Crash Course Psychology #5 is where I learn some of the science behind human vision.
For context, Crash Course inspired me to learn the basics of psychology, so I’ve made it my mission to watch the entire Crash Course Psychology playlist and paraphrase each episode in my own words. This journey wouldn’t have been possible without the Crash Course team, so many thanks to them! To showcase what I learnt, here is my personal paraphrase of episode 5:
Sensing vs Perceiving
Prosopagnosia
A neurological disorder that impairs a person’s ability to perceive or recognise faces, also known as face blindness. The sense of vision is intact, the problem is the perception, at least when it comes to recognising faces. Prosopagnosia is a great example of how sensing and perceiving are connected, but different.
Sensation
The bottom-up process by which our senses (vision, hearing, etc) receive and relay outside stimuli.
Perception
The top-down way our brains organise and interpret that information and put it into context.
Sensory Thresholds
We are constantly bombarded with stimuli, but we can only be aware of what our senses can pick up. For example, we can see what we can see, but we cannot see ultraviolet light (even though they are there!). There’s a lot to sense and every animal has different limitations, allowing us all to sense different things. We also have our…
Absolute Threshold of Sensation
The minimum stimulation needed to register a particular stimulus 50% of the time. Why do our brains do that? Psychologists don’t know yet.
Detecting stimuli isn’t just about the stimuli’s strength alone, it’s also about your psychological state. Things like your alertness and expectations in that moment. This has to do with…
Signal Detection Theory
A model for predicting how and when a person will detect weak stimuli, partly based on context. Conversely, there is…
Sensory Adaptation
This is where your senses will adjust if you are constantly experiencing stimulation.
Difference Threshold
This is referring to our ability to detect the difference between two stimuli. The point in which one can tell the difference is the difference threshold. Part of what helps to explain this is Weber’s Law, where our perception of the stimulus change isn’t as much as the actual stimulus change.
Vision
How it works is a series of long but lightning quick sequence of events. Light bounces off of everything and into your eyes. The eyes then take in all that varied energy and converts that into neural messages that our brains processes and organises into what you see. How we do the eyes transform light waves into meaningful information? Let’s start with the light itself.
Lightwaves
What we humans see is only but a fraction of the entirety of the electromagnetic radiation. Light has many characteristics that determine how we sense it, but for now, we’ll understand light as travelling as waves. The waves’ wavelength and frequency determine their hue, while their amplitude determine their intensity. These light waves hit our lens which then focuses the light rays into specific images. Those specific images are then projected onto the retina, which contains all the receptor cells that begin sensing that visual information. Our retinas don’t receive a full image. It’s more like a bunch of pixel points of light energy that those aforementioned receptor cells then translate into neural impulses which then travel to the brain.
Rods & Cones
That’s what those aforementioned receptor cells are called. The rods detect gray scale and are used in our peripheral vision. The cones detect fine detail and color, and function only in well lit conditions.
Theories as to why Humans are good at this
Human eyes’s difference threshold for colours is exceptional. There is still ongoing research as to why it’s so good or how exactly our colour vision works. So far 2 theories help explain some of what we know.
- Young-Helmholtz Trichromatic Theory
- This theory suggests that the retina houses 3 specific colour receptor cones that register red, green, and blue. And when stimulated together, their combined power allows the eye to register any colour (unless you’re colour-blind).
- Opponent Process Theory
- This theory suggests that we see colour through processes that actually work against each other. So some receptor cells might be stimulated by red, but inhibited by green, while others do the opposite. It’s these combinations that allow us to register colour.
Visual Cortex
When rods and cones get stimulated, they activate the bipolar cells behind them, which then activate the neighbouring ganglion cells. The long axon tails of these ganglion cells braid together to form the ropy optic nerve which carries the neural impulses from the eyeball to the brain.
All this visual information goes through a series of complex levels, from those optic nerves to the thalamus and then the brain’s visual cortex. This visual cortex sits at the back of the brain, in the occipital lobe. Just like in the previous post, the right cortex processes input from the left eye, and vice versa.
Feature Detectors
This visual cortex has specialised nerve cells called Feature Detectors that respond to specific features like shapes, angles, and movements. Since different nerve cells respond to different specific features, this means that different parts of your visual cortex are responsible for identifying different aspects of things. For example, the brain’s object perception occurs in a different place from its face perception.
Also, some cells in a region may respond to just one type of stimulus, such as posture, movement, or facial expression. All this while other clusters of cells weave all of that separate information together. What’s more, this analysis is all done instantly in order to assess a situation. This is called…
Parallell Processing
The ability to process and analyse many separate aspects of the situation at once. In the example of visual processing, this means that the brain works simultaneously on making sense of form, depth, motion, and colour.
Questions after Crash Course Psychology #5
- The Young-Helmholtz Trichromatic Theory suggested that there are cones that register red, green, and blue. Why those 3 colours and not the 3 primary colours, which are red, yellow, and blue?
- When rods and cones get stimulated, they activate the bipolar cells behind them, which then activate the neighbouring ganglion cells. Why is there a middle man whose function it is to turn on the ganglion cells, why not just have the rods and cones activate the ganglions themselves? I’m sure there’s a specific biological reason behind it that was simply not covered in this very quick video.
Danniel’s thoughts on Crash Course Psychology #5
- Sensing and perceiving being related but different was by far the most practical piece of knowledge for me. Now I see myself using it in my daily life, such as seeing what’s on the surface, but my perceiving half allows room for interpretation. Thank you!
Enjoyed this learning of Psychology? Test your knowledge against these quick custom Kahoot! quizzes I’ve made based on the episode above: This is the easy mode and this is the hard mode for Crash Course Psychology #5.
Also, do check out what else Psychology related I’ve learnt from my Psychology blog!
Credits for Crash Course Psychology #5
Original Content & Media by Crash Course
Content Consumed and Paraphrased by Danniel Iskandar
Paraphrase Proofread by
Paraphrase Reviewed by