How to get header values in laravel testing

This is how i get response in test Case

$response = $this->call('POST','/api/auth/login',['username'=>'xx','password'=>'xxx'], [/* cookies */], [/* files */], ['HTTP_ClientSecret' => 'xxxx']);

Then we can get response content by like this

$response->getContents()

i want to know how to get response header data ?