Neural Stimulation

We’ve learned how to record spikes—now let’s try sending signals to neurons. Can the alternating current from a phone’s earbud output make a cockroach leg move?

Devices and Accessories4

About the Experiment

What Will You Learn?

How external electrical currents can excite neurons.
How music signals differ from pure tones—and why that matters.
Why frequency and amplitude jointly determine stimulation thresholds.

Background

In the 1780s, Luigi Galvani showed that electricity makes frog legs twitch, launching the study of bioelectricity. Two centuries later, deep-brain stimulation treats Parkinson’s disease with a related idea: tiny electrical pulses change how neurons fire. Here you’ll swap a lab stimulator for a phone’s headphone output to explore which waveforms move a cockroach leg with the least current.

Experiment

Music Stimulation
Setup
1. Mount a cockroach leg so the tibia can move freely; insert two pins into the femur.
2. Clip the red stimulation cable (included with the SpikerBox) to the pins, then plug the TRRS jack into your phone.
Test 1 – Play a hip-hop track (e.g. one from the Beastie Boys’ Paul’s Boutique). Start at low volume and raise it slowly. Note the volume where the leg twitches.
Test 2 – Repeat with a classical piece (e.g. Bach’s Goldberg Variations). Which genre moves the leg more easily?
Tone Stimulation
Pure-Tone Sweep
1. Install a free tone-generator app.
2. Test frequencies 20 Hz, 50 Hz, 100 Hz, 200 Hz, 500 Hz, 1 kHz, 2 kHz, 5 kHz.
3. For each tone, start at 25% volume and increase to 50%, 75%, and 100% until the leg twitches. Record the lowest volume that works.
Compare your threshold chart to discover the frequency that stimulates neurons with the least current.

Results & Analysis

Music – At what volume did each track evoke movement? Did rhythm-heavy hip-hop or steady classical patterns require less current?

Tones – Plot stimulation threshold (volume) versus frequency. Expect lower frequencies (longer pulses) to recruit neurons more readily than high-frequency tones.

Think about how pulse width, amplitude and electrode placement interact, and how the same principles scale up to medical devices like DBS.

Draw what your stimulus does: phone, cable, the two pins, the nerve in the leg, the leg muscle, movement. Mark where the current enters the nerve, and explain why the leg can move with no brain attached.

Design Challenge

The Most Efficient Stimulus
Medical stimulators, like the ones used for deep-brain stimulation, are designed to work with as little current as possible. Your threshold chart is your test data. Now design the most efficient stimulus you can!
1. Rank your criteria: the leg moves every time, at the lowest volume, with a stimulus you can describe and repeat.
2. Pick at least three candidates from your data (a tone frequency or a song) and test each one three times, recording the lowest volume that moves the leg.
3. Compare your candidates in a table, combine the best features into one final stimulus, and test it.
4. Explain your trade-offs: what did you give up to lower the current?
5. Judge your final stimulus as if it were for a patient’s implant: how long would a battery last at that volume (cost), is it gentle enough to use every day (safety), and who could or could not be helped by it?

NGSS alignment

Performance expectations

HS-LS1-2 Develop and use a model to illustrate the hierarchical organization of interacting systems that provide specific functions within multicellular organisms.

Draw what the current does: phone, pins, nerve, muscle, movement; nerve and muscle in the leg respond together.
MS-ETS1-2 Evaluate competing design solutions using a systematic process to determine how well they meet the criteria and constraints of the problem.

Rank criteria, then compare at least three candidate stimuli in a table (design challenge).
MS-ETS1-3 Analyze data from tests to determine similarities and differences among several design solutions to identify the best characteristics of each that can be combined into a new solution to better meet the criteria for success.

Test at least three stimulus designs, compare their thresholds in a table and combine the best features into a final stimulus (design challenge).
HS-ETS1-3 Evaluate a solution to a complex real-world problem based on prioritized criteria and trade-offs that account for a range of constraints, including cost, safety, reliability, and aesthetics, as well as possible social, cultural, and environmental impacts.

Weigh least current against reliability and judge the stimulus as if for a patient's implant: battery cost, everyday safety, who it could help (design challenge).

Science and engineering practices

1 Asking Questions and Defining Problems

Define the problem: which stimulus moves the leg with the least current, every time (design challenge).
2 Developing and Using Models

Draw what your stimulus does, from phone to moving leg, and mark where the current enters the nerve.
3 Planning and Carrying Out Investigations

Find the volume at which music and pure tones from 20 Hz to 5 kHz move the leg.
4 Analyzing and Interpreting Data

Compare thresholds across tracks and frequencies, plot threshold against frequency, and compare candidate stimuli in a table.
6 Constructing Explanations and Designing Solutions

Design and refine a stimulus from the test data (design challenge).

Crosscutting concepts

Cause and Effect

Frequency and volume decide whether the leg moves.
Influence of Science, Engineering, and Technology on Society and the Natural World Engineering connection

The same principles scale up to medical devices such as deep-brain stimulation.

Disciplinary core ideas

LS1.A Structure and Function

Nerves and muscles in the leg are specialized to respond.
ETS1.B Developing Possible Solutions

Test several solutions and weigh reliability against current.
ETS1.C Optimizing the Design Solution

Use test data to pick and combine the best design.

Backyard Brains mapping using the book's method. Not in the book. Engineering claims rest on the design challenge on this page (Greg, 2026-10-06; real-user judgment step added 2026-10-08), modelled on the SpikerBot curriculum.