Unravelling the neural basis of hearing with Professor Jennifer Bizley at the UCL Ear Institute
Writer: Ebani Dhawan
Editor: Karolay Lorenty
The brain is bombarded with sensory input coming from our eyes, our ears and the like. Our senses are what we rely on, each providing a different aspect of our world view.
Hearing is the sum of a complex series of actions; many parts of the ear work in unison to convert sound waves and vibrations into information the brain understands. It processes this information in the temporal and frontal lobe and interprets those waves and vibrations as auditory information (sound).
Looking into the nitty-gritty, the basilar membrane on the cochlea is deformed by sound wave, opening and closing ion channels, making it one of the fastest sensory reception systems. This speed is crucial – timing is an important cue for us to distinguish where sounds are coming from. Our brain compares the tiny differences in the way that sounds affect each ear. A noise coming from the right is slightly louder in your right ear, and reaches it fractionally earlier than your left. The brain uses these differences, even as small as a 100,000th of a second, to calculate where the sound is coming from.
But what happens when more than one sound is coming from one source? How are we able to distinguish between voices when they are speaking over each other?
Bizley’s lab conducted an experiment in which two gibberish sentences, spoken by different voices, were layered over each other. Participants, both male and female, were still able to separate the two voices. Unmixing sounds requires us to reconstruct the auditory scene by separating the sounds and putting the right ones together. Our brains are attuned to certain cues that allow us to group elements. If they are at a similar pitch, they’ll be grouped together and reconstructed to make a whole sentence.
First, try and watch it with your eyes closed. Can you differentiate the voices? What about with your eyes open?
According to Bizley’s studies, it’s easier to differentiate the voices when we have an added visual element. When a face and mouth are moving in time with sound, the visual element wins. Our brains weigh signals according to reliability, and hence would conclude that the sound is originating from that mouth. Visual signals enhance our ability to segregate and select the sound we want to hear. Visual stimuli are so embedded into our auditory process that they are able to modify auditory cortical responses. This audio illusion illustrates this concept perfectly.
Our senses are not as discrete as we may have assumed them to be. They’re intertwined with each other, and intrinsically influenced by our experiences and our past. So, can we really trust them?