CRITICAL FUSION FREQUENCY IN THE CENTRAL VISUAL FIELD

Visual temporal frequency preference shows a distinct cortical architecture using fMRI

Studies of visual temporal frequency preference typically examine frequencies under 20 Hz and measure local activity to evaluate the sensitivity of different cortical areas to variations in temporal frequencies. Most of these studies have not attempted to map preferred temporal frequency within and across visual areas, nor have they explored in detail, stimuli at gamma frequency, which recent research suggests may have potential clinical utility. In this study, we address this gap by using functional magnetic resonance imaging (fMRI) to measure response to flickering visual stimuli varying in frequency from 1 to 40 Hz. We apply stimulation in both a block design to examine task response and a steady-state design to examine functional connectivity. We observed distinct activation patterns between 1 Hz and 40 Hz stimuli. We also found that the correlation between medial thalamus and visual cortex was modulated by the temporal frequency. The modulation functions and tuned frequencies are different for the visual activity and thalamo-visual correlations. Using both fMRI activity and connectivity measurements, we show evidence for a temporal frequency specific organization across the human visual system.

Critical flicker fusion test of potential vision

PURPOSE

To continue developing a potential vision test based on the critical flicker fusion (CFF) phenomenon by using a brighter stimulus and optimizing its size.

SETTING

Flinders Eye Centre, Flinders Medical Centre, Flinders University, Bedford Park, South Australia, Australia.

METHODS

In a prospective nonrandomized study, 225 participants were assigned to 1 of 4 groups: normal, media opacity only, retinal/neural disease only, and cataract plus retinal/neural disease. Participants were recruited if they were 20 years or older but were excluded if they had a neurological disorder or medication known to affect CFF. The CFF thresholds were measured for 3 stimulus sizes: 0.5 degree, 1.0 degree, and 1.5 degrees. Discrimination between groups was tested by analysis of variance and receiver operating characteristic analysis. The relationship between visual acuity and CFF in eyes without media opacity was determined by linear regression and used to predict visual outcomes in 23 eyes having cataract surgery.

RESULTS

The mean age of the 225 participants was 71.4 years +/- 13.2 (SD); 134 (59.8%) were women. The normal group had 41 participants, and the other 3 groups had 61 participants each. Critical flicker fusion thresholds were reduced in retinal/neural disease but resistant to image degradation from media opacity. The 1.5-degree stimulus had 88% sensitivity and 90% specificity for discriminating groups. Visual acuity after cataract surgery was accurately predicted within +/-1 line in 43% of eyes, +/-2 lines in 83%, and +/-3 lines in 100%. All eyes with poor visual acuity (>0.50 logMAR) or dense cataract (>4.0 Lens Opacities Classification System III) were predicted within +/-2 lines.

CONCLUSIONS

The CFF phenomenon effectively discriminated between subjects with and without retinal/neural disease and accurately predicted visual outcome after cataract surgery. The use of a brighter stimulus enhanced performance in cases of dense media opacity.

Duration thresholds for target detection and identification in the peripheral visual field

PURPOSE

In the current study, we measured threshold duration required to perform the tasks of letter identification, letter detection, grating orientation discrimination, and symmetry detection of targets presented as a function of retinal eccentricity.

METHODS

Targets were presented in the nasal visual field on the horizontal meridian at eccentricities ranging from 0 degree to 22 degrees. Threshold duration was calculated using a modified staircase procedure.

RESULTS

We found that for the task of letter identification, the rate of duration sensitivity loss was independent of the letter size. In contrast, for the tasks of letter detection, grating discrimination, and symmetry detection, the rate of loss was greater for smaller targets than for larger targets.

CONCLUSION

At equivalent target sizes, letter identification always demonstrated the steepest decline in duration sensitivity with increasing eccentricity. This might be the case if the final stages of the identification process are rate limiting. It was hoped that by quantifying duration sensitivity of the peripheral retina, more appropriate temporal stimuli could be used in rehabilitation programs that train reading using eccentric preferred retina loci in patients with advanced macular disease.

Outer-retina locus of increased flicker sensitivity of the peripheral retina

We tested alternative hypotheses concerning the locus of enhanced flicker sensitivity observed in response to stimuli presented to the peripheral retina. The first hypothesis attributes increased temporal frequency sensitivity to ganglion cell and higher-order neural processing, whereas the second hypothesis states that the locus of these temporal effects is at the cone photoreceptors. To test these alternative hypotheses we measured retinal electrophysiological and psychophysical temporal modulation thresholds. We found that sensitivity for temporal frequencies < 30 Hz did not vary as a function of retinal location for either the focal electroretinogram (ERG) or the psychophysical measure. However, for both measures, sensitivity for temporal frequencies > or = 30 Hz was greater in the peripheral retina than in the central retina. In addition, critical flicker frequency for the central retina was linear as a function of retinal illuminance for both the psychophysical and the electrophysiological measures. For the peripheral retina the slopes of critical flicker frequency versus log illuminance functions were steeper than the central slopes for both threshold measures. Eccentrically measured focal ERG and psychophysical critical flicker frequency values showed a relative saturation, deviating from the linear slope above 3.5 log Td. The findings of similar focal ERG and psychophysical temporal sensitivity changes with eccentricity support an outer retinal origin of this phenomenon.

Loss of visual function associated with microalbuminuria in diabetes mellitus: a pilot study

Visual function loss has been documented in diabetes mellitus in relation to flicker and contrast. However, no direct correlation between the degree of loss in sensitivity and the level of retinopathy has been established. It has been suggested that such non-invasive psychophysical procedures actually reflect metabolic disturbances within the diabetic retina. This study investigates the possibility of whether early nephropathy demonstrated by microalbuminuria, is an indicator of microangiopathy which may be a cause of retinal disturbance leading to a loss of visual function. The visual function of a group of diabetics showing microalbuminuria was studied. Contrast and flicker threshold were measured and the results compared with those obtained with an age-matched control diabetic group. The procedures used effectively separated the two groups and raises the issue of incorporating psychophysics in retinal screening programmes.

Changes in the focal electroretinogram with retinal eccentricity

Flicker sensitivity increases in the peripheral retina when relatively large targets are used. This enhancement of cone system-mediated temporal sensitivity persists even when corrections are made for cortical magnification factors. It has been suggested that the differences in temporal frequency response characteristics across the retina are based on differences in receptor morphology between the peripheral and central cones. We have examined a possible retinal origin of this phenomenon by obtaining psychophysical and electroretinographic data at a variety of locations on the temporal retina. Psychophysical results show an increased sensitivity for high temporal frequency stimuli (above 30 Hz) with retinal eccentricity whether or not the stimulus size was scaled. Focal electroretinograms recorded with a constant size stimulus did not show an increase in amplitude with eccentricity. However, when an equal number of receptors were stimulated by scaling the target size, focal amplitudes were larger in the periphery. The electrophysiological findings are consistent with a possible retinal origin for this flicker enhancement phenomenon.