Sensitivities in luminance and amplitude may not predict sensitivity to shape-from-shading

In this study we investigated the effects of age on second-order vision and how the loss of first-order (carrier) visibility and coarse-scale second-order visibility can affect how we sensitive we are to shape-from-shading. Human vision is sensitive to first-order and second-order characteristics. Modulations in luminance are characterised as first-order stimuli whereas contrast and amplitude modulations of luminance patterns are second-order stimuli. It was proposed that first- and second-order signals combine to provide depth information, aiding the observer to discriminate changes in illuminance vs texture (Schofield, Hesse, Rock, & Georgeson, 2006). These visual cues can be replicated by modulating a high spatial frequency carrier with low-frequency amplitude and luminance modulations. To investigate this, observers (18-25 and 60+) completed a 2afc orientation discrimination task with a sinusoidal plaid composed of one in-phase LM+AM grating and one out-of-phase LM-AM grating. Observers' sensitivities were also estimated for noise carriers and course scale amplitude modulations in detection tasks (4, 8 and 12 cpd). We found that performance in the carrier sensitivity and amplitude modulation task did not significantly predict performance in the shape-from-shading orientation discrimination task. In conclusion, we find that older observers’ performance in detecting shape-from-shading cannot be explained by reduced sensitivities to coarse-scale luminance and amplitude modulations. [Economic and Social Research Council.]