27: Intermediate-Level Visual Processing and Visual Primitives

Introduction

• Internal Models of Object Geometry Help the Brain Analyze Shapes

• Depth Perception Helps Segregate Objects from Background

• Local Movement Cues Define Object Trajectory and Shape

• Context Determines the Perception of Visual Stimuli

  • Brightness and Color Perception Depend on Context

  • Receptive-Field Properties Depend on Context

• Cortical Connections, Functional Architecture, and Perception Are Intimately Related

  • Perceptual Learning Requires Plasticity in Cortical Connections

  • Visual Search Relies on the Cortical Representation of Visual Attributes and Shapes

  • Cognitive Processes Influence Visual Perception

• An Overall View

We have seen in the preceding chapter that the eye is not a mere camera but instead contains sophisticated retinal circuitry that decomposes the retinal image into signals representing contrast and movement. These data are conveyed through the optic nerve to the primary visual cortex, which uses this information to analyze the shape of objects. It first identifies the boundaries of objects, represented by numerous short line segments, each with a specific orientation. The cortex then integrates this information into a representation of specific objects, a process referred to as contour integration.

These two steps, local analysis of orientation and contour integration, exemplify two distinct stages of visual processing. Computation of local orientation is an example of low-level visual processing, which is concerned with identifying local elements of the light structure of the visual field. Contour integration is an example of intermediate-level visual processing, the first step in generating a representation of the unified visual field. At the earliest stages of analysis in the cerebral cortex these two levels of processing are accomplished together.

A visual scene comprises many thousands of line segments and surfaces. Intermediate-level visual processing is concerned with determining which boundaries and surfaces belong to specific objects and which are part of the background (see Figure 25–4). It is also involved in distinguishing the lightness and color of a surface from the intensity and wavelength of light reflected from that surface. The physical characteristics of reflected light result as much from the intensity and color balance of the light that illuminates a surface as from the color of that surface. Determining the actual surface color of a single object requires comparison of the wavelengths of light reflected from multiple surfaces in a scene.

Intermediate-level visual processing thus involves assembling local elements of an image into a unified percept of objects and background. Although determining which elements belong together in a single object is a highly complex problem with the potential for an astronomical number of solutions, the brain has built-in logic that allows it to make assumptions about the likely spatial relationships between elements. In certain cases these inherent rules can lead to the illusion of contours and surfaces that do not actually exist in the visual field (Figure 27–1).