Biology subjects

Holz, F. G.

Publications and source records attributed to Holz, F. G..

3 recordsLinked to original sources

bioRxiv2021

Human gaze is precisely aligned with the foveolar cone topography of both eyes

The small physical depression of the human retina, the fovea, is the retinal locus of prime visual resolution, achieved by a peaking topography of the light sensitive cone photoreceptor outer segments 1-3 and a post-receptor wiring scheme preserving high-density sampling 4,5. Humans dynamically direct their gaze such that the retinal images of objects of interest fall onto the foveola, the central one-degree diameter of the fovea 6-8, but it is yet unclear if a relationship between the individual photoreceptor topography at this location and visual fixation behavior exists 9,10. By employing adaptive optics in vivo imaging and micro-stimulation 11-13, we created topographical maps of the complete foveolar cone mosaics in both eyes of 20 healthy participants while simultaneously recording the retinal location of a fixated visual object in a psychophysical experiment with cellular resolution. We found that the locus of fixation was systematically shifted away from the topographical centers towards a naso-superior quadrant on the retina, about 5 minutes of arc of visual angle on average, with a mirror symmetrical trend between fellow eyes. In cyclopean view, the topographical centers were superior to the fixated target, corresponding to areas in the visual field usually more distant 14,15 and thus containing higher spatial frequencies. Given the large variability in foveal topography between individuals, and the surprising precision with which fixation is repeatedly directed to just a small bouquet of cones in the foveola, these findings demonstrate a finely tuned, functionally relevant link between the development of the cellular mosaic of photoreceptors and visual behavior.

neuroscience↗

bioRxiv2021

Retinal factors of visual sensitivity in the human fovea

Humans direct their gaze towards visual objects of interest such that the retinal images of fixated objects fall onto the fovea, a small anatomically and physiologically specialized region of the retina displaying highest visual fidelity. One striking anatomical feature of the fovea is its non-uniform cellular topography, with a steep decline of cone photoreceptor density and outer segment length with increasing distance from its center. We here assessed in how far the specific cellular organization of the foveola is reflected in visual function. Increment sensitivity to small spot visual stimuli (1 x 1 arcmin, 543 nm light) was recorded psychophysically in 4 human participants at 17 locations placed concentric within a 0.2-degree diameter around the preferred retinal locus of fixation with adaptive optics scanning laser ophthalmoscopy based microstimulation. While cone density as well as maximum outer segment length differed significantly among the four tested participants, the range of observed threshold was similar, yielding an average increment threshold of 3.3 {+/-} 0.2 log10 photons at the cornea. Thresholds were correlated with retinal eccentricity, as well as cone density and outer segment length. Biophysical simulation allowed to develop a model of foveal sensitivity based on these parameters, explaining at least 37% of the observed threshold variability. Based on high reproducibility in replicate testing, the residual variability is assumed to be caused by individual cone and bipolar cell weighting at the specific target locations.

neuroscience↗

bioRxiv2019

Eye tracking-based estimation and compensation of chromatic offsets for multi-wavelength retinal microstimulation with foveal cone precision

Multi-wavelength ophthalmic imaging and stimulation of photoreceptor cells requires consideration of chromatic dispersion of the eye, manifesting in longitudinal and transverse chromatic aberrations. Current image-based techniques to measure and correct transverse chromatic aberration (TCA) and the resulting transverse chromatic offset (TCO) in an adaptive optics retinal imaging system are precise, but lack compensation of small but significant shifts in eye position occurring during in vivo testing. Here we present a method that requires only a single measurement of TCO during controlled movements of the eye to map retinal chromatic image shifts to the image space of a pupil camera. After such calibration, TCO can be compensated by continuously monitoring eye position during experimentation and by interpolating correction vectors from a linear fit to the calibration data. The average change rate of TCO per head shift and the correlation between Kappa and the individual foveal TCA are close to the expectations based on a chromatic eye model. Our solution enables continuous correction of TCO with high spatial precision and avoids high light intensities required for re-measuring TCO after eye position changes, which is necessary for foveal cone-targeted psychophysical experimentation.

biophysics↗