Extracellular Electrophysiology Data¶
At the Allen Institute for Brain Science we carry out in vivo extracellular electrophysiology (ecephys) experiments in awake animals using high-density Neuropixels probes. The data from these experiments are organized into sessions, where each session is a distinct continuous recording period. During a session we collect:
- spike times and characteristics (such as mean waveforms) from up to 6 neuropixels probes
- local field potentials
- behavioral data, such as running speed and eye position
- visual stimuli which were presented during the session
- cell-type specific optogenetic stimuli that were applied during the session
The AllenSDK contains code for accessing across-session (project-level) metadata as well as code for accessing detailed within-session data. The standard workflow is to use project-level tools, such as EcephysProjectCache to identify and access sessions of interest, then delve into those sessions' data using EcephysSession.
Project-level¶
The EcephysProjectCache class in allensdk.brain_observatory.ecephys.ecephys_project_cache accesses and stores data pertaining to many sessions. You can use this class to run queries that span all collected sessions and to download data for individual sessions.
- Obtaining an `EcephysProjectCache`
- Querying sessions
- Querying probes
- Querying units
- Surveying metadata
Session-level¶
The EcephysSession class in allensdk.brain_observatory.ecephys.ecephys_session provides an interface to all of the data for a single session, aligned to a common clock. This notebook will show you how to use the EcephysSession class to extract these data.
# first we need a bit of import boilerplate
import os
import numpy as np
import xarray as xr
import pandas as pd
import matplotlib.pyplot as plt
from scipy.ndimage.filters import gaussian_filter
from allensdk.brain_observatory.ecephys.ecephys_project_cache import EcephysProjectCache
from allensdk.brain_observatory.ecephys.ecephys_session import (
EcephysSession,
removed_unused_stimulus_presentation_columns
)
from allensdk.brain_observatory.ecephys.visualization import plot_mean_waveforms, plot_spike_counts, raster_plot
from allensdk.brain_observatory.visualization import plot_running_speed
# tell pandas to show all columns when we display a DataFrame
pd.set_option("display.max_columns", None)
Obtaining an EcephysProjectCache¶
In order to create an EcephysProjectCache object, you need to specify two things:
- A remote source for the object to fetch data from. We will instantiate our cache using
EcephysProjectCache.from_warehouse()to point the cache at the Allen Institute's public web API. - A path to a manifest json, which designates filesystem locations for downloaded data. The cache will try to read data from these locations before going to download those data from its remote source, preventing repeated downloads.
manifest_path = os.path.join("example_ecephys_project_cache", "manifest.json")
cache = EcephysProjectCache.from_warehouse(manifest=manifest_path)
Querying across sessions¶
Using your EcephysProjectCache, you can download a table listing metadata for all sessions.
cache.get_session_table().head()
Querying across probes¶
... or for all probes
cache.get_probes().head()
Querying across channels¶
... or across channels.
cache.get_channels().head()
Querying across units¶
... as well as for sorted units.
units = cache.get_units()
units.head()
# There are quite a few of these
print(units.shape[0])
Surveying metadata¶
You can answer questions like: "what mouse genotypes were used in this dataset?" using your EcephysProjectCache.
print(f"stimulus sets: {cache.get_all_session_types()}")
print(f"genotypes: {cache.get_all_full_genotypes()}")
print(f"structures: {cache.get_structure_acronyms()}")
In order to look up a brain structure acronym, you can use our online atlas viewer. The AllenSDK additionally supports programmatic access to structure annotations. For more information, see the reference space and mouse connectivity documentation.
Obtaining an EcephysSession¶
We package each session's data into a Neurodata Without Borders 2.0 (NWB) file. Calling get_session_data on your EcephysProjectCache will download such a file and return an EcephysSession object.
EcephysSession objects contain methods and properties that access the data within an ecephys NWB file and cache it in memory.
session_id = 756029989 # for example
session = cache.get_session_data(session_id)
This session object has some important metadata, such as the date and time at which the recording session started:
print(f"session {session.ecephys_session_id} was acquired in {session.session_start_time}")
We'll now jump in to accessing our session's data. If you ever want a complete documented list of the attributes and methods defined on EcephysSession, you can run help(EcephysSession) (or in a jupyter notebook: EcephysSession?).
Sorted units¶
Units are putative neurons, clustered from raw voltage traces using Kilosort 2. Each unit is associated with a single peak channel on a single probe, though its spikes might be picked up with some attenuation on multiple nearby channels. Each unit is assigned a unique integer identifier ("unit_id") which can be used to look up its spike times and its mean waveform.
The units for a session are recorded in an attribute called, fittingly, units. This is a pandas.DataFrame whose index is the unit id and whose columns contain summary information about the unit, its peak channel, and its associated probe.
session.units.head()
As a pandas.DataFrame the units table supports many straightforward filtering operations:
# how many units have signal to noise ratios that are greater than 4?
print(f'{session.units.shape[0]} units total')
units_with_very_high_snr = session.units[session.units['snr'] > 4]
print(f'{units_with_very_high_snr.shape[0]} units have snr > 4')
... as well as some more advanced (and very useful!) operations. For more information, please see the pandas documentation. The following topics might be particularly handy:
Stimulus presentations¶
During the course of a session, visual stimuli are presented on a monitor to the subject. We call intervals of time where a specific stimulus is presented (and its parameters held constant!) a stimulus presentation.
You can find information about the stimulus presentations that were displayed during a session by accessing the stimulus_presentations attribute on your EcephysSession object.
session.stimulus_presentations.head()
Like the units table, this is a pandas.DataFrame. Each row corresponds to a stimulus presentation and lists the time (on the session's master clock, in seconds) when that presentation began and ended as well as the kind of stimulus that was presented (the "stimulus_name" column) and the parameter values that were used for that presentation. Many of these parameter values don't overlap between stimulus classes, so the stimulus_presentations table uses the string "null" to indicate an inapplicable parameter. The index is named "stimulus_presentation_id" and many methods on EcephysSession use these ids.
Some of the columns bear a bit of explanation:
- stimulus_block : A block consists of multiple presentations of the same stimulus class presented with (probably) different parameter values. If during a session stimuli were presented in the following order:
- drifting gratings
- static gratings
- drifting gratings then the blocks for that session would be [0, 1, 2]. The gray period stimulus (just a blank gray screen) never gets a block.
- duration : this is just stop_time - start_time, precalculated for convenience.
What kinds of stimuli were presented during this session? Pandas makes it easy to find out:
session.stimulus_names # just the unique values from the 'stimulus_name' column
We can also obtain the stimulus epochs - blocks of time for which a particular kind of stimulus was presented - for this session.
session.get_stimulus_epochs()
If you are only interested in a subset of stimuli, you can either filter using pandas or using the get_stimulus_table convience method:
session.get_stimulus_table(['drifting_gratings']).head()
We might also want to know what the total set of available parameters is. The get_stimulus_parameter_values method provides a dictionary mapping stimulus parameters to the set of values that were applied to those parameters:
for key, values in session.get_stimulus_parameter_values().items():
print(f'{key}: {values}')
Each distinct state of the monitor is called a "stimulus condition". Each presentation in the stimulus presentations table exemplifies such a condition. This is encoded in its stimulus_condition_id field.
To get the full list of conditions presented in a session, use the stimulus_conditions attribute:
session.stimulus_conditions.head()
Spike data¶
The EcephysSession object holds spike times (in seconds on the session master clock) for each unit. These are stored in a dictionary, which maps unit ids (the index values of the units table) to arrays of spike times.
# grab an arbitrary (though high-snr!) unit (we made units_with_high_snr above)
high_snr_unit_ids = units_with_very_high_snr.index.values
unit_id = high_snr_unit_ids[0]
print(f"{len(session.spike_times[unit_id])} spikes were detected for unit {unit_id} at times:")
session.spike_times[unit_id]
You can also obtain spikes tagged with the stimulus presentation during which they occurred:
# get spike times from the first block of drifting gratings presentations
drifting_gratings_presentation_ids = session.stimulus_presentations.loc[
(session.stimulus_presentations['stimulus_name'] == 'drifting_gratings')
].index.values
times = session.presentationwise_spike_times(
stimulus_presentation_ids=drifting_gratings_presentation_ids,
unit_ids=high_snr_unit_ids
)
times.head()
We can make raster plots of these data:
first_drifting_grating_presentation_id = times['stimulus_presentation_id'].values[0]
plot_times = times[times['stimulus_presentation_id'] == first_drifting_grating_presentation_id]
fig = raster_plot(plot_times, title=f'spike raster for stimulus presentation {first_drifting_grating_presentation_id}')
plt.show()
# also print out this presentation
session.stimulus_presentations.loc[first_drifting_grating_presentation_id]
We can access summary spike statistics for stimulus conditions and unit
stats = session.conditionwise_spike_statistics(
stimulus_presentation_ids=drifting_gratings_presentation_ids,
unit_ids=high_snr_unit_ids
)
# display the parameters associated with each condition
stats = pd.merge(stats, session.stimulus_conditions, left_on="stimulus_condition_id", right_index=True)
stats.head()
Using these data, we can ask for each unit: which stimulus condition evoked the most activity on average?
with_repeats = stats[stats["stimulus_presentation_count"] >= 5]
highest_mean_rate = lambda df: df.loc[df['spike_mean'].idxmax()]
max_rate_conditions = with_repeats.groupby('unit_id').apply(highest_mean_rate)
max_rate_conditions.head()
Spike histograms¶
It is commonly useful to compare spike data from across units and stimulus presentations, all relative to the onset of a stimulus presentation. We can do this using the presentationwise_spike_counts method.
# We're going to build an array of spike counts surrounding stimulus presentation onset
# To do that, we will need to specify some bins (in seconds, relative to stimulus onset)
time_bin_edges = np.linspace(-0.01, 0.4, 200)
# look at responses to the flash stimulus
flash_250_ms_stimulus_presentation_ids = session.stimulus_presentations[
session.stimulus_presentations['stimulus_name'] == 'flashes'
].index.values
# and get a set of units with only decent snr
decent_snr_unit_ids = session.units[
session.units['snr'] >= 1.5
].index.values
spike_counts_da = session.presentationwise_spike_counts(
bin_edges=time_bin_edges,
stimulus_presentation_ids=flash_250_ms_stimulus_presentation_ids,
unit_ids=decent_snr_unit_ids
)
spike_counts_da
This has returned a new (to this notebook) data structure, the xarray.DataArray. You can think of this as similar to a 3+D pandas.DataFrame, or as a numpy.ndarray with labeled axes and indices. See the xarray documentation for more information. In the mean time, the salient features are:
- Dimensions : Each axis on each data variable is associated with a named dimension. This lets us see unambiguously what the axes of our array mean.
- Coordinates : Arrays of labels for each sample on each dimension.
xarray is nice because it forces code to be explicit about dimensions and coordinates, improving readability and avoiding bugs. However, you can always convert to numpy or pandas data structures as follows:
- to pandas:
spike_counts_ds.to_dataframe()produces a multiindexed dataframe - to numpy:
spike_counts_ds.valuesgives you access to the underlying numpy array
We can now plot spike counts for a particular presentation:
presentation_id = 3796 # chosen arbitrarily
plot_spike_counts(
spike_counts_da.loc[{'stimulus_presentation_id': presentation_id}],
spike_counts_da['time_relative_to_stimulus_onset'],
'spike count',
f'unitwise spike counts on presentation {presentation_id}'
)
plt.show()
We can also average across all presentations, adding a new data array to the dataset. Notice that this one no longer has a stimulus_presentation_id dimension, as we have collapsed it by averaging.
mean_spike_counts = spike_counts_da.mean(dim='stimulus_presentation_id')
mean_spike_counts
... and plot the mean spike counts
plot_spike_counts(
mean_spike_counts,
mean_spike_counts['time_relative_to_stimulus_onset'],
'mean spike count',
'mean spike counts on flash_250_ms presentations'
)
plt.show()
Waveforms¶
We store precomputed mean waveforms for each unit in the mean_waveforms attribute on the EcephysSession object. This is a dictionary which maps unit ids to xarray dataarrays. These have channel and time (seconds, aligned to the detected event times) dimensions. The data values are millivolts, as measured at the recording site.
We store precomputed mean waveforms for each unit in the mean_waveforms attribute on the EcephysSession object. This is a dictionary which maps unit ids to xarray dataarrays. These have channel and time (seconds, aligned to the detected event times) dimensions. The data values are millivolts, as measured at the recording site.
units_of_interest = high_snr_unit_ids[:35]
waveforms = {uid: session.mean_waveforms[uid] for uid in units_of_interest}
peak_channels = {uid: session.units.loc[uid, 'peak_channel_id'] for uid in units_of_interest}
# plot the mean waveform on each unit's peak channel/
plot_mean_waveforms(waveforms, units_of_interest, peak_channels)
plt.show()
Since neuropixels probes are densely populated with channels, spikes are typically detected on several channels. We can see this by plotting mean waveforms on channels surrounding a unit's peak channel:
uid = units_of_interest[12]
unit_waveforms = waveforms[uid]
peak_channel = peak_channels[uid]
peak_channel_idx = np.where(unit_waveforms["channel_id"] == peak_channel)[0][0]
ch_min = max(peak_channel_idx - 10, 0)
ch_max = min(peak_channel_idx + 10, len(unit_waveforms["channel_id"]) - 1)
surrounding_channels = unit_waveforms["channel_id"][np.arange(ch_min, ch_max, 2)]
fig, ax = plt.subplots()
ax.imshow(unit_waveforms.loc[{"channel_id": surrounding_channels}])
ax.yaxis.set_major_locator(plt.NullLocator())
ax.set_ylabel("channel", fontsize=16)
ax.set_xticks(np.arange(0, len(unit_waveforms['time']), 20))
ax.set_xticklabels([f'{float(ii):1.4f}' for ii in unit_waveforms['time'][::20]], rotation=45)
ax.set_xlabel("time (s)", fontsize=16)
plt.show()
Running speed¶
We can obtain the velocity at which the experimental subject ran as a function of time by accessing the running_speed attribute. This returns a pandas dataframe whose rows are intervals of time (defined by "start_time" and "end_time" columns), and whose "velocity" column contains mean running speeds within those intervals.
Here we'll plot the running speed trace for an arbitrary chunk of time.
running_speed_midpoints = session.running_speed["start_time"] + \
(session.running_speed["end_time"] - session.running_speed["start_time"]) / 2
plot_running_speed(
running_speed_midpoints,
session.running_speed["velocity"],
start_index=5000,
stop_index=5100
)
plt.show()
Optogenetic stimulation¶
session.optogenetic_stimulation_epochs
Local Field Potential¶
We record local field potential on a subset of channels at 2500 Hz. Even subsampled and compressed, these data are quite large, so we store them seperately for each probe.
# list the probes recorded from in this session
session.probes.head()
# load up the lfp from one of the probes. This returns an xarray dataarray
probe_id = session.probes.index.values[0]
lfp = session.get_lfp(probe_id)
print(lfp)
We can figure out where each LFP channel is located in the Brain
# now use a utility to associate intervals of /rows with structures
structure_acronyms, intervals = session.channel_structure_intervals(lfp["channel"])
interval_midpoints = [aa + (bb - aa) / 2 for aa, bb in zip(intervals[:-1], intervals[1:])]
print(structure_acronyms)
print(intervals)
window = np.where(np.logical_and(lfp["time"] < 5.0, lfp["time"] >= 4.0))[0]
fig, ax = plt.subplots()
ax.pcolormesh(lfp[{"time": window}].T)
ax.set_yticks(intervals)
ax.set_yticks(interval_midpoints, minor=True)
ax.set_yticklabels(structure_acronyms, minor=True)
plt.tick_params("y", which="major", labelleft=False, length=40)
num_time_labels = 8
time_label_indices = np.around(np.linspace(1, len(window), num_time_labels)).astype(int) - 1
time_labels = [ f"{val:1.3}" for val in lfp["time"].values[window][time_label_indices]]
ax.set_xticks(time_label_indices + 0.5)
ax.set_xticklabels(time_labels)
ax.set_xlabel("time (s)", fontsize=20)
plt.show()
Current source density¶
We precompute current source density for each probe.
csd = session.get_current_source_density(probe_id)
csd
filtered_csd = gaussian_filter(csd.data, sigma=4)
fig, ax = plt.subplots(figsize=(6, 6))
ax.pcolor(csd["time"], csd["vertical_position"], filtered_csd)
ax.set_xlabel("time relative to stimulus onset (s)", fontsize=20)
ax.set_ylabel("vertical position (um)", fontsize=20)
plt.show()
Suggested excercises¶
If you would hands-on experience with the EcephysSession class, please consider working through some of these excercises.
- tuning curves : Pick a stimulus parameter, such as orientation on drifting gratings trials. Plot the mean and standard error of spike counts for each unit at each value of this parameter.
- signal correlations : Calculate unit-pairwise correlation coefficients on the tuning curves for a stimulus parameter of interest (
numpy.corrcoefmight be useful). - noise correlations : Build for each unit a vector of spike counts across repeats of the same stimulus condition. Compute unit-unit correlation coefficients on these vectors.
- cross-correlations : Start with two spike trains. Call one of them "fixed" and the other "moving". Choose a set of time offsets and for each offset:
- apply the offset to the spike times in the moving train
- compute the correlation coefficient between the newly offset moving train and the fixed train. You should then be able to plot the obtained correlation coeffients as a function of the offset.
unit clustering : First, extract a set of unitwise features. You might draw these from the mean waveforms, for instance:
- mean duration between waveform peak and trough (on the unit's peak channel)
the amplitude of the unit's trough
or you might draw them from the unit's spike times, such as:
median inter-spike-interval
or from metadata
CCF structure
With your features in hand, attempt an unsupervised classification of the units. If this seems daunting, check out the scikit-learn unsupervised learning documention for library code and examples.
population decoding : Using an
EcephysSession(and filtering to some stimuli and units of interest), build two aligned matrices:- A matrix whose rows are stimulus presentations, columns are units, and values are spike counts.
A matrix whose rows are stimulus presentations and whose columns are stimulus parameters.
Using these matrices, train a classifier to predict stimulus conditions (sets of stimulus parameter values) from presentationwise population spike counts. See the scikit-learn supervised learning tutorial for a guide to supervised learning in Python.