Overestimation of stereo thresholds by the TNO stereotest is not due to global stereopsis

<jats:title>Abstract</jats:title><jats:sec><jats:title>Purpose</jats:title><jats:p>It has been repeatedly shown that the <jats:styled-content style="fixed-case">TNO</jats:styled-content> stereotest overestimates stereo threshold compared to other clinical stereotests. In the current study, we test whether this overestimation can be attributed to a distinction between ‘global’ (or ‘cyclopean’) and ‘local’ (feature or contour‐based) stereopsis.</jats:p></jats:sec><jats:sec><jats:title>Methods</jats:title><jats:p>We compared stereo thresholds of a global (<jats:styled-content style="fixed-case">TNO</jats:styled-content>) and a local clinical stereotest (Randot Circles). In addition, a global and a local psychophysical stereotest were added to the design. One hundred and forty‐nine children between 4 and 16 years old were included in the study.</jats:p></jats:sec><jats:sec><jats:title>Results</jats:title><jats:p>Stereo threshold estimates with <jats:styled-content style="fixed-case">TNO</jats:styled-content> were a factor of two higher than with any of the other stereotests. No significant differences were found between the other tests. Bland‐Altman analyses also indicated low agreement between <jats:styled-content style="fixed-case">TNO</jats:styled-content> and the other stereotests, especially for higher stereo threshold estimates. Simulations indicated that the <jats:styled-content style="fixed-case">TNO</jats:styled-content> test protocol and test disparities can account for part of this effect.</jats:p></jats:sec><jats:sec><jats:title>Discussion</jats:title><jats:p>The results indicate that the global – local distinction is an unlikely explanation for the overestimated thresholds of <jats:styled-content style="fixed-case">TNO</jats:styled-content>. Test protocol and disparities are one contributing factor. Potential additional factors include the nature of the task (<jats:styled-content style="fixed-case">TNO</jats:styled-content> requires depth discrimination rather than detection) and the use of anaglyph red/green 3D glasses rather than polarizing filters, which may reduce binocular fusion.</jats:p></jats:sec>