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Functional magnetic resonance imaging (fMRI) has become a popular tool for investigations into the neural correlates of cognitive activity. One limitation of fMRI, however, is that it has difficulty imaging regions near tissue interfaces due to distortions from macroscopic susceptibility effects which become more severe at higher magnetic field strengths. This difficulty can be particularly problematic for language tasks that engage regions of the temporal lobes near the air-filled sinuses. This paper investigates susceptibility-induced signal loss in the temporal lobes and proposes that by defining a priori regions of interest and using the small-volume statistical correction of K. J. Worsley, S. Marrett, P. Neelin, A. C. Vandal, K. J. Friston, and A. C. Evans (1996, Hum. Brain Mapp. 4: 58 -83), activations in these areas can sometimes be detected by increasing the statistical power of the analysis. We conducted two experiments, one with PET and the other with fMRI, using almost identical semantic categorization paradigms and comparable methods of analysis. There were areas of overlap as well as differences between the PET and fMRI results. One anticipated difference was a lack of activation in two regions in the temporal lobe on initial analyses in the fMRI data set. With a specific region of interest, however, activation in one of the regions was detected. These experiments demonstrate three points: first, even for almost identical cognitive tasks such as those in this study, PET and fMRI may not produce identical results; second, differences between the two methods due to macroscopic susceptibility artifacts in fMRI can be overcome with appropriate statistical corrections, but only partially; and third, new data acquisition paradigms are necessary to fully deal with susceptibility-induced signal loss if the sensitivity of the fMRI experiment to temporal lobe activations is to be enhanced.

We present a new account of the fine-grained structure of semantic categories derived from neuropsychological, behavioral, and developmental data. The account places theoretical emphasis on the functions of the referents of concepts. We claim (i) that the distinctiveness of functional features correlated with perceptual features varies across semantic domains; and (ii) that category structure emerges from the complex interaction of these variables. The representational assumptions that follow from these claims make strong predictions about what types of semantic information are preserved in patients showing category-specific deficits following brain damage. These claims are illustrated with a connectionist simulation which, when damaged, shows patterns of preservation of distinctive and shared functional and perceptual information which varies across semantic domains. The data model both dissociations between knowledge for artifacts and for living things and recent neuropsychological evidence concerning the robustness of functional information in the representation of concepts.

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Knowledge of objects in the world is stored in our brains as rich, multimodal representations. Because the neural pathways that process this diverse sensory information are largely anatomically distinct, a fundamental challenge to cognitive neuroscience is to explain how the brain binds the different sensory features that comprise an object to form meaningful, multimodal object representations. Studies with nonhuman primates suggest that a structure at the culmination of the object recognition system (the perirhinal cortex) performs this critical function. In contrast, human neuroimaging studies implicate the posterior superior temporal sulcus (pSTS). The results of the functional MRI study reported here resolve this apparent discrepancy by demonstrating that both pSTS and the perirhinal cortex contribute to crossmodal binding in humans, but in different ways. Significantly, only perirhinal cortex activity is modulated by meaning variables (e.g., semantic congruency and semantic category), suggesting that these two regions play complementary functional roles, with pSTS acting as a presemantic, heteromodal region for crossmodal perceptual features, and perirhinal cortex integrating these features into higher-level conceptual representations. This interpretation is supported by the results of our behavioral study: Patients with lesions, including the perirhinal cortex, but not patients with damage restricted to frontal cortex, were impaired on the same crossmodal integration task, and their performance was significantly influenced by the same semantic factors, mirroring the functional MRI findings. These results integrate nonhuman and human primate research by providing converging evidence that human perirhinal cortex is also critically involved in processing meaningful aspects of multimodal object representations.conceptual knowledge ͉ hierarchical object processing ͉ ventral stream A major outstanding question in the cognitive neurosciences is how different unimodal object features are integrated into coherent, multimodal object representations. Hierarchical models of object processing based on studies with nonhuman primates suggest that the perirhinal cortex, located at the culmination of the ventral occipitotemporal object-processing stream, performs this critical function. Within this stream, increasingly more complex combinations of visual object features are processed from posterior to anterior ventral temporal lobe sites (1-3), with perirhinal cortex of the anteromedial temporal lobe integrating the most complex combinations of features required for fine-grained visual discriminations between objects (4, 5). Recent functional MRI (fMRI) and lesion studies generally support this model in the human system. Lerner et al. (6) demonstrated that the sensitivity of ventral occipitotemporal regions to the scrambling of car images increased significantly from posterior (V1, V2, V3, V4͞V8) to more anteriorly situated sites (lateral occipital sulcus and posterior fusiform gyrus; lateral occipital complex), with scr...

Abstract& How objects are represented and processed in the brain is a central topic in cognitive neuroscience. Previous studies have shown that knowledge of objects is represented in a featurebased distributed neural system primarily involving occipital and temporal cortical regions. Research with nonhuman primates suggest that these features are structured in a hierarchical system with posterior neurons in the inferior temporal cortex representing simple features and anterior neurons in the perirhinal cortex representing complex

Patients with category-specific deficits have motivated a range of hypotheses about the structure of the conceptual system. One class of models claims that apparent category dissociations emerge from the internal structure of concepts rather than fractionation of the system into separate substores. This account claims that distinctive properties of concepts in the living domain are vulnerable because of their weak correlation with other features. Given the assumption that mutual activation among correlated properties produces faster activation in the normal system, the authors predicted a disadvantage for the distinctive features of living things for unimpaired adults. Results of a speeded feature verification study supported this prediction, as did a computational simulation in which networks mapped from orthography to semantics.

It has been widely argued that the left inferior frontal gyrus (LIFG) is involved in the control of retrieval of information from long-term memory. Recent claims that the LIFG is involved in selecting among semantic alternatives have been challenged on the grounds that the manipulation of selection demands may have been confounded with controlled retrieval. The current study used an event-related functional magnetic resonance paradigm to re-examine the possibility that LIFG activation is involved in selection processes. In order to minimize potential confounding effects of controlled retrieval, we used an automatic retrieval task (picture naming) and held retrieval demands constant, while varying selection demands by way of competitor priming from earlier semantically related trials. We found significant activation in LIFG as a function of increased selection demands; activation centred on two peaks, one in anterior LIFG and a second more superior and posterior region. These data support the view that LIFG plays a role in selection among semantic information, even in the absence of controlled retrieval processes.