How perceived egocentric distance varies with changes in tonic vergence.

Area of Science:

• Vision science
• Perceptual psychology
• Oculomotor research

Background:

• The eye muscle potentiation (EMP) hypothesis posits that sustained vergence changes egocentric perceived distance.
• This effect is linked to alterations in the tonic vergence state.

Purpose of the Study:

• To empirically test the EMP hypothesis.
• Quantify the relationship between prism-induced tonic vergence changes and perceived distance shifts.
• Measure the temporal dynamics of these perceptual changes.

Main Methods:

• Thirteen subjects were exposed to 5-diopter base-out prisms for 10 minutes, increasing vergence demand.
• Tonic vergence (phoria) and egocentric distance estimates were measured pre- and post-exposure using pointing tasks without visual feedback.
• Distance aftereffects were assessed for both hands to differentiate visual from visuomotor contributions.

Main Results:

• Distance aftereffects were consistent across both hands, suggesting a visual mediation.
• A significant correlation was found between distance aftereffects and prism-induced phoria changes.
• Perceived distance changes showed monotonic increases during exposure and stability post-exposure, indicating a long time constant.

Conclusions:

• The findings support the EMP hypothesis, confirming vergence's role in reduced-cue distance perception.
• Variations in tonic vergence influence perceived distance by modulating the sensed vergence effort.
• The results align with current models of vergence control system dynamics.