set_channel_types does not properly set the coil_type for eyetracking channels

[scott-huberty]

Issue 1 (Originally posted in https://github.com/mne-tools/mne-python/pull/12744#issue-2425236972 ):

I am important eye tracking data from a tobii device (XDF format), and kept running into this issue when trying to set_channel_types:

KeyError: 0 (FIFFV_COIL_NONE) After some digging, I found that there is a different function that I should call. With this PR I want to make this more obvious in the documentation. However, the above issue also points out that there is probably a bug when setting channel types for eye-tracking.

---

MWE

The code below fails because our use of set_channel_types does not set the coil_type for eyetracking channels:

import numpy as np
import mne

shape = (1, 100)
data = np.vstack([np.full(shape, 960), np.full(shape, 540), np.full(shape, 0)])

info = mne.create_info(
        ch_names=["xpos", "ypos", "pupil"],
        sfreq=100,
        ch_types="eeg"
    )
raw = mne.io.RawArray(data, info)
raw.set_channel_types(dict(xpos="eyegaze", ypos="eyegaze", pupil="pupil"))
epochs = mne.make_fixed_length_epochs(raw)

Stack Trace

---------------------------------------------------------------------------
KeyError                                  Traceback (most recent call last)
Cell In[1], line 14
     12 raw = mne.io.RawArray(data, info)
     13 raw.set_channel_types(dict(xpos="eyegaze", ypos="eyegaze", pupil="pupil"))
---> 14 epochs = mne.make_fixed_length_epochs(raw)

File <decorator-gen-256>:12, in make_fixed_length_epochs(raw, duration, preload, reject_by_annotation, proj, overlap, id, verbose)

File ~/devel/repos/mne-python/mne/epochs.py:4967, in make_fixed_length_epochs(raw, duration, preload, reject_by_annotation, proj, overlap, id, verbose)
   4965 events = make_fixed_length_events(raw, id=id, duration=duration, overlap=overlap)
   4966 delta = 1.0 / raw.info["sfreq"]
-> 4967 return Epochs(
   4968     raw,
   4969     events,
   4970     event_id=[id],
   4971     tmin=0,
   4972     tmax=duration - delta,
   4973     baseline=None,
   4974     preload=preload,
   4975     reject_by_annotation=reject_by_annotation,
   4976     proj=proj,
   4977     verbose=verbose,
   4978 )

File <decorator-gen-247>:12, in __init__(self, raw, events, event_id, tmin, tmax, baseline, picks, preload, reject, flat, proj, decim, reject_tmin, reject_tmax, detrend, on_missing, reject_by_annotation, metadata, event_repeated, verbose)

File ~/devel/repos/mne-python/mne/epochs.py:3568, in Epochs.__init__(self, raw, events, event_id, tmin, tmax, baseline, picks, preload, reject, flat, proj, decim, reject_tmin, reject_tmax, detrend, on_missing, reject_by_annotation, metadata, event_repeated, verbose)
   3563     events, event_id, annotations = _events_from_annotations(
   3564         raw, events, event_id, annotations, on_missing
   3565     )
   3567 # call BaseEpochs constructor
-> 3568 super().__init__(
   3569     info,
   3570     None,
   3571     events,
   3572     event_id,
   3573     tmin,
   3574     tmax,
   3575     metadata=metadata,
   3576     baseline=baseline,
   3577     raw=raw,
   3578     picks=picks,
   3579     reject=reject,
   3580     flat=flat,
   3581     decim=decim,
   3582     reject_tmin=reject_tmin,
   3583     reject_tmax=reject_tmax,
   3584     detrend=detrend,
   3585     proj=proj,
   3586     on_missing=on_missing,
   3587     preload_at_end=preload,
   3588     event_repeated=event_repeated,
   3589     verbose=verbose,
   3590     raw_sfreq=raw_sfreq,
   3591     annotations=annotations,
   3592 )

File <decorator-gen-231>:12, in __init__(self, info, data, events, event_id, tmin, tmax, baseline, raw, picks, reject, flat, decim, reject_tmin, reject_tmax, detrend, proj, on_missing, preload_at_end, selection, drop_log, filename, metadata, event_repeated, raw_sfreq, annotations, verbose)

File ~/devel/repos/mne-python/mne/epochs.py:668, in BaseEpochs.__init__(***failed resolving arguments***)
    666 self.reject = None
    667 self.flat = None
--> 668 self._reject_setup(reject, flat)
    670 # do the rest
    671 valid_proj = [True, "delayed", False]

File ~/devel/repos/mne-python/mne/epochs.py:797, in BaseEpochs._reject_setup(self, reject, flat, allow_callable)
    795 def _reject_setup(self, reject, flat, *, allow_callable=False):
    796     """Set self._reject_time and self._channel_type_idx."""
--> 797     idx = channel_indices_by_type(self.info)
    798     reject = deepcopy(reject) if reject is not None else dict()
    799     flat = deepcopy(flat) if flat is not None else dict()

File ~/devel/repos/mne-python/mne/_fiff/pick.py:873, in channel_indices_by_type(info, picks)
    871 picks = _picks_to_idx(info, picks, none="all", exclude=(), allow_empty=True)
    872 for k in picks:
--> 873     ch_type = channel_type(info, k)
    874     for key in idx_by_type.keys():
    875         if ch_type == key:

File ~/devel/repos/mne-python/mne/_fiff/pick.py:255, in channel_type(info, idx)
    253 if first_kind in _second_rules:
    254     key, second_rule = _second_rules[first_kind]
--> 255     first_kind = second_rule[ch[key]]
    256 return first_kind

KeyError: 0 (FIFFV_COIL_NONE)

---

However the code below does work because mne.create_info properly sets the coil_type for all channels:

code

# Note that I do not use set_channel_types here

import numpy as np
import mne

shape = (1, 100)
data = np.vstack([np.full(shape, 960), np.full(shape, 540), np.full(shape, 0)])

info = mne.create_info(
        ch_names=["xpos", "ypos", "pupil"],
        sfreq=100,
        ch_types=["eyegaze", "eyegaze", "pupil"]
    )
raw = mne.io.RawArray(data, info)

epochs = mne.make_fixed_length_epochs(raw)

---

Issue 2

One work around to the issue above is to use set_channel_types_eyetrack, like so:

Code

import numpy as np
import mne

shape = (1, 100)
data = np.vstack([np.full(shape, 960), np.full(shape, 540), np.full(shape, 0)])

info = mne.create_info(
        ch_names=["xpos", "ypos", "pupil"],
        sfreq=100,
        ch_types=["eyegaze", "eyegaze", "pupil"]
    )

more_info = dict(
    xpos=("eyegaze", "px", "right", "x"),
    ypos=("eyegaze", "px", "right", "y"),
    pupil=("pupil", "au", "right"),
)

raw = mne.io.RawArray(data, info)
# This sets the coil types, loc array, etc.
mne.preprocessing.eyetracking.set_channel_types_eyetrack(raw, more_info)

epochs = mne.make_fixed_length_epochs(raw)

However I personally think that This function has drawbacks:

• 1. It is harder to find for users
• 2. It is more complex (even I have to re-read the docstring each time that I need to use it)
• 3. It can be redundant (I already specified the channel types in mne.create_info, now I need to specify them again?)
• 4. If we make sure set_channel_types properly sets the coil_type for eyetrack channels, then the only extra info set_channel_types_eyetrack adds is the loc array. Now I have to pass a complicated dict and re-set information that might already be correctly set (channel type, coil, unit) just to set the loc array.

---

Proposal

Maybe we can start with fixing issue Number 1, i.e. make sure that set_channel_types properly sets the coil_type for eyetracking channels. And then loop back to see if we can improve the API for setting other info for eyetracking channels, like the loc array, etc.

[sappelhoff]

Thanks Scott, this perfectly summarizes the issues I was facing.

[larsoner]

Maybe we can start with fixing issue Number 1, i.e. make sure that set_channel_types properly sets the coil_type for eyetracking channels

Sounds good, you up for a PR at some point @scott-huberty ?

[scott-huberty]

Maybe we can start with fixing issue Number 1, i.e. make sure that set_channel_types properly sets the coil_type for eyetracking channels

Sounds good, you up for a PR at some point @scott-huberty ?

Yes, I have a backlog of eye-tracking related issues I need to get to 😅 Feel free to assign this to me. I'll have time in August to submit some PR's.