When I call a function with single argument of type bool with empty parentheses, what is the value of a bool parameter?

I just wonder what is the value of bool parameter of member function, when function is called without passing value?
Is it false? Why is it false?

bool CSVread::Reset( bool partial_reset /* = false */ )  

then reset is called as

obj.reset()

No argument is passed so what is value of partial_reset and why? In visual studio it is false, but why?

I cannot find any definition of bool value as being false for function argument?
Isn't function argument part of function scope, so it should be of automatic storage and thus its default value is undefined?