# Younger adult brain utilizes interhemispheric strategy via ipsilateral dorsal premotor cortex for fine control of dexterous finger movements, unlike the aging brain

## TL;DR

Younger adults use a specific brain strategy involving one side's dorsal premotor cortex to control precise finger movements, which changes with age.

## Contribution

The study reveals a novel interhemispheric strategy in younger adults for dexterous finger movements, absent in older adults.

## Key findings

- Younger adults activate ipsilateral PMd, S1, and Area 2 during dexterous tasks, linked to performance.
- Older adults show no performance-linked activation in ipsilateral sensorimotor regions during the same tasks.
- Interhemispheric coupling is stronger in younger adults, while older adults rely on contralateral connectivity.

## Abstract

This study investigated how healthy, right-handed younger adults utilize the typically suppressed ipsilateral sensorimotor cortices [particularly, the dorsal premotor cortex (PMd), primary motor cortex (M1), primary somatosensory cortex (S1), and superior parietal cortex of Area 2] to perform a dexterous finger motor task and its age-related changes.

Functional magnetic resonance imaging was used to measure brain activity in healthy, right-handed younger and older adults during a simple button-press task with the right index finger and a dexterous stick-rotation task involving fine control and coordination of individual right finger movements. The individual performance capacity in stick rotation (the personal trait of finger dexterity) was assessed outside the scanner. The sensorimotor cortices (PMd, M1, S1, and Area 2) in each hemisphere were defined as regions-of-interest (ROIs), and contrast analysis, interparticipant correlation analysis with performance capacity, and interhemispheric functional connectivity analysis were performed.

In the younger group, all ipsilateral sensorimotor cortices were deactivated during the button-press task, whereas during the stick-rotation task, the PMd, S1, and Area 2 were activated, and the ipsilateral M1 remained deactivated. The ipsilateral PMd, S1, and Area 2 activity was correlated with performance capacity. During the stick-rotation task, the anterior ipsilateral PMd consistently enhanced interhemispheric functional coupling with all contralateral sensorimotor cortices. In contrast, in the older group, ipsilateral sensorimotor deactivation was not observed during the button-press task, and all ipsilateral cortices were activated during the stick-rotation task; however, none of the activity was correlated with performance capacity. In addition, functional connectivity within the contralateral sensorimotor cortices (rather than interhemispheric connectivity) increased during the stick-rotation task.

Our findings indicate that ipsilateral sensorimotor activity during the current dexterous task reflects different physiological mechanisms between younger and older adults. When performing the task, younger adults recruited the ipsilateral PMd, S1, and Area 2 by disinhibiting their interhemispheric inhibition to complement for their clumsiness; the ipsilateral PMd appeared important for the interhemispheric interaction, whereas the ipsilateral sensorimotor activity in older adults did not appear to represent proactive interhemispheric interaction to overcome clumsiness.