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.

Localization of cardiac-induced signal change in fMRI.

Signal detection in the analysis of blood oxygen level-dependent (BOLD) functional magnetic resonance imaging (fMRI) may be greatly hindered by cardiac pulsatility artifacts. Vessel pulsation, cerebrospinal fluid movement, and tissue deformation are all associated with the cardiac cycle and all can produce fMRI signal variance. Most cognitive fMRI studies do not utilize a method of cardiac-related noise reduction, in part because of the lack of information on the regional significance and magnitude of cardiac-related signal variance in the brain. In this paper we present a topographical description of the regions showing significant contributions of cardiac-related signal variance. The results are highly consistent across subjects and suggest that reduced sensitivity due to cardiac-induced noise in the BOLD signal is systematically greater in specific areas, typically near major blood vessels. Significant effects of cardiac-related variability were found on average in 27.5 +/- 8.0% of voxels. Strong influences were found along the vertebrobasilar arterial system near the medial areas of the brain, along the middle cerebral artery near the anterior temporal lobes and in the insula, and along the anterior cerebral artery in the anterior interhemispheric fissure in the medial frontal lobes. Significant effects were also observed in the sigmoid transverse and superior sagittal sinus regions. These results identify regions in which fMRI will have reduced sensitivity due to increased signal variation produced by cardiac pulsatility.

Dissociation of saccade-related and pursuit-related activation in human frontal eye fields as revealed by fMRI.

The location of the human frontal eye fields (FEFs) underlying horizontal visually guided saccadic and pursuit eye movements was investigated with the use of functional magnetic resonance imaging in five healthy humans. Execution of both saccadic and pursuit eye movements induced bilateral FEF activation located medially at the junction of the precentral sulcus and the superior frontal sulcus and extending laterally to the precentral gyrus. These findings extend previous functional imaging studies by providing the first functional imaging evidence of a specific activation in the FEF during smooth pursuit eye movements in healthy humans. FEF activation during smooth pursuit performance was smaller than during saccades. This finding, which may reflect the presence of a smaller pursuit-related region area in human FEF than the saccade-related region, is consistent with their relative size observed in the monkey. The mean location of the pursuit-related FEF was more inferior and lateral than the location of the saccade-related FEF. These results provide the first evidence that there are different subregions in the human FEF that are involved in the execution of two different types of eye movements, namely saccadic and pursuit eye movements. Moreover, this study provides additional evidence that the human FEF is located in Brodmann's area 6, unlike the monkey FEF which is located in the posterior part of Brodmann's area 8.

A PC data-base with analytical applications for evoked potentials.

Visual Evoked Potentials (VEPs) are gaining ground in the research for diagnosis of neurological disorders and visual defects, as a non-invasive diagnostic tool. Yet, the methods used towards these goals are not universal and far from able to provide a common ground among researchers in collecting, analyzing and comparing their results. This paper is an attempt to close the gap. We have developed a PC data-base and a set of analysis programs with graphic capabilities, frequency analysis, as well as an objective way of describing the signals obtained during VEP experiments.

Screen design and visual evoked potentials.

In this paper we present a method to complement the sophisticated mathematical analyses on screen designs. The use of an objective measure of screen "goodness" is employed, namely, the Visual Evoked Potential (VEP) of the humans performing the test. After the screen is designed, the text on the screen is transformed into an intensity pattern using a recursive algorithm. This intensity pattern is used as a stimulus to obtain wave forms from the scalp of the subject, and the wave forms are analyzed as to their frequency content. A 2-D Fourier transform of the screen design is performed and the frequency components of the power spectrum are compared to the frequency components of the wave form. Our results indicate that when a "balanced" and well-designed screen is used as a stimulus, the second harmonic of the VEP wave form is always smaller than the first harmonic. The implications of such an analysis are discussed.

Collection and storage of data for evoked potential studies: a database.

A detailed description of a set of programs is presented which is designed for use with subjects tested in a research laboratory primarily with visual evoked potentials (VEPs). It has been written in a way which accommodates other modalities such as brain-stem auditory evoked potentials (BAEPs) and somatosensory evoked potentials (SEPs). It can be used in a clinic or a hospital for collecting patient information, data recordings, and analysis. The programs are implemented on an LSI-11/23 microcomputer and allow for initial subject data entry, entry of data acquired at subsequent visits, data analysis, lists of patient files and data sorts and tabulations. In their present form they are intended to be run by a staff who have little experience with computers and technical skills.