Retinal mechanisms determine the subadditive response to polychromatic light by the human circadian system.

Light is the major synchronizer of circadian rhythms to the 24-h solar day. The intrinsically photosensitive retinal ganglion cells (ipRGCs) play a central role in circadian regulation but cones also provide, albeit indirectly, input to these cells. In humans, spectrally opponent blue versus yellow (b-y) bipolar cells lying distal to the ganglion cell layer were hypothesized to provide direct input to the ipRGCs and therefore, the circadian system should exhibit subadditivity to some types of polychromatic light. Ten subjects participated in a within-subjects 3-night protocol. Three experimental conditions were employed that provided the same total irradiance at both eyes: (1) one unit of blue light (lambda(max)=450 nm, 0.077 W/m(2)) to the left eye plus one unit of green light (lambda(max)=525 nm, 0.211 W/m(2)) to the right eye, (2) one unit of blue light to the right eye plus one unit of green light to the left eye, and (3) 1/2 unit of blue light plus 1/2 unit of green light to both eyes. The first two conditions did not differ significantly in melatonin suppression while the third condition had significantly less melatonin suppression than conditions 1 and 2. Furthermore, the magnitudes of suppression were well predicted by a previously published model of circadian phototransduction incorporating spectral opponency. As was previously demonstrated, these results show that the human circadian system exhibits a subadditive response to certain polychromatic light spectra. This study demonstrates for the first time that subadditivity is due to spectrally opponent (color) retinal neurons.

Effects of light stimulation of embryos on the use of position-specific and object-specific cues in binocular and monocular domestic chicks (Gallus gallus).

Chicks hatched from eggs incubated in the dark (D-chicks) or from eggs exposed to light during the last 3 days before hatching (L-chicks) were trained on day 4 to peck at small cones for food reinforcement. The cones had different patterns (checked or striped) and were located in different positions (either on the left or on the right of a rectangular arena) so as both object-specific (pattern) and position-specific cues could be used to discriminate cones that contained or that did not contain food. After learning, the position of the cones was reversed so that object- and position-specific cues provided contradictory information. No effect of light incubation was observed in binocular chicks that chose cones on the basis of object-specific cues. Monocular D-chicks also tended to approach and peck the cones with the correct pattern in the wrong position, whereas monocular L-chicks did not show any clear choice. Initial choices for one side or other of the arena were mostly determined by the first side visible through the non-occluded eye in D-chicks, particularly when using their left eye. These results suggest that light exposure of the embryo makes neural mechanisms that do not receive direct visual input (i.e., those of the occluded side) more available to be used in assessment of novelty.

Treatment of presbyopia with conductive keratoplasty: six-month results of the 1-year United States FDA clinical trial.

PURPOSE: To provide 6-month results of a 1-year clinical trial evaluating conductive keratoplasty (CK) for the treatment of presbyopic symptoms in emmetropic and hyperopic eyes. METHODS: A total of 143 patients with presbyopic symptoms were enrolled in this 1-year United States FDA clinical trial and treated to improve near vision in 1 eye (unilateral treatment). In addition, 33 fellow eyes were treated to improve distance vision (bilateral treatment). For near vision correction, the target refraction was up to -2.0 D in the nondominant eye, and for distance vision correction, 0.0 D. Enrolled patients had a preoperative spherical equivalent of plano to +2.00 D, no more than 0.75 D of refractive astigmatism, and were 40 years of age or older. No retreatments were performed. RESULTS: Of the eyes treated for near, 77% had uncorrected near vision of J3 or better at 6 months postoperatively. A total of 85% of all patients had binocular distance UCVA of 20/25 or better along with J3 or better near, a combination that represents functional acuity for a presbyope. Sixty-six percent of eyes treated for near had a manifest refractive spherical equivalent (MRSE) within +/- 0.50 D of intended at 6 months. In 89% of eyes, the MRSE changed 0.05 D or less between 3 and 6 months postoperatively. After month 1, the incidence of variables associated with safety was 1% or lower. Seventy-six percent were very satisfied or satisfied with their procedure. CONCLUSIONS: CK appears to be very safe and effective in producing functional visual acuity in presbyopic eyes up to 6 months following the procedure. Patient satisfaction with the procedure is similar to that of monovision LASIK.

Radiotherapy in thyroid eye disease: the effect on the field of binocular single vision.

PURPOSE: Thyroid eye disease (TED) can be a functionally disabling condition if ocular muscle involvement causes diplopia. The extraocular muscle restriction creates a reduced or eccentric field of binocular single vision (BSV). Orbital radiotherapy is now widely used in the treatment of TED, and although it has been reported as improving ocular motility, there have been few quantitative studies of the effect of treatment on ocular motor function. METHODS: Retrospective case note review of patients undergoing orbital radiotherapy for TED between 1992-1998 identified 79 case records. A total of 27 patients had diplopia in primary position or a significantly reduced binocular field before undergoing radiotherapy. The fields of BSV were analyzed pretreatment and at 3, 12, and 24 months after therapy to assess any improvement in function. We used the field of BSV as an outcome measure because it can be quantified and is a good indicator of functional ability. RESULTS: None of the 12 patients with double vision in primary position pretreatment regained a central binocular field with radiotherapy alone. Of the 15 patients with a central but reduced binocular field, 8 (53%) remained unchanged with treatment. In 4 patients (26.6%), there was an improvement in the field, while in 3 (20%) the field deteriorated. In all, 12 patients (44%) went on to require strabismus surgery. CONCLUSIONS: Orbital radiotherapy alone is ineffective in treating restrictive thyroid myopathy and improving binocular function.

Role of primary visual cortex in the binocular integration of plaid motion perception.

This study assessed the early mechanisms underlying perception of plaid motion. Thus, two superimposed gratings drifting in a rightward direction composed plaid stimuli whose global motion direction was perceived as the vector sum of the two components. The first experiment was aimed at comparing the perception of plaid motion when both components were presented to both eyes (dioptic) or separately to each eye (dichoptic). When components of the patterns had identical spatial frequencies, coherent motion was correctly perceived under dioptic and dichoptic viewing condition. However, the perceived direction deviated from the predicted direction when spatial frequency differences were introduced between components in both conditions. The results suggest that motion integration follows similar rules for dioptic and dichoptic plaids even though performance under dichoptic viewing did not reach dioptic levels. In the second experiment, the role of early cortical areas in the processing of both plaids was examined. As convergence of monocular inputs is needed for dichoptic perception, we tested the hypothesis that primary visual cortex (V1) is required for dichoptic plaid processing by delivering repetitive transcranial magnetic stimulation to this area. Ten minutes of magnetic stimulation disrupted subsequent dichoptic perception for approximately 15 min, whereas no significant changes were observed for dioptic plaid perception. Taken together, these findings suggest that V1 is not crucial for the processing of dioptic plaids but it is necessary for the binocular integration underlying dichoptic plaid motion perception.