Spontaneous and training-induced cortical plasticity in MD patients: Hints from lateral masking

Abstract

Macular degeneration (MD) affects central vision and represents the leading cause of visual diseases in elderly population worldwide. As a consequence of central vision loss, MD patients develop a preferred retinal locus (PRL), an eccentric fixation point that replaces the fovea. Here, our aim was to determine whether and to what extent spontaneous plasticity takes place in the cortical regions formerly responding to central vision and whether a visual training based on perceptual learning (PL) can boost this plasticity within the PRL area. Spontaneous and PL-induced cortical plasticity were characterized by using lateral masking, a contrast sensitivity modulation induced by collinear flankers. This configuration is known to be sensitive to neural plasticity and underlies several rehabilitation trainings. Results in a group of 4 MD patients showed that collinear facilitation was similar to what observed in age- and eccentricity-matched controls. However, MD patients exhibited significantly reduced collinear inhibition, a sign of neural plasticity, consistent with the hypothesis of partial cortical reorganization. Three AMD patients from the same group showed a further reduction of inhibition after training, but not controls. This result suggests that PL might further boost neural plasticity, opening promising perspectives for the development of rehabilitation protocols for MD patients.

Conflict of interest statement

The authors declare that they have no competing interests.

Figures

Figure 1

Schematic of the interaction between perceptual field (PF) size and collinear configuration as a function of eccentricity. X-axis represents eccentricity from central fixation. In foveal vision, the standard target-to-flankers distance that induces facilitation is 3λ (i.e., 3 times the wavelength of the Gabor patches of the configuration). In the present study, the spatial frequency was kept fixed at 1 cpd, therefore λ = 1 cpd. In this case, this distance also corresponds to the PF size and facilitation is considered as a between-PFs effect. When the same stimulus is presented in peripheral vision, the PF size increases, so that the 3λ configuration elicits within-PF, collinear inhibition. In order to restore facilitation, flankers must be placed outside the PF. Previous studies– showed that this critical distance is 8λ at an eccentricity of 4°.

Figure 2

OCT procedure for PRL localization. (a) Retinal analysis via OCT: Infrared 2D macular image of the ‘cross fixation’ acquisition showing the macular atrophy (ligther rounded area). In this image, the center of the red dashed cross corresponds to the preferred retinal locus (PRL), the eccentric fixation point replacing the fovea. (b) Autofluorescence imaging with the location of the anatomical fovea (green dot) and the PRL position (red dot). (c) Schematic retinal projection of the stimulus on the PRL location close to the scotoma (dark area); green dot is the location of the fovea.

Figure 3

PRL position for participants in Experiment 1. Coordinates were averaged across three consecutive OCT measurements (see the ‘OCT method’ section).

Figure 4

Experimental paradigm. Patients were instructed to fixate monocularly the point in the center of the screen with their PRL. In order to maximize fixation stablity, we displayed three additional red disks along the internal border of their scotoma (dark grey on the figure), selected individually for each patient within 2 deg from the border of the absolute scotoma. Patients had to adjust their eye position so that these points remained invisible. After 1000 ms, three vertically presented Gabors, either in a collinear (or orthogonal) configuration, appeared at this central position for 133 ms with an Inter Stimulus Interval (ISI) of 500 ms. In the collinear configuration, the flankers were presented above and below the target with the same (vertical) local orientation. In the orthogonal configuration, the flankers appeared above and below the target but their local orientation was horizontal (90° offset with respect to the target). Patients had to report which interval contained the target. Each control participant had to fixate foveally while the configuration was displayed at the eccentricity used in his/her paired MD patient.

Figure 5

Box-and-whisker diagrams for collinear and orthogonal contrast thresholds (Michelson contrast) for MD (red boxes) and Control participants (green boxes) as a function of the target-to-flankers distances (λ). In each boxplot, the central mark is the median. The edges of the box are the 25th and 75th percentiles and the whiskers are the interquartile range (i.e., Q3–Q1). The black circles represent the mean and the vertical bars are +/− SEM.

Figure 6

Box-and-whisker diagrams for collinear (above) and orthogonal (below) Michelson contrast thresholds for MD (red boxes) and Control participants (green boxes), before and after training as a function function of the target-to-flankers distances. See Fig. 5 for the details of the legend.

Figure 7

Single participant data for the training, divided by target-to-flankers distances (3 to 8 lambda, from lighter green to darker green for controls, from lighter red to darker red for MD patients).