The perception of shape-from-stereo is best characterized by the spatial disparity-contrast sensitivity function (DSF). This is the stereo analogue of the well-known luminance-contrast sensitivity function (CSF). In principle, the DSF and CSF portray a visual system's ability to detect spatial modulation as specified by changes in binocular disparity and luminance, respectively. In humans, less fine detail is visible in the stereo domain than is possible in the luminance domain. Here, we characterize for the first time the DSF in a non-human species, viz. the barn owl. At the same time, we re-examined the human DSF with identical apparatus and methods to directly compare between two vertebrate species that evolved stereovision independently. We discovered a close relationship between the owl and human ability to detect shape-from-stereo. In particular, the shift in absolute position between the human and owl DSF, as measured here, closely corresponds to the shift in absolute position between their respective CSFs, as known from the literature. In conclusion, our study establishes unprecedented experimental proof of a striking similarity in the prowess of humans and owls to achieve shape-from-stereo.

^{−2}.

^{−2}). These surfaces consisted of two half-images each covered with a fresh matrix of randomly distributed Gaussian blobs. Blob density was homogeneous, approximating 10 dots/deg

^{2}. All blobs were contained within a circular aperture (diameter size: 15 degree), and thus around 1780 individual blobs were visible in the stimulus. The relative disparity,

*r,*of the individual blobs was calculated as follows:

*x*and

*y*provide the blob coordinates within the circular aperture, and

*A, CF, ρ*and

*θ*represent the peak-to-trough amplitude, spatial frequency, orientation, and phase of the corrugated-RDSs. During each trial, the subject's absolute viewing distance was monitored by means of a magnetic tracking device. Trials during which the viewing distance deviated more than 1 cm form the desired 110 cm were aborted and repeated at a later time.

^{−2}) at the center of a completely darkened stimulus aperture functioned as the observation stimulus.

*SD*); that of O2 was 38 mm (±4 mm,

*SD*). The sign of the depth displacement (or disparity) relative to the fixation plane was made positive when the central corrugation was convex, and made negative in case of a concave central corrugation. In this way, the surfaces contained either negative or, alternatively, positive disparities relative to the plane of fixation.

*SD*: 51 ms; owl 2: mean 815 ms,

*SD*42 ms).

*P*(

*X*≥ 83%) < 0.0001 when

*N*= 70 trials, where

*P*represents the two-sided, independent binomial probability calculated from the number of correct and incorrect responses with an expectation of 0.5 of being correct by chance alone.

*μ*and

*σ*are the mean position and the standard deviation (

*SD*), respectively. In particular, the function

*P*(

*x*) corresponds to the probability of indicating a convex shaped central corrugation. The

*μ*parameter represents the bias towards either negative (

*μ*> 0) or positive (

*μ*< 0) disparities. The stereo acuity parameter,

*σ,*represents a direct measure of the observer's ability to perform the discrimination task. The log-likelihood ratio, based on 10,000 Monte-Carlo simulations, allowed verification of the goodness-of-fit: two-sided

*χ*

_{deviance}

^{2}(7) > 12,

*p*< 0.05 (Wichmann & Hill, 2001b). In other words, the likelihood of finding a deviance greater than 12 (with 7 degrees of freedom) by chance alone for all fitted functions was less than 5%.

*σ*; Equation 2) as a function of spatial frequency, and are plotted for horizontally (black lines) and vertically (gray lines) oriented corrugations. Both human subjects showed small response biases (<30 arcsec),

*μ*(see Equation 2), with 95%-CIs overlapping with zero disparity, indicating a symmetry in the detection of convex and concave corrugations.

^{2}. We therefore were confident that the present procedure and viewing conditions provide sufficient fidelity and resolution to measure the DSF in owls.

*σ*; Equation 2) as a function of spatial frequency, and are plotted for horizontally (black lines) and vertically (gray lines) oriented corrugations. Both owls showed small response biases (<50 arcsec),

*μ*(see Equation 2), with 95%-CIs overlapping with zero disparity, indicating a symmetry in the detection of convex and concave corrugations.

*p*= 0.469, 95%-CI: 0.417–0.522). Owl subject 2 responded in 173 trials correctly (

*p*= 0.519, 95%-CI: 0.467–0.572).