**Definition:** The sensitivity matrix maps local absorption changes within the brain to variations of optical density changes measured by each channel. It is typically represented as a matrix with rows corresponding to the channels of the fNIRS system and columns corresponding to the voxels (or surface elements) in the head. The accuracy of the sensitivity matrix depends on various factors, such as the geometry and spacing of the fNIRS probes, the optical properties of the tissue being measured, and the assumptions made in the model used to estimate the matrix.Definition: The sensitivity matrix maps local absorption changes within the brain to variations of optical density changes measured by each channel. It is typically represented as a matrix with rows corresponding to the channels of the fNIRS system and columns corresponding to the voxels (or surface elements) in the head. The accuracy of the sensitivity matrix depends on various factors, such as the geometry and spacing of the fNIRS probes, the optical properties of the tissue being measured, and the assumptions made in the model used to estimate the matrix.

**Alternative definition:** (if applicable)

**Synonym: **Jacobian

**Related terms:** Sensitivity Map; A matrix, Jacobian