[Show abstract][Hide abstract] ABSTRACT: The spatial extent of visual attention, which facilitates perceptual performances, has been classified into spatial and object-based attention. Object-based attention refers to a type of attention limited within an object that encloses a cued location, which contrasts with spatial attention that spreads around the attentional focus. Whether object-based attention is enabled by the deformation of an attentional “spotlight” or by the prioritization of locations to which the attentional spotlight is directed is a matter of much debate. The present study addresses this issue by employing an EEG (electro-encephalogram) technique called steady-state visual evoked potentials (SSVEP) during a psychophysical experiment. The SSVEP amplitude is modulated by visual attention. During the EEG recording, we asked participants to perform a rapid serial visual presentation task designed to exhibit object-based attention, and a simple detection task to measure the spatial spreading/deployment of attention. The visual stimuli flickered at different frequencies at different spatial locations, which enabled us to tag stimulus locations with temporal-frequency components in the EEG data. We found an effect of object-based attention on SSVEP amplitudes and behavioral performances. Additionally, the absence of event related potential changes at the cued location, triggered by random and frequent presentations of detection-task stimuli throughout a trial, suggests that object-based attention may be based on a steady state mechanism, i.e., spatial spreading, rather than a dynamic one, such as prioritizing shifts of attention to locations within the cued object.
[Show abstract][Hide abstract] ABSTRACT: How attentional modulation on brain activities determines behavioral performance has been one of the most important issues in cognitive neuroscience. This issue has been addressed by comparing the temporal relationship between attentional modulations on neural activities and behavior. Our previous study measured the time course of attention with amplitude and phase coherence of steady-state visual evoked potential (SSVEP) and found that the modulation latency of phase coherence rather than that of amplitude was consistent with the latency of behavioral performance. In this study, as a complementary report, we compared the time course of visual attention shift measured by event-related potentials (ERPs) with that by target detection task. We developed a novel technique to compare ERPs with behavioral results and analyzed the EEG data in our previous study. Two sets of flickering stimulus at different frequencies were presented in the left and right visual hemifields, and a target or distracter pattern was presented randomly at various moments after an attention-cue presentation. The observers were asked to detect targets on the attended stimulus after the cue. We found that two ERP components, P300 and N2pc, were elicited by the target presented at the attended location. Time-course analyses revealed that attentional modulation of the P300 and N2pc amplitudes increased gradually until reaching a maximum and lasted at least 1.5 s after the cue onset, which is similar to the temporal dynamics of behavioral performance. However, attentional modulation of these ERP components started later than that of behavioral performance. Rather, the time course of attentional modulation of behavioral performance was more closely associated with that of the concurrently recorded SSVEPs analyzed. These results suggest that neural activities reflected not by either the P300 or N2pc, but by the SSVEPs, are the source of attentional modulation of behavioral performance.
[Show abstract][Hide abstract] ABSTRACT: An attention shift has been assumed to comprise three stages of neural processes: disengagement from the initially attended location, shift to a new destination, and reengagement on the new location. We developed a novel experimental paradigm to estimate the timings of attentional “disengagement” and “reengagement”. We recorded steady-state visual evoked potentials (SSVEPs) for two flickering stimuli at different frequencies, which were presented on the left and right of the center of the display. Participants were, after attending to the either stimulus, instructed to stay their attention on the same location, or to shift attention toward the other stimulus. We estimated the time course of attentional disengagement and that of attentional reengagement from the difference between the SSVEPs under different attention shift conditions. We conducted two experiments using exogenous (Experiment 1) and endogenous cues (Experiment 2) for controlling visual attention and succeeded to measure the time courses of SSVEP modulations accompanied by disengagement and reengagement of attention. Interestingly, temporal orders of the attentional processes differed between these experiments. In Experiment 1, attention was reengaged to a new object earlier than it was disengaged from the initially attended object. In Experiment 2, on the other hand, attentional reengagement occurred no significantly earlier than disengagement. These results suggest that attention shift processes were not executed in a fixed order; rather, the timings seem to change depending on the types of attention involved in the shift.
[Show abstract][Hide abstract] ABSTRACT: Visual attention is a function that selectively picks up particular information to process from large amounts of visual information projected on the retina. We studied the spatial extent of visual attention by using a component of visual evoked potential called SSVEP (Steady-State Visual Evoked Potential). SSVEP is a sinusoidal modulation in the evoked potential induced by continuously flickering stimulus, which has the same temporal frequency as that of the stimulus. It has been reported that attending to one of the flickering stimuli modulates amplitude of the SSVEP of corresponding temporal frequency. We measured changes in the SSVEP amplitude at various distances from the focus of attention. We used eight stimuli arranged equidistantly along a circle (diameter = 10.0 deg), centered at the fixation point. The stimuli flickered at different temporal frequencies, and changes in the amplitude of the SSVEP were measured. Subjects conducted a detection task at one of the eight stimulus locations while directing attention to a direction designated by a cue. We found clear peaks of SSVEPs corresponding to the temporal frequency of each stimulus. We also found modulations of SSVEP amplitudes by attention. The SSVEP amplitude declined gradually with the distance from the attention focus. These SSVEP measurements succeed in measuring the spatial extent of visual attention across the visual field with an objective measure.
[Show abstract][Hide abstract] ABSTRACT: Endogenous attention modulates the amplitude and phase coherence of steady-state visual-evoked potentials (SSVEPs). In efforts to decipher the neural mechanisms of attentional modulation, we compared the time course of attentional modulation of SSVEP amplitude (thought to reflect the magnitude of neural population activity) and phase coherence (thought to reflect neural response synchronization). We presented two stimuli flickering at different frequencies in the left and right visual hemifields and asked observers to shift their attention to either stimulus. Our results demonstrated that attention increased SSVEP phase coherence earlier than it increased SSVEP amplitude, with a positive correlation between the attentional modulations of SSVEP phase coherence and amplitude. Furthermore, the behavioral dynamics of attention shifts were more closely associated with changes in phase coherence than with changes in amplitude. These results are consistent with the possibility that attention increases neural response synchronization, which in turn leads to increased neural population activity.
Full-text · Article · Feb 2012 · Journal of Cognitive Neuroscience
[Show abstract][Hide abstract] ABSTRACT: Background / Purpose:
Visual attention enhances neural responses to an attended stimulus by neural response gain control. It has been also pointed out that attentional enhancements of neural responses originate from neural response synchronization (e.g. Fries et al 2001; Kim et al 2007). In this study, we adopted a steady-state visual evoked potential (SSVEP) technique in an attempt to investigate the relationship between the neural response amplitude and the neural response synchronization.SSVEP is an oscillatory brain potential evoked by a continuously flickering visual stimulation and can be identified as EEG components corresponding to the flicker frequencies. It has been shown that the visual attention modulates the amplitude and phase of the SSVEP. We recorded SSVEPs for two stimuli flickered at different temporal frequencies which were presented on the left and right sides of the fixation. An arrow-shape cue was presented on the center of the display to control participants’ state of attention. Participants were instructed to shift their attention to a flickering stimulus in the side pointed by the cue. We analyzed the amplitude (thought to reflect the magnitude of neural population response) and phase coherence (thought to reflect neural response synchronization) of SSVEP.
Both the SSVEP amplitude and phase coherence were increased by visual attention and the correlation between the two measures was very high (r > 0.55). Time course analysis of the two SSVEP measures showed that attentional modulation for the phase coherence started earlier than that for the amplitude. These results indicate that visual attention boosts neural population response via neural response synchronization.