Pedestal effect in visual motion discrimination.

Many sensory discriminations, including the discrimination of speed, obey Weber's law and thus become more difficult as the stimuli get larger. Using one-jump apparent motion stimuli, we find that the opposite can occur: displacement discrimination improves with larger jumps. This pedestal effect occurs for small jumps near and below the detection threshold. Finding a pedestal effect in motion discrimination confirms a speed energy model developed in previous experiments on the detection of jump pairs, since the pedestal effect will be observed if the visual system detects the energy of the speed waveform. Once the size of the jumps becomes large enough, the discriminability declines, indicating masking. Masking is just the detectability counterpart of Weber's law; it is not predicted from energy detection. The pedestal effect shows the presence of a squaring nonlinearity for small speed signals, and masking indicates linear transduction for large signals. A half-wave rectifier, when presented with Gaussian noise, behaves this way. The speed energy model can be seen as an approximation, valid for small signals, to a model that includes half-wave rectification.