EPISODIC MEMORY AND THE SELF IN A CASE OF ISOLATED RETROGRADE-AMNESIA

Isolated retrograde amnesia is defined as impaired recollection of exp eriences pre-dating brain injury with relatively preserved anterograde learning and memory. We present findings from a patient (M.L.) with i solated retrograde amnesia following severe traumatic brain injury (TB I) that address hypotheses of the interrelationships of focal neuropat hology, episodic memory and the self. M.L. is densely amnesic for expe riences predating his injury, but shows normal anterograde memory perf ormance on a variety of standard tests of recall and recognition. The cognitive processes underlying this performance were examined with the remember/ know technique, which permits separation of episodic from n on-episodic contributions to memory tests by quantifying subjects' rep orts of re-experiencing aspects of the encoding episode. The results d emonstrated that M.L. does not episodically re-experience post-injury events to the same extent as control subjects, although he can use fam iliarity or other non-episodic processes to distinguish events he has experienced from those he has not experienced. M.L.'s MRI showed damag e to the right ventral frontal cortex and underlying white matter, inc luding the uncinate fasciculus, a frontotemporal band of fibres previo usly hypothesized to mediate retrieval of specific events from one's p ersonal past. Recent functional neuroimaging evidence of an associatio n between right frontal lobe functioning and episodic retrieval demand s suggest that M.L.'s memory deficits are related to this focal injury . This hypothesis was supported by right frontal polar hypoactivation in M.L. in response to episodic retrieval demands when he was examined with a cognitive activation (H2O)-O-15 PET paradigm that reliably act ivated this frontal region in both healthy controls and patients with TBI carefully matched to M.L. (but without isolated retrograde amnesia ). He also showed increased left inferomedial temporal activation rela tive to control subjects, suggesting that his spared anterograde memor y is mediated through increased reliance on medial temporal lobe struc tures. Re-experiencing events as part of one's past is based on autono etic awareness, i.e. awareness of oneself as a continuous entity acros s time. This form of awareness also supports the formulation of future goals and the implementation of a behavioural guidance system to achi eve them. The findings from this study converge to suggest that M.L. h as impaired autonoetic awareness attributable to right ventral frontal lobe injury, including right frontal-temporal disconnection. Reorgani zed brain systems mediate certain preserved cognitive operations in M. L., but without the normal complement of information concerning the se lf with respect to both past and future events.