Welcome to Part II of our ISC Cocktail Party science demo debriefing (check out Part I here). Keep reading for explanations into a few more of the illusions you saw at the party. You can find Part III here.
3D Auditory Experience
First, listen to this video using headphones and with your eyes closed.
Did it feel like the action in the sound clip was happening right around you? This illusion’s eerie effects arise from how it took advantage of the way humans (and other animals) localize sounds in space.
The human auditory system has the incredible ability to determine where a sound is coming from, even when the object that is making the sound cannot be seen. When you cannot find your phone and you have someone call it for you so that you can follow the sound of the ring to its source, you are tapping into this very fine-tuned mechanism. The auditory system uses two cues to determine the location of an incoming sound: intensity (or volume) and timing. If a sound is coming from your right, it will sound louder to your right ear than your left ear. It will also reach your right ear a few milliseconds before it reaches your left ear. Using those tiny differences, the brain can interpret where in space the sound is coming from.
The Virtual Barber Shop recording in the link above demonstrates this ability very clearly. It sounds so real, you’ll get the chills!
This audio clip was made using a recording apparatus like the one pictured below. Each “ear” in this model head contains a microphone, and since they are arranged in the same way our ears are (the same distance apart with a dense object between them) the sounds they pick up are sensitive to the same timing and intensity cues as your actual ears. As a result, your headphones are delivering the sound in the same way you’d hear it if you were actually in the room where it was recorded. The sound you hear matches up succinctly with your real-world perception. If you close your eyes, you feel like you are actually in the Barber Shop, since all of the auditory clues of a real-world situation are right there in the recording.
By the way, we are not the animals with the greatest ability to localize sounds. Since our ears are at the same height, the intensity and timing cues between our ears aren’t as drastic as they could be. Look at the skull of the Boreal Owl below, for example.
Its ears are actually offset from one another. This results in an even greater difference in how they sense sounds, as they are now at different heights. This means that they can sense differences in intensity and timing not only if a sound is to the left or right of them, but also if a sound is above or below them. Since owls fly in vertical space, it’s important for them to be able to sense the elevation of a sound. (Humans have this ability too, but it’s not as finely tuned. We sense elevation by how the sound bounces off the part of the ear that sticks out, or the pinna). This adaptation makes hunting at night much easier, since owls oftentimes rely solely on auditory cues to locate their pray. Maybe if we had offset ears we could avoid having to search the whole house trying to find that ringing cell phone!
If we payed attention to every detail of everything we ever looked at, our brains would be overloaded with information, and we would never be able to function without being constantly distracted. To protect us from this constant bombardment of sensory information, our brain picks and chooses what to attend to and what to ignore. Typically, we pay attention to the important things in or visual field. For instance, if you are standing in a savanna and you see a tiger approaching you, you will definitely notice the tiger, but you might not be able to recall which way the grass around it was blowing in the wind. This selective attention can cause a very strange phenomenon called “change blindness.” Change blindness refers to the phenomenon where we do not detect changes in our visual field due to some disruption. These disruptions can be caused by eye movements, other objects in the visual field, or lapses in attention. Although us confident, humans believe that we attend to everything in our environment, it is typically not the case; we often forget the little details that aren’t important. Can you name what keys are directly above your space bar without looking? You’ve probably looked at your keyboard thousands of times, but have you ever payed that much attention? On top of that, you probably wouldn’t notice right away if someone switched the letter labels on two of your keyboard keys. This latter instance is an example of change blindness.
Above is an example of a change blindness task. You are shown a picture for a short amount of time, then it is briefly disrupted, then a new picture appears that is almost exactly the same as the first picture except for one small feature. You can probably see the difference in this example because the pictures are presented side-by-side, but imagine trying to determine the difference if the images were presented one at a time. Don’t believe us? Check out this website that allows you to control the timing of the task. Even a brief disruption in the display makes it much harder. However, if you change it so that you can see both images at one time, it becomes very easy, since we are good at detecting changes in an uninterrupted scene. For some pretty surprising examples of change blindness, check out this video from Nova!
We still have one more illusion to cover. Check out Part III here!
Will Beischel is a graduate student in psychology at the University of Michigan.