December 11, 2015
A new study published in the Journal of Neuroscience has found that when people concentrate more on visual tasks, they can go deaf momentarily. Based on brain scans from 13 volunteers, the researchers from UCL, the Wellcome Trust and Newcastle University concluded that the senses of hearing and vision share a limited neural resource. So, when people are focussed on a demanding visual task, the brain’s response to sound can be significantly reduced. Examination of people’s ability to detect sounds during the visual demanding task also showed a higher rate of failures to detect sounds, even though the sounds were clearly audible and people did detect them when the visual task was easy. Lead researcher Dr Maria Chait said the phenomenon of ‘inattentional deafness’, where we fail to notice sounds when concentrating on other things, confirms earlier research.
However, for the first time that they have been able to determine, by measuring brain activity in real-time using MEG (magnetoencephalography), that the effects are driven by brain mechanisms at a very early stage of auditory processing which would be expected to lead to the experience of being deaf to these sounds.
“This was an experimental lab study which is one of the ways that we can establish cause and effect. We found that when volunteers were performing the demanding visual task, they were unable to hear sounds that they would normally hear,” explains Dr Chait of the UCL Ear Institute. “The brain scans showed that people were not only ignoring or filtering out the sounds, they were not actually hearing them in the first place.”
“Inattentional deafness is a common experience in everyday life, and now we know why,” says co-author Professor Nilli Lavie (UCL Institute of Cognitive Neuroscience) “For example, if you try to talk to someone who is focusing on a book, game or television programme and don’t receive a response, they aren’t necessarily ignoring you, they might simply not hear you! This could also explain why you might not hear your train or bus stop being announced if you’re concentrating on your phone, book or newspaper.
“This has more serious implications in situations such as the operating theatre, where a surgeon concentrating on their work might not hear the equipment beeping. It also applies to drivers concentrating on complex satnav directions as well as cyclists and motorists who are focusing intently on something such as an advert or even simply an interesting-looking passer-by. Pedestrians engaging with their phone, for example texting while walking, are also prone to inattentional deafness. Loud sounds such as sirens and horns will be loud enough to get through, but quieter sounds like bicycle bells or car engines are likely to go unheard.”
Due to capacity limits on perception, conditions of high perceptual load lead to reduced processing of unattended stimuli (Lavie et al., 2014). Accumulating work demonstrates the effects of visual perceptual load on visual cortex responses, but the effects on auditory processing remain poorly understood. Here we establish the neural mechanisms underlying “inattentional deafness”—the failure to perceive auditory stimuli under high visual perceptual load. Participants performed a visual search task of low (target dissimilar to nontarget items) or high (target similar to nontarget items) load. On a random subset (50%) of trials, irrelevant tones were presented concurrently with the visual stimuli. Brain activity was recorded with magnetoencephalography, and time-locked responses to the visual search array and to the incidental presence of unattended tones were assessed. High, compared to low, perceptual load led to increased early visual evoked responses (within 100 ms from onset). This was accompanied by reduced early (?100 ms from tone onset) auditory evoked activity in superior temporal sulcus and posterior middle temporal gyrus. A later suppression of the P3 “awareness” response to the tones was also observed under high load. A behavioral experiment revealed reduced tone detection sensitivity under high visual load, indicating that the reduction in neural responses was indeed associated with reduced awareness of the sounds. These findings support a neural account of shared audiovisual resources, which, when depleted under load, leads to failures of sensory perception and awareness.
The present work clarifies the neural underpinning of inattentional deafness under high visual load. The findings of near-simultaneous load effects on both visual and auditory evoked responses suggest shared audiovisual processing capacity. Temporary depletion of shared capacity in perceptually demanding visual tasks leads to a momentary reduction in sensory processing of auditory stimuli, resulting in inattentional deafness. The dynamic “push–pull” pattern of load effects on visual and auditory processing furthers our understanding of both the neural mechanisms of attention and of cross-modal effects across visual and auditory processing. These results also offer an explanation for many previous failures to find cross-modal effects in experiments where the visual load effects may not have coincided directly with auditory sensory processing.