VISION

Our exploration of the science of perception will cover all of our senses. However, we will spend most effort to study vision. There are several reasons for this choice:

1. Most of our sensory brain areas are concerned with vision.
2. Vision seems to be more "dominant" than other senses.
3. Even dreams seem to be mostly visual.
4. Vision is relatively easy to study experimentally.
5. Accordingly, vision is the most studied sense.  

We will always follow the same outline for the senses that we will discuss:

First, we will discuss about the stimuli that these senses detect. This implies that we will start with physics and chemistry. Some of that physics will also require us to review certain mathematical principles and laws.

Next, we will explore the psychology of each sense. That is, we will briefly go over what is known from behavioral measurements about how we sense and perceive each sense in isolation or in combination with the other senses.

Then, we will review the anatomy of our sense organs, including their neuroanatomy. This includes a brief review of the basics of systems neuroscience as well.

Lastly, we will survey what is known about the brain mechanisms that go along with the sensation and perception of each sense.

You might be most interested in the last of these steps only. That would be understandable. After all, we started with some puzzling philosophical questions surrounding perception, and you might hope that we will (only) find an answer to these questions once we learn what the brain does when we perceive something.

However, as you will see, we will not find satisfying answers to these philosophical question at that point. In fact, the mystery might deepen. You will also find that starting from physics becomes important and informative at that point.

This is not meant to let you down. In fact, there are suggestions for satisfying answers, and we will discuss them at the end. These proposals will only be comprehensible, though, after we completed the full survey of the physics, anatomy, psychology, and neuroscience of the senses.

So, let us begin.

OPTICS¶

Vision seemingly starts with light (if we put dreams and hallucinations aside for a moment). But what is that - light?

Light¶

The physics of light (optics) is a rabbit hole that is easy to fall down in. One reason for that is that it can quickly lead us from high school physics to quantum physics and relativity. Luckily, we do not have to do that for garnering a basic understanding of physics relevant for our visual perception.

In simple terms, light is electromagnetic radiation.

Light, in fact, is just the part of the electromagnetic spectrum that we can see (due to the biophysics of our photoreceptors).

At this point, we should have a solid set of knowledge about the physics of light (optics). Needless to say, we are just scratching the surface. Light is at the center of all modern physics: The duality of light waves and light particles (photons) is a central conundrum of quantum physics. At the same time, the assumption that the speed of light expansion is fixed inside a vacuum is central to Einstein’s relativity. These are all deeply fascinating (and sometimes highly technical) topics that fill up entire textbooks on their own. Luckily, almost none of that is relevant for human sight. So, we can move on from physics now, and discuss the main star of the show: human vision.

As stated at the outset, all chapters that are sorted by what we commonly define our main senses will be organized as this one: We will first discuss the physics or chemistry that underlies the sensory stimuli that are associated with each sense. Then we will discuss what has been studied about the perception that each of these senses evoke using behavioral techniques, including psychophysics and verbal report. Only then will we discuss what is known about the neural mechanisms (neuroanatomy and neuronal responses) that are associated with each sense.

This is a somewhat usual organization for a textbook on sensation and perception. More commonly, these three separate fields of study (physics/chemistry, neuroscience, and perceptual psychology) are presented as intertwined. Indeed, there are many benefits of presenting some psychophysical phenomena in the context of the physics of stimuli or neuronal connections and responses. We thus will allow for a bit of liberalism and do so occasionally as well. However, the overall goal here is to retain a certain consistent logical structure to our approach. And, in doing so, first think about the differences between the physics or chemistry of sensory stimuli and our perception of the related sensory activations - before thinking about the link between brain mechanisms and perception.

PSYCHOLOGY¶

Sensation and perception is commonly taught by faculty in psychology departments. This is often, though not always, also the main home of perception researchers (many neuroscientists that study perception are housed inside Medical Schools). We thus will go with labeling this chapters as a summary of the psychology of perception (seeing, or vision, in the current case).

However, note, that some psychologists define psychology as the science of human behavior. This definition can still be applied to a large part of perceptual psychology since many researchers exclusively measure perception indirectly via behavioral report, such as button presses or verbal descriptions.

Yet, there is also an aspect of perception that gets lost in this framework - how perception presents itself (to us). And even when we exclusively consider perception-related behavior, this first-hand experience of ours with perception can become part of the equation.

For example, when we learn that some people cannot distinguish colors near red and green (color weakness, or more colloquially - and technically incorrect - color blindness) as well as others, students (and in fact, scientists) will inherently wonder what that is like. Accordingly, attempts are often made to alter color images so that normal observers can experience (via simulation) what a person with color weakness perceives. Likewise, there are attempts at expanding the color experience of people with color weakness to experience what being able to see more shades of hue might be like.

The same goes for learning people that cannot discriminate faces, objects, or who cannot see motion. Part of the process of understanding these phenomena is wondering about what remains, such as whether these people still see objects, faces, or motion like we do, but struggle to make sense of what they see. In other words, since we are ultimately studying perception, and not just its behavioral corollaries, we invariably use our own perception as an additional data point that seems challenging to eliminate from the equation without losing sight of the subject at hand.

Phenomenology¶

One way to account for this interesting issue is to acknowledge that what we mean by “psychology” in this, and similar sections, is not exclusive to behavioral output. Instead, we will also - often implicitly, and sometimes explicitly - leverage our own perceptual experience to interpret these behavioral findings. In fact, we have already done so at the very start of this textbook when we pondered a visual illusion. We will count your own perceptual experience as a valid data point.

Some may call this added data point as being derived from “introspection”. Technically, it seems a bit unclear whether any active process is taking place - let alone what is internal about it - when I look at something red and see red. Yet, it is fair to point out that using your own perception to derive knowledge about perception seems to deviate from using data that is derived from other people. The outcome of this process is sometimes referred to as phenomenology. Many people use this term in a variety of contexts and definitions, but for our purposes, adding this note about “phenomenology” only is meant to expand on a possibly too narrow definition of psychology to incorporate the science of perception.

Back to the topic at hand - what does all that mean for how we see?

Visual Field¶

The first thing one notices when pondering the nature of seeing is that one can quickly derive some simple facts about what we see. First among these is that we do not see all around us. We only see what it is in front of us. That is, the visual world that we encounter (including that of our dreams) is only half.

The reason that we cannot see what is behind our heads is simply due to the fact that both of our eyes face to the front, of course. Interestingly, this is not the rule among animals, let alone mammals. Many animals do not suffer from this limitation. In particular, animals that frequently face predators tend to feature eyes (usually on each side of the head) that allow for seeing almost all around them.

We call this fraction of the entire possible world that we could possibly see (if our eyes were to the side, like the eyes of horses), the visual field. For most humans (and other primates), the visual field roughly reaches 180 degrees around our body horizontally as well as roughly 180 degrees vertically.

You can test this yourself: Keep your eyes steady to the front (e.g., by fixating on an object straight ahead of you). Then, stiffen your arms, hands, and fingers so that they are all in line. Hold them right in front of you as if you are pointing with both arms and hands at what you are looking at. Now, move them apart laterally so that your left and right arms and hand slowly become parallel with your shoulders. Keep your eyes steady as you do so. You will probably be able to move your arms even further beyond the point of being parallel to your shoulders. But as you do so, and you keep looking straight ahead, your hands and arms will vanish from view. You can move your hands a bit back and forth, and you can easily see where the “borders” of your field of you are located. Here is a bonus: Keep your eyes at that point for a moment and close one eye and the other if you can. What do you see now?

This all is very basic and trivial, of course. But it allows us to move towards describing quite rigorously (and indeed mathematically when using degrees of angle) what we see. And this is just a starting point.

Now that we appreciate that what we see is extending 180 degrees horizontally and vertically in front of us, the next step is to think about what that is. And a somewhat trivial, yet surprising answer is: space.

Fronto-Parallel Plane¶

To keep things simple, let us think about two-dimensional space first. In other words, let us consider a plane. One might object that what we see is not just a flat, two-dimensional image. But that actually warrants a bit of consideration. As we will see, the 3D nature of our vision (i.e., the fact that we see depth) actually resembles a bit more of an inference in that is based on a small set of rules. However, more than that, we clearly can just see in 2D. That is, we can have perfectly normal, fine vision and fill our entire visual field with just a plane.

Form¶

Gestalt¶

Multistability¶

Depth¶

Depth Cues¶

Stereopsis¶

Color¶

Color Space¶

Color Anomalies¶

Motion¶

Akinetopsia¶

Objects¶

Agnosias¶

Faces¶

Prosopagnosia¶

Pareidolia¶

ANATOMY¶

NEUROSCIENCE¶

Optic Nerve¶

Optic Chiasm¶