It has been well established that shortly after the onset of visual stimulation, deflections are observed in electroencephalographic (eeg) recordings over the occipital cortex. Consistent with single-cell recordings in this region, the location of the stimulation in the visual scene affects where these deflections are observed, with stimulation of the left hemifield tending to associate with greater deflections in the right occipital cortex, and right hemifield stimulation tending to associate with greater deflections in the left occipital cortex. It has also been observed that visual attention affects the magnitude of these deflections such that an attended stimulus is associated with a larger deflection than an unattended stimulus, suggesting sensory-level modulation of neural activity by attention. It is thereby possible to discern the location of attention by comparing the magnitude of deflections consequent to visual stimulation at different locations. However, insofar as visual stimulation of each location must be achieved at different times, separated by sufficient time for the responses to be discernible, the resulting measure will yield low temporal resolution. An alternative approach recently explored capitalizes on the fact that as the time between successive visual stimuli is shortened, the resulting recording of deflections approaches a sinusiodal pattern with a period equal to that of visual stimulation. When multiple such "steady-state" visual stimuli are employed, each with its own unique frequency, their influence sums in the resulting eeg recording and frequency-domain decomposition permits quantifying the power of each frequency in this sum. In such paradigms, it has been observed that visual attention to a given steady-state stimulus increases power at the frequency of that stimulus, and the presentation of multiple such stimuli in a spatial distributed manner permits quantification of the location of attention by comparison of the power at frequencies associated with these stimuli. However, while more finely time-resolved than single-stimulus methods, existing these steady-state methods of tracking the location of attention are limited by the slowest presented stimulus. That is, if the slowest presented stimulus is 5Hz, it has a period of 200ms, and a time window of at least this long is required to compute the power associated with this stimulus. It is common for such frequency-domain analyses to measure power iteratively from such windows, shifting the window position in the data through time, in an attempt to provide a smooth trace of attention through time; however this approach simply serves as a low-pass filter on changes in the resulting measure of attention, with the filter's response function affected by the window size. Additionally, some criticism may be leveled at existing steady-state approaches for the imbalance of stimulation manifest by the use of steady-state stimuli of discernibly different frequencies (ex. 8Hz vs 20Hz), an imbalance that may affect the operation of attention itself.
To overcome the limitations of existing eeg-based measures of attention, this report presents a new approach that achieves higher temporal resolution and aesthetic balance. The key insight in developing this new method is the observation that while visual evoked potentials are lateralized, they are weakly so, and there remains a discernible deflection in recordings at the central point, Oz, consequent to lateralized stimulation. We propose, then, that when recordings are made at Oz and it's lateral-neighboring locations (O1 & O2) amidst repeated presentation of lateralized visual stimuli, the degree to which the recordings at Oz are similar to those made to it's left or right provide a measure of which side of the visual scene is most attended. In this approach, the visual stimuli need not be frequency-distinct and must only be phase-offset. Given data from a single participant and condition epoched to a time window of interest (ex. the time between presentation of a cue and target in a traditional Posner cuing paradigm), a measure of the location of attention can be computed for each timepoint in that window by collecting together the eeg data recorded at Oz, O1 and O2 from all trials, fitting a simple linear regression predicting the Oz data additively from the O1 data and O2 data, and computing the difference in the resulting regression coefficients ("weights") for O1 and O2; if the weight for O1 is larger, then it may be inferred that there was greater attention to the right hemifield, and if the weight for O2 is larger, then it may be inferred that there was greater attention to the left hemifield. Collecting the resulting weight-difference values from all time points permits a trace of attention's location in space with a time resolution limited merely to the sampling rate at which the eeg data is acquired. The average of such traces across participants provide a view of an overall timecourse of the location of attention from which the individual participant traces deviate, and comparison of such average traces provides a view of how different experimental manipulations affect this timecourse.
The remainder of this report details an experiment to employ this technique as applied to the study of the timecourse of exogenous spatial attention in the context of a traditional Posner cuing experiment. A wealth of research has validated that, amidst a task to detect or discriminate the identity of a visual target stimulus as quickly as possible, performance is affected by the appearance of non-target "cue" stimuli. Even when these cues are presented at random locations with respect to the subsequent target, and thereby provide no spatial information to motivate moving attention in response to their appearance, performance is enhanced for targets that appear at the same location as the cue when the interval between the cue and target is relatively short (100-200ms). When the interval between the cue and target is relatively long (800-1200ms), the opposite pattern is observed whereby performance is diminished fro targets that appear at the same location as the cue. The enhancement of performance for short cue-target intervals is often called "facilitation", while the diminution of performance for longer cue-target intervals is often called "inhibition of return" (IOR). Behavioural investigations of the timecourse of spatial attention in these paradigms is achieved by presentation of multiple cue-target intervals across separate trials. Thus on any given trial a single measure is obtained for a single point on the temporal domain, and temporal resolution is limited to the number of intervals chosen, a choice that will inevitably limited by a finite duration of the experiment and a desire to make multiple observations at each interval to reduce noise the the estimate of mean performance. When the method described above is deployed for using eeg to examine the timecourse of attention, a single long cue-target interval may be chosen whilst mapping the entire timecourse up to that interval.
The central task around which the experiment was designed was a go/no-go task where some identity of the target would have two possible attributes (ex. color: white/black, shape: diamond/square, etc), one of which is mapped to a "go" response in which case participants must make a response, while the other is mapped to a "no-go" response in which case participants refrain from responding. This task is useful as it provides task demands that are greater than simple detection tasks, yet do not involve the extra complication of effector selection associated with 2-alternative forced choice tasks. The use of go/no-go task in a neuroimaging context also provides the benefit of having a subset of trials on which no motor response is made, which can often contaminate examination of brain activity immediately consequent to the appearance of a target. Modifications to the traditional stimulus design of the Posner cuing experiment were necessary to accommodate simultaneous presentation of steady-state stimuli. Lateralized visual stimuli were placed at a relatively large distance from central fixation to minimize any cross-hemispheric influences (i.e. stimulation from the left hemifield yielding left-hemisphere activity, stimulation from the right hemifield yielding right-hemisphere activity). To then diminish participants' need/desire to move their eyes to the target (eye movements are typically discouraged in such experiments to isolate effects of covert from overt visual attention), a luminance discrimination (white circles vs black circles amidst a grey background) was chosen as the focus of the go/no-go task. As a secondary means of incentivizing participants to avoid eye movements, an eye tracker was used to provide realtime feedback when participants either moved their eyes or blinked (which also impair inference on covert visual attention).
Typical experiments examining exogenous spatial attention provide placemarkers at the locations where targets might appear and change the luminance of these markers to provide cue stimuli. In this experiment these placeholders will additionally be used to provide the steady-state stimuli such that the placeholders will consist of light/dark banded rings whose polarity (i.e which bands are light & which bands are dark) will alternate to present the steady-state stimulus. The cue will consist of a brief enhancement of the contrast of these rings, from light-grey-and-dark-grey to white-and-black. As is typical in such experiments, a "cueback" stimulus will be presented at fixation halfway between the time of cue presentation and target presentation, and to achieve this, fixation will be a smaller version of the banded rings, with no steady-state polarity changes and the cueback manifest as a brief contrast increase. Finally, while a fixed cue-target interval will be used, trials will vary in the duration of the pre-cue fixation period; this will not only achieve "jittering" as is typically employed in neuroimaging experiments to eliminate temporal dependencies outside the epoch of interest, but also means that the precise state of each steady-state stimulus will be jittered in the epoch of interest itself, providing across-trial variability on which the above describe metric of attention necessitates.
A total of 10 subjects (age range: 19–23; 6 female; 1 left-handed) were recruited from a local research experience subject pool and compensated for their time with course credit.
Visual stimuli were presented on a Sony OLED display running in its "flicker-free" mode during which it provides image updates at 60Hz while producing a pulse-width modulation of 120Hz. This means that any non-black stimulus is presented at 120Hz with a 50% duty cycle (4ms-on, 4ms-off) and a minimum of 2 cycles. As expected for an OLED display, high speed photometry using a labjack and photosensor conducted prior to initiating this experiment validates that this display provides <1ms response time for transitions to and from any point in the color space, ideal for precise visual stimulus presentation as required for steady-state visual stimulation (c.f. LCDs which have response times of up to 50ms and response time variability dependent on the color/greyscale values being transitioned from and to). EEG was recorded using a BrainVision ActiChamp amplifier recording at 1KHz using a 64 electrode array. Event codes were sent from the stimulus presentation system to the ActiChamp via a labjack. A photostimulus was also presented with critical stimuli for detection by a photometer plugged into the ActiChamp AUX port. Eye movements and blinks were monitored for real-time feedback using an Eyelink 1000 eye tracking system. Manual responses were recorded via the analog triggers on a wired xbox360 gamepad.
The task to be described in detail below involves a white/black color discrimination and consequent go/no-go response. To counterbalance the mapping of color to response requirement, participants were assigned to either "black-go" or "white-go", with this assignment determined by the order in which they were recruited to participate in the experiment, with even-numbered participants assigned to "black-go" and odd-numbered participants assigned to "white-go".
Following consent, participants were fitted with the EEG cap and impedance of all electrodes was brought to below 15 kOhm. The participant was then instructed with regards to task according to the following script:
[Instructions for "black-go" group] In this experiment, you will be playing a simple game. Since we are using this camera to watch your eyes, we have to check that the camera is working ok frequently. A dark-grey-and-light-grey disc with a hole at the center will appear on the screen [at this point the computer shows the fixation stimulus] at which point you can initiate the camera check by looking at the disc's hole and pressing both triggers on this gamepad with your pointer fingers. Sometimes you might see the disc blink momentarily, and that means you should try the check again. If the camera has lost track of your eyes, I'll pause the experiment and re-calibrate the camera. If the camera is ok, two large and flickering rings will appear to the left and right of center, both with dark-grey and light-gray bands [at this point the computer shows the peripheral rings]. When you see these appear, your next job is to try to keep your eyes focused at the center disc's hole, not moving your eyes or blinking. You'll only have to keep this focus for a couple seconds, but if you move your eyes or blink while the big rings are on the screen, the computer will beep at you and start over at the camera check again.
Now, while you're staring at the center disc, other things will happen on the screen. Eventually, either a white or black filled-circle will appear in one of the flickering rings. If the circle is white, you don't need to do anything, but if the circle is black, you need to press both triggers as quickly as you can. When the filled-circle appears, don't look at it, just keep your eyes on the center disc, but use your peripheral vision to notice the filled-circle's color. To help motivate you to respond quickly when the circle is black, after you respond a black number will appear at the center of the screen showing you how long it took you to respond. Smaller numbers are better and you want the numbers to be between 1-4. If you see numbers 5 or larger, that means you're responding too slowly and you need to try to respond faster. If you fail to respond within one second, a black X will appear at the center of the screen, telling you that you took too long. If you press the triggers when you're not supposed to, after a white circle appears, a white X will appear at the center of the screen. If you correctly refrain from responding when a white circle appears, a white zero will appear at the center of the screen.
Before the filled-circle appears, two other things will happen on the screen. First, one of the flickering rings will change contrast briefly, going from being dark-grey and light-grey to being fully black-and-white. About a half second after this, the disc at the center will also change contrast briefly and the circle will appear another half-second later. The location of the ring that changes contrast is completely random and doesn't predict where the subsequent circle is going to appear.
So the full sequence of events will be (1) the camera check, (2) flickering rings appear and stare at center disc, (3) one of the rings will change contrast briefly, (4) the center disc will change contrast briefly, (5) the filled-circle appears and you press both triggers if it's black, do nothing if it's white, and finally (6) you receive feedback on how quickly you responded. After all this, everything will repeat, so you'll be back to the camera check. You can take a moment at each camera check to blink if you need to before continuing the camera check and initiating the next sequence. You'll be given the opportunity for a longer break every few minutes and the first few minutes will be just for practice. Do you have any questions before we start?
After reading these instructions, the eye tracker was then calibrated using a 9-point calibration procedure and participants were given a final opportunity to ask questions before the practice session began. The trial sequence is shown in Figure 1. Throughout the trial, SSVEP stimuli changed polarity every 100ms, with the polarity switch time of each side offset by 50ms (i.e. the left-located SSVEP stimulus changes polarity 50ms after the right-located SSVEP stimulus, and vice versa). If at any point the eye-tracker detected either a blink or a >1° deviation of gaze from the central fixation, the trial was immediately terminated with an auditory alert and the word "Blink!" or "Moved!" (respective) presented at fixation for 500ms, after which a new trial would begin.
Manipulated variables included: cue location (left vs right), target location (left vs right), and target color (white vs black). Repeating the full combination of these variables four times yields 32 trials, which were presented in random order in each block, including one practice and 10 experimental blocks. The total time for the experiment including initial set up was 2 hours.