BCM rule: dw/dt = φ(y, θ_M)·x — the synaptic modification threshold θ_M slides with average activity y². Explains orientation selectivity and ocular dominance in visual cortex.
The sliding threshold θ_M = ⟨y²⟩/y₀ allows LTP when y>θ_M and LTD when y<θ_M. This BCM nonlinearity φ(y,θ_M)=y(y-θ_M) ensures stability and selectivity simultaneously.