Wattam-Bell J, Birtles D, 2005, "The effect of binocular misalignment on cyclopean visual evoked potentials" Perception 34 ECVP Abstract Supplement
The effect of binocular misalignment on cyclopean visual evoked potentials
J Wattam-Bell, D Birtles
Visual evoked potentials (VEPs) to dynamic random-dot correlograms (RDCs) are a well-established method of assessing cortical binocularity, particularly in infants (eg Braddick et al, 1980 Nature 228 363 - 365). The RDC stimulus alternates between a correlated state, with identical patterns in each eye, and an anticorrelated state, in which each eye sees the negative of the other eye's pattern. An absence of a response to this alternation is generally regarded as indicating a lack of cortical binocularity. However, it could instead be a result of inaccurate vergence: if the two eyes' images are sufficiently misaligned, both alternation states will be binocularly uncorrelated and they will be indistinguishable. We have examined the effect of binocular misalignment on adults' VEP responses to RDCs, and to the cyclopean stimulus of Ohzawa and Freeman (1988 Vision Research 28 1167 - 1170). In the latter, gratings drifting in opposite directions are presented to each eye. Binocular summation produces a counterphase grating which generates a VEP. The temporal phase of the counterphase grating, and thus of the VEP, will be altered by binocular misalignment, but the presence and amplitude of the VEP should be unaffected. For both stimuli, binocular misalignment was produced by changing the disparity of a fixation marker. As expected, with RDCs the amplitude of the VEP decreased with increasing misalignment; beyond about 1 deg, no VEP could be detected. With the grating stimulus, VEP amplitude was not affected by misalignment, but phase varied as predicted. These results suggest that the VEP in response to RDCs is likely to be a rather unreliable measure of cortical binocularity in subjects, such as young infants, who have poor control of vergence, whereas the grating stimulus VEP is more robust. Future experiments will compare VEPs to the two stimuli in infants.
[Supported by Medical Research Council programme grant G790850.]
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