Figure 6.

Efficiency as a function of wiggle of orientation (solid symbols), offset (open symbols), and phase (x-in-square symbol). The observers are C.J.C. (squares) and A.S. (circles). We measured threshold contrast energy (integrated square contrast) for letter identification as we perturbed grating orientation, offset, and relative phase. We implemented the ideal observer as a computer program and measured its threshold contrast energy for the same letter sets as those used for the human observer. The ratio of energy threshold of ideal to human is efficiency, an absolute scale that allows us to compare human performance across all our conditions (Pelli & Farell, 1999). Furthermore, it allows us to compare our new results to previous results for letters of various fonts, alphabets, and sizes. The bent line is the best fit of the clipped reciprocal η₀(w) = η₀ / min(1, w/w₀), where w is wiggle, with two degrees of freedom, η₀ = .074 and w₀ = 15 deg, to minimize the fitting error (log η–log η),, where η is efficiency, and rms is root mean square.