What is the difference between synchronized on lockObject and using this as the lock?

I know the difference between synchronized method and synchronized block but I am not sure about the synchronized block part.

Assuming I have this code

class Test {
  private int x=0;
  private Object lockObject = new Object();

  public void incBlock() {
    synchronized(lockObject) {
      x++;
    }
    System.out.println("x="+x);
  }

  public void incThis() {  // same as synchronized method
    synchronized(this) {
      x++;
    }
    System.out.println("x="+x);
  }
}

In this case what is the difference between using lockObject and using this as the lock? It seems to be the same to me..

When you decide to use synchronized block, how do you decide which object to be the lock?