Functional imaging of human crossmodal identification and object recognition.

The perception of objects is a cognitive function of prime importance. In everyday life, object perception benefits from the coordinated interplay of vision, audition, and touch. The different sensory modalities provide both complementary and redundant information about objects, which may improve recognition speed and accuracy in many circumstances. We review crossmodal studies of object recognition in humans that mainly employed functional magnetic resonance imaging (fMRI). These studies show that visual, tactile, and auditory information about objects can activate cortical association areas that were once believed to be modality-specific. Processing converges either in multisensory zones or via direct crossmodal interaction of modality-specific cortices without relay through multisensory regions. We integrate these findings with existing theories about semantic processing and propose a general mechanism for crossmodal object recognition: The recruitment and location of multisensory convergence zones varies depending on the information content and the dominant modality.

A parametric fMRI study of overt and covert shifts of visuospatial attention.

It has recently been demonstrated that a cortical network of visuospatial and oculomotor control areas is active for covert shifts of spatial attention (shifts of attention without eye movements) as well as for overt shifts of spatial attention (shifts of attention with saccadic eye movements). Studies examining activity in this visuospatial network during attentional shifts at a single rate have given conflicting reports about how the activity differs for overt and covert shifts. To better understand how the network subserves attentional shifts, we performed a parametric study in which subjects made either overt attentional shifts or covert attentional shifts at three different rates (0.2, 1.0, and 2.0 Hz). At every shift rate, both overt and covert shifts of visuospatial attention induced activations in the precentral sulcus, intraparietal sulcus, and lateral occipital cortex that were of greater amplitude for overt than during covert shifting. As the rate of attentional shifts increased, responses in the visuospatial network increased in both overt and covert conditions but this parametric increase was greater during overt shifts. These results confirm that overt and covert attentional shifts are subserved by the same network of areas. Overt shifts of attention elicit more neural activity than do covert shifts, reflecting additional activity associated with saccade execution. An additional finding concerns the anatomical organization of the visuospatial network. Two distinct activation foci were observed within the precentral sulcus for both overt and covert attentional shifts, corresponding to specific anatomical landmarks. We therefore reappraise the correspondence of these two precentral areas with the frontal eye fields.

A comparison of visual and auditory motion processing in human cerebral cortex.

Visual and auditory motion information can be used together to provide complementary information about the movement of objects. To investigate the neural substrates of such cross-modal integration, functional magnetic resonance imaging was used to assess brain activation while subjects performed separate visual and auditory motion discrimination tasks. Areas of unimodal activation included the primary and/or early sensory cortex for each modality plus additional sites extending toward parietal cortex. Areas conjointly activated by both tasks included lateral parietal cortex, lateral frontal cortex, anterior midline and anterior insular cortex. The parietal site encompassed distinct, but partially overlapping, zones of activation in or near the intraparietal sulcus (IPS). A subsequent task requiring an explicit cross-modal speed comparison revealed several foci of enhanced activity relative to the unimodal tasks. These included the IPS, anterior midline, and anterior insula but not frontal cortex. During the unimodal auditory motion task, portions of the dorsal visual motion system showed signals depressed below resting baseline. Thus, interactions between the two systems involved either enhancement or suppression depending on the stimuli present and the nature of the perceptual task. Together, these results identify human cortical regions involved in polysensory integration and the attentional selection of cross-modal motion information.

A functional MRI case study of acquired cerebral dyschromatopsia.

Evidence from imaging studies suggests that primary visual cortex and multiple areas in ventral occipitotemporal cortex subserve color perception in humans. To learn more about the organization of these areas, we used structural and functional MRI (fMRI) to examine a patient with damage to ventral cortex. An art professor, KG, suffered a cerebrovascular accident during heart surgery that impaired his ability to perceive color. The Farnsworth-Munsell 100-Hue test was used to assess the extent of his deficit. When tested 12 months after the lesion, KG performed worse than 95% of age-matched normals on the 100-Hue test, but well above chance. Structural and functional MRI studies were conducted 3 years after the lesion to investigate the neuroanatomical correlates of KG'ss remaining color ability. Structural MRI revealed bilateral damage to ventral occipitotemporal cortex. In young and age-matched normal controls, an fMRI version of the 100-Hue reliably activated bilateral, color-selective regions in primary visual cortex and anterior and posterior ventral cortex. In subject KG, color-selective cortex was found in bilateral primary visual cortex. In ventral cortex, no color-selective activity was observed in right ventral cortex, and only a small area of activity was observed in left anterior ventral cortex. However, significant color-selective activity was observed in posterior left ventral cortex spared by the lesion. This posterior left ventral activation was similar in extent, position, and degree of color-selectivity to the posterior left posterior activation observed in normal controls, suggesting that this focus may be the cortical substrate underlying KG's remaining color perception.

An fMRI version of the Farnsworth-Munsell 100-Hue test reveals multiple color-selective areas in human ventral occipitotemporal cortex.

Studies of patients with cerebral achromatopsia have suggested that ventral occipitotemporal cortex is important for color perception. We created a functional magnetic resonance imaging (fMRI) version of a clinical test commonly used to assess achromatopsia, the Farnsworth-Munsell 100-Hue test. The test required normal subjects to use color information in the visual stimulus to perform a color sequencing task. A modification of the test requiring ordering by luminance was used as a control task. Subjects were also imaged as they passively viewed colored stimuli. A limited number of areas responded more to chromatic than achromatic stimulation, including primary visual cortex. Most color-selective activity was concentrated in ventral occipitotemporal cortex. Several areas in ventral cortex were identified. The most posterior, located in posterior fusiform gyrus, corresponded to the area activated by passive viewing of colored stimuli. More anterior and medial color-selective areas were located in the collateral sulcus and fusiform gyrus. These more anterior areas were not identified in previous imaging studies which used passive viewing of colored stimuli, and were most active in our study when visual color information was behaviorally relevant, suggesting that attention influences activity in color-selective areas. The fMRI version of the Farnsworth-Munsell test may be useful in the study of achromatopsia.

Graded effects of spatial and featural attention on human area MT and associated motion processing areas.

Functional magnetic resonance imaging was used to quantify the effects of changes in spatial and featural attention on brain activity in the middle temporal visual area and associated motion processing regions (hMT+) of normal human subjects. When subjects performed a discrimination task that directed their spatial attention to a peripherally presented annulus and their featural attention to the speed of points in the annulus, activity in hMT+ was maximal. If subjects were instead asked to discriminate the color of points in the annulus, the magnitude and volume of activation in hMT+ fell to 64 and 35%, respectively, of the previously observed maximum response. In another experiment, subjects were asked to direct their spatial attention away from the annulus toward the fixation point to detect a subtle change in luminance. The response magnitude and volume dropped to 40 and 9% of maximum. These experiments demonstrate that both spatial and featural attention modulate hMT+ and that their effects can work in concert to modulate cortical activity. The high degree of modulation by attention suggests that an understanding of the stimulus-driven properties of visual cortex needs to be complemented with an investigation of the effects of task-related factors on visual processing.