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Deep Learning — How Neural Networks Actually Think - Leejohnston - 11-17-2025
Thread 3 — Deep Learning: How Neural Networks Actually Think
Understanding the Mind of a Neural Network
Deep learning powers image recognition, natural language models, voice assistants, and modern AI systems.
But how do neural networks actually “think”?
What happens inside the layers?
This thread breaks down the core mechanisms behind deep learning in a clear, intuitive way.
1. The Core Idea: Learning From Patterns
A neural network learns by adjusting millions (or billions) of tiny numerical values called weights.
These weights determine which patterns matter — edges in images, word meanings in text, shapes, sounds, etc.
Neural networks don’t follow rigid rules.
They learn structure from data.
Example:
• A network sees thousands of cat images
• It keeps adjusting weights
• Eventually, some neurons fire strongly for cat-like patterns
• It forms abstract concepts: shapes → edges → fur → full cat
This hierarchy of concepts is called feature learning.
2. Neurons, Layers & Activations
A neural network is built from:
• Input layer
• Hidden layers
• Output layer
Each neuron applies:
1. A weighted sum
2. A nonlinear activation function
Common activations:
• ReLU — fast and simple
• Sigmoid — probabilities
• Tanh — centred signals
Nonlinearity is what makes the network capable of learning complex patterns.
3. Forward Pass: How a Network Makes a Prediction
When data enters the model:
1. Each layer transforms it
2. Patterns become more abstract
3. The final layer produces the prediction
Example:
Image → edges → shapes → object → “cat: 0.97 confidence”
This process is called the forward pass.
4. Backpropagation: How the Model Learns
Learning happens during training via:
– Loss function
Measures how wrong the model’s prediction is.
– Backpropagation
Calculates how each weight contributed to the error.
– Gradient descent
Updates weights in the direction that reduces error.
The model improves by gradually lowering its loss.
5. Deep Networks Learn Hierarchically
Early layers learn:
• lines
• edges
• colors
Middle layers learn:
• patterns
• shapes
• textures
Late layers learn:
• objects
• semantics
• concepts
This mirrors the structure of the human visual cortex.
6. Why Deep Learning Works So Well
Deep learning succeeds because it can:
• learn representations automatically
• find patterns humans would never notice
• scale with massive data
• adapt to many tasks
It’s flexible, powerful, and general.
7. Real-World Applications
Deep learning powers:
• self-driving cars
• medical image analysis
• voice assistants
• translation systems
• robotics
• ChatGPT-like large language models
Its capabilities continue to grow rapidly.
Final Thoughts
Deep learning is the backbone of modern AI.
Understanding how networks think reveals not just “what AI does,” but why it works.
If you’d like, we can dive deeper into:
• loss landscapes
• optimisers
• layer architectures
• or advanced model training
Just let me know. ?