking manifestations have been observed in the visual domain, a number of studies demonstrate that neglect can affect virtually any sensory modality, in particular touch and audition. Furthermore, a few recent studies have reported a correlation in severity between visual and non-visual neglect-related deficits evaluated in the same patients, providing some preliminary support for a multisensory conception of neglect. Sensory stimulation and sensorimotor adaptation techniques, aimed at alleviating neglect, have also been shown to affect several sensory modalities, including some that were not directly affected by the intervention. Finally, in some cases neglect can bias multisensory interactions known to occur in healthy individuals, leading to abnormal behaviour or uncovering multisensory compensation mechanisms. This evidence, together with neurophysiological and neuroimaging data revealing the multisensory role played by the areas that are most commonly damaged in neglect patients, seems to speak in favour of neglect as a multisensory disorder. However, since most previous studies were not conducted with the specific purpose of systematically investigating the multisensory nature of neglect, we conclude that more research is needed to appropriately assess this question, and suggest some methodological guidelines that we hope will help clarify this issue. At present, the conception of neglect as a multisensory disorder remains a promising working hypothesis that may help define the pathophysiology of this syndrome.

Attention in neglect and extinction: assessing the degree of correspondence between visual and auditory impairments using matched tasks

Claims have been made for associated degrees of impairment on both visual and auditory performance in unilateral neglect and extinction. Since this evidence is primarily based on different tests in each modality, it is difficult to properly quantify the degree of association between performance in vision and audition. The current study compares visual and auditory extinction and temporal order judgments (TOJs) in two cases with clinical visual neglect. Stimuli in both modalities were precisely matched in their temporal and spatial parameters. The results reveal a mixed pattern of association between different auditory tests and their visual counterparts. This suggests that associations between visual and auditory neglect can occur but these are neither obligatory nor pervasive. Instead, our data support models of spatial impairment in neglect and extinction that acknowledge differences in the contribution of spatial information to performance in each modality in responses to changing task demands.

Studying Multisensory Processing and Its Role in the Representation of Space through Pathological and Physiological Crossmodal Extinction

The study of crossmodal extinction has brought a considerable contribution to our understanding of how the integration of stimuli perceived in multiple sensory modalities is used by the nervous system to build coherent representations of the space that directly surrounds us. Indeed, by revealing interferences between stimuli in a disturbed system, extinction provides an invaluable opportunity to investigate the interactions that normally exist between those stimuli in an intact system. Here, we first review studies on pathological crossmodal extinction, from the original demonstration of its existence, to its role in the exploration of the multisensory neural representation of space and the current theoretical accounts proposed to explain the mechanisms involved in extinction and multisensory competition. Then, in the second part of this paper, we report recent findings showing that physiological multisensory competition phenomena resembling clinical crossmodal extinction exist in the healthy brain. We propose that the development of a physiological model of sensory competition is fundamental to deepen our understanding of the cerebral mechanisms of multisensory perception and integration. In addition, a similar approach to develop a model of physiological sensory competition in non-human primates should allow combining functional neuroimaging with more invasive techniques, such as transient focal lesions, in order to bridge the gap between works done in the two species and at different levels of analysis.