New Discoveries in Cortical Motor Maps

Background Introduction

Humans and other primates are capable of performing a wide range of complex body movements, the initiation and control of which rely on multiple cortical and subcortical structures. Among these, the primary motor cortex (M1), located in the precentral gyrus, is the core region for executing movements. A key feature of M1 is its somatotopic organization, meaning there is a systematic relationship between the location of neurons and the body parts they control. For example, from the top of the precentral gyrus to the ventral side, neurons sequentially control the feet, legs, hands, arms, upper torso, face, and head. Early evidence for this organization came from direct electrical stimulation experiments on patients undergoing awake craniotomy.

In recent years, studies have suggested that, in addition to M1, the lateral occipitotemporal cortex (LOTC) and the precuneus may also contain similar somatotopic motor maps. These findings are significant for understanding the organization of motor behavior, particularly in revealing the interaction between visual and motor-related activities. However, evidence for these newly proposed motor maps remains limited. Therefore, this study aims to re-evaluate the existence of somatotopic motor maps in the LOTC and precuneus by analyzing an open functional magnetic resonance imaging (fMRI) dataset.

Source of the Paper

This study was conducted by Deyan Ivaylov Mitev, Kami Koldewyn, and Paul E. Downing from the Department of Psychology at Bangor University, UK. The paper was first published on December 3, 2024, in the Journal of Neurophysiology, with the DOI 10.1152/jn.00160.2024.

Research Process

1. Dataset and Participants

This study analyzed an open fMRI dataset comprising 62 participants, each performing 12 different body part movements. All participants were right-handed, aged between 19 and 29 years. The research team conducted behavioral training before scanning to minimize head motion artifacts in the data.

2. fMRI Experimental Design

Participants were asked to perform simple body part movements, including toes, ankles, left leg, right leg, fingers, wrists, forearms, upper arms, jaw, lips, tongue, and eyes. All movements were bilateral, except for leg movements, which were divided into left and right legs. The experiment used a block design, with each movement block lasting 16 seconds, and each scanning run consisting of 6 movement blocks and rest blocks.

3. Data Analysis

The study employed a surface-based analysis method, mapping each participant’s fMRI data to a standardized cortical surface (fsaverage template). The research team designed a series of quantitative metrics to test whether the LOTC and precuneus exhibit somatotopic motor maps similar to M1. Specific methods included:

• Split-half reliability analysis: The data were divided into odd and even runs to calculate the correlation of responses to different body part movements in each small region.
• Spatial autocorrelation analysis: This assessed changes in the correlation of movement responses across cortical regions at different distances.
• Representational similarity analysis: This tested whether the response patterns in the LOTC and precuneus aligned with “top-to-bottom” or “out-in” somatotopic models.
• Background connectivity analysis: By analyzing the residual time series from the GLM, this evaluated the functional connectivity between M1 and the LOTC and precuneus.

Key Findings

1. Somatotopic Motor Maps in M1 and S1

The study first validated the somatotopic motor maps in M1 and the primary somatosensory cortex (S1). The results showed that the response patterns in M1 and S1 were consistent with the classical somatotopic organization, with the dorsal to ventral regions corresponding to movements of the legs, arms, fingers, and head, respectively. Split-half reliability analysis demonstrated high reliability in the responses of M1 and S1 to different body part movements. Spatial autocorrelation analysis also indicated that as distance increased, the correlation of response patterns decreased, even becoming negative.

2. Response Patterns in LOTC and Precuneus

In the LOTC, the study found weak movement responses and little evidence for a somatotopic motor map. Split-half reliability analysis showed that the LOTC had some reliability in responding to different body part movements, but spatial autocorrelation analysis revealed no significant distance-dependent changes in response patterns within the LOTC. Representational similarity analysis also found no alignment of LOTC response patterns with the “top-to-bottom” or “out-in” somatotopic models.

In the precuneus, the study found limited evidence for a somatotopic motor map. Split-half reliability analysis indicated some reliability in the precuneus’s responses to different body part movements, and spatial autocorrelation analysis showed a trend of decreasing correlation with increasing distance in some regions. Representational similarity analysis revealed that the response patterns in certain regions of the precuneus aligned with the “out-in” somatotopic model.

3. Background Connectivity Analysis

Background connectivity analysis showed weak connectivity between M1 and the LOTC, with no clear somatotopic pattern. However, connectivity between M1 and the precuneus exhibited some somatotopic features, particularly in the right hemisphere, where the inferior-posterior precuneus showed stronger connectivity with the head movement region of M1, and the superior-anterior precuneus showed stronger connectivity with the arm movement region of M1.

Conclusions and Significance

This study re-evaluated the existence of somatotopic motor maps in the LOTC and precuneus through quantitative analysis of fMRI data. The results showed little evidence for a somatotopic motor map in the LOTC, while the precuneus exhibited limited somatotopic features, particularly in functional connectivity analysis. These findings provide new insights into the organization of human motor representations, especially in revealing the complex relationship between visual and motor-related activities.

Research Highlights

1. Large-scale dataset: This study analyzed a large-scale fMRI dataset comprising 62 participants, providing results with high statistical power.
2. Quantitative analysis methods: The study designed a series of quantitative metrics to objectively assess the existence of somatotopic motor maps.
3. Surface-based analysis: The use of surface-based analysis improved sensitivity to continuous response patterns on the cortical surface.
4. Background connectivity analysis: This analysis revealed somatotopic features in the connectivity between M1 and the precuneus.