---
title: "The Trolley Problem · Fly Brain Experiment · TheFlyBrain"
url: https://theflybrain.com/experiments/trolley-problem/
description: "The fly stands at the switch lever of a trolley junction with selectable items on both tracks. A measured readout over a 4 s deliberation window, either its…"
language: en
---

# The Trolley Problem

The fly stands at the switch lever of a trolley junction with selectable items on both tracks. A measured readout over a 4 s deliberation window, either its integrated DNa02 right-minus-left rate or any DNp01 giant-fiber spike, decides whether the lever is pulled.

**An eight-second loop from the workstation. Full recording pending publication.** [Follow along](#recordings) for the release.

## What the fly saw

Frames rendered from the fly’s eye camera at four points in the timeline, each with the 32-column retinal input the connectome model receives at that moment.

![Eye-camera frame at 0:05](https://theflybrain.com/images/eye/trolley-problem-0.jpg) ![32-column retinal input at 0:05](https://theflybrain.com/images/eye/trolley-problem-0-retina.png)

0:05 The dilemma

![Eye-camera frame at 0:15](https://theflybrain.com/images/eye/trolley-problem-1.jpg) ![32-column retinal input at 0:15](https://theflybrain.com/images/eye/trolley-problem-1-retina.png)

0:15 Deliberation window (4 s)

![Eye-camera frame at 0:25](https://theflybrain.com/images/eye/trolley-problem-2.jpg) ![32-column retinal input at 0:25](https://theflybrain.com/images/eye/trolley-problem-2-retina.png)

0:25 Through the junction

![Eye-camera frame at 0:35](https://theflybrain.com/images/eye/trolley-problem-3.jpg) ![32-column retinal input at 0:35](https://theflybrain.com/images/eye/trolley-problem-3-retina.png)

0:35 Riding off

## Timeline

7 steps, with their times in the experiment

1.  0:00
    
    **The dilemma**
    
    A trolley is coming. Track A runs straight on; track B is diverted toward the viewer. The fly stands at the switch lever, facing both tracks. The board lists what is on each one.
    
2.  0:08
    
    **The trolley approaches**
    
    The trolley rolls toward the junction, bell ringing. Only luminance reaches the fly’s 32-column retina: the items are patches of brightness.
    
3.  0:15
    
    **Deliberation window (4 s)**
    
    Twenty measured 200 ms windows. Measured steering integrates DNa02 R − L; the giant-fiber source watches for any DNp01 spike. The scene waits for each window.
    
4.  0:19
    
    **The lever**
    
5.  0:22
    
    **Through the junction**
    
    The trolley takes the selected route through the points, or drifts across both tracks at once.
    
6.  0:26
    
    **Cushioned stop**
    
    The trolley brakes. Its padded cowcatcher scoops everything on its path into a cushioned bin: BONK. Sleeping flies wake up and climb aboard.
    
7.  0:32
    
    **Riding off**
    
    The trolley rolls away with its passengers waving from the cowcatcher. The session tally counts decisions per item pairing.
    

## Setup

-   Philippa Foot posed the runaway-tram case in 1967 and Judith Jarvis Thomson named it the trolley problem in 1976; the layout follows the internet comic's diagram of a trolley, two tracks and a figure at the lever, attributed to Jesse Prinz.
-   Seven cues over 39 s: the dilemma, the approach, a 4 s deliberation window, the lever, the junction, a cushioned stop and the ride-off; a red-and-cream early-1900s streetcar numbered 237 runs through a pastoral junction at fly scale.
-   Both tracks draw on one catalogue of 24 entries, including nothing, sleeping flies in beds, rubber ducks, lobsters, a cat in a box, a Stradivarius violin, a replica Mona Lisa, a pile of cash, an AI server rack, a snowman and a goldfish in a bowl; countable items take 1–5, and the default puts five sleeping flies on track A and one on track B.
-   The fly faces both tracks' items; during deliberation the switch stand, label board and approaching trolley are outside its eye field, and the lever enters the eye image only after the decision.
-   Outcomes are non-graphic: the trolley takes the selected route and brakes, a padded cowcatcher scoops that track's items into a cushioned bin, and sleeping flies wake and ride off waving.
-   Cameras: Classic side-on diagram, Lever close-up, Trolley-front POV, Fly POV / both tracks, Overhead junction, Items on both tracks, Label board.

## Measurement

-   Retinal-only input; measured readouts are [DNa02](https://theflybrain.com/neurons/dna02/) (steering) and [DNp01](https://theflybrain.com/neurons/dnp01-giant-fiber/) (giant-fiber escape).
-   The deliberation window is twenty 200 ms windows anchored at its start; the scene waits at each anchor until that window is measured, and windows off the anchors are shown but never counted.
-   Steering rule: pull if Σ (DNa02 R − L) × 0.2 s over the 20 windows exceeds the threshold (0–5 Hz·s, default 0.5); giant-fiber rule: pull if any anchored window contains a DNp01 spike; without all 20 windows the result is "not measured" and the lever stays at track A.
-   The lever throw and the fly's foreleg and head movements are pre-programmed; always pull, never pull, a seeded coin flip and multi-track drifting are pre-programmed alternatives, labeled as such.

## Limitations

-   The items reach the 32-column retina only as luminance: a dark server rack against bright grass contributes negative luminance and a flat lobster barely registers, so the readout responds to brightness, not to what the items are.
-   Every complete deliberation window measured live on 16 October 2026 ended in no pull: DNa02 fired as sparse single 5 Hz spikes, mostly on the left.
-   No DNp01 spike occurred in 20 anchored windows.
-   Whether the readout pulls depends on spike noise and the threshold; neural weights are fixed and nothing is learned.
-   The 32 × 18 eye image point-samples the scene, so objects thinner than a pixel row can drop out; the sleeping flies lie in beds with tall head- and footboards so that they register.

## Sources

-   [Judith Jarvis Thomson (1976), Killing, Letting Die, and the Trolley Problem, The Monist](https://doi.org/10.5840/monist197659224)
-   [Judith Jarvis Thomson (1985), The Trolley Problem, Yale Law Journal](https://openyls.law.yale.edu/entities/publication/f07d4886-dee1-40df-a492-0c6151f292d0)
-   [Know Your Meme, The Trolley Problem (comic attributed to Jesse Prinz)](https://knowyourmeme.com/memes/the-trolley-problem)

## About

The fly stands at the switch lever of a trolley junction with selectable items on both tracks. A measured readout over a 4 s deliberation window, either its integrated DNa02 right-minus-left rate or any DNp01 giant-fiber spike, decides whether the lever is pulled.

[theflybrain.com/experiments/trolley-problem](https://theflybrain.com/experiments/trolley-problem/)

[labs-and-senses](https://theflybrain.com/experiments/labs-and-senses/) [drosophila](https://theflybrain.com/methods/) [connectome](https://theflybrain.com/methods/) [malecns](https://theflybrain.com/methods/) [specimen-237](https://theflybrain.com/specimen-237/)

-   Subject [Specimen 237](https://theflybrain.com/specimen-237/)
-   Readme
-   MaleCNS data · CC BY 4.0
-   Views not yet tallied
-   Duration **39 s**
-   Cues **7**
-   Deliberation window **4 s (20 × 200 ms)**
-   Track items **24**
-   Default threshold **0.5 Hz·s**

## Recordings

No recordings published yet.

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## Packages

[workstation source](https://www.patreon.com/cw/ParanoidAmerican/membership)Included for Research Associates · [setup guide](https://theflybrain.com/workstation/setup/)

## Credits 2

[![Specimen 237](https://theflybrain.com/images/specimen-237.jpg)](https://theflybrain.com/specimen-237/) [PA](https://paranoidamerican.com)

Official Experiment

## Notes

-   Setup33%
-   Measurement22%
-   Limitations28%
-   Sources17%

## Neurons

-   [DNp01](https://theflybrain.com/neurons/dnp01-giant-fiber/)
-   [DNa02](https://theflybrain.com/neurons/dna02/)

[All neuron pages](https://theflybrain.com/neurons/)

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---
Source: https://theflybrain.com/experiments/trolley-problem/ · TheFlyBrain (theflybrain.com) · Site summary for language models: https://theflybrain.com/llms.txt