How is the signal sent to the brain of the salmon and where does is it come from?

Question: How is the signal sent to the brain of the salmon and where does is it come from?
- Keywords:
  salmon
Asked by confusedoctopus to Madgie on 11 Nov 2013.
- Marlene Lorgen answered on 11 Nov 2013:
  
  Great question! So the light signal comes from the sun. In humans, we receive light signals via the retina in our eye. However, salmon can receive light directly to the brain due to having very thin skulls and things called ‘photoreceptor cells’ in the brain, as well as still getting light through the eyes. This means the salmon can tell if it is day or night, and changes in the length of the light signal lets the salmon know what season it is 🙂

Question: How is the signal sent to the brain of the salmon and where does is it come from?