ValueChanged() signal is not emitted by QScrollBar when Page Up/Dn is Hit in QPlainTextEdit having scrollbar

#include <QtGui>

class myPlainTextEdit:public QPlainTextEdit
{
	Q_OBJECT
public:
	myPlainTextEdit(QWidget *parent=0)
	{
		QString s;
      for (int i=0; i<1000; i++)
		  s += QString("Line #%1\n").arg(i);
	  
     setPlainText(s);
	 connect(verticalScrollBar(),SIGNAL(valueChanged(int)),this,SLOT(myScrollBarSlot(int)));
	
	}

	public slots:
		void myScrollBarSlot(int value)
		{
			qDebug()<<"changed value ="<<value;
		}
};

#include "main.moc"
int main(int argc, char ** argv)
{
   QApplication app(argc, argv);
   myPlainTextEdit w;
   w.show();
   return app.exec();
}