David Eagleman is guest blogging this week.
When light strikes your eyes, it takes some hundreds of milliseconds before you become conscious of the event. As a consequence, you are always living in the past. This strange fact of our existence is well known is neuroscience, but there’s an interesting, underappreciated consequence: you may not ever become aware of the thing that kills you.
Cormac McCarthy addresses this point in his post-apocalyptic novel The Road, in a scene in which the main character has his pistol leveled on a miscreant. The malefactor challenges: “you won't shoot....they [my companions] will hear the shot.”
The protagonist replies, “Yes they will. But you won’t.”
“How do you figure that?”
“Because the bullet travels faster than sound. It will be in your brain before you can hear it. To hear it you will need a frontal lobe and things with names like colliculus and temporal gyrus and you won't have them anymore. They’ll just be soup.”
One way to appreciate the slowness of your perception is to compare it to the speed of mechanical devices. Take this incredible, sobering "anatomy of a crash," as described in an Australian magazine and echoed on Tom Vanderbilt’s blog. With fine-grained temporal resolution, it analyzes what happens when a stationary Ford Falcon XT sedan is struck in the driver’s door by another vehicle traveling at 50 kilometers per hour:
0 milliseconds - An external object touches the driver’s door.
1 ms - The car’s door pressure sensor detects a pressure wave.
2 ms - An acceleration sensor in the C-pillar behind the rear door also detects a crash event.
2.5 ms - A sensor in the car’s centre detects crash vibrations.
5 ms - Car’s crash computer checks for insignificant crash events, such as a shopping trolley impact or incidental contact. It is still working out the severity of the crash. Door intrusion structure begins to absorb energy.
6.5 ms - Door pressure sensor registers peak pressures.
7 ms - Crash computer confirms a serious crash and calculates its actions.
8 ms - Computer sends a “fire” signal to side airbag. Meanwhile, B-pillar begins to crumple inwards and energy begins to transfer into cross-car load path beneath the occupant.
8.5 ms - Side airbag system fires.
15 ms - Roof begins to absorb part of the impact. Airbag bursts through seat foam and begins to fill.
17 ms - Cross-car load path and structure under rear seat reach maximum load.
Airbag covers occupant’s chest and begins to push the shoulder away from impact zone.
20 ms - Door and B-pillar begin to push on front seat. Airbag begins to push occupant’s chest away from the impact.
27 ms - Impact velocity has halved from 50 km/h to 23.5 km/h. A “pusher block” in the seat moves occupant’s pelvis away from impact zone. Airbag starts controlled deflation.
30 ms - The Falcon has absorbed all crash energy. Airbag remains in place. For a brief moment, occupant experiences maximum force equal to 12 times the force of gravity.
45 ms - Occupant and airbag move together with deforming side structure.
50 ms - Crash computer unlocks car’s doors. Passenger safety cell begins to rebound, pushing doors away from occupant.
70 ms - Airbag continues to deflate. Occupant moves back towards middle of car.
Engineers classify crash as “complete”.
150-300 ms - Occupant becomes aware of collision.
The last line is the zinger. Early studies by Benjamin Libet suggest that the last line should perhaps read as high as 500 ms, although others, such as Daniel Dennett, have correctly pointed out that it is impossible to measure the moment of onset of conscious experience, so the exact timing will never be known.
Just as the explorer David Livingstone appreciated the biological kindness of stress-induced analgesia, there may an equivalent kindness in the slowness of perception.
David Eagleman is a neuroscientist and a writer. His book of literary fiction, Sum, debuted internationally this month.