Tonotopy in human auditory cortex examined with functional magnetic resonance imaging.

Tonotopic organization within the human auditory cortex was investigated with functional magnetic resonance imaging (fMRI) using the blood oxygenation level-dependent (BOLD) contrast mechanism. Single-frequency pulsed tones were alternated with no-tone conditions to elicit stimulus-specific functional activity. Differential frequency-specific activity was imaged within the auditory cortex Activations for high-frequency tones were located more posteriorly and medially than those for low-frequency tones. Such a pattern is consistent with descriptions of tonotopic organization suggested by other nonneuroimaging methodologies used with human and nonhuman primates. Furthermore, these results demonstrate that fMRI can be used to reliably investigate functional organization of the human auditory cortex. Hum. Brain Mapping 4:18-25, 1997. (c) 1997 Wiley-Liss, Inc.

Covariations in ERP and PET measures of spatial selective attention in human extrastriate visual cortex.

In a previous study using positron emission tomography (PET), we demonstrated that focused attention to a location in the visual field produced increased regional cerebral blood flow in the fusiform gyrus contralateral to the attended hemifield (Heinze et al. [1994]: Nature 372:543). We related these effects to modulations in the amplitude of the P1 component (80-130 msec latency) of the visual event-related brain potentials (ERPs) recorded from the same subjects, under the identical stimulus and task conditions. Here, we replicate and extend these findings by showing that attention effects in the fusiform gyrus and the P1 component were similarly modulated by the perceptual load of the task. When subjects performed a perceptually demanding symbol-matching task within the focus of spatial attention, the fusiform activity and P1 component of the ERP were of greater magnitude than when the subjects performed a less perceptually demanding task that required only luminance detection at the attended location. In the latter condition, both the PET and ERP attention effects were reduced. In addition, in the present data significant activations were also obtained in the middle occipital gyrus contralateral to the attended hemifield, thereby demonstrating that multiple regions of extrastriate visual cortex are modulated by spatial attention. The findings of covariations between the P1 attention effect and activity in the posterior fusiform gyrus reinforce our hypothesis that common neural sources exist for these complementary, but very different measures of human brain activity.

Developmental dyslexia: passive visual stimulation provides no evidence for a magnocellular processing defect.

Livingstone et al. [Livingstone, M. S., Rosen, G. D., Drislane, F. W. and Galaburda, A. M. Physiological and anatomical evidence for a magnocellular defect in developmental dyslexia. Proceedings of the National Academy of Science U.S.A. 88, 7943-7947, 1991] presented evidence for a defect of the magnocellular visual processing stream in developmental dyslexia. We attempted to replicate this effect using transient and steady-state VEPs to checkerboard reversal stimuli in a group of adult developmental dyslexics. Several different reversal rates and contract levels were utilized. No differences were found between the dyslexic and control groups for the low-contrast, rapidly reversing patterns, nor for any combination of stimulus rate or contrast that was tested. Thus, these findings do not support a magnocellular processing deficit in developmental dyslexia.

Estimation of interpolation errors in scalp topographic mapping.

Topographic maps are commonly constructed from electrical scalp recordings (such as EEGs and ERPs) using several different interpolation methods. It is important to determine the accuracy of such maps. Previous assessments of interpolation methods have been based on global error measures and the visual appearance of the topographic maps. However, the relationship of interpolation error to local contributing factors requires a more detailed analysis. In this paper, we use simulations to explore and quantify the relationship of error to global and local factors for different interpolation methods. We find that among the best interpolation methods, adequate electrode density is more important than the method used. For shallow sources, we show that local interpolation error is most correlated with potential gradient, and has a lesser correlation with distance to nearest electrode. The greatest correlation, however, is with the product of gradient and distance. Thus, interpolation error can be controlled locally by making the interelectrode distance inversely proportional to the expected potential gradient. With shallow sources, areas far from any electrode and having high apparent gradient are likely to have high interpolation error. Moreover, all areas far from any electrode may contain high interpolation errors, and should be interpreted with caution.

Brain potentials in developmental dyslexia: differential effects of word frequency in human subjects.

The differences of word processing between a group of adult developmental dyslexics and control subjects were examined with the event-related potential (ERP) technique. In particular, the effects of word frequency and word recognition were assessed. The subjects viewed a series of frequently and infrequently used words, most of which were repeated after some intervening items and they discriminated between first and second presentations of the words. It can be shown that in the range from 300 to 550 ms post stimulus the amplitude of the N400 component, an ERP measure of semantic processing, is reduced for high frequency words. This effect is more pronounced in the dyslexic group and the effects of word recognition are also reduced in the dyslexic group for high frequency words. These findings are discussed with respect to current concepts of dyslexia and of semantic processing.