The First Steps in Vision: From Light to Neural Signals
As you look at your computer monitor, you are seeing points of light—pixels—that form the image of this webpage. Each pixel projects a spot of light onto your retina which your visual system interprets as the words you are reading now. At a very basic level, points of light are all you ever see. While we commonly speak of seeing objects in the world, your visual system senses not the objects themselves, but spots 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 points of light, the visual world is first registered by your eyes in that format.
Be forewarned that this chapter includes a lot of basic anatomical 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 (Activity 2.1) 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 (Activity 2.2) describes how light reaches your eyes in the first place, a fundamental precursor to vision that you may have never fully considered. The activities on Eye Structure (Activity 2.3) and Retinal Structure (Activity 2.5) 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.
Three other activities in this chapter will help you to understand crucial concepts in retinal function. The process of Phototransduction (Activity 2.6)—converting light energy into neural energy—is the crucial first step in sensing light so that objects and surfaces can eventually be perceived. The activity on Acuity versus Sensitivity (Activity 2.7) demonstrates how different types of neural circuits in the retina allow us to detect objects in low light and see objects accurately under bright lighting conditions. The activity on Ganglion Receptive Fields (Activity 2.8) illustrates the patterns of light that ganglion cells, the neurons that actually transmit information from the eye to the brain, respond to.
To help you understand what age-related macular degeneration (AMD) is like, the fourth activity in this chapter lets you experience a Simulated Scotoma (Activity 2.4). 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 (Essay 2.3). Another essay includes a description of classic experiments that determined How Many Quanta (Essay 2.1) of light are necessary to activate a single cone, and more information about AMD and retinitis pigmentosa (RP) is presented in the essay called Clinical Case: The Man Who Couldn’t Read (Essay 2.2).