By Emma Young

It’s well-known that we can miss apparently obvious objects in our visual field if other events are hogging our limited attention. The same has been shown for sounds: in a nod to Daniel Simons’ and Christopher Chabris’ famous gorilla/basketball study that demonstrated “inattentional blindness”, distracted participants in the first “inattentional deafness” study failed to hear a man walking through an auditory scene for 19 seconds saying repeatedly “I am a gorilla”. Now, two new studies separately show that a very similar effect occurs in relation to touch (inattentional numbness) and to smell (inattentional anosmia).

Sandra Murphy and Polly Dalton (a co-author on the inattentional deafness paper) at Royal Holloway, University of London report in the journal Cognition on inattentional numbness. They wanted to go beyond the way we rapidly tune out ongoing tactile stimulation, like the sensation of our clothes, and explore what happens when we’re touched more than once out of the blue. (“If someone taps us on the shoulder, are we less likely to notice their other hand going into our pocket?” they write.)

To investigate, they recruited 37 women and 45 men aged 18-47. The participants sat with their hands in front of them, palms up. Bone-conduction hearing aids, which would deliver vibration stimulations, were taped to their palms, and a black cloth was placed over the top, so they couldn’t see any movement. The participants also listened to white noise, to mask any sounds.

On each trial, the participants were instructed to count the number of vibrations presented to their hands (until the end of each block of trials, the sensations were always to just one of their hands – either the left, or right, counterbalanced across participants). Half the participants were given a relatively easy, low attentional-load version of this task: to count all the vibrations applied to their palms. The others had to keep separate counts of the number of constant vibrations and the number of pulsed vibrations – a task that placed a bigger demand on their attentional resources.

Each experimental block consisted of 16 trials. On the 16th, an unexpected extra stimulus was applied to the other hand at the same time as the target sequence on the usual hand. Ninety-two per cent of participants in the low-load condition noticed this extra stimulus, whereas 69 per cent of those in the high-load group completely missed it – suggesting they had experienced “inattentional numbness”.

“The present study provides the first robust demonstration of inattentional numbness to a one-off tactile event that is completely unexpected and therefore genuinely unattended,” Murphy and Dalton write.

This study investigated touch awareness when the brain was already focusing on a touch task. But there’s evidence from earlier work that, for inattentional effects to occur, the two stimuli do not have to involve the same senses, and the new paper in Psychological Science on inattentional anosmia also finds this.

Sophie Forster, at the University of Sussex, and Charles Spence, head of the Crossmodal Research Laboratory at the University of Oxford, looked at the effects of performing a high vs. low attentional-load visual task on scent awareness.

Across a series of experiments, groups of participants had to repeatedly search for a target letter – X or N – arranged in a circle together with either five small letter “o”s (the easy, low attentional-load version of the task) or five similarly-sized angular non-target letters, such as W and Z (the high-load version), while exposed to the background scent of coffee (open bags of coffee beans were concealed in the room).

Forster and Spence found that those in the high-load groups were much less likely to afterwards report having detected that scent, suggesting that the attention they’d used up on the visual task had left them with a temporary lack of smell, or anosmia. (In two of the experiments, 42.5 per cent fewer participants in the high visual load condition, compared to the low-load condition, reported noticing the smell.)

“The results of the present study establish the phenomenon of inattentional anosmia for the first time,” Forster and Spence write. “In real-world terms, our study implies that people are significantly less likely to notice ambient smells in their surroundings when they are engaged in a visually-demanding task.”

One of the experiments revealed a further intriguing detail: participants could become habituated to the coffee scent even while their attention was absorbed elsewhere, suggesting that the scent had been processed at some level even though it had not reached conscious awareness. This habituation to the scent meant that even after the task was over, and they had plenty of attention to spare, they were less likely to detect the aroma.

“The present study reveals that olfactory attention shares a key common characteristic with visual and auditory attention, in terms of the effect of perceptual capacity limits on irrelevant processing,” the researchers write (my italics).

It’s worth noting that in these new studies, neither the smell of coffee nor the gentle, brief secondary touch stimulus were in any way threatening. Not only were they irrelevant to the task at hand, they were also irrelevant to survival.

It remains to be confirmed whether inattentional effects on our multi-sensory awareness are the same for more important or threatening incoming stimuli. There’s reason to think they will be – for instance, there’s evidence that pain perception is reduced when other activities or events are hogging our attention (when playing a VR game, for example). It would be interesting to see work exploring whether we can fail even to notice potentially dangerous smells – such as burning – and potentially damaging touch stimuli while we’re concentrating on something else.

—Inattentional numbness and the influence of task difficulty

—“What Smell?” Temporarily Loading Visual Attention Induces a Prolonged Loss of Olfactory Awareness

Emma Young (@EmmaELYoung) is Staff Writer at BPS Research Digest