Table 4 Type of study and outcome for intervention studies

Chung 2007 ²⁸

Cohort study Perceptual Learning (PL)

N = 8Adults NV

Training: Identifying middle letter trigram at 10° in inferior visual field (0.8× x-height letter separation). 6 sessions = 6000 trials (6 days)Pre-test/Post-test:1) reading speed for 6 print sizes;2) flanked letters identification at 5 separations (0.8×,1×,1.25×,1.6×, and 2×).

1) Maximum reading speed did not improve significantly. Significant reduction in critical print size after training.2) Accuracy for identifying target in a trigram improved significantly (88% improvement).Spatial extent decreased significantly from 1.12x to 0.69x the letter size after training.

Green 2007 ²⁹

Non-Randomised controlled trial (Non-RCT) Video-game playing (VGP)

Exp. Group: N = 16 Adults NV Control GroupN = 16 Adults NV (all non-videogame players)

Training:-Experimental group: high intensity action videogame;-Control group: less visually intense videogame.30 h training (4–6 weeks) Pre-test/Post-test:Identification middle T trigram at 0°, 10° and 25° (VGPs vs. non-VGPs).

Only the action videogame group showed a significant decrease in crowding region (all eccentricities).No improvement single T acuity after training.

Huckauf 2007 ¹³

Non-RCT PL

Training 1: N = 10 (no training); N = 10 (training with feedback target); N = 10 (training with feedback flanker).Training 2: N = 4

Training 3: N = 24 Training 4: N = 11

Adults NV

Training 1: Identify flanked target letters. Always same target/flank combination at 4° and 7°, 1° center-to-center spacing (25 min. training). Training 2: Random assignment trained flankers (letters and unfamiliar symbols). Eccentricities 1°,4° and 7°.2 h/day (3 days; 1980 trials).Training 3: String training at a defined eccentricity of 3° in one of the two visual fields. Participants were measured at three time points: after 144 trials (short training), and an additional 720 trials (long training). Twelve were retested 24 h after training.Training 4: Same as Experiment 3, but also presentation of isolated letters during training and test.

Training 1: Crowding significantly reduces for trained strings and less for untrained strings (specificity effect). No difference between training groups.Training 2: Crowding effects do not reduce when letter combinations differ from trial to trial (specificity effect).

Training 3: 16% improvement after short training and 28% improvement after long training. After 24 h, performance was significantly better than at baseline, but did not differ from performance after short or long training.

Training 4: Transfer occurred earlier when words were used as stimuli. Isolated letter recognition showed no significant improvement after training, flanked letter recognition improved significantly.

Maniglia 2011 ³⁰

Cohort study PL

N = 8 Adults NV

Training: Contrast detection of a Gabor target presented in at 4° in the presence of co-oriented and co-aligned high contrast Gabors.160 sessions ≈ 60.000 trials (8 weeks)Pre-test/Post-test1) Visual Acuity2) Crowded acuity3) Contrast sensitivity

1) Visual Acuity did not improve in peripheral vision.2) Crowding reduced significantly in peripheral vision. Observers could better identify a target in a cluttered background.3) Training lateral interactions only reduced contrast sensitivity at the highest spatial frequency used.

Li 2011 ¹⁷

Non-RCT VGP

N = 10(action videogame group) N = 3(non action videogame group) N = 7(crossover control group; 20 h occlusion, 40 h video game therapy) Adults with amblyopia

Training: Action videogame group (n = 10), non-action videogame group (n = 3) and cross-over control group (n = 7).40-80 h videogame playing.Pre-test/Post-test1)Visual Acuity (Bailey-Lovie logMAR charts)2) Positional acuity;3) Spatial attention;4) Stereopsis.

1.1) On average 1.4 to 1.6 lines improvement of acuity after action videogame;1.2) Non-action videogame players improved 1.5 lines on crowded letters and 0.8 lines for single letters. Patching group no improvement in visual acuity after 20 h. Recovery crowded acuity slightly faster than single. Mean crowding index did not significantly improve.2) Positional acuity improved significantly;3) Spatial attention improved significantly;4) Stereopsis improved significantly.

Sun 2011 ¹⁶

Cohort study PL

N = 6Adults NV

Training: Same as Chung²⁸.Pre-test/Post-test Identification letter in 2 flanking conditions (unflanked/flanked) crossed with four noise levels.

Accuracy improvement in identifying letters in flanked condition without noise (22%). Training improves efficiency or equivalent input noise in a subject-dependent matter.-Retained improvements after 1–6 months.

Hussain 2012 ³¹

Non-RCT PL

N = 10(of which 5 served as a control group that trained after performing 2 pre-tests). Adults with amblyopiaN = 10 (training group)N = 7 (control group) Adults NV

Training: Identifying central target letter (1.4 × threshold size) surrounded by 4 letter in each cardinal orientation. Adults with amblyopia = foveal training. Adults with NV = 4° eccentricity.8–14 sessions (3600–9600 trials)Pre-test/Post-test1) unflanked acuity fellow eye;(2) unflanked acuity amblyopic eye;(3) flanked acuity fellow eye at a spacing of 1.1× letter size; (4) flanked acuities amblyopic eye at spacing of 1.1×, 1.2×, and 1.4× letter size.

1) Unflanked and flanked acuity both significantly improved in the fellow eye. Difference not significant.2) Unflanked acuity improved significantly.3) More progress for flanked than unflanked acuity.Significant improvements on Bailey-Lovie chart on average 1.5 lines.Comparable results for adults with NV in periphery (no improvement for control group). Two follow up participants performed additional sessions and showed a further significant decrease in their crowding ratio’s (after performing 1–11 additional sessions).