Negative PWM(is it possible?)

Hi friends,
I am working on kind of line follower which should also be capable of moving backwards in some cases. I have defined directions of the motors in the void setup() funciton i.e. :

pinMode(direction1,HIGH) ;
pinMode(direction2,HIGH) ;

My question is if I want to direct my robot backwards, can I just set the speed to ''negative'' value? . Or should I change the direction parameter rather than speed. By the way the motor driver I am using has 2 direciton parameter for each driving motor.

The later, you must control by using both direction and speed commands, no negative pwm allowed.