HFF is an uncommon modality-specific and stimulus-specific syndrome identified in 9 patients with epilepsy or a tonic-clonic seizure. The diagnosis rests on a selective false familiarity for multiple faces; delusions are not present and neither voices nor nonface visual objects falsely elicit hyperfamiliarity in these patients. Lesions or seizure foci usually involved the left hemisphere (left 5, bilateral 2, unknown 2) and temporal lobe (6/7 with known localization) ().2–5
The MRIs are normal or show nonspecific changes in 2/8 cases with imaging; lesions include remote trauma (1), stroke (2), and low-grade tumor (1).
The duration of HFF ranged from seconds to minutes in the ictal case and from 2 days to more than 7 years in the other cases. Duration did not correlate with the presence or extent of a lesion. Symptoms lasting hours to days may have resulted from a Todd paralysis5
(cases 4, 8, and 9). Paradoxically, persistent HFF occurred in 2 cases (2, 6) after seizures; these patients had no abnormal MRI results or remote head injuries and the mechanisms are unknown.
The physiology of recollection and familiarity are relevant to HFF. Recollection is the retrieval of specific contextual or related details associated with a prior episode. Familiarity is a feeling of recognition for a particular item without accompanying associated episodic details or any specific context. Selective lesions, electrical stimulation, and functional MRI studies reveal that the hippocampus is important for recollection while the perirhinal cortex is implicated in the feeling of familiarity.6–8
Déjà vu (already seen) and déjà vécu (already lived through) are hyperfamiliarity for a present experience with an undefined and often nonexistent past (familiarity without retrieval). They occur during seizures and stimulation of the amygdala, hippocampus, and perirhinal cortex, more common with right temporal hyperexcitability,9
supporting right dominance in familiarity.9
The neural response to a familiar stimulus is suppression as compared to the activation with a novel stimulus.8
Familiarity strength is signaled through graded repetition suppression.8
Epileptic discharges during déjà vu and vécu may suppress function and thereby mimic the experience of familiarity.
and HFF may result from increased activity in right relative to left medial temporal areas. HFF may be produced by impaired left hemisphere identification of unique facial features and excessive right hemisphere processes that link individual faces with emotional and personal meaning, leading to spurious familiarity feelings.10
Left temporal lobe dysfunction may impair novelty signaling and detection of specific facial features while disinhibiting right temporal regions that falsely signal familiarity.
Patients with HFF quickly accept that their recognition is false, likely reflecting preserved right hemispheric and frontal areas. In contrast, fixed delusions of misidentification or reduplication disorders usually result from lesions in bifrontal and/or right hemispheric areas.10
Lesions that destroy or isolate stimuli from right perirhinal cortex may impair familiarity (e.g., Capgras syndrome) while hyperfamiliarity (e.g., Fregoli syndrome) may result from overactivity in right perirhinal cortex,10
consistent with the effects of left temporal lesions in HFF.
Finally, HFF is probably much more common than clinically identified. The 4 cases described here were seen by a single author (O.D.) over a 2-year period without any systematic survey of patients. Thus, it is likely that the vast majority of cases go unrecognized and include cases with and without epilepsy.