There is increasing evidence that interocular suppression plays a key role in the visual loss experienced by patients with amblyopia.1–7 In particular, it seems that the neural architecture required for improved visual function may be present but suppressed within the amblyopic visual system,6,8–10 and that interventions aimed at modulating suppression can improve both binocular vision and monocular vision in children and adults with amblyopia.3,11–15 Given the potential importance of suppression, there is a need for tools that can provide an accurate quantitative measure of inhibitory interactions between the two eyes. We have recently developed such a technique that involves dichoptic presentation of global motion stimuli16 and manipulation of the relative contrast of the stimulus components seen by each eye.6 The stimulus is constructed from two populations of moving dots, one population of “signal” dots that all move in the same direction and one population of “noise” dots that move randomly. The observer’s task is to identify the direction of the signal population, and the relative proportion of signal dots relative to noise dots required to perform this task is known as the motion coherence threshold. When the signal dots are shown to one eye and the noise dots to the other eye at equal contrast, the dot population shown to the nonsuppressed eye dominates task performance. However, when the interocular contrast is offset sufficiently in favor of the suppressed eye, it does not matter which eye sees signal and which sees noise, motion coherence thresholds remain constant6 and reflect conditions under which binocular fusion is operative. The size of the contrast offset required to “balance” the two eyes provides an objective assessment of suppression.6,7,17–19
Our initial work with this technique used a series of multiple interleaved staircases whereby motion coherence thresholds were measured for a range of contrast offsets between the two eyes, with signal dots presented randomly to either the fellow or amblyopic eye.6,18–20 Functions were then fit to these data to calculate the balance point contrast. This technique is accurate but time consuming, and we subsequently developed a rapid technique for clinical use17 that consists of two steps. We first measure motion coherence thresholds under binocular viewing conditions (i.e., both eyes seeing the same images at the same contrast). Then, using this threshold, we fix the number of signal dots presented at high contrast to the amblyopic eye and gradually increase the contrast of noise dots presented to the fellow eye using a staircase procedure until task performance is at threshold. Black and colleagues17 have described this abbreviated technique and a comparison with the multiple interleaved staircase technique in detail. This approach is rapid17 and is suitable for assessment of suppression in children.7,14
During a recent series of clinical measurements using the new rapid technique, we found surprisingly low suppression measurements in amblyopic patients with levels of anisometropia of greater than 5 D SED between the eyes. As we had previously found robust suppression in amblyopic patients with comparable levels of anisometropia using our original suppression measurement technique,18 this result was unexpected. One possible explanation was that aniseikonia resulting from optical correction with spectacle lenses21 was providing a cue for task performance, whereby dots seemed larger in the amblyopic eye and could be easily segregated from noise dots presented to the fellow eye. As the rapid technique always presents noise dots to the fixing eye and also gradually increases contrast in the fellow eye over trials, differences in perceived dot size between the two eyes could provide a cue for the segregation of signal and noise dots. This effect would not be so pronounced for the more time-consuming randomly interleaved staircase technique because the type (signal vs. noise) and contrast of dots presented to the amblyopic eye are unpredictable from trial to trial.6
To test this possibility, we assessed suppression in two groups of adult observers with anisometropic amblyopia: one group with less than 5 D SED between the two eyes and one group with more than 5 D SED who were therefore likely to experience aniseikonia.21 Suppression was measured using the standard version of our rapid measurement technique for which dot size is kept constant between the two eyes17 and a modified version of the technique whereby dot size was randomized within the stimulus to render element size uninformative as a cue for segregating signal from noise.
Eleven participants (see Table 1 for clinical details) with anisometropic amblyopia took part in this study and were split into a low anisometropia group (n = 6; mean SED, 3.7 D; SD, 1.7; mean age, 23 years) and a high anisometropia group (n = 5; mean SED, 6.7 D; SD, 0.8; mean age, 22 years). Anisometropic amblyopia was defined according to the Preferred Practice Protocol from The American Academy of Ophthalmology22 with a visual acuity (VA) loss in the worst eye of no worse than 20/200. A group of 10 control participants (five males; mean age, 23.8 years) with normal binocular function also completed the study. Participants provided full written informed consent, and all study protocols adhered to the tenets of the Declaration of Helsinki.
Suppression measurements were made using the rapid technique that we have previously reported17 whereby dot size was kept constant at 0.5 degrees between the two eyes. Measurements were also made using a modified technique whereby dot size was randomized between 0.88 and 1.32 degrees (±20% of the mean dot size of 1.1 degrees). Fig. 1 shows a screenshot of the two different stimuli. A mean dot size of 1.1 degrees was used so that the smaller dots were clearly visible to the amblyopic eye. Because of the larger dot size, the number of dots in the stimulus was reduced from 100 in the original version to 50, and the limited lifetime was set at a 30% chance of any dot being redrawn in a random position from one frame to the next. Stimulus duration was 500 milliseconds for all stimuli, and the dot speed was 6 degrees per second. Stimuli were presented using a video goggle apparatus (Z800 3D Visor; eMagin Corp, Washington, DC) driven by a laptop computer (MacBook Pro; Apple Computer, Cupertino, Calif, running MatLab; The MathWorks, Natick, MA) and the Psychophysics Toolbox, version3.17,23
Participants were familiarized with the task, fully corrected using spectacle lenses, and suppression was measured using the original and modified technique in a randomized sequence. The first stage of the measurement was to assess motion coherence thresholds under binocular viewing conditions (i.e., both eyes seeing the same image). A three-down, one-up staircase with six reversals and a proportional step size of 50% before the first reversal and 25% thereafter was used for this purpose. There were six reversals, and the final five reversals were averaged to provide a threshold estimate. This procedure was repeated a minimum of three times to provide an average binocular motion coherence threshold. During the second dichoptic stage of the measurement, the threshold number of signal dots identified by the binocular motion coherence threshold was presented to the amblyopic eye at 100% contrast, and the remaining noise dots were presented to the fellow eye. The contrast of the noise dots varied using a three-down, one-up staircase technique with a step size of 10% contrast before the first reversal and 5% thereafter. The starting contrast was 0% in the fellow eye (i.e., only the signal dots shown to the amblyopic eye were visible) and the threshold contrast that could be tolerated in the fellow eye was estimated by averaging the final five staircase reversals. This procedure was repeated a minimum of three times to provide an average contrast threshold that represents a measure of interocular suppression.17 In addition, distance VA was measured using a tumbling E chart with logarithmic progression. For control participants, the dominant eye (assessed using the hole-in-card test) was designated as the fellow eye.
A two-way analysis of variance conducted on the suppression measurements revealed a significant interaction between measurement type (original vs. modified) and anisometropia group (low anisometropia vs. high anisometropia) (F1,9 = 61.9, p < 0.0001). As can be seen in Fig. 2, this effect was caused by a significant increase in suppression for the high anisometropia group for the randomized dot size technique (t4 = 7.4, p = 0.002), whereas the low anisometropia group showed no significant difference between the two techniques (t5 = 2.2, p = 0.08). There was no difference between the two techniques for controls (t9, = 0.8, p = 0.9). Fig. 2 also shows measurements made on a separate group of participants using the randomized staircase technique from a previous study.18 The measurements made using the randomized dot size technique are in good agreement with this previous data set.
Importantly, the measurements made with the modified technique correlated with interocular acuity difference, whereby deeper suppression was associated with poorer acuity in the amblyopic eye (Pearson r = −0.8, p = 0.003) (Fig. 3). This is consistent with previous measurements made using the version of the technique that used multiple interleaved staircases.18
An examination of the binocular motion coherence thresholds (Fig. 4) revealed that the modified technique resulted in significantly higher thresholds for all groups (F1,18 = 38.1, p < 0.001); however, within the data for the modified technique, there were no significant differences in motion coherence threshold between controls, low anisometropes, and high anisometropes (controls vs. low anisometropes, t7 = 2, p = 0.09, corrected for inequality of variances; controls vs. high anisometropes, t13 = 1.7, p = 0.1; low vs. high anisometropes, t9 = 0.7, p = 0.9), indicating that the selective increase in suppression for the high anisometropia group for the modified test could not be accounted for by task difficulty.
The aim of this study was to assess the use of a signal/noise technique for measuring suppression that we have recently developed6,17 in amblyopic patients with large degrees of anisometropia (>5 D SED). Our results indicated that suppression could be measured in these patients if interocular size difference cues were overcome by randomizing the element sizes within the stimulus. This modification was only necessary for patients in the high anisometropia group (>5 D SED). Once the size cue was accounted for, we found a strong relationship between the degree of suppression and the acuity loss in the amblyopic eye, providing further support for the importance of suppression in the visual loss that occurs in amblyopia.1,3,18
Two recent studies have used our abbreviated technique to assess suppression in children.7,14 We do not expect aniseikonia to have had a major influence on these results because only 1 of 14 patients in the patient group of Knox et al.14 and 3 of 19 patients in the anisometropic patient group of Narasimhan et al.7 had an SED of more than 5 D. However, based on the data presented here, we recommend that elements with randomized size are used to assess suppression in patients with anisometropia of greater than 5 D SED when using techniques based on the principles of signal/noise segregation.
State Key Laboratory of Ophthalmology
Zhongshan Ophthalmic Center
Department of Optometry and Vision Science
Sun Yat-sen University
People’s Republic of China
This work was supported by the Thrasher Research Fund for Early Career Award to Dr Jinrong Li; the Fundamental Research Funds of State Key Lab of Ophthalmology, Sun Yat-sen University to Jinrong Li; the Guangdong Province International Collaboration Project Grant to Dr Daming Deng (2010B050100014); and a Guangdong Province Medical Science Research Grant (B2011105) to Dr Jinrong Li.
Received September 19, 2012; accepted October 29, 2012.
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