Motor Maps
Source: Laboratories of Jonas T. Kaplan and Sarah I. Gimbel—University of Southern California
One principle of brain organization is the topographic m…
1. Recruit 20 participants.
- Confirm that the participants have no history of neurological or psychological disorders.
- Confirm that the participants are all right-handed using the handedness questionnaire.
- To ensure the participants can see the visual cues properly, make sure they have normal or corrected to normal vision.
- Make sure the participants have no metal in their body. This is an important safety requirement due to the high magnetic field involved in fMRI.
- Since the fMRI requires lying in the small space of the scanner bore, confirm that the participants don't suffer from claustrophobia.
2. Pre-scan procedures
- Fill out the pre-scan paperwork.
- When participants come in for their fMRI scan, have them 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.
- Have the participants remove all metal from their body (including belts, wallets, phones, hair clips, coins, and all jewelry) to prepare to go in the scanner.
3. Provide instructions for the participant.
- Tell the participant that when they see a hand on the screen, they are to begin moving their hand until the visual cue disappears. Inform the participant that the hand movement involves touching the thumb to each finger of the same hand in order and repeating this sequence in reverse. When the cue appears on the left side of the screen, they are to move their left hand, and when the cue appears on the right side of the screen, they are to move their right hand.
- Tell the participant that when they see a foot on the screen, they are to begin moving their foot and continue to do so until the visual cue disappears. Inform the participant that the foot movement involves repetitively pressing the foot down, as if pressing on an imaginary pedal. When the cue appears on the left side of the screen, they are to move their left foot, and when the cue appears on the right side of the screen, they are to move their right foot.
- Stress to the participant the importance of keeping their head still, even while they are moving their hand or foot.
4. Guide the participant into the scanner.
- Give the participant earplugs (to protect their ears from the noise of the scanner) and earphones (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.
- Secure the participant's head in the coil with foam pads 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 the stimulus presentation with the start of the scanner.
- Present the visual cues via a laptop connected to a projector. The participant should have a mirror above their eyes, reflecting a screen at the back of the scanner bore.
- Present each visual cue for 12 s, followed by 12 s of resting baseline. Alternate between the left hand, right hand, left foot, and right foot.
- Repeat four repetitions of each condition, for a total of 6.5 min.
6. Post-scan procedures
- Bring the participant out of the scanner.
- Debrief the participant.
- Pay 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.
- 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.
- Register the participant's brain to a standard atlas in order to combine data across participants.
- Combine statistical maps across subjects for a group level analysis of the data.
Motor information is organized according to anatomical divisions in the primary motor cortex, creating a topographical map in the brain.
Located in the precentral gyrus, cortical representations of the body are organized into a motor homunculus?"little man"?and are arranged in an inverted manner, such that the areas that control the toes are found in the medial wall and the tongue is located near the lateral sulcus.
Furthermore, body parts that require finer voluntary motor control, such as the hands and their associated digits, have larger representations in the cortex, compared to anatomical features that don?t require such precise manipulation?like the hip.
The homunculus is also lateralized, with neurons in the left primary motor cortex?shown here?controlling the right side of the body, and vice versa. Thus, when an individual moves their right hip, there is increased cortical activation on their left precentral gyrus within a discrete region.
This video details an experiment that uses modern functional neuroimaging to demonstrate the body-mapped organization of the human primary motor cortex, including how to collect and analyze brain activity when participants move their hands or feet.
In this experiment, brain activity is measured using functional magnetic resonance imaging, abbreviated as fMRI, while participants are repetitively cued to move different body parts?like the digits on their left or right hands.
This technique relies on changes in blood oxygenation levels, referred to as the BOLD?Blood-Oxygenation-Level-Dependent?response. For an in-depth look at the principles behind the method, please refer to another video in JOVE?s SciEd Essentials of Neuroscience Collection, fMRI: Functional Magnetic Resonance Imaging.
In the context presented here, when a body part, such as the left foot, is flexing back and forth, oxygenated cerebral blood flow?supplied by arteries in the brain?increases to neural regions that are active during this movement, like the primary motor cortex.
However, this hemodynamic response occurs more slowly than the actual physical motion, which warrants that actions be separated with periods of rest.
Thus, each body movement is precisely timed to distinguish the four conditions from one another: left hand, left foot, right hand, and right foot.
For example, participants in a fMRI machine are asked to start gesturing their left hand when one appears on the left side of a presentation screen.
The required hand movement is actually complex, and involves touching the thumb to each finger, in order, starting with the pointer. Then, the participant must repeat these actions in the opposite direction, starting with the pinky.
Movement is stopped when the cue?in this instance, the picture of the left hand?disappears from the screen.
Likewise, when they see a foot on the right, they are instructed to move their right foot by pushing it down repeatedly, until the image disappears.
Here, the dependent variable is the intensity of the BOLD response after a movement from the hand or foot, which can then be localized to specific brain regions.
For a left hand movement, brain activation is primarily expected on the right dorsolateral surface of the precentral gyrus. In contrast, for a right hand movement, brain activation is anticipated on the left dorsolateral surface. These results would align with the lateralized motor homunculus.
Prior to the experiment, recruit participants who are right-handed, have normal or corrected-to-normal vision, do not have any metallic implants in their body, or suffer from claustrophobia on account of experimental control and safety concerns.
Have them fill out pre-scan paperwork, which includes questions related to health and safety issues during the session, such as consent for a radiologist to look at their images in the case of incidental findings, as well as detailing the risks and benefits of the study.
Ask the participant to also remove all metal objects from their body?including watches, phones, wallets, keys, belts, and coins?to prepare for entering into the scanning room.
Next, explain the task rules: the appendage they need to move?in this case their foot?will appear as a visual cue on the corresponding side of the screen. Demonstrate how they should move their foot by repeatedly pressing it down, as if pushing on an imaginary pedal.
When a hand cue appears, they must touch the thumb to each finger of the same hand in order and then repeat this sequence in reverse.
Now, bring the participant into the imaging room. Provide earplugs to protect their ears from loud noises and earphones so that they can hear any additional communication during the session. Have them lie down on the bed with their head in the coil and secure it with foam pads to avoid excess movement and blur during the scan.
Above the participants? eyes, place a mirror that reflects a screen at the back of the scanner bore. Then, give them a squeeze ball to use in case of emergency. Also remind them that it is very important to stay as still as possible the entire time.
After guiding the participant inside the machine, first collect high-resolution, anatomical images. To begin the functional portion, synchronize the stimulus presentation with the start of the scanner.
Present the visual cues via a laptop connected to a projector, each for 12 s, followed by 12 s of resting baseline. Alternate between the four conditions: left hand, right hand, left foot, and right foot?repeating each four times within 6.5 min.
Once the scan is completed, direct the participant out of the room. Debrief them and provide compensation for their participation in the study.
As the first step of the analysis, preprocess the data by performing motion correction to reduce artifacts, temporal filtering to remove signal drifts, and spatial smoothing to increase the signal-to-noise ratio.
Using these data, create a model of the expected hemodynamic response for each task condition. Then, fit the data to this model, resulting in a statistical map for each subject, where the value at each voxel?a 3D pixel of volume?represents the extent to which that voxel was involved in the task condition.
Register the participant?s brain to a standard atlas in order to combine data across each participant. Then, combine all statistical maps across participants for a group level analysis. Note that changes in blood flow are represented by different colors on the surface of the brain.
Movements of the right hand, shown in blue, produced the greatest activation on the left lateral surface of the precentral gyrus, whereas engaging the left hand, represented in green, produced the greatest activation on the right lateral surface.
Additionally, flexing the right foot, indicated by light blue, produced activation on the left medial surface, while the greatest activation for left foot movements, in yellow, were on the right medial surface.
These results suggest that motor actions can be localized to discrete regions of the primary motor cortex in both hemispheres, supporting the motor homunculus.
Now that you are familiar with executing an fMRI experiment to observe the organization of the primary motor cortex, let?s consider how the brain manages movement after damage, or after the attachment of prosthetic limbs.
Damage to the left precentral gyrus, such as from a stroke, can lead to difficulty in moving the right side of the body.
As you?ve learned in this video, the specific parts that are impacted depend upon the extent of injury: impairments could be small and affect a single finger, or large enough to influence all of the digits and the entire arm.
While the representations seem straightforward, the primary motor cortex does not work alone, as it?s just a segment within a wider network of regions that are involved in the selection, planning, and coordination of movement. Thus, localizing damage may not be as easy as it seems.
One potential therapeutic approach for improving limb function in amputees involves brain-computer interfaces. This technically advanced method is based on electromyographic, or EMG signals?the electrical communication between motor neurons and muscle movements.
Researchers are developing ways to integrate EMG recordings with limb prostheses to more seamlessly control motor behaviors, like standing, or even walking up a ramp.
You?ve just watched JoVE?s introduction to motor maps. Now you should have a good understanding of how to design and conduct the fMRI experiment, and finally how to analyze and interpret the brain activation results.
Thanks for watching!
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Q1: What is the motor homunculus and how is it organized in the brain?
The motor homunculus, Latin for 'little man,' is a topographical map of the body represented in the primary motor cortex located in the precentral gyrus. Body parts are organized in an inverted manner, with toes on the medial wall and the tongue near the lateral sulcus. Body parts requiring finer control, like hands and digits, occupy larger cortical areas than those requiring less precision, such as the hip.
Q2: Why do hands and fingers have larger representations in the motor cortex than other body parts?
Hands and fingers require finer voluntary motor control and precise manipulation compared to body parts like the hip. The primary motor cortex allocates cortical space proportionally to the precision demands of each body part. This disproportionate representation reflects the brain's organization of motor control based on functional complexity and dexterity requirements.
Q3: How does brain lateralization affect motor control?
The motor homunculus is lateralized: neurons in the left primary motor cortex control the right side of the body, and vice versa. When an individual moves their right hip, increased cortical activation occurs in the left precentral gyrus within a discrete region. This contralateral organization is a fundamental principle of motor system anatomy.
Q4: What is the BOLD response and why is timing important in motor fMRI experiments?
The BOLD (Blood-Oxygenation-Level-Dependent) response measures changes in blood oxygenation levels in active brain regions. Because this hemodynamic response occurs more slowly than actual physical movement, researchers separate movement periods with rest intervals. Precise timing ensures that different movement conditions—left hand, right hand, left foot, right foot—can be distinguished from each other and baseline activity.
Q5: How are participants prepared and instructed during a motor mapping fMRI scan?
Participants receive pre-scan paperwork covering health, safety, and consent. They remove all metal objects and receive earplugs and earphones. Inside the scanner, their head is secured with foam pads in the coil. Visual cues on a screen instruct them when to move: touching thumb to fingers sequentially for hand movements or repeatedly pressing down for foot movements, with 12-second movement and 12-second rest periods.
Q6: What preprocessing and analysis steps reveal motor cortex organization from fMRI data?
Data preprocessing includes motion correction, temporal filtering, and spatial smoothing to reduce artifacts and improve signal quality. Researchers create hemodynamic response models for each task condition and fit data to these models, generating statistical maps. Brain registration to a standard atlas enables group-level analysis, where color-coded activation patterns show discrete motor regions corresponding to different body parts.
Q7: What do motor mapping results reveal about brain damage and therapeutic approaches?
Motor mapping demonstrates that specific body parts localize to discrete precentral gyrus regions, so stroke damage affects only the corresponding body part. However, the primary motor cortex works within a larger movement network, making damage localization complex. Brain-computer interfaces using electromyographic signals represent an emerging therapeutic approach for improving limb function in amputees and restoring motor control. Understanding motor excitability during action observation through techniques like TMS can further inform rehabilitation strategies.