THE ROUGH GUIDE TO PSYCHOLOGYHearing
Sound ripples through the air, causing three tiny bones inside your
ears to vibrate. From here, the sound is passed to your hair-lined inner
ear – known as the cochlear – which translates the signal into neural
activity. This neural signal then travels to the brain stem, onwards to
the thalamus before reaching the auditory cortex. To localize sounds,
your brain compares information arriving at the two ears in terms of
loudness and timing. A sound originating from straight ahead, for
example, will arrive at the two ears at the same time, whereas a sound
coming from the left side of space would obviously reach your left ear
first and would be perceived as louder in that ear compared with the
right ear.
The auditory channel provides another powerful demonstration of
how our perceptual experiences are based not just on what is served up
by our senses, but also on what our brains bring to the table. Listen to an
excerpt of music played backwards (Led Zeppelin’s “Stairway to Heaven”
is available on a website run by Jeff Milner: http://www.jeffmilner.com/back-
masking.htm) and it will sound like nonsense. However, if you’re then
told which words to listen out for in the backward music (the website
provides them) and you hear the song again, the words immediately
become clear. In fact, once you’ve been told the backward lyrics, you can’t
unhear them – the incoming sounds are permanently filtered through
your “top-down” expectations.
Although the situation is not as well understood as it is for vision, it’s
looking increasingly likely that, as with visual information, the auditory
pathway is divided along two parallel branches – one for “what” the
sound is and the other for “where” it is. Animal lovers should look away
now, because the most compelling evidence to date for this dual-pathway
account of auditory processing actually comes from a rather invasive
study with cats. In 2008 Stephen Lomber and Shveta Malhotra inserted
ultra-cold tubes into the brains of cats to selectively freeze brain-activity
in either the rear or frontal regions of their auditory cortex, which is
the part of the brain, near the ears, that processes sounds. Crucially,
the researchers found that when they froze activity in the frontal part
of the auditory cortex, the cats lost their ability to localize sounds, but
they could still discriminate between sounds. By contrast, the opposite
pattern of impairment was found when the researchers froze the rear
part of the cats’ auditory cortex. Cats are mammals, and if they have