Professor Nilli Lavie (UCL Institute of Cognitive Neurosciene) explains that too much visual information causes a phenomenon known as 'load induced blindness',
with an effect akin to dimming the lights.
Read: Daily Mail,
More: BBC News,
UCL News,
Listen: BBC Radio 4 'All in the Mind',
More: BBC World Service 'The Science Hour' (from 12 mins 44 secs)