Show one image exclusively to one eye and a different image exclusively to the other eye and rather than experiencing a merging of the images, an observer’s percept will flit backwards and forwards randomly and endlessly between the two. This “binocular rivalry“, as it’s known, has been of particular interest to psychologists because it shows how the same incoming sensory information can give rise to two very different conscious experiences. Now, in a research first, psychologists have shown that a similar process occurs with our sense of smell. If one odour is presented to one nostril and another odour is presented to the other nostril, a person will experience “binaral rivalry” – sensing one smell and then the other, backwards and forwards, rather than a blending of the two.
Wen Zhou and Denise Chen presented twelve participants with the smell of rose to one of their nostrils and the smell of a marker pen to their other nostril. The odours were presented intermittently, every twenty to thirty seconds, to prevent “adaptation”, which is the tendency for brain cells to gradually reduce their response to a continuous stimulus. After each break in the smells, the participants indicated on a visual scale whether they had detected the scent of rose or of marker pen. Just as with binocular rivalry, the participants’ perceptual experience fluctuated back and forth randomly between the two scents.
The researchers believe this nostril rivalry is related in some way to the process of adaptation, both in the receptor cells in the nose and in the part of the brain that processes smells. For example, when repeatedly presented with a balanced mix of both smells, the participants’ sensory experience fluctuated between rose and marker pen, presumably because of adaptation in the brain: as central neurons tired of one odour, their response to the other became more dominant and back again. The researchers also showed that adaptation occurs in the nose: swapping the bottles of odour around from one nostril to the other reinstated participants’ experience of a given smell after it had previously faded through continuous sniffing.
“Our work sets the stage for future studies of this phenomenon,” the researchers said.
Zhou, W., & Chen, D. (2009). Binaral Rivalry between the Nostrils and in the Cortex. Current Biology, 19 (18), 1561-1565 DOI: 10.1016/j.cub.2009.07.052