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During postnatal refractive development, an emmetropization mechanism uses refractive error to modulate the growth rate of the eye. Hyperopia (image focused behind the retina) produces what has been described as "GO" signaling that increases growth. Myopia (image focused in front of the retina) produces "STOP" signaling that slows growth. The interaction between GO and STOP conditions is non-linear; brief daily exposure to STOP counteracts long periods of GO. In young tree shrews, long-wavelength (red) light, presented 14 h per day, also appears to produce STOP signals. We asked if red light also shows temporal non-linearity; does brief exposure slow the normal decrease in hyperopia in infant animals? At 11 days after eye opening (DVE), infant tree shrews (n = 5/group) began 13 days of daily treatment (red LEDs, 624 ± 10 or 636 ± 10 nm half peak intensity bandwidth) at durations of 0 h (normal animals, n = 7) or 1, 2, 4, or 7 h. Following each daily red period, colony lighting resumed. A 14 h red group had no colony lights. Refractive state was measured daily; ocular component dimensions at the end of the 13-day red-light period. Even 1 h of red light exposure produced some hyperopia. The average hyperopic shift from normal rose exponentially with duration (time constant 2.5 h). Vitreous chamber depth decreased non-linearly with duration (time constant, 3.3 h). After red treatment was discontinued, refractions in colony lighting recovered toward normal; the initial rate was linearly related to the amount of hyperopia. The red light may produce STOP signaling similar to myopic refractive error., (Copyright © 2018 Elsevier Ltd. All rights reserved.)

The local-global map hypothesis states that locally organized response properties--such as orientation preference--result from visuotopically organized local maps of non-retinotopic response properties. In the tree shrew, the lateral extent of horizontal patchy connections is as much as 80-100% of V1 and is consistent with the length summation property. We argue that neural signals can be transmitted across the entire extent of V1 and this allows the formation of maps at the local scale that are visuotopically organized. We describe mechanisms relevant to the formation of local maps and report modeling results showing the same patterns of horizontal connectivity, and relationships to orientation preference, seen in vivo. The structure of the connectivity that emerges in the simulations reveals a 'hub and spoke' organization. Singularities form the centers of local maps, and linear zones and saddle-points arise as smooth border transitions between maps. These findings are used to present the case for the local-global map hypothesis for tree shrew V1.

To study the correlation between blood-retinal barrier (BRB) permeability and development of form deprivation (FD) myopia, FD was induced in tree shrews. The refractive error and the axial dimensions of the optical elements were measured. Ocular fluorescence was measured before and after fluorescein-Na injection. The inward permeability (P(in)) of the BRB was measured before and 15, 30, and 45 days after FD was induced. FD eyes became significantly myopic 15 days after FD was induced (P<0.01), and myopia progressed 45 days after FD was induced compared with untreated controls. Neither anterior chamber length nor lens thickness changed significantly. The vitreous chamber in FD eyes, however, was significantly elongated from 15 days after FD was induced (P<0.01) compared with controls. The P(in) ratio (P(in) [FD eye]/P(in) [untreated control]), increased significantly 45 days after FD was induced (P<0.05). In FD myopia in tree shrews, the BRB permeability increases abnormally. Impaired BRB function might be a secondary effect of myopia development rather than the cause of myopia.

To examine the susceptible period for deprivation-induced myopia, six groups of tree shrew pups (Tupaia glis belangeri) were monocularly deprived for 12 days with an opaque occluder starting 7, 15, 21, 33, 48, or 63 days after natural eyelid opening. Compared to the untreated fellow control eye, significant myopia and vitreous chamber elongation were produced by the deprivation in all six groups. The effect was greater in the middle three groups in comparison with the youngest and the two oldest groups and the amount of induced myopia and axial elongation was not proportional to the normal rate of axial growth. The peak period of susceptibility was between approximately 15 and 45 days after eye opening during the juvenile, slow-elongation phase of ocular development when the eye is within 7% of its adult axial length. Significant myopia and axial elongation were also induced in adult animals by 70 days of monocular deprivation. To examine recovery from deprivation-induced myopia, the occluder was removed at the end of the 12 day deprivation period. After an additional 48 days of binocular visual experience, no significant myopia was present in the previously deprived eyes in any experimental group. During the recovery period, the elongation rate of the previously deprived eyes was reduced in comparison with the control eyes while normal corneal flattening and lens development continued, thus reducing the myopia. No difference in corneal curvature, relative to the untreated control eyes, was found after deprivation or after the recovery period. Data are presented which suggests that changes in the thickness of the choroid may occur in this mammal during deprivation and recovery that are in the same direction, but of smaller magnitude, than those reported in the chicken. The results of this study provide evidence that visually guided emmetropization occurs in this mammalian species during a period of ocular development analogous to the juvenile period in humans.

The retina of the tree shrew (Tupaia belangeri) is heavily cone dominated, rods comprising less than 4% of the total photoreceptors. Spectral mechanisms and color vision were investigated in this species in both behavioral and electrophysiological experiments. In confirmation of an earlier investigation, the tree shrew was found to have a clear spectral neutral point (at ca 505 nm) and is thus a dichromat. Spectral sensitivity functions determined in an increment threshold discrimination task show two clear peaks (at ca 440 and 550-560 nm) with an intermediate region of lowered sensitivity centered at about 500 nm. Spectral sensitivity of the two cone types in this animal were determined using ERG flicker photometry. One of these cone classes has a peak at 556 nm; the other has a 444 nm peak.