Corpus Callosum (CC) and amblyopia: background and aims

1. UNILATERAL CATHODAL tDCS APPLICATION REDUCES TRANSCALLOSAL INHIBITION AND IMPROVES VISUAL ACUITY IN AMBLYOPIC PATIENTSLA STIMOLAZIONE TRANSCRANICA A CORRENTE DIRETTA (tDCS) UNILATERALE RIDUCE L’INIBIZIONE TRANSCALLOSALE E MIGLIORA L’ACUITÀ VISIVA IN PAZIENTI AMBLIOPI Ferdinando Sartucci 1,2,3, Francesco Nasini 4, Matteo Caleo 2, Laura Restani 2, Alberto Priori 5,6, Lamberto Maffei 2, Marco Nardi 4, Tommaso Bocci 1,5 1 Section of Neurophysiopathology, Department of Clinical and Experimental Medicine, University of Pisa, Italy; 2 CNR Institute of Neuroscience, Pisa, Italy; 3 Integrated Care Department Medical Specialty, AOUP, Pisa, Italy 3 Department of Surgical, Medical, and Molecular Pathology and Critical Care, University of Pisa, Pisa, Italy; 5 Clinical Center for Neurotechnologies, Neuromodulation, and Movement Disorders, Fondazione IRCCS Ca‘ Granda Ospedale Maggiore Policlinico, Milano, Italy; 6 Department of Health Sciences, University of Milan & Ospedale San Paolo, Milan, Italy

2. Corpus Callosum (CC) and amblyopia: background and aims • Amblyopia is a disorder of central visual acuity and contrast sensitivity that develops in the first decade of life, persistent despite lens correction, but potentially reversible if treated early with occlusive dressing. • Therapy, however, has not undergone substantial changes over the past two centuries, while exposing patients to irreversible side effects such as occlusive amblyopia and induced strabismus. • The ability to interfere in adulthood with the plasticity of the ipsilateral visual cortex at the amblyopic eye has been demonstrated in the animal by studies that have experienced partial recovery of visual acuity after environmental enrichment (Sale et al., 2007), and after administration of selective serotonin re-uptake inhibitors (Fluoxetine; Maya Vetencourt et al., 2008). • To investigate the role of interhemispheric pathways in amblyopic patients, we studied Visual Acuity (logMAR) and the response of the visual cortex (VEPs) to transcranial Direct Current Stimulation (tDCS) applied over the primary visual area (V1) contralateral to the “lazy eye”,assessed before (T0), immediately after (T1) and 60’ (T2) following the application of Cathodal tDCS (2.0 mA, 20’) in twelve amblyopic patients. At each time point, Visual Evoked Potentials (VEPs), triggered by grating stimuli of different contrasts (K90%, K20%), were recorded in both hemispheres and compared to those obtained in ten healthy volunteers, together with Visual Acuity.

3. Corpus Callosum and amblyopia: animal models Corpus Callosum and monocular deprivation (modified by Restani et al., 2009). In A the simplified experimental protocol is reported: the muscimol, or a saline solution in the control experiment, is injected into the healthy visual cortex of the rat subjected to monocular deprivation within the first week of life. On the right are the changes in peak firing rates (A), representative examples of VEP responses for both eyes, before and after muscimol (musc) administration to the opposite site (B) and mean VEP amplitudes in the contralateral hemisphere and in the ipsilateral injection of muscimol, before and after drug administration: note the marked increase in amplitude of PEV in the contralateral visual cortex, afferent to the deprived eye. Restani et al., 2009; Neuron 64: 1-12 Visual system and non-invasive neuromodulation techniques (tDCS) Antal et al., 2004 IOVS 45 (2): 702-7; Kanai et al., 2008 Curr Biol 18(23): 1839-43; Bocci et al., 2014 J Neural Transm 121 (3):221-31. Spiegel et al., 2012 PLoS One7(5):e36220. Experimental Protocol (A) Schematic diagram of the rat visual system and description of the experimental protocol. The striped areas indicate the binocular portion of the primary visual cortex in both hemispheres. Binocularity was recorded before and after injection of either saline or muscimol (musc) into the contralateral cortex. (B) Representative examples of VEP recordings demonstrating blockade of injected visual cortex after muscimol (musc) delivery.

4. Figure 2 - Correlation with baseline values. Changes in logMAR score linearly correlated with baseline values: patients with greater impairment at baseline showed a more robust enhancement of the visual acuity (p = 0.0004, Pearson’s correlation). Figure 1 – Changes in Visual acuity (LogMAR). Amblyopic patients showed a significant improvement of visual acuity following cathodal tDCS compared to sham polarization, with effects lasting for up to one hour. Data are given as mean values ± standard error (S.E.); ** p < 0.001.

5. A. T0 T1 T2 P1 5 µV Amblyopic side 100 ms N1 Not amblyopic side B. Figure 3 - Changes inVEP amplitudes in amblyopic subjects. A. Representative VEP responses to central stimulation (contrast, 90%), in the amblyopic (top traces) and not amblyopic side (bottom traces), respectively in the hemisphere contralateral and ipsilateral to tDCS intervention. B. VEP amplitudes significantly increased ipsilaterally to the amblyopic eye, at high contrasts (K90%), and remained persistently elevated at T2 (p < 0.0001, Holm-Sidak post-hoc test). On the opposite side, we observed a reduction of VEP amplitudes at T1, but at T2 all values returned to baseline, both for high (Holm-Sidak test, T1 vs T2: p < 0.0001 ) and low contrasts (p = 0.001).

6. tDCS and amblyopia: results Conclusions tDCS is a promising treatment for amblyopia in adults. The rapid recovery of excitability and the persistent trans-callosal disinhibition following perturbation of cortical activity support a critical role of interhemispheric balance in the pathophysiology of amblyopia. • If the data from the present study are confirmed, the tDCS could be included in the treatment options of amblyopic patients; this on the one hand would allow to exclude the important side effects, albeit sporadic, linked to conservative therapy, on the other hand would also allow adult patients to take advantage of a valid therapeutic alternative, to date not yet available Bocci T., Nasini F., Caleo M., Restani L., Barloscio D., Ardolino G., Priori A., Maffei L., Nardi M., Sartucci F.: Unilateral Application of Cathodal tDCS Reduce Transcallosal Inhibition and Improves Visual Acuity in Amblyopic Patients. Front Behav Neurosci. 2018, 29;12: 109. The histogram on the left shows the course of visual acuity in the amblyopic eye in five subjects, immediately after (2) and after about one hour (3) from the end of the tDCS session. It should be noted that the inhibition of the contralateral cortex at the amblyopic eye, obtained thanks to a continuous stimulation of direct current and cathodic polarity, determines a significant and lasting improvement in visual acuity in terms of the number of words identified at the octetric table. The two histograms on the right, on the other hand, always show the amplitude variations of the two main components of the VEP at high (K90%), medium (K50%) and low (K20%) luminance contrasts on the eye side of the eye; note that, similarly to healthy subjects and patients with photosensitive epilepsy, there is a strengthening of the electrophysiological response limited to medium-high contrasts, which is statistically greater than that obtained in the control group (p = 0.0051).

7. Thanks to: Tommaso Bocci Department of Clinical and Experimental Medicine, Section of Neurophysiopathology, University of Pisa, Italy Matteo Caleo Lamberto Maffei Laura Restani CNR Neuroscience Institute S. Cataldo - Pisa Marco Nardi Francesco Nasini Department of Surgical, Medical, and Molecular Pathology and Critical Care, University of Pisa, Italy Alberto Priori Clinical Center for Neurotechnologies, Neuromodulation, and Movement Disorders, Fondazione IRCCS Ca‘ Granda Ospedale Maggiore Policlinico, Milano, Italy Thank you for your kind attention Save the date: SINC Nat. Congr. Pisa, May 27-30, 2020. For further informations: ferdinando.sartucci@med.unipi.it f.sartucci@ao-pisa.toscana.it