Strabismic amblyopia disrupts the hemispheric asymmetry for spatial stimuli in cortical visual processing

Introduction

Amblyopia is a highly complex, developmental disease in which visual acuity is reduced in one eye leading to deprivation (form-deprivation/opacification or blur) or strabismus in the absence of eye disease (Kanonidou, Proudlock, & Gottlob, 2010). Since the classical Hubel and Wiesel studies showing reduction in cortical cells receiving input from the amblyopic eye (Hubel, 1979, 1982; Hubel & Wiesel, 1964) and impairment of binocularity in recordings of cortical neurons (Hubel & Wiesel, 1965, 1970, 1973), no other relevant changes were reported.

In recent years, a body of studies has been found interesting results suggesting a possible other dramatic alteration occurring in the visual cortex of amblyopic subjects. In addition to the motion impairment related to a decrease in the middle temporal (MT)/V5 are activation (El-Shamayleh, Kiorpes, Kohn, & Movshon, 2010; Thompson, Villeneuve, Casanova, & Hess, 2012), the loss in contrast sensitivity (CS) related to parvocellular (PC) (Hess, Li, Lu, Thompson, & Hansen, 2010; Hess, Li, Mansouri, Thompson, & Hansen, 2009) and, sometimes, also to magnocellular (MC) (Zele, Pokorny, Lee, & Ireland, 2007; Zele, Wood, & Girgenti, 2010) visual pathways that do not add much to the Hubel and Wiesel results, damage in a more selective and complex functions as decision-making (Farzin & Norcia, 2011) and deficits of reading (Barban et al., 2010; Farzin & Norcia, 2011). However, the most relevant finding was the deficits in neuron network present in all levels of the visual pathway tested (thalamo-striate and striate-extra striate networks), but the more strong effects were found for thalamo-striate feed forward input and the presence of an anomalous connectivity asymmetry ipsilateral to the amblyopic eye. Previous studies also reported the absence of motion processing symmetry in primary visual cortex area (V1) (Hou, Pettet, & Norcia, 2008; Norcia, 1996; Norcia, Wesemann, & Manny, 1999) and reading impairment dyslexia-like (Barban et al., 2010) in strabismic and not in anisometropic children.

Considering that some studies found hemispheric asymmetries for cognitive functions as judgment of simple visual elements (Avrahami, Argaman, & Weiss-Chasum, 2004), global perception in right hemisphere, and local processing in left hemisphere (Charras & Lupianez, 2009, 2010; Evert & Kmen, 2003; Weissman & Woldorff, 2005), we argued the possibility of an earlier asymmetry in the processing of basic visual elements. Thereby, we decided to investigate the very plausible hemispheric asymmetries in visual processing, related to spatial stimuli considering the presence of anisometropia or strabismus. The presence of such asymmetry supports high-level processing impairment, as decision-making or reading had not been correlated with the impairment in the visual function presented by the strabismic subjects (Avrahami et al., 2004; Hou et al., 2008; Norcia, 1996; Norcia et al., 1999).

To date, functional connectivity patterns in primary visual area have been reported in amblyopic subjects showing significant decreases in functional connectivity with the primary visual area in the inferior parietal lobule and the posterior cerebellum in both anisometropic amblyopia and mixed amblyopia (J. Ding, Klein, & Levi, 2013). Based on that background suggesting cortical disruption in connectivity and behavioral reduction in reading performances in strabismic amblyopia are not found in anisometropic amblyopia, we decided to investigate how functional cortical activation and behavioral performances in visual tasks preferentially stimulating ventral and dorsal pathways occur in strabismic and anisometropic amblyopia.

Considering the previous reports of global-local processing impairment, motion asymmetry persistence in strabismic patients, and the anomalous thalamo-striate connectivity in strabismic amblyopia, we hypothesize that strabismus promotes an impairment on the cortical functional architecture not presented in the anisometropia. Thus, we investigated the functional processing for spatial stimulus for the two main streams of visual information, the MC and PC pathways, using low and high spatial frequency measurements.

The study was conducted in accordance with the Declaration of Helsinki. All participants provided verbal and written informed consent before participating in the study. Participants lay supine in the scanner and were given earplugs and padding to maximize comfort and minimize involuntary head movements. A video recording system (RealEye 5721, Avotec Inc.) inside the scanner was used to ensure safety and wakefulness. At any time, participants had access to a button signaling that they wished to interrupt the session.

Psychophysics

We measured the CS from both eyes, monocularly, using a stimulus with eccentricity of 6°, to the left and right visual fields from a central fixation point. A staircase procedure was used and the test ends when 11 reversals were obtained. Threshold was calculated considering the last five reversals. We tested four spatial frequencies including low (0.3 and 0.5 cycles per degree [cpd]), middle (2.0 cpd), and high (8.3 cpd) in each hemifield (Figure 1(a)).

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Figure 1.

Timeline of the psychophysical and functional magnetic resonance imaging stimulus presentation: (a) sinusoidal gratings were used for the contrast sensitivity measurements for the right and left visual fields; (b) chessboard was used to evoke cortical responses. Low spatial frequency squares with high temporal rate of polarity reversal and low luminance were used to isolate the MC-mediated responses. High spatial frequency with low temporal rate of reversal squares was used to isolate the PC-mediated responses.

Functional magnetic resonance imaging

Stimuli were pattern-reversed checkerboards filling the entire screen (30.3 cm × 23.1 cm) designed using the Psychophysics Toolbox for MATLAB. To bias the MC pathway, we used stimuli with low spatial frequency, 0.25 cpd fundamental frequency, and high temporal frequency (18 Hz), with low contrast (18%) that preferentially activates the MC pathway. The bias to PC pathway was obtained by stimulation using a higher spatial frequency (2 cpd), lower temporal frequency (2 Hz), and 100% contrast. Stimuli were presented in a block design composed of six blocks (Figure 1(b)). The blood oxygenation level–dependent (BOLD) effect was measured, for each hemisphere in a contrast of MC versus PC over the visual primary area.

Scanning was performed on a 3T Siemens TimTrio scanner at the Portuguese Brain Imaging Network, using a 12-channel birdcage head coil. For each participant, we acquired (a) two T1-weighted (T1w) magnetization-prepared rapid acquisition gradient echo (MP-RAGE) sequences, 1 × 1 × 1 mm voxel size, repetition time (TR) = 2.3 s, echo time (TE) = 2.98 ms, flip angle (FA) = 9u, field of view (FOV) = 256 × 256, 160 slices; (b) a T2-weighted (T2w) fluid attenuation inversion recovery (FLAIR) sequence, 1 × 1 × 1 mm voxel size, TR = 5 s, TE = 2.98 ms, inversion time (TI) = 1.8 s, FOV = 250 × 250, 160 slices; and (c) one run of functional magnetic resonance imaging (fMRI) scanning for the MC/PC-biased stimuli and two runs of the polar angle stimuli using single shot echo planar imaging (EPI) acquired in the axial plane orthogonal to the anterior commissure covering the occipital, temporal, and frontal cortices, with 2 × 2 × 2 mm voxel size, TR = 2 s, TE = 39 ms with a 128x128 matrix, FA = 90u, FOV = 256 × 256, 26 slices.

For anatomical image processing, a high-resolution T1w anatomical images were averaged and intensity normalized and reoriented into a space where the anterior commissure and the posterior commissure lie in the same plane (AC-PC). Afterward, cortex was segmented using automatic segmentation routines, and mesh representations of each hemisphere were created and inflated for polar angle map projection.

For functional image processing, we applied slice scan time correction, linear trend removal, temporal high-pass filtering (two cycles per run), modest spatial smoothing (full width at half maximum [FWHM] = 2 mm, yielding a small but clear smoothing effect), and a correction for small interscan head movements. Participants were excluded from further analysis if any within-run movement exceeding 2 mm was detected (n = 2 NF1; four controls).

The data were preprocessed and analyzed using Brainvoyager QX 2.3 (Brain Innovation, Maastricht, The Netherlands). Slice-scan-time correction, head-motion-correction, as well as temporal high-pass filtering were applied and subsequently anatomical and functional data were spatially normalized to the Talairach coordinate system (Lancaster et al., 1997; Lancaster et al., 2000). The synchronic interpolation option was used to transform each brain into the size of the standard Talairach brain using manually specified reference points. Within-run movement exceeding more than 3 mm led to the exclusion of a run for further analysis. In addition, spatial smoothing (8 mm) of individual datasets was applied only for the group analysis.

Statistical analyses were performed on individual and group data using the general linear model (GLM). Predictors for the response to MC- and PC-biased stimulations were obtained by convolution of a condition box-car time course with a two-gamma function. Pre-defined areas were used as regions-of-interest (ROI) to perform the ROI analysis on each subject in the AC-PC space. Within each ROI, a GLM for the MC/PC-biased stimuli experiment, corrected for temporal serial correlations, was computed and the individual beta values evoked by each stimulus were retrieved and analyzed. In the first stage, a whole-volume random effect (RFX) GLM was performed to estimate condition effects (beta values) separately for each subject and stimuli. At the second level, an independent sample t test was used to compare patients and controls for each stimulus category. Statistical maps were corrected for multiple comparisons using cluster-size thresholds. Hemispherical asymmetries were defined by probabilistic comparisons between PC × MC cortical activation based on Statistical Surface Maps, in which a contrast surface analysis was performed at each voxel, outputting that the probability of PC or MC dominates the responses in that cortical area.

Normal subjects (n = 10; mean age = 25.3; standard deviation [SD] = 3.8; four females) presented a higher CS for low spatial frequency when presented in the left visual field – right hemisphere (LVF-RH) (F = 12.548; p = .002), whereas high spatial frequencies showed higher CS when presented in right visual field – left hemisphere (RVF-LH) for two spatial frequencies – 2.0 cpd (F = 4.582; p = .021) and 8.3 cpd (F = 8.561; p = .001) (Figure 2(a)). Accordant results were obtained in the fMRI. Contrast analysis found more MC activity (related to low spatial frequencies processing – in yellow) on the right hemisphere, whereas the PC activity (related to high spatial frequencies processing – in blue) was found more intense in the left hemisphere (Figure 2(b)).

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Figure 2.

(a) In psychophysics, significant higher CS were obtained for low spatial frequency presented at the left visual field – right hemisphere (LVF-RH) and for high spatial frequencies when presented at right visual field – left hemisphere (RVF-LH). (b) BOLD effect at the fMRI measurements for the calculated MC × PC contrasts at visual primary area (V1). The left panel shows a statistical activation dominated by the MC pathway (yellow activation) and the right panel shows the activation dominated by the PC pathway (blue activation), evidencing a lateral hemispheric asymmetry for spatial frequencies processing. Low spatial frequency (inferred MC pathway) was markedly processed at the right hemisphere, whereas the high spatial frequency (inferred PC pathway) was processed at the left hemisphere.

The hemispheric asymmetry found in fMRI normal subjects has no correlates in the literature. Since asymmetries were only measured for perceptual and cognitive functions as judgment for simple visual elements related to their diagonal spanning from top right to bottom left or vice-versa (Avrahami et al., 2004) inferring a more detailed processing against a more spatial configuration and for the recurrent and widely validated studies showing that spatial processing is related to right hemisphere and local processing to the left hemisphere (Charras & Lupianez, 2010; Evert & Kmen, 2003; Weissman & Woldorff, 2005), our data support the idea of an earlier differentiation in the sensory pathway.

We also studied the hemispheric asymmetry in the two main types of amblyopia, a developmental disease that reduces the detail discrimination ability for spatial elements with clinically normal eyes. Since reduction in cortical cells receiving input from the amblyopic eye (Hubel, 1979, 1982; Hubel & Wiesel, 1964) and impairment of binocularity in recordings of cortical neurons (Hubel & Wiesel, 1965) are well established and there are reports of spatial impairment that are more related to MC pathways (Aaen-Stockdale & Hess, 2008; Aaen-Stockdale, Ledgeway, & Hess, 2007; Barban et al., 2010) adding new findings to the PC pathway damage, related to the visual acuity reduction, we argue whether the hemispheric asymmetry could be affected by the disease.

We tested five amblyopic subjects – three subjects with anisometropic and two with strabismic amblyopia. All subjects were diagnosed with a mild amblyopia in which the visual loss was around 1–2 octaves worse than the normal visual acuity (Table 1).

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Table 1.

Orthoptic data of the control and amblyopic subjects.

Group	ID	Sex	Age (years)	VA/OD	VA/OE	Stx	RX OD	RX OE	History
A	CPP	M	24	20/15	20/30	160″	Plano	−3.25	XT2; a occlusion since 10 years for 3 months; RX since 10 years
S	CO	F	32	20/20	20/40	250″	−1.25	Plano: −1.25X115°	ET10; a occlusion since 8 years for 2 years; RX since 4 years
S	AS	F	23	20/50	20/20	Absent	+2.00; −0.75X30	+1.75: −1.75X180	ET12 a HTe/d10; a occlusion since 3 years for 2 years; Rx since 3 years; surgery correction with 7 years
A	RL	M	25	20/80	20/25	250″	−1.75	−0.50	Ortho
A	TP	F	25	20/25	20/60	250″	−7.50	−6.75	XT4; a RX since 5 years

A: anisometropic amblyopia; S: strabismic amblyopia; RX: refraction; Stx: stereopsis (seconds of arc); ortho: eye alignment under cover-uncover test; X: exoforia; E: endoforia; XT: exotropia; ET: endotropia; VA: Snellen visual acuity; OD: oculus dexter.

a

Prismatic dioptry.

Psychophysical and the fMRI results showed different cortical processing regarding the amblyopic type. In anisometropic amblyopia, the CS is more impaired compared to the strabismic amblyopia mainly for the higher spatial frequencies tested. However, we found a preserved hemispheric asymmetry in spatial frequency processing in the anisometropic amblyopia. For low spatial frequencies, the CS is better for the LVF-RH. Reduced CS for high spatial frequencies is not allowed in accessing the hemispheric asymmetry in psychophysics. Subjects with strabismic amblyopia showed similar CS values to the normal subjects, but no asymmetry in psychophysical measurement of the CS (Figure 3, right panel).

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Figure 3.

Upper panel: The mean values of the CS measured (a) for the two subjects with strabismic amblyopia and (b) for the three subjects with anisometropic amblyopia were compared with the mean CS obtained for 10 normal subjects – black line. The dark gray line indicates the CS measured for the LVF-RH and the light gray line indicates the CS measured for the RVF-LH. Normal CSs were measured in the strabismic amblyopia, but no evidences of hemispheric asymmetries were found. An opposite profile was obtained for the anisometropic amblyopia. Reduced CSs for almost all spatial frequencies compared with adults, but there is a strong evidence for hemispheric asymmetry. Lower panel: The BOLD effect at the fMRI measurements for the calculated MC × PC contrasts at visual primary area (V1) in subjects with anisometropic amblyopia (line A) and in subjects with strabismic amblyopia (line B). The left panel shows the activation dominated by the MC pathway and the right panel shows the activation dominated by the PC pathway. Lateral hemispheric asymmetry was present only for the anisometropic subjects. The cortical processing in strabismic subjects shows no asymmetry and an activity dominated by the MC pathway.

The BOLD effect of the fMRI in the amblyopic subjects supports the psychophysical findings in which the loss of hemispheric asymmetry was measured for strabismic amblyopia instead of a similar to normal hemispheric asymmetry measured for anisometropic amblyopia (Figure 3, left panel).

Our data support the idea of a hemispheric asymmetry for the processing of simple visual elements. These findings project forward our understanding of the functional architecture of primary visual cortex, greatly expanding our understanding of visual cortical functioning. Our results also point to the existence of fundamental differences in the pathophysiology of amblyopia, showing that the presence of strabismus generates a large damage of anatomical and functional development of visual pathways, not observed in anisometropia.

Hubel and Wiesel were pioneers in studying the cortical architecture of the visual cortex, identifying different visual cells based on their physiological and functional response to visual stimuli, as simple cells, complex cells, and hypercomplex cells (Hubel & Wiesel, 1968, 1969), and their anatomical organization in eye-dominance columns (Hubel & Wiesel, 1962, 1963). More recent important contributions in understanding the visual cortex structure and functions were achieved since their works but almost confirm their results of columnar architecture and the physiological function variety they found (Adams & Horton, 2009; Hess, Li, et al., 2010; Hess, Thompson, Gole, & Mullen, 2010). Recently, a significant decrease in functional connectivity at the primary visual area in the inferior parietal lobe and in the posterior cerebellum has been shown in both anisometropic amblyopia and mixed amblyopia subjects (K. Ding, Liu, Yan, Lin, & Jiang, 2013).

The loss we found in the anatomical and functional connectivity patterns can give us clues to understand other effects related to performance on visual tasks in patients with strabismus in which younger children and adults with strabismus, unlike younger children and adults with anisometropia, present as motion perception impairment (Norcia, 1996; Thompson et al., 2011; Thompson et al., 2012), global spatial perception (Hamm, Black, Dai, & Thompson, 2014), visual decision-making tasks (Farzin & Norcia, 2011), and reading difficulties (Barban et al., 2010). Considering the strabismic amblyopia, reduced cytochrome oxidase, indicating decrease in cortical activity, was found in cortical layers 2/3/4A and 4B compared to layers 4C 5/6 of the amblyopic eye (Adams, Economides, & Horton, 2015) and could be related with our imaging results. There are also recent evidences that the contralateral non-amblyopic eye also have functional impairments in strabismic amblyopia (Gao et al., 2014). It is important to take in mind that the lack of cortical asymmetry in strabismus suggests that a different developmental process occurs in those patients than in the anisometropic amblyopia patients. Strabismus affects more deeply the developmental process of visual pathways that we and our data suggest that these occur in the functional level and, most likely, anatomical too.

In conclusion, we presented disruption in the cortical hemispheric asymmetry in processing low and high spatial frequency components of the spatial vision in V1 and in spatial function of patients with strabismic amblyopia. Higher level functions as motion perception, visual task decision-making, and reading difficulties impaired in strabismic amblyopia but spared in anisometropic amblyopia could be related to a more lower level impairment. It is also important to say that even after several decades of study on the primary visual system and the deprivation and strabismic effects in visual function, many remains to be understood about their normal development and in disease conditions.