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Strabismus induces an abnormal pattern of alternating light and dark columns of cytochrome oxidase (CO) activity in macaque striate cortex. This pattern may arise because visual perception is suppressed in one eye to avoid diplopia. To test whether CO activity is reduced in the ocular dominance columns of the suppressed eye, we performed monocular enucleation to co-label the ocular dominance columns with Zif268 immunohistochemistry in seven exotropic adult Macaca fascicularis. This approach was unsuccessful, for two reasons. First, Zif268 yielded inconsistent labelling, that was usually greater in the enucleated eye's ocular dominance columns, but was sometimes greater in the intact eye's columns. Therefore, Zif268 was not a reliable method for identifying the ocular dominance columns serving each eye. Second, in three control animals we found that a brief survival period following monocular enucleation (needed for Zif268 levels to change) was long enough to alter CO staining. For example, a survival time of only 3 h was sufficient to induce CO columns, indicating that the activity of this enzyme fluctuates more rapidly than realized previously. Independent of these findings, we have also discovered that acute monocular enucleation produces a vivid pattern of ocular dominance columns visible in unstained or CO-stained sections under dark-field illumination. The ocular dominance columns of the acutely enucleated eye appear dark. This was verified by labelling the ocular dominance columns with [3H]proline. Dark-field illumination of the cortex after acute monocular enucleation offers a new, easy method for identifying the ocular dominance columns in macaques.

Misalignment of the ocular axes induces double vision and rivalry. To prevent these unpleasant sensations, most subjects fixate preferentially with one eye and suppress entirely the deviating eye or else suppress portions of the visual field of either eye. To explore the mechanism of visual suppression, a divergent strabismus (exotropia) was induced in six normal, adult Macaca fascicularis by disinserting the medial rectus muscles. After 4-8 weeks, each animal was chaired to measure its exotropia and to determine its ocular fixation preference. Five of the monkeys developed a clearly dominant eye. It was injected with [(3)H]proline. Alternate sections from flat-mounts of striate cortex were then processed either for autoradiography to label the ocular dominance columns or for cytochrome oxidase (CO) to assess local metabolic activity. Two CO patterns were seen, often in the same cortex. The first consisted of thin dark columns alternating with wide pale columns. This pattern arose from reduced CO activity in the suppressed eye's monocular core zones and both eyes' binocular border strips. The second pattern consisted of thin pale bands from reduced metabolic activity in both eyes' border strips. The thin dark-wide pale CO pattern was more widespread in the three animals with a strong fixation preference. The dark CO columns usually fit in register with the ocular dominance columns of the fixating eye, suggesting that perception was suppressed in the deviating eye. In most animals, however, the correlation switched in peripheral cortex contralateral to the deviating eye, implying local suppression of the fixating eye's temporal retina (beyond 10 degrees), as reported in humans with divergent strabismus. In the two animals with a weak fixation preference, pale border strips were found within the central visual field representation in both hemispheres. This CO pattern was consistent with alternating visual suppression. These experiments provide the first anatomical evidence for changes in cortical metabolism that can be correlated with suppression scotomas in subjects with strabismus.

In primate striate cortex, geniculocortical afferents in layer IVc terminate in parallel stripes called ocular dominance columns. We propose that this segregation of ocular inputs generates a related but distinct columnar system of monocular core zones alternating with binocular border strips. Evidence for this functional parcellation was obtained by comparing the effects of enucleation, eyelid suture, and retinal laser lesions on cytochrome oxidase (CO) activity in eight macaques. Enucleation produced a high-contrast pattern of dark and light columns in layer IVc, corresponding precisely to the ocular dominance columns, whereas eyelid suture produced a low-contrast pattern of thin dark columns alternating with wide pale columns. [3H]Proline eye injection showed that the thin dark columns corresponded to the core zones of the open eye's ocular dominance columns. The wide pale columns resulted from loss of CO activity in the sutured eye's core zones and within both eyes' border strips. Loss of CO activity within both eyes' border strips suggested that these regions are binocular. To confirm our findings, we compared different CO patterns in the same cortex by making retinal laser lesions in four animals. They produced a CO pattern tantamount to "focal" enucleation, although contrast was low when laser damage was confined to the outer retina. CO levels in cortical scotomas remained severely depressed for months after retinal lesions, even when the other eye was enucleated. This observation provided little anatomical support for the notion of topographic plasticity after visual deafferentation. In a single human subject with macular degeneration, CO revealed a low-contrast pattern of ocular dominance columns, resembling the pattern in monkeys with laser-induced photoreceptor damage.

We examined cytochrome oxidase (CO) activity in striate cortex of four macaque monkeys after monocular enucleation at ages 1, 1, 5, and 12 weeks. These animal experiments were performed to guide our interpretation of CO patterns in occipital lobe specimens obtained from two children who died several years after monocular enucleation during infancy for tumor. In the macaques, the ocular dominance columns were labelled by injecting [3H]proline into the remaining eye. After enucleation at age 1 week, ocular dominance columns were eliminated in layer IVc(beta), resulting in a uniform pattern of autoradiographic label and CO staining. However, columns could still be seen in wet, unstained sections and with the Liesegang silver stain. Autoradiographs through layers IVc(alpha) and IVa showed residual, shrunken columns belonging to the missing eye, indicating that enucleation has less drastic effects in these layers. In the two human cases, enucleation at age 1 week also resulted in uniform CO staining in layer IVc. In the macaque after enucleation at age 5 weeks, ocular dominance columns belonging to the missing eye were severely narrowed, but still occupied 20% of layer IVc(beta). CO revealed wide, dark columns alternating with thin, pale columns in layer IVc(beta). The CO pattern and the columns labelled by autoradiography matched perfectly. After enucleation at age 12 weeks, only mild shrinkage of ocular dominance columns occurred. Enucleation at ages 1, 5, and 12 weeks did not alter the pattern of thin-pale-thick-pale stripes in V2. The main findings from this study were that (1) CO histochemistry accurately labels the boundaries of columns in layer IVc(beta) of macaque striate cortex after early monocular enucleation, making it a suitable technique for defining the critical period for plasticity of ocular dominance columns in human striate cortex; (2) enucleation causes more severe shrinkage of ocular dominance columns than eyelid suture; (3) early monocular enucleation obliterates ocular dominance columns in layer IVcbeta, but their pattern remains visible in wet sections and with the Liesegang stain; and (4) enucleation does not affect CO staining in V2.

Unilateral eyelid suture, a model for amblyopia induced by congenital cataract, produces shrinkage of the deprived eye's ocular dominance columns in the striate cortex. Loss of geniculocortical projections are thought to account for the poor vision in the amblyopic eye. It is uncertain whether ocular dominance columns become shrunken in other forms of amblyopia. We examined the striate cortex in a pigtailed macaque with natural anisometropia discovered at age 5 months. Amblyopia in the left eye was documented at 1 year by behavioral testing. At age 6 years, the left eye was injected with [3H]proline and the striate cortex was processed for autoradiography and cytochrome oxidase (CO). The ocular dominance columns in layer IVc labelled with [3H]proline were normal. CO staining showed a novel pattern of thin dark bands in layer IV. These bands occupied the core zones at the center of the ocular dominance columns. Their appearance resulted from relative loss of CO activity along the borders of the ocular dominance columns, regions specialized for binocular processing. These findings indicate that not all forms of amblyopia are accompanied by shrinkage of ocular dominance columns. The unusual pattern of CO staining in layer IVc reflected a subtle alteration in metabolic activity which may have resulted from impairment of binocular function in anisometropic amblyopia.

Visual deprivation induced by monocular eyelid suture, a laboratory model for congenital cataract, results in shrinkage of ocular dominance columns serving the closed eye. We performed monocular suture in macaques at ages 1, 3, 5, 7, and 12 weeks to define the critical period for plasticity of ocular dominance columns. After a minimum survival of 8 months, complete montages of [3H]proline-labeled columns were reconstructed from flat-mounts of striate cortex in both hemispheres. In any given monkey, visual deprivation induced the columns throughout striate cortex (V1) to retract the same distance from their original borders in layer IVcbeta. After deprivation, the widest columns remained in the foveal representation and along the V1/V2 border, where columns are widest in control animals. The narrowest deprived columns belonged to the ipsilateral eye, especially along the horizontal meridian and in the periphery, where columns are narrowest in control animals. At the earliest age that we tested (1 week), visual deprivation reduced the columns to fragments. These fragments always coincided with a cytochrome oxidase patch, or a short string of patches, in the upper layers. More severe column shrinkage occurred in layer IVcbeta (parvo) than layer IVcalpha (magno). The geniculate input to the patches in layer III (konio) appeared normal after deprivation, despite loss of CO activity. Surprisingly, the blind spot representation of the open eye was shrunken by monocular deprivation, although binocular competition is absent in this region. Our principal finding was that eyelid suture at age 1 week caused the most severe column shrinkage. With suture at later ages, the degree of column shrinkage showed a progressive decline. Deprivation commencing at age 12 weeks caused no column shrinkage. These results imply that primate visual cortex is most vulnerable to deprivation during the first weeks of life. Our experiments should provide further impetus for the treatment of children with congenital cataract at the earliest possible age.

A pattern of alternating light and dark columns was observed in wet, unstained sections of macaque striate cortex after monocular enucleation. The columns were clearest in layer IV, but could be detected through the full thickness of the cortex. Subsequent processing for cytochrome oxidase (CO) showed that the light columns in wet sections viewed under darkfield illumination matched the ocular dominance columns serving the enucleated eye. These columns labeled preferentially with an antibody to myelin basic protein, suggesting that greater myelin content accounted for their brighter appearance. However, when sections were counterstained with luxol fast blue, Gallyas and Woelcke myelin techniques, the enucleated eye's columns appeared pale. It is unclear why classical myelin stains and myelin basic protein immunohistochemistry yielded opposite results. Discrepant patterns of myelin distribution were also found in normal animals using different myelin stains. Luxol fast blue showed homogeneous staining in layer IVc of macaque striate cortex, but the Gallyas stain revealed a pattern of thin pale bands alternating with wide dark bands, matching the pattern seen with the Liesegang stain. The CO patches in layers II and III fit in register with the wide dark myelin bands. In layers II and III of striate cortex, the Gallyas and luxol fast blue methods both labeled the CO patches. However, in squirrel monkey V2 the Gallyas stain labeled the pale CO stripes, whereas luxol fast blue labeled the dark CO stripes. These results indicate that pattern of myelin staining in visual cortex can vary according to the choice of technique, and may not reflect the true distribution of myelin. Studies of myeloarchitecture should employ a variety of myelin techniques, including examination of unstained sections, to obtain the most accurate picture of cortical myelin content.

Little is known about intrinsic variation from animal to animal in the periodicity of columnar systems within various regions of the mammalian cerebral cortex. To address this issue, complete mosaics of the ocular dominance columns were reconstructed from flat-mounts of the left and right striate cortex (V1) in six normal adult macaques (Macaca fascicularis). To identify the columns, we enucleated the right eye and subsequently processed striate cortex for cytochrome oxidase (CO) activity. Average column areas for the intact eye and the missing eye were nearly equal, confirming that monocular enucleation in adult macaques produces negligible column shrinkage. The contralateral eye's columns occupied more territory than the ipsilateral eye's columns, even in the central visual field representation (0 degree to 8 degrees), where they predominated by 52 to 48%. The column mosaics showed remarkable variation in periodicity. The number of column pairs along the V1/V2 border ranged from 101 sets in one monkey to 154 sets in another. Average column width along the V1/V2 border ranged between 670 and 395 microns, a nearly twofold difference. The widest columns were found in the foveal representation. This high degree of innate variability should be taken into account when considering the effects of various sensory manipulations (e.g., strabismus, anisometropia), which have been reported to alter the periodicity of ocular dominance columns. We found pronounced intrinsic variation in the width and number of ocular dominance columns in a sample of six M. fascicularis, indicating that the number of hypercolumns within a given cortical area can range widely among normal members of the same species.

The squirrel monkey is the only primate reported to lack ocular dominance columns. Nothing anomalous about the visual capacity of squirrel monkeys has been found to explain their missing columns, leading to the suggestion that ocular dominance columns might be "an epiphenomenon, not serving any purpose" (Livingstone et al., 1995). Puzzled by the apparent lack of ocular dominance columns in squirrel monkeys, we made eye injections with transneuronal tracers in four normal squirrel monkeys. An irregular mosaic of columns, averaging 225 microns in width, was found throughout striate cortex. They were double-labeled by placing wheat germ agglutinin-horseradish peroxidase into the left eye and [3H]proline into the right eye. The tracers labeled opposite sets of interdigitating columns, proving they represent ocular dominance columns. The columns were much clearer in layer IVc alpha (magno-receiving) than IVc beta (parvo-receiving). In the lateral geniculate body, the parvo laminae showed extensive mixing of ocular inputs, suggesting that increased label spillover contributes to the blurred columns in layer IVc beta. The cytochrome oxidase (CO) patches were organized into distinct rows, but they bore no consistent relationship to the ocular dominance columns. These experiments indicate that ocular dominance columns are less well segregated in squirrel monkeys than macaques, but they are present. This fact is pertinent to a recent study reporting that ocular dominance columns are absent in normal squirrel monkeys, but induced to form by strabismus (Livingstone, 1996).

Previous experiments in animals have shown that early unilateral eyelid suture, a model of amblyopia induced by cataract, causes shrinkage of ocular dominance columns serving the deprived eye in the striate cortex. It is unknown whether the ocular dominance columns are affected in amblyopia produced by strabismus. We examined specimens of striate cortex obtained postmortem from a 79-year-old woman with a history of amblyopia in her left eye (20/800) since age 2 from accommodative esotropia. Four years prior to her death, she suffered an ischemic infarct of the left optic disc. This injury to the left optic disc made it possible to label the ocular dominance columns using cytochrome oxidase histochemistry. The pattern of ocular dominance columns was reconstructed throughout most of the right striate cortex. No shrinkage of columns was found. In the left cortex only half the column mosaic was labelled, because the patient had some residual vision in the temporal retina of her left eye. The columns within the labelled portion of the overall mosaic appeared normal. These findings indicate that shrinkage of ocular dominance columns does not occur in humans with amblyopia caused by accommodative esotropia. The ocular dominance columns are probably no longer susceptible to shrinkage at the age when most children with this condition begin to develop amblyopia.