def _get_ch_type_mapping(fro='mne', to='bids'):
    """Map between BIDS and MNE nomenclatures for channel types.

    Parameters
    ----------
    fro : str
        Mapping from nomenclature of `fro`. Can be 'mne', 'bids'
    to : str
        Mapping to nomenclature of `to`. Can be 'mne', 'bids'

    Returns
    -------
    mapping : dict
        Dictionary mapping from one nomenclature of channel types to another.
        If a key is not present, a default value will be returned that depends
        on the `fro` and `to` parameters.

    Notes
    -----
    For the mapping from BIDS to MNE, MEG channel types are ignored for now.
    Furthermore, this is not a one-to-one mapping: Incomplete and partially
    one-to-many/many-to-one.

    Bio channels are supported in mne-python and are converted to MISC
    because there is no "Bio" supported channel in BIDS.
    """
    if fro == 'mne' and to == 'bids':
        mapping = dict(eeg='EEG', misc='MISC', stim='TRIG', emg='EMG',
                       ecog='ECOG', seeg='SEEG', eog='EOG', ecg='ECG',
                       resp='RESP', bio='MISC', dbs='DBS',
                       # MEG channels
                       meggradaxial='MEGGRADAXIAL', megmag='MEGMAG',
                       megrefgradaxial='MEGREFGRADAXIAL',
                       meggradplanar='MEGGRADPLANAR', megrefmag='MEGREFMAG',
                       ias='MEGOTHER', syst='MEGOTHER', exci='MEGOTHER')

    elif fro == 'bids' and to == 'mne':
        mapping = dict(EEG='eeg', MISC='misc', TRIG='stim', EMG='emg',
                       ECOG='ecog', SEEG='seeg', EOG='eog', ECG='ecg',
                       RESP='resp',
                       # No MEG channels for now
                       # Many to one mapping
                       VEOG='eog', HEOG='eog', DBS='dbs')
    else:
        raise ValueError('Only two types of mappings are currently supported: '
                         'from mne to bids, or from bids to mne. However, '
                         'you specified from "{}" to "{}"'.format(fro, to))

    return mapping