Chapter 37 - Natural Scene Statistics and Salient Visual Features

Abstract

The control of attention and of the associated saccadic fixations is to a large degree determined by the image content, that is, by salient local features in the image. The nature of this salient features and of the underlying strategy for their selection can be investigated within the context of natural scene statistics. For this, saccadic eye movements of human observers are recorded for a variety of natural and artificial test images, and the statistical properties of the fixated image regions are analyzed. The second-order statistics indicate that regions with higher spatial variance have a significantly higher probability to be fixated. Therefore, local contrast is a salient feature. It was difficult to find additional differences in the local power spectra that can yield unequivocal information about a preference for specific local form properties. Differences that can be more easily interpreted as structural properties are derived from an investigation with higher-order statistics (bi-spectral density). The results indicate that non-redundant, intrinsically two-dimensional image features, such as, curved lines and edges, occlusions, isolated spots, and so on, play an important role in the saccadic selection process. Such features cannot be extracted by the classic spatial frequency selective filter mechanisms but require nonlinear AND-like interactions among frequency components as possibly provided by end-stopping or hyper-complex properties related to the extra-classical receptive field.