Abstract
Similar to contrast, disparity sensitivity varies as a function of spatial frequency. The psychophysical ‘disparity sensitivity function’ peaks around 0.5cpd but drops off for low and high frequency disparity-defined gratings. Behavioural measures have described its shape as the envelope of at least two spatial frequency channels, but a linkage to possible temporal channels has not been established.
We reconstructed the disparity sensitivity function from a neural readout, measuring steady-state visual evoked potentials in normally-sighted individuals (N=25). Stimuli were dynamic random dot stereograms alternating at 2Hz between a flat plane at zero disparity, and a crossed-disparity sine-wave grating increasing in disparity amplitude. Gratings of seven different spatial frequencies were shown (0.1 - 2cpd), with an additional absolute disparity condition.
We applied reliable component analysis to identify cortical sources that responded in a consistent manner across trials, revealing two primary components. The first was focused over the occipital pole and the second was right-lateralized over extrastriate cortex.
Across both components, the first harmonic was strongly tuned for spatial frequency, where the magnitude of the response depended both on the stimulus disparity amplitude and its spatial frequency. The characteristic U-shaped disparity sensitivity function was mirrored by responses in the occipital 1f1 component. We suggest this response harmonic captures the dynamics of a sustained relative disparity mechanism.
In contrast, the second harmonic was invariant across spatial frequency. Responses were generally weaker to all stimuli except the absolute disparity stimulus. Responses to a flat plane oscillating in depth were best captured by the occipital source, suggesting an early, transient absolute disparity mechanism.
Our electrophysiological approach revealed at least two parallel mechanisms differing in their spatio-temporal tuning, adding a further dimension to the current description of the spatial frequency channels thought to underlie disparity processing.