The polarity inversion of scalp potentials evoked by upper and lower half-field stimulus patterns: latency or surface distribution differences?

Exploiting individual primary visual cortex geometry to boost steady state visual evoked potentials

OBJECTIVE

The steady-state visual evoked potential (SSVEP) is an electroencephalographic response to flickering stimuli generated partly in primary visual area V1. The typical 'cruciform' geometry and retinotopic organization of V1 is such that certain neighboring visual regions project to neighboring cortical regions of opposite orientation. Here, we explored ways to exploit this organization in order to boost scalp SSVEP amplitude via oscillatory summation.

APPROACH

We manipulated flicker-phase offsets among angular segments of a large annular stimulus in three ways, and compared the resultant SSVEP power to a conventional condition with no temporal phase offsets. (1) We divided the annulus into standard octants for all subjects, and flickered upper horizontal octants with opposite temporal phase to the lower horizontal ones, and left vertical octants opposite to the right vertical ones; (2) we individually adjusted the boundaries between the eight contiguous segments of the standard octants condition to coincide with cruciform-consistent, early-latency topographical shifts in pattern-pulse multifocal visual-evoked potentials (PPMVEP) derived for each of 32 equal-sized segments; (3) we assigned phase offsets to stimulus segments following an automatic algorithm based on the relative amplitudes of vertically- and horizontally-oriented PPMVEP components.

MAIN RESULTS

The three flicker-phase manipulations resulted in a significant enhancement of normalized SSVEP power of (1) 202%, (2) 383%, and (3) 300%, respectively.

SIGNIFICANCE

We have thus demonstrated a means to obtain more reliable measures of visual evoked activity purely through consideration of cortical geometry. This principle stands to impact both basic and clinical research using SSVEPs.

Effects of long-time reading experience on reaction time and the recognition potential

The proposition that long-time experience in reading a language gradually builds up rapidly acting neural processes that facilitate the processing of words in that language and speed them into conscious awareness was examined. Behavioral reaction time (RT) and electrophysiological responsiveness to visually displayed words and non-language images were measured in persons who differed in how much experience they had in reading English. The electrophysiological response was the recognition potential (RP). Behavioral RT and the latency of the RP to English words were both expected to depend upon how much English reading experience a person had. The short latency of the RP was expected to free it from the influence of non-perceptual factors that affect RT, such as speed/accuracy tradeoff. This expectation yielded the prediction that the behavioral and electrophysiological results would differ in a specific way. Long-time readers of English were expected to show shorter RP latency to English words than less experienced (China-educated) readers of English but no RP latency difference for non-language images, with which neither group had greater experience. In contrast, due to speed accuracy tradeoff, the China-educated subjects were expected to show longer RT for both the words and the non-language images. The prediction was confirmed. The amount of language experience that a person had showed a stronger relationship to RP latency than it did to RT. This helped to validate the use of the RP as a tool for investigating perception and demonstrated definite advantages that it has for studying acquired perceptual processes in humans.

Source estimates for MEG/EEG visual evoked responses constrained by multiple, retinotopically-mapped stimulus locations

Studying the human visual system with high temporal resolution is a significant challenge due to the limitations of the available, noninvasive measurement tools. MEG and EEG provide the millisecond temporal resolution necessary for answering questions about intracortical communication involved in visual processing, but source estimation is ill-posed and unreliable when multiple; simultaneously active areas are located close together. To address this problem, we have developed a retinotopy-constrained source estimation method to calculate the time courses of activation in multiple visual areas. Source estimation was disambiguated by: (1) fixing MEG/EEG generator locations and orientations based on fMRI retinotopy and surface tessellations constructed from high-resolution MRI images; and (2) solving for many visual field locations simultaneously in MEG/EEG responses, assuming source current amplitudes to be constant or varying smoothly across the visual field. Because of these constraints on the solutions, estimated source waveforms become less sensitive to sensor noise or random errors in the specification of the retinotopic dipole models. We demonstrate the feasibility of this method and discuss future applications such as studying the timing of attentional modulation in individual visual areas.

Generators of visual evoked potentials investigated by dipole tracing in the human occipital cortex

Current source generators (dipoles) of the human visual evoked potentials to pattern-onset stimuli were investigated with the dipole tracing method, using a realistic four-layer head model of scalp-skull-fluid-brain, which can equate the surface potential distributions on a scalp to one or two corresponding equivalent dipoles. Three healthy adult human subjects were used, and 29 electrodes were set on a scalp of each subject. Visual stimulus of a checkerboard pattern was presented for 250 ms in each of eight different visual fields (central and peripheral parts of each of four quadrant fields). The visual evoked potentials consisting of initial positive-late negative waves (CI and CII components designated by Jeffreys and Axford) were recorded mainly on the occipital region contralateral to stimulated visual fields. The initial positive wave (CI) of visual evoked potentials were divided into two components: early component of the CI (e-CI--an early small positive deflection with approximate peak latency of 70-90 ms) and late component of the CI (l-CI--a late large positive deflection with approximate peak latency of 100-120 ms). The dipole with a fit exceeding 98% dipolarity with our model at the shortest latencies was defined as an "earliest dipole" of the evoked potentials, produced by the primary responses in the occipital cortex to an afferent volley from the lateral geniculate body. These earliest dipoles, for eight different visual field stimulations, were estimated at the approximate peak of the e-CI. Estimated dipoles were superimposed on a three-dimensional magnetic resonance image of each subject's brain. Earliest dipoles for right upper and right lower quadrant-field stimulations were located at the left calcarine cortices below and above the calcarine fissure, respectively; earliest dipoles for left upper and left lower quadrant-field stimulations were located at the right calcarine cortices below and above the calcarine fissure, respectively. Furthermore, earliest dipoles for central and peripheral quadrant-field stimulations were located posteriorly and anteriorly in the calcarine cortex, respectively. The results from these non-invasive analyses of visual evoked potentials indicated topographic localization of the dipoles around the calcarine fissure based on the loci of the visual fields. This was comparable to the retinotopy of the human occipital lobe based on clinicopathological studies.

Vertical neglect: behavioral and electrophysiological data

Both behavioral and electrophysiological methods were used to assess altitudinal neglect. In the first experiment, 100 patients with neglect completed Albert's Barrage test. Most omissions were present in the lower left quadrant. In 16 patients, visual evoked potentials to stimuli in the four quadrants were separately recorded (Exp. 2). Latencies in the lower left quadrant were longer than those in the other quadrants. A third experiment provided electrophysiological normative data from 13 young normal subjects. Overall, the results showed that both the horizontal and vertical dimensions of space are affected in neglect patients.

The recognition potential and word priming

The effect of priming on the latency of the recognition potential (RP) was tested using rapid stream stimulation. Subjects detected five-letter words in a stream of nonword images. Lifting the right index finger signalled detection of a word. Rapid responses were rewarded and false alarms were penalized. Just before generating an image stream, a computer briefly displayed either the specific target word or or five-letter string that indicated the target was any one of ten previously studied words. Precise target specification was expected to produce more rapid detection than the provision of less definite information. Since the RP was thought to reflect the speed of perception, it was predicted that its latency would be less when the target word was beforehand than when less specific information was provided. The results for 10 subjects confirmed the hypothesis.

A new procedure for the registration of the visual-evoked cortical potential by multichannel recording of the gradient distribution

A new bipolar multichannel visual-evoked cortical potential (VECP) procedure is presented, which can supply objective information on visual field losses. The electrical potential is recorded from 11 electrodes applied to the back of the head in the form of an equidistant rectangular grid. Each neighbouring pair of electrodes feeds one of 14 bipolar channels. The adjacent horizontal and vertical channels are used to calculate an approximation to the direction and amplitude of the electrical field gradients. The gradient distribution is represented by a map of arrows (gradient map) for every instant of the sweep, so that the whole sweep can be plotted as a time series of gradient maps. The maps are easy to scale and are well suited for visual evaluation. Twenty normal subjects were stimulated using checkerboard reversal, partial field patterns to simulate visual field defects. The stimulated area varied between full-, half- and quarter-field, and the particular area stimulated could be clearly seen in the resulting gradient maps. Additionally, we developed a computerized classification procedure that detected 86% of the disturbed visual fields from the gradient recordings.

Infant visual development: an overview of studies using visual evoked potential measures from Harter to the present

Studies of sensory and perceptual abilities in infants require creative, innovative techniques. Although the young infant's response repertoire may appear limited to the naive individual, a number of highly refined procedures have been developed and implemented with these non-verbal humans over the last twenty years. The most successful protocols for evaluating visual development rely either on behavioral responses or on electrophysiological recordings. The first published report using visual evoked potentials to study the development of pattern vision in human infants was presented by M. Russell Harter. This work provided the impetus for a wealth of studies exploring issues of visual information processing abilities in early infancy. The available range of data and experimental techniques are now sufficiently refined that many clinical issues are currently being addressed. The purpose of this review is to document the evolution of scientific studies since Harter's seminal work. The selection of protocols presented focuses on those with either current clinical applications or those which hold promise for future applications in the evaluation and treatment issues of abnormal visual development.

Chronotopographical analysis of the pattern onset visual evoked magnetic response (VEMR): implications for waveform peak identification

The topography of the visual evoked magnetic response (VEMR) to a pattern onset stimulus was studied in five normal subjects using a single channel BTi magnetometer. Topographic distributions were analysed at regular intervals following stimulus onset (chronotopography). Two distinct field distributions were observed with half field stimulation: (1) activity corresponding to the C11 m which remains stable for an average of 34 msec and (2) activity corresponding to the C111 m which remains stable for about 50 msec. However, the full field topography of the largest peak within the first 130 msec does not have a predictable latency or topography in different subjects. The data suggest that the appearance of this peak is dependent on the amplitude, latency and duration of the half field C11 m peaks and the efficiency of half field summation. Hence, topographic mapping is essential to correctly identify the C11 m peak in a full field response as waveform morphology, peak latency and polarity are not reliable indicators.

Recognition potential: sensitivity to visual field stimulated

The recognition potential (RP) was distinguished from P3 and eye blink responses by its sensitivity to visual area stimulated. Images were flashed in upper and lower hemifields. Current source density profiles were computed, using 16 midline scalp electrodes. For P3 and eye blink profiles, the hemifield stimulated was not a significant factor. For the recognition potential, upper and lower field stimulation produced radically different profiles. An improved recognition potential signal was obtained by a new mathematical procedure. It used the difference in sensitivity to visual area stimulated to reject P3 and eye blink responses.

The extrastriate generators of the EP to checkerboard onset. A source localization approach

The cortical origin of the pattern onset EP has been investigated over a time window which covers the entire positive-negative-positive complex of the pattern onset EP. On the basis of a dipole source localization approach, the position, orientation and strength of the underlying sources of the pattern onset EP were estimated. For large check stimuli, chosen to have a weak edge specific component in the response, still two components are needed to account for the variance of the responses. Each component corresponds to a single dipole source, and both originate in the extrastriate cortex. These components dominate, respectively, the initial and the late positive peaks of the pattern onset EP. The equivalent dipole sources of the two components show different behaviors with respect to the position of the stimulus in the visual field. The topography and behavior of the equivalent dipole source underlying the early positive component suggest an origin in area 18. The invariance with stimulus location of the dipole source underlying the late positive component suggests an origin beyond area 18. The different topographies of the components also account for the differences in surface distribution of the pattern onset EP to large check stimulation of the upper and lower sectors of the visual field.

Cortical generators of the CI component of the pattern-onset visual evoked potential

Thirteen-channel visual evoked potentials (VEPs) to pattern-onset were recorded with stimuli restricted to individual octants of the peripheral field, to halves and to quadrants of the fovea. The voltage of the CI component was measured in each channel to define its topography for each stimulated sector. The potential fields so obtained were then analysed to find the orientation and location of a dipole that would produce a corresponding pattern of voltages at the scalp. The locations of the computed dipoles are consistent with the hypothesis that CI is generated in striate cortex. The computed locations and orientations are not compatible with alternative arrangements of sources in extrastriate cortex. A significant problem remains. If CI is indeed generated by the striate cortex then the orientation of the dipoles excited by stimulation of the peripheral field indicates that the cortex is surface negative. This leads to the prediction that foveal stimuli will elicit a CI which is negative at posterior electrodes. The experiments reported here confirm that CI is positive with such stimuli, and its source is calculated as a horizontal dipole with its positive pole oriented posterolaterally. Two possible explanations are considered for the reversed polarity of foveal CI: (a) that macropotentials associated with stimulation of the fovea are opposite in polarity from those associated with stimulation of the peripheral field; (b) that the foveal area of the retinotopic map extends into the lateral calcarine fissure with the effect that much of it faces in the reverse direction from the cortex at the pole.

Interindividual variation and additivity of the visual evoked potentials to the local checkerboard stimulation of the central and paracentral retina

Visually evoked cortical potentials to reversing checkerboard stimulation were recorded from normal subjects. Different locations of the central visual field of maximally 8 degrees radius were stimulated. Stimulation of various parts of the central visual field changed the waveform and the amplitude of the responses of different subjects very individually. This makes it impossible to make a universal decision, how large field or which part of the central field contributes most to pattern evoked cortical potentials. The responses to the upper half field stimulation showed greatest variation making the VEP recording worthless in detecting altitudinal visual field defects. The computed sum of the half field responses was of similar waveform and amplitude to the response to the full field stimulation. The good additivity of the responses applied to all parts of the central visual field tested.

Localization of visually evoked cortical activity in humans

The locations of cortical activity evoked by visual stimuli presented at different positions in the visual field are deduced from the scalp topography of visually evoked potentials in humans. To accomplish this, the Laplacian evoked potential is measured using a multi-electrode array. It is shown that the Laplacian response has the following useful attributes for this purpose. It is reference-free. Its spatial resolution is approximately 2 cm referred to the surface of the cortex. Its spatial sensitivity characteristic is that of a spatial band-pass filter. It is relatively insensitive to source--sink configurations that are oriented tangentially to the surface of the scalp. Only modest assumptions about the source--sink configuration are required to obtain a unique inversion of the scalp topography. Stimuli consisting of checkerboard-filled octant or annular octant segments are presented as appearance-disappearance pulses at sixteen different positions in the visual field in randomized order. The locations of evoked cortical activity in the occipital, parietal and temporal lobes are represented on a Mercator projection map for each octant or octant segment stimulated. Lower hemifield stimuli activate cortex which lies mainly on the convexity of the occipital lobe contralateral to the side of stimulus presentation in the visual field. The more peripheral the stimulus is in the visual field, the more rostral is the location of the active cortex. The rostral-to-caudal location of the evoked activity varies from subject to subject by as much as 3 cm on the surface of the occipital cortex. Furthermore, in any single subject there is a substantial amount of hemispheric asymmetry. Upper hemifield stimuli activate cortex that lies on the extreme caudal pole of the occipital lobe. This activity is relatively weak, and in some subjects it is almost unmeasurable. It is suggested that the representation of the upper hemifield in the cortex lies mostly on the inferior and mesial walls of the occipital lobe and possibly within the calcarine fissures. Those locations are inaccessible to the Laplacian analysis because the current generators therein may be oriented tangentially to the surface of the overlying scalp. Posterior parietal lobe activity and/or inferior temporal lobe activity is frequently evoked. Different subjects have different patterns of evoked activity. Unilateral or bilateral posterior parietal lobe activity is the most common pattern. Unilateral inferior temporal lobe activity is a less common pattern.(ABSTRACT TRUNCATED AT 400 WORDS)

Correlation functions in the analysis of visual evoked potentials

Our "vision" laboratory has been working for several years on stimulation methods and data processing. We present here the results of an analysis of correlation functions between VEPs obtained by flashes and patterned stimulations. This study provides additional information about possible interpretations.

Measurements of eccentricity of fixation in normals and in amblyopes by evoked potentials

A method to estimate the eccentricity of fixation, i.e. the position of the center of the fovea relative to the point of fixation, based on visually evoked potentials is described and applied to 14 normal and 17 amblyopic subjects. Eye position was simultaneously recorded. In normal subjects, the estimates of fixational eccentricity distributed unimodally with mean 12.1' and range 1-36'. The estimates from the nonamblyopic eyes of amblyopic subjects distributed bimodally with peaks near 5 and 55' and those amblyopic subjects with larger estimates were anisometropic. The results suggest that the anisometropic amblyopes have an asymmetry of retinocortical projections. When corrected for the fixational eccentricity of the nonamblyopic eye. 5 of 17 amblyopic eyes had fixational eccentricity greater than 40'. Since only 1 of these amblyopic eyes was found to fixate eccentrically by conventional clinical testing, it is suggested that eccentric fixation may be more common in amblyopia than has heretofore been appreciated.