Yes / Reduced binocularity is a prominent feature of amblyopia and binocular cues are thought to be important for prehension. We examine prehension in individuals with amblyopia when the target-object was flanked, thus mimicking everyday prehension. Methods: amblyopes (n=20, 36.4±11.7 years; 6 anisometropic, 3 strabismic, 11 mixed) and visually-normal controls (n=20, 27.5±6.3 years) reached forward, grasped and lifted a cylindrical target-object that was flanked with objects on either (lateral) side of the target, or in front and behind it in depth. Only 6 amblyopes (30%) had measurable stereoacuity. Trials were completed in binocular and monocular viewing, using the better eye in amblyopic participants. Results: Compared to visual normals, amblyopes displayed a longer overall movement time (p=0.031), lower average reach velocity (p=0.021), smaller maximum aperture (p=0.007) and longer durations between object contact and lift (p=0.003). Differences between groups were more apparent when the flankers were in front and behind, compared to either side, as evidenced by significant group-by-flanker configuration interactions for reach duration (p<0.001), size and timing of maximum aperture (p≤0.009), end-of-reach to object-contact (p<0.001), and between object contact and lift (p=0.044), suggesting that deficits are greatest when binocular cues are richest. Both groups demonstrated a significant binocular advantage, in that in both groups performance was worse for monocular compared to binocular viewing, but interestingly, amblyopic deficits in binocular viewing largely persisted during monocular viewing with the better eye. Conclusions: These results suggest that amblyopes either display considerable residual binocularity or that they have adapted to make good use of their abnormal binocularity.

Yes / Binocularity in the human visual system poses two interesting and extremely challenging questions. The first, and perhaps most obvious stems from the singularity of perception even though the neural images we see originate as two separate images in the right and left eyes. Mechanistically we can ask how and where do we convert two images into one? The second question is more of a “why” question. By converting lateral eyes with their inherent panoramic visual field into frontal eyes with overlapping binocular visual fields, primates have developed an extremely large blind region (the half of the world behind us). We generally accept that this sacrifice in visual field size was driven by the potential benefit of extracting information about the 3rd dimension from overlapping right and left eye visual fields. For some people, both of these core processes of binocularity fail: a single fused binocular image is not achieved (when diplopia or suppression is present), and the ability to accurately represent the 3rd dimension is lost (stereo-blindness). In addition to these failures in the core functions of the human binocular system, early imbalances in the quality of right and left eye neural images (e.g. due to anisometropia, monocular deprivation, and/or strabismus), can precipitate profound neurological changes at a cortical level which can lead to serious vision loss in one eye (amblyopia). Caring for patients with malfunctioning binocular visual systems is a core therapeutic responsibility of the eye care professions (optometry, ophthalmology and orthoptics) and significant advances in patient care and subsequent visual outcomes will be gained from a deeper understanding of how the human brain accomplishes full binocular integration. This feature issue on binocular vision brings together original articles and reviews from leading groups of neuroscientists, psychophysicists and clinical scientists from around the world who embrace the multidisciplinary nature of this topic. Our authors have taken on the big issues facing the research community tasked with understanding how binocular vision is meant to work, how it fails, and how to better treat those with compromised binocularity. These studies address deep issues about how the human brain functions and how it fails, as well as how it can be altered by therapy.

As disturbed binocular functions in small children may lead to severe amblyopia it is of interest to detect it as early as possible. Most tests for binocular functions, however, demand active cooperation and may be unreliable in children up to 4-5 years of age. This study therefore aims to employ visual evoked potentials (VEP) to enable the examiner to evaluate the binocular function in a subject without need of active cooperation from the subject. Initially we studied the relation of suprathreshold contrast to the latency of the transient pattern VEP (tpVEP). Although suprathreshold contrast independently influenced the tpVEP latency, interindividual variation was too large to suggest tpVEP as a possible method for objectively measuring contrast sensitivity in a subject. The tpVEP latency in normal and microstrabismic adult subjects was examined. It was significantly shorter with binocular viewing in normals, but not in the microstrabismic group. Contrast sensitivity and tpVEP latency was examined in adults, both with normal binocularity and with microstrabismus, using both luminance (black-and-white) contrast and colour contrast patterns. The tpVEP latency to colour contrast, like that to luminance contrast, is shorter in normal subjects who view the stimulus binocularly. Interindividual variation or overlap between the normal and microstrabismic groups did not improve with colour contrast. The most significant features of the tpVEP are amplitude and latency. Depending on stimulus conditions, the response may show variations in configuration, amplitude and, to a lesser degree, latency. To decrease the influence of such variations steady-state VEP (ssVEP) can be used. The stimulus is presented in a fast repetitive manner, yielding a VEP response shaped as a continuous curve. The frequency components of this curve can be analysed using Fast Fourier Analysis. Fast Fourier analysis of ssVEP in children aged 8-15 years with normal binocularity and with microstrabismus showed that the power of the second harmonic (the double frequency of stimulus frequency) of the response with binocular viewing was larger than with monocular viewing, both in normals and microstrabismic subjects. For higher stimulus frequencies, microstrabismic subjects showed a significantly lower power of the second harmonic compared with subjects with normal binocularity, when the stimulus was presented binocularly. Finally, Fast Fourier analysed ssVEP in pre-school children aged 4-5 years was studied. A normal group was compared with a group with microstrabismus and a group with significant amblyopia. Amblyopic subjects had significant interocular differences in the first harmonic. We confirmed the significant difference found between microstrabismic subjects and subjects with normal binocularity regarding the second harmonic’s power with higher temporal frequency binocular stimulation, although at a slightly lower frequency than for older children. A low power of the second harmonic in the ssVEP to a binocular stimulus with high temporal frequency is a strong indicator of disturbed binocular function. / För att förhindra amblyopi (ensidig synsvaghet) hos barn är det viktigt att upptäcka störningar i samsynsfunktionerna så tidigt som möjligt. Samsynstester kräver dock aktiv medverkan och kan ge osäkra resultat hos barn upp till 4-5 års ålder. Den här avhandlingen studerar möjligheterna att utifrån tidsmässiga (temporala) egenskaper hos visual evoked potentials (VEP) undersöka samsynsfunktioner objektivt, utan att den undersökte behöver medverka aktivt. Första delstudien visar att ett synstimulus kontrastnivå i relation till kontrastkänslighetströskeln oberoende påverkar latensen i VEP, men variationer mellan individer gör metoden olämplig som objektiv kontrastkänslighetstest. Andra delstudien jämför latensen i VEP hos individer med normal samsyn med den hos personer med mikroskelning. Stimulering av båda ögonen gav signifikant kortare latens än stimulering av ett öga hos normala, men inte hos mikroskelare. I den tredje delstudien jämfördes känslighet för luminanskontrast och färgkontrast hos individer med normal samsyn och personer med mikroskelning. Både luminansmönster (svart-vita) och färgkontrastmönster upptäcktes vid lägre kontrast om båda ögonen stimulerades istället för ett i taget hos normalseende. Mikroskelare uppfattade mönstren sämre med båda ögonen än med ett öga (det dominanta). Latensen i VEP mättes i båda grupperna för både luminans- och färgkontrastmönster. Båda typerna av kontrast gav förkortning av latensen när båda ögonen stimulerades vid normal samsyn, men denna förkortning uteblev hos mikroskelare. Både luminans- och färgkontrast gav för varierande resultat för att utnyttja metoden för objektiv undersökning av samsynen. Fourier-analys innebär att en kurvform delas upp i sinuskurvor med olika frekvens, amplitud och fas. Om ett stimulus växlar hastigt får man ett steady-state VEP (ssVEP), dvs en kontinuerligt vågformad VEP-kurva, som kan delas upp i delsinuskurvor med Fourieranalys. Detta gör att man särskilt kan studera frekvenser som är relaterade till stimuleringsfrekvensen, till exempel grundton och övertoner (multipler av grundtonsfrekvensen). Barn 8-15 år gamla, med normal samsyn och med mikroskelning undersöktes med ssVEP i det fjärde delarbetet. Den första övertonen (= ”second harmonic”) var statistiskt signifikant svagare hos mikroskelare jämfört med normala individer, när stimuleringsfrekvensen var hög. Det femte och sista delarbetet undersökte ssVEP hos 4-5 år gamla förskolebarn på motsvarande sätt. I denna studie deltog också en grupp barn med amblyopi (synsvaghet) på ena ögat. På en något lägre stimuleringsfrekvens bekräftades den svagare första övertonen hos barnen med mikroskelning jämfört med barnen med normal samsyn. De amblyopa barnen visade tydligast förändringar vid lägre stimuleringsfrekvenser och i ssVEP:s grundtonsfrekvens (= ”first harmonic” eller ”fundamental harmonic”). Resultaten i de olika grupperna är så pass åtskilda att metoden verkar lämpa sig för objektiv undersökning av samsynsfunktioner, i det att en svag första överton i binokulärt ssVEP utlöst av hög stimuleringsfrekvens inger en stark misstanke om störd samsyn, medan en stor skillnad i grundtonens styrka i höger respektive vänster ögas ssVEP tyder på amblyopi.