For us to sense the world around us, our sensory organs receive stimuli. The messages those organs produce go through a process known as transduction. This means that the signals are transformed into impulses that travel through the thalamus to the appropriate cortices. Our bodies are constantly sending signals to the brain, but if it were to register every sensation, it would be overwhelming. Imagine feeling, seeing, hearing, touching, etc.. everything. This doesn’t happen due to the combination of sensory adaptation (the decrease in responsiveness to stimuli due to constant stimulation) and sensory habituation (our perception of sensation is partially due to how focused we are on them.) Now that I mention it, you likely will start to feel the socks on your feet, even though you hadn’t noticed that before, as you weren’t focused on the sensation. This connects to the cocktail-party phenomenon which is the idea that as your focus changes, you ignore certain stimuli, for example, at a party, if you’re focused on your friend talking, and someone yells your name across the room, your focus may involuntarily shift to whoever is yelling at you, causing you not to hear what your friend is saying.
Step 1: Gathering Light: Light reflects off of surfaces and hits our eyes. The colour we perceive that surface to depend on a few things; one, the light intensity which is to say, how much energy is in the light that hits the eye. The second is the light wavelength, which determines the hue that we see. (Or if we even see it at all- wavelengths longer than visible light such as infrared waves, microwaves and radio waves or shorter than visible light such as UV waves and X-rays aren’t registered by the eye) A mix of all the visible hues of light produces white. When an object appears a certain colour, it has absorbed all other colours and reflected the colour we see it as. A green plant, for example, would not be able to survive in the green light, as it would reflect the green, causing it to starve.
Step 2: Within the Eye: Reflected light enters the eye through the cornea which is a clear protective covering over the eye. The light travels through a hole known as the pupil. The iris (what make your eyes a certain colour) is a collection of muscles which work to constrict the pupil in extremely light situations, and expand the pupil in dark places- this is known as accommodation. The curved lens focuses the light like a camera focuses on an image. The image is flipped upside down as it travels through the lens. This now focused, inverted image project itself on the retina which works as a screen. On this screen are specialised neurons which are activated by light.
Step 3: Transduction: The light needs to be translated into neural impulses to be registered by the brain. There are two kinds of neurons which do that job. Cones, which are activated by colour and rods, which respond to black and white. Rods outnumber cones, and are especially useful in dark situations when colour cannot be registered at all; this is why in the dark, it is nearly impossible to figure out what colour an object is. There just isn’t enough coloured light being reflected for the cones to be activated properly. Cones are concentrated in the centre of the retina. At the very centre of the retina is the fovea; where the highest concentration of cones exists. Peripheral vision depends on cones, while your centred of vision depends on cones (in light, of course). Once enough rods and cones fire, they activate the next layer of bipolar cells; ganglion cells, whose axons make up the optic nerve, sending impulses to the lateral geniculate nucleus (LGN) of the thalamus. Messages are relayed to the visual cortex in the occipital lobes. The optic nerve has no rods or cones and is known as your blind spot (different to the driving blind spot). There are two parts to the optic nerve. Impulses from the right side of both retinas go to the right hemisphere, while impulses from the left side of both retinas go to the left hemisphere; it is that separation of the optic nerve that allows this to take place. The spot where the optic nerves cross each other is the optic chiasm.
Here’s a fun experiment to show where the blind spot is. (Here is the original link for anyone curious: https://visionaryeyecare.wordpress.com/2008/08/04/eye-test-find-your-blind-spot-in-each-eye/- it has a few more interesting experiments to try as well)
Sit with your nose between the cross and black circle. Cover your left eye, and look at the cross with your right eye. Move towards the computer while still using your right eye. Eventually, the black circle will disappear and reappear, showing you your blind spot. Do the same with your left eye, except now focus on the black circle. Weird right?? (Science rules!)
Step 4: In the Brain: There is still a lot of debate as to how our visual cortex actually works to register vision. Some say the visual cortex is where sensation ends and perception begins. Some say interpretation begins at the retina. Some say it begins at the thalamus. Scientists David Hubel and Torsten Wiesel were responsible for finding that groups of neurons in the visual cortex respond to different types of visual images. There are feature detectors for vertical lines, curves, motion, and so on.
Theories of Colour Vision
A lot is still unknown about how we see colour. The oldest theory is the Young-Helmholtz Trichromatic theory which hypothesised that we have 3 different typed of cones which detect, red, blue and green. They are activated in different combinations to produce the spectrum of colour we see. There are issues with this theory; namely afterimages and colour blindness. That’s where the opponent-process theory of colour vision comes in. (Not to be confused with the opponent-process theory of emotion. I’ll get to that in a later post). This theory states that sensory receptors are arranged in pairs: red/green, yellow/blue, and black/white. When one sensor is stimulated, its pair is inhibited. This explains after images. Try this illusion out: Stare at this image for 30 seconds, and then look at a white surface. If the flag doesn’t immediately show up try blinking a few times.
What’s happening is that you’ve exhausted your cones, and so the opponents will fire. This is why dichromatic colour blindness also tends to be consistent as to which two colours are affected such as red-green colour blindness, or yellow-blue colour blindness
While both vision and hearing are considered energy senses, they are very different stimuli. Sound waves are vibrations in the air, instead of electromagnetic waves used to produce images. The vibrations are captured by our ears and transduced into neural impulses which allow the brain to register and interpret the sound. Sound waves have amplitude and frequency. Amplitude is the height of the wave and determines how loud the sound will be. Frequency is the length of the waves and determines the pitch of the sound. High-frequency waves are densely packed together, while low-frequency waves are spaced apart.
The process of hearing works much like a Rube Goldberg machine, where one thing moving activates the next part of the sequence. The pinna (outer ear) collects sound, which travels down the auditory canal. The sound reaches the tympanic membrane (the eardrum) which vibrates as sound hits it. Attached to the tympanic membrane are the ossicles; the smallest bones in your body. The tympanic membrane is connected to the hammer/malleus, which is connected to the anvil/incus, which is connected to the stirrup/stapes, which all transmit the vibration to the oval window, a similar membrane to the eardrum. The windows membrane is connected to the cochlea, a snail-like structure filled with perilymph, and endolymph. The vibration of the oval window causes these fluids to move. At the base of the cochlea is the basilar membrane, lined with hair cells connected to the Organ of Corti, which are neurons activated by the moving hair cells. The Organ of Corti fires neural impulses and those impulses travel to the brain via the auditory nerve.
Place Theory: Place theory states that hair cells in the cochlea respond to different frequencies based on their location. Some bend to high pitches, and others to low pitches.
Frequency Theory: Research has shown that while place theory is accurate for high pitches, but not for low tones, who are sensed by the rate at which the cells fire. We sense pitch because hair cells fire at different rates (frequencies) in the cochlea.
For each step in the hearing process, there are just as many ways those steps can go wrong. Conduction deafness refers to a problem conducting sound to the cochlea (so any point in the ear canal, to the eardrum, ossicles, or oval window). Sensorineural deafness occurs when the hair cells in the cochlea are damaged. Often this is caused by overstimulation, causing a high pitched ringing. Prolonged exposure to that much stimulation can severely damage the hair cells which do not regenerate.
There is one more type of energy sense and it is just as different as the other two. Our skin contains millions of different types of receptors. Some (nociceptors) respond to pain, some (thermoreceptors) respond to temperature. Some (Messiner’s Corpuscles) respond to plain old touch. Some (Pacinian Corpuscles) respond to pressure, all of which can be found all over the skin. The relationship between these receptors is not well understood. What we do know is that the brain interprets the amount of indentation (temperature change) and the intensity of touch. Where nerve endings fire also determines where the touch is coming from. Certain parts of our body have more concentrated amounts of receptors.
For example, in my physiology class, we did an interesting experiment where a friend took a paper clip and unfolded it until it had 2 ends. The friend would poke me on different parts of my skin with either 1 or 2 of the ends and see if I could guess how many ends I was being poked with. On the fingertip, I guessed with 100% accuracy, while on my shoulder, I was guessing the whole time.
Gate-control theory explains how pain is experienced. Some pain messages are more important than others. When a priority message is sent, the “gate” swings open for it and swings shut for lower priority messages. This is why scratching an itch gets rid of the itch. The scratching is a higher priority message, and so the itch is temporarily ignored in order to prioritise the itching. That’s also why you shouldn’t scratch an itch- you’re not doing yourself any good. Endorphins also work to shut this gate, working like morphine to shut off any pain.
Fun fact before I start: I remember that taste is gestation because of Gusteau from Ratatouille who was named after the French word for taste. When you eat food, the tastebuds are not responding to energy, but rather the chemicals on the tongue. These tastebuds are located on papillae, which are bumps on the tongue. There are 5 different kinds of tastebuds scattered throughout the tongue which respond to Sweet, Salty, Bitter, and Umami (savoury). The more densely packed the taste buds, the more intense the taste is. The tongue is not unique to taste; you’ll know this if you’ve ever tried to eat a jalapeno because spicy food stimulates nociceptors, which is why the experience is agonising for some people. Weaker nociceptors increase a person’s spice tolerance, which is why certain cultures do better eating spicy foods; they’ve adapted to the pain.
Olfaction is created by chemicals entering the nose and being absorbed by receptors on the mucous membrane. A lot is not known about these receptor cells- some researchers estimate there are as many as 100 different types. These cells are linked to the olfactory bulb which gathers messages from olfactory receptor cells and relays them to the brain. The nerve fibres from the olfactory bulb connect to the brain differently than all the other senses; instead of going to the thalamus, the message is sent to the amygdala and then the hippocampus. This could be why smells can trigger memories much more strongly than other senses can.
Let's get something out of the way: you do not have 5 senses. How many you have depends on who you ask, but a prominent one is the sense of position.
Your vestibular sense helps you determine where your body is in space. The semicircular canals in the inner ear are responsible for this. The canals are tubes partially filled endolymph. When the head moves, the fluid moves too. The hair cells activate, sending signals to the brain telling it that the head has moved. Dizziness is caused when the brain receives several conflicting signals in a short period of time, caused by rapid movement, or when you’re in a boat, and the eyes register a static view, while the inner ear is registering all kinds of movement.
While vestibular sense tracks the overall orientation of the body, kinesthetic sense tracks the position and orientation of specific body parts. Receptors in muscles and joints send information to the brain, helping the brain track the body.
Perception is the process of understanding and interpreting sensations. Psychophysics is a branch of psychology which studies the interaction between sensations and how we experience them.
Senses have limits. The absolute threshold is the smallest amount of stimulus we can detect. There are several videos online helping people track the absolute threshold of pitch that they can detect by playing higher and higher frequencies. The proper definition of the absolute threshold is the minimal amount of stimulus we can detect 50% of the time as things such as other sensory inputs can make studying the absolute threshold extremely difficult. Stimuli below the absolute threshold are referred to as subliminal.
Another important threshold is the difference threshold. This is also referred to as the just-noticeable difference and is rather self-explanatory. It is the smallest amount of change in a stimulus needed for us to detect a change. This threshold is determined by Weber’s Law, discovered by psychophysicist Ernst Weber (it is sometimes called the Weber-Fechner law.) The change needed is proportional to the original intensity of the stimulus. The more intense a stimulus, the more it will need to change before we notice a difference. Each sense varies according to a constant (which differs between the senses). The constant for hearing is 5%- when listening to a 50-decibel tone, it would need to reach 52.5 decibels to notice a difference.
Signal Detection Theory: This theory investigates the effects of distractions and interference we normally experience in our lives. This area of research revolves around attempting to predict what we will perceive. It takes into account how motivated we are to perceive specific stimuli and our expectation. These are known as response criteria or receiver operating characteristics. A false positive is when we think we are perceiving a non-present stimulus. While a false negative is the failure to perceive a present stimulus.
Top-Down Processing: When we use top-down processing, we perceive by filling in gaps what we sense. For example, when reading this sentence: I _m t_king p_ych_ _ gy, you can pretty easily fill in the blanks based on the context of what you read before, and the letters I did give you. In other words, top-down processing uses background knowledge to fill in gaps. Our schemata are mental representations of how we think the world should be. They can create a perceptual set or a predisposition to experience something a specific way. https://www.youtube.com/watch?v=Bafuyg98no4- Watch this video about backmasking (”hidden” messages in songs when played backwards.) Try looking away when the song starts playing and then reading the supposed message. I know personally, when I did this, I couldn’t understand the hidden message until I read what I was supposed to be hearing, and then I could hear it clearly. Several parents became worried about this phenomenon, likely because they had schemata of the music their kids were listening to as dangerous.
Bottom-Up Processing: This is also known as feature analysis, where only the features of the object are used to build a perception. This is an automatic process, however, is much slower than top-down processing. As a result, it is less prone to error.
Principles of Visual Perception
There are numerous rules for visual perception. One of the first decisions the brain makes is the figure-ground relationship. This is self-explanatory; what is the figure and what is the background? A famous illusion connected to this principle is the vase face illusion. Do you see the background as black or white? Do you see the figure as the vase or the face?
Gestalt psychologists at the beginning of the twentieth century noted several principles which explain how we perceive groups of objects, and theorised that we perceive images as groups rather than individuals. (Hence the name “Gestalt”) Here is a great diagram from https://www.verywellmind.com/gestalt-laws-of-perceptual-organization-2795835 illustrating those principles.
Proximity: Objects that are close together are more likely to be perceived as a part of the same group
Similarity: Objects that look similar to each other are likely to be perceived as part of one group
Continuity: Objects creating a continuous form are likely to be perceived as part of the same group
Closure: Similar to the sentence connected to top-down processing. Objects making a recognisable image are likely to be viewed as part of the same group
Law of Pragnanz: You are more likely to perceive and interpret complex images as the simplest form possible
Law of Common Region: Objects grouped together within the same region of space tend to be viewed as their own group.
Our ability to maintain a constant perception of an object despite a constantly changing world is known as constancy. Here are a few types.
Our brains can detect how fast images move across the retinas and use our own motion to determine the speed the object is moving. This causes some interesting phenomena where the brain thinks a still object is moving. For example, you may be familiar with the stroboscopic effect. This is what causes perceived motion in flipbooks. There is also the phi phenomenon. Movie Marquees use this phenomenon by turning on and off lights sequentially, creating the appearance of one moving light.
Depth perception is what allows us to perceive the world in 3-dimensions rather than 2. Eleanor Gibson performed an interesting experiment known as the visual cliff experiment, which found that crawling toddlers can perceive depth. In this experiment, the infant is placed onto one side of a glass table, creating the appearance of a cliff. The infant would not crawl onto the glass. There are two categories of cues that we use; monocular (needing one eye) and binocular (needing two)
Artists tend to use monocular cues to create the illusion of depth in their art. A wonderful artist on deviant art created a really helpful diagram, illustrating each of these ideas. https://www.deviantart.com/akenon/art/Monocular-Cues-34065283
Linear Perspective- In order to draw a faraway house, artists tend to draw two lines which converge at a focal point, taking advantage of the linear perspective cue.
Relative Size Cue- When drawing two similarly sized cats, one far away and one nearby, an artist will draw the faraway cat as significantly smaller, creating the illusion of distance.
Interposition Cue- If the artist draws a cat that is partially obscured by a tree, we will view the tree as closer than the cat.
Texture Gradient- Another way artists create the illusion of distance is by making close up objects more detailed than those faraway
Shadowing- By using shading in an image, an artist can imply a light source, further implying depth in the image.
Binocular Disparity/Retinal Disparity- Each eye sees an object at a slightly different angle. The farther away an object is, the more similarly the eyes will perceive it, the closer it is, the more retinal disparity there will be. Notice in this diagram how the trees don’t seem to move, but the finger moves drastically?
Convergence- As an object comes closer to our face, the eyes must move towards each other to keep it in focus. The more the eyes converge, the closet the object is.
How Culture Affects Perception
One of the more interesting (in my opinion) parts of psychology is exploring how culture affects us. Some of the perceptual rules that for years psychologists believed to be innate are actually learnt. Cultures which don’t use monocular depth cues in their art don’t see depth in pictures using these cues. It also affects how optical illusions affect our brains. The famous Muller-Lyer illusion often does not affect cultures that don’t use right angles and corners in architecture.
