From Phineas Gage and Monsieur Leborgne to H.M.: Revisiting Disconnection Syndromes

On the 50th anniversary of Norman Geschwind's seminal paper entitled ‘Disconnexion syndrome in animal and man’, we pay tribute to his ideas by applying contemporary tractography methods to understand white matter disconnection in 3 classic cases that made history in behavioral neurology. We first documented the locus and extent of the brain lesion from the computerized tomography of Phineas Gage's skull and the magnetic resonance images of Louis Victor Leborgne's brain, Broca's first patient, and Henry Gustave Molaison. We then applied the reconstructed lesions to an atlas of white matter connections obtained from diffusion tractography of 129 healthy adults. Our results showed that in all 3 patients, disruption extended to connections projecting to areas distant from the lesion. We confirmed that the damaged tracts link areas that in contemporary neuroscience are considered functionally engaged for tasks related to emotion and decision-making (Gage), language production (Leborgne), and declarative memory (Molaison). Our findings suggest that even historic cases should be reappraised within a disconnection framework whose principles were plainly established by the associationist schools in the last 2 centuries.

Reliability of MRI-derived cortical and subcortical morphometric measures: effects of pulse sequence, voxel geometry, and parallel imaging

Advances in magnetic resonance imaging (MRI) have contributed greatly to the study of neurodegenerative processes, psychiatric disorders, and normal human development, but the effect of such improvements on the reliability of downstream morphometric measures has not been extensively studied. We examined how MRI-derived neurostructural measures are affected by three technological advancements: parallel acceleration, increased spatial resolution, and the use of a high bandwidth multiecho sequence. Test-retest data were collected from 11 healthy participants during 2 imaging sessions occurring approximately 2 weeks apart. We acquired 4 T1-weighted MP-RAGE sequences during each session: a non-accelerated anisotropic sequence (MPR), a non-accelerated isotropic sequence (ISO), an accelerated isotropic sequence (ISH), and an accelerated isotropic high bandwidth multiecho sequence (MEM). Cortical thickness and volumetric measures were computed for each sequence to assess test-retest reliability and measurement bias. Reliability was extremely high for most measures and similar across imaging parameters. Significant measurement bias was observed, however, between MPR and all isotropic sequences for all cortical regions and some subcortical structures. These results suggest that these improvements in MRI acquisition technology do not compromise data reproducibility, but that consistency should be maintained in choosing imaging parameters for structural MRI studies.

Neuroimaging H.M.: a 10-year follow-up examination

In 1997, Corkin et al. described the anatomical boundaries of the amnesic patient H.M.'s surgical resection, based on a comprehensive analysis of magnetic resonance imaging (MRI) scans collected in 1992 and 1993 (Corkin et al. (1997) J Neurosci 17:3964-3979). We subsequently scanned H.M. on several occasions, employing more advanced data acquisition and analysis methods, and now describe additional details about his brain anatomy and pathology. This account combines results from high-resolution T1-weighted scans, which provide measures of cortical and subcortical morphometry, diffusion tensor images, which provide quantitative information about white matter microstructure and the anatomy of major fasciculi, and T2-weighted images, which highlight damage to deep white matter. We applied new MRI analysis techniques to these scans to assess the integrity of areas throughout H.M.'s brain. We documented a number of new changes, including cortical thinning, atrophy of deep gray matter structures, and a large volume of abnormal white matter and deep gray matter signal. Most of these alterations were not apparent in his prior scans, suggesting that they are of recent origin. Advanced age and hypertension likely contributed to these new findings.

Age-related changes in prefrontal white matter measured by diffusion tensor imaging

Age-related degeneration of brain white matter (WM) has received a great deal of attention, with recent studies demonstrating that such changes are correlated with cognitive decline and increased risk for the development of age-related neurodegenerative disease. Past studies have used magnetic resonance imaging (MRI) to measure the volume of normal and abnormal tissue signal as an index of tissue pathology. More recently, diffusion tensor MRI (DTI) has been employed to obtain regional measures of tissue microstructure, such as fractional anisotropy (FA), providing better spatial resolution and potentially more sensitive metrics of tissue damage than traditional volumetric measures. We used DTI to examine the regional basis of age-related alterations in prefrontal WM. As expected from prior volumetric and DTI studies, prefrontal FA was reduced in older adults (OA) compared to young adults (YA). Although WM volume has been reported to be relatively preserved until late aging, FA was significantly reduced by middle age. Much of prefrontal WM showed reduced FA with increasing age. Ventromedial and deep prefrontal regions showed a somewhat greater reduction compared to other prefrontal areas. Prefrontal WM anisotropy correlated with prefrontal WM volume, but the correlation was significant only when the analysis was limited to participants over age 40. This evidence of widespread and regionally accelerated alterations in prefrontal WM with aging illustrates FA's potential as a microstructural index of volumetric measures.

Age-related alterations in white matter microstructure measured by diffusion tensor imaging

Cerebral white matter (WM) undergoes various degenerative changes with normal aging, including decreases in myelin density and alterations in myelin structure. We acquired whole-head, high-resolution diffusion tensor images (DTI) in 38 participants across the adult age span. Maps of fractional anisotropy (FA), a measure of WM microstructure, were calculated for each participant to determine whether particular fiber systems of the brain are preferentially vulnerable to WM degeneration. Regional FA measures were estimated from nine regions of interest in each hemisphere and from the genu and splenium of the corpus callosum (CC). The results showed significant age-related decline in FA in frontal WM, the posterior limb of the internal capsule (PLIC), and the genu of the CC. In contrast, temporal and posterior WM was relatively preserved. These findings suggest that WM alterations are variable throughout the brain and that particular fiber populations within prefrontal region and PLIC are most vulnerable to age-related degeneration.

Bilateral medial temporal lobe damage does not affect lexical or grammatical processing: evidence from amnesic patient H.M

In the most extensive investigation to date of language in global amnesia, we acquired data from experimental measures and examined longitudinal data from standardized tests, to determine whether language function was preserved in the amnesic patient H.M. The experimental measures indicated that H.M. performed normally on tests of lexical memory and grammatical function, relative to age- and education-matched control participants. Longitudinal data from four Wechsler subtests (Information, Comprehension, Similarities, and Vocabulary), that H.M. had taken 20 times between 1953 (preoperatively) and 2000, indicated consistent performance across time, and provided no evidence of a lexical memory decrement. We conclude that medial temporal lobe structures are not critical for retention and use of already acquired lexical information or for grammatical processing. They are, however, required for acquisition of lexical information, as evidenced in previous studies revealing H.M.'s profound impairment at learning new words.

Beware of frontal lobe deficits in hippocampal clothing

The Wisconsin card-sorting test (WCST) is a commonly used clinical tool for the detection of frontal lobe dysfunction, specifically executive dysfunction. Patients with lesions outside the frontal lobes sometimes show deficits on the WCST, however, and some researchers have implicated hippocampal dysfunction as the cause of the deficit. But a critical role for the hippocampus seems to be untenable because amnesic patients with bilateral medial temporal lobe (MTL) lesions perform the WCST normally. In the case of epileptic patients, an alternative explanation of the card-sorting impairment is the propagation of abnormal discharges from MTL to frontal lobe structures, causing remote interference with executive circuits.

H.M. revisits the Tower of Hanoi Puzzle

To address the controversy of whether an intact procedural memory system alone can support the learning of the recursive strategy for solving the Tower of Hanoi Puzzle, the authors tested 2 amnesic patients, H.M. and P.N. Contrary to the report of N. J. Cohen, H. Eichenbaum, B. S. Deacedo, and S. Corkin (1985), both patients failed to master the recursive strategy under the active-interaction condition. In contrast, normal control participants were able to master the strategy under identical testing conditions. The failure of H.M. and P.N. could not be attributed to the differences between the original and current testing conditions. In addition, neither patient showed frontal lobe dysfunction or impairment in procedural memory. Together with evidence provided by theoretical analyses of this puzzle as well as studies on normal participants, the authors conclude that declarative memory plays a vital role in the acquisition of the recursive strategy for solving the Tower of Hanoi Puzzle.

Referred phantom sensations and cortical reorganization after spinal cord injury in humans

To test the hypothesis that cortical remapping supports phantom sensations, we examined referred phantom sensations and cortical activation in humans after spinal-cord injury (SCI) at the thoracic level (T3-T12). Of 12 SCI subjects, 9 reported phantom sensations, and 2 reported referred phantom sensations. In both of these subjects, referred phantom sensations were evoked by contact in reference zones (RZ) that were not adjacent in the periphery and were not predicted to be adjacent in the postcentral gyrus (PoCG), suggesting that representations separated by centimeters of cortical space were simultaneously engaged. This finding was supported by functional MRI (fMRI). In a subject with a T6-level complete SCI, contact in RZ on the left or right forearm projected referred phantom sensations to the ipsilateral chest. During fMRI, contact in either forearm RZ evoked activity in the central PoCG (the position of the forearm representation) and the medial PoCG (the position of the chest representation) with >/=1.6 cm of nonresponsive cortex intervening. In contrast, stimulation in non-RZ forearm and palm regions in this subject and in lesion-matched SCI subjects evoked central but not medial PoCG activation. Our findings support a relation between PoCG activation and the percept of referred phantom sensations. These results, however, present an alternative to somatotopic cortical reorganization, namely, cortical plasticity expressed in coactivation of nonadjacent representations. The observed pattern suggests that somatotopic subcortical remapping, projected to the cortex, can support perceptual and cortical reorganization after deafferentation in humans.

Segregation of somatosensory activation in the human rolandic cortex using fMRI

The segregation of sensory information into distinct cortical areas is an important organizational feature of mammalian sensory systems. Here, we provide functional magnetic resonance imaging (fMRI) evidence for the functional delineation of somatosensory representations in the human central sulcus region. Data were collected with a 3-Tesla scanner during two stimulation protocols, a punctate tactile condition without a kinesthetic/motor component, and a kinesthetic/motor condition without a punctate tactile component. With three-dimensional (3-D) anatomical reconstruction techniques, we analyzed data in individual subjects, using the pattern of activation and the anatomical position of specific cortical areas to guide the analysis. As a complimentary analysis, we used a brain averaging technique that emphasized the similarity of cortical features in the morphing of individual subjects and thereby minimized the distortion of the location of cortical activation sites across individuals. A primary finding of this study was differential activation of the cortex on the fundus of the central sulcus, the position of area 3a, during the two tasks. Punctate tactile stimulation of the palm, administered at 3 Hz with a 5.88(log10.mg) von Frey filament, activated discrete regions within the precentral (PreCG) and postcentral (PoCG) gyri, corresponding to areas 6, 3b, 1, and 2, but did not activate area 3a. Conversely, kinesthetic/motor stimulation, 3-Hz flexion and extension of the digits, activated area 3a, the PreCG (areas 6 and 4), and the PoCG (areas 3b, 1, and 2). These activation patterns were observed in individual subjects and in the averaged data, providing strong evidence for the existence of a distinct representation within area 3a in humans. The percentage signal changes in the PreCG and PoCG regions activated by tactile stimulation, and in the intervening gap region, support this functional dissociation. In addition to this distinction within the fundus of the central sulcus, the combination of high-resolution imaging and 3-D analysis techniques permitted localization of activation within areas 6, 4, 3a, 3b, 1, and 2 in the human. With the exception of area 4, which showed inconsistent activation during punctate tactile stimulation, activation in these areas in the human consistently paralleled the pattern of activity observed in previous studies of monkey cortex.

An fMRI investigation of cortical contributions to spatial and nonspatial visual working memory

The experiments presented in this report were designed to test the hypothesis that visual working memory for spatial stimuli and for object stimuli recruits separate neuronal networks in prefrontal cortex. We acquired BOLD fMRI data from subjects while they compared each serially presented stimulus to the one that had appeared two or three stimuli previously. Three experiments failed to reject the null hypothesis that prefrontal cortical activity associated with spatial working memory performance cannot be dissociated from prefrontal cortical activity associated with nonspatial working memory performance. Polymodal regions of parietal cortex (inferior and superior parietal lobules), as well as cortex surrounding the superior frontal sulcus (and encompassing the frontal eye fields), also demonstrated equivalent levels of activation in the spatial and object conditions. Posterior cortical regions associated with the ventral visual processing stream (portions of lingual, fusiform, and inferior temporal gyri), however, demonstrated greater object than spatial working memory-related activity, particularly when stimuli varied only along spatial or featural dimensions. These experiments, representing fMRI studies of spatial and object working memory in which the testing procedure and the stimuli were identical in the two conditions, suggest that domain-specific visual working memory processing may be mediated by posterior regions associated with domain-specific sensory processing.

Differences between Pick disease and Alzheimer disease in clinical appearance and rate of cognitive decline

OBJECTIVES

To define the cognitive characteristics of Pick disease (PcD), and to determine which features distinguish PcD from Alzheimer disease (AD), in a cross-sectional and longitudinal study.

METHODS

The participants were 44 patients with PcD (10 pathologically verified), 121 patients with AD (14 pathologically verified), and 60 normal control subjects. We obtained information regarding the initial symptom of dementia from each patient's caregiver, estimated global dementia severity by the Blessed Dementia Scale and the Activities of Daily Living Scale, and assessed specific cognitive domains by administering 10 tests of memory, language, visuospatial, and reasoning abilities and selective attention.

RESULTS

Among initial symptoms reported by caregivers, personality change and language impairment were significantly more common in PcD than AD; deficits in memory were common in both groups but more prevalent in AD (P<.001). At initial cognitive testing, the scores of patients with PcD were inferior to those of normal controls on all tests, except on a measure of visuospatial function; the scores of patients with AD were inferior to those of controls on all tests. Patients with PcD were superior to patients with AD on measures of explicit memory (P<.001) and visuospatial function (P = .001) but had greater impairments on the Activities of Daily Living Scale (P<.05). During the course of illness, patients with PcD declined significantly faster than those with AD on language tests and on global measures of dementia severity (P<.05), whereas measures of explicit memory and visuospatial and reasoning abilities worsened equally in both patient groups.

CONCLUSIONS

There is a characteristic cognitive profile and course of dementia in PcD. Nonetheless, cognitive test performance does not clearly distinguish PcD from AD.

An evaluation of the concurrent discrimination task as a measure of habit learning: performance of amnesic subjects

Habit learning has been defined as an association between a stimulus and a response that develops slowly and automatically through repeated reinforcement. Concurrent discrimination (CD) learning, in which subjects learn to choose the rewarded objects in a series of pairs, is believed to be an example of habit learning in monkeys. Studies of human amnesic subjects, however, have produced equivocal results, revealing impaired or absent learning on the same CD tasks that monkeys with medial temporal-lobe (MTL) lesions learn normally. One possible explanation for impaired performance in human amnesic subjects is that, unlike monkeys, human subjects use explicit memory to solve CD problems. To test this hypothesis, we administered a 10-object pair CD learning task to two amnesic subjects, HM and PN, and normal control subjects (NCS). Both amnesic subjects have severe anterograde amnesia with little ability to form explicit memories. On the CD task, they demonstrated little or no learning and acquired no explicit knowledge of the task procedures or reward contingencies. In contrast, NCS learned the task quickly and easily using explicit memory strategies. These results suggest that CD tasks cannot be learned by habit in human subjects, and emphasize the discrepancies between the human and monkey literature on habit learning.

Visual discrimination after anterior temporal lobectomy in humans

OBJECTIVE

To determine whether right anterior temporal lobectomy (RTL) results in perceptual deficits, and whether the perception of particular stimulus features (i.e., shape, motion, color) is affected differentially.

BACKGROUND

RTL results in abnormal visual discrimination, recognition, and recall of pictorial material that cannot be easily specified verbally, such as designs and faces. It is unclear whether stimuli must be conceptually meaningful to elicit perceptual deficits.

METHODS

Tests were constructed to assess a wide spectrum of basic visual discrimination abilities with simple, meaningless stimuli. The performance of nine patients who underwent left temporal lobectomy (LTL) and nine patients who underwent RTL were compared with that of normal control individuals. The mean excision size along lateral cortex was 3.7 cm for the LTL group and 5.6 cm for the RTL group; mean mesial excision size was 5.2 cm for LTL and 4.6 cm for RTL.

RESULTS

Basic visual discrimination capacities were demonstrated to be essentially intact after LTL and RTL, except for a mild loss of blue color discrimination after RTL.

CONCLUSIONS

There is little evidence that RTL produces perceptual impairments limited to the domain of pattern perception, or generalizable to nonmeaningful stimuli. The perceptual loss after RTL may be largely restricted to extraction of meaning, and related to the disruption of the circuits that connect the outcome of visual analysis to previously stored semantic information.

Manipulation of familiarity reveals a necessary lexical component of the word-stem completion priming effect

These experiments were motivated by the idea that many types of nondeclarative memory are by-products arising from the plasticity that is inherent in much of the nervous system. We hypothesized that two types of repetition priming, word-stem completion (WSC) priming and perceptual identification (PI) priming, rely on different mechanisms because the WSC task and the PI task engage different cognitive and brain processes. We tested this hypothesis by manipulating word familiarity. The results, impaired WSC priming but intact PI priming with unfamiliar words, indicate that WSC priming relies primarily on a modification mechanism, whereas PI priming relies primarily on an acquisition mechanism. Our conclusions are consistent with component processes theories of nondeclarative memory.

Visual discrimination and attention after bilateral temporal-lobe lesions: a case study

We studied a woman (Case 1) who acquired achromatopsia, prosopagnosia, and memory loss after sustaining bilateral temporal-lobe lesions. Given her symptoms and locus of lesion, the affected area may be related to the monkey visual area IT. In order to examine her deficits, we assessed her basic discrimination capacities in several domains. She performed normally when stimuli differed in contrast, size, or motion. her performance was abnormal for patterned targets, and was markedly impaired when the patterned targets were less prominent than distractors. This impairment decreased with practice. These symptoms partially resemble the deficits that have been found in monkeys with lesions in visual cortical area V4.

Functional MRI for studying episodic memory in aging and Alzheimer's disease

Anatomic and functional neuroimaging have a potential for clarifying the differential diagnosis of dementia and for evaluating new treatments for memory impairment. Our neuroimaging research examines the neural abnormalities underlying memory impairment in aging and Alzheimer's disease. We use data from high-resolution magnetic resonance imaging (MRI) to measure hippocampal volumes, and data from high-speed echo-planar imaging to evaluate cortical physiology. The results obtained with neuroimaging techniques are then related to the subject's performance on memory tasks (encoding and retrieval) performed inside the scanner during functional MRI.

Time-dependent motor memory processes in amnesic subjects

Functional properties of motor memory change with the passage of time. The time-dependent nature of memories in humans has also been demonstrated for certain "declarative" memories. When the declarative memory system is damaged, are the time-dependent properties associated with motor memories intact? To approach this question, we examined five subjects with global amnesia (AMN), including subject H.M., and a group of age-matched control subjects. The task was to make reaching movements to visually presented targets. We found that H.M. (but not the other subjects) was significantly impaired in the ability to perform the visuomotor kinematic transformations required in this task, to accurately move the hand in the direction specified by a target. With extensive practice, H.M.'s performance improved significantly. At this point, a force field was imposed on the hand. With practice in field A, H.M. and other AMN subjects developed aftereffects and maintained these aftereffects for 24 h. To quantify postpractice properties associated with motor memories, subjects learned field B on day 2 and at 5 min were retested in field A. In both subject groups, performance in field A was significantly worse than their own naive performance a day earlier. The aftereffects indicated persistence of the just-learned but now inappropriate motor memory. After 4 h of rest, subjects were retested in B. Performance was now at naive levels. The aftereffects at 4 h indicated a reduced influence of the memory of field A. The time-dependent patterns of motor memory perseveration, as measured at 5 min and 4 h, were not different in the AMN and normal control groups.

Impaired word-stem completion priming but intact perceptual identification priming with novel words: evidence from the amnesic patient H.M

We hypothesized that word-stem completion (WSC) priming and perceptual identification (PI) priming, two types of repetition priming, rely on different cognitive and neural mechanisms: WSC priming on a modification mechanism that influences lexical retrieval, and PI priming on plasticity in pre-lexical perceptual systems. We compared the priming performance of the amnesic patient H.M. with words that came into common usage after the onset of his amnesia, and thus were novel to him (post-1965 words), and with familiar (pre-1953) words. We also tested age- and education-matched normal control subjects (NCS) and a patient with anterograde amnesia of recent onset (P.N.). The modification hypothesis predicted that H.M. would fail to show WSC priming with post-1965 words because pre-existing lexical representations of the test stimuli would be necessary for priming to occur. H.M.'s WSC priming score in the post-1965 condition did not differ from 0, and was inferior to the performance of NCS and of P.N. In contrast, H.M. displayed normal WSC priming in the pre-1953 condition. H.M. also showed robust and equivalent levels of PI priming in both conditions. A final experiment demonstrated preserved post-1965 word PI priming in H.M. when his baseline performance was matched with his post-1965 WSC priming baseline score. Our results challenge models that assume that most kinds of verbal repetition priming rely on the same or similar perceptual mechanisms.

The time course of spatial and object learning in Parkinson's disease

Parkinson's disease (PD) is characterized by spatial memory dysfunction, but the selectivity of the deficit remains unclear. We addressed this issue by comparing performance on spatial and object variants of a conditional associative learning task, and by analysing the data with time series analytical techniques. The 11 PD subjects and 15 normal control subjects learned stimulus-stimulus pairings through trial-and-error learning. PD subjects were selectively impaired on the spatial condition: they required more trials to achieve criterion, learned at a slower rate and displayed a working memory deficit. The groups did not differ in the object condition. These results suggest a distinction between material-specific spatial and object visual memory systems. Further, they indicate that spatial learning and memory are selectively impaired in early PD, suggesting that interactions between the basal ganglia and prefrontal cortex are important for the mediation of high-level cognition.

Spatial, but not object, delayed response is impaired in early Parkinson's disease

The authors hypothesized that the pathophysiology of early Parkinson's disease (PD) may selectively target structures that support visual working memory for spatial relations but leave structures that support working memory for featural characteristics of objects relatively intact. Fifteen PD and 15 normal control participants took a visual delayed-response test with a spatial condition and a (nonspatial) object condition, equating the perceptual difficulty of the tests for each participant. The stimuli were irregular polygons presented at different locations on a computer screen. Results revealed a selective impairment of spatial delayed response in PD, indicating a disruption of spatial working memory unconfounded by sensory processing difficulties. The selectivity of this deficit may reflect the circumscribed nature of pathophysiological change affecting the caudate nucleus in early PD.

H. M.'s medial temporal lobe lesion: findings from magnetic resonance imaging

Although neuropsychological studies of the amnesic patient H. M. provide compelling evidence that normal memory function depends on the medial temporal lobe, the full extent of his surgical resection has not been elucidated. We conducted magnetic resonance imaging studies to specify precisely the extent of his bilateral resection and to document any other brain abnormalities. The MRI studies indicated that the lesion was bilaterally symmetrical and included the medial temporal polar cortex, most of the amygdaloid complex, most or all of the entorhinal cortex, and approximately half of the rostrocaudal extent of the intraventricular portion of the hippocampal formation (dentate gyrus, hippocampus, and subicular complex). The collateral sulcus was visible throughout much of the temporal lobe, indicating that portions of the ventral perirhinal cortex, located on the banks of the sulcus, were spared; the parahippocampal cortex (areas TF and TH) was largely intact. The rostrocaudal extent of the ablation was approximately 5.4 cm (left) and 5.1 cm (right). The caudal 2 cm, approximately, of the hippocampus body (normal length, approximately 4 cm) was intact, although atrophic. The temporal stem was intact. Outside the temporal lobes, the cerebellum demonstrated marked atrophy, and the mammillary nuclei were shrunken. The lateral temporal, frontal, parietal, and occipital lobe cortices appeared normal for age 66 years. The mediodorsal thalamic nuclei showed no obvious radiological changes. These findings reinforce the view that lesions of the hippocampal formation and adjacent cortical structures can produce global and enduring amnesia and can exacerbate amnesia beyond that seen after more selective hippocampal lesions.

Spatial, but not object, delayed response is impaired in early Parkinson's disease

The authors hypothesized that the pathophysiology of early Parkinson's disease (PD) may selectively target structures that support visual working memory for spatial relations but leave structures that support working memory for featural characteristics of objects relatively intact. Fifteen PD and 15 normal control participants took a visual delayed-response test with a spatial condition and a (nonspatial) object condition, equating the perceptual difficulty of the tests for each participant. The stimuli were irregular polygons presented at different locations on a computer screen. Results revealed a selective impairment of spatial delayed response in PD, indicating a disruption of spatial working memory unconfounded by sensory processing difficulties. The selectivity of this deficit may reflect the circumscribed nature of pathophysiological change affecting the caudate nucleus in early PD.

Intact implicit memory for novel patterns in Alzheimer's disease

Repetition priming is a kind of implicit memory (learning without awareness) that does not depend on the medial temporal-lobe system. For example, the amnesic patient H.M., who underwent bilateral medial temporal-lobe resection, shows intact priming with novel patterns, suggesting that perceptual priming with nonverbal material does not depend on areas critical for explicit memory. A logical candidate for the neural substrate that supports this kind of priming is the peristriate cortex, an area that is relatively spared in Alzheimer's disease (AD). We therefore predicted that AD subjects would be unimpaired on pattern priming. Subjects copied each of six target figures onto dot patterns. After performing a 3-min distractor task, they were given the same dot patterns (without lines) and asked to draw the first figure that came to mind by connecting the dots with straight lines. Subsequently, in a test of recognition (explicit) memory, subjects viewed each of the six patterns of dots that they had copied previously and were asked to indicate which of four possible completions corresponded to the figure that they had copied 3 min earlier. The AD and control groups achieved comparable priming scores, but AD subjects were significantly impaired in recognizing the patterns explicitly. Our finding of intact pattern priming in AD provides, for the first time, evidence that pattern priming depends on the peristriate cortex.

The hippocampal formation participates in novel picture encoding: evidence from functional magnetic resonance imaging

Considerable evidence exists to support the hypothesis that the hippocampus and related medial temporal lobe structures are crucial for the encoding and storage of information in long-term memory. Few human imaging studies, however, have successfully shown signal intensity changes in these areas during encoding or retrieval. Using functional magnetic resonance imaging (fMRI), we studied normal human subjects while they performed a novel picture encoding task. High-speed echo-planar imaging techniques evaluated fMRI signal changes throughout the brain. During the encoding of novel pictures, statistically significant increases in fMRI signal were observed bilaterally in the posterior hippocampal formation and parahippocampal gyrus and in the lingual and fusiform gyri. To our knowledge, this experiment is the first fMRI study to show robust signal changes in the human hippocampal region. It also provides evidence that the encoding of novel, complex pictures depends upon an interaction between ventral cortical regions, specialized for object vision, and the hippocampal formation and parahippocampal gyrus, specialized for long-term memory.

Apolipoprotein E genotype does not influence rates of cognitive decline in Alzheimer's disease

BACKGROUND

Inheritance of the apolipoprotein E (apoE) epsilon 4 allele is a risk factor for developing Alzheimer's disease (AD) and is associated with a lower age of dementia onset. The purpose of this study was to determine whether apoE genotypes differentially influence the course of cognitive decline in AD dementia.

METHODS

We administered nine cognitive tests that assessed explicit memory, attention, language, visuospatial function, frontal-lobe function, and logical reasoning abilities to 66 probable AD patients every 6 to 24 months over a span of up to 5.5 years. We identified apoE genotype by a PCR-based method; there were 16 patients with epsilon 3/3, 34 with epsilon 3/4, and 16 with epsilon 4/4. Using regression statistical methods, we computed the change in performance for each test for each patient over time. We then analyzed the mean change in each test in patients grouped according to apoE genotype.

RESULTS

For the AD patients as a group, performance on all cognitive tests declined significantly over time, but the rate of decline did not vary significantly across apoE genotypes on any cognitive test. Specifically, the rate of cognitive decline was not faster in patients with an epsilon 4 allele than in those with epsilon 3/3.

CONCLUSIONS

These results indicate that the mechanism placing individuals with an epsilon 4 allele at risk for developing AD does not influence the rate of cognitive decline. These observations imply that the influence of apoE epsilon 4 either precedes or occurs at an early point in the AD disease process.

Cognitive test performance in detecting, staging, and tracking Alzheimer's disease

OBJECTIVES

To identify the specific cognitive deficits that characterize Alzheimer's disease (AD) and determine which cognitive tests, or combination of tests, are best for detecting AD (ie, distinguishing patients with AD from normal control subjects), staging AD (ie, distinguishing different severities of dementia), and tracking disease progression.

SUBJECTS

Patients with AD (n = 123) and normal control subjects (n = 60) of comparable age, education, and gender distribution.

SETTING

Outpatient care.

MEASURES

Ten cognitive tests of memory, language, visuospatial abilities, and reasoning; the Information, Memory and Concentration subtest of the Blessed Dementia Scale, and the total score on an activities of daily living questionnaire.

DESIGN

Patients with AD were tested every 6 to 24 months over a span of up to 5.5 years.

RESULTS

Patients with AD were significantly inferior to normal control subjects on all cognitive tests. The scores of patients with AD worsened over time. Delayed recall of stories and figures showed sharp deterioration to an early floor, consistent with the finding that these tests discriminated patients with mild AD from normal control subjects well but were poor for staging. Confrontation naming, semantic fluency, and immediate recognition of geometric figures showed steady linear decline across time for patients with AD, consistent with these tests being found best for staging dementia severity.

CONCLUSIONS

We postulate that the pathologic bases of impairment in delayed recall are atrophy of cholinergic ventral forebrain neurons and partial deafferentation of the hippocampus, both of which occur early in the course of AD. Worsening language and visuospatial abilities likely reflect progressive loss of neocortical neurons and their connections.

Double dissociation of memory capacities after bilateral occipital-lobe or medial temporal-lobe lesions

Memory for recently encountered information can be reflected in conscious recall and recognition of that material, or in facilitated reprocessing of that material, an effect known as repetition priming. Repetition priming may be perceptual (form-based) or conceptual (meaning-based). A patient with bilateral occipital-lobe lesions (L.H.) and a patient with bilateral medial-temporal lobe lesions (H.M.) showed a double dissociation between visuoperceptual priming (impaired in L.H. and intact in H.M.) and visual recognition memory (intact in L.H. and impaired in H.M.). L.H. showed intact conceptual priming for visually presented words; his pattern of impaired visuoperceptual priming and intact conceptual priming is the reverse dissociation to that observed in prior studies of patients with Alzheimer's disease, in whom occipital cortices are relatively spared. These double dissociations suggest that a memory system localized to the occipital lobe mediates visuoperceptual priming effects, and that this system is independent of neural circuits mediating conceptual priming effects, and independent of the limbic-diencephalic system supporting conscious recognition of recently encountered information.

Prevalence of visual deficits in Alzheimer's disease

This study addressed the issue of prevalence and pattern of visual deficits in 77 subjects with Alzheimer's disease (AD) and 111 healthy control subjects. We defined cutoff scores that would be expected from only 1 control subject of 100 (p = 0.01). The percentage of AD subjects who performed at or worse than this level varied across the 16 visual tests from a high of 58% to a low of 0%. The distribution of impairment across tests suggests a high vulnerability in AD of pattern vision, moderate vulnerability of spatial vision, and low vulnerability of motion and flicker perception. We found evidence for heterogeneity in the AD subject group: a subgroup (N = 14) emerged that was characterized by poor performance on the Backward Pattern Masking test, relatively young age, and relatively short duration of AD. Overall, the results indicate that visual dysfunction, especially on Backward Masking, is a common sign of AD.

Visual dysfunction predicts cognitive deficits in Alzheimer's disease

Deficits in basic visual capacities are prevalent in Alzheimer's disease (AD), raising the question of their impact on cognitive function. We examined the relation between vision and cognition in 72 patients with AD. Vision tests assessed color discrimination, stereoacuity, contrast sensitivity, and backward pattern masking. For cognitive tests of object recognition, at least 25% (up to 50%) of score variance was predicted by performance on a vision test. For tests of spatial localization, only 2 to 11% of the variance was predicted by performance on a vision test. The results indicated that: (1) visual dysfunction was a significant predictor of cognitive dysfunction in AD, and (2) visual deficits in AD may have a strong functional impact on performance in specific cognitive domains.

Broad-band visual capacities are not selectively impaired in Alzheimer's disease

Histological examination of the optic nerves of Alzheimer's disease (AD) patients has revealed a selective degeneration of large axon ganglion cells. This morphological abnormality raises the possibility of a selective impairment of broad-band channel visual function. To test this hypothesis, we administered visual psychophysical tests associated with either the color-opponent or the broad-band retinocortical channel to 14 AD patients and 29 elderly control subjects (ECS). In previous studies in monkeys, these tests had been sensitive to the effects of either parvocellular or magnocellular LGN lesions. In the present study, the color-opponent channel was assessed by tests of texture and color discrimination; the broad-band channel was assessed by tests of flicker and motion detection. Logistic regression analysis indicated that all tests collectively discriminated diagnostic groups at a borderline level of significance (p = 0.09). ANOVA also indicated a trend towards overall depressed function for AD patients on some capacities tested. Analyses comparing the prevalence of deficits in the AD and ECS groups showed that a significantly greater number of AD patients than ECS had deficits on texture discrimination, blue-violet discrimination, and 4.72 degrees/s motion detection. No individual subject demonstrated a selective impairment of broad-band channel function. The visual deficits in AD did not resemble those caused by lesions of magnocellular LGN in monkeys, indicating that the visual impairment in AD is not a functional reflection of damage limited to the broad-band channel.(ABSTRACT TRUNCATED AT 250 WORDS)

Impaired problem solving in Parkinson's disease: impact of a set-shifting deficit

Parkinson's disease (PD) is associated with specific cognitive deficits in the absence of dementia, including the inability to suppress previously learned responses in a changed context. Our goal was to determine whether this set-shifting deficit is sufficient to account for impaired performance on a problem-solving task, or, instead, whether it is necessary to postulate deficits in one or more other cognitive capacities, such as logical deduction. Deductive reasoning and other conceptual abilities were assessed in 15 nondemented subjects with PD who had never been medicated, 15 nondemented subjects with PD who were currently receiving medication, and 15 healthy elderly control subjects. On a deductive reasoning task, Poisoned Food Problems, the PD groups made more errors than the control group. The PD groups' error pattern was characterized by intrusions of information from previous problems. By contrast, the PD groups made appropriate assessments of redundant and irrelevant information that appeared in these problems, and performed normally on other tests of concept formation and problem solving that did not require set shifting, indicating that the capacities for logical deduction and concept formation were intact. The set-shifting deficit, conceptualized as a difficulty in suppressing a prepotent response, appears to be a primary cognitive impairment in PD and presumably arises from dysfunction of the nigrostriatal-dorsolateral prefrontal cortex complex loop.

Intact acquisition and long-term retention of mirror-tracing skill in Alzheimer's disease and in global amnesia

The ability of patients with Alzheimer's disease (AD) or global amnesia (AMN) to acquire skill for tracing a pattern seen in mirror-reversed view and to retain that skill over 24-h intervals was examined. Both patient groups had poor recall and recognition of their mirror-tracing experience, but they acquired and retained mirror-tracing skill as well as normal control subjects. One AMN patient (H.M.) retained the skill over a year-long interval. Furthermore, the patients transferred their skill normally to an alternate pattern. These results indicate that the memory system underlying mirror-tracing skill learning is separable from medial-temporal structures compromised in AMN and AD and from neocortical areas compromised in AD. Brain regions relatively spared in early AD, such as the basal ganglia or cerebellum, may mediate critical aspects of the learning of novel sensorimotor associations that underlie skilled mirror tracing.

Dissociations among structural-perceptual, lexical-semantic, and event-fact memory systems in Alzheimer, amnesic, and normal subjects

Patients with Alzheimer's disease (AD), patients with global amnesia (AMN), and normal control (NC) subjects received tests of recall and recognition, word-completion priming, and incomplete-picture priming. The AD and AMN patients had impaired recall and recognition. The AD patients, but not the AMN patients, had impaired word-completion priming. In contrast, the AD patients had intact incomplete-picture priming, a form of priming shown to be perceptual in normal subjects. These results provide neuropsychological evidence for a dissociation between two components of repetition priming, perceptual priming as measured with identification tasks and nonperceptual priming as measured with generation tasks. Preserved perceptual priming in AD may be mediated by the occipital regions that are relatively spared in AD; compromised nonperceptual priming may be mediated by temporal regions that show dense neuropathological changes early in AD.

Priming in perceptual identification of pseudowords is normal in Alzheimer's disease

Patients with Alzheimer's disease (AD) have deficits in recall and recognition memory, and show dissociable performance in repetition priming tasks: They exhibit impaired priming in word-completion and word-generation tasks, but normal priming is perceptual identification of words. In order to examine whether AD patients can show normal priming with novel, unfamiliar stimuli, the present study examined their performance in perceptual identification of pseudowords. Despite impaired recognition memory performance, AD patients showed normal priming in perceptual identification of pseudowords. These results extend the boundaries of intact repetition priming in AD, demonstrating that such priming is not limited to stimuli that are pre-morbidly represented in long-term knowledge. Preserved repetition priming in AD may reflect the operation of perceptual processes localized to posterior visual circuits that are relatively spared in AD.

Intact acquisition and long-term retention of mirror-tracing skill in Alzheimer's disease and in global amnesia

The ability of patients with Alzheimer's disease (AD) or global amnesia (AMN) to acquire skill for tracing a pattern seen in mirror-reversed view and to retain that skill over 24-h intervals was examined. Both patient groups had poor recall and recognition of their mirror-tracing experience, but they acquired and retained mirror-tracing skill as well as normal control subjects. One AMN patient (H.M.) retained the skill over a year-long interval. Furthermore, the patients transferred their skill normally to an alternate pattern. These results indicate that the memory system underlying mirror-tracing skill learning is separable from medial-temporal structures compromised in AMN and AD and from neocortical areas compromised in AD. Brain regions relatively spared in early AD, such as the basal ganglia or cerebellum, may mediate critical aspects of the learning of novel sensorimotor associations that underlie skilled mirror tracing.

Auditory function in Alzheimer's disease

The pattern of cerebral degeneration in Alzheimer's disease (AD) patients suggests that basic auditory capacities should be normal in AD, whereas progressively higher levels of auditory function should be increasingly impaired. To test this hypothesis, we administered tests of auditory capacities associated with primary auditory cortex (sound localization and perception of complex tones) and auditory association cortex (phoneme discrimination, timbre discrimination, and tonal memory) to 19 mildly to moderately demented AD patients, 21 elderly control subjects (ECS), and 14 young control subjects (YCS). The results showed significant differences between YCS and ECS on phoneme discrimination with synthetic speech and on tonal memory. The AD group differed from the ECS group on sound localization, one measure of synthetic speech discrimination, and timbre discrimination. Performance did not correlate with age, dementia severity, or duration of illness on any test condition. These findings indicate that although AD is accompanied by specific auditory deficits, the increase in neuropathologic change between primary auditory and auditory association cortices is not reflected in an increased impairment of functions that are mediated by these areas. Degraded aural language comprehension, which is characteristic of AD, likely reflects disruption of language processes, rather than dysfunction specific to auditory circuits.

Incomplete achromatopsia in Alzheimer's disease

We report that patients with Alzheimer's disease (AD) have a selective deficit in blue hue discrimination, as assessed with three clinical measures of color vision. The Farnsworth D-15 Test, the Lanthony New Color Test, and the City University Color Vision Test were administered to 32 patients with AD (ranging in dementia severity from mild to severe) and 32 age-matched normal control subjects (NCS). Of the AD patients, 11 who were representative of the larger group for age, education level, and dementia severity received a complete neuro-ophthalmological examination that ruled out obvious disorders of the anterior visual structures. AD patients made significantly more tritan (blue) errors than NCS on all three color vision tests but did not make more protan (red) or deutan (green) errors on two of the three tests. The results support the conclusion that there is a deficit in color discrimination in AD that is specific to blue hues, and oppose the hypothesis that AD does not deleteriously affect the color-opponent visual channel. In the absence of obvious damage to anterior visual structures, the likely substrates for the observed deficit are peristriate and inferotemporal visual cortices, which are subject to significant neuropathology in AD.

Retinocalcarine function in Alzheimer's disease. A clinical and electrophysiological study

Impaired visual function in Alzheimer's disease (AD) could result from either precortical or cortical lesions, or both. In a parallel psychophysical study of visual function in AD, we found that contrast sensitivity function, color vision, stereoacuity, and backward masking were impaired relative to the performance of age-matched control subjects, whereas performance on a critical flicker fusion test was normal. The intent of the present study was to determine whether abnormalities of the retinocalcarine pathway contribute to visual dysfunction. We performed neuro-ophthalmological examinations on 38 patients with AD; from this group, 25 received additional psychophysical testing and 13 underwent electrophysiological testing. Clinical neuro-ophthalmological examinations, full-field electroretinograms, focal electroretinograms, and pattern visual evoked potentials were normal in all patients tested. There was no evidence of retinocalcarine abnormality specific to AD. We conclude that the visual impairment experienced by some patients with AD primarily results from involvement of the visual association cortices rather than from precortical damage, at least before the end stage of the disease.

Category knowledge in Alzheimer's disease: Normal organization and a general retrieval deficit

Three hypotheses that could account for deficits in the retrieval of category information in Alzheimer's disease (AD) were evaluated: abnormal organization, class- or category-specific vulnerability, and limitation by general factors, such as decreased processing speed. Relative to 18 elderly control subjects, 18 patients with AD produced fewer items in a category fluency task and had longer reaction times in a category decision task. The pattern of performance across categories on both tasks was normal in the AD group: The same categories elicited the most (or fastest) responses in both the control group and the AD group. AD patients showed normal performance in ranking of category exemplars by typicality. There was no evidence for differential accessibility by category or by class of information (animate vs. inanimate). The authors conclude that a general factor or factors limit(s) retrievability equally across all categories.

Visual function in Alzheimer's disease and normal aging

We examined a wide range of visual behaviors in 59 patients with Alzheimer's disease (AD), 35 elderly control subjects, and 12 young control subjects. A subset of the patients with AD received neuro-ophthalmologic and electrophysiologic examinations in order to evaluate the integrity of the retino-calcarine pathway. Patients with AD showed significant, selective losses in visual function, including color discrimination, stereoacuity, contrast sensitivity, and backward masking, but not in critical flicker fusion. The deficits were not attributable to clinically apparent lesions of the retina or optic nerve. We therefore suggest that AD lesions in primary visual and posterior association cortices underlie the observed behavioral abnormalities.

Evidence for a dissociation between perceptual and conceptual priming in Alzheimer's disease

The status of perceptual priming and word-completion priming in patients with Alzheimer's disease (AD) was examined. Experiment 1 established the reliability of the perceptual priming measure in normal subjects. In Experiment 2, AD patients showed a normal magnitude of perceptual priming. In Experiment 3, a single group of AD patients showed a normal magnitude of perceptual priming and impaired word-completion priming. Further, word-completion priming, but not perceptual priming, was correlated with verbal fluency performance in AD. These results suggest a dissociation between two components of verbal priming. Perceptual priming may reflect the operation of a structural-perceptual memory system mediated by occipital lobe regions relatively spared in AD. Word-completion priming may reflect the operation of a lexical-semantic memory system mediated by temporoparietal lobe regions compromised in AD.