Processing Deficits in Primary Visual Cortex of Amblyopic Cats

Abstract

Early esotropic squint frequently results in permanent visual deficits in one eye, referred to as strabismic amblyopia. The neurophysiological substrate corresponding to these deficits is still a matter of investigation. Electrophysiological evidence is available for disturbed neuronal interactions in both V1 and higher cortical areas. In this study, we investigated the modulation of responses in cat V1 to gratings at different orientations and spatial frequencies (SFs; 0.1–2.0 cycles/°) with optical imaging of intrinsic signals. Maps evoked by both eyes were well modulated at most spatial frequencies. The layout of the maps resembled that of normal cats, and iso-orientation domains tended to cross adjacent ocular dominance borders preferentially at right angles. Visually evoked potentials (VEPs) were recorded at SFs ranging from 0.1 to 3.5 cycles/° and revealed a consistently weaker eye for the majority of squinting cats. At each SF, interocular differences in VEP amplitudes corresponded well with differences in orientation response and selectivity in the maps. At 0.7–1.3 cycles/°, population orientation selectivity was significantly lower for the weaker eye in cats with VEP differences compared with those with no VEP amplitude differences. In addition, the cutoff SF, above which gratings no longer induced orientation maps, was lower for the weaker eye (≥1.0 cycles/°). These data reveal a close correlation between the loss of visual acuity in amblyopia as assessed by VEPs and the modulation of neuronal activation as seen by optical imaging of intrinsic signals. Furthermore, the results indicate that amblyopia is associated with altered intracortical processing already in V1.

Selectivity as well as sensitivity loss characterizes the cortical spatial frequency deficit in amblyopia

“…However, the more broadly tuned single cell explanation does have neurophysiological support. Studies of strabismic animals have shown that neurons driven by the amblyopic eye are less selective for spatial frequency [Chino et al, 1983;Schmidt et al, 2004], although it has to be said that comparable psychophysical evidence [Hess and Campbell, 1980;Levi and Harwerth, 1982] on strabismic humans is lacking. However, in humans the methods are not only confined to detection threshold (here we use suprathreshold stimuli) but also they necessarily involve indirect measures, such as adaptation [Hess and Campbell, 1980] and masking [Levi and Harwerth, 1982] both of which are open to alternative network-based interpretations [Meese and Holmes, 2007].…”

“…Other studies have suggested that there may be subtle deficits. Both the optimum spatial frequency of some neurons within the central field driven by the strabismic eye, at least in severe cases, is displaced to lower spatial frequencies [Kiorpes and McKee, 1999;Kiorpes et al, 1998;Movshon et al, 1987] and the spatial tuning bandwidth may be broader when driven by the strabismic eye [Chino et al, 1983;Schmidt et al, 2004]. Human brain imaging also has the potential to address the question of the spatial frequency dependence of the cortical loss of function in amblyopia.…”

Association of Optic Radiation Integrity with Cortical Thickness in Children with Anisometropic Amblyopia

“…It is accepted that a central rather than a retinal deficit is the major factor in the pathogenesis of amblyopia. Animal studies suggest that amblyopia represents functional and morphological influences of visual deprivation on the lateral geniculate nucleus (LGN) and visual cortex [3,4]. However, the neuroanatomical information on structural changes in the human brain remains limited and inconclusive.…”

The Maximum Tolerated Dose of Gamma Radiation to the Optic Nerve during Gamma Knife Radiosurgery in an Animal Study

“…VEP is a reliable method for detecting the visual changes in animals and has been standardized for cats and used in previous studies [26,27,28,29,30,31,32,33,34,35,36,37,38]. VEP is an important objective reference for assessing the electrophysiological functions of the visual pathway.…”