Waveform Extractor

SpikeInterface provides an efficient mechanism to extract waveform snippets.

The WaveformExtractor class:

• randomly samples a subset spikes with max_spikes_per_unit

• extracts all waveforms snippets for each unit

• saves waveforms in a local folder

• can load stored waveforms

• retrieves template (average or median waveform) for each unit

Here the how!

import matplotlib.pyplot as plt

from spikeinterface import download_dataset
from spikeinterface import WaveformExtractor, extract_waveforms
import spikeinterface.extractors as se

Traceback (most recent call last):
  File "/home/docs/checkouts/readthedocs.org/user_builds/spikeinterface/checkouts/0.98.0/examples/modules_gallery/core/plot_4_waveform_extractor.py", line 21, in <module>
    import spikeinterface.extractors as se
  File "/home/docs/checkouts/readthedocs.org/user_builds/spikeinterface/conda/0.98.0/lib/python3.9/site-packages/spikeinterface/extractors/__init__.py", line 1, in <module>
    from .extractorlist import *
  File "/home/docs/checkouts/readthedocs.org/user_builds/spikeinterface/conda/0.98.0/lib/python3.9/site-packages/spikeinterface/extractors/extractorlist.py", line 15, in <module>
    from .neoextractors import *
  File "/home/docs/checkouts/readthedocs.org/user_builds/spikeinterface/conda/0.98.0/lib/python3.9/site-packages/spikeinterface/extractors/neoextractors/__init__.py", line 1, in <module>
    from .alphaomega import AlphaOmegaRecordingExtractor, AlphaOmegaEventExtractor, read_alphaomega, read_alphaomega_event
  File "/home/docs/checkouts/readthedocs.org/user_builds/spikeinterface/conda/0.98.0/lib/python3.9/site-packages/spikeinterface/extractors/neoextractors/alphaomega.py", line 3, in <module>
    from .neobaseextractor import NeoBaseRecordingExtractor, NeoBaseEventExtractor
  File "/home/docs/checkouts/readthedocs.org/user_builds/spikeinterface/conda/0.98.0/lib/python3.9/site-packages/spikeinterface/extractors/neoextractors/neobaseextractor.py", line 332, in <module>
    class NeoBaseSortingExtractor(_NeoBaseExtractor, BaseSorting):
  File "/home/docs/checkouts/readthedocs.org/user_builds/spikeinterface/conda/0.98.0/lib/python3.9/site-packages/spikeinterface/extractors/neoextractors/neobaseextractor.py", line 480, in NeoBaseSortingExtractor
    def _infer_t_start_from_signal_stream(self, segment_index: int, stream_id: Optional[str] = None) -> float | None:
TypeError: unsupported operand type(s) for |: 'type' and 'NoneType'

First let’s use the repo https://gin.g-node.org/NeuralEnsemble/ephy_testing_data to download a MEArec dataset. It is a simulated dataset that contains “ground truth” sorting information:

repo = 'https://gin.g-node.org/NeuralEnsemble/ephy_testing_data'
remote_path = 'mearec/mearec_test_10s.h5'
local_path = download_dataset(repo=repo, remote_path=remote_path, local_folder=None)

Let’s now instantiate the recording and sorting objects:

recording = se.MEArecRecordingExtractor(local_path)
print(recording)
sorting = se.MEArecSortingExtractor(local_path)
print(recording)

The MEArec dataset already contains a probe object that you can retrieve an plot:

probe = recording.get_probe()
print(probe)
from probeinterface.plotting import plot_probe

plot_probe(probe)

A WaveformExtractor object can be created with the extract_waveforms() function:

folder = 'waveform_folder'
we = extract_waveforms(
    recording,
    sorting,
    folder,
    ms_before=1.5,
    ms_after=2.,
    max_spikes_per_unit=500,
    overwrite=True
)
print(we)

Alternatively, the WaveformExtractor object can be instantiated directly. In this case, we need to set_params() to set the desired parameters:

folder = 'waveform_folder2'
we = WaveformExtractor.create(recording, sorting, folder, remove_if_exists=True)
we.set_params(ms_before=3., ms_after=4., max_spikes_per_unit=1000)
we.run_extract_waveforms(n_jobs=1, chunk_size=30000, progress_bar=True)
print(we)

To speed up computation, waveforms can also be extracted using parallel processing (recommended!). We can define some 'job_kwargs' to pass to the function as extra arguments:

job_kwargs = dict(n_jobs=2, chunk_duration="1s", progress_bar=True)

folder = 'waveform_folder_parallel'
we = extract_waveforms(
    recording,
    sorting,
    folder,
    ms_before=3.,
    ms_after=4.,
    max_spikes_per_unit=500,
    overwrite=True,
    **job_kwargs
)
print(we)

The 'waveform_folder' folder contains:

• the dumped recording (json)

• the dumped sorting (json)

• the parameters (json)

• a subfolder with “waveforms_XXX.npy” and “sampled_index_XXX.npy”

import os

print(os.listdir(folder))
print(os.listdir(folder + '/waveforms'))

Now we can retrieve waveforms per unit on-the-fly. The waveforms shape is (num_spikes, num_sample, num_channel):

unit_ids = sorting.unit_ids

for unit_id in unit_ids:
    wfs = we.get_waveforms(unit_id)
    print(unit_id, ':', wfs.shape)

We can also get the template for each units either using the median or the average:

for unit_id in unit_ids[:3]:
    fig, ax = plt.subplots()
    template = we.get_template(unit_id=unit_id, mode='median')
    print(template.shape)
    ax.plot(template)
    ax.set_title(f'{unit_id}')

Or retrieve templates for all units at once:

all_templates = we.get_all_templates()
print(all_templates.shape)


'''
Sparse Waveform Extractor
-------------------------

'''

For high-density probes, such as Neuropixels, we may want to work with sparse waveforms, i.e., waveforms computed on a subset of channels. To do so, we two options.

Option 1) Save a dense waveform extractor to sparse:

In this case, from an existing waveform extractor, we can first estimate a sparsity (which channels each unit is defined on) and then save to a new folder in sparse mode:

from spikeinterface import compute_sparsity

# define sparsity within a radius of 40um
sparsity = compute_sparsity(we, method="radius", radius_um=40)
print(sparsity)

# save sparse waveforms
folder = 'waveform_folder_sparse'
we_sparse = we.save(folder=folder, sparsity=sparsity, overwrite=True)

# we_sparse is a sparse WaveformExtractor
print(we_sparse)

wf_full = we.get_waveforms(we.sorting.unit_ids[0])
print(f"Dense waveforms shape for unit {we.sorting.unit_ids[0]}: {wf_full.shape}")
wf_sparse = we_sparse.get_waveforms(we.sorting.unit_ids[0])
print(f"Sparse waveforms shape for unit {we.sorting.unit_ids[0]}: {wf_sparse.shape}")

Option 2) Directly extract sparse waveforms:

We can also directly extract sparse waveforms. To do so, dense waveforms are extracted first using a small number of spikes ('num_spikes_for_sparsity')

folder = 'waveform_folder_sparse_direct'
we_sparse_direct = extract_waveforms(
    recording,
    sorting,
    folder,
    ms_before=3.,
    ms_after=4.,
    max_spikes_per_unit=500,
    overwrite=True,
    sparse=True,
    num_spikes_for_sparsity=100,
    method="radius",
    radius_um=40,
    **job_kwargs
)
print(we_sparse_direct)

template_full = we.get_template(we.sorting.unit_ids[0])
print(f"Dense template shape for unit {we.sorting.unit_ids[0]}: {template_full.shape}")
template_sparse = we_sparse_direct.get_template(we.sorting.unit_ids[0])
print(f"Sparse template shape for unit {we.sorting.unit_ids[0]}: {template_sparse.shape}")

As shown above, when retrieving waveforms/template for a unit from a sparse 'WaveformExtractor', the waveforms are returned on a subset of channels. To retrieve which channels each unit is associated with, we can use the sparsity object:

# retrive channel ids for first unit:
unit_ids = we_sparse.unit_ids
channel_ids_0 = we_sparse.sparsity.unit_id_to_channel_ids[unit_ids[0]]
print(f"Channel ids associated to {unit_ids[0]}: {channel_ids_0}")

However, when retrieving all templates, a dense shape is returned. This is because different channels might have a different number of sparse channels! In this case, values on channels not belonging to a unit are filled with 0s.

all_sparse_templates = we_sparse.get_all_templates()

# this is a boolean mask with sparse channels for the 1st unit
mask0 = we_sparse.sparsity.mask[0]
# Let's plot values for the first 5 samples inside and outside sparsity mask
print("Values inside sparsity:\n", all_sparse_templates[0, :5, mask0])
print("Values outside sparsity:\n", all_sparse_templates[0, :5, ~mask0])

plt.show()