Models of the human brain and the surrounding media: their influence on the reliability of source localization.

This article is a review of the evolution of models of the head as used in dipole source localization. Models fall into two classes: those that can be expressed in simple analytic form, such as the homogeneous sphere or spherical three-shell models, or those that can only be solved by numerical methods, such as the finite element approach. The latter models always involve heavy procedural and computational burdens. The trend over the last decade has been to use these more advanced models to estimate the error that would be incurred if one of the simpler spherical models were used instead for dipole source localization. An estimate is presented of the magnitudes of the random and systematic errors of localization that may be expected when using these methods.

Dependence of Panum's fusional area on local retinal stimulation.

The range of fusion in the human binocular vision system has been shown by many research groups to be capable of extension. Although each of these research groups tested the extension of fusion under different conditions, the majority agreed that an extension of fusion is possible, and most have noted a variety of variables that contribute to the phenomenon. The extension of fusion may be interpreted in at least two ways. The first is that the range over which fusion is possible is enlarged, and the second is that the range is shifted. Therefore a study of human binocular fusional ranges was performed to clarify the nature of the extension of Panum's fusional area. The following results were obtained: (1) A shift of the fusional area occurs when one of a pair of fused retinally stabilized images is moved into disparate locations on the retinas. Results for one observer demonstrate a corresponding small enlargement of the fusional area. Thus the extension of the fusional area not only involves the recruitment of retinal locations into the fusional area that are not normally in the area but also involves the loss from the fusional area of certain retinal locations that are normally within it. (2) A continually present stimulus fuses over a wider range with a flashed stimulus than with another continually present stimulus.

Hysteresis in human binocular fusion: temporalward and nasalward ranges.

Fender and Julesz [J. Opt. Soc. Am. 57, 819 (1967)] moved pairs of retinally stabilized images across the temporalward visual fields and found significant differences between the disparities that elicited fusion and the disparities at which fusion was lost. They recognized this phenomenon as an example of hysteresis. In the work reported in this paper, binocular retinally stabilized images of vertical dark bars on white backgrounds were moved into horizontal disparity in both the nasalward and the temporalward directions. The limits of Panum's fusional area and the hysteresis demonstrated by these limits were measured for two observers. The following results were obtained: (1) the nasalward limits of Panum's fusional area and the hysteresis demonstrated by the nasalward limits do not differ significantly from the temporalward limits and the hysteresis demonstrated by the temporalward limits; (2) the limits of Panum's fusional area and the hysteresis demonstrated by these limits are not significantly different if one stimulus moves across each retina or if one stimulus is held still on one retina and the other stimulus is moved across the other retina; (3) the use of nonstabilized cross hairs for fixation decreases the hysteresis; and (4) the full hysteresis effect can be elicited with a rate of change of disparity of 2 arcmin/sec.

Eye movements and neural remapping during fusion of misaligned random-dot stereograms.

Fender and Julesz [J. Opt. Soc. Am. 57, 819 (1967)] found that fused retinally stabilized binocular line targets could be misaligned on the two retinas in the temporalward direction by at least 30 min of arc without loss of fusion and stereopsis and that random-dot stereograms could be misaligned 2 deg before fusion was lost. To test these results in normal vision, we recorded eye motions of four observers while they viewed a random-dot stereogram that subtended about 10 deg. The observers misaligned overlaid vectograph stereo images by moving them apart in a temporalward direction until fusion was lost. They then returned the vectographs to the overlaid position. Throughout this cycle the observers reported at frequent intervals if they could perceive strong or weak depth, loss of depth, or loss of fusion. For some observers the image separation could be increased to 5 deg beyond parallel before fusion was lost. The visual axes diverged to follow the image centers and varied from overconverged to overdiverged with respect to the image centers while the observers still reported depth and fusion. We call the difference between the image separation and eye vergence the vergence error. If a vergence error persisted for at least 10 sec without loss of the percepts of fusion and depth, we postulate that neutral remapping occurred that compensated for the retinal misalignment. We found that the average maximum neural remapping was 3.0 deg.(ABSTRACT TRUNCATED AT 250 WORDS)

Spatio-temporal visually evoked scalp potentials in response to partial-field patterned stimulation.

Visually evoked scalp potentials (VESP) have been recorded at 40 electrode sites from each of 3 subjects. Pattern appearance/disappearance was used for whole field and partial field stimulation. The data are displayed as equipotential maps. The topographical features of the equipotential maps show periods of stable organization followed by periods of relatively rapid change. The structure of the maps changes in a consistent pattern with the region of the retina stimulated; the first peak fits well within the framework of the cruciform model of striate cortex. The first and second peaks of the VESP appear to be caused by independent neural generators. The work reported in this paper shows major agreement with other authors and reconciles some points of disagreement between them.

Nonlinear kernels of the human ERG.

In this paper we examine the use of a symmetric binary random stimulus for eliciting the ERG, and for calculating the first-order and second-order kernels of a nonlinear functional expansion of the response. We show that if the stimulus is represented in a non-dimensional form, then the units in which all kernels are measured are the same as the units used to measure the response, microvolts in the case of the ERG; further, contributions from all kernels to the response can be added without scale factors. We present the first-order and second-order kernels measured for a population of 15 normal subjects in a clinical setting. The measurements were made at various levels of adaptation ranging from photopic to scotopic conditions. The second-order kernels illustrate the processes of rapid adaptation (less than 100 ms) in the retina.

Spatiotemporal mapping of scalp potentials.

Computerized analysis and display techniques are applied to the problem of identifying the origins of visually evoked scalped potentials (VESP's). A new stimulus for VESP work, white noise, is being incorporated in the solution of this problem. VESP's for white-noise stimulation exhibit time domain behavior similar to the classical response for flash stimuli but with certain significant differences. Contour mapping algorithms are used to display the time behavior of equipotential surfaces on the scalp during the VESP. The electrical and geometrical parameters of the head are modeled. Electrical fields closely matching those obtained experimentally are generated on the surface of the model head by optimally selecting the location and strength parameters of one or two dipole current sources contained within the model. Computer graphics are used to display as a movie the actual and model scalp potential field and the parameters of the dipole generators whithin the model head during the time course of the VESP. These techniques are currently used to study retinotopic mapping, fusion, and texture perception in the human.

The dimensionality of the human visual evoked scalp potential.

A small number of processes can account for most of the evoked potentials activity in the two subjects studied. Principal components analysis indicates that six independent processes can account for approximately 97% of the variability in the data. Moreover, the factor analysis and plots of the factor coefficients yield indications that the times during which these principal factors are active agree quite well with the times at which the equipotential maps show some organized activity. The question of dipoles being the underlying cause of the observed activity is not answered by the factor analysis. The principal factors are not unique, but models which have a small number of parameters are more justifiable in light of the results of this study.