Comparison of Randomized Multifocal Mapping and Temporal Phase Mapping of Visual Cortex for Clinical Use

fMRI is becoming an important clinical tool for planning and guidance of surgery to treat brain tumors, arteriovenous malformations, and epileptic foci. For visual cortex mapping, the most popular paradigm by far is temporal phase mapping, although random multifocal stimulation paradigms have drawn increased attention due to their ability to identify complex response fields and their random properties. In this study we directly compared temporal phase and multifocal vision mapping paradigms with respect to clinically relevant factors including: time efficiency, mapping completeness, and the effects of noise. Randomized, multifocal mapping accurately decomposed the response of single voxels to multiple stimulus locations and made correct retinotopic assignments as noise levels increased despite decreasing sensitivity. Also, multifocal mapping became less efficient as the number of stimulus segments (locations) increased from 13 to 25 to 49 and when duty cycle was increased from 25% to 50%. Phase mapping, on the other hand, activated more extrastriate visual areas, was more time efficient in achieving statistically significant responses, and had better sensitivity as noise increased, though with an increase in systematic retinotopic mis-assignments. Overall, temporal phase mapping is likely to be a better choice for routine clinical applications though random multifocal mapping may offer some unique advantages for selected applications

Are Neuronal Mechanisms of Attentional Modulation Universal Across Human Sensory and Motor Brain Maps?

One\u27s experience of shifting attention from the color to the smell to the act of picking a flower seems like a unitary process applied, at will, to one modality after another. Yet, the unique experience of sight vs smell vs movement might suggest that the neural mechanisms of attention have been selectively optimized to employ each modality to greatest advantage. Relevant experimental data can be difficult to compare across modalities due to design and methodological heterogeneity. Here we outline some of the issues related to this problem and suggest how experimental data can be obtained across modalities using more uniform methods and measurements. The ultimate goal is to spur efforts across disciplines to provide a large and varied database of empirical observations that will either support the notion of a universal neural substrate for attention or more clearly identify to what degree attentional mechanisms are specialized for each modality

The attentional field revealed by single-voxel modeling of fMRI time courses

The spatial topography of visual attention is a distinguishing and critical feature of many theoretical models of visuospatial attention. Previous fMRI-based measurements of the topography of attention have typically been too crude to adequately test the predictions of different competing models. This study demonstrates a new technique to make detailed measurements of the topography of visuospatial attention from single-voxel, fMRI time courses. Briefly, this technique involves first estimating a voxel's population receptive field (pRF) and then “drifting” attention through the pRF such that the modulation of the voxel's fMRI time course reflects the spatial topography of attention. The topography of the attentional field (AF) is then estimated using a time-course modeling procedure. Notably, we are able to make these measurements in many visual areas including smaller, higher order areas, thus enabling a more comprehensive comparison of attentional mechanisms throughout the full hierarchy of human visual cortex. Using this technique, we show that the AF scales with eccentricity and varies across visual areas. We also show that voxels in multiple visual areas exhibit suppressive attentional effects that are well modeled by an AF having an enhancing Gaussian center with a suppressive surround. These findings provide extensive, quantitative neurophysiological data for use in modeling the psychological effects of visuospatial attention

Effects of Thresholding on Voxel-Wise Correspondence of Breath-Hold and Resting-State Maps of Cerebrovascular Reactivity

Functional magnetic resonance imaging for presurgical brain mapping enables neurosurgeons to identify viable tissue near a site of operable pathology which might be at risk of surgery-induced damage. However, focal brain pathology (e.g., tumors) may selectively disrupt neurovascular coupling while leaving the underlying neurons functionally intact. Such neurovascular uncoupling can result in false negatives on brain activation maps thereby compromising their use for surgical planning. One way to detect potential neurovascular uncoupling is to map cerebrovascular reactivity using either an active breath-hold challenge or a passive resting-state scan. The equivalence of these two methods has yet to be fully established, especially at a voxel level of resolution. To quantitatively compare breath-hold and resting-state maps of cerebrovascular reactivity, we first identified threshold settings that optimized coverage of gray matter while minimizing false responses in white matter. When so optimized, the resting-state metric had moderately better gray matter coverage and specificity. We then assessed the spatial correspondence between the two metrics within cortical gray matter, again, across a wide range of thresholds. Optimal spatial correspondence was strongly dependent on threshold settings which if improperly set tended to produce statistically biased maps. When optimized, the two CVR maps did have moderately good correspondence with each other (mean accuracy of 73.6%). Our results show that while the breath-hold and resting-state maps may appear qualitatively similar they are not quantitatively identical at a voxel level of resolution

Functional Magnetic Resonance Imaging: Applications

Functional magnetic resonance imaging (FMRI) produces dynamic images of the brain that reflect local differences in neuronal activity. Although there is still much to be learned about the biological processes underlying FMRI, the technology is sufficiently advanced to permit its use in exploratory studies of human brain function. A fruitful focus for this research lies in the study of the visual system. Previous work using other methods has generated a vast body of information about vision that can be used to make predictions about the results of FMRI experiments. At the same time, much of this knowledge derives from animal work and needs to be tested and extended with human studies. The following presents a review of some oft he recent applications of FMRI with an emphasis on studies of human visual cortex

Functional Magnetic Resonance Imaging: A Prime

Functional magnetic resonance imaging (FMRI) is a new technology for producing images of the brain that reflect local differences in neuronal activity. As a consequence of this and other brain scan technologies, a new discipline has emerged for the study of cerebral function and pathology. The FMRI technique is potentially revolutionary because it can produce high resolution pictures and movies of dynamic brain activation during sensory input, motor performance, or cognitive activity. Since it is non-invasive and has no known health risks, hundreds of images can be obtained from a single subject, thus permitting detailed and thorough studies of brain function in humans. Moreover, by performing similar FMRI studies in both humans and animals, knowledge obtained from subhuman species eventually may be applied more directly to studies of normal and pathological brain conditions in humans. Clinically, the FMRI technology may prove to be one of the more accessible of the new scanning techniques since many hospital include MR imaging facilities that could be modified to permit functional imaging. To acquaint the non-specialist with this technology, we have presented here and in ""Functional Magnetic Resonance Imaging: Applications"" an introduction to FMRI techniques and their application to vision research

A physiological correlate of the 'spotlight' of visual attention

nature neuroscience • volume 2 no 4 • april 1999 articles Helmholtz 1 and William James 2 each noted the potential dissociation between the point of gaze fixation and the focus of attention within the field of view. More recently, this phenomenon has been compared to a 'spotlight' that evokes attentional enhancement of visual information within a circumscribed region of visual space or within the confines of a target object RESULTS To study visuospatial attention, we used a task in which the subject's gaze remained fixated on a central marker while spatial attention was directed to a cued location (target segment) within an array of segments greater eccentricities The region of attentional modulation extended throughout medial occipital cortex as well as ventrally into, and surrounding, the collateral sulcus. Based on previous retinotopic mapping 24 , this swath traversed portions of V1 and proposed extrastriate visual areas V2, V3, VP, V4v and sometimes cortex anterior to V4v. Attentional modulation was also seen deep within the left calcarine fissure To determine conclusively if the pattern of attentional enhancement followed the cortical retinotopy of single cued segments, we repeated the experiment but with only one cued segment present at a tim

Estimation of FMRI response delays

Abstract We present an efficient algorithm using the Hilbert Transform for estimating the delay of the BOLD response to neuronal stimulation. With minimal additional computations, the algorithm estimates parameters generated in the widely used cross-correlation method and simplifies the interpolation required to estimate the response delay from the cross-correlation function. We examined errors in the Hilbert-based delay estimate associated with the use of DFT on short-duration discrete signals and proposed a method for minimizing these errors. Furthermore, we compared the delay estimates obtained with the Hilbert method to those obtained using the onset of the BOLD response. The Hilbert method resulted in less variance in the delay estimate despite the potential for higher variability in the latter part of the BOLD response. This improved delay estimate was attributed to the reduced sensitivity of the Hilbert method to noise contamination compared to the onset method