Visual evoked responses to the upper and lower half-field stimulation in a dark-adapted man
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The averaged visual evoked responses (VERs) to flashed blank and checkerboard-patterned (32′ checkers) stimulation of the upper and lower half-field of 6° angular subtense were recorded referentially (Oz−A1+2) in seven dark adapted subjects using a luminance range of 3.69 log units (maximum luminance=2220 cd/m2).
With the logarithmical rise of stimulation luminance the peak latencies of the maximum positive-negative deflection of the VERs to the upper and lower half-field blank and to the upper half-field patterned stimulation display a monotonick shortening. These three response types do not differ in waveform, polarity and amplitude. Their amplitudes show no significant luminance-dependent changes.
The peak latencies of the VERs to the lower half-field patterned stimulation exhibit an “U” shaped course with the increase of the luminance. At lower luminances they are by about 30 ms shorter in comparison with the VERs to the other three stimulation types, at higher luminances a gradual lengthening is observable. Consequently, at low luminances these VERs have a reversed polarity, at higher luminances the same polarity as the VERs to the other three stimulation types. The amplitude values of the lower half-field patterned responses are the highest and show a triphasic luminance-dependent course.
From these and further differences between the VERs to the upper and lower half-field patterned stimulation in connection with the reaction to defocusing and to the subtraction of the luminance-related part of the response it is concluded that the VERs to the lower half-field patterned stimulation only contain a pattern-related component.
Key wordsVisual evoked potentials Human visual system Pattern vision Half-field stimulation
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