EEG & Electrophysiology

ERPs dissociate Semantic and Syntactic Processing in Scenes

Melissa Vo and Jeremy Wolfe

In sentence processing, semantic and syntactic violations elicit differential brain responses in ERP recordings: An N400 has been associated with issues of semantic content, while a P600 marks syntactic structure. Does the brain register similar distinctions in scene perception? To test this, participants viewed “semantic inconsistencies” created by including objects that are incongruent with a scene’s meaning like a fire-hydrant in the kitchen. They also viewed “syntactic inconsistencies” in which an object violated structural rules, like a fork misplaced on the kitchen chair. Compared to consistent controls, semantic inconsistencies produced negative deflections similar to the “N390/N400 scene congruity effect” (Ganis & Kutas, 2003; Mudrik, Lamy, & Deouell, 2010). In addition, we observed a late positivity for our syntactic inconsistencies, which might resemble the P600 found for syntax manipulations in sentence processing. Interestingly, extreme syntax violations such as a floating toast, showed an initial increase in attentional deployment, but failed to produce the late positivity we observed for more modestly mislocated objects. We therefore conclude that 1) different neural populations are active during the semantic and syntactic processing of objects in scenes and 2) impossible placements of objects may be categorically different from the syntactically inconsistent placements that produce a P600.

Fine-grained face discrimination and categorical face perception as evidenced by steady-state visual evoked potentials

Adélaïde de Heering, Joan Liu-Shuang, Anthony M. Norcia and Bruno Rossion

Steady-state visual evoked potentials (SSVEP) can be used to rapidly and objectively measure individual face discrimination (Rossion & Boremanse, 2011, JOV). Here we extended this observation to fine-grained discrimination of morphed faces using an “event-related” SSVEP design. We recorded 128-channel EEG in 10 adult observers presented with 40-s sequences of faces flickering at 6Hz (sinusoidal contrast modulation). Each sequence was made of 4 repetitions of an identical face (A) followed by a morphed face (B) whose difference from the original face (A) within each sequence ranged from 0% (same identity) to 100% (different identity), in 10% steps. In line with the introduction of morphed faces at fixed intervals (AAAAB = every 4 stimuli, or 6Hz/5 = 1.2Hz), we found an enhancement of the signal-to-noise ratio (SNR) with increasing distance between the original and the morphed faces at exactly 1.2Hz and its harmonics (2F = 2.4Hz; 3F = 3.6Hz…). Categorical perception was revealed by a sharp SNR increase at these frequencies around the perceptual boundary (50%), mainly at right occipito-temporal channels. Event-related SSVEP designs are therefore particularly useful to examine fine-grained face discrimination and categorical face perception thresholds.

Association of pre-stimulus brain activity and perceptual organization on multiple temporal scales

Andrei Nikolaev, Sergei Gepshtein and Cees van Leeuwen

We studied the dynamics of visual perceptual organization while recording electrical brain activity (EEG) of human observers. Observers perceived grouping of multistable dot lattices into strips and reported their orientation: the probability that one of several possible orientations prevailed at a time, followed a quantitative, law of proximity. Within trials, the power of pre-stimulus alpha activity in EEG predicted observers’ orientation bias: their tendency to report vertical orientations more often than horizontal. The predictions were most reliable in the trials where the reported groupings were inconsistent with dot proximity. Between two successive trials, the probability that the same orientation was reported (“response duplets”) was higher than chance for horizontal orientations, and in this case pre-stimulus alpha power was higher than for vertical duplets. Over an entire experimental session, orientation bias steadily decreased while observers’ grouping sensitivity and pre-stimulus alpha power increased. Consistent with this observation, pre-stimulus alpha power predicted orientation in single trials and duplets only in the initial part of the session. These results indicate that the dynamics of perceptual organization is governed by lasting states of the visual system and perceptual learning.

Correlated variability in laminar cortical circuits

Bryan Hansen, Mircea Chelaru and Valentin Dragoi

The amount of information encoded by cortical circuits depends critically on the capacity of nearby neurons to exhibit correlations in responses. Despite the fact that strong trial-by-trial correlated variability in response strength has been reported in many cortical areas, recent evidence suggests that neuronal correlations are much lower than previously thought. However, the responses of cortical neurons are known to depend on their network environment, such as cortical layer. For example, in primary visual cortex (V1), the granular layer receives feedforward thalamic inputs. Subsequently, neuronal impulses from the granular layer are transmitted to neurons in supragranular layers and then infragranular layers; both layers constitute outputs of V1. We reasoned that one important distinction between cortical networks in the middle and superficial/deep layers is the spatial spread of intracortical connections. In the granular layers, where neurons receive geniculate input, the spatial spread of connections is small, whereas in supragranular and infragranular layers neurons receive recurrent input from larger distances (up to several mm) via horizontal and feedback circuitry. Therefore, we revisited the issue of correlated variability in V1 circuits by using laminar probes (16 contacts with a diameter of 25µm and an inter-contact distance of 100µm) to record single-units from two monkeys during a fixation task. Cortical layers were identified by measuring the evoked-response potentials (ERP) and computing the current-source density to locate a sink-driven inversion in the amplitude of the ERP. We examined the laminar dependence of correlated variability, and measured spike count correlations by using multi-contact laminar probes. We found that correlations between neurons depend strongly on local network context. For our population of 327 pairs, we found that correlated variability in the supragranular layers was 0.29 ± 0.03, similar to the values previously reported in V1. However, in the granular layer, correlation values were exceedingly low 0.03 ± 0.01. In infragranular layers, correlated variability was high again 0.24 ± 0.03. Cells in granular layer, which have only local projections to other layers within V1, encode incoming stimuli optimally by exhibiting extremely low correlated variability. In contrast, output layers sending projections to other cortical and subcortical areas encode information suboptimally by exhibiting large correlations. These results argue that correlations are generated within recurrent circuits, and that populations in different cortical layers play different roles in network computations.

MEG Beta band activity discriminates perceptual form and motion binding

Jean Lorenceau, Charles Aissani, Jacques Martinerie and Anne-Lise Paradis

Scalp recordings of electrophysiological oscillations in behaving humans reveals activity in different frequency bands thought to play functional roles in perceptual, motor and cognitive processes. We conducted a MEG study with a protocol well designed to probe the hypothetical roles of Gamma and Beta band activity in perceptual form/motion binding. Briefly, we employed an ambiguous visual moving stimulus perceived either as a whole shape moving along a Lissajou’s figure (bound percept) or as independent bars oscillating along horizontal and vertical trajectory (unbound percept). Participants were to report their bound, unbound or unclassified percept after each trial. Importantly, by randomizing the position of color coded responses, thus imposing a remapping of the motor response on each trial, our protocol avoids motor preparation that could contaminate the data. We report that bilateral parietal oscillations in the Beta range (15-25 Hz) discriminate perceptual states at the individual level. Moreover, significant trial-by-trial classification of Beta band activity suggests it provides a marker of perceptual states, uncontaminated by motor preparation. In contrast, activity in the Gamma band (40-80 Hz), although significantly higher during visual stimulation than during base line, does not distinguish perceptual states. Altogether our results suggest that Beta activity plays a functional role in perceptual integration of motion and shape within and across both hemispheres. In line with recent proposals, our findings support the view that Beta activity serves to facilitate the long-range communication between distinct brains regions specialized in motion and form processing.

Eccentricity-dependent variations of the N2p in target detection

Susann Schaffer and Cristina Meinecke

In ERP studies, the N2p, a negativity occurring around 200 to 300 ms after stimulus onset on posterior electrode sites, shows sensitivity to target eccentricity: foveal targets elicit larger N2p amplitudes than peripheral targets (N2p eccentricity effect). We were interested in (1) whether the N2p eccentricity effect is task-specific and compared the N2p in texture segmentation with visual search, and (2) we investigated whether the N2p eccentricity effect is only observable in target-present trials or also in target-absent trials. We also examined the role of (3) eccentricity-dependent variations in target detection difficulty (foveal vs. peripheral advantage) and (4) cortical magnification. Our results suggest that the N2p eccentricity effect is (1) not task-specific, (2) shows up only in target-present trials, and can neither be due (3) to variations in detection difficulty nor (4) to cortical magnification. We conclude that the N2p might be an indicator for the existence of a foveal vs. peripheral subsystem in target detection processes.