![]() For an active region, the pattern is present for a single frame, and hence can be considered an impulse of contrast, against a zero-contrast baseline. An active region could receive one of three stimulus conditions: a pattern pulse to the left eye, to the right eye, or to both eyes (denoted OD, OS, BIN, respectively). Within each frame pair, one or more regions could be active, the inactive regions remaining at the mean luminance. Recording run duration, unless otherwise stated, was 109 seconds, equal to 8192 frame pairs at 75 Hz. The ferro-electric shutter goggle system delivers left-right–frame pairs at a rate of 75 Hz, comprising first a frame to the left eye, then an interleaved frame to the right eye, \(\) second later. The temporal stimulus delivered in each of the 60 regions was a sequence of pattern pulses, consisting of the presentation for one frame of a full contrast 4 × 4 checkerboard within the region. Subjects sat in a curtained space having low-background illumination, at a viewing distance of 40 cm, fixating a small cross at the screen’s center. Against this baseline, checkerboards pulsed on, with light and dark luminances of 20 and 0.1 cd/m 2, equivalent to a Michaelson contrast of 98%. The mean level of luminance measured with a radiometer (CS-100 Minolta, Osaka, Japan) through the goggles while toggling was 10 cd/m 2, a low level, but still within the photopic range. ![]() The shutter goggles allowed approximately 25% transmission in the open-shutter phase and almost no transmission in the closed-shutter phase hence, allowing on average only one eighth of the monitor luminance to be used. The monitor’s framerate was 150 Hz, toggled by the shutter goggles to produce 75 Hz stimulation at each eye. Stimuli were displayed on a monitor (Clinton Monoray CRT Clinton Electronics Corporation, Loves Park, IL), driven by a dichoptic system consisting of a VSG2/5 graphics board and ferro-electric (FE-1) shutter goggles (Cambridge Research Systems, Rochester, UK). 3 4 Such fine-grained retinotopic analysis of cortical evoked potentials is of interest both for basic research in the processing of visual information in early visual areas and for clinical application. Multifocal visual evoked potential (MVEP) analysis refers to the simultaneous characterization of response properties for multiple visual field locations, by concurrently applying separate test stimulus waveforms to each location, and decomposing the overall response into components due to each location. The use of focal stimuli, restricted to smaller regions of the visual field, activates smaller regions of cortex, but substantial recording times are necessary if many locations are to be studied. The morphology of early cortical visual areas varies significantly between individuals 1 2 hence, the resultant superposition leads to significant interindividual variation in evoked potential waveforms. Wide-field visual stimulation generates a complex superposition of evoked potentials due to the varying orientation of generators within the retinotopically organized early visual areas. A general methodology is illustrated that allows multifocal analysis with flexible choice of stimulus conditions. The PPMVEP can simultaneously characterize 60 regions of the visual field for both eyes in less than 7 minutes. Root mean square (RMS) signal-to-noise ratio (SNR) was 1.9 times higher with pattern-pulse stimulation, corresponding to a reduction of 73% in recording time to achieve the same SNR.Ĭonclusions. In a direct comparison with a contrast-reversal multifocal analysis, the pattern-pulse responses had similar waveforms and scalp topography, but were 15 times larger in amplitude. A stereotypical distribution of waveforms held in most subjects, depending primarily on the polar angle location of the stimulus within the visual field. ![]() Response waveform sets for 12 subjects varied in maximum amplitude from 1.8 to 6.8 μV. A direct comparison was then made with contrast-reversal stimulation. Left-eye, right-eye, and binocular viewing conditions were concurrently tested by dichoptic stimulation. Responses were recorded from normal subjects, by using a 32-channel electroencephalogram recording system, and elementary responses to each region were estimated by multiple regression of each of the response channel signals on stimulus signals. VEPs were obtained by concurrently stimulating 60 regions of a cortically scaled dartboard with pulses of pattern contrast. To define the pattern-pulse multifocal visual evoked potential (PPMVEP) and determine its characteristics in a sample of normal subjects in terms of amplitude of response attainable, the variation in waveform across visual field, and distribution of potential over the scalp and to compare pattern-pulse with contrast-reversal multifocal stimuli. ![]()
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