The Adelson - Bergen energy model (1985 Journal of the Optical Society of America A 2 284 – 299) is a standard framework for understanding 1st-order motion processing. Its output, the opponent energy for a given input, is calculated by subtracting one directional energy measure (R) from its opposite (L), and its sign indicates the direction of motion of the input. Our observers viewed a dynamic sequence of gratings (1 cycle deg−1) equivalent to the sum of two gratings moving in opposite directions with different contrasts, and had to report the dominant direction of motion. The ratio of contrasts was varied across trials. We found that opponent energy was a very poor predictor of direction discrimination performance. Heeger (1992 Visual Neuroscience 9 181 – 197) has suggested that divisive inhibition amongst striate cells requires a contrast gain control in the energy model. A new metric can be formulated in the spirit of Heeger's model by normalising the opponent energy (L-R) with flicker energy, the sum of the directional motion energies (L+R). This new measure, motion contrast (L-R)/(L+R), was found to be a good predictor of direction discrimination performance over a wide range of contrast levels, but opponent energy was not. Discrimination thresholds expressed as motion contrast were around 0.5 for the sampled drifting gratings used in our experiments, corresponding to an energy ratio (L:R or R:L) of 3:1 at discrimination threshold. Such high values suggest that the outputs of motion energy filters are very noisy (variable over space and time) or that the use of them is inefficient.
