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Posted by Jef Akst
[Entry posted at 20th August 2009 05:00 PM GMT]When the nostrils sense different smells, they may be duking it out for the brain’s attention, according to a study published online today (August 20) in Current Biology. Such rivalry, well-documented in other sensory systems, has never before been shown for the olfactory system.
“It’s an interesting study,” said Jay Gottfried, a neuroscientist and neurologist at Northwestern University in Illinois, who was not involved in the research. “It provides some challenging and exciting new ways of thinking about olfactory processing.”
Rivalry between the two eyes – so-called binocular rivalry – occurs when the eyes are presented with conflicting stimuli. Because two objects cannot coexist in the same physical location, the brain has difficulty processing the nonsensical input, resulting in a repeated alteration of which image the viewer actually perceives.
Cognitive psychologist Denise Chen of Rice University in Texas and her graduate student Wen Zhou, currently at the Institute of Psychology, Chinese Academy of Sciences, demonstrated a similar effect in the olfactory system. In 12 subjects, they presented each nostril simultaneously with two different bottled scents – one that smelled like roses, and the other like markers. Subjects reported the smell to be more like markers, then more like roses, and then more like markers again, suggesting that the nostrils may actually be alternating which signal is sent to the brain for processing.
However, when the researchers presented a bottled mixture of both odors to both nostrils simultaneously, they found a similar effect. That might mean the rivalry is occurring within the brain after both nostrils deliver their smells to the olfactory bulb or cortex, and not in how the olfactory epithelium is sending the signals, other researchers say. In other words, there may be a rivalry between the smells, and not the nostrils per se, said neuroscientist Frank Tong of Vanderbilt University in Tennessee, who did not participate in the study.
“To really distinguish between binaral” – between the nostrils – “or just olfactory rivalry, they need to do other manipulations,” agreed perception psychologist Sheng He of the University of Minnesota, who was also not involved in the research, “[but] it’s probably not very easy to do that.” Because olfaction is a “slower” sense than vision, he explained, it does not lend itself to the quick switching of stimuli used to confirm binocular rivalry.
Chen agreed that the cause of the observed rivalry effect is still unclear. “We’re showing an olfactory phenomenon that resembles binocular rivalry, [but] the exact mechanism of this phenomenon needs to be studied in future studies,” she said.
Still, the alternating perceptual interpretations of olfactory stimuli is a novel finding, said Gottfried. “There have been a lot of studies on odor mixtures,” he said, and while many have shown that the intensity of the components greatly affects perception, “none of these reports have observed fluctuations.”
This effect, however, does not appear to be ubiquitous. Although Chen and Zhou’s subjects often reported that they experienced a predominately rose-like or predominately marker-like smell, there were several reports of a smell that was somewhere in between. They appear to be “experiencing more blending than they would in binocular rivalry,” Tong said. “It’s not totally exclusive.”
Gottfried believes this result jives well with the inherent differences between the visual and olfactory systems. In vision, “it’s a very artificial experimental arrangement where you’re playing a trick on the visual system, but in olfaction, it’s not so artificial.” Because smells, unlike objects, can overlap in space, the two nostrils might sense slightly different odors. “Receiving conflicting inputs – and creating synthetic integrated percepts – is precisely what the olfactory system is meant to do in the first place.”
This then begs the question of “How much does this [rivalry effect] depend on the particular smells that were chosen?” said Tong. “Some smells might lead to blends [while] others might lead to rivalry.”
Clearly, many questions remain, Chen admits, but still, she said, “it’s a very interesting olfactory illusion” which should shed further light on human olfaction.