Temporopolar regions of the human brain

Following prolonged neglect during the formative decades of behavioural neurology, the temporopolar region has become a site of vibrant research on the neurobiology of cognition and conduct. This turnaround can be attributed to increasing recognition of neurodegenerative diseases that target temporopolar regions for peak destruction. The resultant syndromes include behavioural dementia, associative agnosia, semantic forms of primary progressive aphasia and semantic dementia. Clinicopathological correlations show that object naming and word comprehension are critically dependent on the language-dominant (usually left) temporopolar region, whereas behavioural control and non-verbal object recognition display a more bilateral representation with a rightward bias. Neuroanatomical experiments in macaques and neuroimaging in humans show that the temporoparietal region sits at the confluence of auditory, visual and limbic streams of processing at the downstream (deep) pole of the 'what' pathway. The functional neuroanatomy of this region revolves around three axes, an anterograde horizontal axis from unimodal to heteromodal and paralimbic cortex; a radial axis where visual (ventral), auditory (dorsal) and paralimbic (medial) territories encircle temporopolar cortex and display hemispheric asymmetry; and a vertical depth-of-processing axis for the associative elaboration of words, objects and interoceptive states. One function of this neural matrix is to support the transformation of object and word representations from unimodal percepts to multimodal concepts. The underlying process is likely to start at canonical gateways that successively lead to generic (superordinate), specific (basic) and unique levels of recognition. A first sign of left temporopolar dysfunction takes the form of taxonomic blurring where boundaries among categories are preserved but not boundaries among exemplars of a category. Semantic paraphasias and coordinate errors in word-picture verification tests are consequences of this phenomenon. Eventually, boundaries among categories are also blurred and comprehension impairments become more profound. The medial temporopolar region belongs to the amygdalocentric component of the limbic system and stands to integrate exteroceptive information with interoceptive states underlying social interactions. Review of the pertinent literature shows that word comprehension and conduct impairments caused by temporopolar strokes and temporal lobectomy are far less severe than those seen in temporopolar atrophies. One explanation for this unexpected discrepancy invokes the miswiring of residual temporopolar neurons during the many years of indolently progressive neurodegeneration. According to this hypothesis, the temporopolar regions become not only dysfunctional but also sources of aberrant outputs that interfere with the function of areas elsewhere in the language and paralimbic networks, a juxtaposition not seen in lobectomy or stroke.

Conceptual priming in semantic dementia: A window into the cognitive and neural basis of conceptual implicit memory

The present study examined the impact of impairment in semantic memory on conceptual repetition priming by means of the longitudinal study of a patient with semantic dementia. ST was tested in four consecutive years, during which his semantic memory progressively deteriorated. On each occasion, he performed an abstract/concrete verification task and a verb generation task. In both of these tasks, performance during a test phase was compared for stimuli previously processed in a study phase and stimuli first seen in the test phase. Control subjects showed priming, as indicated by faster responses to studied than to baseline stimuli. ST showed intact priming that was of normal magnitude in the first two years of testing, but failed to show priming in the subsequent two years of testing. This pattern of results is interpreted with reference to the differential decline of item-specific and superordinate knowledge. The implications of these findings for the neural basis of conceptual priming are also discussed.

A metanalysis of impaired and spared naming for different categories of knowledge in patients with a visuo-verbal disconnection

The study of the neuroanatomical correlates of category-specific semantic disorders has strongly supported the 'sensory/motor model of semantic knowledge,' which assumes that the cortical areas that have critically contributed to the development of various categories are also implicated in their semantic representation. However, if the anatomo-clinical correlates are consistent with the model, less clearcut results have been obtained by functional neuroimaging experiments. In the present metanalysis, I addressed the question from a different viewpoint, shifting attention from the anatomical lesion in patients with category-specific semantic disorders to the pattern of naming impairment shown by patients suffering from a disconnection between visual areas and lexical output mechanisms. According to the model, living entities should be particularly impaired, since their semantic representations are mainly based upon visual perceptual attributes. On the contrary, actions and body parts (and to a lesser extent artefacts) should be relatively spared, as their semantic representations are mainly based upon motor, somato-sensory or functional attributes. These predictions were checked by reviewing the categorical pattern of naming impairment shown by patients with a visuo-verbal disconnection and a category-specific naming impairment published in the last 20 years. The pattern of impaired and spared categories observed in these patients was consistent with the hypothesis, since: (1) 'actions' and 'body parts' were systematically spared in comparison to all the other categories; (2) 'artefacts' were relatively spared with respect to the 'living categories'; and (3) within the biological categories, 'plants' were usually more impaired than animals.

Differences in word associations to pictures and words

Normal subjects were asked to produce the "first word that comes to mind" in response to pictures or words that differed with respect to manipulability and animacy. In separate analyses across subjects and items, normal subjects produced a significantly higher proportion of action words (that is, verbs) to pictures as compared to words, to manipulable as compared to non-manipulable stimuli and to inanimate as compared to animate stimuli. The largest proportion of action words was elicited by pictures of non-living, manipulable objects. Furthermore, associates to words matched standard word associates significantly more often than those elicited by pictures. These data suggest that pictures and words initially contact different forms of conceptual information and are consistent with an account of semantic organization that assumes that information is distributed across different domains reflecting the mode of acquisition of that knowledge.

An electrophysiological study of scene effects on object identification

The meaning of a visual scene influences the identification of visual objects embedded in it. We investigated the nature and time course of scene effects on object identification by recording event-related brain potentials (ERPs) and response times (RTs). In three experiments, participants identified objects within a scene that were either semantically congruous (e.g., a pot in a kitchen) or incongruous (e.g., a desk in a river). As expected, RTs were faster for congruous than incongruous objects. The earliest sign of reliable scene congruity effects in the ERPs (greater positivity for congruous pictures between 300 and 500 ms) was around 300 ms. Both the morphology and time course of the N390 scene congruity effect are reminiscent of the N400 sentence congruity effect typically observed in sentence context paradigms, suggesting a functional similarity of the neural processes involved. Overall, these results support theories postulating that visual scenes do not appreciably affect object identification processes before associated semantic information is activated. We speculate that the N390 scene congruity effect reflects the action of visual scene schemata stored in the anterior temporal lobe.

Role of the anterior temporal lobe in repetition and semantic priming: evidence from a patient with a category-specific deficit

Neuroimaging studies in healthy participants have implicated anterior temporal lobe regions and the fusiform gyrus in repetition priming and semantic priming. Only the investigation of patients with selective lesions, however, can establish the necessity of these particular regions. To this end, we administered three tests of repetition priming (pseudoword identification; masked-form priming; category-exemplar generation) and a test of semantic priming to a patient (J.P.) with a category-specific deficit stemming from bilateral damage to the anterior fusiform gyrus and anterior temporal regions. On all of the repetition priming tasks, J.P. showed priming effects within 1 S.D. of 10 age- and education-matched CON; ANOVAs indicated no interaction between group and prime condition. These findings suggest that the anterior fusiform and anterior temporal lobe are not required for these priming effects. J.P. also showed normal repetition priming even for items that he had never been able to name or to provide semantic information about. On the semantic priming task, J.P. showed normal levels of priming across categories. When we separately analyzed his priming for items he could never name or access information about versus items that he had been able to name on at least two testing sessions, we found priming for the latter items, but not for the former. This result suggests that category-specific deficits resulting from damage to the anterior temporal lobes may disrupt the automatic, rapid access of semantic information of some items.

Successive memory test performance and priming in Alzheimer's disease: evidence from the word-fragment completion task

This study assessed the performance of patients with Alzheimer's disease and healthy controls in a successive memory test paradigm. Subjects studied lists of words. Following study, tests of recognition (an explicit memory task) and primed word fragment completion (an implicit memory task) were administered. Since the same words were used in the two tasks, we were able to calculate the degree of dependence between recognition performance and primed word fragment completion. AD patients evidenced impaired recognition memory. In contrast, priming was intact. The pattern of correlation between the two tasks was similar in healthy controls and in AD. Independence between recognition and fragment completion was obtained when recognition preceded the fragment completion task, but not when fragment completion preceded recognition.

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.

Visual priming within and across symbolic format using a tachistoscopic picture identification task: a PET study

The present work was aimed at characterizing picture priming effects from two complementary behavioral and functional neuroimaging (positron emission tomography, PET) studies. In two experiments, we used the same line drawings of common living/nonliving objects in a tachistoscopic identification task to contrast two forms of priming. In the within-format priming condition (picture-picture), subjects were instructed to perform a perceptual encoding task in the study phase, whereas in the cross-format priming condition (word-picture), they were instructed to perform a semantic encoding task. In Experiment 1, we showed significant priming effects in both priming conditions. However, the magnitude of priming effects in the same-format/perceptual encoding condition was higher than that in the different-format/semantic encoding condition, while the recognition performance did not differ between the two conditions. This finding supports the existence of two forms of priming that may be subserved by different systems. Consistent with these behavioral findings, the PET data for Experiment 2 revealed distinct priming-related patterns of regional cerebral blood flow (rCBF) decreases for the two priming conditions when primed items were compared to unprimed items. The same-format priming condition involved reductions in cerebral activity particularly in the right extrastriate cortex and left cerebellum, while the different-format priming condition was associated with rCBF decreases in the left inferior temporo-occipital cortex, left frontal regions, and the right cerebellum. These results suggest that the extrastriate cortex may subserve general aspects of perceptual priming, independent of the kind of stimuli, and that the right part of this cortex could underlie the same-format-specific system for pictures. These data also support the idea that the cross-format/semantic encoding priming for pictures represents a form of lexico-semantic priming subserved by a semantic neural network extending from left temporo-occipital cortex to left frontal regions. These results reinforce the distinction between perceptual and conceptual priming for pictures, indicating that different cerebral processes and systems are implicated in these two forms of picture priming.

Perceptual and semantic components of memory for objects and faces: a pet study

Previous studies have suggested differences in the neural substrates of recognition memory when the contributions of perceptual and semantic information are manipulated. In a within-subjects design PET study, we investigated the neural correlates of the following factors: material type (objects or faces), semantic knowledge (familiar or unfamiliar items), and perceptual similarity at study and test (identical or different pictures). There was consistent material-specific lateralization in frontal and temporal lobe regions when the retrieval of different types of nonverbal stimuli was compared, with objects activating bilateral areas and faces preferentially activating the right hemisphere. Retrieval of memories for nameable, familiar items was associated with increased activation in the left ventrolateral prefrontal cortex, while memory for unfamiliar items involved occipital regions. Recognition memory for different pictures of the same item at study and test produced blood flow increase in left inferior temporal cortex. These results have implications for our understanding of the neural correlates of perceptual and semantic contributions to recognition memory.

Detection and explanation of sentence ambiguity are unaffected by hippocampal lesions but are impaired by larger temporal lobe lesions

We address the recent suggestion that the "hippocampal system" is important for understanding ambiguities in language (MacKay et al., J Cogn Neurosci 1998;10:377-394). Seven amnesic patients and 11 controls first decided whether a sentence was ambiguous and then tried to explain the ambiguity. Three amnesic patients with damage limited to the hippocampal formation and one amnesic patient with primarily diencephalic damage performed like the controls in all respects. Thus, the ability to comprehend ambiguity is independent of the hippocampal formation. By contrast, three patients with larger temporal lobe lesions, which extended beyond the medial temporal lobe, were impaired to about the same degree as the noted amnesic patient H.M. (as reported by Lackner, Neuropsychologia 1974;12:199-207; MacKay et al., J Cogn Neurosci 1998;10:377-394). Patient H.M., like our 3 impaired patients, has some damage outside the medial temporal lobe. However, patient H.M. also had additional difficulties on these and other language tests that the patients with larger temporal lobe lesions did not exhibit. Accordingly, it is possible that H.M.'s impairment has a different basis.

Profound amnesia after damage to the medial temporal lobe: A neuroanatomical and neuropsychological profile of patient E. P

E. P. became profoundly amnesic in 1992 after viral encephalitis, which damaged his medial temporal lobe bilaterally. Because of the rarity of such patients, we have performed a detailed neuroanatomical analysis of E. P.'s lesion using magnetic resonance imaging, and we have assessed his cognitive abilities with a wide range of neuropsychological tests. Finally, we have compared and contrasted the findings for E. P. with the noted amnesic patient H.M, whose surgical lesion is strikingly similar to E. P.'s lesion.

Is a picture worth a thousand words? Evidence from concept definitions by patients with semantic dementia

Nine patients with semantic dementia (the temporal lobe variant of frontotemporal dementia) were asked to define concrete concepts either from presentation of a picture of the object or from its spoken name. As expected, the patients with the most severe semantic impairment produced the least detailed definitions, and the quality of the definitions overall was significantly related to concept familiarity. Further analyses of the definitions were designed to assess two key theoretical aspects of semantic organization. (i) Do objects and their corresponding names activate conceptual information in two neuroanatomically separable (modality-specific) semantic systems? If so, then-apart from any expected commonality in performance attributable to factors such as concept familiarity-one would not predict striking item-specific similarities in a patient's picture- and word-elicited definitions. (ii) Do sensory/perceptual features and more associative/functional attributes of conceptual knowledge form two neuroanatomically separable subsystems? If so, then one would predict significant dissociations in the prominence of these two types of information in the patients' definitions. The results lead us to favor a model of the semantic system that is divided by attribute type but not by modality.

A functional neuroimaging study of the variables that generate category-specific object processing differences

Brain damage can cause remarkably selective deficits in processing specific categories of objects, indicating the high degree of functional segregation within the brain. The neuroimaging study presented here investigates differences in the neural activity associated with two categories of natural objects (animals and fruit) and two categories of man-made objects (vehicles and tools). Stimuli were outline drawings and the tasks were naming and word-picture matching. For man-made objects, the only category-specific effect was in the left posterior middle temporal cortex, which was most active for drawings of tools, as previously reported. For natural objects, drawings of animals and fruit (relative to drawings of man-made objects) enhanced activity in bilateral anterior temporal and right posterior middle temporal cortices. Critically, these effects with natural objects were not observed when the stimuli were coloured appropriately to facilitate identification. Furthermore, activation in the same right hemisphere areas was also observed for viewing and matching unfamiliar non-objects relative to naming and matching man-made objects. These results indicate that, in the right hemisphere, differences between processing natural relative to man-made objects overlap with the effects of increasing demands on object identification. In the left hemisphere, the effects are more consistent with functional specialization within the semantic system. We discuss (i) how category-specific differences can emerge for multiple reasons and (ii) the implications of these effects on the interpretation of functional imaging data and patients with category-specific deficits.

Episodic memory: new insights from the study of semantic dementia

The study of patients with semantic dementia, the temporal variant of frontotemporal dementia, has provided new insights into the interaction between episodic and semantic memory, and the different roles played by various structures in the temporal lobe. Recent findings indicate that the syndrome of semantic dementia can inform us about the organisation of long-term memory and the relationship between semantic memory and other cognitive systems.