Cognitive impairment following frontal lobe damage and its relevance to human amnesia

Whether frontal lobe pathology can account for some of the cognitive impairment observed in amnesic patients with Korsakoff's syndrome was investigated. Various cognitive and memory tests were given to patients with circumscribed frontal lobe lesions, patients with Korsakoff's syndrome, non-Korsakoff amnesic patients, and control Ss. Patients with frontal lobe lesions were not amnesic. Nevertheless they exhibited 2 deficits that were also exhibited by patients with Korsakoff's syndrome but not by other amnesic patients: (a) impairment on the Wisconsin Card Sorting Test and (b) impairment on the Initiation and Preservation subscale of the Dementia Rating Scale. Thus, frontal lobe pathology can explain some of the cognitive deficits observed in patients with Korsakoff's syndrome.

Short-latency somatosensory evoked potentials

Short-latency components of the somatosensory evoked potential (SEP) were studied in 20 subjects who had median nerve stimulation using knee, forehead, and ear reference recordings. Six potentials were identified (P10, P12, P14, N19, P20, P23). Potential P10 seems to originate in the brachial plexus, P12 most likely is generated in dorsal column nuclei and medial lemniscus, P14 is probably thalamic in origin but is frequently bilobed and may have a second generator source, and N19 may originate in sensory radiation or cortex. The origin of P20 is unclear, and P23 appears to be generated in the contralateral somatosensory cortex. Clinically, we recommend using right, left, and bilateral median nerve stimulation at 4 Hz. Intensity of stimulus should produce at least a small thumb twitch, it was only with knee reference recording that all early potentials were seen.

Description of brain injury in the amnesic patient N.A. based on magnetic resonance imaging

N.A. has been amnesic since 1960 when at the age of 22 years he sustained a penetrating brain injury with a miniature fencing foil. The amnesia primarily affects verbal material and occurs in the absence of other detectable cognitive deficits. Previous CT scans demonstrated a lucency in the region of the left mediodorsal thalamic nucleus, but no additional damage was revealed. Beginning in 1986 when he was 48 years old, N.A. was evaluated with a series of magnetic resonance imaging (MR) studies. Three major areas of damage were identified. In the left thalamus there is a prominent 3- to 4-mm-wide linear lesion that approximates the position and orientation of the internal medullary lamina. The defect extends for approximately 20 mm anteroposteriorly and likely involves the rostral group of intralaminar nuclei (central medial, paracentral, central lateral, rhomboid, and reuniens nuclei), the caudal group of intralaminar nuclei (centrum medianum and parafascicular nuclei), the ventral aspect of the mediodorsal nucleus, and the ventral lateral and ventral anterior nuclei. It also likely interrupts the trajectories of the mammillothalamic tract and postcommissural fornix. The posterior hypothalamus is markedly disrupted and the mammillary nuclei appear to be missing bilaterally. Finally, the right anterior temporal lobe is damaged for a distance of about 3.5 cm from the pole to midway through the amygdaloid complex. This damage probably occurred during exploratory neurosurgery done at the time of N.A.'s injury. The hippocampal formation appears intact on both sides. A comparison of these findings with those from other patients with diencephalic amnesia suggests that amnesia can result when several diencephalic structures are damaged conjointly, including the internal medullary lamina, the intralaminar nuclei, the mediodorsal nucleus, and the mammillothalamic tract. Whether amnesia as severe as N.A.'s would result from selective damage to any one of these structures remains to be determined.

Transient global amnesia: characterization of anterograde and retrograde amnesia

Five patients with transient global amnesia (TGA) were given neuropsychological tests during and after their episode. During TGA, all patients were impaired on tests of new learning ability for both verbal and nonverbal material. Retrograde amnesia was patchy and covered a variable period of time before the onset of the episode: from about 36 hours in one case to 4 years in two cases. Some cognitive impairment in addition to amnesia was present during the episode. For example, patients copied a complex figure more poorly during the episode than afterwards. All patients had normal memory at follow-up testing. These data are consistent with the idea that the amnesic symptoms of TGA are caused by transient bilateral dysfunction of medial temporal brain structures important for memory.

Normal memory after damage to medial thalamus

We studied two patients with nonhemorrhagic infarcts of the thalamus and assessed their cognitive functions comprehensively using standardized neuropsychological probes. Neither patient had any discernible memory impairment for verbal or nonverbal material. Analysis of magnetic resonance images with a stereotaxic method revealed that one subject had a right-sided lesion involving about 15% of the dorsomedial nucleus (DM). The other had bilateral lesions that affected about 15% of the left DM and less than 5% of the right DM. The mamillothalamic tract appeared intact in both patients. Considering that medial thalamic lesions commonly cause amnesia in human beings as well as nonhuman primates, there are two possible reasons, alone or in combination, that may explain why these patients failed to have amnesia: the amount of DM damage was less than required to cause amnesia; or the amnesia related to thalamic lesions requires damage to a second structure, such as the mamillothalamic tract or the anterior nucleus.

Transient global amnesia: evidence for extensive, temporally graded retrograde amnesia

We gave six patients with transient global amnesia (TGA) neuropsychological tests during and after their episodes. During TGA, all patients had severe anterograde amnesia for verbal and nonverbal material and a patchy but temporally graded retrograde amnesia for personal and public events dating back to at least 1960. In addition, they were unusually passive during TGA, had impaired ability to copy a complex figure, and possibly had mild impairment of confrontation naming. All exhibited complete recovery of memory and other cognitive abilities after the episode. There are similarities between the transient amnesia of patients with TGA and the chronic amnesia of patients with presumed bilateral damage to the medial temporal region or the diencephalic midline.

Functional amnesia: clinical description and neuropsychological profile of 10 cases

We carried out the first neuropsychological study of a series of patients with functional amnesia. We evaluated 10 patients, first with a neurological examination and then with three tests of anterograde amnesia and four tests of retrograde amnesia. Excluding one patient who later admitted to malingering, all patients had a significant premorbid psychiatric history and one or more possible precipitating factors for their amnesia. Eight of the 10 patients still had persistent retrograde amnesia at our last contact with them (median = 14 mo after the onset of amnesia). On tests of anterograde amnesia, the patients performed normally as a group, though some patients scored poorly on tests of verbal memory. On tests of retrograde amnesia, all patients had difficulty re-collecting well-formed autobiographical memories of specific events from their past. In contrast, patients performed as well as controls at distinguishing the names of cities from fictitious city names. On remote memory tests for past public events and famous faces, different patients exhibited different but internally consistent patterns of impaired and spared performance. The variability in the clinical and neuropsychological findings among our patients may be understood by supposing that memory performance is poor in proportion to how directly a test appears to assess a patient's common sense concept of memory. The presentation of patients with functional amnesia is as variable as humankind's concept of what memory is and how it works.

Transient global amnesia and functional retrograde amnesia: contrasting examples of episodic memory loss

We studied 11 patients with transient global amnesia (TGA) and ten patients with functional retrograde amnesia (FRA). Patients with TGA had a uniform clinical picture: a severe, relatively isolated amnesic syndrome that started suddenly, persisted for 4-12 h, and then gradually improved to essentially normal over the next 12-24 h. During the episode, the patients had severe anterograde amnesia for verbal and non-verbal material and retrograde amnesia that typically covered at least two decades. Thirty hours to 42 days after the episode, the patients had recovered completely and performed normally on tests of anterograde and retrograde amnesia. By contrast, patients with FRA had a sudden onset of memory problems that were characterized by severe retrograde amnesia without associated anterograde amnesia and with a clinical presentation that otherwise varied considerably. The episodes persisted from several weeks to more than two years, and some of the patients had not recovered at the time of our last contact with them. The uniform clinical picture of TGA and the variable clinical picture of FRA presumably reflect their respective neurologic ('organic') and psychogenic ('non-organic') aetiologies.

Permanent global amnesia with unknown etiology

Three patients developed severe and selective memory impairment with no known cause, one during a period of a few days and two others during a period of 1 to 2 years. In two of these patients, the amnesia has been stable and circumscribed for 5 to 6 years. The third patient appears to have declined in cognitive functions during the past year, at the age of 78, after 6 years of stable, circumscribed amnesia. Neuropsychological testing reveals severe impairment in the ability to learn verbal and nonverbal material as well as retrograde amnesia covering at least 20 years. CT and routine brain MRIs were uninformative. Subsequently, a high-resolution protocol for imaging human hippocampus with MR revealed that the hippocampal formation was markedly reduced in size in all three patients. The pattern of cognitive impairment and the MR findings are similar to the findings in other patients with chronic amnesia due to a known anoxic or ischemic episode, and differ from the findings in amnesic patients with alcoholic Korsakoff's syndrome. We suggest that the amnesia may be due to ischemic damage to medial temporal lobe brain structures important for memory.

Alexia without agraphia or hemianopia in parietal infarction

Images

Discourse deficits following right hemisphere damage in deaf signers

Previous findings have demonstrated that hemispheric organization in deaf users of American Sign Language (ASL) parallels that of the hearing population, with the left hemisphere showing dominance for grammatical linguistic functions and the right hemisphere showing specialization for non-linguistic spatial functions. The present study addresses two further questions: first, do extra-grammatical discourse functions in deaf signers show the same right-hemisphere dominance observed for discourse functions in hearing subjects; and second, do discourse functions in ASL that employ spatial relations depend upon more general intact spatial cognitive abilities? We report findings from two right-hemisphere damaged deaf signers, both of whom show disruption of discourse functions in absence of any disruption of grammatical functions. The exact nature of the disruption differs for the two subjects, however. Subject AR shows difficulty in maintaining topical coherence, while SJ shows difficulty in employing spatial discourse devices. Further, the two subjects are equally impaired on non-linguistic spatial tasks, indicating that spared spatial discourse functions can occur even when more general spatial cognition is disrupted. We conclude that, as in the hearing population, discourse functions involve the right hemisphere; that distinct discourse functions can be dissociated from one another in ASL; and that brain organization for linguistic spatial devices is driven by its functional role in language processing, rather than by its surface, spatial characteristics.

A case of 'sign blindness' following left occipital damage in a deaf signer

We report on a right-handed, deaf, life long signer who suffered a left posterior cerebral artery (PCA) stroke. The patient presented with right homonymous hemianopia, alexia and a severe sign comprehension deficit. Her production of sign language was, however, virtually normal. We suggest that her syndrome can be characterized as a case of 'sign blindness', a disconnection of the intact right hemisphere visual areas from intact left hemisphere language areas. This case provides strong evidence that the neural systems supporting sign language processing are predominantly in the left hemisphere, but also suggests that there are some differences in the neural organization of signed vs spoken language within the left hemisphere.

Neuropsychological presentation of Ganser symptoms

We describe a patient with memory complaints whose history of head injury, polysubstance abuse and positive neurological findings suggested the presence of dementia. Neuropsychological testing revealed inconsistent performance plus multiple verbal and visuospatial approximate answers leading to the diagnosis of a Ganser episode. This case illustrates the importance of neuropsychological assessment in diagnosing Ganser symptoms and in differentiating this disorder from organic conditions and malingering.

Transient global amnesia: a complication of incremental exercise testing

Incremental exercise testing is routinely used for diagnosis, rehabilitation, health screening, and research. We report the case of a 71-yr-old patient with chronic obstructive pulmonary disease (COPD) who suffered an episode of transient global amnesia (TGA) several minutes after successfully completing an incremental exercise test on a cycle ergometer. TGA, which is known to be precipitated by physical or emotional stress in about one-third of cases, is a transient neurological disorder in which memory impairment is the prominent deficit. TGA has a benign course and requires no treatment although 24-h observation is recommended. Recognition of TGA as a potential complication of incremental graded exercise testing is important to both aid diagnosis of the amnesia and to spare a patient unnecessary evaluation.

Three-dimensional trajectory analysis of congenital mirror movements in a single subject

Mirror movements are involuntary movements executed by one side of the body that occur with voluntary activation of homologous muscles of the other side. Although such movements have been described qualitatively and with surface EMG recordings, the spatial and temporal characteristics of these movements remain relatively unexplored. We studied selected simple and complex upper limb movements in a 20-yr.-old woman with congenital mirror movements and no other neurological disorder. Movements were digitized in three-dimensional space, reconstructed computergraphically, and analyzed numerically and graphically. Mirror movements had smaller amplitudes than did the corresponding voluntary movements, and there was, in general, temporal coupling between mirror and voluntary movements. Nonetheless, mirror movements were not always a perfect mirror image of the corresponding voluntary movements and sometimes differed in timing and trajectory shape from the original movement. Substantially larger mirror movements were elicited by distal than by proximal movements, and mirror movements were enhanced when loads were applied to the hand executing the voluntary movement. These data support the proposal that congenital mirror movements are produced by a partial failure of decussation of the pyramidal tract. We suggest that the variability in the extent to which mirror movements correspond to the voluntary movements is due to propriospinal and descending extrapyramidal input.

Transient global amnesia. When memory temporarily disappears

Transient global amnesia is a benign condition of sudden onset that resolves spontaneously. Retrograde amnesia prevents recall of events antedating the episode by hours to years, and anterograde amnesia produces the characteristic features of inability to learn new material and repetitious questioning. Laboratory investigation of these patients is generally unrewarding. Transient global amnesia is easily distinguished from amnesia caused by head trauma or transient ischemic attack, confusional state, and functional amnesia. Although transient global amnesia is most likely caused by transient ischemia of brain structures important for memory, thromboembolic cerebrovascular disease is not the cause. The patient with transient global amnesia should be treated conservatively.

Sign language aphasia following right hemisphere damage in a left-hander: a case of reversed cerebral dominance in a deaf signer?

Recent lesion studies have shown that left hemisphere lesions often give rise to frank sign language aphasias in deaf signers, whereas right hemisphere lesions do not, suggesting similar patterns of hemispheric asymmetry for signed and spoken language. We present here a case of a left-handed, deaf, life-long signer who became aphasic after a right-hemisphere lesion. The subject exhibits deficits in sign language comprehension and production typically associated with left hemisphere damaged signers. He also exhibits evidence of local versus global deficits similar to left-hemisphere lesioned hearing patients. This case represents reversed lateralization for sign language and also may represent reversed lateralization for visuo-spatial abilities in a deaf signer.