PURPOSE. To determine the influence of the spatial extent and the depth of localized field loss on the computation of the General Height (GH) method for estimating the diffuse component of visual field loss and, therefore, on the subsequent appearance of the Pattern Deviation (PD) map. METHODS. Varying shapes and depths of localized glaucomatous field loss were modeled in each of 82 Humphrey Field Analyzer Program 30-2 fields (82 normal eyes) by superimposing, on the fields from the normal

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... the PD defect depth (P Ͻ 0.05) of one visual field from each of 123 patients with glaucoma. The difference in GH between each of the 10,086 modeled fields and the GH in the corresponding measured normal field was derived and the relationship to possible differences in the location-by-location significance levels of each pair of PD maps determined. RESULTS. For the group mean overestimation in the GH of Ϫ0.79 dB, the 50th, 90th, and 95th percentiles of the modeled fields described 23.4%, 37.9%, and 42.6% locations, respectively, exhibiting an underestimation in PD statistical significance. As the size of the superimposed field loss increased, the overestimation of the GH increased, and the underestimation of PD statistical significance became more apparent. This error was independent of defect depth. CONCLUSIONS. The localized component of field loss in glaucoma produced an overestimation of diffuse loss and a consequent underestimation of the severity of focal loss by PD analysis. This effect increased as the spatial extent of the loss became more extensive and will lead to an underestimation of progressive localized field loss. (Invest Ophthalmol Vis Sci.