The spatial tuning of opponent-motion normalization

AbstractThe final stage of the Adelson–Bergen model [J. Opt. Soc. Am. A 2 (1985) 284] computes net motion as the difference between directionally opposite energies EL and ER. However, Georgeson and Scott-Samuel [Vis. Res. 39 (1999) 4393] found that human direction discrimination is better described by motion contrast (Cm)––a metric where opponent energy (EL−ER) is divided by flicker energy (EL+ER). In the present paper, we used a lateral masking paradigm to investigate the spatial properties of flicker energy involved in the normalization of opponent energy. Observers discriminated between left and right motion while viewing a checkerboard in which half of the checks contained a drifting sinusoid and the other half contained flicker (i.e. a counterphasing sinusoid). The relative luminance contrasts of flicker and motion checks determined the checkerboard's overall motion contrast Cm. We obtained selectivity functions for opponent-motion normalization by measuring Cm thresholds whilst varying the orientation, spatial frequency, or size of flicker checks. In all conditions, performance (percent correct) decayed lawfully as we decreased motion contrast, validating the Cm metric for our stimuli. Thresholds decreased with check size and also improved as we increased either the orientation or spatial-frequency difference between motion and flicker checks. Our data are inconsistent with Heeger-type normalization models [Vis. Neurosci. 9 (1992) 181] in which excitatory inputs are normalized by a non-selective pooling of inhibitory inputs, but data are consistent with the implicit assumption in Georgeson and Scott-Samuel's model that flicker normalization is localized in orientation, scale, and space. However, our lateral masking paradigm leaves open the possibility that the spatial properties of flicker normalization would be different if opponent and flicker energies spatially overlapped. Further characterization of motion contrast will require models of the spatial, temporal, and joint space–time properties of mechanisms mediating opponent-motion and flicker normalization

Dermo-optical perception: the non-synesthetic "palpability of colors" a comment on Larner (2006)

We comment on Larner's (2006) recent description of the seventeenth-century case of a blind man who could differentiate the color of objects by touch. This ability is generally known as "dermo-optical perception" and is due to the cutaneous temperature sense rather than to synesthetic processing. Although devoid of references to the phenomenon of dermo-optical perception, Larner's communication is highly valuable because it raises several issues relevant to present-day neurosciences. These comprise functional reorganization after sensory loss, handedness effects, and differences between single fingers in the sensitivity to thermal changes