sentence: To generate a population of OFF neurons , we can reflect the tuning curves in the x = 0 axis by multiplying α by − 1 , which yields the following pair of transformations :( 14 ) α i = ± f − 1 ( ri ) 1 − ai ( 15 ) β i = − aif − 1 ( ri ) 1 − aiwhere the ± indicates ON / OFF , respectively and by f ( x) .