Orientation characteristics of a mechanism in the human visual system sensitive to cyclopean form

Human observers viewed two superimposed cyclopean gratings in dynamic random noise. Neither grating was visible monocularly. The disparities of the two gratings were counterphase-modulated at different temporal frequencies. We recorded frequency domain brain responses at ultra-high frequency resolution (0.008 Hz) by means of nondestructive zoom-FFT. The response to the superimposed gratings was quite different to the sum of responses to the two individual gratings. We used this strong nonlinearity to investigate the orientation tuning of the cyclopean mechanism that responded to the gratings. One grating had a fixed orientation while the other was rotated. Half-sensitivity full bandwidth was greater than 90 degrees. This is considerably greater than the corresponding bandwidth (30 degrees) for luminance-defined (LD) gratings [Vis. Res. 27 (1987) 2181]. Furthermore, the cyclopean nonlinearity was different from the corresponding nonlinearity for LD gratings. Brain responses to a single cyclopean grating showed an inverse oblique effect over a 1.2-1.9 degrees range of bar widths, but not for smaller or larger bar widths.