Alex Kim, Hyelin Kim, Izzy Castner
Vision and Illusion
Lily Benedict, Leigha Phillips, Helen Tang
How accurately do your eyes see the world? You instinctively know that your eyes are powerful tools. They allow you to recognize three-dimensional objects, notice small movements, and appreciate fine details from a distance. But your eyes can be deceived. What happens to your perception of this gray bar when it is give a new background? While the bar is still the same solid shade of gray, it appears darker on one side and lighter on the other. If it seems strange that your eyes can be tricked by such a simple illusion, rest assured. This is not a flaw in your vision, but the product of a feature that is very useful in the real world. Let’s look at how this illusion works. Light reflecting off it enters your eye and is projected as an image on the retina. The retina is made up of photoreceptors that respond to light like the pixels of a digital camera. These photoreceptors are grouped into receptive fields. Each one is attached to a ganglion cell which generates signals that travel to the brain. The frequency of the signals tells the brain how light or dark the area perceived by the receptive field is. A medium shade elicits a medium response, a dark shade elicits a weak response, and a light shade elicits a strong response. But contrasting shades within the receptive field change the signals in an interesting way. You might expect a darker shade at the edge of the field to weaken the signal. It actually does the opposite - increasing the signal’s frequency so the brain perceives the gray shade as lighter than it really is. A lighter shade at the edge of the field decreases the signal’s frequency and makes the gray appear darker. This function increases the contrast between the two shades. Your brain receives all these signals and pieces them together to generate this image. Because the ganglion cells are signaling at different rates along the gray bar, you mistakenly perceive it as a gradient from light to dark. This feature of your vision comes in handy in the real world, like when you are walking through the woods. When you are moving through the world, you need to be able to recognize objects in your path so you can avoid them. Fortunately, your eyes increase contrast so the edges of objects are easier to see. So while your eyes might not show you the world exactly as it is, they do something even better. They highlight the most important features of your environment so that you can best respond to them.
Sam Bellamy, Emma Funk, Lya Lim, Kelly Scruggs
Phoebe Draper, Peyton North, Jeffrey Wu
Migraines are much more than just headaches. A migraine is preceded by strange sensations and peculiar brain activity. The extreme pain that ensues is a result of activation of receptors and blood vessels outside the brain feeding into an overly sensitive pain pathway within the brain. Let’s look at these aspects in greater detail. It is generally accepted that changes in the blood flow to vessels surrounding the brain are associated with migraine pain. The walls of blood vessels are lined with sensory neurons called nociceptors. When blood flow in the head changes, these nociceptors are activated and send an electrical message to the brain that is interpreted as pain. But changes in blood flow happen all the time and don’t always lead to a migraine. There has to be more to the story. Recent theories propose that the trigemino-vascular pain pathway - brain wiring that connects blood vessels to the part of the brain that processes pain - is the major player. In migrainers, this pathway is overly sensitized, so stimuli like stress, exercising, and certain foods that don’t usually trigger pain activate the pain pathway. There are other migraine symptoms, known as auras, that seem to have nothing to do with the pain pathway. Often, migraines report one part of their vision becoming blurry - the blurriness slowly spreading to more distant parts of their visual field. The timing of this visual disturbance perfectly matches that of a well-characterized electrical disturbance called cortical spreading depression. This abnormal wave of electrical activity marches across the brain at about five millimeters per minute. When it invades the occipital lobe, it affects vision. If it marches over the parietal lobe, it affects sense of touch and illicits a tingling sensation. Beyond pain and aura, migraine symptoms are numerous and vary among sufferers. It is difficult for researchers to pin down a unifying explanation. Strings of commonality have allowed us to piece together what we do know. That nociceptors and blood vessels outside the brain are part of the story and sensitization of pain pathways may be the condition that predisposes people to migraines. However, scientists are still searching for answers as to why some people are wracked with migraines daily while others may never experience one.