Eyewitnesses and Dark Vision

eyewitness-19061Question: An eyewitness has departed a brightly-lit café at mid-night and walked down a street dimly-lit by weak street lamps. 30 minutes after leaving the café, they witness a hit-and-run. The eyewitness has given a description of the incident and the vehicle involved to investigators. How would their account be expected to differ from a day-time incident? Would their report be more or less detailed if they had seen the hit-and-run only five minutes after leaving the café? Why?

Answer: Cone receptors in our eyes are adapted for colour vision, daytime and detailed vision, whereas rod receptors are adapted for vision in dim light. Dark adaptation is the gradual improvement in the ability to see in dim light, as the rod receptors (needed for the dark) regenerate their retinaldehydes (regenerating molecules) slower than cone receptors. This is how we can move into daylight (or a brightly-lit café) and our vision adjusts virtually straight away, as opposed to suddenly enter a dark area (dimly-lit street at mid-night). With this in mind, the eyewitness would be able to see clearer 30 minutes after leaving the café than if it were only 5 minutes after leaving as it takes between 5 – 30 minutes for full dark adaptation to occur.

Something to consider with eyewitness testimony in the dark is that colour perception after twilight is unreliable to non existent, and the Purkinje effect can change the relative brightness of colours (e.g. reds appear darker and blue-greens appear brighter) which can make testimonies more unreliable. Red light however does not interfere with scotopic (dark adapted) vision.

About Stu Dunn

With a background in sales and behavioural science, I enjoy learning more about people, behaviour, psychology - which led into motivation - and more recently - sales again. Having started my own real estate company with my wife, it's time to merge interests.
This entry was posted in Psychology and tagged , . Bookmark the permalink.