Topographic visually evoked potentials induced by stereoptic stimulus

Can whole brain nerve conduction velocity be derived from surface-recorded visual evoked potentials?

Reed, Vernon, and Johnson [Reed, T. E., Vernon, P. A., & Johnson, A. M. (2004). Sex difference in brain nerve conduction velocity in normal humans. Neuropsychologia, 42, 1709-1714] reported that "nerve conduction velocity" (NCV) of visual transmission from retina to the primary visual area (V1) is significantly faster in males than females. The authors estimated the NCV by dividing head length (nasion-to-inion distance) by the latency of the well-known P100 component of the visual evoked potential (VEP). Here, we critically examine these metrics and we contend that knowledge of the underlying physiology of neural transmission across the initial stages of the visual processing hierarchy dictates that a number of their assumptions cannot be reasonably upheld. Alternative, and we believe, more parsimonious interpretations of the data are also proposed.

Stereoscopic visual evoked potentials in normal subjects and patients with open-angle glaucomas

PURPOSE

To evaluate stereoscopic visual evoked potentials (S-VEP) in normal controls and in patients with glaucomatous optic nerve damage.

METHODS

Computer-generated dynamic random-dot stereograms were used to elicit cortical visual evoked potentials using wireless electric liquid crystal shutter glasses. Normal subjects (n=22) and patients with glaucoma (n=22) were investigated using five different disparities from 9 to 40 arc min. Statistical dependency of measurements with different stimulus at identical patients was adjusted for.

RESULTS

Peak times of onset and offset response of S-VEP can be significantly delayed in glaucomas. A general linear regression model confirmed that differences between patients and normals depend on disparity. S-VEP onset shows no significant difference between controls and glaucomas at 9 arc min disparity. At high disparities, however, peak time of the onset response was significantly (p<0.01) delayed in glaucomas when compared with normals (normals: 125.8+/-13 ms, glaucomas: 148.2+/-25.6 ms at 40 arc min).

CONCLUSIONS

Visual evoked potential elicited by the onset of a random-dot stereogram can be used for objective measurement of stereoacuity in a clinical setting. Differences between controls and glaucomas in high and low disparities could indicate a stereo-specific deficit in glaucoma.

Assessment of stereopsis in rhesus monkeys using visual evoked potentials

Rhesus monkeys can have deficiencies in stereo vision, making it necessary to screen monkey subjects intended for single cell studies of stereo-based depth processing. We measured VEPs in two monkeys using a dynamic random-dot display in which a stereo-defined checkerboard reversed in depth. Monkeys fixated upon a small dot during stimulus presentation. One monkey showed clear evoked potentials in response to changes in disparity that were similar to those obtained in human subjects, using an identical stimulus paradigm. Controls with presentations of the monocular stimulus sequences (in which no depth reversal can be perceived) yielded no or much weaker VEPs. In the other animal, however, there was no difference in evoked potential between the two conditions. These electrophysiological findings closely match the performance of these same two subjects in a disparity discrimination task in which they were previously trained. We conclude that VEPs using this type of stimulus display can be used to screen monkeys for single cell or behavioral studies of stereopsis.

Depth perception and evoked brain activity: the influence of horizontal disparity and visual field location

The perception of dynamic random-dot stereograms (RDS) depends on the physiological fusion of horizontally disparate binocular visual input. Thus, the use of RDS offers the possibility to study selectively cortical processing of visual information in man. We investigated the influence of horizontal disparity on the scalp topography of RDS evoked brain activity in 33 healthy subjects. Stereoscopic checkerboard patterns were presented in the center or lateralized in the left or right visual field with horizontal disparities changing at temporal frequencies of six or eight depth reversals/s using different disparity values ranging from 3.5 to 28 min of arc. In 11 subjects evoked potential fields were recorded from 16 electrodes, and 21 subjects participated in 30-channel recordings with electrodes located over the parietal and occipital brain areas. Stimulation frequency-related brain activity was obtained with all disparity values; however, with large or small disparities the potential field strength decreased significantly while largest responses were obtained with intermediate disparities. Significant differences were observed in RDS evoked brain activity when central and lateralized stimulus locations were compared. With lateral stimuli (extending from the fovea to 17.1-deg eccentricity) maximal amplitudes were obtained at larger disparities than with central stimuli. In addition there were pronounced differences between brain activity evoked with stimuli presented in the left or right visual field; however, there were very similar evoked potential signals recorded from electrodes located over the left and right hemispheres. Our findings indicate that the processing of disparity information with lateralized stimuli is different from the processing in the center of the visual field. In addition, lateralized stimulation yields a significant disparity tuning mainly with stereoscopic targets occurring to the right from the fixation point (but not with stimuli to the left) suggesting a functional difference between the visual half-fields.

Topography of the evoked potential to spatial localization cues

Visual tasks that are perceptually diverse might be expected to elicit unique evoked-potential waveforms that exhibit differing topographic maps. To investigate this possibility, multichannel visual-evoked potentials (VEPs) were recorded in response to several dot spatial localization stimuli that are physically similar yet produce different percepts (vernier offsets, steroscopic disparity, bisection, orientation, and relative displacement) to determine if the unique percepts arising from these stimuli reflect the activation of different cortical neural populations. The resulting evoked potentials were all similar in waveform, although the stereoscopic VEPs were relatively delayed. Topographic maps of the evoked-potential activity to each stimulus revealed a late major component with two independent foci: one 7 or more centimeters above the inion lateral to the midline, and the other at least 6 cm lateral to OZ. The scalp localization of both peaks was independent of both the position of the stimulus in the visual field and the particular stimulus cue presented. An asymmetric response to pattern appearance vs. disappearance indicated strong pattern specificity for each stimulus type except unreferenced motion. The timing of the VEP responses and relative insensitivity to retinal locus of stimulation suggest the involvement of higher cortical areas. The two map foci might be interpreted as activation of inferotemporal and parietal cortices whose roles are thought to be visual object interpretation and spatial attention and localization, respectively.

Discrimination of motion in depth trajectory following acute alcohol ingestion

Evoked potentials were recorded at four scalp sites O1, O2, T5, T6; before and after the consumption of one of two alcohol dose levels, to the occurrence of disparate stimuli in dynamic random dot stereograms. Twenty young adult subjects, all of whom had vision which was normal or corrected to normal, were randomly assigned to the two dosage groups. Subjects viewed stimuli which embodied crossed disparity and followed one of three trajectories in depth. The task was to distinguish between and identify, by specific button presses, these trajectories. One half of the stimuli were presented with monocular cues to motion in depth (dark condition); in the remainder only stereoscopic depth information was available (uniform condition). Responses to both conditions showed alcohol-related reduction of evoked responses across all sites. It was concluded that the visual systems associated with the processing of motion in depth were susceptible to alcohol effects, as revealed by reductions in the amplitude of evoked responses.

Scalp response topography to dynamic random dot stereograms

Responses were recorded to dynamic random dot stereograms from 32 sites (acquired in separate 8-channel montages). The disparate stimulus comprised a central, 3 degrees square of 30 arc min crossed disparity, superimposed on a 10.7 degrees square background field. Stimulus onset occurred 100 msec into a 500 msec recording epoch, with a 100 msec stimulus duration. Wave forms for 8 subjects were normalised across the 100 msec pre-stimulus period. Average amplitudes were calculated for the post-onset intervals: 120-140, 140-180, 180-220, 220-260 and 260-300 msec. Contour lines of evoked response amplitude were plotted from these data. These plots are compared with theoretical plots generated from a '3 concentric sphere' model of the head. The results lend support to the proposal that preliminary stereoscopic processing takes place in the primary visual cortex, followed by secondary processing in more anterior cortical regions.

Topographical study of stereo-related potentials

In order to estimate objectively binocular vision and especially stereopsis, random dot stereograms generated by a personal computer were used. Brain activity during stereopsis was topographically studied by visually evoked potentials (VEPs). The potentials evoked by binocular viewing of patterns without disparity, e.g. correlogram, were very similar to the potentials evoked from patterns with disparity, i.e. stereogram, as many authors have already indicated. To derive the stereo-related potentials from the VEP elicited by stereograms, the potentials evoked by correlograms were subtracted from the potentials evoked by stereograms, and the differences of topographical distribution between normal and stereoblind subjects were investigated.