Colour and luminance interactions in the visual perception of motion

We sought to determine the extent to which red–green, colour–opponent mechanisms in the human visual system play a role in the perception of drifting luminance–modulated targets. Contrast sensitivity for the directional discrimination of drifting luminance–modulated (yellow–black) test sinusoids was measured following adaptation to isoluminant red–green sinusoids drifting in either the same or opposite direction. When the test and adapt stimuli drifted in the same direction, large sensitivity losses were evident at all test temporal frequencies employed (1–16 Hz). The magnitude of the loss was independent of temporal frequency. When adapt and test stimuli drifted in opposing directions, large sensitivity losses were evident at lower temporal frequencies (1–4 Hz) and declined with increasing temporal frequency. Control studies showed that this temporal–frequency–dependent effect could not reflect the activity of achromatic units. Our results provide evidence that chromatic mechanisms contribute to the perception of luminance–modulated motion targets drifting at speeds of up to at least 32°s-1. We argue that such mechanisms most probably lie within a parvocellular–dominated cortical visual pathway, sensitive to both chromatic and luminance modulation, but only weakly selective for the direction of stimulus motion.

Humans assume a mixture of diffuse and point-source lighting when viewing sinusoidal shading patterns

Observers perceive sinusoidal shading patterns as being due to sinusoidally corrugated surfaces, and perceive surface peaks to be offset from luminance maxima by between zero and 1/4 wavelength. This offset varies with grating orientation. Physically, the shading profile of a sinusoidal surface will be approximately sinusoidal, with the same spatial frequency as the surface, only when: (A) it is lit suitably obliquely by a point source, or (B) the light source is diffuse and hemispherical--the 'dark is deep' rule applies. For A, surface peaks will be offset by 1/4 wavelength from the luminance maxima; for B, this offset will be zero. As the sum of two same-frequency sinusoids with different phases is a sinusoid of intermediate phase, our results suggest that observers assume a mixture of two light sources whose relative strength varies with grating orientation. The perceived surface offsets imply that gratings close to horizontal are taken to be lit by a point source; those close to vertical by a diffuse source. [Supported by EPSRC grants to AJS and MAG].

The role of texture amplitude in shape from shading

When a textured surface is modulated in depth and illuminated, parts of the surface receive different levels of illumination; the resulting variations in luminance can be used to infer the shape of the depth modulations-shape from shading. The changes in illumination also produce changes in the amplitude of the texture, although local contrast remains constant. We investigated the role of texture amplitude in supporting shape from shading. If a luminance plaid is added to a binary noise texture (LM), shape from shading produces perception of corrugations in two directions. If the amplitude of the noise is also modulated (AM) such that it is in-phase with one of the luminance sinusoids and out-of-phase with the other, the resulting surface is seen as corrugated in only one directionöthat supported by the in-phase pairing. We confirmed this subjective report experimentally, using a depth-mapping technique. Further, we asked naïve observers to indicate the direction of corrugations in plaids made up of various combinations of LM and AM. LM+AM was seen as having most depth, then LM-only, then LM-AM, and then AM-only. Our results suggest that while LM is required to see depth from shading, its phase relative to any AM component is also important.