How should I name cardinal directions to correspond to character movement?

Hi,

I'm new to programming and have been piecing together sample code to create a game while I learn the ropes. I'm using EX2D to animate my sprites. I'm trying to use the following script to assign a specific animation to each direction the character moves while up, down, left, or right is pressed. How should I name Vertical and Horizontal in the GetButton statements to correspond to each cardinal direction?

Right now, the only error I get is "Object reference not set to an instance of an object"

Thanks!

 var sprite: exSprite;
 
     function Start () {
 
     }
 
     function Update () {
 
 while(Input.GetButton("Vertical"))
  { 
  sprite.spanim.Play("PlayerWalkUpAnimation");
  }
 while(Input.GetButton("Vertical"))
  { 
  sprite.spanim.Play("PlayerWalkDownAnimation");
  }
 while(Input.GetButton("Horizontal"))
  { 
  sprite.spanim.Play("PlayerWalkLeftAnimation");
  }
 while(Input.GetButton("Horizontal"))
  { 
  sprite.spanim.Play("PlayerWalkRightAnimation");
  }
 
 }