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. 2005 May;89(5):569–574. doi: 10.1136/bjo.2004.050120

Figure 1.

Figure 1

 A schematic of Tranquair’s analogy “an island of vision in a sea of blindness” displays the essential elements behind TAG testing. Reducing contrast reduces the stimulus; this is analogous to cutting through the island near its highest point. The standard AG cuts the lower part of the island where retinal sensitivity is low, while the TAG cuts the island where retinal sensitivity is higher. Relative scotoma is represented by the black area. Suprathreshold standard Amsler grid (AG), red Amsler grid (RAG), and threshold Amsler grid (TAG).