Strabismus Does Not Impair Ability to Judge Depth

Patients with strabismus may have similar susceptibility to perspective convergence information as those without the condition, according to findings published in Royal Society Open Science.

Researchers conducted a controlled task-based study to find out how the judgement of  2-dimensional (2D) and 3-dimensional (3D) relative depth differs among those with strabismus and those who have typically-developing binocular vision. The tasks performed were a 2-alternative forced-choice (2AFC), and the method of adjustment for the 3D condition, with outcome measures of susceptibility, measured by examining 3D bias in making 2D interval equidistance judgements, and accuracy, measured by examining 3D interval equidistance judgements. 

The study participants included 1 group of 24 normal stereovision patients and 1 group of 8 patients with limited or no stereovision. Both groups responded accurately in the 2D condition and the 2D perception condition showed that judgements about interval equidistance on a 2D plane of the test monitor were affected by perspective information in the stimuli for all participants (P <.001). 

For the 3D task, participants’ ratios for judgements were similar in value to the 2D judgements. Interval equidistance error was almost identical in the 3D pictorial space to that of the 2D pictorial space for those with no or limited stereovision. There was a larger difference in ratio for normal stereovision patients, but this difference was not significant (P =.083). 

The researchers explain that, although all participants demonstrated accurate judgement of interval equidistance on the 2D plane without perspective information in the stimuli, perspective convergence information affected their responses despite being required to ignore it. This fact suggests that automatic, bottom-up processing takes effect at the onset of perspective convergence cues and that those with typical stereovision cannot suppress the resulting bias in 2D judgements. 

There was no significant difference between the 2 groups in terms of interval equidistance in both the 2D and 3D spaces, suggesting that those with strabismus do not experience impairments in these judgements. However, the researchers explain that some limitations of the study may have led to confounded results.

“It is possible that the order of testing, the 2D task followed by the 3D task, may have caused participants to misinterpret the initial 2D task to be a 3D task because of the strong pictorial background (in comparison to the control and CP condition where the 2D instruction was more salient),” the study says. “This may explain the relatively large bias in the 2D PP condition which yielded ratios almost the same as the 3D PP judgement.”

On this point, the researchers say that the 2D task instructions could have been more salient if participants had performed the 2 tasks in reverse order, with a more natural 3D task first and a subsequent, different 2D task.

As another limitation, the study explains that the 3D PP and 2D PP conditions could not be compared directly because of varying methodologies used for measurement.

Reference

Zlatkute G, de la Bastida VCS, Vishwanath D. Unimpaired perception of relative depth from perspective cues in strabismus. R Soc Open Sci. 2020;23;7(12):200955. doi:10.1098/rsos.200955.