(A) Responses were simulated for four model neurons. The dominance weight for modality 1, *d*_{1}, was fixed at unity while the dominance weight for modality 2, *d*_{2}, decreased from left to right (denoted by the number of receptive field contours). Input1 (‘+’) and Input2 (‘×’) were presented in the center of the receptive fields. (B) Responses as a function of intensity are shown for Input1 (red), Input2 (blue), and both inputs together (black). Format as in . (C) Additivity Index is plotted as a function of intensity for the four model neurons; format as in . (D) Summary of multisensory integration properties for a population of neurons from area VIP (black symbols), re-plotted from ref ^{}. The ordinate shows a measure of response additivity: (Bi – (U1 + U2)) / (Bi + (U1 + U2)) × 100, for which positive and negative values indicate superadditive and sub-additive interactions, respectively. Bi: bimodal response; U1, U2: unimodal responses. The abscissa represents a measure of response enhancement: (Bi – max (U1, U2)) / (Bi + max (U1, U2)) × 100, for which positive and negative values denote cross-modal enhancement and cross-modal suppression, respectively. Colored symbols represent predictions of the normalization model for units that vary in the ratio of dominance weights (d_{2}/d_{1}, ranging from 0 to 1) and the semi-saturation constant, α, ranging from 1 to 16. The exponent, *n*, was 2.5. Numbered symbols correspond to model neurons for which responses are shown as bar graphs (right).

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