A comparative analysis of vertical and horizontal fixation disparity in sentence reading

Abstract

Humans have two, frontally placed eyes and during reading oculomotor and sensory processes are needed to combine the two inputs into a unified percept of the text. Generally, slight vergence errors, i.e., fixation disparities, occur but do not cause double vision since disparate retinal inputs fall into Panum's fusional area, that is, a range of disparity wherein sensory fusion of the two retinal images is achieved. In this study, we report benchmark data with respect to the mean magnitude and range of vertical compared to horizontal fixation disparities for natural reading. Our data clearly fit to an elliptical pattern of Panum's fusional area that corresponds with theoretical estimates. Furthermore, when we examined disparity-driven vergence adjustments during fixations by comparing monocular with binocular reading conditions, we found that only horizontal fixation disparities increased significantly under conditions of monocular stimulation. Also, no significant vertical fine-tuning (vergence adjustment) was observed for vergence eye movements during reading fixations. Thus, horizontal and vertical fixation disparities and vergence adjustments during reading showed quite different characteristics, and this dissociation is directly related to the functional role of vergence adjustments: vertical fusion - and vertical vergence - subserve the maintenance of a single percept and stereopsis by keeping the eyes in register and allowing for horizontal fusional processes to successfully operate over a vertically aligned input. A reliable and stable vertical alignment is, thus, a pre-requisite over which horizontal fusional responses (and depth perception) can work most efficiently - even in a task like reading.

Binocular coordination and return-sweep saccades among skilled adult readers

“…The proportion of alignment for each fixation population further quantifies the role of the vergence system during the reading of multiline texts. Consistent with see also Blythe et al, 2010;Blythe et al, 2006;Jainta et al, 2015;Juhasz et al, 2006;Kirkby et al, 2010;Kirkby et al, 2013;Nikolova et al, 2015;Nikolova et al, 2017Nikolova et al, , 2018, we observed more aligned than unaligned fixations for intraline reading. However, a higher proportion of fixations were unaligned following a return-sweep saccade at fixation onset.…”

Binocular coordination and reading performance during smartphone reading in intermittent exotropia

“… 18 Previous studies commonly showed that the horizontal fixation disparity, measured by VOG, had a variance of 2°. 19 , 20 Therefore, monocular viewing was tentatively defined in the present study as a horizontal disparity greater than 2°. The proportion of monocular viewing was calculated using the following formula: monocular viewing time/total reading time.…”

Using a dichoptic moving window presentation technique to investigate binocular advantages during reading.

“…Thus, for a large range of tasks including reading, motor fusion usually serves to reduce disparities and sensory fusion occurs when disparity falls within the functional fusional range (Jainta, Blythe, Nikolova, Jones, & Liversedge, 2014).…”