# Separating the forest from the palm trees: Individual variation in a presurgical language mapping task

## TL;DR

This study evaluates a language mapping task using fMRI to understand individual brain activity variations in neurosurgical patients.

## Contribution

The study introduces threshold-weighted overlap mapping to assess fMRI task reliability across individuals and scanners.

## Key findings

- Task-related language network activations were consistent across individuals and test-retest sessions.
- Some brain regions showed variable activation, indicating atypical language dominance or processing differences.
- Overlap mapping effectively visualizes individual neural variations, aiding pre- and intra-operative alignment.

## Abstract

•We evaluated a fMRI semantic association task for presurgical language mapping.•Threshold weighted overlap mapping shows activation reliability over people & time.•Overlap mapping reveals meaningful individual variations in task fMRI activity.

We evaluated a fMRI semantic association task for presurgical language mapping.

Threshold weighted overlap mapping shows activation reliability over people & time.

Overlap mapping reveals meaningful individual variations in task fMRI activity.

Selecting optimal tasks for language mapping in neurosurgical patients poses challenges that are exacerbated by mismatches in practice between presurgical and intraoperative evaluations. To help align practices, we evaluated a functional MRI version of a semantic association task increasingly used during intra-operative assessment of awake neurosurgery patients. Using a recently proposed consistency mapping approach, we characterise task fMRI activation reliability across individuals, visits, and scan cohorts.

FMRI data were acquired during an adapted Pyramids and Palm Trees Task (PPTT) in 15 healthy controls and 54 pre-surgical patients with a glioma. A new implementation of threshold-weighted overlap mapping (TWOM) was used to evaluate: 1. inter-individual variability in task activations among individuals; 2. test–retest variability in controls scanned twice (16 ± weeks apart); 3. between-scanner reliability across two patient cohorts scanned on a 3 T Siemens Prisma (n = 27) or Verio (n = 24) scanner using standard (TR = 3 s, voxel size 3 × 3 × 3 mm) or advanced (TR = 0.93 s, voxel size 2x2x2 mm) fMRI acquisitions, respectively.

Task-related activations in the core language network were highly consistent between individuals and across test–retest sessions. Several brain regions showed variable activations, reflecting atypical language dominance (confirmed during neurosurgery), or differences in regional involvement during semantic processing.

The PPTT engaged widespread brain networks including but not limited to regions implicated in semantic processing. Overlap mapping is a powerful way to visualise meaningful variations in neural processing at the individual level, supporting alignment of pre- and intra-operative mapping for any given task.