fmri_downsample
===============

.. currentmodule:: fmri_toolbox

Description
-----------
FMRI_DOWNSAMPLE  Spatially downsamples vectorized fMRI data by integer factor.
Retains every Nth voxel along each spatial dimension (x, y, z), reducing
data size for computationally expensive operations. The output is a
sub-sampled vectorized brain (fewer voxels x T). Inverse: fmri_upsample.

Usage
-----
.. code-block:: matlab

    fmri2 = fmri_downsample(fmri)
    fmri2 = fmri_downsample(fmri, factor)

Inputs
------
- **FMRI**: Vectorized fMRI data (228453 x T).
- **FACTOR**: Downsampling factor (integer). Every FACTOR-th voxel is kept in each dimension. Default: 2 (keeps 1/8 of voxels).

Outputs
-------
- **FMRI2**: Downsampled vectorized data. Number of rows depends on FACTOR: factor=2: ~28542 voxels, factor=3: ~8468, etc.

Examples
--------
.. code-block:: matlab

    small = fmri_downsample(data);         % factor 2
    small = fmri_downsample(data, 3);      % factor 3
    orig  = fmri_upsample(small);          % restore full resolution

See Also
--------
- fmri_upsample