Explicit and implicit motor simulations are impaired in individuals with aphantasia

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Dupont, W., Papaxanthis, C., Madden-Lombardi, C., & Lebon, F. (2024). Explicit and implicit motor simulations are impaired in individuals with aphantasia. Brain Communications, 6(2). doi:10.1093/braincomms/fcae072

Abstract

Individuals with aphantasia report having difficulties or an inability to generate visual images of objects or events. So far, there is no evidence showing that this condition also impacts the motor system and the generation of motor simulations. We probed the neurophysiological marker of aphantasia during explicit and implicit forms of motor simulation, i.e. motor imagery and action observation, respectively. We tested a group of individuals without any reported imagery deficits (phantasics) as well as a group of individuals self-reporting the inability to mentally simulate images or movements (aphantasics). We instructed the participants to explicitly imagine a maximal pinch movement in the visual and kinaesthetic modalities and to observe a video showing a pinch movement. By means of transcranial magnetic stimulation, we triggered motor-evoked potentials in the target right index finger. As expected, the amplitude of motor-evoked potentials, a marker of corticospinal excitability, increased for phantasics during kinaesthetic motor imagery and action observation relative to rest but not during visual motor imagery. Interestingly, the amplitude of motor-evoked potentials did not increase in any of the conditions for the group of aphantasics. This result provides neurophysiological evidence that individuals living with aphantasia have a real deficit in activating the motor system during motor simulations.

Authors

William Dupont2
Charalambos Papaxanthis2
Carol Madden-Lombardi2
Florent Lebon3

What This Study Is About

Researchers investigated whether aphantasia—the inability to visualize images in the mind—also affects the motor system, which controls body movement. They wanted to see if people with aphantasia show the same "brain-to-muscle" activity as others when they imagine moving or when they watch someone else perform a task.

How They Studied It

The study compared two groups: "phantasics" (people with typical mental imagery) and "aphantasics" (people who cannot visualize). Participants performed three tasks:

Kinaesthetic imagery: Imagining the feeling of doing a pinch movement.

Visual imagery: Imagining what a pinch movement looks like.

Action observation: Watching a video of someone else making a pinch movement.

The researchers used Transcranial Magnetic Stimulation (TMS)—a non-invasive way to stimulate the brain with magnets—to measure "corticospinal excitability." This is essentially a measure of how "primed" or ready the muscles are to move based on activity in the brain.

What They Found

In people with typical imagery, the motor system became more active and "excited" when they imagined the feeling of moving or when they watched someone else move. However, in people with aphantasia, this physical response was absent. Their motor systems did not show any increased activity during any of the tasks. Specifically, while phantasics showed a significant increase in muscle readiness during kinaesthetic imagery, aphantasics showed almost no change from their resting state.

What This Might Mean

This suggests that aphantasia is not just a "visual" difference, but a broader condition that affects how the brain simulates actions. It provides neurophysiological evidence that aphantasia involves a real difference in how the brain's motor system functions during mental tasks, rather than just a difference in how people describe their thoughts. However, the study was small, and the researchers noted that some aphantasics might still have a limited, though measurable, ability to simulate movement that this specific test didn't fully capture.

One Interesting Detail

The study found that even when aphantasics were just watching a video of a movement (an "implicit" task that doesn't require trying to imagine anything), their motor systems remained quiet, whereas typical visualizers' brains automatically "mirrored" the action they saw.

Explicit and implicit motor simulations are impaired in individuals with aphantasia

Abstract

Authors

Explicit and implicit motor simulations are impaired in individuals with aphantasia

Abstract

Authors

Explicit and implicit motor simulations are impaired in individuals with aphantasia

Abstract

Authors

Explicit and implicit motor simulations are impaired in individuals with aphantasia

Abstract

Authors

What This Study Is About

How They Studied It

What They Found

What This Might Mean

One Interesting Detail

What This Study Is About

How They Studied It

What They Found

What This Might Mean

One Interesting Detail

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