Two disparity channels in human visual cortex with different contrast and blur sensitivity

Abstract

PurposeOur goal is to describe the contrast and blur sensitivity of multiple horizontal disparity sub-systems and to relate them to the contrast and spatial sensitivities of their monocular inputs. MethodsSteady-State Visual Evoked Potentials (SSVEPs) amplitudes were recorded in response to Dynamic Random Dot Stereograms (DRDS) alternating at 2 Hz between zero disparity and varying magnitudes of crossed disparity for disparity plane and disparity grating stimuli. Half-image contrasts ranged between 2.5 and 80% and over a range of Gaussian blurs from 1.4 to 12 arcmin. Separate experiments measured contrast and blur sensitivity for the monocular half-images. ResultsThe first and second harmonics disparity responses were maximal for disparity gratings and for the disparity plane condition, respectively. The first harmonic of the disparity grating response was more affected by both contrast and blur than was the second harmonic of the disparity plane response which had higher contrast sensitivity than the first harmonic. ConclusionThe corrugation frequency, contrast and blur tuning of the first harmonic suggest that it reflects activity of neurons tuned to higher luminance spatial frequencies that are selective for relative disparity, whereas the second harmonic reflects the activity of neurons sensitive to absolute disparity that are driven by low monocular spatial spatial frequencies. Translational RelevanceSSVEPs to DRDS provide two objective neural measures of disparity processing, the first harmonic - whose stimulus preferences are similar to those of behavioral stereoacuity - and the second harmonic that represents an independent disparity-specific, but not necessarily stereoscopic mechanism.