Early VEP magnitude is modulated by structural sparseness and the distribution of spatial frequency contrast in natural scenes

摘要

The contrast response function of early visual evoked potentials (VEPs) elicited by sinusoidal gratings is known to exhibit characteristic potentials associated with the parvocellular and magnocellular pathways.Specifically, the N1 component has been linked with parvocellular processes, while the P1 component has been linked with magnocellular processes (Ellemberg, et al., 2001, Spatial Vision). However, little is known regarding the response of those pathways during the encoding of complex (i.e., broadband) stimuli such as natural scenes. Natural scenes are known to vary in terms of: 1) the amount of structural content (i.e., structural sparseness) contained within each image, and 2) the distribution of contrast across spatial frequency (i.e., 1/f slope of the amplitude spectrum) across each image. Thus, the present study was designed to examine the extent to which the physical characteristics of natural scenes mentioned above modulate early VEPs in humans. The stimuli consisted of 50 natural scene images, grouped according to the slope of their amplitude spectra (five different levels of slope: a??0.76, a??0.87, a??1.0, a??1.2, & a??1.4) and degree of structural sparseness (two levels of structural sparseness: high and low) contained within each image. We recorded EEGs while participants viewed each natural scene image for 500ms, preceded by a 500ms mean luminance blank from which base-line measurements were taken. The results show that: 1) the relative magnitude of the early VEPs was highly dependent on the amount of structure contained within the scenes, independent of amplitude spectrum slope; 2) the overall magnitude of the early VEPs was dependent on the slope of the amplitude spectrum such that the presence of more contrast at the higher spatial frequencies yielded higher overall early VEP magnitude. These results suggest that it is the amount of structure at the higher spatial frequencies in natural scenes that dominate early VEPs.