Opposite Hemifield Research Articles

Contralesional distractors enhance ipsilesional target processing after right-hemispheric stroke

Stroke can result in marked impairments in the processing of information presented in contralesional space. The present prospective study investigated how a contralesional distractor affects ipsilesional perception in patients with a right-hemispheric stroke. In a simple target detection task, the influence of the distractor on reaction times (RTs) was examined in stroke patients and compared to the performance of healthy elderly controls. Distractor interference effects were related to measures of neglect and extinction using a regression analysis. Moreover, the magnitude of the behavioural distractor effect entered a voxel-based lesion-symptom mapping (VLSM) analysis with the hypothesis that parietal and temporoparietal lesions are related to altered distractor processing. While the presence of a distractor in the opposite hemifield slowed down RTs in healthy controls for left and right targets, this effect was only observed for contralesional left targets in the group of right-hemispheric patients. In stark contrast, the presence of a distractor in the contralesional hemifield expedited ipsilesional (i.e., right) target detection. This effect was significantly related to lesions in the anterior middle temporal and temporoparietal cortex, external and internal capsule, as well as the superior longitudinal fascicle (SLF). These results elucidate the nature of the disruption of attentive processing in the contralesional hemifield after right-hemispheric stroke. More specifically, they shed light on the abnormal prioritisation of ipsilesional information: our data suggest that damage to the temporal and temporoparietal cortex and white matter tracts may transform contralesional stimulation into an unspecific saliency signal contributing to facilitated information processing in ipsilesional space.

How Eye Dominance Strength Modulates the Influence of a Distractor on Saccade Accuracy.

Neuroimaging studies have shown that the dominant eye is linked preferentially to the ipsilateral primary visual cortex. However, its role in perception still is misunderstood. We examined the influence of eye dominance and eye dominance strength on saccadic parameters, contrasting stimulations presented in the two hemifields. Participants with contrasted eye dominance (left or right) and eye dominance strength (strong or weak) were asked to make a saccade toward a target displayed at 5° or 7° left or right of a fixation cross. In some trials, a distractor at 3° of eccentricity also was displayed either in the same hemifield as the target (to induce a global effect on saccade amplitude) or in the opposite hemifield (to induce a remote distractor effect on saccade latency). Eye dominance did influence saccade amplitude as participants with strong eye dominance showed more accurate saccades toward the target (weaker global effect) in the hemifield contralateral to the dominant eye than in the ipsilateral one. Such asymmetry was not found in participants with weak eye dominance or when a remote distractor was used. We show that eye dominance strength influences saccade target selection. We discuss several arguments supporting the view that such advantage may be linked to the relationship between the dominant eye and ipsilateral hemisphere. French Abstract.

Saccades Follow Perception When Judging Location.

An unresolved question in vision research is whether perceptual decision making and action are based on the same or on different neural representations. Here, we address this question for a straightforward task, the judgment of location. In our experiment, observers decided on the closer of two peripheral objects—situated on the horizontal meridian in opposite hemifields—and made a saccade to indicate their choice. Correct saccades landed close to the actual (physical) location of the target. However, in case of errors, saccades went in the direction of the more distant object, yet landed on a position approximating that of the closer one. Our finding supports the notion that perception and action-related decisions on object location rely on the same neural representation.

Opponent Coding of Sound Location (Azimuth) in Planum Temporale is Robust to Sound-Level Variations.

Coding of sound location in auditory cortex (AC) is only partially understood. Recent electrophysiological research suggests that neurons in mammalian auditory cortex are characterized by broad spatial tuning and a preference for the contralateral hemifield, that is, a nonuniform sampling of sound azimuth. Additionally, spatial selectivity decreases with increasing sound intensity. To accommodate these findings, it has been proposed that sound location is encoded by the integrated activity of neuronal populations with opposite hemifield tuning (“opponent channel model”). In this study, we investigated the validity of such a model in human AC with functional magnetic resonance imaging (fMRI) and a phase-encoding paradigm employing binaural stimuli recorded individually for each participant. In all subjects, we observed preferential fMRI responses to contralateral azimuth positions. Additionally, in most AC locations, spatial tuning was broad and not level invariant. We derived an opponent channel model of the fMRI responses by subtracting the activity of contralaterally tuned regions in bilateral planum temporale. This resulted in accurate decoding of sound azimuth location, which was unaffected by changes in sound level. Our data thus support opponent channel coding as a neural mechanism for representing acoustic azimuth in human AC.

Refining The Resource Model: Cortical Competition Could Explain Hemifield Independence

A growing number of studies find that we can select independently from the right and left hemifields, prompting some to suggest separate resources in the two hemispheres. Here we report an experiment suggesting that competition between representations, rather than exhaustion of resources, underlies these effects. Subjects searched a briefly displayed array for one of two targets. Subjects were slower to detect a target when it shared a hemifield with the distractors than when the distractors appeared in the opposite hemifield. Importantly however, this only occurred when the array was dense, suggesting that competition, rather than hemifield-specific resources, explains the decrement.

Clinical assessment of retinal changes by spectral-domain OCT.

To evaluate optical coherence tomography changes in patients with retinal thinning at the posterior pole. In this cross-sectional and retrospective study, 648 files were reviewed, and 67 patients were selected. Optical coherence tomography images that showed an area with a retinal thickness reduction at the macular region by using the Asymmetry Analysis Map in Heidelberg Spectralis were selected. The presence of hemisphere asymmetry in the same eye and asymmetry between the paired eyes were calculated and used for the analysis. Retinal thickness was measured in 3 different retinal areas (squares): (1) the area (square) involved by the pathology (IA), (2) the specular area (square) in the opposite hemifield (SA), and (3) the corresponding IA in the contralateral eye (CIA) (area used to recruit the patients). Retinal layer morphology was analyzed observing the Spectralis screen. The thickness of the IA was 235.54 ± 39.95 µm (mean ± standard deviation), while it was 269.84 ± 36.16 µm and 293.81 ± 37.52 µm for SA and CIA, respectively. Different retinal layers could be involved in reduction of the retinal thickness: a reduction of the inner layers was related to disease in which ciliary or retinal arterial vessel flow was involved, while a reduction of the outer retinal layer was related to pathologies related to choroidal flow diseases.

Keeping in touch with the visual system: spatial alignment and multisensory integration of visual-somatosensory inputs.

Correlated sensory inputs coursing along the individual sensory processing hierarchies arrive at multisensory convergence zones in cortex where inputs are processed in an integrative manner. The exact hierarchical level of multisensory convergence zones and the timing of their inputs are still under debate, although increasingly, evidence points to multisensory integration (MSI) at very early sensory processing levels. While MSI is said to be governed by stimulus properties including space, time, and magnitude, violations of these rules have been documented. The objective of the current study was to determine, both psychophysically and electrophysiologically, whether differential visual-somatosensory (VS) integration patterns exist for stimuli presented to the same versus opposite hemifields. Using high-density electrical mapping and complementary psychophysical data, we examined multisensory integrative processing for combinations of visual and somatosensory inputs presented to both left and right spatial locations. We assessed how early during sensory processing VS interactions were seen in the event-related potential and whether spatial alignment of the visual and somatosensory elements resulted in differential integration effects. Reaction times to all VS pairings were significantly faster than those to the unisensory conditions, regardless of spatial alignment, pointing to engagement of integrative multisensory processing in all conditions. In support, electrophysiological results revealed significant differences between multisensory simultaneous VS and summed V + S responses, regardless of the spatial alignment of the constituent inputs. Nonetheless, multisensory effects were earlier in the aligned conditions, and were found to be particularly robust in the case of right-sided inputs (beginning at just 55 ms). In contrast to previous work on audio-visual and audio-somatosensory inputs, the current work suggests a degree of spatial specificity to the earliest detectable multisensory integrative effects in response to VS pairings.

An inability to set independent attentional control settings by hemifield.

Recent evidence suggests that people can simultaneously activate attentional control setting for two distinct colors. However, it is unclear whether both attentional control settings must operate globally across the visual field or whether each can be constrained to a particular spatial location. Using two different paradigms, we investigated participants' ability to apply independent color attentional control settings to distinct regions of space. In both experiments, participants were told to identify red letters in one hemifield and green letters in the opposite hemifield. Additionally, some trials used a "relevant distractor"-a letter that matched the opposite side's target color. In Experiment 1, eight letters appeared (four per hemifield) simultaneously for a brief amount of time and then were masked. Relevant distractors increased the error rate and resulted in a greater number of distractor intrusions than irrelevant distractors. Similar results were observed in Experiment 2 in which red and green targets were presented in two rapid serial visual presentation streams. Relevant distractors were found to produce an attentional blink similar in magnitude to an actual target. The results of both experiments suggest that letters matching either attentional control setting were selected by attention and were processed as if they were targets, providing strong evidence that both attentional control settings were applied globally, rather than being constrained to a particular location.

Spatial attention increases the pupillary response to light changes

We measured pupil size in adult human subjects while we manipulated both the luminance of the visual scene and the location of attention. We found that, with central fixation maintained, pupillary constrictions and dilations evoked by peripheral luminance increments and decrements are larger when spatial attention is covertly (i.e., with no eye movements) directed to the stimulus region versus when it is directed to the opposite hemifield. Irrespective of the size of the attended region (focused at the center of the stimulus or spread within and outside the stimulus), the attentional enhancement is large: more than 20% of the response to stimuli in the unattended hemifield. This indicates that a sizable portion of this simple ocular behavior—often considered a subcortical "reflex"—in fact depends on cortical processing. Together, these features indicate that pupillometry is not only an index of retinal and brainstem function, but also an objective measure of complex constructs such as attention and its effects on sensory processing.

Neural competition via lateral inhibition between decision processes and not a STOP signal accounts for the antisaccade performance in healthy and schizophrenia subjects.

A commentary on Re-starting a neural race: anti-saccade correction by Noorani, I., and Carpenter, R. H. S. (2014). Eur. J. Neurosci. 39, 159–164. doi: 10.1111/ejn.12396 Decision making is the process of accumulating evidence about the world and the utility of possible outcomes (Cutsuridis, 2010). A paradigm often used by behavioral neuroscientists to investigate decision processes is the antisaccade paradigm (see Figure 1A; Hallett, 1978). In the antisaccade paradigm subjects are required to suppress an erroneous saccade (error prosaccade) toward a peripheral stimulus and instead make an eye movement to a position in the opposite hemifield (antisaccade). The response repertoire of a subject performing the antisaccade task has been reported to be: (1) the subject makes an erroneous response (i.e., looking toward the peripheral stimulus), (2) the subject makes the antisaccade (i.e., looking in the opposite direction of the peripheral stimulus, and (3) the subject makes an erroneous response followed by a corrected antisaccade (Evdokimidis et al., 2002).

Nasotemporal ERP differences: evidence for increased inhibition of temporal distractors

Previous research has demonstrated behavioral advantages for stimuli in the temporal relative to the nasal visual hemifield. To investigate whether this nasotemporal asymmetry reflects a genuinely attentional bias, we recorded event-related potentials in a task where participants identified a color-defined target digit in one visual hemifield that was accompanied by an irrelevant distractor in the opposite hemifield (experiment 1). To dissociate the processing of stimuli in nasal and temporal visual hemifields, an eye-patching procedure was used. Targets triggered N2pc components that marked their attentional selection. Unexpectedly, these N2pc components were larger and emerged earlier for nasal relative to temporal targets. Experiment 2 provided evidence that this nasotemporal asymmetry for the N2pc is linked to an increased attentional inhibition of temporal distractors. Relative to nasal distractors, temporal distractors elicited an increased inhibition-related contralateral positivity, resulting in more pronounced differences between contralateral and ipsilateral event-related potentials on trials with temporal distractors and nasal targets. These results provide novel evidence for a genuinely attentional contribution to nasotemporal asymmetries and suggest that such asymmetries are associated with top-down controlled distractor inhibition.

Decreased visual detection during subliminal stimulation.

What is the perceptual fate of invisible stimuli-are they processed at all and does their processing have consequences for the perception of other stimuli? As has been shown previously in the somatosensory system, even stimuli that are too weak to be consciously detected can influence our perception: Subliminal stimulation impairs perception of near-threshold stimuli and causes a functional deactivation in the somatosensory cortex. In a recent study, we showed that subliminal visual stimuli lead to similar responses, indicated by an increase in alpha-band power as measured with electroencephalography (EEG). In the current study, we investigated whether a behavioral inhibitory mechanism also exists within the visual system. We tested the detection of peripheral visual target stimuli under three different conditions: Target stimuli were presented alone or embedded in a concurrent train of subliminal stimuli either at the same location as the target or in the opposite hemifield. Subliminal stimuli were invisible due to their low contrast, not due to a masking procedure. We demonstrate that target detection was impaired by the subliminal stimuli, but only when they were presented at the same location as the target. This finding indicates that subliminal, low-intensity stimuli induce a similar inhibitory effect in the visual system as has been observed in the somatosensory system. In line with previous reports, we propose that the function underlying this effect is the inhibition of spurious noise by the visual system.

Understanding how visual attention locks on to a location: Toward a computational model of the N2pc component.

Competing theories regarding the neural mechanism underlying the N2pc suggest that it reflects target enhancement or distractor suppression, but we suggest a different theory. In our study, participants viewed a bilateral dual-stream rapid serial visual presentation (RSVP). Each trial had either one or two targets (T1, T2) presented either in the same or different streams. We found that the N2pc elicited by T1 was not affected by an immediately following T2 at the same location. However, if T2 was on the opposite hemifield, a second N2pc was elicited. These results suggest that the N2pc reflects neural processes involved in localizing a target to initiate the deployment of attention, rather than the sustained enhancement of processing at that location or distractor inhibition. We have created a computational model of a convergent gradient-filter model of attention that reproduces the observed event-related potential traces and makes five testable predictions to be addressed in future work.

Refining the resource model: Cortical competition could explain hemifield independence

A growing number of studies find that we can select independently from the right and left hemifields, prompting some to suggest separate resources in the two hemispheres. Here we report an experiment suggesting that competition between representations, rather than exhaustion of resources, underlies these effects. Subjects searched a briefly displayed array for one of two targets. Subjects were slower to detect a target when it shared a hemifield with the distractors than when the distractors appeared in the opposite hemifield. Importantly however, this only occurred when the array was dense, suggesting that competition, rather than hemifield-specific resources, explains the decrement.

Learning to Read Vertical Text in Peripheral Vision

English-language text is almost always written horizontally. Text can be formatted to run vertically, but this is seldom used. Several studies have found that horizontal text can be read faster than vertical text in the central visual field. No studies have investigated the peripheral visual field. Studies have also concluded that training can improve reading speed in the peripheral visual field for horizontal text. We aimed to establish whether the horizontal vertical differences are maintained and if training can improve vertical reading in the peripheral visual field. Eight normally sighted young adults participated in the first study. Rapid serial visual presentation (RSVP) reading speed was measured for horizontal and vertical text in the central visual field and at 10 degrees eccentricity in the upper or lower (horizontal text) and right or left (vertical text) visual fields. Twenty-one normally sighted young adults split equally between two training groups and one control group participated in the second study. Training consisted of RSVP reading using either vertical text in the left visual field or horizontal text in the inferior visual field. Subjects trained daily over 4 days. Pre- and post- horizontal and vertical RSVP reading speeds were carried out for all groups. For the training groups, these measurements were repeated 1 week and 1 month posttraining. Before training, RSVP reading speeds were faster for horizontal text in the central and peripheral visual fields when compared with vertical text. Training vertical reading improved vertical reading speeds by an average factor of 2.8. There was partial transfer of training to the opposite (right) hemifield. The training effects were retained for up to a month. Rapid serial visual presentation training can improve RSVP vertical text reading in peripheral vision. These findings may have implications for patients with macular degeneration or hemianopic field loss.

Nasal-Temporal Asymmetries in Suprathreshold Facial Expressions of Emotion

In this study, we investigated nasal-temporal asymmetries resulting from exposure to visible facial expressions of emotion. The literature has so far reported attentional asymmetries on spatial perception, and nasal-temporal asymmetries resulting from backward masked visual stimuli activated through non-conscious perception. To our knowledge however, no attempt has been made to test such asymmetries with un-masked, consciously visible emotional stimuli. Here, we report on response differences in binocular and monocular viewing on the perception of visible facial expressions of affect. In Study 1, 24 right-handed adults completed a speeded forced-choice paradigm, in which they binocularly viewed bilateral displays of a neutral face in one hemifield, simultaneously paired either with a happy or angry face of variable emotional salience in the opposite hemifield, for 50ms; the task was to indicate the left-right location of the emotional face. In Study 2 (N=23, right-handed), participants completed the same paradigm mononocularly while alternately patching either left and right eye in successive blocks. Under binocular viewing, we found an overall advantage for localising happy faces, further intensified when displayed in the right visual hemifield, and evident even when the emotional expression was extremely subtle. For monocularly viewed stimuli, we observed a response latency asymmetry with faster responses to temporally viewed happy faces. However there was higher overall accuracy for nasal stimuli regardless of emotion, further intensified when the emotional face appeared on the left. Our findings show that the nasal-temporal asymmetries previously associated exclusively with the non-conscious perception of emotional stimuli, manifest as ‘trigger-happy’ responses when emotional stimuli become visible. This asymmetrical reduction in response latency for nasal stimuli could be attributed an overall attentional bias towards the temporal area ipsilateral to the open eye. This however does not appear to increase our detection accuracy, suggesting that a subcortically driven increase in vigilance does not necessarily make for more efficient responses.

EEG Correlates of Decision-Making and Preparation of Saccades in Human

A modificated version of experimental scheme double step was used to study the decision-making process in saccade programming. Two short visual stimuli were presented sequentially in opposite hemifields (scheme pulse overshoot). The pattern of response (double saccade or single saccade at the second target) and latency of the first saccade depended on the duration of the first stimulus (150 or 50 ms). Positive ERP components were revealed within the interval 100 - 120 ms before the saccade onset by using the method of selective EEG averaging. These potentials can be considered as markers of the decision-making process. The parameters and topography of these components indicate involvement of the frontal- parietal neural network of saccadic control and frontal- parietal-and medial-thalamic activation systems of selective attention at the stage of decision-making process.

Redundancy gain in binocular rivalry.

A behavioural advantage is found across a wide range of stimuli when two targets are presented in opposite hemifields compared with those targets being presented together in one hemifield, or one target being presented alone. This advantage for responses to multiple targets versus a single target is often termed redundancy gain. Here we report on the findings of two experiments investigating redundancy gain in binocular rivalry. Experiment 1 presented a rival pair in one hemifield with an additional image presented to both eyes in the opposite hemifield. There was a weak effect of this stable image on the perceived dominance of the images within the rival pair. Experiment 2 presented a second rival pair in either the same or opposite hemifield and showed that instances of joint predominance were greater when the two pairs were presented in opposite hemifields than within the same hemifield. Therefore, the findings suggest that redundancy gain may be extended to stimuli presented under binocular rivalry conditions.

FMRI correlates of object-based attentional facilitation vs. suppression of irrelevant stimuli, dependent on global grouping and endogenous cueing

Theories of object-based attention often make two assumptions: that attentional resources are facilitatory, and that they spread automatically within grouped objects. Consistent with this, ignored visual stimuli can be easier to process, or more distracting, when perceptually grouped with an attended target stimulus. But in past studies, the ignored stimuli often shared potentially relevant features or locations with the target. In this fMRI study, we measured the effects of attention and grouping on Blood Oxygenation Level Dependent (BOLD) responses in the human brain to entirely task-irrelevant events. Two checkerboards were displayed each in opposite hemifields, while participants responded to check-size changes in one pre-cued hemifield, which varied between blocks. Grouping (or segmentation) between hemifields was manipulated between blocks, using common (vs. distinct) motion cues. Task-irrelevant transient events were introduced by randomly changing the color of either checkerboard, attended or ignored, at unpredictable intervals. The above assumptions predict heightened BOLD signals for irrelevant events in attended vs. ignored hemifields for ungrouped contexts, but less such attentional modulation under grouping, due to automatic spreading of facilitation across hemifields. We found the opposite pattern, in primary visual cortex. For ungrouped stimuli, BOLD signals associated with task-irrelevant changes were lower, not higher, in the attended vs. ignored hemifield; furthermore, attentional modulation was not reduced but actually inverted under grouping, with higher signals for events in the attended vs. ignored hemifield. These results challenge two popular assumptions underlying object-based attention. We consider a broader biased-competition framework: task-irrelevant stimuli are suppressed according to how strongly they compete with task-relevant stimuli, with intensified competition when the irrelevant features or locations comprise the same object.

Retinotopy of facial expression adaptation.

The face aftereffect (FAE; the illusion of faces after adaptation to a face) has been reported to occur without retinal overlap between adaptor and test, but recent studies revealed that the FAE is not constant across all test locations, which suggests that the FAE is also retinotopic. However, it remains unclear whether the characteristic of the retinotopy of the FAE for one facial aspect is the same as that of the FAE for another facial aspect. In the research reported here, an examination of the retinotopy of the FAE for facial expression indicated that the facial expression aftereffect occurs without retinal overlap between adaptor and test, and depends on the retinal distance between them. Furthermore, the results indicate that, although dependence of the FAE on adaptation-test distance is similar between facial expression and facial identity, the FAE for facial identity is larger than that for facial expression when a test face is presented in the opposite hemifield. On the basis of these results, I discuss adaptation mechanisms underlying facial expression processing and facial identity processing for the retinotopy of the FAE.