Before revision:  A critical feature of this architecture is that it is both higher-order and asymmetric: there is a vast number of perceptual states is nested under "present", but a much smaller number of possible states is nested under "absent".
After revision:  A critical feature of this architecture is that it is both higher-order and asymmetric: a vast number of perceptual states is nested under "present", but a much smaller number of possible states is nested under "absent".