Functional magnetic resonance imaging of human visual cortex during face matching: a comparison with positron emission tomography

Temporal dynamics of early perceptual processing

Recordings of electrical and magnetic brain responses to sensory stimulation provide high-resolution measures of the time course of early perceptual processing. Spatio-temporal analyses of brain activity patterns during the first 200 ms after stimulus presentation have characterized the timing of attentional selection processes and different stages of feature encoding and pattern analyses. Recent studies that incorporate blood flow neuroimaging techniques provide support for mechanisms of early selection of attended visual inputs in extrastriate cortical pathways. The spatial tuning properties of early auditory selection have also been delineated. Electrical and magnetic responses that index the encoding of higher-order pattern information have been identified in both visual and auditory modalities and localized to specific cortical areas.

A neural system for human visual working memory

Working memory is the process of actively maintaining a representation of information for a brief period of time so that it is available for use. In monkeys, visual working memory involves the concerted activity of a distributed neural system, including posterior areas in visual cortex and anterior areas in prefrontal cortex. Within visual cortex, ventral stream areas are selectively involved in object vision, whereas dorsal stream areas are selectively involved in spatial vision. This domain specificity appears to extend forward into prefrontal cortex, with ventrolateral areas involved mainly in working memory for objects and dorsolateral areas involved mainly in working memory for spatial locations. The organization of this distributed neural system for working memory in monkeys appears to be conserved in humans, though some differences between the two species exist. In humans, as compared with monkeys, areas specialized for object vision in the ventral stream have a more inferior location in temporal cortex, whereas areas specialized for spatial vision in the dorsal stream have a more superior location in parietal cortex. Displacement of both sets of visual areas away from the posterior perisylvian cortex may be related to the emergence of language over the course of brain evolution. Whereas areas specialized for object working memory in humans and monkeys are similarly located in ventrolateral prefrontal cortex, those specialized for spatial working memory occupy a more superior and posterior location within dorsal prefrontal cortex in humans than in monkeys. As in posterior cortex, this displacement in frontal cortex also may be related to the emergence of new areas to serve distinctively human cognitive abilities.

Functional MRI studies of word-stem completion: reliability across laboratories and comparison to blood flow imaging with PET

Functional magnetic resonance imaging (fMRI) based on blood oxygen level-dependent (BOLD) contrast has become an increasingly popular technique for mapping the brain. The relationship between BOLD-fMRI imaging and imaging of blood flow activation with positron emission tomography (PET) remains unclear. Moreover, BOLD imaging strategies and analysis procedures vary widely across laboratories. To examine the relationship between these different methods, we compared brain activation maps of a word-stem completion task obtained both using PET and using fMRI across two separate institutions (Washington University and Massachusetts General Hospital) with different acquisitions (gradient-refocused echo and asymmetric spin echo) and different analysis techniques. Overall, activation maps were highly similar across both fMRI methods and PET. A set of activated brain areas, in consistent locations in Talairach atlas space, were identified across all three studies, including visual striate and extrastriate, left prefrontal, supplementary motor area (SMA), and right cerebellar areas. Decreases in activation were also consistently observed in medial parietal, posterior insular, and medial inferior frontal areas. Some differences were noted that may be related to the silent performance of the task with fMRI. The largely consistent results suggest that comparisons can be made appropriately across imaging modalities and laboratory methods. A further implication of the consistencies, which extended to both increases and decreases in signal, is that the underlying brain physiology leading to BOLD contrast may be more similar to blood flow than originally appreciated.

How the brain learns to see objects and faces in an impoverished context

A degraded image of an object or face, which appears meaningless when seen for the first time, is easily recognizable after viewing an undegraded version of the same image. The neural mechanisms by which this form of rapid perceptual learning facilitates perception are not well understood. Psychological theory suggests the involvement of systems for processing stimulus attributes, spatial attention and feature binding, as well as those involved in visual imagery. Here we investigate where and how this rapid perceptual learning is expressed in the human brain by using functional neuroimaging to measure brain activity during exposure to degraded images before and after exposure to the corresponding undegraded versions. Perceptual learning of faces or objects enhanced the activity of inferior temporal regions known to be involved in face and object recognition respectively. In addition, both face and object learning led to increased activity in medial and lateral parietal regions that have been implicated in attention and visual imagery. We observed a strong coupling between the temporal face area and the medial parietal cortex when, and only when, faces were perceived. This suggests that perceptual learning involves direct interactions between areas involved in face recognition and those involved in spatial attention, feature binding and memory recall.

In vivo studies on striatal dopamine D1 and D2 site binding in L-dopa-treated Parkinson's disease patients with and without dyskinesias

Dyskinesias are usually seen in Parkinson's disease (PD) patients after several years of L-dopa therapy. Their presence has been attributed to supersensitivity of striatal D1 and D2 receptors. We have used PET to assess striatal D2 receptor binding in untreated PD patients and striatal D1 and D2 binding in L-dopa-treated PD patients. Untreated patients showed a 14% increase in mean D2 receptor binding in the putamen contralateral to the more affected limbs (p < 0.02). Treated patients were segregated into subgroups according to the presence or absence of dyskinesias. There were no differences in mean caudate and putamen D1 and D2 binding between dyskinetic and nondyskinetic patients, matched for duration of clinical disease. Both dyskinetic and nondyskinetic PD subgroups showed a similar 16% reduction of mean caudate D2 binding (p < 0.01) with normal D2 binding in putamen. Mean caudate and putamen D1 binding potentials of both subgroups were reduced by 10% compared with those of controls, though this trend did not reach significance. Putamen D1 binding, however, showed a negative correlation with duration and L-dopa treatment (p < 0.03). These findings suggest that, while exposure of PD patients to L-dopa may be associated with reductions in caudate D2 and caudate and putamen D1 receptor, dyskinesias are unlikely to result from alterations in striatal dopamine receptor binding.

Brain mechanisms associated with top-down processes in perception

Perception arises through an interaction between sensory input and prior knowledge. We propose that at least two brain areas are required for such an interaction: the 'site' where analysis of afferent signals occurs and the 'source' which applies the relevant prior knowledge. In the human brain, functional imaging studies have demonstrated that selective attention modifies activity in early visual processing areas specific to the attended feature. Early processing areas are also modified when prior knowledge permits a percept to emerge from an otherwise meaningless stimulus. Sources of this modification have been identified in parietal cortex and in prefrontal cortex. Modification of early processing areas also occurs on the basis of prior knowledge about the predicted sensory effects of the subject's own actions. Activity associated with mental imagery resembles that associated with response preparation (for motor imagery) and selective attention (for sensory imagery) suggesting that mental imagery reflects the effects of prior knowledge on sensory processing areas in the absence of sensory input. Damage to sensory processing areas can lead to a form of sensory hallucination which seems to arise from the interaction of prior knowledge with random sensory activity. In contrast, hallucinations associated with schizophrenia may arise from a failure of prior knowledge about motor intentions to modify activity in relevant sensory areas. When functioning normally, this mechanism permits us to distinguish our own actions from those of independent agents in the outside world. Failure to make this distinction correctly may account for the strong association between hallucinations and paranoid delusions in schizophrenia; the patient not only hears voices, but attributes (usually hostile) intentions to these voices.

What fMRI has taught us about human vision

The recent application of functional magnetic resonance imaging (fMRI) to visual studies has begun to elucidate how the human visual system is anatomically and functionally organized. Bottom-up hierarchical processing among visual cortical areas has been revealed in experiments that have correlated brain activations with human perceptual experience. Top-down modulation of activity within visual cortical areas has been demonstrated through studies of higher cognitive processes such as attention and memory.

The fusiform face area: a module in human extrastriate cortex specialized for face perception

Using functional magnetic resonance imaging (fMRI), we found an area in the fusiform gyrus in 12 of the 15 subjects tested that was significantly more active when the subjects viewed faces than when they viewed assorted common objects. This face activation was used to define a specific region of interest individually for each subject, within which several new tests of face specificity were run. In each of five subjects tested, the predefined candidate "face area" also responded significantly more strongly to passive viewing of (1) intact than scrambled two-tone faces, (2) full front-view face photos than front-view photos of houses, and (in a different set of five subjects) (3) three-quarter-view face photos (with hair concealed) than photos of human hands; it also responded more strongly during (4) a consecutive matching task performed on three-quarter-view faces versus hands. Our technique of running multiple tests applied to the same region defined functionally within individual subjects provides a solution to two common problems in functional imaging: (1) the requirement to correct for multiple statistical comparisons and (2) the inevitable ambiguity in the interpretation of any study in which only two or three conditions are compared. Our data allow us to reject alternative accounts of the function of the fusiform face area (area "FF") that appeal to visual attention, subordinate-level classification, or general processing of any animate or human forms, demonstrating that this region is selectively involved in the perception of faces.