Leisa M. Schmid

Retinal detachment induces massive immediate reorganization in visual cortex

Large inactive regions of the retina of adult cats were produced by the novel method of inducing monocular retinal detachment. Within a few hours, neurones throughout the detachment projection zone in primary visual cortex (55-136 mm2), including some > 4.5 mm from its boundary, were found to have large receptive fields displaced onto intact retina. The new receptive fields of some neurones represented shifts of up to 9 mm across the retinotopic representation. For these rapid changes to occur pre-existing viable circuits must provide a cortical locus with inputs from a wide extent of the retina. Receptive fields, and the retinotopic map, for stimulation of the other eye were unchanged.

Monocular Focal Retinal Lesions Induce Short-term Topographic Plasticity in Adult Cat Visual Cortex

Electrophysiological recording in primary visual cortex (V1) was performed both prior to and in the hours immediately following the creation of a discrete retinal lesion in one eye with an argon laser. Lesion projection zones (LPZs; 21 to 64 mm2) were defined in visual cortex by mapping the extent of the lesion onto the topographic representation in cortex. There was no effect on neuronal responses to the unlesioned eye or on its topographic representation. However, within hours of producing the retinal lesion, receptive fields obtained from stimulation of the lesioned eye were displaced onto areas surrounding the scotoma and were enlarged compared with the corresponding field obtained through the normal eye. The proportion of such responsive recording sites increased during the experiment such that 8 to 11 hours post lesion 56% of recording sites displayed neurons responsive to the lesioned eye. This is an equivalent proportion to that previously reported with long–term recovery (three weeks to three months). Responsive neurons were evident as far as 2.5 mm inside the border of the LPZ. The reorganization of the lesioned eye representation produced binocular disparities as great as 15° suggesting interactions between sites in V1 up to 5.5 mm apart.

Responsiveness of cat area 17 after monocular inactivation: limitation of topographic plasticity in adult cortex.

1. Recordings were made from neurones in the splenial sulcus of normal adult cats and adult cats which had one eye inactivated by enucleation or photocoagulation of the optic disc. Two visually responsive regions were observed, corresponding to the peripheral representation of visual area 1 (V1) and the splenial visual area. In normal animals, responses to the ipsilateral eye in V1 were restricted to the medial half of the splenial sulcus, up to 45‐50 deg eccentricity. Thus, by inactivating the eye contralateral to the experimental hemisphere, we created a region in V1, 1‐2 mm wide, that lacked normal inputs. 2. In contrast to results from previous experiments where lesions were placed in the central retina, neurones in the deprived peripheral representation remained unresponsive to light stimuli for up to 12 h after deactivation of the contralateral eye. 3. In animals that were allowed to recover from the monocular deactivation for periods of 2 days to 16 months, there was rearrangement of the retinotopic maps. Receptive fields in regions of cortex that normally represented the monocular crescent were displaced to the temporal border of the binocular field of vision. However, most neurones in the deprived peripheral representation remained unresponsive to visual stimuli even more than 1 year after treatment. This is also in marked contrast with the extensive reorganization that is observed in the central representation of V1 after restricted retinal lesions. Analysis of the cortical magnification factor demonstrates that the change in visual topography is local, and does not involve an overall centro‐peripheral shift of the retinotopic map. 4. Among the neurones that did show displaced receptive fields, the response properties were clearly abnormal. They showed a notable lack of spontaneous activity, low firing rates and rapid habituation to repeated stimulation. 5. The low potential for reorganization of the monocular sector of V1 demonstrates that the capacity for plasticity of mature sensory representations varies with location in cortex. Even relatively small pieces of cortex, such as the monocular crescent representations, may not reorganize completely if certain conditions are not met. These results suggest the existence of natural boundaries that may limit the process of reorganization of sensory representations.

Topography and extent of visual-field representation in the superior colliculus of the megachiropteran Pteropus

It has been proposed that flying foxes (genus Pteropus) have a primate-like pattern of representation in the superficial layers of the superior colliculus (SC), whereby the visual representation in this structure is limited by the same decussation line that limits the retino-geniculo-cortical projection (Pettigrew, 1986). To test this hypothesis, visual receptive fields were plotted based on single- and multi-unit recordings in the SC of ten flying foxes. A complete representation of the contralateral hemifield was observed in the SC. Although the binocular hemifield of vision in Pteropus is 54 deg wide, receptive-field centers invaded the ipsilateral hemifield by only 8 deg, and the receptive-field borders by 13 deg. This invasion is similar to that observed at the border between visual areas V1 and V2 in the occipital cortex. The extent of the ipsilateral invasion was not affected by a lesion that completely ablated the occipital visual areas, thus suggesting that this invasion may be consequence of a zone of nasotemporal overlap in the retinal projections to the two colliculi. Neurones located in the superficial layers typically responded briskly to stimulation of both eyes, with a bias towards the contralateral eye. After cortical lesions the neuronal responses to the ipsilateral eye were depressed, and the ocular-dominance histograms shifted towards an even stronger dominance by the contralateral eye. However, cells located in the rostral pole of the SC remained responsive to the ipsilateral eye after cortical lesions. Responses in the stratum opticum and stratum griseum intermediale were more severely affected by cortical lesions than those in the stratum griseum superficiale. Our results demonstrate that the SC in flying foxes retain some generalized mammalian characteristics, such as the stronger direct projections of the contralateral eye and the location of the upper, lower, central, and peripheral representations in the SC. Nonetheless, the extent of visual representation in the SC demonstrates a specialized, primate-like pattern. These observations are consistent with the hypothesis that megachiropterans are members of a group that branched off early during the differentiation of primates from basal mammals.

The detection of diabetic retinopathy by Australian optometrists

Background: Diabetes mellitus is a systemic disease affecting approximately 750,000 Australians of whom more than 70,000 are Queenslanders. It can have serious ocular consequences and patients with diabetes require regular eye examinations to determine the degree of ocular involvement and the stage of retinopathy, if present. It is important that optometrists detect diabetic retinal changes and refer appropriately. We sought to determine the proficiency of optometrists at detecting retinal changes caused by diabetes.

Retinal Topography in the Koala (Phascolarctos cinereus)

Nissl-stained retinal wholemounts were used to investigate the topographical organization of the ganglion cell layer of the koala (Phascolarctos cinereus); the visual resolution limit of this animal was subsequently estimated from retinal ganglion cell density data. Two types of cells could be differentiated on the basis of their size and staining characteristics: a subpopulation of presumed ganglion cells, consisting of medium to large cells with Nissl substance in the cytoplasm and pale uniformly staining nuclei, and a further subpopulation of small, densely staining cells. The latter group were presumed to be neuroglia and displaced amacrine cells. Iso-density contour maps were prepared from total cell counts and also counts of presumed ganglion cells; in all cases, the density of cells was greatest in the inferior retina where there was an area of peak density occurring as a poorly developed, horizontal streak that extended across the inferior retina. The inferior position of the streak in the koala contrasts with reports of the superior position of streaks in other marsupials. Peak cell densities of 2370 cells/mm2 and 1480 cells/mm2 were recorded for the total cell population and the presumed ganglion cell subpopulation, respectively. The latter value is equivalent to a visual resolution of 2.4 cycles/degree, based on sampling theory and a square packing paradigm, placing the koala close in visual performance to two other marsupials, the Australian Northern native cat and the American Virginia opossum.

Ocular allergy: causes and therapeutic options.

Ocular allergic eye conditions are among the most common anterior eye problems encountered in optometric practice. There are six common forms of ocular allergy: seasonal allergic conjunctivitis, perennial allergic conjunctivitis, vernal keratoconjunc‐tivitis, atopic keratoconjunctivitis, contact lens associated papillary conjunctivitis and contact ocular allergy. Here, we review the current understanding of the pathophysiol‐ogy underlying ocular allergic conditions and describe the different causes and forms of allergic eye disease and different treatment options.

A survey of ocular therapeutic pharmaceutical agents in optometric practice

Background: In all Australian states optometrists are permitted to use diagnostic topical ocular drugs in the practice of their profession. In addition, legislation has just been passed in Victoria allowing optometrists to prescribe topical S4 medications for ocular conditions. Changing optometric legislation to incorporate S4 ophthalmic agents is a topical issue within optometry.