Fix cnt response (mne-tools#13007)

Co-authored-by: pre-commit-ci[bot] <66853113+pre-commit-ci[bot]@users.noreply.github.com>
Co-authored-by: Eric Larson <[email protected]>

doc/changes/devel/13007.bugfix.rst

@@ -0,0 +1 @@
+Correct :func:`mne.io.read_raw_cnt` to read responses and fix exceptions by `Jacob Woessner`_. 

mne/io/cnt/cnt.py

@@ -14,7 +14,7 @@
 from ..._fiff.utils import _create_chs, _find_channels, _mult_cal_one, read_str
 from ...annotations import Annotations
 from ...channels.layout import _topo_to_sphere
-from ...utils import _check_option, _validate_type, fill_doc, warn
+from ...utils import _check_option, _explain_exception, _validate_type, fill_doc, warn
 from ..base import BaseRaw
 from ._utils import (
     CNTEventType3,
@@ -150,7 +150,22 @@ def _update_bad_span_onset(accept_reject, onset, duration, description):
             np.array([e.KeyPad_Accept for e in my_events])
         )
 
-        description = np.array([str(e.StimType) for e in my_events])
+        # Check to see if there are any button presses
+        description = []
+        for event in my_events:
+            # Extract the 4-bit fields
+            # Upper nibble (4 bits) currently not used
+            # accept = (event.KeyPad_Accept[0] & 0xF0) >> 4
+            # Lower nibble (4 bits) keypad button press
+            keypad = event.KeyPad_Accept[0] & 0x0F
+            if str(keypad) != "0":
+                description.append(f"KeyPad Response {keypad}")
+            elif event.KeyBoard != 0:
+                description.append(f"Keyboard Response {event.KeyBoard}")
+            else:
+                description.append(str(event.StimType))
+
+        description = np.array(description)
 
         onset, duration, description = _update_bad_span_onset(
             accept_reject, onset / sfreq, duration, description
@@ -532,7 +547,8 @@ def __init__(
             )
         except Exception:
             raise RuntimeError(
-                "Could not read header from *.cnt file. mne.io.read_raw_cnt "
+                f"{_explain_exception()}\n"
+                "WARNING: mne.io.read_raw_cnt "
                 "supports Neuroscan CNT files only. If this file is an ANT Neuro CNT, "
                 "please use mne.io.read_raw_ant instead."
             )

mne/io/cnt/tests/test_cnt.py

@@ -57,7 +57,8 @@ def test_auto_data():
     third = pytest.warns(RuntimeWarning, match="Omitted 6 annot")
     with first, second, third:
         raw = read_raw_cnt(input_fname=fname_bad_spans)
-
+    # Test that responses are read properly
+    assert "KeyPad Response 1" in raw.annotations.description
     assert raw.info["bads"] == ["F8"]
 
     with _no_parse, pytest.warns(RuntimeWarning, match="number of bytes"):