Oculomotor changes following learned use of an eccentric retinal locus

Abstract

People with bilateral central vision loss sometimes develop a new point of oculomotor reference called a preferred retinal locus (PRL) that is used for fixating and planning saccadic eye movements. How individuals develop and learn to effectively use a PRL is still debated; in particular, the time course of learning to plan saccades using a PRL and learning to stabilize peripheral fixation at the desired location. Here we address knowledge limitations through research describing how eye movements change as a person learns to adopt an eccentric retinal locus. Using a gaze-contingent, eye tracking-guided paradigm to simulate central vision loss, 40 participants developed a PRL by engaging in an oculomotor and visual recognition task. After 12 training sessions, significant improvements were observed in six eye movement metrics addressing different aspects involved in learning to use a PRL: first saccade landing dispersion, saccadic re-referencing, saccadic precision, saccadic latency, percentage of useful trials, and fixation stability. Importantly, our analyses allowed separate examination of the stability of target fixation separately from the dispersion and precision of the landing location of saccades. These measures explained 50% of the across-subject variance in accuracy. Fixation stability and saccadic precision showed a strong, positive correlation. Although there was no statistically significant difference in rate of learning, individuals did tend to learn saccadic precision faster than fixation stability. Saccadic precision was also more associated with accuracy than fixation stability for the behavioral task. This suggests effective intervention strategies in low vision should address both fixation stability and saccadic precision.

Keywords: Eye movements; Fixation stability; Macular degeneration; Saccade; Scotoma; Training.

Figure 1.. Training task used in the study.

Participants were asked to recognize a target as it changed location and identity by directing the TRL clear window (located in this example to the left of the simulated scotoma) onto the target. This was performed under three conditions: A. Face Recognition, B. Object Recognition, and C. Word Recognition. In each case, the target was obscured (top image) until revealed (bottom image) by directing the trained TRL over the target. In A, a face is shown. In B, a bell pepper is shown. In C, the word “bulb” is shown.

Figure 2.. Overview of the oculomotor metrics used in the study (adapted from Maniglia, Visscher and Seitz, 2020).

These metrics were extracted from the eye movement data collected during each training block. First saccade landing dispersion: blue dots represent the end points of absolute first saccades during each trial of a training block. The BCEA is represented by a red ellipse and encompasses 68% of total eye positions. Saccadic re-referencing: green dots represent ‘absolute’ first fixations of a trial that place the target outside of the scotoma, red dots are ‘absolute’ first fixations of a trial that place the target within the scotoma. Saccadic precision: dots represent the end points of saccades that first place the target outside of the scotoma. A green dot means the saccade was an ‘absolute’ first saccade (same as Saccadic re-referencing), whereas a red dot means that location was from a second or later saccade. Latency of target acquisition: reflects how long it takes to make a saccade which places the target in a visible location. Percentage of useful trials: indicates what percentage of trials include at least one saccade placing the target in a visible location. Fixation stability: a within-trial measure of dispersion after the first saccade of each trial, normalized to center each trial starting point to the average across-trial TRL location. It is visually represented using a kernel density estimator (KDE).

Figure 3.. Oculomotor changes with training.

Block average of metrics scores for each of the six oculomotor metrics as a function of training (comparison between the first training session (Block 1) and the last training session (Block 12)).

Figure 4.. Principal components analysis on the oculomotor metric scores.

Plot of the two principal components; red dots represent scores for individual participants and blue dots represent the weighting of each metric. Principal Component 1 weighs heavily on a cluster of three metrics that are highly correlated with each other: fixation stability, saccadic precision, and first saccade landing dispersion.

Figure 5.. Correlation matrix of oculomotor metrics and mean accuracy.

A correlation analysis was completed to measure the strength and direction of association between the oculomotor metrics at the last training session and the overall mean accuracy between the three tasks. Colors indicate Pearson’s R.

Figure 6.. Learning curves of the six oculomotor metrics.

The blue line represents the mean value of a given metric as a function of training session. Orange lines represent the learning curves for each participant (fit to Equation 1). A. First saccade landing dispersion, B. Percentage of useful trials, C. Saccadic Precision, D. Saccadic re-referencing, E. Fixation stability, and F. Saccadic latency.

Figure 7.. Learning curves of the three behavioral tasks.

The blue line represents the mean accuracy value of a given behavioral task as a function of training session. Orange lines represent the learning curves for each participant (fit to Equation 1). A. Face accuracy, B. Object accuracy, C. Letter accuracy.