Loki does pause, stop. It's Frigga's voice. And she solidifies in front of him. For a second he lets his magic fizzle. Then he regathers it, fire in his eyes.

"You dare to try to use my mother's form to trick me?! You will pay for that, defiler of memories."

He throws an even stronger spell in an arc in front of him, again meant to destroy any illusions. He's not watching, though; while he can attack his fake mother, he can't look while doing it.