Arduino : Finding ADC & Pin Change in same pin
Some MCU from Atmel have an embedded analog comparator.
The purpose of this module is to detect the sign of the voltage diff present between the + input and the - input. 

With the analog comparator enabled, you can put its threshold at Vcc/2 for instance and you will get an interrupt when the analog voltage passes below or above the threshold.

Usually, the analog inputs are multiplexed with the analog comparator.
I am not 100% sure that you can enable the ADC and the analog comparator at the same time on the same pin. It's something you will have to check in the MCU datasheet.

In any case, it's not needed to be able to enable both at the same time. If you have your ADC running, you can check the voltage level returned from you ADC in your code and trigger a soft interrupt if the voltage passe above of below you defined threshold. Using this approach, you can easily implement a shmitt trigger also.

The analog comparator is useful only if you want to stop your ADC.