A neurophysiological model for anomalous correspondence based on mechanisms of sensory fusion

Torsional anomalous retinal correspondence effectively expands the visual field in hemianopia

PURPOSE

Exotropia in congenital homonymous hemianopia has been reported to provide field expansion that is more useful when accompanied with harmonious anomalous retinal correspondence (HARC). Torsional strabismus with HARC provides a similar functional advantage. In a subject with hemianopia demonstrating a field expansion consistent with torsion, we documented torsional strabismus and torsional HARC.

METHODS

Monocular visual fields under binocular fixation conditions were plotted using a custom dichoptic visual field perimeter. The dichoptic visual field was also modified to measure perceived visual directions under dissociated and associated conditions across the central 50° diameter field. The field expansion and retinal correspondence of a subject with torsional strabismus (along with exotropia and right hypertropia) with congenital homonymous hemianopia was compared with that of another exotropic subject with acquired homonymous hemianopia without torsion and to a control subject with minimal phoria. Torsional rotations of the eyes were calculated from fundus photographs and perimetry.

RESULTS

Torsional anomalous retinal correspondence documented in the subject with congenital homonymous hemianopia provided a functional binocular field expansion up to 18°. Normal retinal correspondence was mapped for the full 50° visual field in the control subject and for the seeing field of the acquired homonymous hemianopia subject, limiting the functional field expansion benefit.

CONCLUSIONS

Torsional strabismus with anomalous retinal correspondence, when occurring with homonymous hemianopia provides useful field expansion in the lower and upper fields. Dichoptic perimetry permits documentation of ocular alignment (lateral, vertical, and torsional) and perceived visual direction under binocular and monocular viewing conditions. Evaluating patients with congenital or early strabismus for HARC is useful when considering surgical correction, particularly in the presence of congenital homonymous hemianopia.

Binocular Interactions in the Lateral Suprasylvian Visual Area of Strabismic Cats Following Section of the Corpus Callosum

Visually responsive neurons have been recorded in the lateral suprasylvian area (LSA) of cats raised with either a convergent or a divergent strabismus. In contrast to areas 17 and 18, where many studies have documented a profound loss of binocularly activated neurons following early strabismus, in the LSA the majority of cells could still be binocularly driven. Acute or chronic section of the splenium of the corpus callosum reduced but did not abolish binocularity in the LSA. We propose that the widespread callosal connections, the large size of the receptive fields and the peculiar internal circuitry of the LSA all concur in permitting the maintenance of binocular coding in spite of early misalignment of the eyes.

Binocular interactions and steady-state VEPs. A study in normal and defective binocular vision (Part II)

BACKGROUND

Recent evidence indicates that an index of binocular activity may be found in some properties of steady-state visual evoked potentials (VEPs), such as amplitude facilitation and phase shortening. We evaluated binocular interactions with steady-state VEPs in normal subjects as well as in patients with concomitant strabismus and defective binocular vision.

METHODS

Steady-state (8-Hz) VEPs to counterphased sinusoidal gratings (1.2 c/deg spatial frequency) of low contrast (3.2%) were recorded in 19 esotropic patients and in 18 age-matched controls. Patients had either anomalous retinal correspondence (ARC, n = 10) or suppression (n = 9) in casual seeing conditions (striated glasses). In all subjects, both binocular and monocular VEPs displayed a major component at twice the stimulation frequency (second harmonic), whose amplitude and phase were measured. A binocular interaction index was obtained by comparing binocular VEPs (BVEPs) with the sum (vectorial) of the two monocular VEPs (SMVEPs).

RESULTS

In normal subjects, BVEPs were larger in amplitude than SMVEPs (facilitation), and shortened in latency (phase). On average, both ARC and suppression patients displayed loss in amplitude facilitation and absence of phase shortening. However, 50% of ARC patients showed clear VEP facilitation. In both ARC and suppression patients, the amplitude ratio BVEP/SMVEP was negatively correlated with the amount of the angle of deviation.

CONCLUSION

These results suggest that losses in amplitude facilitation and phase shortening of binocular steady-state VEPs reflect abnormal binocular interactions associated with different forms of sensorial adaptation in concomitant strabismus.

Mechanism of anomalous retinal correspondence: maintenance of binocularity with alteration of receptive-field position in the lateral suprasylvian (LS) visual area of strabismic cats

We have examined the effects of rearing kittens with a unilateral convergent strabismus, induced surgically at 3 weeks of age, on the binocularity (ocular dominance) and receptive-field position of neurons in the motion-sensitive lateral suprasylvian (LS) area of cat extrastriate cortex. Data were compared to those obtained from area 17 in the same animals, and from the two areas of cortex in normal adult cats. Interocular alignment of the operated cats was assessed in alert adults using corneal reflex photography and during recording from the positions of retinal landmarks under paralysis. The strabismus magnitude in each operated cat was calculated by comparison with equivalent data from the normal animals. Strabismus always caused a major loss of binocularity in area 17. The remaining binocular neurons had receptive-field (RF) pairs arising from positions of normal correspondence in the two retinae and would thus have been responsive to different regions of visual space through the misaligned eyes in the alert animal. In area LS, the effects were dependent on the strabismus magnitude. In the group of four cats with pronounced strabismus (18-30 deg crossed), a loss of binocularity occurred in area LS equivalent in severity to that in area 17. The majority of the remaining binocular LS neurons possessed RF pairs in normal retinal correspondence and would thus, in the alert animal, have received spatially disparate visual input through the two eyes. This also occurred in three other cats with more moderate strabismus (11-15 deg crossed), although only a small breakdown in the binocularity of area LS was apparent. The group of cats with mild strabismus (less than or equal to 10 deg crossed) had normal proportions of binocular neurons in area LS. In three of these cats, the maintenance of binocularity was accompanied by shifts in RF position, with visual inputs arising from anomalous retinal locations. These shifts compensated, in part, for the strabismus angle present in each cat, so that most of the binocular LS neurons would have received inputs from regions of visual correspondence through the misaligned eyes when the animal was alert. Similar mechanisms could afford a basis for the binocular visual compensations that occur in humans with small-angle strabismus of early onset. If so, anomalous retinal correspondence in such individuals would have as a locus areas of extrastriate cortex with a role in motion perception, and would involve alterations to the neural substrate underlying normal binocular vision.

Field processes in stereovision. A description of stereopsis appropriate to ophthalmology and visual perception

There is, as yet, no satisfactory theory of stereopsis, despite the fact that our overt knowledge of "solid seeing" is now about 150 years old, and that contributions to our understanding come today from many fields: ophthalmology, psychology, psychophysics, neurophysiology, computer modelling, and optical-TV display technology. We review herein, and demonstrate for the reader whenever possible, certain key perceptual properties of the stereoscopic event of which any general theory must take account: vector stereoscopy and the neural grid, depth in empty visual fields, the relationship between stereoscopic and cognitive contours, stereoscopic contour formation in the presence of blur (thus, at low levels of central visual acuity), the phenomenon of cortical locking and of neural grid evocation in the presence of either peripheral or central rivalry, certain unusual ranges of figural mismatch and the concept of the horopter in relation to modern single cell electroneurophysiology in animals and to the constancy of visual directions. Some comments are also made on the concept of disparity processing by single cortical neurons, together with a short discussion of the implications of certain views of the genetics of stereovision for the perception of novel random texture sine-wave stereograms. We conclude that any theory pertinent to ophthalmology and visual science must combine the global concepts of cortical integration, the neural lock and the neural grid, herein introduced, with the more classical concepts of particulate or local binocular cortical correspondence. Certain preliminary steps in this direction are presented.

Surgical correction of convergent strabismus: its relationship to prism compensation

Esotropic patients whose angle of strabismus has been corrected by prisms frequently increase their angle deviation to compensate for the prismatic correction. This sensorio-motorial reaction to prism correction has been given the name of anomalous movements (a.m.). Quantification of a.m. has been made according to the amount of prisms that an esotropic patient is capable of compensating for (progressive prism compensation test--p.p. test). Some esodeviation does not compensate for any prisms at all since a.m. have not yet developed. Other cases compensate for as much as 40 or 60 prism diopters and more of over-correction of the angle deviation and they therefore have powerful a.m. The interference of these innervational forces acting on the medial recti to corrective surgery has been studied in 126 operated esotropic patients. A significant decrease from the expected surgical result (p less than 0.001) has been found in patients having powerful a.m., as can be judged by the p.p. test. It is believed that a.m. are an important drawback contributing to vitiate any formula on the amount of muscle surgery to be performed in patients having no possibilities of restoring normal binocular vision. Practical advice on how to eliminate this drawback and theoretical reasoning on the significance of a.m. are offered.

Objective evaluation of sensorial and sensorimotorial status in esotropia: their importance in surgical prognosis

Appropriate use of base-out prisms may be useful objective test for detecting persistence of normal binocular vision (4-dioptre prism test). By prolonged observation of prismatic correction of an esotropic patient one may infer the presence of an anomalous sensorial status. This can be done when the prismatic correction is compensated for by an increase of the angle of esotropia (prism adaptation test). The increase in the angle of esotropia induced by base-out prisms, here called anomalous movements, is probably related to a type of anomalous movement fusional in nature. When anomalous movements are present, it is important to realise how powerfully they have developed. This may be inferred by determining what amount of prism overcorrection of the esotropic angle the patient is capable of compensating for (progressive prism compensation test). This has important implications for surgery. It has been statistically demonstrated that esotropia with strong anomalous movements tends to respond less effectively to surgery than esotropia without or with weak anomalous movements.