Chords of perception

Your brain is endowed with the fascinating ability to detect saliency in the world.  This means that it works to recognize patterns in the presence of excessive sensory information, which usually contains abundant noise.  Each of your sensory systems is tuned to maximize this signal to noise ratio.

In the visual system, for example, you can infer that continuous objects exist even when their appearance is distorted by the presence of other objects in the visual scene or by the absence of lines or contours in the perceived object.  Given this image of a Kanizsa Triangle, what shapes do you see?

Most people would report observing an arrangement of circles and triangles.  However, there are no complete circles or triangles present in the image!  Instead, parts of these shapes are arranged such that the mind has to fill in the gaps to construct continuous perceptual objects.  Similarly, a dashed line is perceived as continuous when shapes cover the breaks in the line.

This type of mental “filling in” is not exclusive to the visual system, as it is also observed in the auditory system.  One example is the auditory continuity illusion.  A periodic auditory stimulus is perceived as an uninterrupted tone when the gap between the periodic beeps is filled with noise.  This noise is often called an occluder because it masks the gap and occludes the perception of this break in the periodic stimulus.  In this sound clip, you can hear an example of an auditory stimulus that produces this illusion.  As the sound bite progresses, it seems as if the periodic stimulus is continuous and can be heard playing through the noise!

There are several neural mechanisms one could posit for how this illusion manifests.  One school of thought holds that in order for this illusion to occur, the occluder (the noise between the periodic beeps) must provoke peripheral neural responses in the cochlea and auditory nerves that completely mask the gap.  This popular idea is known as the peripheral masking theory.  It is analogous to the way that we perceive the green line in the figure above as continuous simply because the breaks in the line are “occluded” by the purple triangle and square.

Although there is significant evidence corroborating the peripheral masking theory, some recent evidence contradicts this notion.  A study published in the March 2012 issue of the Journal of Neuroscience by Riecke and colleagues showed that an occluder that does not completely mask breaks in the periodic tone, is still capable of producing the illusion!  The researchers suggested that, in this case, the illusion was the result of the “failure of the auditory system to analyze the fine-grained temporal structure of the peripheral neural responses to sound.”

While the mechanisms underlying this illusion are still being determined, some of the benefits of this type of perceptual processing are clear.  Rather than interpreting this phenomenon as evidence that the auditory system is a “gullible” or ineffective sensory modality, this type of auditory processing may have evolved as a means for us to extract valuable information from sounds, such as speech, even when we cannot hear the sounds clearly.  Just think of the times when you were listening intently to a lecture in your favorite class, or to a character in your favorite TV show; you were able to hear what the person was saying even in the presence of competing sounds.  So, next time your Mom tells you to clean you room, don’t expect your MP3 player to be a viable excuse!!