Welcome to Simon Fraser University
You have reached this page because we have detected you have a browser that is not supported by our web site and its stylesheets. We are happy to bring you here a text version of the SFU site. It offers you all the site's links and info, but without the graphics.
You may be able to update your browser and take advantage of the full graphical website. This could be done FREE at one of the following links, depending on your computer and operating system.
Or you may simply continue with the text version.

*Windows:*
FireFox (Recommended) http://www.mozilla.com/en-US/firefox/
Opera http://www.opera.com/

*Macintosh OSX:*
FireFox (Recommended) http://www.mozilla.com/en-US/firefox/
Opera http://www.opera.com/

*Macintosh OS 8.5-9.22:*
The only currently supported browser that we know of is iCAB. This is a free browser to download and try, but there is a cost to purchase it.
http://www.icab.de/index.html
Close x
Searching... Please wait...
  • banner-fun3

Human Electrophysiology Lab

.

 

Electrical neuroimaging of attention control

Functional MRI has pinpointed brain regions involved in the control of spatial attention, but this method is unable to shed light on the precise timing of neuro-cognitive operations because of the sluggishness of the hemodynamic response. By contrast, EEG/ERPs reveal precisely the timing of brain activity but traditionally have failed to provide precise information about the locations of active neurons. My former student, Jessica Green, and I used newly developed methods for estimating the distributed brain activities that give rise to the recorded EEG/ERPs. In a pair of related studies, we used a new signal-processing technique called beamforming to track attention control activity in the brain over time.

Control of attention in visual space

In the first study of its kind, we imaged sources of low-frequency (theta-band; 5-7 Hz) EEG rhythms while participants shifted attention to different spatial locations in anticipation of a visual target (Green & McDonald, 2008, PLoS Biology). On different trials, participants viewed visually presented symbolic cues that provided reliable information about the location of the impending target (shift-cue trials) or not (neutral-cue trials). Our beamformer analysis revealed the following sequence of attention-shift activity: occipital lobe -> parietal lobe (IPL and SPL) -> frontal lobe -> parietal lobe (IPL only) -> occipital lobe. Remarkably, our beamformer brain activity measures accounted for nearly all of the variability in the attentional effect on perceptual performance (R = .97). The control regions highlighted by this beamformer analysis mapped onto the well-known control network that has been highlighted using functional neuroimaging (fMRI). Importantly, we found that parietal cortex contributed to attention control before frontal cortex, thereby disconfirming a popular view of ‘top-down’ attention control. 

 

Control of attention in auditory space

We reported a similar sequence of attentional control activity in an auditory attention task, except that parietal and frontal control activity was followed by preparatory activity in auditory cortex rather than in visual cortex (Green, Doesburg, Ward, & McDonald, 2011, J Neuroscience). 

 


 

Visual Search

In everyday life, we must often search cluttered and continually changing visual environments for objects of interest (targets). The search for a target is particularly challenging when other highly salient objects (distractors) are also present in our field of view. We can deal with the surplus of information and attempt to avoid distraction by focusing our attention on specific items or at specific locations in the visual field.

When an observer pays attention to one of many items in the visual field, the selection processes taking place in the brain give rise to an ERP component called the posterior contralateral N2, or N2pc. If you focus your attention covertly (that is, without moving your eyes) on an item on the left side of the visual field the ERP recorded over the posterior scalp becomes more negative on the right side of the head than on the left side of the head. If you shift your focus of attention over to an item on the right side of the visual field, the posterior ERP becomes more negative on the left side of the head than on the right side of the head. This electrophysiological marker, discovered by Steve Luck and Steve Hillyard at UC San Diego (Luck & Hillyard, 1994a, 1994b), enables research to track attention while people search for targets and try to ignore distractors.

Separable target and distractor subcomponents of the N2pc

 

 

 

The "capture" of attention by salient - but irrelevant - stimuli

 

We use ERPs to track people’s attention while they search for visual targets and try to ignore other, potentially distracting stimuli. We have found that, under some conditions, attention is momentarily diverted to salient-but-irrelevant stimuli before being deployed to the target stimulus (Hickey, McDonald, & Theeuwes, 2006). Specifically, we discovered that a salient distractor elicited an ERP component called the N2pc, which is known to reflect attentional selection (Luck & Hillyard, 1994). Although this distractor-elicited N2pc was interpreted initially as evidence for the salience-driven capture of attention, we later learned that this capture can be prevented (Jannati et al., 2013; McDonald et al., 2013). For example, when the features of the visual search stimuli remain fixed across trials, the distractor no longer elicits the N2pc but instead elicits an ERP component called the distractor positivity (PD), which is known to reflect attentional suppression. 

updated 04-Mar-13