Spatial Vision: From Spots to Stripes
As you learned in Chapter 2, ganglion cells in the retina analyze the visual world as an array of spots of light. Information about the mosaic of 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 terms of lines, stripes, and edges—translating from spots to stripes.
Before we learn how this translation occurs, two activities will introduce you to the important concepts of visual angle and foveal acuity. Visual Angle (Activity 3.1) 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 but containing no rods. The fovea is also privileged in that it is overrepresented in the brain relative to other areas of the retina. The combination of these two factors leads foveal vision to be particularly acute, a fact that will become obvious when you do the activity on Foveal Acuity (Activity 3.2).
Eventually, information from the fovea and other parts of the retina is transferred to striate cortex and other parts of the brain (to get an “inside view” of the brain, check out the site linked from The Whole Brain Atlas [Essay 3.2]). Early research on the neurons in striate cortex showed that they respond best to bars of light rather than to spots of light. Furthermore, each neuron is “tuned” to respond to light bars of a particular orientation. You can explore the orientation selectivity of striate cortex neurons, and some of their other characteristics, in the activity on Striate Receptive Fields (Activity 3.4). 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 (Activity 3.3). Researchers have also gone a long way toward detailing the remarkable organizational principles of striate cortex neurons, which is explored in the activity on Hypercolumns (Activity 3.5).
The two additional essays in this chapter cover Hyperacuity (Essay 3.1), the ability to make incredibly precise judgments about the relative locations of objects, and the innovative procedures researchers have used to take snapshots of the visual cortex in action as it processes images (Essay 3.3: Seeing Images on the Cortex).