On the right, black lines flank a green star. This star sports a “neon glow” around it, so it looks a little like a green disk.
You will also observe changes – the outer lines disappear, or leave a gap, or are rotated. Also the colour settings vary.
By operating the check boxes, you can see the neon glow vanish when the black inducers disappear (‘autorun’ is automatically disabled). Any gap (due to shrinkage or rotation) destroys the illusion as well. The colourpicker helps you to assess the effect of colour. Any saturated colour works for the inner star.
While I sorted this phenomenon into the colour section, it works also with gray levels. Van Tuijl (1975) was the first to describe this illusion, the gap and rotation effects were described by Redies & Spillmann 1981. The mechanism is unclear.
Varin D (1971) Fenomeni di contrasto e diffusione cromatica nell organizzazione spaziale del campo percettivo. Rivista di Psicologia 65:101–128
Tuijl HFJM van (1975) A new visual illusion: Neonlike color spreading and complementary color induction between subjective contours. Acta Psychologica 39:441–445
Redies C & Spillmann L (1981) The neon color effect in the Ehrenstein illusion. Perception 10:667–681
Redies C, Spillmann L & Kunz K (1984) Colored neon flanks and line gap enhancement. Vision Res 24:1301–1309
Bressan P, Mingolla E, Spillmann L & Watanabe T (1997) Neon color spreading: A review. Perception 26:1353–1366 [PDF]
Kitaoka A, Gyoba J, Kawabata H & Sakurai K (2001) Two competing mechanisms underlying neon color spreading, visual phantoms and grating induction. Vision Res 41:2347–2354