Chapter 3 Overview

Spatial Vision: From Spots to Stripes

Click here to toggle the bars behind the text.

As you learned in Chapter 2, ganglion cells in the retina analyze the visual world as an array of light spots. Information about the spots of light striking the eye is then sent along the optic nerve to the brain. In Chapter 3, you will discover that neurons in an area of the brain known as striate cortex receive this information and reanalyze light patterns in the visual world in terms of lines, stripes, and edges—translating from spots to stripes.

Before we see how this translation occurs, two activities will introduce you to the important concepts of visual angle and foveal acuity. Visual Angle refers to the area of the retina covered by the image of an object that is being viewed. The extent of this area is related to the size of an object and its distance from the observer and must be understood before approaching a number of other concepts in visual perception. The fovea, as you learned in Chapter 2, is a small portion of the retina located directly behind the pupil that is distinguished from other parts of the retina by being densely packed with cones and containing no rods. The fovea is also privileged in that it is overrepresented relative to other areas of the retina, in the brain. The combination of these two factors leads vision to be particularly acute in the foveal area, a fact that will become obvious when you do the activity on Foveal Acuity.

Eventually, information from the fovea and other parts of the retina makes it back to striate cortex and other parts of the brain (to get an “inside view” of the brain, check out the site linked from Essay 3.2, The Whole Brain Atlas). Early research on the neurons in striate cortex showed that they respond best to bars of light such as those you see behind the text on this page. Furthermore, each neuron is “tuned” to respond to light bars at a particular orientation. Thus, each bar on the page excites a series of different neurons as it rotates. You can explore the orientation specificity of striate cortex neurons, as well as a number of other characteristics of these cells in the activity on Striate Receptive Fields. Later research found that stimuli called sine wave gratings were even more effective than bars of light at stimulating striate cortex neurons. Explore the nature of these stimuli in the activity on Gabor Patches. Researchers have also gone a long way toward detailing the remarkable organization of striate cortex neurons; read about these organizational principles in the activity on Hypercolumns.

The two additional essays in this chapter cover hyperacuity—the ability to make incredibly precise judgments about the relative locations of objects, and innovative procedures researchers have used to take snapshots of the visual cortex in action as it processes images.

Once you have read the chapter in the textbook and done the activities here, use the study aids (Study Questions, Flashcards, and Chapter Summary) to review.