Figure 9. Color sensitivity indices of V4 neurons and the goodness of fit of linear cone excitation models with or without rectification. A, Color sensitivity index, SI, is the ratio of the maximum response among equiluminant stimuli, Re, to the maximum response among achromatic stimuli, Ra. Each point shows SI for one cell. Color represents the classification from Figure 4. The dotted lines identify the range of sensitivity indices (0.5 < SI < 2.0) that were classified as color luminance cells by Johnson et al. (2001). Sensitivity indices of the vast majority of V4 neurons, except most equiluminance cells, fell in this range. Most equiluminance cells, however, had sensitivity indices >2.0. For 14 cells (3 bright, 1 dark, 9 equiluminance, and 1 flat cell), Ra was zero; these cells are shown at the right extreme (note broken x-axis). B, Histogram of goodness of fit values for bright (top left), dark (top right), contrast (bottom left), and equiluminance (bottom right) cells. For each cell, we fit the neuronal responses with linear cone excitation models with or without rectification (see Materials and Methods). Goodness of fit (x-axis) is given by the percentage of variance explained by the best fitting model. Some bright, dark, and contrast cells were well described by a rectified linear model, but for most equiluminance cells, linear cone excitation-based models provided a poor fit of the data (see Results). Mean goodness of fit for bright, dark, contrast, and equiluminance cells were 35.7, 50.1, 32.0, and 17.9, respectively.