The principle
Tishby, Pereira and Bialek (1999) framed learning as finding a compressed representation Z of an input X that keeps as much information as possible about a target Y: minimise I(X;Z) while keeping I(Z;Y) high, with a single parameter trading the two off.
The tool's formulas
Retention = 45% × (bottleneck ÷ 16) + 55% × (1 − noise × (1 − threshold)). Accuracy starts at 99%, loses 18 points per unit below a bottleneck of 3, and loses noise × 25 × (1 − threshold). At the defaults (4 units, threshold 0.65, noise 0.35) that gives 59.5% retained, 34.1% discarded and 95.9% accuracy. These are illustrative rules of thumb, not measurements from a trained network.
What's random on the page
The 64-cell input matrix, the 'Input Shannon Entropy' figure and which links are drawn as pruned are regenerated with random numbers on every pass, so they change each time you move a slider. The entropy value is also divided by 10 for display, so treat it as a visual cue rather than a real measurement in bits.
A debated theory of deep learning
Shwartz-Ziv and Tishby (2017) reported that deep networks first fit their training data and then compress their internal representations. Saxe and colleagues (ICLR 2018) found that this compression phase depends on the activation function and is not needed for good generalisation, so the bottleneck view is a useful lens rather than a settled law.
Bottlenecks you can train
The Deep Variational Information Bottleneck (Alemi et al., 2016) turns the idea into a practical training objective, and a narrow latent layer is the core of autoencoders. In real models, too wide a bottleneck lets noise through and too narrow a one loses the signal; this tool only models the second failure.
Numbers in the video come from the simulator's simplified formulas at the Noisy Digit Recognition defaults; real networks do not report retention or accuracy this way.