Spatial integration within and between first- and second-order stimuli

The detection of first-order (luminance-modulated—LM) and second-order (contrast-modulated—CM) stimuli is believed to involve separate mechanisms that interact weakly or are entirely independent; detection of an LM-only stimulus is barely improved by the addition of CM. However, little is known about the integration of stimuli comprising non-overlapping regions of LM and CM. Spatial summation of LM, CM and LM+CM targets was assessed using (i) full 1.25 c deg -1gratings of different sizes (1–16 cycles), (ii) fixed-diameter targets whose signal area was controlled by modulating a large (8 or 16 cycles) ‘full’ grating with a raised plaid pattern. The noise carrier (also present for LM stimuli) was bandpass-filtered white noise (8 c deg -1, ±0.5 octaves). We find that sensitivity improves with target size more rapidly for LM than for CM. When area was constant, comparing full and modulated stimuli yielded summation of ∼5dB for both CM and LM. We also investigated cross-order summation, which was weak (∼2dB) for full CM+LM (threshold adjusted) stimuli, but stronger (∼3dB) when first and second order stimuli were interdigitated over area. This suggests a mechanism capable of integrating textures with attributes that vary over space, perhaps owing to changes in illumination or material properties.