NSCell KVO monitoring highlighted

Question

I want to monitor the buttons cell highlighted state, but the callback gets never call, although the highlight changes

monitoring the keyPath "state" works, but why doesn't monitoring highlighted?

- (void)awakeFromNib {

    [super awakeFromNib];

    [self.cell addObserver:self
              forKeyPath:@"highlighted"
                 options:(NSKeyValueObservingOptionNew | NSKeyValueObservingOptionOld)
                 context:NULL];


}

- (void)observeValueForKeyPath:(NSString *)keyPath
                      ofObject:(id)object
                        change:(NSDictionary *)change
                       context:(void *)context
{


    ...

}