Due to the nature of the reincarnation cycle, the Avatar can recall memories of past lives. This can be done directly through deliberate meditation in order to connect with a specific past Avatar, as demonstrated when Korra meditated in order to connect with Aang and relive Yakone's trial .  As a variation of this, the Avatar can call forth a spirit-like apparition of a past life, or even their current self, in order to communicate with them.  Recalling a past memory can also be done subconsciously when in the presence of people or objects important in a past life, or other certain triggers which can force the recollection of a memory. This was demonstrated by Aang when he recognized Roku upon seeing his statue, despite not knowing what he looked like,  as well as when Korra recognized Wan's teapot as her own and Iroh as a friend, despite having seen neither in her current lifetime.  The Air Nomads used this concept of subconscious recognition to find the next Avatar, which was the child that chose the Avatar relics due to their familiarity. 
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