A PET study of stimulus- and task-induced semantic processing

To investigate the neural correlates of semantic processing, previous functional imaging studies have used semantic decision and generation tasks. However, in addition to activating semantic associations these tasks also involve executive functions that are not specific to semantics. The study reported in this paper aims to dissociate brain activity due to stimulus-driven semantic associations and task-induced semantic and executive processing by using repetition and semantic decision on auditorily presented words in a cognitive conjunction design. The left posterior inferior temporal, inferior frontal (BA 44/45), and medial orbital gyri were activated by both tasks, suggesting a general role in stimulus-driven semantic and phonological processing. In addition, semantic decision increased activation in (i) left ventral inferior frontal cortex (BA 47), right cerebellum, and paracingulate, which have all previously been implicated in executive functions, and (ii) a ventral region in the left anterior temporal pole which is commonly affected in patients with semantic impairments. We attribute activation in this area to the effortful linkage of semantic features. Thus, our study replicated the functional dissociation between dorsal and ventral regions of the left inferior frontal cortex. Moreover, it also dissociated the semantic functions of the left posterior inferior temporal gyrus and anterior temporal pole: The posterior region subserves stimulus-driven activation of semantic associations and the left anterior region is involved in task-induced association of semantic information.

Retrieval of visual, auditory, and abstract semantics

Conceptual knowledge is thought to be represented in a large distributed network, indexing a range of different semantic features (e.g., visual, auditory, functional). We investigated the anatomical organization of these features, using PET, by contrasting brain activity elicited by heard words with (i) visual (e.g., blue), (ii) auditory (e.g., noise), or (iii) abstract (e.g., truth) meaning. The activation task was either repetition or semantic decision (e.g., does the meaning of the word relate to religion?). In the baseline conditions, the sound track of the words was reversed and subjects had to say "OK" (control for repetition) or make an acoustic decision (control for semantic decision). Irrespective of task, words relative to their corresponding controls activated the left posterior inferior temporal and inferior frontal cortices. In addition, semantic decisions on words with sensory (visual and auditory) meanings enhanced activation in a ventral region of the left anterior temporal pole. These results are consistent with neuropsychological studies showing that anterior temporal lobe damage can cause deficits for items that are mainly defined by their sensory features (i.e., concrete, particularly living items). Since modality-specific activation was observed only during the semantic decision task, we discuss whether it reflects retrieval of sensory semantics per se or the degree to which semantic associations are triggered during effortful retrieval.

Anatomic constraints on cognitive theories of category specificity

Many cognitive theories of semantic organization stem from reports of patients with selective, category-specific deficits for particular classes of objects (e.g., fruit). The anatomical assumptions underlying the competing claims can be evaluated with functional neuroimaging but the findings to date have been inconsistent and insignificant when standard statistical criteria are adopted. We hypothesized that category differences in functional brain responses might be small and task dependent. To test this hypothesis, we entered data from seven PET studies into a single multifactorial design which crossed category (living vs man-made) with a range of tasks. Reliable category-specific effects were observed but only for word retrieval and semantic decision tasks. Living things activated medial aspects of the anterior temporal poles bilaterally while tools activated a left posterior middle temporal region. These category-by-task interactions provide robust evidence for an anatomical double dissociation according to category and place strong constraints on cognitive theories of the semantic system. Furthermore they reconcile some of the apparent inconsistencies between lesion studies and functional neuroimaging data.

Are subitizing and counting implemented as separate or functionally overlapping processes?

Enumeration of small groups of four or fewer objects is very fast and accurate (often called "subitizing"), but gets slower and more error prone for more than four items ("counting"). Many theories have been proposed to account for this dichotomy, most suggesting that "subitizing" and "counting" are two qualitatively different and separable processes. Others, in contrast, have proposed that the two operations reflect two different levels along a continuum of complexity. In this paper we present a PET study that attempts to characterize subitizing and counting at a neural level in order to investigate whether they are implemented as separate or functionally overlapping processes. Subjects performed an enumeration task on visual arrays of dots that varied in numerosity (1-4 and 6-9 dots) and spatial arrangement (canonical and random). The results demonstrated a common network for subitizing and counting that comprises extrastriate middle occipital and intraparietal areas. The intensity and spatial extent of this network were modulated by the number of dots and their spatial arrangement: activation increased as the number of items in the visual array increased, reaching maximum peak and extent for counting 6-9 randomly arranged items. Direct comparison between subitizing and counting showed that counting, relative to subitizing, was correlated with increased activity in this occipitoparietal network, while subitizing did not show areas of increased activation with respect to counting. Results speak against the idea of the two processes being implemented in separable neural systems.

Is there an anatomical basis for category-specificity? Semantic memory studies in PET and fMRI

Patients with semantic impairments sometimes demonstrate category-specific deficits suggesting that the anatomical substrates of semantic memory may reflect categorical organisation, however, neuroimaging studies have failed to provide consistent data in support of a category-based account. We conducted three functional neuroimaging experiments to investigate the neural correlates of semantic processing, two with positron emission tomography (PET) and a third with functional magnetic resonance imaging (fMRI). The first experiment used a lexical decision task to search for brain regions selectively activated by concepts from four different categories--animals, fruit, tools, and vehicles. The second experiment used a semantic categorisation task to increase the demands on the semantic system and to look for evidence of consistent activations for the domains of natural kinds or man-made items. The final experiment was a replication of the semantic categorisation task using fMRI to increase the spatial resolution and statistical sensitivity of the experiment. The results of these experiments reliably identified a distributed neural system common to both natural kinds and artifacts but failed to find robust evidence of functional segregation by domain or categories. Category effects were neither reliable nor consistently present across experiments although some were consistent with previous studies. We discuss the implications of these findings, arguing that they are most consistent with a semantic system undifferentiated by category at the neural level.

Functional imaging studies of neuropsychological patients: applications and limitations

Functional imaging studies of neuropsychologically impaired patients have not enjoyed the immediate success that was attained by functional imaging studies of normal subjects. This is largely because it has taken time to appreciate some of the deeper issues surrounding study design, analysis and interpretation. The most significant limitation is that functional imaging experiments with patients need tasks that the patients can perform. This precludes direct investigations of the physiological correlates of cognitive deficits. Nevertheless, functional imaging studies of brain-damaged patients who retain task competence can provide information that is not available from structural imaging, behavioural assessments or functional imaging with normal subjects. This is because intact task performance, following a brain lesion, does not necessarily entail normal neuronal responses in undamaged cortical areas. Abnormal neuronal responses, in the context of normal performance, can indicate alternative neuronal and cognitive mechanisms for supporting the same task. This, in turn, has important implications for understanding the mechanisms that mediate recovery and the organizational principles that underlie functional architectures in the human brain.

Detecting latency differences in event-related BOLD responses: application to words versus nonwords and initial versus repeated face presentations

We introduce a new method for detecting differences in the latency of blood oxygenation level-dependent (BOLD) responses to brief events within the context of the General Linear Model. Using a first-order Taylor approximation in terms of the temporal derivative of a canonical hemodynamic response function, statistical parametric maps of differential latencies were estimated via the ratio of derivative to canonical parameter estimates. This method was applied to two example datasets: comparison of words versus nonwords in a lexical decision task and initial versus repeated presentations of faces in a fame-judgment task. Tests across subjects revealed both magnitude and latency differences within several brain regions. This approach offers a computationally efficient means of detecting BOLD latency differences over the whole brain. Precise characterization of the hemodynamic latency and its interpretation in terms of underlying neural differences remain problematic, however.

The effect of botulinum toxin type A on the functional ability of the child with spastic hemiplegia a randomized controlled trial

It has been demonstrated that botulinum toxin type A (BTX-A) injections reduce spasticity and improve muscle growth in children with spasticity. It has been postulated that BTX-A allows the learning of more normal movement patterns. The aim of this study was to measure the effect of this treatment on functional ability, as measured by the Gross Motor Function Measure (GMFM), in children with spastic hemiplegic cerebral palsy. Children of 3--13 years and meeting the selection criteria were randomly allocated to the control or injection group using a matched pair design. A match constituted a child within 6 months of age with the same Modified Ashworth Score (MAS) for the gastroc-soleus and within 10% of the same goal scores on the Gross Motor Function Measure. Twelve matched pairs were enrolled. Outcomes were measured on enrolment and at 1, 3 and 6 months post injection. The time course of the response to BTX-A was assessed with measurements of the MAS, dynamic range of motion (R1) and static muscle length (R2). Motor function was assessed using the 88-item GMFM and parental satisfaction with a 10-point visual analogue scale. Within pair comparisons of the GMFM using the Wilcoxon signed rank test indicated that the treatment group made significantly greater gains than controls at 3 months (P=0.02) with even greater differences seen at 6 months (P=0.004). Using parametric statistics, the intrapair difference in proportional change of GMFM increased from 35% (4 to 65) at 3 months to 52% (17--87) at 6 months. Response to injection was confirmed by a decrease in MAS in the treatment group and very little change in controls. This difference was significant (P=0.002) at 3 months and was attenuated but still significant (P=0.016) at 6 months; the difference in proportional change decreased from 44% at 3 months to 22% at 6 months. Changes in R1 reflected those of MAS in the treatment group and deteriorated significantly over the study period in controls. Parents of children in the treatment group were more satisfied than controls, but satisfaction scores did not correlate with changes in function or technical outcomes suggesting that this may be a placebo effect. The changes in GMFM correlated with changes in technical outcomes at 3 months, suggesting a causal relationship. The intrapair differences in GMFM continued to increase even after the local response to injection had started to wane.

Nonlinear coupling between evoked rCBF and BOLD signals: a simulation study of hemodynamic responses

The aim of this work was to investigate the dependence of BOLD responses on different patterns of stimulus input/neuronal changes. In an earlier report, we described an input-state-output model that combined (i) the Balloon/Windkessel model of nonlinear coupling between rCBF and BOLD signals, and (ii) a linear model of how regional flow changes with synaptic activity. In the present investigation, the input-state-output model was used to explore the dependence of simulated PET (rCBF) and fMRI (BOLD) signals on various parameters pertaining to experimental design. Biophysical simulations were used to estimate rCBF and BOLD responses as functions of (a) a prior stimulus, (b) epoch length (for a fixed SOA), (c) SOA (for a fixed number of events), and (d) stimulus amplitude. We also addressed the notion that a single neuronal response may differ, in terms of the relative contributions of early and late neural components, and investigated the effect of (e) the relative size of the late or "endogenous" neural component. We were interested in the estimated average rCBF and BOLD responses per stimulus or event, not in the statistical efficiency with which these responses are detected. The BOLD response was underestimated relative to rCBF with a preceding stimulus, increasing epoch length, and increasing SOA. Furthermore, the BOLD response showed some highly nonlinear behaviour when varying stimulus amplitude, suggesting some form of hemodynamic "rectification." Finally, the BOLD response was underestimated in the context of large late neuronal components. The difference between rCBF and BOLD is attributed to the nonlinear transduction of rCBF to BOLD signal. Our simulations support the idea that varying parameters that specify the experimental design may have differential effects in PET and fMRI. Moreover, they show that fMRI can be asymmetric in its ability to detect deactivations relative to activations when an absolute baseline is stipulated. Finally, our simulations suggest that relative insensitivity to BOLD signal in specific regions, such as the temporal lobe, may be partly explained by higher cognitive functions eliciting a relatively large late endogenous neuronal component.

Identification of famous faces and buildings: a functional neuroimaging study of semantically unique items

Several functional imaging experiments have clearly established that the fusiform gyri are preferentially responsive to faces, whereas the parahippocampal/lingual gyri are more responsive to buildings. Other studies have demonstrated that famous faces additionally activate the anterior temporal cortex relative to unfamiliar faces, animals, tools, body parts and maps. One explanation for this apparent specialization for known people could be that famous faces are 'semantically unique items'. In other words, they carry unique semantic associations that are not shared by other perceptually similar category members. If this hypothesis is correct, the anterior temporal cortex should also respond to other semantically unique items, such as famous buildings. In this PET study, we investigated the effect of fame (famous relative to non-famous) on activation elicited by famous and non-famous faces and buildings during a same-different matching task. We found that, when the task was held constant, category-specific activations in the fusiform and parahippocampal/lingual areas were not modulated by fame. In contrast, in the left anterior middle temporal gyrus there was an effect of fame that was common to faces and buildings. These results suggest that the identification of famous faces and buildings involves category-specific perceptual processing in the fusiform and parahippocampal/lingual regions, respectively, and shared analysis of unique semantic attributes in the left anterior temporal cortex.

The effects of presentation rate during word and pseudoword reading: a comparison of PET and fMRI

The effect of stimulus rate and its interaction with stimulus type on brain activity during reading was investigated using functional magnetic resonance imaging (fMRI). This (i) enabled the segregation of brain regions showing differential responses, (ii) identified the optimum experimental design parameters for maximizing sensitivity, and (iii) allowed us to evaluate further the sources of discrepancy between positron emission tomography (PET) and fMRI signals. The effect of visual word rate has already been investigated in a previous PET study. However, rate effects can be very different in PET and fMRI, as seen in previous studies of auditory word processing. In this work, we attempt to replicate rate-sensitive activations observed with PET using fMRI. Our objective was to characterize the discrepancies in regionally specific rate-sensitive effects between the two imaging modalities. Subjects were presented with words and pseudowords at varying rates while performing a silent reading task. The analysis specifically identified regions showing (i) an effect of stimulus rate on brain activity during reading; (ii) modulation of this effect by word type; and (iii) increased activity during reading relative to rest, but with no dependence on stimulus rate. The results identified similar effects of rate for words and pseudowords (no interactions between rate and word type reached significance). Irrespective of word type, strong positive linear effects of rate (i.e., activity increasing with rate) were detected in visual areas, right superior temporal gyrus, and bilateral precentral gyrus. These findings replicate the results of the previous PET study, confirming that activation in regions associated with visual processing and response generation increases with the number of stimuli. Likewise, we detected rate-independent effects reported in the previous PET study in bilateral anterior middle temporal, inferior frontal, and superior parietal regions. These results differentiate the functionally specific responses in rate-dependent and rate-independent areas. However, for negative effects of rate, fMRI did not replicate the effects seen in PET, suggesting some form of hemodynamic "rectification." The discussion focuses on differences between evoked rCBF and BOLD signals.

Functional-imaging studies of the 19th Century neurological model of language

This paper will consider the consistencies and disparities between the classic 19(th) Century model of language and the results of functional neuro-imaging studies on auditory and visual word repetition. The functional imaging studies show that, as predicted by the 19(th) Century neurologists, auditory and visual word repetition engage perisylvian regions in the left posterior superior temporal and posterior inferior frontal cortices. More specifically, the roles that Wernicke and Broca assigned to these regions lie respectively in the upper banks of the left posterior superior temporal sulcus and the left anterior insula/frontal operculum. In addition, a region in the left posterior inferior temporal cortex is activated for word retrieval and provides a second route to reading, emphasised by 20(th) Century cognitive models of language. There is no firm evidence as yet to link the function of a visual word form area to a specific neural substrate. The angular gyrus, previously linked to the visual word form system, is shown to be part of a distributed semantic system that can be accessed by objects and faces as well as speech. Overall, functional neuroimaging demonstrates that the 19(th) Century neurological model of language is remarkably insightful.

Dopamine D4 receptor activation inhibits presynaptically glutamatergic neurotransmission in the rat supraoptic nucleus

Oxytocin and vasopressin release from magnocellular neurons of the supraoptic nucleus is under the control of glutamate-dependent excitation. The supraoptic nucleus also receives a generalized dopaminergic input from hypothalamic sources. To determine if dopamine can influence this excitatory drive onto the magnocellular neurons, we used whole-cell patch clamp to record the effect of dopamine on evoked and miniature excitatory postsynaptic currents in rat hypothalamic slices. Dopamine exposure (30 microM to 1 mM) induced a large and reversible reduction in the amplitude of evoked excitatory postsynaptic current in nearly all magnocellular cells tested. D4 receptors appeared to mediate dopamine's activity, based on inhibition of the response with 50 microM clozapine, but not by SCH 23390 or sulpiride, and mimicry of dopamine's action with the D4 specific agonist, PD 168077. Analysis of paired-pulse experiments and miniature postsynaptic currents indicated that dopamine's action involved a presynaptic mechanism, since the frequency of miniature postsynaptic currents was reduced with dopamine exposure without any change in current kinetics or amplitude, while the paired-pulse ratio increased. We therefore have demonstrated for the first time a role for dopamine D4 receptors in the supraoptic nucleus in the presynaptic inhibition of glutamatergic neurotransmission onto magnocellular neurons.

The constraints functional neuroimaging places on classical models of auditory word processing

Several previous functional imaging experiments have demonstrated that auditory presentation of speech, relative to tones or scrambled speech, activate the superior temporal sulci (STS) bilaterally. In this study, we attempted to segregate the neural responses to phonological, lexical, and semantic input by contrasting activation elicited by heard words, meaningless syllables, and environmental sounds. Inevitable differences between the duration and amplitude of each stimulus type were controlled with auditory noise bursts matched to each activation stimulus. Half the subjects were instructed to say "okay" in response to presentation of all stimuli. The other half repeated back the words and syllables, named the source of the sounds, and said "okay" to the control stimuli (noise bursts). We looked for stimulus effects that were consistent across task. The results revealed that central regions in the STS were equally responsive to speech (words and syllables) and familiar sounds, whereas the posterior and anterior regions of the left superior temporal gyrus were more active for speech. The effect of semantic input was small but revealed more activation in the inferior temporal cortex for words and familiar sounds than syllables and noise. In addition, words (relative to syllables, sounds, and noise) enhanced activation in the temporo-parietal areas that have previously been linked to modality independent semantic processing. Thus, in cognitive terms, we dissociate phonological (speech) and semantic responses and propose that word specificity arises from functional integration among shared phonological and semantic areas.

The effects of case mixing on word recognition: evidence from a PET study

The early stages of visual word recognition were investigated by scanning participants using PET as they took part in implicit and explicit reading tasks with visually disrupted stimuli. CaSe MiXiNg has been shown in behavioral studies to increase reaction times (RTs) in naming and other word recognition tasks. In this study, we found that during both an implicit (feature detection) task and an explicit word-naming task, mixed-case words compared to same-case words produced increased activation in an area of the right parietal cortex previously associated with visual attention. No effect of case was found in this area for pseudowords or consonant strings. Further, lowering the contrast of the stimuli slowed RTs as much as case mixing, but did not lead to the same increase in right parietal activation. No significant effect of case mixing was observed in left-hemisphere language areas. The results suggest that reading mixed-case words requires increased attentional processing. However, later word recognition processes may be relatively unaffected by the disruption in presentation.

Generative models, brain function and neuroimaging

The representational capacity and inherent function of any neuron, neuronal population or cortical area in the brain is dynamic and context-sensitive. Functional integration, or interactions among brain systems, that employ driving (bottom up) and backward (top-down) connections, mediate this adaptive and contextual specialisation. A critical consequence is that neuronal responses, in any given cortical area, can represent different things at different times. This can have fundamental implications for the design of brain imaging experiments and the interpretation of their results. Our arguments are developed under generative models of brain function, where higher-level systems provide a prediction of the inputs to lower-level regions. Conflict between the two is resolved by changes in the higher-level representations, which are driven by the ensuing error in lower regions, until the mismatch is "cancelled". From this perspective the specialisation of any region is determined both by bottom-up driving inputs and by top-down predictions. Specialisation is therefore not an intrinsic property of any region but depends on both forward and backward connections with other areas. Because the latter have access to the context in which the inputs are generated they are in a position to modulate the selectivity or specialisation of lower areas. The implications for classical models (e.g., classical receptive fields in electrophysiology, classical specialisation in neuroimaging and connectionism in cognitive models) are severe and suggest these models may provide incomplete accounts of real brain architectures. Here we focus on the implications for cognitive neuroscience in the context of neuroimaging.

MR diffusion tensor imaging of white matter tract disruption in stroke at 3 T

Recent advances in MR diffusion weighted imaging (DWI) enable the identification of anisotropic white matter tracts with diffusion tensor imaging (DTI). We aimed to use a novel DTI technique to safely study patients with recent stroke in a high field (3 T) MR machine with its intrinsically higher spatial resolution and signal-to-noise ratio. Of ten patients studied, six had disruption of white matter tracts as determined by DTI. A further patient had distortion of white matter tracts around an infarct rather than actual disruption of the tracts themselves. The lack of tract destruction may imply a beneficial prognosis, information that is not available with conventional DWI.

Developmental toxicity evaluation of 1,2,3,4-butanetetracarboxylic acid in Sprague Dawley (CD) rats

1,2,3,4-butanetetracarboxylic acid (BTCA), proposed as a formaldehyde substitute in the treatment of permanent press fabrics, was evaluated for developmental toxicity. Timed-mated CD rats (25 per group) received BTCA 250, 500, or 1000 mg/kg/day or vehicle (deionized/distilled water) by gavage on gestational days (gd) 6 through 19. Maternal feed and water consumption, body weight, and clinical signs were monitored throughout gestation. At termination (gd 20), confirmed-pregnant females (21 to 25 per group) were evaluated for clinical status and gestational outcome; live fetuses were examined for external, visceral, and skeletal malformations. One maternal death, reduced body weight, and reduced weight gain were noted at the high dose; confirmed pregnancy rates were 84 to 100% for each group. There were no treatment-related effects on fetal growth, survival, or morphologic development. The maternal toxicity NOAEL and LOAEL are 500 and 1000 mg/kg/day, respectively. The developmental toxicity NOAEL is > or = 1000 mg/kg/day, and the LOAEL was not established in this study.

Functional plasticity of language-related brain areas after cochlear implantation

Using PET, the cerebral network engaged by heard language processing in normal hearing subjects was compared with that in patients who received a cochlear implant after a period of profound deafness. The experimental conditions were words, syllables and environmental sounds, each controlled by a noise baseline. Four categories of effect were observed: (i) regions that were recruited by patients and controls under identical task conditions: the left and right superior temporal cortices and the left insula were activated in both groups in all conditions; (ii) new regions, which were recruited by patients only: the left dorsal occipital cortex showed systematic activation in all conditions versus noise baselines; (iii) regions that were recruited by both groups with a different functional specificity; e.g. Wernicke's area responded specifically to speech sounds in controls but was not specialized in patients; and (iv) regions that were activated in one group more than the other: the precuneus and parahippocampal gyrus (patients more than controls) and the left inferior frontal, left posterior inferior temporal and left and right temporoparietal junction regions (controls more than patients). These data provide evidence for altered functional specificity of the superior temporal cortex, flexible recruitment of brain regions located within and outside the classical language areas and automatic contribution of visual regions to sound recognition in implant patients.

Cross-modal plasticity underpins language recovery after cochlear implantation

Postlingually deaf subjects learn the meaning of sounds after cochlear implantation by forming new associations between sounds and their sources. Implants generate coarse frequency responses, preventing place-coding fine enough to discriminate sounds with similar temporal characteristics, e.g., buck/duck. This limitation imposes a dependency on visual cues, e.g., lipreading. We hypothesized that cross-modal facilitation results from engagement of the visual cortex by purely auditory tasks. In four functional neuroimaging experiments, we show recruitment of early visual cortex (V1/V2) when cochlear implant users listen to sounds with eyes closed. Activity in visual cortex evolved in a stimulus-specific manner as a function of time from implantation reflecting experience-dependent adaptations in the postimplant phase.

Dynamic diaschisis: anatomically remote and context-sensitive human brain lesions

Functional neuroimaging was used to investigate how lesions to the Broca's area impair neuronal responses in remote undamaged cortical regions. Four patients with speech output problems, but relatively preserved comprehension, were scanned while viewing words relative to consonant letter strings. In normal subjects, this results in left lateralized activation in the posterior inferior frontal, middle temporal, and posterior inferior temporal cortices. Each patient activated normally in the middle temporal region but abnormally in the damaged posterior inferior frontal cortex and the undamaged posterior inferior temporal cortex. In the damaged frontal region, activity was insensitive to the presence of words but in the undamaged posterior inferior temporal region, activity decreased in the presence of words rather than increasing as it did in the normal individuals. The reversal of responses in the left posterior inferior temporal region illustrate the context-sensitive nature of the abnormality and that failure to activate the left posterior temporal region could not simply be accounted for by insufficient demands on the underlying function. We propose that, in normal individuals, visual word presentation changes the effective connectivity among reading areas and, in patients, posterior temporal responses are abnormal when they depend upon inputs from the damaged inferior frontal cortex. Our results serve to introduce the concept of dynamic diaschisis; the anatomically remote and context-sensitive effects of focal brain lesions. Dynamic diaschisis reveals abnormalities of functional integration that may have profound implications for neuropsychological inference, functional anatomy and, vicariously, cognitive rehabilitation.

Cognitive neuropsychology and functional brain imaging: implications for functional and anatomical models of cognition

We discuss the relations between functional imaging and cognitive neuropsychological research. We begin by elaborating on some of the problems of traditional neuropsychological research, which attempted to provide accounts of cognitive performance at a neural as well as at a functional level of description. The difficulties in making neural-level arguments from neuropsychological data include: problems of associated deficits, problems due to interactive effects between brain regions, problems with analyses based on behavioural syndromes, problems due to the influence of compensatory strategies, and problems in separating damaged from disconnected representations. We discuss how cognitive neuropsychology by-passed many of these problems by emphasising functional rather than neural-level theories, though problems with inferences at the neural-level remain. We then consider the contribution that functional imaging can make to cognitive neuropsychology. Using evidence drawn from studies of language, object recognition and visual attention, we argue that functional imaging complements cognitive neuropsychology by: (i) not being reliant on accidents of nature and by enabling effects of lesions on 'distant' neural areas to be measured, (ii) revealing the brain systems necessary and sufficient for a given task, (iii) providing tests of neural-level models of cognition, and by (iv) providing novel evidence on the mechanisms of functional recovery in patients. In addition to this, imaging studies can contribute directly to functional-level theories, by providing converging evidence on the neural locus of cognition--knowing 'where' can allow new inferences about 'how' a given task is performed.

Voxel-based morphometry of herpes simplex encephalitis

Voxel-based morphometry (VBM) is a powerful tool for analyzing changes in gray or white matter density of the brain. By using an automated segmentation procedure and standardized parametric statistics it avoids biases inherent in operator-dependent morphological operations (J. Ashburner and K. J. Friston, 2000, NeuroImage 11, 805-821). Since its introduction in 1995, VBM has been used to examine anatomical changes in a variety of diseases associated with neurologic and psychiatric dysfunction. Given the power of this technique for discerning subtle anatomical changes, we wanted to assess its performance on brains with gross structural abnormalities. Such results could have implications regarding the difficulties to be faced when examining other types of distorted brains (e.g., brains with changes due to degenerative disease). This report describes the use of VBM for examining individual and group changes in gray matter concentration in five patients who had recovered from herpes simplex encephalitis (HSE) compared with age- and sex-matched controls. Because HSE tends to affect a specific set of brain regions we thought that this would (1) provide an opportunity to assess the anatomical face validity of VBM, (2) allow us to assess the problems of this technique when used on distorted brains, and (3) provide an in vivo demonstration of the gray matter changes due to HSE. We found that, despite problems in normalizing and segmenting these severely distorted brains, VBM was able to identify correctly a number of the regional gray matter abnormalities in HSE. The results, while consistent with the well-known histopathology of the disease, also demonstrate potential difficulties with this method.