Norman Forschack

Object-based attention

Scenes are so rich in information that the visual system cannot process all inputs simultaneously. Visual attention is the process that allows us to select the information that is relevant to our current behavior from the multiple inputs available at any given moment. Previous chapters have focused on the role of spatial and feature-based attention in selection. Here we show that attention can operate not only on the basis of locations and visual features but also by selecting an object as a whole. One way to study object-based attention is to require attention to a stimulus that is spatially superimposed on another, a situation which precludes spatial selection. Several studies have shown that it is possible to attend selectively to one of two transparent surfaces, finding that it is easier to divide attention between attributes of the same surface (e.g., direction and speed of motion) than between identical attributes for different surfaces. Moreover, attending to one feature causes other features of the same object to be processed efficiently. Electrophysiological and functional imaging findings show that the effects of surface-based attention are present from the early stages of visual processing. The units of attentional competition

Alpha-Band Brain Oscillations Shape the Processing of Perceptible as well as Imperceptible Somatosensory Stimuli during Selective Attention

Attention filters and weights sensory information according to behavioral demands. Stimulus-related neural responses are increased for the attended stimulus. Does alpha-band activity mediate this effect and is it restricted to conscious sensory events (suprathreshold), or does it also extend to unconscious stimuli (subthreshold)? To address these questions, we recorded EEG in healthy male and female volunteers undergoing subthreshold and suprathreshold somatosensory electrical stimulation to the left or right index finger. The task was to detect stimulation at the randomly alternated cued index finger. Under attention, amplitudes of somatosensory evoked potentials increased 50–60 ms after stimulation (P1) for both suprathreshold and subthreshold events. Prestimulus amplitude of peri-Rolandic alpha, that is mu, showed an inverse relationship to P1 amplitude during attention compared to when the finger was unattended. Interestingly, intermediate and high amplitudes of mu rhythm were associated with the highest P1 amplitudes during attention and smallest P1 during lack of attention, that is, these levels of alpha rhythm seemed to optimally support the behavioral goal (“detect” stimuli at the cued finger while ignoring the other finger). Our results show that attention enhances neural processing for both suprathreshold and subthreshold stimuli and they highlight a rather complex interaction between attention, Rolandic alpha activity, and their effects on stimulus processing. SIGNIFICANCE STATEMENT Attention is crucial in prioritizing processing of relevant perceptible (suprathreshold) stimuli: it filters and weights sensory input. The present study investigates the controversially discussed question whether this attention effect extends to imperceptible (subthreshold) stimuli as well. We found noninvasive EEG signatures for attentional modulation of neural events following perceptible and imperceptible somatosensory stimulation in human participants. Specifically, stimulus processing for both kinds of stimulation, subthreshold and suprathreshold, is enhanced by attention. Interestingly, Rolandic alpha rhythm strength and its influence on stimulus processing are strikingly altered by attention most likely to optimally achieve the behavioral goal.

Global enhancement but local suppression in feature-based attention

A key property of feature-based attention is global facilitation of the attended feature throughout the visual field. Previously, we presented superimposed red and blue randomly moving dot kinematograms (RDKs) flickering at a different frequency each to elicit frequency-specific steady-state visual evoked potentials (SSVEPs) that allowed us to analyze neural dynamics in early visual cortex when participants shifted attention to one of the two colors. Results showed amplification of the attended and suppression of the unattended color as measured by SSVEP amplitudes. Here, we tested whether the suppression of the unattended color also operates globally. To this end, we presented superimposed flickering red and blue RDKs in the center of a screen and a red and blue RDK in the left and right periphery, respectively, also flickering at different frequencies. Participants shifted attention to one color of the superimposed RDKs in the center to discriminate coherent motion events in the attended from the unattended color RDK, whereas the peripheral RDKs were task irrelevant. SSVEP amplitudes elicited by the centrally presented RDKs confirmed the previous findings of amplification and suppression. For peripherally located RDKs, we found the expected SSVEP amplitude increase, relative to precue baseline when color matched the one of the centrally attended RDK. We found no reduction in SSVEP amplitude relative to precue baseline, when the peripheral color matched the unattended one of the central RDK, indicating that, while facilitation in feature-based attention operates globally, suppression seems to be linked to the location of focused attention.

It takes two to tango: Suppression of task-irrelevant features requires (spatial) competition

Abstract In a recent electrophysiological study, we reported on global facilitation but local suppression of color stimuli in feature‐based attention in human early visual cortex. Subjects attended to one of two centrally located superimposed red/blue random dot kinematograms (RDKs). Task‐irrelevant single RDKs in the same colors were presented in the left and right periphery, respectively. Suppression of the to‐be‐ignored color was only present in the centrally located RDK but not in the one with the same color in the periphery. This result was at odds with the idea of active suppression of task‐irrelevant features across the entire visual field. In the present study, we introduced competition in the periphery by superimposing the RDKs at the task‐irrelevant location as well. With such competition, we found suppression of the task‐irrelevant color in the centrally and peripherally located RDKs. Results clearly demonstrate that suppression of task‐irrelevant features at task‐irrelevant locations requires (spatial) competitive interactions and is not an inherent neural mechanism in feature‐based attention as was found for global facilitation. HighlightsEffects of global suppression in feature‐based attention in early visual cortex.Attention leads to global facilitation of the attended feature.Suppression in periphery requires (spatial) competition of stimuli.Facilitation and suppression are localized differently.