Zeki claims that he and his collaborators managed to localize and quantify subjective mental states for the first time in human history. That means, to operationalize perceptions, thoughts, prejudices, biases, beliefs, desires, or emotions. When Semir Zeki refers to the localization of a subjective mental state, he means that methods such as fMRI identify statistically significant changes of brain activation. For example, ''neuro-realism'' implies that if a psychological construct cannot be associated with an identifiable brain response, then it cannot really exist.
Given the normative and institutional interests involved, it is understandable that many psychiatrists hope that genetic and neuroimaging research might yield biomarkers. However, out of the 150–600 disorders of the DSM-5, not a single one can be diagnosed reliably with a biomarker. It is difficult to objectify and validate the construct of Major Depressive Disorder since psychologists noticed a lack of specificity - a certain physiological signal or process can be associated with numerous psychological states or processes (reverse inference problem). When talking about Major Depressive Disorder, it is not clear how to be sure whether a person is diagnosed properly or not. Neuroimaging cannot replace the clinical expert’s diagnosis, as there are no reliable biomarkers. Dennett proposed the method of heterophenomenology to resolve this challenge - to scientifically test people’s accounts of their subjective experience. What could this process look like for MDD?
Depressive mood, diminished interest in activities and loss of energy do have a subjective component. However, these symptoms also have an observable side - we would not expect somebody with them to cheerfully engage in many social activities or to make plans for the future. Furthermore, a depressed person would not be able to complete cognitively demanding tasks. While the construct of MDD is not completely objectifiable, it is also not absolutely subjective either.
