Is additional damage from a spell counted as part of the same attack?
For a specific example, let's look at witch bolt (PHB, page 289). There may be others, but I am far too lazy to look through each and every one for other examples. Witch bolt says:
Make a ranged spell attack against that creature. On a hit, the target takes 1d12 lightning damage, and on each of your turns for the duration, you can use your action to deal 1d12 lightning damage to the target automatically.
In connection with this question, if you were to score a critical hit on the initial attack roll for witch bolt, would the additional damage rolls on your other turns be counted as part of the same attack and thus also be critical hits?