Transcranial Magnetic Stimulation applied to the primary motor cortex demonstrates involvement mental rotation of Shepard figures and hands

Nam Nguyen

1. Fundamental concepts

Mental rotation (MR) is an imagery task that requires the participant to rotate visual stimuli to an orientation that is distinct from which they are presented [1]. This ability to mentally form and rotate objects/images enables people to compare the area of two different polygons in geometry [3] or visualize their direction by mapping their environment [2]. Studies have since investigated the cognitive process underlying MR through transcranial magnetic stimulation (TMS), which disrupts continuous neural activity in a brain region to disclose when its activity occurs during a cognitive process [4]. TMS can be achieved by applying the figure-eight coil on a specific brain area (see figure 1), which generates multiple electrical pulses precisely targeting its activity.

MR was first investigated when a study asked participants to determine the congruence between pairs of three-dimensional, multi-armed cube figures, known as Shepard figures (See figure 2) [5]. There were two types of Shepard cubes’ pairs, in which the “same” pairs could be rotated into congruence with each other while the “different” pairs could not rotate into congruence with each other. The study found that the reaction time increased linearly with the angular difference of the Shepard cubes, whatever pairs. When asked about their process, they perceived the two-dimensional pictures as objects in three-dimensional space to rotate in every possible axis. The study demonstrated that MR is a mental process that requires people’s cognition.

2. Hypotheses and studies’ outcomes

The primary motor cortex (M1) was discovered to be activated during MR [4]. One study delivered a single-pulse TMS to the M1 to determine its Motor evoked potentials (MEPs), referring to its electrical activity during MR [4]. Participants completed the MR task by assessing whether two pictures of Shepard figures were identical or mirrored in two experiments. The experiment required them to engage in MR, besides spoken tasks, and inner speech tasks. Since both reading tasks utilize M1, the study compared three tasks to investigate whether M1 is indeed involved in MR. The results demonstrated that MR of Shepard cubes obtained higher MEPs in M1, compared to reading aloud or silently. Despite M1’s activation in each task, it was unclear why the M1’s involvement was more inherent in MR than reading aloud or silently.

Additionally, the M1 is also responsible for the MR of hand, yet, its involvement is still ambiguous [6]. By applying single-pulse TMS, another study found that TMS impaired the MR of hands more than that of feet, indicating that the M1 contributes more to the hands’ task [6]. However, the study required participants to deliver motor responses via feet, which compromised TMS’s impairment in M1 and thus obtained TMS effects at a later stage. In contrast, a separate research did not find any disruption from single-pulse TMS to the M1 when participants judged the laterality of hands’ stimuli [7]. Because participants answered verbally, TMS pulses impacted the activity of M1’s region that is responsible for initiating vocal movement instead of hands. This could suggest that M1’s involvement in MR of hand demands further clarification because of feet and verbal response’s interferences.

Perhaps M1’s role in MR is dependent on the strategic application in rotating visual stimuli [8]. A study speculated that in general, the MR involved either internal strategy, referring to the MR of an object with hands that trigger M1, or external strategy, indicating a self-rotating object driven by an external force that may not require M1 [8]. The internal strategy is applied to simple stimuli, such as hands or tools whereas abstract figures, like houses or Shepard cubes, involve external strategy. Nonetheless, their study found TMS pulses applied to M1 evoke stronger MEPs during MR for every category of objects except for hands. Subjectively, most participants reported using the external strategy for Shepard figures, tools, and 2-D figures, yet it was not widely applied to hands. Although the study pointed out that external strategy is not essential for M1 during MR, the M1’s causal role in MR of hands warrants further research.

M1 has a lower reaction time under the hand condition compared to Shepard cubes’ condition may be attributable to the coordination in motor brain areas adjacent to M1. Specifically, the premotor cortex (Area 6) and M1 were activated in preparatory hand movements, suggesting that M1 may receive anticipatory signals from area 6 to prepare for hand rotation [9]. Furthermore, the dorsal premotor cortex mediates the comprehension of the differences between stimuli and processes the concluding information for M1 to respond [10]. Since multiple brain regions including M1 facilitate MR of the hand, M1’s strategic role in hand rotation can be an avenue for MR.

It is reasonable to suppose that M1 did not entirely contribute to hands’ rotation, which may explain why the effect size in M1 activation in the hands condition is moderate compared to Shepard figures. In one research, MR of the hand stimulated M1, area 6, and the posterior motor cortex under fMRI, which construct images of the functioning brain by measuring neural activity through the level of oxygenated blood [1]. As their brains were scanned during MR, M1’s activation alongside other motor areas supported their imagination in how to move the hand, e.g in what sequence or direction. Further studies should compare the brain activity during tasks involving MR of hands and Shepard Cubes using the fMRI.

3. Conclusion

The MR of Shepard cubes was found to be slower than that of hands when TMS was applied to the M1. M1 contributed to MR of abstract figures but was less engaged in rotating hands. M1’s engagement in MR of hands suggests that brain regions exhibit relative, not absolute specialisation in specific tasks, which demand the cooperation of other brain areas to achieve the tasks. As studies revealed the function of M1, and how involved the area is with distinct stimuli, they expanded the knowledge regarding M1’s functionality in helping humans utilize imagery skills in everyday life. Mental rotation provides greater clarity into our mental process that enables us to practice our geometric skills, such as navigation, sketching and more.

Editor: Thoa Đinh

Translation: Thi Bui

Illustration: Quynh Theresa Huong Do

References

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