Visual Attention: fMRI Investigation of Object-based Attentional Control
Source: Laboratories of Jonas T. Kaplan and Sarah I. Gimbel— University of Southern California
The human visual system is incredibly sophisticated and…
1. Participant recruitment
- Recruit 20 participants.
- Participants should be right-handed and have no history of neurological or psychological disorders.
- Participants should have normal or corrected-to-normal vision to ensure that they will be able to see the visual cues properly.
- Participants should not have metal in their body. This is an important safety requirement due to the high magnetic field involved in fMRI.
- Participants should not suffer from claustrophobia, since the fMRI requires lying in the small space of the scanner bore.
2. Pre-scan procedures
- Fill out pre-scan paperwork.
- When participants come in for their fMRI scan, instruct them to first fill out a metal screen form to make sure they have no counter-indications for MRI, an incidental-findings form giving consent for their scan to be looked at by a radiologist, and a consent form detailing the risks and benefits of the study.
- Prepare participants to go in the scanner by removing all metal from their body, including belts, wallets, phones, hair clips, coins, and all jewelry.
3. Provide instructions for the participant.
- Tell the participant that in the scanner, they will see images of faces and houses.
- For the initial localizer runs, instruct participants that they will passively view faces and houses.
- Explain to the participants that in the task runs, they will see a face and a house superimposed on each other. Their task will be to pay attention to either the house or the face, as follows.
- When the task first begins, they will be told via text instructions whether to pay attention to houses or faces.
- Participants will have four runs where they are instructed to pay attention to houses, and four runs where they are instructed to pay attention to faces.
- The schedule of face and house runs will be random for each subject.
- Stress to the participant the importance of keeping their head still throughout the scan.
4. Put the participant in the scanner.
- Give the participant ear plugs to protect their ears from the noise of the scanner and ear phones to wear so they can hear the experimenter during the scan, and have them lie down on the bed with their head in the coil.
- Give the participant the emergency squeeze ball and instruct them to squeeze it in case of emergency during the scan.
- Use foam pads to secure the participants head in the coil to avoid excess movement during the scan, and remind the participant that it is very important to stay as still as possible during the scan, as even the smallest movements blur the images.
5. Data collection
- Collect a high-resolution anatomical scan.
- Begin functional scanning.
- Synchronize the start of stimulus presentation with the start of the scanner.
- Present pictures via a laptop connected to a projector. The participant has a mirror above their eyes, reflecting a screen at the back of the scanner bore.
- Present each participant with two localizer runs, in which they passively see blocks of faces and houses. These localizer runs are used to identify the FFA and PPA in each individual participant.
- Present each set of faces and houses in a 30-s block, with 20 s of fixation between blocks. Show each stimulus for 750 ms, followed by a 250-ms inter-stimulus interval, in which only a fixation cross is present on the screen. Repeat blocks in alternating order to obtain five blocks of faces and five blocks of houses.
- Present each participant with eight functional runs of the attention task.
- Begin each run with a superimposed face and house (Figure 1) in the center of the screen, and an instruction to begin by attending to either the face or the house. The initial attention target will be random from run to run.
- Each run will contain 300 pairs of superimposed faces and houses. Pairs of faces and houses are repeated between runs, but not within runs.
- Show each superimposed stimulus pair for one second. Every second, replace the superimposed house and face with a new superimposed house and face.
Figure 1. Face stimulus and house stimulus superimposed together. Each stimulus presented was a superimposed face and house. The participant was instructed to focus on either the face or the house.
6. Post-scan procedures
- Bring the participant out of the scanner.
- Debrief the participant.
7. Data analysis
- Preprocess the data.
- Perform motion correction to reduce motion artifacts.
- Perform temporal filtering to remove signal drifts.
- Smooth the data to increase signal-to-noise ratio.
- Model the data for each participant.
- Create a model of what the expected hemodynamic response should be for each task condition (faces and houses) in the localizer scan.
- Fit the data to this model, resulting in a statistical map, where the value at each voxel represents the extent to which that voxel was involved in the task condition.
- Identify the clusters for each subject that correspond to FFA and PPA based on their anatomical locations. The FFA will include all contiguous voxels in the mid-fusiform gyrus (which responds significantly more to faces than to houses) and the PPA included all voxels in the parahippocampal gyrus that responded more significantly to houses than to faces. To establish these regions of interest, a minimum significance threshold of p < 10-6 was used for each voxel.
- Use each individual's FFA and PPA masks as an ROI to extract signal activations during the four focus-on-faces runs and the four focus-on-houses runs.
- Quantify percent signal change for face-focus and house-focus conditions in FFA and PPA for each subject.
- Create a model of what the expected hemodynamic response should be for each task condition (faces and houses) in the localizer scan.
- Perform a two-way analysis of variance (ANOVA) on percent signal change values to test for differences among the conditions. The factors in this test are ROI (PPA vs. FFA) and attention (faces vs. houses).
Visual attentional control refers to our deliberate state of choosing what to pay attention to.
If, for instance, the goal of an observer is to pick out all of the onions in his soup, then he may not notice the fly that is swirling about.
Even though both were spatially coincident, the item of focus?the onions?stood out because of the individual?s objective. This is an example of object-based attentional control.
Interestingly, the brain?and the visual cortex, in particular?can process the objects separately. But it?s the attended object that gains stronger activation in its associated specialized processing area.
Using functional magnetic resonance imaging, fMRI, and methods originally developed by Nancy Kanwisher and colleagues, this video demonstrates how to locate dedicated brain regions that process particular objects.
We will also investigate how attentional control modulates neural activity in the same regions using voxel-based analysis, and even discuss how mindfulness training can enhance the ability to control attention over time.
In this experiment, participants lie in an fMRI scanner and are shown images of faces and houses in two different phases: passive viewing and superimposed.
During the first phase, they are asked to simply observe images one at a time in a block design, that is, a number of faces are presented followed by a sequence of houses. This type of viewing serves to localize activity within specific regions of interest.
For example, the fusiform face area, the FFA, has been shown to be more active when individuals view faces compared to other common objects, whereas the parahippocampal place area, PPA for short, responds more strongly to houses and places rather than to faces.
Given that these regions respond to specific types of stimuli, the patterns of voxel-based activity?or areas representing some level of activation?are expected to change, depending on the images shown.
Such expectations set-up the second phase, where superimposed images of a face and a house are shown. Over several trials, participants are asked to pay attention to only one of the items at a time, and therefore, must switch their focus between either the house or the face.
In this case, the dependent variable is the amount of activation recorded across image conditions, which can be converted to the magnitude of signal change to observe variation in activation from baseline to face-focused blocks and those centered on the house.
Although both images are presented in a superimposed manner, it is predicted that patterns of activity in the participant?s FFA and PPA will change, based on the specific item they attended to. Such results would highlight object-based attentional control.
After recruiting participants for this study, greet them in the laboratory and verify that they meet the safety requirements as they complete the necessary consent forms. Please refer to another fMRI project in this collection for more details on how to prepare individuals to enter the scanning room and imaging bore.
With the participant now in the scanner, explain the task instructions: They must first passively view a number of images on the screen. During the second phase, text instructions will prompt them to pay attention to either the house or face when they appear superimposed.
Following these directions, begin the scanning protocol by first collecting a high-resolution anatomical scan.
Then, initiate the functional portion with two localizer runs, where participants passively view images in 30-second blocks. For instance, in the first segment, display faces, each for 750 ms, and a fixation cross in between, during an inter-stimulus-interval, or ISI, of 250 ms.
At the end of every block, present the fixation cross for 20 s before alternating the series of images, which should now be houses. Note that this sequence repeats with different images five times, for a total of 10 blocks within one run.
Next, proceed with eight functional runs of the attentional control task. During this phase, instruct participants which object to attend to via text onscreen, and then cycle a superimposed face and house every second, with each run containing 300 overlaid images.
To conclude the study, bring the participant out of the scanner and debrief them.
To preprocess the data, perform motion correction to reduce movement artifacts, temporal filtering to remove signal drifts, and spatial smoothing to increase signal-to-noise ratio.
Subsequently, create a general linear model based on what the expected hemodynamic response should be for each task condition, either faces or houses, in the localizer scan.
Generate a statistical map by fitting the data to this model, where the value at each voxel represents the extent to which it was involved in the task condition.
Based on the regions of interest, identify clusters for each subject with a minimum statistical threshold for each voxel that responded either to faces or houses.
Specifically, focus on the FFA, in the mid-fusiform gyrus, which responds significantly more to faces than to houses, as well as the PPA, which includes all voxels in the parahippocampal gyrus that responds more significantly to houses than to faces.
Then, quantify and graph the percentage of signal change for face- and house-focused conditions in the FFA and PPA for each subject.
During the localizer phase, notice that the bilateral FFA was more active when subjects viewed faces compared to houses. Conversely, the PPA was more active when subjects observed houses compared to faces.
Now, from the functional runs, use the same measure?percent signal change?plotted against the brain regions.
When the face was attended to, increased activity was found in the FFA, but not the PPA. Conversely, when the house was focused on, increased activity occurred in the PPA but not the FFA. These findings indicate that neural activity is modulated, depending on what item is being attended to.
Now that you are familiar with how to use functional neuroimaging to study object-based attentional control, let?s look how researchers study other types of attentional processing.
In addition to focusing on static visual images, researchers are also interested in how brain activity is modulated when individuals attend to moving objects?especially relevant for operating a motorized vehicle and avoiding accidents.
For example, if the driver is told to look out for movement?like a dog crossing the street?the motion itself will capture their attention; however, they may not remember other identifying details about the canine. After all, it?s more important to avoid tragedy than to remember fur color.
Another practice, mindfulness, incorporates key elements of attentional switching, by encouraging astute focus away from more stressful thoughts. While engaging in instructor-led meditation, individuals have been shown to enhance their ability to control attention, especially away from adverse views.
However, for individuals with anxiety disorders, including post-traumatic stress, attentional control is more difficult. That is, they are biased towards emotionally negative stimuli, like tragic events in the news, rather than neutral stories.
Such poor attentional control makes them more vulnerable to the effects of threatening images?perpetuating situations that they can?t seem to get off their mind.
You?ve just watched JoVE?s video on how attention modulates neural activity. Now you should have a good understanding of how to design and conduct an attentional control experiment using functional neuroimaging, and finally how to analyze and interpret specific patterns of brain activity related to object-based attention.
Thanks for watching!
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Q1: What is object-based attentional control and how does it work?
Object-based attentional control refers to deliberately choosing what to pay attention to based on your goals. For example, if you focus on picking onions from soup, you may not notice a fly nearby, even though both are in the same location. The brain processes objects separately, but the attended object gains stronger activation in its specialized processing area, allowing selective focus despite competing stimuli.
Q2: What are the fusiform face area and parahippocampal place area?
The fusiform face area (FFA) is a brain region in the mid-fusiform gyrus that responds significantly more to faces than other objects. The parahippocampal place area (PPA) is located in the parahippocampal gyrus and responds more strongly to houses and places than faces. These specialized regions demonstrate how visual cortex is organized to process specific types of stimuli.
Q3: How does fMRI help researchers study attentional control?
Functional magnetic resonance imaging (fMRI) allows researchers to measure neural activity patterns across brain regions during different tasks. By using voxel-based analysis to track signal changes, researchers can observe how brain activation varies when participants attend to different objects. This technique reveals how attentional control modulates neural activity in specialized processing areas like the FFA and PPA.
Q4: What happens in the brain when you attend to one object over another?
When you attend to a specific object, neural activity increases in the brain region specialized for processing that object type. In studies with superimposed faces and houses, attending to faces increased FFA activity but not PPA activity, while attending to houses increased PPA activity but not FFA activity. This demonstrates that attentional control selectively modulates neural responses based on what you choose to focus on.
Q5: How does mindfulness training affect attentional control?
Mindfulness training, which incorporates attentional switching through instructor-led meditation, enhances individuals' ability to control attention over time. This practice encourages focused awareness away from stressful thoughts. However, individuals with anxiety disorders or post-traumatic stress show biased attention toward emotionally negative stimuli, making attentional control more difficult and increasing vulnerability to threatening images.
Q6: What is the difference between passive viewing and attentional control phases in fMRI experiments?
During passive viewing, participants simply observe images in block design without directing attention, allowing researchers to localize which brain regions respond to specific stimuli like faces or houses. In the attentional control phase, superimposed images appear and participants must focus on one object at a time. This comparison reveals how top-down attention modulates activity in specialized brain regions beyond their baseline responses.
Q7: Why is attention important for visual perception?
The brain's capacity to process visual information is limited, making attention essential for selectively processing goal-relevant information while ignoring irrelevant details. Some attentional processes are automatic, while others involve voluntary, conscious control. This selective processing allows efficient navigation of complex visual environments, such as focusing on relevant hazards while driving rather than processing every visual detail.