Attentional selection of relative SF mediates global versus local processing: evidence from EEG

Skill and experience impact neural activity during global and local biological motion processing

During biological motion perception, individuals with perceptual experience learn to use more global processing, simultaneously extracting information from multiple body segments. Less experienced observers may use more local processing of individual body segments. The parietal lobe (e.g., alpha and beta power) has been shown to be critical to global and local static stimulus perception. Therefore, in this paper, we examined how skill impacts motion processing by assessing behavioral and neural responses to degrading global or local motion information for soccer penalty kicks. Skilled (N = 21) and less skilled (N = 19) soccer players anticipated temporally occluded videos of penalty kicks under normal, blurred (degraded local information), or spatially occluded (hips-only; degraded global information) viewing conditions. EEG was used to measure parietal alpha and beta power. Skilled players outperformed less skilled players, albeit both skill groups were less accurate in the blurred and hips-only conditions. Skilled performers showed significant decreases in bilateral parietal beta power in the hips-only condition, suggesting a greater reliance on global motion information under normal viewing conditions. Additionally, the hips-only condition elicited significantly greater beta relative to alpha power (beta - alpha), lower beta power, and lower alpha power than the control condition for both skill groups, suggesting spatial occlusion elicited a shift towards more local processing. Our novel findings demonstrate that skill and experience impact how motion is processed.

Skilled Performers Show Right Parietal Lateralization during Anticipation of Volleyball Attacks

Global and local biological motion processing are likely influenced by an observer's perceptual experience. Skilled athletes anticipating an opponent's movements use globally distributed motion information, while less skilled athletes focus on single kinematic cues. Published reports have demonstrated that attention can be primed globally or locally before perceptual tasks; such an intervention could highlight motion processing mechanisms used by skilled and less skilled observers. In this study, we examined skill differences in biological motion processing using attentional priming. Skilled (N = 16) and less skilled (N = 16) players anticipated temporally occluded videos of volleyball attacks after being primed using a Navon matching task while parietal EEG was measured. Skilled players were more accurate than less skilled players across priming conditions. Global priming improved performance in both skill groups. Skilled players showed significantly reduced alpha and beta power in the right compared to left parietal region, but brain activity was not affected by the priming interventions. Our findings highlight the importance of right parietal dominance for skilled performers, which may be functional for inhibiting left hemispheric local processing or enhancing visual spatial attention for dynamic visual scenes. Further work is needed to systematically determine the function of this pattern of brain activity during skilled anticipation.

Motivational intensity and visual word search: Layout matters

Motivational intensity has been previously linked to information processing. In particular, it has been argued that affects which are high in motivational intensity tend to narrow cognitive scope. A similar effect has been attributed to negative affect, which has been linked to narrowing of cognitive scope. In this paper, we investigated how these phenomena manifest themselves during visual word search. We conducted three studies in which participants were instructed to perform word category identification. We manipulated motivational intensity by controlling reward expectations and affect via reward outcomes. Importantly, we altered visual search paradigms, assessing the effects of affective manipulations as modulated by information arrangement. We recorded multiple physiological signals (EEG, EDA, ECG and eye tracking) to assess whether motivational states can be predicted by physiology. Across the three studies, we found that high motivational intensity narrowed visual attentional scope by altering visual search strategies, especially when information was displayed sparsely. Instead, when information was vertically listed, approach-directed motivational intensity appeared to improve memory encoding. We also observed that physiology, in particular eye tracking, may be used to detect biases induced by motivational intensity, especially when information is sparsely organised.

Time course of spatial frequency integration in face perception: An ERP study

Face perception is based on the processing and integration of multiple spatial frequency (SF) ranges. However, the temporal dynamics of SF integration to form an early face representation in the human brain is still a matter of debate. To address this issue, we recorded event-related potentials (ERPs) during the presentation of spatial frequency-manipulated facial images. Twenty-six participants performed a gender discrimination task on non-filtered, low-, high-, and band-pass filtered face images, corresponding, respectively, to the full range, spatial frequencies up to 8 cycles/image, above 32 cycles/image, and from 8 to 16 cycles/image. Behaviorally, the task related-performance was more accurate and faster for non-filtered (NF) and mid-range SF (MSF) than for low SF (LSF) and high SF (HSF) stimuli. At both behavioral and electrophysiological levels, response to MSF contained in faces did not differ from the responses to full spectrum non-filtered (NF) facial images. In ERPs, LSF facial images evoked the largest P1 amplitude while HSF facial images evoked the largest N170 amplitude compared with the other three conditions. Since LSFs and HSFs would transmit global and local information respectively, our observations lend further support to the "coarse-to-fine" processing theory of faces. Furthermore, they offer original evidence of the effectiveness and adequacy of the mid-range spatial frequency in face perception. Possible theoretical interpretations of our findings are discussed.

Recurrent Activation of Neural Circuits during Attention to Global and Local Visual Information

The attentional selection of different hierarchical level within compound (Navon) figures has been studied with event related potentials (ERPs), by controlling the ERPs obtained during attention to the global or the local echelon. These studies, using the canonical Navon figures, have produced contradictory results, with doubts regarding the scalp distribution of the effects. Moreover, the evidence about the temporal evolution of the processing of these two levels is not clear. Here, we unveiled global and local letters at distinct times, which enabled separation of their ERP responses. We combine this approach with the temporal generalization methodology, a novel multivariate technique which facilitates exploring the temporal structure of these ERPs. Opposite lateralization patterns were obtained for the selection negativities generated when attending global and local distracters (D statistics, p < .005), with maxima in right and left occipito-temporal scalp regions, respectively (η2 = .111, p < .01; η2 = .042, p < .04). However, both discrimination negativities elicited when comparing targets and distractors at the global or the local level were lateralized to the left hemisphere (η2 = .25, p < .03 and η2 = .142, p < .05 respectively). Recurrent activation patterns were found for both global and local stimuli, with scalp topographies corresponding to early preparatory stages reemerging during the attentional selection process, thus indicating recursive attentional activation. This implies that selective attention to global and local hierarchical levels recycles similar neural correlates at different time points. These neural correlates appear to be mediated by visual extra-striate areas.

Relative Spatial Frequency Processing Drives Hemispheric Asymmetry in Conscious Awareness

Visual stimuli with different spatial frequencies (SFs) are processed asymmetrically in the two cerebral hemispheres. Specifically, low SFs are processed relatively more efficiently in the right hemisphere than the left hemisphere, whereas high SFs show the opposite pattern. In this study, we ask whether these differences between the two hemispheres reflect a low-level division that is based on absolute SF values or a flexible comparison of the SFs in the visual environment at any given time. In a recent study, we showed that conscious awareness of SF information (i.e., visual perceptual selection from multiple SFs simultaneously present in the environment) differs between the two hemispheres. Building upon that result, here we employed binocular rivalry to test whether this hemispheric asymmetry is due to absolute or relative SF processing. In each trial, participants viewed a pair of rivalrous orthogonal gratings of different SFs, presented either to the left or right of central fixation, and continuously reported which grating they perceived. We found that the hemispheric asymmetry in perception is significantly influenced by relative processing of the SFs of the simultaneously presented stimuli. For example, when a medium SF grating and a higher SF grating were presented as a rivalry pair, subjects were more likely to report that they initially perceived the medium SF grating when the rivalry pair was presented in the left visual hemifield (right hemisphere), compared to the right hemifield. However, this same medium SF grating, when it was paired in rivalry with a lower SF grating, was more likely to be perceptually selected when it was in the right visual hemifield (left hemisphere). Thus, the visual system’s classification of a given SF as “low” or “high” (and therefore, which hemisphere preferentially processes that SF) depends on the other SFs that are present, demonstrating that relative SF processing contributes to hemispheric differences in visual perceptual selection.

Spatial frequency selection and integration of global and local information in visual processing: A selective review and tribute to Shlomo Bentin

Previous research has suggested a relationship between processing lower versus higher spatial frequencies (SFs) and global/local perception, respectively. Here we honor Shlomo Bentin by reviewing the work we conducted with him regarding this issue. This work was aimed at investigating the mechanisms by which selective attention to spatial frequency (SF) mediates global and local perception in general and how these perceptual levels are integrated with the shapes that define them. The experiments demonstrate that attention to global and local aspects of a hierarchical display biases the flexible selection of relatively lower and relatively higher SFs during image processing. Additionally, attentional selection of SF allows for the shapes in a hierarchical display to be integrated with the level (global/local) at which they occur. The studies reviewed here provide strong evidence that the flexible, top-down selection of low-level SF channels mediates the perception of global and local elements of visual displays. The studies also support a hemisphere asymmetry in this process, with right hemisphere functions biased toward global perception and left hemisphere functions biased toward local.

The effects of a transition between local and global processing on vigilance performance

Sixty participants performed a sustained attention task in which they were required to perform either global or local feature discrimination. Two groups required just one type of discrimination, while the remaining two groups started on one type of discrimination before transitioning to the other type halfway through. A transition resulted in worse performance when compared to no transition. It was also found that the local discrimination group showed improved performance over time compared to the global discrimination group. Functional near-infrared spectroscopy (fNIRS) was used to measure blood oxygenation during the task and was used as an index of cerebral hemodynamic activity. Total oxygenation was found to increase more in global discrimination tasks. It was also found that the left prefrontal cortex showed little change in nontransition tasks while in transition tasks it followed the same trend as the right prefrontal cortex. Combined with performance data, it suggests that an increased utilization of bilateral resources may in some cases improve performance over time.

Asynchronous presentation of global and local information reveals effects of attention on brain electrical activity specific to each level

The neural basis of selective attention within hierarchically organized Navon figures has been extensively studied with event related potentials (ERPs), by contrasting responses obtained when attending the global and the local echelons. The findings are inherently ambiguous because both levels are always presented together. Thus, only a mixture of the brain responses to two levels can be observed. Here, we use a method that allows unveiling of global and local letters at distinct times, enabling estimation of separate ERPs related to each level. Two interspersed oddball streams were presented, each using letters from one level and comprised of frequent distracters and rare targets. Previous work and our Experiment 1 show that it is difficult to divide attention between two such streams of stimuli. ERP recording in Experiment 2 evinced an early selection negativity (SN, with latencies to the 50% area of about 266 ms for global distracters and 276 ms for local distracters) that was larger for attended relative to unattended distracters. The SN was larger over right posterior occipito-temporal derivations for global stimuli and over left posterior occipito-temporal derivations for local stimuli (although the latter was less strongly lateralized). A discrimination negativity (DN, accompanied by a P3b) was larger for attended targets relative to attended distracters, with latencies to the 50% area of about 316 ms for global stimuli and 301 ms for local stimuli, which presented a similar distribution for both levels over left temporo-parietal electrodes. The two negativities apparently index successive stages in the processing of a selected level within a compound figure. By resolving the ambiguity of traditional designs, our method allowed us to observe the effects of attention for each hierarchical level on its own.

Attentional selection of levels within hierarchically organized figures is mediated by object-files

Objects frequently have a hierarchical organization (tree-branch-leaf). How do we select the level to be attended? This has been explored with compound letters: a global letter built from local letters. One explanation, backed by much empirical support, is that attentional competition is biased toward certain spatial frequency (SF) bands across all locations and objects (a SF filter). This view assumes that the global and local letters are carried respectively by low and high SF bands, and that the bias can persist over time. Here we advocate a complementary view in which perception of hierarchical level is determined by how we represent letters in object-files. Although many properties bound to an object-file (i.e., position, color, even shape) can mutate without affecting its persistence over time, we posit that same object-file cannot be used to store information from different hierarchical levels. Thus, selection of level would be independent from locations but not from the way objects are represented at each moment. These views were contrasted via an attentional blink paradigm that presented letters within compound figures, but only one level at a time. Attending to two letters in rapid succession was easier if they were at the same-compared to different-levels, as predicted by both accounts. However, only the object-file account was able to explain why it was easier to report two targets on the same moving object compared to the same targets on distinct objects. The interference of different masks on target recognition was also easier to predict by the object-file account than by an SF filter. The methods introduced here allowed us to investigate attention to hierarchical levels and to object-files within the same empirical framework. The data suggests that SF information is used to structure the internal organization of object representations, a process understood best by integrating object-file theory with previous models of hierarchical perception.

Attending to global versus local stimulus features modulates neural processing of low versus high spatial frequencies: an analysis with event-related brain potentials

Spatial frequency (SF) selection has long been recognized to play a role in global and local processing, though the nature of the relationship between SF processing and global/local perception is debated. Previous studies have shown that attention to relatively lower SFs facilitates global perception, and that attention to relatively higher SFs facilitates local perception. Here we recorded event-related brain potentials (ERPs) to investigate whether processing of low versus high SFs is modulated automatically during global and local perception, and to examine the time course of any such effects. Participants compared bilaterally presented hierarchical letter stimuli and attended to either the global or local levels. Irrelevant SF grating probes flashed at the center of the display 200 ms after the onset of the hierarchical letter stimuli could either be low or high in SF. It was found that ERPs elicited by the SF grating probes differed as a function of attended level (global versus local). ERPs elicited by low SF grating probes were more positive in the interval 196–236 ms during global than local attention, and this difference was greater over the right occipital scalp. In contrast, ERPs elicited by the high SF gratings were more positive in the interval 250–290 ms during local than global attention, and this difference was bilaterally distributed over the occipital scalp. These results indicate that directing attention to global versus local levels of a hierarchical display facilitates automatic perceptual processing of low versus high SFs, respectively, and this facilitation is not limited to the locations occupied by the hierarchical display. The relatively long latency of these attention-related ERP modulations suggests that initial (early) SF processing is not affected by attention to hierarchical level, lending support to theories positing a higher level mechanism to underlie the relationship between SF processing and global versus local perception.

Persistent hemispheric differences in the perceptual selection of spatial frequencies

Previous research has shown that the right hemisphere processes low spatial frequencies more efficiently than the left hemisphere, which preferentially processes high spatial frequencies. These studies have typically measured RTs to single, briefly flashed gratings and/or have directed observers to attend to a particular spatial frequency immediately before making a judgment about a subsequently presented stimulus. Thus, it is unclear whether the hemispheres differ in perceptual selection from multiple spatial frequencies that are simultaneously present in the environment, without bias from selective attention. Moreover, the time course of hemispheric asymmetry in spatial frequency processing is unknown. We addressed both of these questions with binocular rivalry, a measure of perceptual selection from competing alternatives over time. Participants viewed a pair of rivalrous orthogonal gratings with different spatial frequencies, presented either to the left or right of central fixation, and continuously reported which grating they perceived. At the beginning of a trial, the low spatial frequency grating was perceptually selected more often when presented in the left hemifield (right hemisphere) than in the right hemifield (left hemisphere), whereas the high spatial frequency grating showed the opposite pattern of results. This hemispheric asymmetry in perceptual selection persisted for the entire 30-sec stimulus presentation, continuing long after stimulus onset. These results indicate stable differences in the resolution of ambiguity across spatial locations and demonstrate the importance of considering sustained differences in perceptual selection across space when characterizing conscious representations of complex scenes.

Neural substrates of perceptual integration during bistable object perception

The way we perceive an object depends both on feedforward, bottom-up processing of its physical stimulus properties and on top-down factors such as attention, context, expectation, and task relevance. Here we compared neural activity elicited by varying perceptions of the same physical image--a bistable moving image in which perception spontaneously alternates between dissociated fragments and a single, unified object. A time-frequency analysis of EEG changes associated with the perceptual switch from object to fragment and vice versa revealed a greater decrease in alpha (8-12 Hz) accompanying the switch to object percept than to fragment percept. Recordings of event-related potentials elicited by irrelevant probes superimposed on the moving image revealed an enhanced positivity between 184 and 212 ms when the probes were contained within the boundaries of the perceived unitary object. The topography of the positivity (P2) in this latency range elicited by probes during object perception was distinct from the topography elicited by probes during fragment perception, suggesting that the neural processing of probes differed as a function of perceptual state. Two source localization algorithms estimated the neural generator of this object-related difference to lie in the lateral occipital cortex, a region long associated with object perception. These data suggest that perceived objects attract attention, incorporate visual elements occurring within their boundaries into unified object representations, and enhance the visual processing of elements occurring within their boundaries. Importantly, the perceived object in this case emerged as a function of the fluctuating perceptual state of the viewer.

Hemispheric Asymmetry in Perception: A Differential Encoding Account

Hemispheric asymmetry in the processing of local and global features has been argued to originate from differences in frequency filtering in the two hemispheres, with little neurophysiological support. Here we test the hypothesis that this asymmetry takes place at an encoding stage beyond the sensory level, due to asymmetries in anatomical connections within each hemisphere. We use two simple encoding networks with differential connection structures as models of differential encoding in the two hemispheres based on a hypothesized generalization of neuroanatomical evidence from the auditory modality to the visual modality: The connection structure between columns is more distal in the language areas of the left hemisphere and more local in the homotopic regions in the right hemisphere. We show that both processing differences and differential frequency filtering can arise naturally in this neurocomputational model with neuroanatomically inspired differences in connection structures within the two model hemispheres, suggesting that hemispheric asymmetry in the processing of local and global features may be due to hemispheric asymmetry in connection structure rather than in frequency tuning.

Top-down control in contour grouping

Human observers tend to group oriented line segments into full contours if they follow the Gestalt rule of 'good continuation'. It is commonly assumed that contour grouping emerges automatically in early visual cortex. In contrast, recent work in animal models suggests that contour grouping requires learning and thus involves top-down control from higher brain structures. Here we explore mechanisms of top-down control in perceptual grouping by investigating synchronicity within EEG oscillations. Human participants saw two micro-Gabor arrays in a random order, with the task to indicate whether the first (S1) or the second stimulus (S2) contained a contour of collinearly aligned elements. Contour compared to non-contour S1 produced a larger posterior post-stimulus beta power (15–21 Hz). Contour S2 was associated with a pre-stimulus decrease in posterior alpha power (11–12 Hz) and in fronto-posterior theta (4–5 Hz) phase couplings, but not with a post-stimulus increase in beta power. The results indicate that subjects used prior knowledge from S1 processing for S2 contour grouping. Expanding previous work on theta oscillations, we propose that long-range theta synchrony shapes neural responses to perceptual groupings regulating lateral inhibition in early visual cortex.

Functional hemispheric asymmetries of global/local processing mirrored by the steady-state visual evoked potential

While hemispheric differences in global/local processing have been reported by various studies, it is still under dispute at which processing stage they occur. Primarily, it was assumed that these asymmetries originate from an early perceptual stage. Instead, the content-level binding theory (Hübner & Volberg, 2005) suggests that the hemispheres differ at a later stage at which the stimulus information is bound to its respective level. The present study tested this assumption by means of steady-state evoked potentials (SSVEPs). In particular, we presented hierarchical letters flickering at 12Hz while participants categorised the letters at a pre- cued level (global or local). The information at the two levels could be congruent or incongruent with respect to the required response. Since content-binding is only necessary if there is a response conflict, asymmetric hemispheric processing should be observed only for incongruent stimuli. Indeed, our results show that the cue and congruent stimuli elicited equal SSVEP global/local effects in both hemispheres. In contrast, incongruent stimuli elicited lower SSVEP amplitudes for a local than for a global target level at left posterior electrodes, whereas a reversed pattern was seen at right hemispheric electrodes. These findings provide further evidence for a level-specific hemispheric advantage with respect to content-level binding. Moreover, the fact that the SSVEP is sensitive to these processes offers the possibility to separately track global and local processing by presenting both level contents with different frequencies.