>Wut. (let's disregard that a claim could also be seen as an object too).
A statement about reality can be based on a process of empirical observation, but the objects which the statements describe are not empirical. They are phenomena. It's a problem of scope. Light isn't empirical, but claims about light like "Light is both a particle and a wave" can be described as empirical based on the process by which they are derived.
>How is "the brain received a signal" not an empirical claim by nature of the objects it concerns?
Because we cannot empirically describe the process by which the signal is generated. If apply a signal to a computer we can describe that we generated a signal, but we cannot create an objective reference frame to describe conscious phenomena because language processes are subjective and cannot be directly converted into objectively understandable claims. Therefore, even if we did an empirical test, such as telling a person X and observing the reaction of the electrons in the brain, we can't actually empirically describe the process by which the conscious mind interacts with the language statement X, which means we can't create a description the induced change to the system state.
Imagine a 1x1x1 meter cube in space which emits no light, heat, sound, gravitation, radio waves, etc. etc. such that no physical process can be used to describe what occurs inside the cube. You throw a sledgehammer into the cube. You observe that a sledgehammer emerges from the far side the cube. You may heuristically determine that the space inside the cube is empty and the sledgehammer passed through it, but you cannot make an empirical claim about the contents of the inside of the cube. You may even time, measure, and in all ways observe the process of throwing objects through the cube, but since the inside of the cube cannot be described empirical claims about the inside still cannot be generated.