Fade Out (Get Ready For Death)

"Detecting a tumor is hard and there will always be some amount of uncertainty. There are two kinds of noise factors that contribute to the uncertainty: internal noise and external noise.
External noise: There are many possible sources of external noise. There can be noise factors that are part of the photographic process, a smudge, or a bad spot on the film. Or something in the person's lung that is fine but just looks a bit like a tumor. All of these are to be examples of external noise. While the doctor makes every effort possible to reduce the external noise, there is little or nothing that they can do to reduce internal noise.
Internal noise: Internal noise refers to the fact that neural responses are noisy. To make this example really concrete, let's suppose that our doctor has a set of tumor detector neurons and that they monitor the response of one of these neurons to determine the likelihood that there is a tumor in the image (if we could find these neurons then perhaps we could publish and article entitled ``What the radiologist's eye tells the radiologist's brain''). These hypothetical tumor detectors will give noisy and variable responses. After one glance at a scan of a healthy lung, our hypothetical tumor detectors might fire 10 spikes per second. After a different glance at the same scan and under the same conditions, these neurons might fire 40 spikes per second.
Internal response: Now I do not really believe that there are tumor detector neurons in a radiologist's brain. But there is some internal state, reflected by neural activity somewhere in the brain, that determines the doctor's impression about whether or not a tumor is present. This is a fundamental issue; the state of your mind is reflected by neural activity somewhere in your brain. This neural activity might be concentrated in just a few neurons or it might be distributed across a large number of neurons. Since we do not know much about where/when this neural activity is, let's simply refer to it as the doctor's internal response.
This internal response is inherently noisy. Even when there is no tumor present (no-signal trials) there will be some internal response (sometimes more, sometimes less) in the doctor's sensory system. "