Learning may occur as a result of habituation or classical conditioning, seen in many animal species, or as a result of more complex activities such as play, seen only in relatively intelligent animals. Learning may occur consciously or without conscious awareness. There is evidence for human behavioral learning prenatally, in which habituation has been observed as early as 32 weeks into gestation, indicating that the central nervous system is sufficiently developed and primed for learning and memory to occur very early on in development. According to James Zull, Professor of Biology and Biochemistry at Case Western University, "Learn­ing is phys­i­cal. Learn­ing means the mod­i­fi­ca­tion, growth, and prun­ing of our neu­rons, connectionscalled synapsesand neu­ronal net­works, through expe­ri­ence".