Altered visual population receptive fields in human albinism

Albinism is a congenital disorder where misrouting of the optic nerves at the chiasm gives rise to abnormal visual field representations in occipital cortex. In typical human development, the left occipital cortex receives retinal input predominantly from the right visual field, and vice-versa. In albinism, there is a more complete decussation of optic nerve fibers at the chiasm, resulting in partial representation of the temporal hemiretina (ipsilateral visual field) in the contralateral hemisphere. In this study, we characterize the receptive field properties for these abnormal representations by conducting detailed fMRI population receptive field mapping in a rare subset of participants with albinism and no ocular nystagmus. We find a nasal bias for receptive field positions in the abnormal temporal hemiretina representation. In addition, by modelling responses to bilateral visual field stimulation in the overlap zone, we found evidence in favor of discrete unilateral receptive fields, suggesting a conservative pattern of spatial selectivity in the presence of abnormal retinal input. Highlights We characterized population receptive fields in albinotic participants with no ocular nystagmus Confirmed overlapping representations of left and right visual fields in cortex Detected nasal bias in receptive field location for temporal hemiretina representation Evidence in favor of discrete unilateral receptive fields and against double coding


Abstract
Albinism is a congenital disorder where misrouting of the optic nerves at the chiasm gives rise to abnormal visual field representations in occipital cortex. In typical human development, the left occipital cortex receives retinal input predominantly from the right visual field, and viceversa. In albinism, there is a more complete decussation of optic nerve fibers at the chiasm, resulting in partial representation of the temporal hemiretina (ipsilateral visual field) in the contralateral hemisphere. In this study, we characterize the receptive field properties for these abnormal representations by conducting detailed fMRI population receptive field mapping in a rare subset of participants with albinism and no ocular nystagmus. We find a nasal bias for receptive field positions in the abnormal temporal hemiretina representation. In addition, by modelling responses to bilateral visual field stimulation in the overlap zone, we found evidence in favor of discrete unilateral receptive fields, suggesting a conservative pattern of spatial selectivity in the presence of abnormal retinal input.

Introduction
Albinism is a congenital disorder associated with misrouting of the optic nerves during embryogenesis, which leads to abnormal retinotopic organization in sub-cortical and cortical visual areas (Carroll, Jay, McDonald, & Halliday, 1980;Creel, Witkop, & King, 1974;Hedera et al., 1994;Morland, Hoffmann, Neveu, & Holder, 2002). In humans, retinal projections are normally divided at the optic chiasm, with temporal hemiretina fibers projecting to the hemisphere ipsilateral to the eye, and nasal hemiretina fibers crossing the midline and projecting to the contralateral hemisphere. In albinism, however, the line of decussation is shifted, leading to an over-crossing of temporal hemiretina projections to the contralateral hemisphere and leaving a weaker ipsilateral projection (Guillery, Okoro, & Witkop, 1975;Neveu & Jeffery, 2007). Despite this gross anatomical abnormality, individuals with albinism have relatively normal visual spatial perception and behavior, aside from peripheral effects such as reduced acuity as a result of foveal hypoplasia (Kinnear, Jay, & Witkop, 1985;Summers, 1996).
The functional consequences of this misrouting are not fully understood. It is clear from studies in the cat that a distinct temporal visual field representation forms in the contralateral lateral geniculate body (Hubel & Wiesel, 1971) and visual cortex (Kaas & Guillery, 1973) and the topology of the abnormal representation is variable (Cooper & Blasdel, 1980). Three models of functional organization in mammalian albinism have been proposed; (1) a contiguous representation, where ipsilateral and contralateral visual fields form a continuous map, (2) an interleaved representation, where mirror-symmetric ipsilateral and contralateral visual field locations are represented on the same cortical territory and (3) an interleaved suppressed representation, where input to the abnormally-routed representation is suppressed (Guillery, 1986;Guillery, Casagrande, & Oberdorfer, 1974;Hoffmann & Dumoulin, 2015;Kaas, 2005;Shatz & LeVay, 1979). However, these models are largely derived from experiments in the cat, 4 and evidence for their applicability in primates is limited. Nevertheless, a single case study in a green monkey (Guillery et al., 1984) and more recently human functional MRI (fMRI) studies (Hoffmann, Tolhurst, Moore, & Morland, 2003;Kaule et al., 2014;Morland et al., 2002) suggest the presence of overlapping representations of ipsilateral and partial contralateral visual fields on the same cortical territory, in agreement with model (2); an interleaved representation.
While these recent advances shed light on the topographic organization of the albinotic visual cortex, relatively little is known about the nature of these abnormal representations.
First, it is unclear if the abnormal contralateral representation has a coarse or fine sampling of the visual field. If coarse, it might merely increase spatial integration. If fine, it might support a duplicate coding of fine spatial detail within the same hemisphere (Kaas, 2005). Second, the neural encoding model for overlapping receptive fields remains poorly understood. One interpretation of the interleaved pattern is that two maps representing different parts of the visual field overlap, resulting in ipsilateral and contralateral representations within the same cortical territory, perhaps organized in hemifield columns (Guillery, 1986;Guillery et al., 1984).
However, the normal integration across adjacent columns would be confounded by the dual, offset visual field representations, and would not obviously support inter-ocular integration for standard stereoscopic vision (Klemen, Hoffmann, & Chambers, 2012). instead, integrating neurons might have ipsilateral and contralateral receptive fields -a "dual receptive field".
Alternatively, binocular cells in early visual cortex may be solely modulated by classical receptive field cells capturing sensory input from a single hemifield. Such integration cells would require substantial intracortical plasticity in order to suppress hemifield crosstalk and give rise to integrated stereoscopic vision. The presence or absence of evidence for dual receptive fields would therefore inform how abnormal retinal input is integrated to produce stereoscopic vision and to what degree are retinotopic representations plastic in the abnormally developed visual cortex.

5
In order to carefully characterize receptive field properties in human albinism, we used a fMRI population receptive field (pRF) mapping approach in a rare sub-population of participants with albinism and no nystagmus. Consistent fixation is critical for delivering visual stimulation in a retinotopic fashion, as the position of the stimulus on the retina must be known in order to correctly reconstruct activity patterns in cortex (Dumoulin & Wandell, 2008;Wandell & Winawer, 2015). In the presence of involuntary eye movements, the correspondence between the stimulus presented and the retinal image cannot be ensured. Thus, fixation stability is a concern for visuotopic mapping in general (Binda, Thomas, Boynton, & Fine, 2013) and pRF mapping in particular (Hummer et al., 2016;Levin, Dumoulin, Winawer, Dougherty, & Wandell, 2010). As nystagmus is a primary clinical feature of albinism, present in 89-95% of albinism cases (Apkarian & Shallo-Hoffmann, 1991;Charles, Green, Grant, Yates, & Moore, 1993;Lee, King, & Summers, 2001), it presents a significant challenge for delivering retinotopic stimulation in the presence of involuntary eye movements. In light of these considerations, we have opted to study a small group of individuals with clinically albinotic phenotypes, but not presenting with overt nystagmus and consistent fixation behavior.
Following monocular stimulation of the temporal hemiretina, significant blood-oxygen level dependent (BOLD) responses were observed in contralateral occipital cortex, demonstrating retinotopic organization with the temporal hemiretina representation largely overlapping the nasal representation, in agreement with the interleaved representation model. We report no differences in pRF size for nasal or temporal hemiretina representation in the shared territory of V1, V2 and V3, pointing to similar computational roles for the overlapping representations.
We detected a nasal bias for pRF position in the abnormal temporal hemiretina representation.
Finally, by modelling responses to bilateral visual field stimulation we found no evidence in favor of a dual receptive field model. Instead, our data points to interleaved and unsuppressed representations of both nasal and temporal hemiretina, occupying the same cortical territory 6 (model 2), suggesting a conservative pattern of functional organization in early visual areas under grossly abnormal retinal input.

Participants
Five participants with albinism (A1-A5) took part in the study (2 females, mean age=23.80, SD=15.90, age range=9-50). A diagnosis of albinism was reached after an ophthalmological assessment that investigated the presence or absence of phenotypic markers including; iris transillumination, photophobia, skin color, hair color at assessment and at birth, evidence, appearance of macula, imaging revealing foveal hypoplasia and presence or absence of a crossed asymmetry by comparison of monocular visual evoked potential (VEP) distribution.
Based on their phenotypic features, A1, A4 and A5 were diagnosed with oculocutaneous albinism (OCA), and A2 and A3 were diagnosed with ocular albinism type OA1. A4 and A5 are full siblings. An overview of the key phenotypic description is provided in 0. No genotype information was available at the time of testing.
In addition to participants with albinism, 10 healthy adult controls (4 males, mean age=26.30, SD=4.95, age range: 21-36) with normal visual acuity took part in the study. All adult participants provided written informed consent. Parents or legal guardians provided written consent on behalf of underage participants. This study was approved by the London -City and East Research Ethics Committee of the UK Health Research Authority. Participants with albinism displayed VEP asymmetry and foveal hypoplasia consistent with a shift of the line of decussation.

Ophthalmological screening
All participants underwent ophthalmological screening prior to taking part in the study. Best corrected monocular visual acuity was assessed with an ETDRS chart at 4 m distance to threshold. The best monocular acuity for the control group was M=0.03, SD=0.21 LogMAR, and all participants with albinism performed at a minimum of 0.3 LogMAR. All fMRI stimulation was delivered monocularly to the best-performing eye. All five participants with albinism failed to demonstrate binocular stereopsis when assessed with the Frisby stereotest (Frisby Near Stereotest, Sheffield, UK).
Visual field perimetry was assessed with automated perimetry equipment (Octopus 900, Haag-Streit Diagnostics, Koeniz, Switzerland) to two standard isopters (outer target size=I4e, inner target size=I2e, target speed=5°/s). Resulting visual fields showed normal outer perimeter 8 detection for A1-A5 (see Appendix A). Inner perimeter detection showed a small reduction in the nasal visual field of A2 and the temporal visual field of A4.
Consistent eye fixation is a requirement for visuotopic mapping, particularly if fixation behavior differs between two groups being investigated (Bressler & Silver, 2010;Crossland, Morland, Feely, Hagen, & Rubin, 2008). We assessed the stability of eye movement at fixation outside the scanner for participants with and without albinism to establish whether differences were expected during fMRI acquisition.
Eye movement recordings were carried out with a head-mounted infrared eye tracker (JAZZnovo, Ober Consulting, Poznan, Poland) sampling relative eye position during visual stimulation on a plasma display. The eye with best monocular acuity was tracked, that is, the eye to which monocular stimulation was delivered during the fMRI experiment. First, a 1.15° fixation target was presented for 10 s in two locations, 14° to left or to the right from central fixation for calibration. A total of 5 events were presented in each position. Next, a central fixation target was presented for 300 s, with the participant instructed to maintain a constant head position and fixate to the targets as they appeared. In order to quantify stability at central fixation, horizontal axis traces were visually inspected, and blink artefacts identified and removed at their amplitude peak. Time points 200 ms immediately before and after blink peaks were also removed from the analysis. Resulting traces were then de-trended to remove linear drifts introduced by the apparatus. Standard deviations of horizontal axis displacement for the eye with best monocular acuity during central fixation were 0.57° for A1, 0.86° for A2, 0.84° for A3, 0.59° for A4, and 0.81° for A5. Fixation stability in the control group averaged 0.64° (95% 9 CI=0.15° -1.13°). Participants with albinism showed comparable horizontal stability to the control group, with no systematic deviation of gaze at central fixation.

fMRI visual stimulation
Stimuli were generated in MATLAB (v8.0, Mathworks Inc., Natick, MA, USA) using Psychtoolbox (Brainard, 1997;Pelli, 1997) and displayed on a back-projection screen in the bore of the magnet via an LCD projector. This arrangement ensured the peripheral visual field was stimulated.
The stimulus pattern consisted of a 62° radius disc of a dynamic, high-contrast tessellated pseudo-checkerboard with a drifting ripple-like pattern that varied across time in spatial frequency (Alvarez, de Haas, Clark, Rees, & Schwarzkopf, 2015). This pattern was presented in two configurations; a hemifield mapping stimulus for pRF mapping and full-field configuration for estimation of the subject-specific hemodynamic response function (HRF). Stimuli were delivered monocularly to the eye with best-uncorrected visual acuity, while the poorer- volumes acquired per run. A total of 4 runs were conducted, two presented ipsilateral and two contralateral to the occluded eye. Each pair contained one run with clockwise wedge rotation and expanding rings, and one run with counter-clockwise wedge rotation and contracting rings.
The order of presentation was randomized across participants, with a total of 1240 volumes acquired per participant.
The full field configuration consisted of the stimulus pattern presented through a 62° radius circular aperture for 3 volumes, followed by 32 volumes of equiluminant grey background. This was repeated 10 times, totaling 350 volumes acquired.
Throughout both conditions, a fixation cross spanning 1.8° was presented to aid fixation.
Participants engaged in an attentional task, consisting of a brief (200 ms) change in color of the fixation cross, occurring semi-randomly with a probability of 5% for any given 200 ms epoch, with no consecutive events. Participants were instructed to attend to the cross and provide a response via an MRI-compatible response button every time they witnessed an event.
Participant responses were monitored to ensure engagement with the task.

MRI acquisition and pre-processing
MR images were acquired on a 1.5T Avanto MRI system using a 32-channel head coil (Siemens Healthcare, Erlangen, Germany). During visual stimulation, the top elements of the head coil were removed (remaining coils=20) to avoid visual field restrictions. A gradient echo EPI sequence (TR=1 s, TE=55 ms, 36 interleaved slices, resolution=2.3 mm isotropic) with parallel multiband acquisition of 4 simultaneous slices (Breuer et al., 2005)  High-resolution anatomical images were processed with FreeSurfer (v5.3.0, http://www.freesurfer.net) , creating a cortical surface render for each participant. A manual definition of the occipital lobe surface was created for each hemisphere in order to restrict data analysis to the occipital cortex.
Functional data were pre-processed in SPM12 (Wellcome Trust Centre for Neuroimaging, http://www.fil.ion.ucl.ac.uk/spm). All images were intensity bias-corrected, realigned to the first image of the run and unwarped to correct for movement artefacts and field distortions. This signal was then fitted with a double gamma function (Friston, Frith, Turner, & Frackowiak, 1995), resulting in an individual HRF for each participant's hemisphere.

pRF modeling
Hemifield mapping runs were analyzed with a forward model pRF approach (Alvarez et al., 2015;Dumoulin & Wandell, 2008). In brief, model predictions were generated from the a priori knowledge of stimulus position at each volume acquired, and under the assumption of an isotropic two-dimensional Gaussian pRF. Predictions were convolved with the individual HRF and compared to the observed signal in a two-stage procedure. First, a coarse fit was conducted by sampling predictions generated from an exhaustive grid of combinations of three pRF parameters (X and Y coordinates, and spatial spread, (σ) and correlating them with a smooth version (FWHM=8.3 mm on the spherical surface mesh) of the observed BOLD time courses.
The parameters resulting in the highest correlation at each vertex then formed the starting point for a subsequent fit to the unsmooth data, using a constrained non-linear minimization procedure (Lagarias et al., 1998). Best-fitting model predictions therefore provided estimates of retinotopic location (X and Y coordinates) and pRF size (σ) for each vertex, as well as a scaling factor (β). The coefficient of determination (R 2 ) was taken as the model metric for goodness of fit, and only vertices with R 2 > 0.2 were considered in further analysis.
Data resulting from stimulation to each hemifield were analyzed independently, incorporating BOLD time series from clockwise and counter-clockwise runs in each hemisphere. While the stimulated eye differed between participants, all subsequent analyses and discussion refers to hemispheres ipsilateral and contralateral to the stimulated eye, and in the interest of clarity all further illustrations are presented as if stimulation was through the left eye.
An additional analysis was conducted to assess the validity of the dual pRF model of retinotopic encoding in human albinism. Data recorded during stimulation to left and right hemifields were collated and fitted with a) single 2D Gaussian pRF model, as described above and b) dual horizontally-mirrored 2D Gaussian pRF model, which incorporates a second receptive zone of the same size (σ) in the equivalent mirror location across the vertical meridian. Both models were fitted independently for each hemisphere.

Visual area delineation
We manually delineated retinotopic maps on the inflated cortical surface based on polar angle and eccentricity representations derived from the pRF model. Maps were drawn in each hemisphere from data obtained under stimulation to the contralateral visual field. Early visual areas V1, V2 and V3 were reliably identified in all participants, while defining boundaries for 13 further extrastriate areas was variable across participants. Therefore, in order to ensure consistent and sufficient sampling, we restricted our analysis to visual areas V1, V2 and V3.

Overlapping representations of ipsilateral and contralateral visual fields
We presented participants with a monocular retinotopic mapping stimulus under two conditions; one stimulating the nasal hemiretina and one stimulating the temporal hemiretina Abnormal temporal hemiretina responses in the contralateral hemisphere were largely parafoveal, with the more eccentric representations reverting to the normal uncrossed pattern and appearing in the ipsilateral hemisphere. Substantial overlap was observed in the contralateral hemisphere, in agreement with some animal models of albinism (Guillery, 1986;Guillery et al., 1974;Shatz & LeVay, 1979) and previous fMRI studies (Hoffmann et al., 2003;Kaule et al., 2014). Despite this overlap, no structured representation of polar angle reversals was detected in the abnormal representation ( Figure 3).

Nasal bias in receptive field position
Is the temporal hemiretina projection of early albinotic visual cortex abnormally organized? In order to answer this question, we examined the pRF properties of visual areas V1, V2 and V3.
First, overlapping representations of the nasal and temporal retina in albinotic contralateral cortex may contain systematic variation in receptive field position. While it is clear from Figure   2 that the two contralateral representations follow the same broad retinotopic plan, preferred retinal locations may vary systematically, for example biased towards the shifted line of decussation. To assess this, we examined the estimated pRF location for the same cortical location under nasal and temporal hemiretina stimulation, and calculated the mean difference in horizontal pRF position between conditions, across visual areas V1, V2 and V3 for each participant (see Figure 4). This approach revealed a bias for nasal pRFs in the contralateral representation of the temporal hemiretina for all participants with albinism (mean horizontal shift=7. 90°,SEM=4.19°). No compelling evidence emerged for bias towards temporal pRFs in the temporal hemiretina representation.

Variability in receptive field size
In addition to biases in pRF location, another potential indicator for altered functional roles in cortical processing may be enlarged or constricted pRF sizes in the abnormal retinal representation zone. To examine this, the significant vertices for all subjects in each group were collated and fitted with a linear model ( Figure 5). Following stimulation to the temporal hemiretina, both controls and participants with albinism displayed a positive monotonic relationship between pRF size and eccentricity in the hemisphere contralateral to stimulation.
In order to assess these trends statistically, individual participant estimates of pRF size were summarized by averaging values into 30 bins, each spanning 2° of eccentricity. Binned pRF sizes were introduced to a mixed ANOVA model as the dependent variable, with group (control, albinotic) as the between-subject independent variable, and both pRF eccentricity and visual area (V1, V2, V3) as within-subject independent variables. No statistically significant interaction between group and eccentricity was detected (F(1, 13) = 0.01, p = 0.948, η 2 = 10 -3 ), 20 nor significant 3-way interaction between group, visual area and eccentricity (F(2, 26) = 0.84, p = 0.445, η 2 = 0.06). The individual-level trends in pRF size were assessed by comparing estimates of binned pRF size in single albinotic participants against the control group mean estimate in visual areas V1, V2 and V3 (see Figure 6). The contralateral response to nasal retina stimulation and the ipsilateral response to temporal retina stimulation were found to be broadly similar between albinotic participants and controls, albeit with high inter-individual variability in the albinotic group. These results indicate the abnormal temporal hemiretina representation in the contralateral hemisphere follows the same organization principles at the group level, as regularly organized cortex in both controls and participants with albinism.
21 Figure 5. pRF size as a function of receptive field eccentricity in cortical visual areas V1, V2 and V3 for controls (N=10, blue) and albinotic (N=5) participants. Bestfitting linear model (blue = control, red = albinotic) and matching coefficient of determination (R 2 ) shown for each condition. Both groups show a monotonic increase in pRF size with eccentricity. Contralateral responses to temporal retina stimulation omitted for controls, as no significant responses were detected under that condition. No significant differences between groups in the relationship between pRF size and eccentricity were observed. 22

No evidence for dual receptive fields
In order to further explore the underlying organization of visual receptive fields in albinotic visual cortex, an additional analysis was performed where responses to nasal and temporal hemiretina stimulation were modeled using both a classic 2D Gaussian pRF model, as implemented in the previous section, and alternatively with a dual pRF model, with two receptive fields, mirror-symmetric across the vertical median, as a neuronal population-level approximation of the dual receptive field model (Klemen et al., 2012). To assess differences in fits between the two models, we compared goodness of fit (R 2 ) estimates for each participant with a series of matched samples tests. As the distribution of R 2 were non-normal (Figure 7

Discussion
We identified and characterized the abnormal visual field representation in five rare participants with albinism and no nystagmus. Following monocular stimulation of the temporal hemiretina, significant BOLD responses were observed in both ipsilateral and contralateral occipital cortices, reproducing the widely reported asymmetric lateralization of responses in albinism (Apkarian et al., 1983;Carroll et al., 1980;Creel et al., 1974;Hagen et al., 2008;Hedera et al., 1994). The abnormal response was found to be retinotopically organized, with the temporal hemiretina representation largely overlapping the nasal representation in the contralateral hemisphere, in agreement with previous electrophysiological (Guillery et al., 1984) and fMRI findings (Hagen, Houston, Hoffmann, Jeffery, & Morland, 2005;Hoffmann et al., 2003;Kaule et al., 2014;Morland et al., 2002). This abnormal temporal representation was characterized by a shifted line of decussation, with foveal portions represented contralaterally, and eccentric portions represented in the ipsilateral hemisphere. This is again consistent with previous human fMRI literature (Hagen, Houston, Hoffmann, & Morland, 2007;Hoffmann et al., 2003;Kaule et al., 2014).
One advantage of performing visual mapping with participants with unimpaired fixation is the potential for careful mapping of receptive field properties. In this study, we implemented a hemifield-restricted stimulus where in each run, a pair of de-phased, contrast-reversing apertures were presented to a single hemifield while participants fixated, therefore exclusively stimulating either the nasal or temporal hemiretina.

28
Three findings of interest emerge from the analysis presented here. First, no evidence for systematically altered pRF sizes point towards a conservative role for visual areas representing the abnormally overlapping nasal and temporal hemiretina representations. The lack of altered pRF properties in area V1 is surprising given previous reports of reduced calcarine fissure length (Neveu & Jeffery, 2007) and increased grey matter volume and cortical thickness in albinotic V1 (Bridge et al., 2014;Hagen et al., 2005). While one may expect such structural abnormalities to be linked with concomitant functional differences such as receptive field spatial sensitivity, as approximated by pRF metrics, it is worth highlighting that albinism is a heterogeneous disorder (Carroll et al., 1980;Neveu, Jeffery, Burton, Sloper, & Holder, 2003). It is therefore possible that individuals with no nystagmus in the primary position, such as those reported on here, may represent a unique population with reduced chiasmal abnormalities and therefore reduced anatomical or functional consequences at the level of cortical representations.
Non-altered receptive field properties in albinism raise the question of functional significance for overlapping visual field representations in contralateral cortex. While it was originally postulated that the abnormal temporal representation might be suppressed (Guillery et al., 1974;Kaas, 2005), electrophysiological and functional imaging evidence in humans suggests this abnormal representation is indeed encoding valuable retinotopic information (Kaule et al., 2014). Here we show that both overlapping responses in contralateral V1 display similar pRF size increases with eccentricity, indicating a similar functional role in encoding of retinotopic information at the earliest stage of cortical processing. Significant levels of activation, retinotopic organization and coherent receptive field increases with eccentricity all indicate this region plays an active role in the representation of the temporal hemiretina. It is also worth noting that inspection of responses to temporal hemiretina stimulation reveals a split representation across the cortical hemispheres, rather than a duplicate representation. This highlights the importance of the abnormal temporal representation in maintaining continuity 29 of the spatial representation of the visual field, which may not be afforded by a suppressed representation.
How is visual information encoded when overlapping retinotopic representations are present in albinism? One possibility is that cortical cells with conservative organization represent all the retinal inputs they receive, giving rise to dual receptive field cells. Such cells would be responsive to mirrored nasal and temporal retinal locations and non-discriminatory of originating hemifield. Imaging evidence has suggested such cells may be present in achiasma, a related condition of optic nerve misrouting . Alternatively, hemifield discrimination can take place at the early cortical level, with single receptive field cells responding to a single retinal location in order to integrate ocular dominance column information. Here, we modeled BOLD responses to bilateral visual field stimulation in participants with albinism with two models; a single receptive field model with a single pRF zone and a dual receptive field model with two mirror-symmetrical pRFs divided by the vertical meridian. While other models of multiple spatial encoding are possible, this model was chosen in order to assess the explicit hypothesis of horizontally-mirrored receptive fields postulated by (Klemen et al., 2012). When assessed for model evidence, the single receptive field model outperformed the dual receptive field model in all participants, both control and participants with albinism. Vertices where the dual receptive field model provided a better fit were fewer than 9% of those sampled, and displayed no spatial clustering in cortex. Overall, evidence in this study suggests a rejection of the dual receptive field encoding model for human albinism, in agreement with previous psychophysical evidence (Klemen et al., 2012).
These results, therefore, suggest hemifield information is segregated at an early cortical level.
Given the normal or sub-normal stereovision reported in some patients with albinism (Lee et al., 2001;Summers, 1996), successful integration of ocular dominance column information can occur without major sensory conflict (cf. (Tsytsarev, Arakawa, Zhao, Chédotal, & Erzurumlu, 2017). In order to achieve this, a degree of cortical plasticity must come into play, as contralateral cortical territories normally represent retinotopically mirror-equivalent positions in left and right visual fields that must be functionally segregated. If the 'true albino' pattern (Guillery et al., 1984) is an accurate model of visual field representation in the human, and the present study as well as recent fMRI findings suggest that may be the case (Hagen et al., 2007;Hoffmann et al., 2003;Kaule et al., 2014;Wolynski, Kanowski, Meltendorf, Behrens-Baumann, & Hoffmann, 2010), hemifield segregation may be achieved through the presence of hemifield columns. Nevertheless, the correct selection of sensory information across hemifield columns and between ocular dominance columns for stereopsis is not easily achieved with a conservative retinotopic organization. Instead, these findings suggest a degree of developmental plasticity involved in providing a disambiguated sensory input to early visual processing, leading to normal or near-normal visual perception in human albinism.