Stimulation Of Eye Research Articles

The Effects of L-dopa on the Functional Magnetic Resonance Imaging Response of Patients With Amblyopia: A Pilot Study

We investigated the effects of a single dose of levodopa (L-dopa) on the level and extent of visual cortex activation of subjects with amblyopia and normal subjects using blood oxygenation level-dependent functional magnetic resonance imaging (fMRI). Six patients with amblyopia and 9 control patients were recruited. A baseline fMRI session was followed by a second session 90 minutes after the dose of L-dopa. Visual stimuli included vertical sinusoidal gratings with spatial frequencies of 1 and 2 cpd that were counterphased at 4 Hz. Stimuli were presented monocularly and binocularly. The fMRI response was characterized by the total volume and the average level of activation within the occipital cortex. An interocular absolute difference (IDIF) was defined in terms of said measures for between-population analysis of monocular data. After the administration of L-dopa, visual acuity improved significantly ( P = 0.026) from 0.72 +/- 0.21 (mean +/- SD) to 0.64 +/- 0.24 LogMAR in the amblyopic eye, although remaining the same in the dominant eye and in the eyes of control subjects. The response to L-dopa was found to be population-specific, as indicated by a significant treatment-by-population interaction for the volume of activation IDIF ( P = 0.018) measure: subjects with amblyopia exhibited a post-treatment increase in the volume of activation IDIF whereas control subjects showed a less prominent decrease. This post-treatment increase of IDIF in subjects with amblyopia may be explained by the decrease in the volume of activation found for the amblyopic eye after L-dopa ( P = 0.038). No L-dopa-related activation changes were detected for dominant eye or binocular stimulation in the amblyopic group, and no change was detected in control subjects. L-dopa elicits a population-specific modulation of the fMRI response, namely, a reduced total volume of activation from the amblyopic eye despite improvement in visual acuity.

Differential activation of cerebral blood flow by stimulating amblyopic and fellow eye

Positron emission tomography (PET), the blood flow response in the primary visual cortex (V1) to two visual stimuli, low temporal frequency (6 Hz) to activate the parvocellular system, and high temporal frequency (25 Hz) to activate the magnocellular system were used to investigate pathophysiologic mechanism of amblyopia. Five women and one man who were aged between 26 and 60 years, who were ophthalmologically normal except for amblyopia, and who had corrected visual acuity in the amblyopic eye of 0.6 or worse were examined. An intravenous injection of the H(2)(15)O was given, and the regional cerebral blood flow was measured by PET during full-field stimulation with either 6 Hz or 25 Hz flicker to the amblyopic or the sound eye. The activation of blood flow in the contra-lateral area V1 by the 6-Hz stimulation of the sound eye was greater than that during the stimulation of the amblyopic eye (P<0.05, small volume correction, n=6). With 25-Hz stimulation of the sound and amblyopic eyes, the blood flow in the contra-lateral and ipsi-lateral areas V1 was not significantly different. The decreased activation of blood flow in the contra-lateral V1 by low temporal frequency stimuli supports the hypothesis that the parvocellular pathway in amblyopic eyes is depressed.

The effect on human eye blink frequency of exposure to limonene oxidation products and methacrolein

Oxidation products of terpenes (e.g. limonene) contain unidentified irritants, which may be responsible for a fraction of the reported eye and airway complaints in indoor environments. Here we report exposure to parts per billion (ppb) levels of limonene oxidation products (LOPs) and the terpene oxidation product methacrolein using blink frequency (BF) as a measure of trigeminal stimulation of the human eye. Ten male subjects averaging 43 (standard deviation 10.5) years were exposed for 20 min to LOPs, methacrolein, and clean air, respectively. A baseline BF was measured prior to and following each exposure (8 min and 4 min, respectively). The subjects were exposed locally in the non-dominant eye and single blind at 20% relative humidity (RH), while viewing an educational film. Blinking was video recorded and evaluated for full sessions of 36 min. Mean BF increased significantly during exposure to LOPs and methacrolein compared to the baseline of clean air, and the findings coincided with weak eye irritation symptoms. Lowest observed effect levels were 286 ppb methacrolein and a 10-min-old LOPs mixture of initially 92 ppb limonene and 101 ppb ozone (O 3), which increased the BF comparably by 18% ( p = 0.001) and 17% ( p = 0.003), respectively. The increase in BF was smaller, although not significantly different, during exposure to LOPs at 50% RH to 20% RH in mixtures prepared from ca. 350 ppb limonene and 300 ppb O 3. LOPs may cause trigeminal stimulation and possibly eye irritation at O 3 and limonene concentrations, which are close to high-end values measured in indoor settings. The effects may be exacerbated by low RH.

Topographic Analysis of Flash Visual Evoked Potentials in Dogs

Visual evoked potentials by flash stimulation (flash VEP) were analyzed in dogs using a topographic method. The flash VEP consisted of 3 positive (P1, P2 and P3) and 2 negative (N1 and N2) components by 150 msec after the flash stimuli. On the topographic mappings, a negative response area was observed in the frontal region of the scalp in the stimulated site followed by the shifting of the area to the contralateral frontal region and occipital region, during the first 100 msec. The negative response area in the frontal region in the stimulated site, contralateral frontal and temporal region, and occipital region were corresponded to N1, P2, and N2 on the flash VEP, respectively, according to their latencies. In the dogs with experimentally impaired the right lateral geniculate body, the latency of P2 was prolonged, and N2 and P3 were disappeared after the left eye stimulation. On the topographic mapping, only the early negative response area was detected on the stimulated site of the frontal region of the brain. Therefore, it is concluded that P1 and N1, P2, and N2 are referred to the retinal potentials, the potentials from the retina to the brainstem included the lateral geniculate body, and those from the brainstem to the visual cortex, respectively.

Changes in the EEG Spectrum and Subjective Characteristics of the General State after Stimulation with Variable Frequency Photic Stimuli with Two Types of Organization

Two sinusoidal signals, one with a constant frequency of 13 Hz and the other with a frequency continuously changing from 1 to 6 Hz and back, were presented simultaneously to subjects through spectacles with light-emitting diodes either to both eyes as a product (amplitude modulation of a constant frequency by a variable one) or to each eye separately. Both kinds of variable frequency exposure revealed a rhomboid pattern of the resonance activation of the EEG spectrum, similar to the spectral dynamics of a signal subject to amplitude modulation. This testifies to the key role of EEG amplitude modulation in the responses of the nervous system to variable frequency rhythmic stimuli. Both types of photic stimulation led to a substantial increase in EEG spectral density and improved the subjects' self-rating of the overall state of well-being, activity, and mood. In addition, separate stimulation of each eye led to an improvement in the anxiety and exercise performance indices (the Luscher color test) and a significant correlation between the intensity of EEG responses and changes in the general state. These differences are explained in terms of the involvement of the interhemispheric interaction mechanisms in the processing of complex rhythmic signals by the brain.

Comparison of pupil responses to luminance and colour in severe optic neuritis

Objective: The pupil response to light flux increments is abnormal in severe optic neuritis, but little is known about the effects of this condition on the pupil colour response. The aim of this study was to examine how optic neuritis affects pupil responses to light flux and colour modulation and the extent to which such pupil responses mirror the loss and recovery of vision. Methods: A new pupil examination technique that makes use of sinusoidal modulation of either luminance contrast or chromatic saturation was employed. This technique enables the automatic extraction of both pupil response amplitude and latency and achieves a high signal to noise ratio with fewer averages. Results: The study reveals a greater loss of pupil response amplitude and significantly longer latencies to chromatic modulation (i.e. ∼80 ms). Stimulation of the unaffected eye in the optic neuritis group results in smaller response amplitudes when compared to the normal group for both light flux and colour modulation. Conclusions: Pupil response components can be affected differently in optic neuritis. These findings suggest that the pupil colour response, in particular, may provide a useful, objective estimator to judge the extent of damage and recovery in diseases of the optic nerve.

Non-dominant eye responses in the dorsal lateral geniculate nucleus of the cat: an intracellular study

While binocularity has been established as an important characteristic of cat visual cortical neurons, neurons in the dorsal lateral geniculate nucleus (LGNd) are commonly believed to be monocular. To test whether binocularity exists at the level of the LGNd, postsynaptic potentials (PSPs) of 101 cells were intracellularly recorded in eight normal and eight monocularly deprived cats while presenting stimuli to either the dominant or non-dominant eyes. The results showed that: (1) About 92% of neurons (45 out of 49) responded to a flashing spot presented to the non-dominant eye. In contrast to the dominant eye responses, the non-dominant eye PSPs usually exhibited the same polarization tendency (hyperpolarization or depolarization) to flashing spot stimuli of light increment or decrement, and most of them were inhibitory (hyperpolarization, 35 out of 45, 78%). (2) The response field (RF) of the non-dominant eye overlapped that of the dominant eye. (3) For most binocular cells, peak-to-peak amplitudes of non-dominant eye PSPs were about half the size (46%) of those of the dominant eye. The peak latencies and half-peak latencies of non-dominant eye PSPs were significantly longer than those of the dominant eye (mean differences were 5.4 ms and 5.6 ms respectively). (4) Most of the binocular cells responded well to contrast reversing gratings presented to the non-dominant eye, and the responses were clearly spatial-frequency tuned. No null phase could be found for non-dominant eye PSPs, no matter the neuron was classified as X or Y type according to dominant eye elicited responses. Some of the cells responded well to drifting gratings presented to the non-dominant eye. (5) We also recorded 52 cells in monocularly deprived cats, and found that 49 cells (94%) showed significant responses to flashing spots presented to the non-dominant eye, a similar percentage to that found in normal cats (92%). Conclusion: as strongly monocular neurons, most of LGNd cells could also be driven by the non-dominant eye. The responses evoked by non-dominant eye stimulation differ greatly from those evoked by dominant eye stimulation, and remain intact even without visual experience. These observations suggest an important role of the perigeniculate nucleus in providing binocular inputs to LGNd cells.

Molecular mechanism for loss of visual cortical responsiveness following brief monocular deprivation.

A dramatic form of experience-dependent synaptic plasticity is revealed in visual cortex when one eye is temporarily deprived of vision during early postnatal life. Monocular deprivation (MD) alters synaptic transmission such that cortical neurons cease to respond to stimulation of the deprived eye, but how this occurs is poorly understood. Here we show in rat visual cortex that brief MD sets in motion the same molecular and functional changes as the experimental model of homosynaptic long-term depression (LTD), and that prior synaptic depression by MD occludes subsequent induction of LTD. The mechanisms of LTD, about which there is now a detailed understanding, therefore contribute to visual cortical plasticity.

Topographic Plasticity in Primary Visual Cortex Is Mediated by Local Corticocortical Connections

The placement of monocular laser lesions in the adult cat retina produces a lesion projection zone (LPZ) in primary visual cortex (V1) in which the majority of neurons have a normally located receptive field (RF) for stimulation of the intact eye and an ectopically located RF (displaced to intact retina at the edge of the lesion) for stimulation of the lesioned eye. Animals that had such lesions for 14-85 d were studied under halothane and nitrous oxide anesthesia with conventional neurophysiological recording techniques and stimulation of moving light bars. Previous work suggested that a candidate source of input, which could account for the development of the ectopic RFs, was long-range horizontal connections within V1. The critical contribution of such input was examined by placing a pipette containing the neurotoxin kainic acid at a site in the normal V1 visual representation that overlapped with the ectopic RF recorded at a site within the LPZ. Continuation of well defined responses to stimulation of the intact eye served as a control against direct effects of the kainic acid at the LPZ recording site. In six of seven cases examined, kainic acid deactivation of neurons at the injection site blocked responsiveness to lesioned-eye stimulation at the ectopic RF for the LPZ recording site. We therefore conclude that long-range horizontal projections contribute to the dominant input underlying the capacity for retinal lesion-induced plasticity in V1.

Simultaneous mapping of binocular and monocular receptive fields in awake monkeys for calibrating eye alignment in a dichoptical setup

We developed a modified Wheatstone stereoscope for simultaneous dichoptical and binocular stimulation in awake monkey. We, therefore, extended the conventional two-screen Wheatstone stereoscope to a setup with an additional third screen viewed binocularly via semi-transparent mirrors. With a sparse noise stimulation we mapped classical receptive field (CRF) positions via each screen independently but simultaneously. This was done for multiple recording positions (16 electrodes) at once in primary visual cortex based on multiple unit spike activity (MUA) and local field potentials (LFP), respectively. The technique can be used to (1) quickly and simultaneously determine binocular as well as left and right eye CRFs, including ocular dominance characteristics (net recording time for the given examples: ∼2 min), (2) precisely adjust dichoptical stimulation by evaluating offsets between monocular and binocular CRF positions (average spatial incongruency between binocular and left/right eye stimulation after calibration: ∼0.025° visual angle), and (3) investigate left and right eye interaction in forming binocular CRFs. Due to the precise adjustment of the dichoptical and the simultaneous binocular stimulation investigations on the basis of stereo vision can be done with appropriate eye vergence alignment matching normal binocular viewing conditions in awake animals.

Temporal Constraints on Visually Directed C-Start Responses: Behavioral and Physiological Correlates

To study the modulatory influences of visual information on Mauthner (M-) initiated C-start responses, short interval visual stimuli were presented to individual cichlid fish prior to being startled with a sound pulse. Because the axon of each Mauthner neuron activates trunk musculature contralateral to the soma, the initial direction of an ensuing startle response provides a behavioral measure of which cell has been driven closer to threshold by visual cues at the time the sound pulse causes one cell to fire. When an LED was illuminated on one side of the fish for 10 ms prior to a startling sound pulse, cichlids reliably turned toward the visual cue. At durations of 15 and 20 ms, fish turned away from the same stimulus. Thus, behavioral evidence suggests that the M-cell contralateral to a visual stimulus appears to be excited first but visual excitation of the M-cell ipsilateral to the visual stimulus follows and predominates. Consistent with the behavioral results, visually evoked excitatory potentials recorded intracellularly in the cichlid M-cells were complex, with initial PSPs showing latencies of about 11.6 ms from contralateral eye stimulation and 15.5 ms from stimulating the ipsilateral eye. PSP latencies in goldfish were longer and more similar for stimuli to the two eyes (about 22 ms). For contrast, sound-evoked PSPs begin within 2 ms. The relative long latencies for visually regulating M-cell function suggest that vision is most adaptive for biasing response direction prior to rather than during a predator’s attack.

Functional MRI of amblyopia before and after levodopa

Functional magnetic resonance imaging (fMRI) was applied to five older amblyopes with monocular amblyopia before and after levodopa treatment. During the experiment, images were acquired in two runs with visual stimulation delivered through the sound and the amblyopic eyes, respectively. The experiment was performed on each of the subjects, before and after their oral administration of levodopa/carbidopa (0.5/0.12 mg/kg) three times per day for 7 weeks. Our study demonstrated that there was no effect on the spatial extent of the visual cortical activation during the sound eye stimulation ( P=0.17), but some improvement during the amblyopic eye stimulation ( P=0.06). The volume ratio between the amblyopic and sound eye stimulation significantly increased after the treatment ( P<0.05). This finding supports the previous studies of levodopa effect on amblyopia at the visual cortical level, and suggests that fMRI can be a useful tool in assessing changes of visual cortical activity after the treatment

Functional magnetic resonance imaging as a tool for investigating amblyopia in the human visual cortex: A pilot study

Purpose: This study investigates interocular differences in the level and extent of cortical activation in amblyopic and normal subjects. Subjects and Methods: Blood oxygenation level dependent functional magnetic resonance imaging (FMRI) was performed at 1.5 T. A total of 5 subjects with amblyopia and 6 control subjectswere recruited. Visual stimuli included a homogeneous field flickering at 8 Hz and vertical sinusoidal gratings with spatial frequencies of 0.5, 1, and 2 cycles per degree of visual angle (counterphased at 8 Hz). Baseline images were taken in darkness. Stimuli were presented monocularly and binocularly. Data analysis was constrained to the occipital cortex, including striate and extrastriate areas. An absolute percent difference measure defined interocular differences in terms of total area, average level of activation, and a pooled activation parameter. Results: Subjects with amblyopia exhibited a significantly larger (P <.05) interocular activation difference compared to controls. The level of activation driven by monocular stimulation of the amblyopic eye was reduced by 8.25% ± 1.3% relative to the dominant eye. Controls showed an interocular difference in level of activation of 4.82% ± 0.74%. The total area of activation driven by the amblyopic eye was reduced by 34.86% ± 6.25% relative to the dominant eye. Controls showed an interocular difference in total area of activation of 20.80% ± 3.67%. Subjects with amblyopia also manifested significantly greater (P <.05) cortical area and level of activation differences between the binocular and monocular states; the dominant eye response differed less from the binocular response than did the amblyopic eye response. Conclusion: FMRI is sensitive to amblyopia-related deficits in the human occipital cortex and, therefore, has potential as a tool for basic amblyopia research. (J AAPOS 2002;6:300-8)

Investigations into the source of binocular input to the nucleus of the optic tract in an Australian marsupial, the wallaby Macropus eugenii.

Recordings from direction-selective neurons in the nucleus of the optic tract (NOT) of the marsupial wallaby, Macropus eugenii, show that 53% of cells are sensitive to visual stimulation of both eyes. Anatomical tracing studies using horseradish peroxidase reveal many retinal terminals in the contralateral NOT but very few in the ipsilateral nucleus. There was no convincing evidence of cortical inputs to the ipsilateral NOT despite large injections of tracer into the visual cortex. During visual stimulation in the visual field of the contralateral eye with moving patterns, the excitatory responses in the NOT generated by ipsiversive motion (right-to-left when recording from the left NOT) were usually larger than the inhibitory responses produced by contraversive motion. Conversely, during ipsilateral eye stimulation, the negative motion components to contraversive motion were usually larger than the positive components to ipsiversive motion. This response pattern resembles that observed in the NOT of the American opossum, Didelphis aurita, where binocularity appears to arise through a commissural subcortical pathway that connects the two nuclei and inverts the directional tuning of the transmitted signals. We propose that the lack of significant input from the ipsilateral eye and cortex in the wallaby suggests that binocularity must arise from another pathway, possibly a commissural route between the nuclei. As directional information appears not to be carried by the internucleus pathway in rats and cats, our results suggest that binocularity in the NOT arises from different sources in marsupials as compared to eutherians.

Adenosine Enhances Functional Activation of Blood Flow in Cat Optic Nerve Head during Photic Stimulation Independently from Nitric Oxide

Blood flow studies in the brain, heart, and other organs suggest that there could be interaction between nitric oxide (NO) and adenosine. This possibility was investigated in the optic nerve head (ONH) during photic stimulation of the dark-adapted cat eye. Functional activation of ONH blood flow was measured by laser Doppler flowmetry, simultaneously with NO and PO 2 using double-barrel recessed electrochemical sensors. Photic stimulation (diffuse luminance flickering light at 30 Hz) increased ONH blood flow to 127.4 ± 4.7% (mean ± SEM) of baseline with a transient increase in NO by 79.8 ± 12.8 nM, while PO 2 decreased from 24.5 ± 2.7 to 22.7 ± 2.4 Torr (control responses, 15 trials, 10 cats). Adenosine (3 mg/kg iv) increased baseline ONH blood flow to 113.8 ± 8.4% of control within 5 min. Functional activation of ONH blood flow was enhanced during photic stimulation, reaching a maximum of 155.8 ± 8.1% within 5 min, and remained enhanced for 30 to 45 min. NO responses during photic stimulation were not different from control responses. Treatment with a nonspecific NO synthase inhibitor ( N G-nitro- l-arginine methyl ester, 40 mg/kg iv, 5 cats) did not alter the increase in resting ONH blood flow or the enhanced functional activation after adenosine. We conclude that there is no interaction between NO and adenosine during functional activation of cat ONH blood flow by photic stimulation.

CAMP/Ca2+Response Element-Binding Protein Function Is Essential for Ocular Dominance Plasticity

The monocular deprivation model of amblyopia is characterized by a reduction in cortical responses to stimulation of the deprived eye. Although the effects of monocular deprivation on the primary visual cortex have been well characterized physiologically and anatomically, the molecular mechanisms underlying ocular dominance plasticity remain unknown. Previous studies have indicated that the transcription factor adenosine cAMP/Ca(2+) response element-binding protein (CREB) is activated during monocular deprivation. However, it remains unknown whether CREB function is required for the loss of cortical responses to the deprived eye. To address this issue, we used the herpes simplex virus (HSV) to express a dominant negative form of CREB (HSV-mCREB) containing a single point mutation that prevents its activation. Quantitative single-unit electrophysiology showed that cortical expression of this mutated form of CREB during monocular deprivation prevented the loss of responses to the deprived eye. This effect was specific and not related to viral toxicity, because overexpression of functional CREB or expression of beta-galactosidase using HSV injections did not prevent the ocular dominance shift during monocular deprivation. Additional evidence for specificity was provided by the finding that blockade of ocular dominance plasticity was reversible; animals treated with HSV-mCREB recovered ocular dominance plasticity when mCREB expression declined. Moreover, this effect did not result from a suppression of sensory responses caused by the viral infection because neurons in infected cortex responded normally to visual stimulation. These findings demonstrate that CREB function is essential for ocular dominance plasticity.

Ocular dominance in extrastriate cortex of strabismic amblyopic cats

Ocular dominance in extrastriate visual cortex of cats with behaviorally defined strabismic amblyopia was studied using extracellular recording techniques. In area 18, the amblyopic eye drove about as many cells as the normal one. In area posteromedial lateral suprasylvian area (PMLS), about 60% of the cells responded exclusively to stimulation of the normal eye and 30% to stimulation of the amblyopic eye. In area 21a more than 75% of the cells were monocularly driven by the non-amblyopic eye while only 5% were monocularly driven by the amblyopic eye. These findings suggest that ventral pathways (area 21a) are more affected in amblyopia than dorsal pathways (area PMLS).

Optical imaging of orientation and ocular dominance maps in area 17 of cats with convergent strabismus.

Strabismus (or squint) is both a well-established model for developmental plasticity of the brain and a frequent clinical symptom. While the layout and topographic relationship of functional domains in area 17 of divergently squinting cats has been analyzed extensively in recent years (e.g. Löwel et al., 1998), functional maps in convergently squinting animals have so far not been visualized with comparable detail. We have therefore investigated the functional organization of area 17 in adult cats with a surgically induced convergent squint angle. In these animals, visual acuity was determined by both behavioral tests and recordings of visual evoked potentials, and animals with comparable acuities in both eyes were selected for further experiments. The functional layout of area 17 was visualized using optical imaging of intrinsic signals. Monocular iso-orientation domains had a patchy appearance and their layout was different for left and right eye stimulation, so that segregated ocular dominance domains could be visualized. Iso-orientation domains exhibited a pinwheel-like organization, as previously described for normal and divergently squinting cats. Mean pinwheel density was the same in the experimental and control animals (3.4 pinwheel centers per mm2 cortical surface), but significantly (P < 0.00001) higher than that reported previously for normal and divergently squinting cats (2.7/mm2). A comparison of orientation with ocular dominance maps revealed that iso-orientation domains were continuous across the borders of ocular dominance domains and tended to intersect these borders at steep angles. However, in contrast to previous reports in normally raised cats, orientation pinwheel centers showed no consistent topographical relationship to the peaks of ocular dominance domains. Taken together, these observations indicate an overall similarity between the functional layout of orientation and ocular dominance maps in area 17 of convergently and divergently squinting cats. The higher pinwheel densities compared with previous reports suggest that animals from different gene pools might generally differ in this parameter and therefore also in the space constants of their cortical orientation maps.

Human Ocular Dominance Columns as Revealed by High-Field Functional Magnetic Resonance Imaging

We mapped ocular dominance columns (ODCs) in normal human subjects using high-field (4 T) functional magnetic resonance imaging (fMRI) with a segmented echo planar imaging technique and an in-plane resolution of 0.47 × 0.47 mm 2. The differential responses to left or right eye stimulation could be reliably resolved in anatomically well-defined sections of V1. The orientation and width (∼1 mm) of mapped ODC stripes conformed to those previously revealed in postmortem brains stained with cytochrome oxidase. In addition, we showed that mapped ODC patterns could be largely reproduced in different experiments conducted within the same experimental session or over different sessions. Our results demonstrate that high-field fMRI can be used for studying the functions of human brains at columnar spatial resolution.

Interocular rivalry revealed in the human cortical blind-spot representation

To understand conscious vision, scientists must elucidate how the brain selects specific visual signals for awareness. When different monocular patterns are presented to the two eyes, they rival for conscious expression such that only one monocular image is perceived at a time1,2. Controversy surrounds whether this binocular rivalry reflects neural competition among pattern representations or monocular channels3,4. Here we show that rivalry arises from interocular competition, using functional magnetic resonance imaging of activity in a monocular region of primary visual cortex corresponding to the blind spot. This cortical region greatly prefers stimulation of the ipsilateral eye to that of the blind-spot eye. Subjects reported their dominant percept while viewing rivalrous orthogonal gratings in the visual location corresponding to the blind spot and its surround. As predicted by interocular rivalry, the monocular blind-spot representation was activated when the ipsilateral grating became perceptually dominant and suppressed when the blind-spot grating became dominant. These responses were as large as those observed during actual alternations between the gratings, indicating that rivalry may be fully resolved in monocular visual cortex. Our findings provide the first physiological evidence, to our knowledge, that interocular competition mediates binocular rivalry, and indicate that V1 may be important in the selection and expression of conscious visual information.