module indexsolutionsall modules1. The recording →

Notebook 0 — Roadmap

SWC ENC 2026 · ephys-pop module

Welcome. Many of you will soon be recording from hundreds of neurons at once with Neuropixels probes, then running a spike sorter — most likely Kilosort — to turn a wall of voltage into sorted spike trains. This module demystifies that black box. Over the day you'll build picosort: a miniature spike sorter, loosely following the stages of Kilosort4, that takes a raw recording all the way to sorted units. This first notebook has no exercises; it's a map.

The problem

A Neuropixels probe measures extracellular voltage at hundreds of sites along a thin shank. When a neuron near the probe fires, it produces a tiny (~tens of µV) biphasic blip that appears — at different amplitudes — on the handful of channels closest to it. Many neurons fire, their blips overlap in time and space, and the whole thing sits in noise.

Spike sorting is the inverse problem: given only the voltage, recover which neuron fired when. That means (1) cleaning up the signal, (2) detecting spikes, (3) describing each spike compactly, (4) grouping spikes into units (putative neurons), and (5) resolving overlapping spikes. Kilosort does all of this; picosort does a simplified, transparent version of each step.

Here's the physical picture — a probe studded with hundreds of recording sites, and a nearby neuron whose spike lands on several channels at once, largest on the closest:

import numpy as np
import matplotlib.pyplot as plt
import picosort as ps

fig = plt.figure(figsize=(11, 6))
ps.plotting.draw_probe_schematic(plt.subplot(1, 2, 1))
ps.plotting.draw_spike_on_probe(plt.subplot(1, 2, 2))
plt.tight_layout(); plt.show()

Why synthetic data?

We generate the recording from a forward model whose ground truth we choose: known neuron positions, known waveforms, known spike times. There's a deep pedagogical reason — you learn a method best when you can check it against a known answer. So every stage you build gets tested the same way: did we recover the spikes we put in? At the end we score picosort against the truth, exactly the thing you can never do on a real recording. That question — recover-the-truth — is the spine of the whole module.

The generator lives in a small backend package, picosort (imported as ps), which also holds a reference implementation of every stage — so any notebook runs on its own, and a stuck exercise has a ps.<name> fallback to keep you moving.

The pipeline, notebook by notebook

Every notebook is short and self-contained — a bit of signal processing or statistics, an exercise or two, a figure at the end — and any stage can be skipped and left for self-study. The stages chain together into the full picosort pipeline; Notebook 8 runs the whole thing and grades it against ground truth.

#	Stage	What you build	What you'll learn
1	The recording	probe geometry + the forward model	how a spike spreads across channels; what ground truth looks like
2	Preprocessing	high-pass filter → common average reference → whitening	why filtering and whitening; removing spatial noise correlations
3	Spike detection	threshold crossings + snippet extraction	thresholds in noise units; aligning and cutting waveforms
4	Feature extraction	PCA on waveforms	dimensionality reduction; why a few components capture spike shape
5	Clustering → templates	a simple graph/greedy clusterer; t-SNE for QC	unsupervised clustering; templates as cluster centroids
6	Template matching	matching pursuit deconvolution	resolving overlapping spikes by iterative peeling
7	Merging & cleanup	cross-correlograms to catch duplicates	refractory periods; over-splitting vs merges
8	Scoring	match sorted spikes to ground truth	precision/recall, hits/misses; what "good sorting" means

The Kilosort4 stages we're simplifying are highlighted along the way; the paper is Pachitariu et al. (2024), Nature Methods. The stages chain left to right:

fig, ax = plt.subplots(figsize=(12, 2.2))
ps.plotting.draw_pipeline(ax); plt.show()

How the notebooks work

Read at your own depth. The material comes in three layers, so you can go as deep as you like on each technique:

1. The big picture — the plain-language prose and the code cells. Enough to understand what each step does and run it. If that's all you want, skim and go.
2. The intuition — worked examples and figures that build up why a method works (e.g. seeing a signal as a mixture of frequencies before we filter it).
3. The math — collapsible ▸ Go deeper blocks with the underlying equations, for those who want them. They're folded away by default; open them if you're curious, skip them freely if not.

• Each notebook comes in two versions: a student copy with # YOUR CODE HERE blanks, and a solutions copy with everything worked out.
• The backend package picosort generates the synthetic recording and holds reference implementations of every stage. Each notebook imports what it needs, so you can run any notebook on its own.
• Wherever we can, an exercise ends by checking your result against ground truth, and tells you roughly what to expect so you can spot a bug.
• Stuck on an exercise? Don't let it block your day. Every stage also lives in the backend as ps.<name> (and fully worked out in the solutions copy), so you can drop that in, keep pace, and circle back later.

Run the cell below to confirm your environment is set up (select the "SWC Ephys-Pop (.venv)" kernel if prompted). If it prints a recording's shape and a plot appears, you're ready for Notebook 1.

import numpy as np
import matplotlib.pyplot as plt
import picosort as ps

# Generate one synthetic recording and take a quick look -- no exercise here,
# just a check that everything is installed and importable.
rec = ps.make_recording(n_units=6, duration_s=5.0, seed=0)
gt = rec.ground_truth
print("environment OK — generated a recording")
print(f"  traces: {rec.traces.shape} samples x channels, {rec.duration_s:.1f} s at {rec.fs/1e3:.0f} kHz")
print(f"  ground truth: {gt.n_units} units, {len(gt.spike_times)} spikes total")

ps.plotting.plot_traces(rec, channels=range(8), t0=0.10, t1=0.16)
plt.show()

Glossary — terms you'll meet

You don't need these yet; come back when a word trips you up. Each is defined properly where it first appears (in parentheses).

term	in plain words
channel / site	one recording electrode on the probe; it has a position (NB1)
template / footprint	a unit's stereotyped spike shape across channels and time; the footprint is its amplitude per channel (NB1)
unit	a putative single neuron — a cluster of spikes the sorter believes came from one cell (NB0)
common average reference (CAR)	subtract the across-channel average to kill shared artefacts (NB2)
high-pass filter	remove slow (<300 Hz) fluctuations so fast spikes stand out (NB2)
whitening	de-correlate the noise across channels so a threshold means the same thing everywhere (NB2)
threshold crossing	a candidate spike: voltage dips past N noise standard deviations (NB3)
snippet / waveform	the short multi-channel voltage clip cut out around a detected spike (NB3)
PCA / principal component	a few directions that capture most of the variation in waveform shape (NB4)
feature space	each spike as a handful of numbers (its PC scores) instead of a full waveform (NB4)
clustering	grouping spikes with similar features into units, with no labels (NB5)
t-SNE	a 2-D layout for eyeballing whether clusters are well separated (NB5)
matching pursuit	explain the trace by repeatedly subtracting the best-fitting template (NB6)
cross-correlogram / refractory period	spike-timing histogram between two units; a real neuron can't fire twice within ~1–2 ms (NB7)
precision / recall	of the sorted spikes, how many are real; of the real spikes, how many were found (NB8)

Ready

That's the whole pipeline: from a wall of voltage to sorted spike trains, every step checked against a truth you control. On to Notebook 1, where we build the recording itself.

 module indexsolutionsall modules1. The recording →