Chapter 2 Overview

The First Steps in Vision: From Light to Neural Signals

You should currently be seeing “stars”—spots of light scattered across your computer screen that are twinkling on and off. (Click here to toggle them on and off.) At a very basic level, this is all you ever see. While we commonly speak of seeing objects in the world, your visual system senses not the objects themselves, but points of light reflecting off the objects and into your eyes. And as you will learn in this chapter, while you ultimately perceive much more than just stars, the visual world is first registered by your eyes as mere spots of light.

Be forewarned that this chapter includes a lot of basic anatomic terms, as this will be your first detailed look at the visual system. The first four activities are dedicated to helping you master this terminology. The Visual System Overview provides a road map of what is to come, charting the structures that you will be learning about in this chapter and the next. Starting with the eyes, these structures collect information about the visual world and send it on to the cerebral cortex for more detailed processing. From Sun to Eye describes how light reaches your eyes in the first place, a fundamental precursor to vision that you may have never fully considered. Activities on Eye Structure and Retinal Structure detail the mechanisms by which light is projected and focused on the back of your eye and then transduced into neural action potentials, which are the language of the brain.

Two other activities in this chapter will help you to understand crucial concepts in retinal function. The activity on Acuity versus Sensitivity demonstrates how different types of neural circuits in the retina allow us to detect objects in low light and see objects accurately under better lighting conditions. Finally, the activity on Ganglion Receptive Fields illustrates the patterns of light that ganglion cells, the neurons that actually transmit information from the eye to the brain, respond to.

Finally, to help you understand what age-related macular degeneration (AMD) is like, the last activity in this chapter lets you experience a Simulated Scotoma. AMD results in central visual field loss which can make it difficult to perform everyday tasks, such as recognizing faces.

The essays dive deeper into some of the topics introduced in this chapter, including information on Seeing Illusory Stripes and Spots and several diseases of the eye. Another essay includes a description of classic experiments that determined How Many Quanta of light are necessary to activate a single cone. More information about AMD and RP is presented in the essay called The Man Who Couldn’t Read, and information about the causes and treatments for Glaucoma, a disorder affecting the optic nerve, is also described.

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.