If I understand the argument of this study correctly (1) a depressed condition entails (at least in part) a system of how one processes information; (2) “updating beliefs” is a/the key to moving out of a depressed condition; (3) ingestion of ketamine facilitated the fact of one updating their beliefs:

“In the suite of the study, TRD patients received three administrations of ketamine at a subanesthetic dose (0.5 mg/kg over 40 minutes) in one week. Only four hours after the first administration, patients’ ability to update their beliefs after receiving a positive information was increased. They became less sensitive to negative information and recovered an ability to update their knowledge com parable to that of control subjects.

“Moreover, improvement in depressive symptoms after ketamine treatment was associated with these changes in belief updating, suggesting a link between clinical improvement and changes in this cognitive mechanism. “In other words, the more patients’ belief updating ability was increased, the greater the improvement in symptoms was”.”

The question would then the relationship between the drug the updated beliefs. Does the drug perform a placebo function? Does the drug somehow alter brain chemistry [whatever that might mean or entail in this particular instance] such that beliefs can be updated? Is there something which is preventing the “updating” of beliefs [beyond the belief that I cannot do so]? Does the drug do something intermediate, such as relaxing the patient which makes them more willing to “update their beliefs” [which would then suggest that the failure to update beliefs was more akin to a refusal than an inability]?