is 2 < ce : hsp sp = 0 . 12 > < / ce : hsp > + < ce : hsp sp = 0 . 12 > < / ce : hsp > 2 < ce : hsp sp = 0 . 12 > < / ce : hsp > = < ce : hsp sp = 0 . 12 > < / ce : hsp > 4 ? meta - analyses of brain areas needed for numbers and calculations .
Persistent activity in the prefrontal cortex during working memory