Biomedical subjects

Darren R Brinkworth

Publications and source records attributed to Darren R Brinkworth.

3 recordsLinked to original sources

PubMed2006

What image properties regulate eye growth?

The growth of the eye, unlike other parts of the body, is not ballistic. It is guided by visual feedback with the eventual aim being optimal focus of the retinal image or emmetropization . It has been shown in animal models that interference with the quality of the retinal image leads to a disruption to the normal growth pattern, resulting in the development of refractive errors and defocused retinal images . While it is clear that retinal images rich in pattern information are needed to control eye growth, it is unclear what particular aspect of image structure is relevant. Retinal images comprise a range of spatial frequencies at different absolute and relative contrasts and in different degrees of spatial alignment. Here we show, by using synthetic images, that it is not the local edge structure produced by relative spatial frequency alignments within an image but rather the spatial frequency composition per se that is used to regulate the growth of the eye. Furthermore, it is the absolute energy at high spatial frequencies regardless of the spectral slope that is most effective. Neither result would be expected from currently accepted ideas of how human observers judge the degree of image "blur" in a scene where both phase alignments and the relative energy distribution across spatial frequency (i.e., spectral slope) are important.

Animals↗

PubMed2005

The effect of manipulations to target contrast on emmetropization in chick.

Emmetropization is dependent on visual feedback and presumably some measure of the optical and image quality of the eye. We investigated the effect of simple alterations to image contrast on eye growth and refractive development. A 1.6 cyc/deg square-wave-grating target was located at the end of a 3.3 cm cone, imaged by a +30 D lens and applied monocularly to the eyes of 8-day-old chicks. Eleven different contrast targets were tested: 95, 67, 47.5, 33.5, 24, 17, 12, 8.5, 4.2, 2.1, and 0%. Refractive error (RE), vitreous chamber depth (VC) and axial length (AL) varied with the contrast of the image (RE diff: F(10,86)=12.420, p<0.0005; VC diff: F(10,86)=8.756, p<0.0005; AL diff: F(10,86)=9.240, p<0.0005). Target contrasts 4.2% and lower produced relative myopia (4.2%: RE diff=-7.48+/-2.26 D, p=0.987; 2.1%: RE diff=-7.22+/-2.77 D, p=0.951) of similar amount to that observed in response to a featureless 0% contrast target (RE diff=-9.11+/-4.68 D). For target contrast levels 47.5% and greater isometropia was maintained (95%: RE diff =1.83+/-2.78 D; 67%: RE diff=0.14+/-1.84 D; 47.5%: RE diff=0.25+/-1.82 D). Contrasts in between produced an intermediate amount of myopia (33.5%: RE diff=-2.81+/-1.80 D; 24%: RE diff=-3.45+/-1.64 D; 17%: RE diff=-3.19+/-1.54 D; 12%: RE diff=-4.08+/-3.56 D; 8.5%: RE diff=-4.09+/-3.60 D). We conclude that image contrast provides important visual information for the eye growth control system or that contrast must reach a threshold value for some other emmetropization signal to function.

Animals↗

PubMed2003

Emmetropisation responses when visual information is presented at only one or two near target planes in chick.

PURPOSE: When visual information is confined to one object plane and zero or hyperopic defocus is present, emmetropisation is directed towards this plane, when myopic defocus is present, emmetropisation processes fail. We investigated the effect of introducing information at a second nearer plane on emmetropisation responses under these conditions. METHODS: The visual environment was controlled using lenses (+30 D, +40 D and +50 D) and cones with a Maltese cross target (MX) at one or two distances. Four different target configurations were used: 1. a single target was located at 3.3 cm, 2. a vertical hemi-field target was added at 2.5 cm, 3. a transparent target was added at 2.5 cm or 4. a single target was located at 2.5 cm. An additional cone length of 4.0 cm and nearer target distance of 3.3 cm was used with the +50 D lens. The imaging devices were applied monocularly to eight-day-old chicks and worn for four days. At the end of the treatment, refractive errors and eye growth were measured. Potential regional differences in growth were also assessed. RESULTS: The configuration of the target and the interaction between target configuration and lens power had significant refractive and axial growth effects. With a single target plane, myopic and hyperopic defocus resulted in myopia. When defocus was experienced at two planes, refractive errors shifted towards the plane with the lower defocus and emmetropisation responses, although still not normal, were more consistent. CONCLUSIONS: When visual information is provided at two distances, the target with the lesser incident defocus has the greater influence on the resultant refractive error. Emmetropisation responses are more accurate when information is presented at many distances.

Accommodation, Ocular↗