some context for graphics

Author: Susan Vanderplas

part-wrangling/02c-good-graphics.qmd

@@ -31,8 +31,21 @@ Finally, @sec-checklist provides a guide to evaluating graphics for clarity, eff
 Before we discuss how to create good charts, there is a certain amount of background information that must be considered.
 Charts make use of the visual system within the human brain, which means we need to understand some basic attributes of human perception and cognition in order to make the best use of this "wetware" [@wikipediacontributorsWetwareBrain2025] processing power. 
 
+First, let's set the stage. 
+Information in the form of light bounces off objects in the world and lands on our retina (there are lenses and focusing mechanisms that we'll skip). 
+There are four types of light detectors in the retina: three types of cones that respond to red, green, and blue light wavelengths, and rods, which respond to light intensity across wavelengths. Cones are concentrated in one area, while rods are spread across the surface of the retina. 
+The rods and cones turn light into neural impulses, which are transmitted along the optic nerve to the **visual cortex** located in the back of the brain (roughly where your head would hit the floor when you are lying down).
+The visual cortex contains special neurons called feature detectors which organize the information from the retina and reconstruct this information into a mental representation of the world. 
+Some feature detectors respond to specific angles, signals from specific parts of the retina corresponding to specific parts of the outside world, and many other low-level features. 
+Signals from these feature detectors are then aggregated into higher-level concepts that form our visual experience of the world.
+
+The initial light signals and lower-level information are sometimes called "sensation", and the ability to detect higher-level concepts is called "perception". We can also think of "top-down" perception, where our experience shapes what we perceive and how we experience the world, compared to "bottom-up" perception, where we construct higher-level concepts solely from lower-level signals. 
+
 <!-- Basic overview of components of the visual system - eye, optic nerve, processing centers of the brain, visual cortex (hardware) and software -->
 
+Perception tends to require a few other mental resources beyond the visual detection and processing equipment (eyes, visual cortex, etc.): attention and memory (short and long-term) are vital for processing the visual input and making sense out of it.
+The next few subsections provide specific examples of why it's important to understand the basics of the visual system when thinking about how to construct charts and graphs. 
+
 ### Preattentive Features
 
 You've almost certainly noticed that some graphical tasks are easier than others.