29: Visual Processing and Action

Introduction

• Successive Fixations Focus Our Attention in the Visual Field

  • Attention Selects Objects for Further Visual Examination

  • Activity in the Parietal Lobe Correlates with Attention Paid to Objects

• The Visual Scene Remains Stable Despite Continual Shifts in the Retinal Image

• Vision Lapses During Saccades

• The Parietal Cortex Provides Visual Information to the Motor Syste

• An Overall View

Vision requires eye movements. Small eye movements are essential for maintaining the contrast of objects that we are examining. Without these movements the perception of an object rapidly fades to a field of gray, a phenomenon correlated with the decreased response of neurons in area V1 (see Chapter 25). Large eye movements direct the fovea from one object to another. These movements or saccades bring the high resolution of the fovea to bear on different regions of the visual field, exploiting the high density of photoreceptors in the central fovea. Without saccades this high-resolution processing could be achieved only by moving the head or body.

The preceding chapters have described how visual images are constructed, beginning with the processing of intensity and contrast, then the integration of visual primitives, and finally the high-level processing that leads to the recognition of objects. But the visual system involves more than just object recognition. It must also support the brain's goal of assigning significance to objects in order to develop strategies for interacting with the environment. Thus the brain must be able to select some objects for greater examination while ignoring others.

In this chapter we consider how saccades support that goal. We first consider the essential benefits that saccades provide, shifting attention in the visual field and assisting with the preparation to grasp objects. We then consider the brain mechanisms that solve a major problem created by saccades—the fact that the retinal image is abruptly displaced with every saccade.

In shifting our attention from how the brain constructs a visual scene to how it uses visual information to plan actions, we now concentrate on the region of the brain referred to as the dorsal visual pathway (Figure 29–1). This pathway extends from V1 to the regions in parietal cortex that continue the intermediate level of visual processing, such as the middle temporal area, and then to other regions of parietal and frontal cortex. The regions particularly relevant to this chapter are in the parietal cortex, such as the lateral intraparietal area, but include also the frontal eye field region of the frontal cortex.