CHAPTER 2244 The Hippocampus, Memory, and Consciousness Bradley R. Postle OUTLINE Prologue: Road Trip 326 LTM 331 STM 334 Background 327 Imagining New Experiences 335 The Neurological Exam and its Implications for What it is Like to Be Amnesic? 335 Consciousness 328 Conclusions 336 The Implications of MTL Damage for Neuroanatomical and Neurophysiological Epilogue: A Final Word 336 Correlates of Consciousness 329 Acknowledgements 337 The Effects of MTL Damage on Various Mental Functions and Their Implications References 337 for Consciousness 331 ABSTRACT This chapter reviews the cognitive and neurological profile of medial temporal-lobe (MTL) amnesia as relates to conscious phenomenology. Topics include the selectivity of the memory deficit vis-à-vis other cognitive functions; the selectivity of the deficit, within the myriad abilities that can be called mnemonic, to the conscious accessing of information acquired post trauma; the seeming normalcy of many aspects of the patient’s conscious experience; and the severe constraints that damage to the MTL places on the candidate information and qualia that can make up the contents of consciousness. Particularly relevant to the latter are relatively recent, still controversial ideas that ascribe to the hippocampus important functions that extend beyond its traditionally accepted role in memory encoding – retrieval of autobiographical episodic memory, relational binding, and imagining new experiences. PROLOGUE: ROAD TRIP patient H.M. from and to his home, a handful of hours distant, for his roughly semiannual research visits 1 . His renown derived from the profound influence that One of the privileges afforded graduate students at the study of his global amnesia, first described by MIT’s Behavioural Neuroscience Laboratory has been Scoville and Milner in 1957 [2] , had had on the neuro- the opportunity to transport the renowned amnesic science and psychology of memory. For example, it had provided the impetus for lesion studies of the role 1 Anecdotes relating to H.M.’s life as a celebrated patient can be of the hippocampus and adjacent structures in learn- found in [1] . ing and memory in rodents, nonhuman primates, S. Laureys & G. Tononi (Eds.) The Neurology of Consciousness 326 © 2009, Elsevier Ltd. CCh24-P374168.inddh24-P374168.indd 332626 88/11/2008/11/2008 22:07:56:07:56 PPMM BACKGROUND 327 and humans [3] , for electrophysiological studies of At that point, sensing a ‘ teachable moment ’ , the long-term potentiation (LTP) [4, 5] , for the idea of the driver of the car interjected with a question of his hippocampus as a cognitive map [6] , and for many own: theoretical models of learning and memory [7] . ‘ H_____, do you remember what game we were playing The convention for H.M.’s transport was for the a few minutes ago? ’ designated graduate student to travel with a com- No, he didn’t. panion, and there was never a shortage of volunteers ‘ It involved counting cars of different colours; do you (typically a fellow student or a postdoc) eager to take remember what your colour was? ’ a ‘ road trip ’ with this famous patient. On one such No. trip the two scientists-in-training and their charge ‘ The three colours were green, blue, and white; do you sought to pass the time by playing a game in which remember which was yours? ’ each player selects a colour – on this occasion, green, No, the cues didn’t help. blue, and white – and accumulates points for each car ‘ Do you remember who won the game? ’ painted in his or her colour that passes in the opposite No recollection even of the triumph that had, only direction on the highway. Each player counts aloud, a few minutes before, produced in H.M. a modest and the gaps between passing cars are typically chuckle and satisfied smile. filled with cheering and good-natured banter. H.M. This vignette illustrates several points about the participated fully in the game, selecting his colour, cognitive profile and conscious phenomenology of accurately keeping track of his running total, and par- medial temporal-lobe (MTL) amnesia that will be ticipating in the debate about whether a teal-coloured taken up in this chapter: the selectivity of the memory car should be scored as blue, green, or neither. Indeed, deficit vis-à-vis other cognitive functions; the selectiv- on this occasion H.M. won, accruing a score of 20 first. ity of the deficit, within the myriad abilities that can A round of congratulations was exchanged, followed be called mnemonic, to the conscious accessing of by a lull as the car rolled through the undulating cen- information acquired post trauma; the seeming nor- tral Massachusetts countryside. A few minutes later, malcy of many aspects of the patient’s conscious experi- the guest traveller, eager to maximize his once-in-a- ence; and the severe constraints that damage to the MTL lifetime opportunity to gain first-hand insight from places on the candidate information and qualia that this famous patient, asked ‘ H___, what are you think- can make up the contents of consciousness. ing about right now? ’ H.M. replied that his count of white cars had now increased to 36. The driver and guest were both impressed that BACKGROUND this patient, famously incapable of remembering vir- tually anything that had occurred in his life since his The study of patients with anterograde amne- 1953 surgery, had accurately maintained and updated sia resulting from damage to the hippocampus and a running count of arbitrarily selected ‘ target stimuli ’ adjacent structures of the MTL has contributed enor- across a span of several minutes, with no evident mously to our understanding of the organization of source of external support or reinforcement. The three memory, of its relation to other aspects of cognition travellers commented on this before the guest traveller and behaviour, and of its neural bases. Perhaps the redirected the conversation to a line of questions that most important principle to derive from the study of was typical of these trips: Do you know what today’s date is? Do you know who the current President is? Do you anterograde amnesia is that it is inaccurate to depict memory as a unitary domain of cognition, in the way know who we are; where we’re going today? H.M. com- that one might characterize vision or language . Indeed, plied with good-natured responses, as always, clearly a hallmark of a ‘ pure ’ case of anterograde amnesia enjoying the interaction with and attention from these is the inability to encode (or learn) new information, young, engaged travelling companions. Very quickly, despite relatively intact abilities to retrieve premorbid however, H.M. initiated another typical element in the memories 2 , to remember a small amount of informa- driving-with-H.M. script, by steering the conversation tion, such as a phone number, for tens of seconds or towards a reminiscence from his youth, the portion of his life still mentally accessible after his surgery. (The story, about riding the Silver Meteor passenger 2 train on a multi-day trip to visit an Aunt in Florida, As we shall see, although there is ongoing controversy about the quality of some types of premorbid memories in these patients, had already been told several times at that point in neurologists find it ‘ clinically useful to describe amnesia as a failure the trip, a product of the teller not remembering the to learn new information, which is distinct from a retrieval deficit ’ previous tellings.) [8] p. 41. IV. SEIZURES, SPLITS, NEGLECTS AND ASSORTED DISORDERS CCh24-P374168.inddh24-P374168.indd 332727 88/11/2008/11/2008 22:07:56:07:56 PPMM 328 24. THE HIPPOCAMPUS, MEMORY, AND CONSCIOUSNESS even longer, and to demonstrate the improvements in the forward order, and lists of five when instructed that accompany repeated performance of routine to recall them in reverse order. On the Mini Mental behaviours or repeated exposure to stimuli. Also State Exam (MMSE), a dementia screen that evalu- spared in anterograde amnesia is every other major ates knowledge of where one is and when it is (year, domain of cognition – sensory perception, language season, month, date, day of the week) at the time of comprehension and production, motor control, intel- testing, counting backward from 100 by sevens, and ligence, and so on. Because this condition produces following simple instructions, she responded cor- so circumscribed a deficit of cognition, it provides an rectly to 28 out of 30 questions, missing only two that interesting case with which to examine the relation of required recall of information provided earlier in the consciousness to memory vs. other domains of cog- exam. Her vocabulary was intact, as assessed by the nition. The analysis in this chapter will begin with a ability to name 59 of 60 black-and-white drawings of review of the neurological exam and its implications, objects from the Boston Naming Test. When asked to followed by the anatomical and physiological pro- name as many animals as possible within a minute, file of the amnesic brain, followed by considerations an index of retrieval from semantic memory and, of cognitive effects of damage to the MTL. It will end particularly, the control of this retrieval, she named with a consideration of phenomenology ( Box 24.1 ). 19. Executive function was intact, as assessed by tests evaluating the ability to perform mental arith- metic, to change strategy after covert changes of the THE NEUROLOGICAL EXAM rule in the Wisconsin Card-Sorting Test, and to with- AND ITS IMPLICATIONS hold responses on a speeded responding test. Finally, FOR CONSCIOUSNESS and perhaps most striking for one not familiar with such cases, her full-scale intelligence quotient (IQ) as assessed by the Wechsler Adult Intelligence Scale- Anterograde amnesia is diagnosed when a neuro- Revised (WAIS-R) was within the normal range.