fmri_xcorr
==========

.. currentmodule:: fmri_toolbox

Description
-----------
FMRI_XCORR  Voxelwise lagged cross-correlation between regressor and fMRI data.
Correlates a regressor with every brain voxel across a range of time lags,
returning a (228453 x 2*maxlag+1) correlation matrix. Column k corresponds
to a temporal lag of (-maxlag + k - 1) scans. Useful for detecting
haemodynamic delays or testing the HRF shape voxelwise.

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

    c = fmri_xcorr(regr, data, maxlag)

Inputs
------
- **REGR**: Regressor (T x 1) or (T x R) for R regressors.
- **DATA**: fMRI time series, vectorized (228453 x T).
- **MAXLAG**: Maximum lag in scans (integer). The function computes correlations for lags in [-maxlag, +maxlag].

Outputs
-------
- **C**: Cross-correlation matrix (228453 x (2*maxlag+1)). Column (maxlag+1) is the zero-lag correlation (= fmri_corregressor).

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

    c = fmri_xcorr(stim_ts, data, 5);
    [peak_r, peak_lag] = max(c, [], 2);
    peak_lag = peak_lag - (maxlag+1);     % convert to signed lag
    fmri_showslices(peak_lag .* (peak_r > 0.3), 1, 2, [-5 5])

See Also
--------
- fmri_corregressor
- fmri_doublegamma
- fmri_simmat