Alex showed surprise and anger when confronted with a nonexistent object or one different from what he had been led to believe was hidden during the tests .
Alex passed increasingly difficult tests measuring whether humans have achieved Piaget 's Substage 6 object permanence .
Alex showed surprise and anger when confronted with a nonexistent object or one different from what Alex had been led to believe was hidden during the tests .
Alex passed increasingly difficult tests measuring whether humans have achieved Piaget 's Substage 6 object permanence .