Token receptive fields on the LayerNorm hypersphere

Toy model with n_hidden = 4: LayerNorm's mean-subtraction removes the 1 direction, so normalised hidden states live on a visible 2-sphere. Each dot is a token: direction ui, dot size ∝ scale si (row norm after folding γ into lm_head). Drag to rotate; click to place a probe hidden state.
Orthographic view (front hemisphere). Click = probe. Ring = perturbation cone of angle ε.
Lambert equal-area map of the whole sphere: visual area is proportional to true catchment area. + marks current view centre.
Vocabulary
Softmax
Softmax gauge (the "lost" dof)
Adds the same vector c to every lm_head row. Probabilities are identical (fields don't move), but naive "normalise each row" directions and norms (dots, orange in chart) drift. Fix the gauge before step (2).
Pruning
Brittleness probe
Statistics
Probe
Catchment area per token (sorted)
Bars = argmax area (full vocab). White dots = gauge-fixed scale si. Orange = naive row norm under shift c. Grey bars = pruned.
What is being drawn

Cohere's LayerNorm gives x̂ = (h − mean(h)) / std(h), so x̂ ⟂ 1 and ‖x̂‖ = √d. Folding the gain γ into lm_head and keeping only the part of each row that x̂ can see: w̃ᵢ = P⊥1(γ ⊙ wᵢ) − c, sᵢ = ‖w̃ᵢ‖, uᵢ = w̃ᵢ / sᵢ, giving logitᵢ = logit_scale·√d · sᵢ · cos θᵢ (+ a term common to all tokens). With d = 4 the unit sphere in the 3-dim subspace ⟂ 1 is an ordinary S², which is this picture.

Softmax's invariance to adding a constant to all logits is a freedom in parameters (the shift c), not in hidden space, so it does not cost a dimension of the sphere. It does mean "row norm" is only meaningful after fixing c (e.g. subtract the mean row).

The hard argmax fields are a multiplicatively weighted spherical Voronoi diagram: equal sᵢ gives ordinary Voronoi cells, and larger sᵢ inflates a cell. A token is reachable by greedy decoding iff its point sᵢuᵢ is a vertex of the convex hull of all {sⱼuⱼ}, which is gauge-invariant. Try σ ≈ 1 to see tokens with zero area. β sets how soft the boundaries are; for Command-R 35B, 0.0625·√8192 ≈ 5.66 multiplies sᵢ.

Caveat: areas here assume hidden states uniformly distributed on the sphere. Real hidden states are concentrated, so for actual pruning, weight catchment by argmax counts or softmax mass over real calibration activations.