Capgras syndrome is a strange disorder in which the sufferer becomes convinced that someone close to them has been replaced by an impostor.

Yet now, a new and even stranger variant of the syndrome has been reported - "Cat-gras". This is the name coined by Harvard neurologists R. Ryan Darby and David Caplan in

a new paper in the journal Neurocase

. The authors describe the case of a man who believed that his cat was in fact a different cat. According to Darby and Caplan, the patient is a 71 year old man with a history of heavy drinking, ice hockey-related head traumas, and bipolar disorder.

Six years prior to presentation, he became acutely paranoid in the setting of stopping his psychiatric medications. He passed his wife the written notes stating that their house was being monitored, and often mistook persons in parking lots for Federal Bureau of Investigation agents. He then became obsessed with the idea that his pet cat had been replaced by an imposter cat that was involved in the conspiracy against him. He knew that the current cat resembled his pet cat physically, but that the personality or psychic core of his cat had been replaced. His symptoms improved with medications and he has had no further delusions of imposters replacing his cat.

Neurological tests showed evidence of memory and cognitive decline, and a brain scans showed atrophy of the cerebral cortex, possibly indicating the development of dementia. The patient's past head traumas may have contributed to this. But what explains the patient's past attitude towards his cat? Could this really be a case of feline Capgras? Darby and Caplan say that this is a very rare syndrome, but not unheard of:

Capgras delusions have rarely been reported with animals. Review of the literature reveals two cases reported in pet cats, two cases in pet birds, and one in a pet dog. The majority of these cases occurred during a psychotic episode with other paranoid and persecutory delusions, as in our patient.

They say that their patient is unique, however, because he is the first case of animal Capgras associated with verifiable brain injury. Darby and Caplan then go on to propose a new theory of the Capgras (including Cat-gras) syndrome and other related delusions, the so-called delusional misidentification syndromes (DMS). They hold that

The delusional belief content in DMS results from dysfunctional linking between externally perceived objects and appropriately retrieved internal autobiographical memories associated with an object, leading to an erroneous learned belief that a familiar external object is a new, distinct entity... An inability of the external object to trigger the appropriate retrieval of autobiographical memory would lead to the erroneous belief that the external object is an imposter or replica (leading to hypo-familiar delusions such as Capgras.)

In other words, delusional misidentification syndromes results from a failure of the system that normally allows us to perceive that a person (or animal or object) has an identity that persists over time. This system involves the integration of memory with perception. Darby and Caplan have little to say about how all this happens, however. Their theory also has little to say specifically about cats.

Darby, R., & Caplan, D. (2016). "Cat-gras" delusion: a unique misidentification syndrome and a novel explanation Neurocase , 1-6 DOI: 10.1080/13554794.2015.1136335