Monocular Stimulation Research Articles

White matter and latency of visual evoked potentials during maturation: A miniature pig model of adolescent development

BackgroundContinuous myelination of cerebral white matter (WM) during adolescence overlaps with the formation of higher cognitive skills and the onset of many neuropsychiatric disorders. We developed a miniature-pig model of adolescent brain development for neuroimaging and neurophysiological assessment during this critical period. Minipigs have gyroencephalic brains with a large cerebral WM compartment and a well-defined adolescence period. MethodsEight Sinclair™ minipigs (Sus scrofa domestica) were evaluated four times during weeks 14–28 (40, 28 and 28 days apart) of adolescence using monocular visual stimulation (1 Hz)-evoked potentials and diffusion MRI (dMRI) of WM. The latency for the pre-positive 30 ms (PP30), positive 30 ms (P30) and negative 50 ms (N50) components of the flash visual evoked potentials (fVEPs) and their interhemispheric latency (IL) were recorded in the frontal, central and occipital areas during ten 60-second stimulations for each eye. The dMRI imaging protocol consisted of fifteen b-shells (b = 0–3500 s/mm2) with 32 directions/shell, providing measurements that included fractional anisotropy (FA), radial kurtosis, kurtosis anisotropy (KA), axonal water fraction (AWF), and the permeability-diffusivity index (PDI). ResultsSignificant reductions (p < 0.05) in the latency and IL of fVEP measurements paralleled significant rises in FA, KA, AWF and PDI over the same period. The longitudinal latency changes in fVEPs were primarily associated with whole-brain changes in diffusion parameters, while fVEP IL changes were related to maturation of the corpus callosum. ConclusionsGood agreement between reduction in the latency of fVEPs and maturation of cerebral WM was interpreted as evidence for ongoing myelination and confirmation of the minipig as a viable research platform. Adolescent development in minipigs can be studied using human neuroimaging and neurophysiological protocols and followed up with more invasive assays to investigate key neurodevelopmental hypotheses in psychiatry.

Resolving a "W-Shaped" P100 Waveform: Is It Normal or Abnormal?

Pattern-reversal visual evoked potentials are used to assess the visual pathways. The main waveform of interest is the P100, which is best recorded with electrodes over the mid-occipital region. Most often, the P100 waveform has negative-positive-negative components. Occasionally, it is "W-shaped," with positive-negative-positive components. Although most often a W-shaped P100 waveform indicates an abnormality in the visual pathway, occasionally, it can be normal. A case is presented in which a W-shaped P100 waveform is seen after monocular full-field stimulation of both eyes with 30' checks. To resolve this finding, the pattern-reversal visual evoked potentials is repeated with 60' and 15' checks. With 15' checks a single typical single-peak P100 waveform is seen with normal latency. Evaluation of a W-shaped P100 waveform should involve analysis of various montages, stimulation with different check sizes, and hemifield stimulation to confirm whether the W-shaped waveform is normal or abnormal.

Cortical excitability in patients with migraine with aura and depressive symptoms: a visual evoked potentials study

Background: Migraine is a brain disorder characterized by recurring headache attacks, and emotional comorbidities, such as anxiety and depression, may influence the repetition of these attacks. The lack of sensory habituation is a common neurophysiological abnormality in migraine, and research suggests that it is under the influence of serotonin and other monoamines that are also involved in mood disorders. This study aimed to investigate the influence of emotional symptoms on cortical information processing in patients with migraine with aura by correlating cortical activity with self-perceived emotional distress. Methods: Visual evoked potentials from monocular stimulation were recorded in 16 patients with migraine with aura (MA) and 22 healthy volunteers (HV). The visual stimulus consisted of a full-screen black-and-white checkerboard pattern with a reversal rate of 1.55 Hz. 600 consecutive traces were collected and divided into six sequential blocks of 100 sweeps. Before the recording session, both MA patients and HV completed the Beck Depression Inventory (BDI) and State-Trait Anxiety Inventory (STAI). Pearson's correlation test was used to find correlations between electrophysiological and psychometric variables in HV and MA patients. Results: Compared to HV, MA patients showed a significant difference in the degree of habituation to repeated visual stimulation between the two groups, resulting in a habitation deficit. Psychometric test results showed that trait STAI and BDI values were significantly higher in MA patients. In the MA group, BDI correlated negatively with the amplitude of the first block and positively with the degree of habituation. Conclusions: These results highlight a link between the level of brain responsiveness and depressive symptoms in patients with MA. Further research is required to confirm whether the same correlations exist in patients with other headache disorders.

Ocular dominance-dependent binocular combination of monocular neuronal responses in macaque V1

Primates rely on two eyes to perceive depth, while maintaining stable vision when either one eye or both eyes are open. Although psychophysical and modeling studies have investigated how monocular signals are combined to form binocular vision, the underlying neuronal mechanisms, particularly in V1 where most neurons exhibit binocularity with varying eye preferences, remain poorly understood. Here, we used two-photon calcium imaging to compare the monocular and binocular responses of thousands of simultaneously recorded V1 superficial-layer neurons in three awake macaques. During monocular stimulation, neurons preferring the stimulated eye exhibited significantly stronger responses compared to those preferring both eyes. However, during binocular stimulation, the responses of neurons preferring either eye were suppressed on the average, while those preferring both eyes were enhanced, resulting in similar neuronal responses irrespective of their eye preferences, and an overall response level similar to that with monocular viewing. A neuronally realistic model of binocular combination, which incorporates ocular dominance-dependent divisive interocular inhibition and binocular summation, is proposed to account for these findings.

Ocular dominance-dependent binocular combination of monocular neuronal responses in macaque V1.

Primates rely on two eyes to perceive depth, while maintaining stable vision when either one eye or both eyes are open. Although psychophysical and modeling studies have investigated how monocular signals are combined to form binocular vision, the underlying neuronal mechanisms, particularly in V1 where most neurons exhibit binocularity with varying eye preferences, remain poorly understood. Here, we used two-photon calcium imaging to compare the monocular and binocular responses of thousands of simultaneously recorded V1 superficial-layer neurons in three awake macaques. During monocular stimulation, neurons preferring the stimulated eye exhibited significantly stronger responses compared to those preferring both eyes. However, during binocular stimulation, the responses of neurons preferring either eye were suppressed on the average, while those preferring both eyes were enhanced, resulting in similar neuronal responses irrespective of their eye preferences, and an overall response level similar to that with monocular viewing. A neuronally realistic model of binocular combination, which incorporates ocular dominance-dependent divisive interocular inhibition and binocular summation, is proposed to account for these findings.

How should visual function monitoring be performed and interpreted in surgery for suprasellar tumors?

Preservation of visual function is important in surgery for suprasellar tumors. Visual evoked potentials (VEPs) are expected to play an important role in monitoring visual function during surgery. Given the lack of information in this field, the authors aimed to investigate the effects of optic nerve compression caused by suprasellar tumors to understand the possible usefulness of VEP monitoring using off-response (OFR) VEP. Eleven healthy volunteers who underwent surgery for standard record confirmation and 32 patients with optic chiasm lesions who underwent surgery were examined. Preoperative, postoperative, and intraoperative VEPs were recorded. Propofol anesthesia was administered during intraoperative VEP monitoring. Patients who underwent surgery were monitored using the same stimulation method during surgery. Light stimulation was given from a luminant pad on the eyelids, and low-intensity stimulation with continuous 500-msec emission and 500 msec off was performed. The luminescence intensity of the stimulation was at a maximum of 8000 lx with three attenuation steps, each of which was recorded repeatedly. The OFR potentials and delay latencies decreased as stimulus intensity decreased. In the patient with temporal hemianopia, monocular stimulation produced the highest OFR in the contralateral occipital lobe of the stimulated eye. The authors recorded preoperative, intraoperative, and postoperative VEP in 32 patients and observed intraoperative changes in 23 patients. In the cases where VEP declined during intraoperative recording, it recovered when surgery was discontinued. Furthermore, 3 patients eventually achieved a higher VEP than that achieved at the beginning of the surgery, and rapid recovery was confirmed with visual field examination immediately after surgery. Of the 5 patients in whom VEP did not recover during surgery, 3 showed decreased visual field and acuity after surgery. In 15 cases, potential dropped temporarily but returned to the original potential, and their visual field recovered after surgery. OFR has a diagnostic element in the visual field, in which the maximal potential was recorded on the opposite side of the stimulus with monocular stimulation. Unambiguous determination required stimulation of different intensities in both eyes or 1 eye and multiple recording electrodes placed in the occiput. Monitoring the OFR provides real-time alerts, making it a valuable tool for visual function evaluation in suprasellar surgery.

Effects of Monocular Flicker on Binocular Imbalance in Amblyopic and Nonamblyopic Adults.

This study aimed to evaluate the effects of monocular flicker stimulation on binocular imbalance in both amblyopic and nonamblyopic adults. Seven amblyopic patients (28.3 ± 3.3years; four females) and seven normally sighted participants (27.3 ± 4.1years; five females) participated in the study. We used liquid crystal spectacles to create externally-generated monocular flicker (4, 7, 10, 15, or 20Hz) and used the metric of log balance point (logBP) to determine whether imposed flicker could change the eyes' equilibrium interocular contrast ratio. Flicker was applied to either the fellow eye vs. the amblyopic eye or dominant eye (DE) vs. non-DE (non-DE) of amblyopic and nonamblyopic participants, respectively. We defined a logBP of 0 to indicate complete binocular balance and an increase in logBP relative to baseline to indicate a relative strengthening of the non-DE or amblyopic eye. Monocular flicker applied to the DE or fellow eye increased logBP, whereas when applied to the non-DE or amblyopic eye, reduced the logBP. These effects were more pronounced at low temporal frequencies than that at high temporal frequencies. The interaction between eye and temporal frequency was significant in both normals, F(4, 24) = 58.082, P<0.001, η2 = 0.906, and amblyopes, F(1.923, 11.538) = 60.555, P<0.001, η2 = 0.91. Monocular flicker diminishes the contribution of the flickered eye in binocular combination, resulting in a relative dominance of the nonflickered eye in interocular interactions. Furthermore, a more pronounced temporally modulated effect was observed at lower temporal frequencies.

Simulating the Effects of Partial Neural Conduction Delays in the Visual Evoked Potential.

The purpose of this study was to understand the double peaks or broadening of P100 observed in some cases of optic neuritis by inducing conduction delays in healthy eyes through stimulus luminance manipulation in analogy to the perceptual Pulfrich effect. Checkerboard pattern reversal visual evoked potentials (VEPs) with check sizes of 0.8degrees, 0.4degrees, and 0.2degrees were recorded in healthy participants using two experiment variants. Variant (1) involved binocular stimulation with inter-ocular luminance difference achieved by a 1.8 neutral density (ND) filter, along with monocular control conditions. Variant (2) included monocular stimulation with hemifields having a luminance difference (half of monitor with ND filter), along with single-hemifield control conditions. In both variants, VEP curves under mixed stimulation were compared to synthesized VEPs computed from offline summation of curves from the relevant control conditions, followed by assessing P100 characteristics. Despite considerable variability between participants, the binocular variant demonstrated marked differences between VEPs from mixed recordings and synthesized curves, whereas in the hemifield variant, agreement was strong. The anticipated double peak or broadened deflection pattern was observed to varying extents in participants, often contingent on check size, with nominal peak time frequently failing to indicate partial conduction delays. The present findings corroborate the hypothesis that nominal peak time does not always reflect conduction delays if only a subset of fiber bundles is affected. Peak shape might provide additional diagnostic evidence of a partial conduction delay. Enhancing the understanding of VEP waveform changes associated with partial conduction delays could offer diagnostic insights for optic neuritis.

Poster Session I: Binocular facilitation of the BOLD response to melanopsin stimulation in the suprachiasmatic nucleus.

In a recent analysis of archival data, Spitschan and Cajochen (2019) identify what appears to be substantial binocular facilitation of melatonin suppression due to melanopic light stimulation. This putative effect likely originates in the melanopsin-containing intrinsically photosensitive retinal ganglion cells (ipRGCs) which project directly to the suprachiasmatic nucleus (SCN) of the hypothalamus. We asked whether we could measure a direct physiological correlate of this binocular facilitation using a binocular, MRI-compatible, 10-primary spectral stimulation device. We present preliminary findings from a functional magnetic resonance imaging (fMRI) study designed to explore the blood oxygen level dependent (BOLD) response to monocular and binocular melanopic light stimulation. The study used a 30 s on/off design with three 'ocularity' conditions (binocular-low, monocular-high, binocular-high) and two classes of targeted photoreceptors (melanopsin and LMS cones). Throughout each scan, subjects (N=18) also responded to brief, cone-directed sinusoidal modulations of varying intensity. We report that binocular vs. monocular melanopsin stimulation induced significant BOLD activation in SCN but that this effect was not seen for cone-directed stimulation. This is consistent with the binocular facilitation effect described by Spitschan and Cajochen (2019) and provides the first direct evidence of melanopsin-driven activation and binocular facilitation in human subcortical nuclei.

The Value of VR-PVEP in Objective Evaluation of Monocular Refractive Visual Impairment.

To study the virtual reality-pattern visual evoked potential (VR-PVEP) P100 waveform characteristics of monocular visual impairment with different impaired degrees under simultaneous binocular perception and monocular stimulations. A total of 55 young volunteers with normal vision (using decimal recording method, far vision ≥0.8 and near vision ≥0.5) were selected to simulate three groups of monocular refractive visual impairment by interpolation method. The sum of near and far vision ≤0.2 was Group A, the severe visual impairment group; the sum of near and far vision <0.8 was Group B, the moderate visual impairment group; and the sum of near and far vision ≥0.8 was Group C, the mild visual impairment group. The volunteers' binocular normal visions were set as the control group. The VR-PVEP P100 peak times measured by simultaneous binocular perception and monocular stimulation were compared at four spatial frequencies 16×16, 24×24, 32×32 and 64×64. In Group A, the differences between P100 peak times of simulant visual impairment eyes and simultaneous binocular perception at 24×24, 32×32 and 64×64 spatial frequencies were statistically significant (P<0.05); and the P100 peak time of normal vision eyes at 64×64 spatial frequency was significantly different from the simulant visual impairment eyes (P<0.05). In Group B, the differences between P100 peak times of simulant visual impairment eyes and simultaneous binocular perception at 16×16, 24×24 and 64×64 spatial frequencies were statistically significant (P<0.05); and the P100 peak time of normal vision eyes at 64×64 spatial frequency was significantly different from the simulant visual impairment eyes (P<0.05). In Group C, there was no significant difference between P100 peak times of simulant visual impairment eyes and simultaneous binocular perception at all spatial frequencies (P>0.05). There was no significant difference in the P100 peak times measured at all spatial frequencies between simulant visual impairment eyes and simultaneous binocular perception in the control group (P>0.05). VR-PVEP can be used for visual acuity evaluation of patients with severe and moderate monocular visual impairment, which can reflect the visual impairment degree caused by ametropia. VR-PVEP has application value in the objective evaluation of visual function and forensic clinical identification.

Widespread and Multifaceted Binocular Integration in the Mouse Primary Visual Cortex.

The brain combines two-dimensional images received from the two eyes to form a percept of three-dimensional surroundings. This process of binocular integration in the primary visual cortex (V1) serves as a useful model for studying how neural circuits generate emergent properties from multiple input signals. Here, we perform a thorough characterization of binocular integration using electrophysiological recordings in the V1 of awake adult male and female mice by systematically varying the orientation and phase disparity of monocular and binocular stimuli. We reveal widespread binocular integration in mouse V1 and demonstrate that the three commonly studied binocular properties-ocular dominance, interocular matching, and disparity selectivity-are independent of each other. For individual neurons, the responses to monocular stimulation can predict the average amplitude of binocular response but not its selectivity. Finally, the extensive and independent binocular integration of monocular inputs is seen across cortical layers in both regular-spiking and fast-spiking neurons, regardless of stimulus design. Our data indicate that the current model of simple feedforward convergence is inadequate to account for binocular integration in mouse V1, thus suggesting an indispensable role played by intracortical circuits in binocular computation.SIGNIFICANCE STATEMENT Binocular integration is an important step of visual processing that takes place in the visual cortex. Studying the process by which V1 neurons become selective for certain binocular disparities is informative about how neural circuits integrate multiple information streams at a more general level. Here, we systematically characterize binocular integration in mice. Our data demonstrate more widespread and complex binocular integration in mouse V1 than previously reported. Binocular responses cannot be explained by a simple convergence of monocular responses, contrary to the prevailing model of binocular integration. These findings thus indicate that intracortical circuits must be involved in the exquisite computation of binocular disparity, which would endow brain circuits with the plasticity needed for binocular development and processing.

Rapid adaptation of primate LGN neurons to drifting grating stimulation.

The visual system needs to dynamically adapt to changing environments. Much is known about the adaptive effects of constant stimulation over prolonged periods. However, there are open questions regarding adaptation to stimuli that are changing over time, interrupted, or repeated. Feature-specific adaptation to repeating stimuli has been shown to occur as early as primary visual cortex (V1), but there is also evidence for more generalized, fatigue-like adaptation that might occur at an earlier stage of processing. Here, we show adaptation in the lateral geniculate nucleus (LGN) of awake, fixating monkeys following brief (1 s) exposure to repeated cycles of a 4-Hz drifting grating. We examined the relative change of each neuron's response across successive (repeated) grating cycles. We found that neurons from all cell classes (parvocellular, magnocellular, and koniocellular) showed significant adaptation. However, only magnocellular neurons showed adaptation when responses were averaged to a population response. In contrast to firing rates, response variability was largely unaffected. Finally, adaptation was comparable between monocular and binocular stimulation, suggesting that rapid LGN adaptation is monocular in nature.NEW & NOTEWORTHY Neural adaptation can be defined as reduction of spiking responses following repeated or prolonged stimulation. Adaptation helps adjust neural responsiveness to avoid saturation and has been suggested to improve perceptual selectivity, information transmission, and predictive coding. Here, we report rapid adaptation to repeated cycles of gratings drifting over the receptive field of neurons at the earliest site of postretinal processing, the lateral geniculate nucleus of the thalamus.

Monocular transcorneal electrical stimulation induces ciliary muscle thickening in contralateral eye

Transcorneal electrical stimulation (TES) is used as therapy for retinal diseases such as retinitis pigmentosa (RP) and was suggested for assessing retinal sensitivity by determining phosphene thresholds, subjective luminance impressions caused by retinal stimulation. Further applications concerned the accommodation process, revealing an improved accommodative amplitude in presbyopic eyes after TES treatment. The respective changes of the ciliary muscle (CM), the structure most important for near vision, during TES are yet unknown. In a pilot study, we aimed to assess whether monocular TES leads to morphological and functional CM changes and whether central accommodation control is affected. Ten healthy, near-emmetropic adults participated in the trial (4 females, age 26.3 ± 3.6 years). Using a wavefront and a stimulus generator, a biphasic square-wave stimulus (2 s positive and 6 s negative amplitude) of 0 μA average current was produced and transferred to the eye by means of a Dawson-Trick & Litzkow electrode. Prior to the stimulation, an individual determination of phosphene thresholds served to define individual TES current amplitudes, which ranged between 60 and 100 μA. Optical coherence tomography (OCT) imaging of the right eye's temporal ciliary muscle was performed before and during ipsi- as well as contralateral monocular TES in randomized order in the morning and afternoon of the same day. During imaging, subjects fixated a target at 4 m distance and refraction was simultaneously recorded via eccentric infrared photorefraction. OCT images were assessed using previously published custom-developed software, allowing the definition of selective CM thickness (CMT) readings, and plotting of continuous CMT profiles along the muscle border. CMT profiles revealed that both stimulations, on the ipsi- and contralateral eye, induced a thickening of the CM compared to the non-stimulated state. The selective CMT readings confirmed a significant increase with ipsi- (31 ± 30 μm; p = 0.010) and contralateral (25 ± 16 μm; p = 0.001) TES. However, refraction during far vision was not significantly affected by either stimulation (ipsilateral [n = 5]: median Δw/-w/o = 0 D; contralateral [n = 7]: Δw/-w/o = 0.13 D). Pupil size on average increased during TES, but without reaching significance (ipsilateral [n = 5] median Δw/-w/o = 0.23 mm, contralateral [n = 7] Δw/-w/o = 0.39 mm). Ipsilateral CM thickening could be explained by local changes within the stimulated ciliary muscle, such as increased blood flow or interstitial fluid rise induced by TES. However, the CMT increase in the right eye when TES was performed contralaterally, on the left eye, indicates an involvement of the central control circuit of accommodation. Further possible explanations for this finding are a synchronization of neuronal activities in the visual pathway, the release of vasoactive neuropeptides, or effects on the central blood pressure regulation. Given a neuromodulation effect on the CM function, TES might have implications for children with accommodation insufficiencies and as additional therapy in myopia control management, e.g. in combination with multifocal contact lens treatment. Our study is important for the clinical application of TES, and the outcome might add crucial knowledge to the current understanding of the accommodation process and inform research and treatment of both myopia and presbyopia.

Neural circuits for binocular vision: Ocular dominance, interocular matching, and disparity selectivity.

The brain creates a single visual percept of the world with inputs from two eyes. This means that downstream structures must integrate information from the two eyes coherently. Not only does the brain meet this challenge effortlessly, it also uses small differences between the two eyes' inputs, i.e., binocular disparity, to construct depth information in a perceptual process called stereopsis. Recent studies have advanced our understanding of the neural circuits underlying stereoscopic vision and its development. Here, we review these advances in the context of three binocular properties that have been most commonly studied for visual cortical neurons: ocular dominance of response magnitude, interocular matching of orientation preference, and response selectivity for binocular disparity. By focusing mostly on mouse studies, as well as recent studies using ferrets and tree shrews, we highlight unresolved controversies and significant knowledge gaps regarding the neural circuits underlying binocular vision. We note that in most ocular dominance studies, only monocular stimulations are used, which could lead to a mischaracterization of binocularity. On the other hand, much remains unknown regarding the circuit basis of interocular matching and disparity selectivity and its development. We conclude by outlining opportunities for future studies on the neural circuits and functional development of binocular integration in the early visual system.

Temporal synchrony discrimination is abnormal in dichoptic but not monocular visual processing in treated anisometropic amblyopes.

To evaluate whether temporal synchrony processing deficits remain when normal visual acuity is restored in adults with unilateral anisometropic amblyopia. We recruited 14 clinically treated anisometropic amblyopes (mean age 23.17 ± 2.53 years) with best-corrected visual acuity ≤ 0.1 logMAR and 15 age-matched emmetropes (mean age 24.40 ± 1.92 years) with normal vision to participate in our experiment. We presented two pairs of flicking Gaussian dots (1Hz) as visual stimuli: one pair of dots was synchronous (reference), and the other pair of dots was asynchronous (signal). Subjects were asked to determine the position of the asynchronous pair. We applied the constant stimuli method to measure the temporal synchrony threshold under monocular and dichoptic viewing conditions. There were eight temporal phase lags in the asynchronous pair. The minimum degree of the temporal phase at which a participant can discriminate a signal pair is defined as the temporal synchrony threshold. Under monocular viewing conditions where both the reference and signal pairs were presented to one eye, the temporal synchrony thresholds of previous amblyopic eyes and fellow eyes were not significantly different (p= 0.15). Under dichoptic viewing conditions where both the reference and signal pairs were dichoptically presented to both eyes, the temporal synchrony threshold in the treated anisometropic amblyopes was significantly higher than that of the controls (119.34 ± 20.43 vs. 99.78 ± 16.60 ms, p= 0.009). There was no significant correlation between the monocular and dichoptic viewing conditions in the treated amblyopes (r= -0.22, p= 0.94). Temporal synchrony discrimination is abnormal under dichoptic but not under monocular visual stimulation in treated anisometropic amblyopes with normalised visual acuity.

Ketamine promotes adaption-induced orientation plasticity and vigorous network changes

Adult primary visual cortex features well demonstrated orientation selectivities. However, the imposition of a non-preferred stimulus for many minutes (adaptation) or the application of an antidepressant drug, such as ketamine, shifts the peak of the tuning curve, assigning a novel selectivity to a neuron. The effect of ketamine on V1 neural circuitry is not yet ascertained. The present investigation explores (in control, post-adaptation, and following local ketamine application) the modification of orientation selectivities and its outcome on functional relationships between neurons in mouse and cat. Two main results are revealed. Electrophysiological neuronal responses of monocular stimulation show that in cells exhibiting large orientation shifts after adaptation, ketamine facilitates the cell’s recovery. Whereas in units displaying small shifts following adaptation, the drug increases the magnitude of orientation shifts.In addition, pair-wise cross correlogram analyses show modifications of functional relationships between neurons revealing updated micro-circuits as a consequence of ketamine application. We report in cat but not in mouse, that ketamine significantly increases the connectivity rate, their strengths, and an enhancement of neuronal synchrony.

GABAergic inhibition in the human visual cortex relates to eye dominance

Binocular vision is created by fusing the separate inputs arriving from the left and right eyes. ‘Eye dominance’ provides a measure of the perceptual dominance of one eye over the other. Theoretical models suggest that eye dominance is related to reciprocal inhibition between monocular units in the primary visual cortex, the first location where the binocular input is combined. As the specific inhibitory interactions in the binocular visual system critically depend on the presence of visual input, we sought to test the role of inhibition by measuring the inhibitory neurotransmitter GABA during monocular visual stimulation of the dominant and the non-dominant eye. GABA levels were measured in a single volume of interest in the early visual cortex, including V1 from both hemispheres, using a combined functional magnetic resonance imaging and magnetic resonance spectroscopy (combined fMRI-MRS) sequence on a 7-Tesla MRI scanner. Individuals with stronger eye dominance had a greater difference in GABAergic inhibition between the eyes. This relationship was present only when the visual system was actively processing sensory input and was not present at rest. We provide the first evidence that imbalances in GABA levels during ongoing sensory processing are related to eye dominance in the human visual cortex. Our finding supports the view that intracortical inhibition underlies normal eye dominance.

Reduced surround suppression in monocular motion perception.

Motion discrimination of large stimuli is impaired at high contrast and short durations. This psychophysical result has been linked with the center-surround suppression found in neurons of area MT. Recent physiology results have shown that most frontoparallel MT cells respond more strongly to binocular than to monocular stimulation. Here we measured the surround suppression strength under binocular and monocular viewing. Thirty-nine participants took part in two experiments: (a) where the nonstimulated eye viewed a blank field of the same luminance (n = 8) and (b) where it was occluded with a patch (n = 31). In both experiments, we measured duration thresholds for small (1 deg diameter) and large (7 deg) drifting gratings of 1 cpd with 85% contrast. For each subject, a Motion Suppression Index (MSI) was computed by subtracting the duration thresholds in logarithmic units of the large minus the small stimulus. Results were similar in both experiments. Combining the MSI of both experiments, we found that the strength of suppression for binocular condition (MSIbinocular = 0.249 ± 0.126 log10 (ms)) is 1.79 times higher than under monocular viewing (MSImonocular = 0.139 ± 0.137 log10 (ms)). This increase is too high to be explained by the higher perceived contrast of binocular stimuli and offers a new way of testing whether MT neurons account for surround suppression. Potentially, differences in surround suppression reported in clinical populations may reflect altered binocular processing.

Performance evaluation of multifocal electroretinography for monocular and binocular stimulation in anisochromoartiphakia

To evaluate the difference and nature of changes in the amplitude (P1) and latency (P1, L1) parameters of mfERG in monocular (MS) and binocular stimulation (BS) in individuals with anisochromoartiphakia. The study included 24 patients aged 76.3±7.9 years with anisochromoartiphakia [one eye - IOL with yellow spectrum SN60AT (yIOL); the other is a transparent IOL - Rayacryl (trIOL)] comprising the main group, and 16 patients without artiphakia aged 51.6±4.6 years in the control group [one eye with the addition of a yellow light filter (YF) ZHS1 - a spectral analog of SN60AT, the other - without YF]. Recording of mfERG was done with MS and BS. Parameters under evaluation were DP1 (nV/deg2), amplitude P1 (mV) and latency LP1, LN1 (ms) in rings R1-R5. In general, when switching from one ring to another under both types of stimulation and spectral correction, all indicators decrease. DP1 and P1 in MS and BS in the control group (49.2±17.7 and 53.6±17.5 nV/deg2) compared to the main group were higher (42.9±15.0 and 47.7±17.3 nV/deg2), LP1 and LN1 were shorter (41.2±5.0 and 44.8±4.3 ms in the control group, 44.0±4.1 and 50.7±21.1 ms in the main group). There was a significant difference between DP1R3 and DP1R4 in the control group (p≤0.003-0.050). In the control group without YF, P1R2 under MS was statistically significantly higher than under BS (p≤0.028). LP1 of R1, R3, R5 in the intergroup comparison was statistically significantly shorter in the control group without YF and MS (p≤0.033) and with YF in R1-R3, R5 (p≤0.003-0.023). LP1R2, LP1R4 were statistically significantly higher with trIOL in the main group under MS (p≤0.015; 0.047) and with yIOL in LP1R1, LP1R4, and LR5 (p≤0.053). The intergroup comparison showed that in the main group with yIOL, LP1R5 was statistically significantly lower under MS than BS (p≤0.032). When comparing LN1 between the groups, the statistically significant difference under MS without YF was observed in R5 (p≤0.002), while the difference under BS was insignificant. For YF, the increase in LN1 was statistically significant for MS in R2 and R3 (p≤0.010; 0.037) and for BS - in R1, R2 and R4 (p≤0.037; 0.003; 0.020). The intergroup comparison revealed that in the control group without YF, LN1R2 was statistically significantly lower under MS than BS (p≤0.028), and in the main group with trIOL, LN1R3 under MS was statistically significantly lower than under BS (p≤0.041). In anisochromoartiphakia, spectral differences in the optical environment of the eye lead to changes in the parameters of mfERG in the form of a decrease in DP1 and elongation of LP1 and LN1 in comparison with healthy eye and non-spectral intraocular correction. MS or BS in mfERG does not have a significant impact on the character and value of these parameters. The use of a single-stage binocular recording of mfERG reduces the duration of the study, which is especially important when examining elderly patients.

Ultra-high field fMRI reveals origins of feedforward and feedback activity within laminae of human ocular dominance columns

Ultra-high field MRI can functionally image the cerebral cortex of human subjects at the submillimeter scale of cortical columns and laminae. Here, we investigate both in concert, by imaging ocular dominance columns (ODCs) in primary visual cortex (V1) across different cortical depths. We ensured that putative ODC patterns in V1 (a) are stable across runs, sessions, and scanners located in different continents, (b) have a width (~1.3 mm) expected from post-mortem and animal work and (c) are absent at the retinotopic location of the blind spot. We then dissociated the effects of bottom-up thalamo-cortical input and attentional feedback processes on activity in V1 across cortical depth. Importantly, the separation of bottom-up information flows into ODCs allowed us to validly compare attentional conditions while keeping the stimulus identical throughout the experiment. We find that, when correcting for draining vein effects and using both model-based and model-free approaches, the effect of monocular stimulation is largest at deep and middle cortical depths. Conversely, spatial attention influences BOLD activity exclusively near the pial surface. Our findings show that simultaneous interrogation of columnar and laminar dimensions of the cortical fold can dissociate thalamocortical inputs from top-down processing, and allow the investigation of their interactions without any stimulus manipulation.