Flicker sensitivity and cardiovascular function in healthy middle-aged people

Serial multifocal electroretinograms during long-term elevation and reduction of intraocular pressure in non-human primates

The purpose of this study was to evaluate the relationship between elevations of intraocular pressure (IOP) and the multifocal electroretinogram (mfERG) in non-human primates. Experimental glaucoma was induced in 4 rhesus and 4 cynomolgus monkeys by laser trabecular meshwork destruction (LTD) in one eye. To evaluate the contribution of ganglion cells to mfERG changes, one monkey of each species had previously underwent unilateral optic nerve transection (ONT). After >or=44 weeks of elevation, the IOP was reduced by trabeculectomy in 2 non-transected animals. In the intact (non-transected) animals, there was an increase in the amplitude of the early mfERG waveforms (N1 and P1) of the first-order kernel (K1) throughout the period of IOP elevation in all of the rhesus, but not all of the cynomolgus monkeys. A species difference was also present as a decrease of the second-order kernel, first slice (K2.1) in all of the cynomolgus monkeys but only in 1 of the rhesus monkeys (the 1 with the ONT). Similar IOP effects on the mfERG were seen in the ONT animals. Surgical lowering of IOP resulted in a return of the elevated K1 amplitudes to baseline levels. However, the depressed K2.1 RMS in the cynomolgus monkeys did not recover. These results demonstrate species-specific changes in cone-driven retinal function during periods of elevated IOP. These IOP-related effects can occur in the absence of retinal ganglion cells and may be reversible.

Central visual field changes using flicker perimetry in type 2 diabetes mellitus

PURPOSE

To evaluate the efficacy of flicker perimetry compared with that of static perimetry in determining central visual field losses in participants with type 2 diabetes mellitus.

METHODS

Flicker and static perimetry were performed using the Medmont field analyser on 20 participants with type 2 diabetes mellitus and 24 age-matched control participants.

RESULTS

Flicker, but not static, perimetry demonstrated significant reductions in thresholds in those participants who had a recent diagnosis of diabetes or who had no or only minimal diabetic retinopathy compared to the control group at eccentricities close to fixation.

CONCLUSION

Flicker perimetry is recommended for the evaluation of visual field losses in participants with recent onset of diabetes and minimal diabetic retinopathy.

Critical Flicker Fusion Frequency: Relation to Resting Systolic Blood Pressure

OBJECTIVE

Past studies have shown that a number of factors, both acute and chronic, influence individual differences in critical flicker fusion frequency (CFF). In this study, we assessed whether CFF co-varied with resting systolic blood pressure (SBP).

METHODS

SBP and CFF were assessed in one session in 221 subjects (mean age, 19.6 +/- 2.5 years), and SBP and CFF were assessed over 10 sessions in 12 subjects. CFF values were determined psychophysically using a 570 nm circular 1 degrees test field centrally fixated. Blood pressure was determined with an automated sphygmomanometer.

RESULTS

There was a significant (p < 0.0002) positive relation between CFF and SBP for the larger sample (n = 221). Significant within-subject relations were found for 6 of the 12 subjects tested repeatedly.

CONCLUSIONS

The combination of results strongly suggests that CFF and resting SBP are positively related.

High blood pressure and visual sensitivity

The study had two main purposes: (1) to determine whether the foveal visual sensitivities of people treated for high blood pressure (vascular hypertension) differ from the sensitivities of people who have not been diagnosed with high blood pressure and (2) to understand how visual adaptation is related to standard measures of systemic cardiovascular function. Two groups of middle-aged subjects--hypertensive and normotensive--were examined with a series of test/background stimulus combinations. All subjects met rigorous inclusion criteria for excellent ocular health. Although the visual sensitivities of the two subject groups overlapped extensively, the age-related rate of sensitivity loss was, for some measures, greater for the hypertensive subjects, possibly because of adaptation differences between the two groups. Overall, the degree of steady-state sensitivity loss resulting from an increase of background illuminance (for 580-nm backgrounds) was slightly less for the hypertensive subjects. Among normotensive subjects, the ability of a bright (3.8-log-td), long-wavelength (640-nm) adapting background to selectively suppress the flicker response of long-wavelength-sensitive (LWS) cones was related inversely to the ratio of mean arterial blood pressure to heart rate. The degree of selective suppression was also related to heart rate alone, and there was evidence that short-term changes of cardiovascular response were important. The results suggest that (1) vascular hypertension, or possibly its treatment, subtly affects visual function even in the absence of eye disease and (2) changes in blood flow affect retinal light-adaptation processes involved in the selective suppression of the flicker response from LWS cones caused by bright, long-wavelength backgrounds.

Adaptation of spatiotemporal mechanisms by increment and decrement stimuli

Sawtooth modulation has been used in the past to examine visual sensitivity to luminance increments and decrements. The threshold elevation caused by adaptation depends on the spatial profile of the stimulus field and the polarities of the adaptation and test stimuli. We hypothesized that the adaptation effects reflect a change in the sensitivity of the spatiotemporal channels that detect the stimuli. We used a 2-deg disk centered in a larger surround field. Five levels of contrast between the test field and surround were investigated: equiluminant, three intermediate levels, and dark. At each contrast, observers adapted for 5 s to 2-Hz sawtooth modulation (rapid-on or rapid-off). Immediately after adaptation, thresholds were measured for detection of a single cycle of either a rapid-on or a rapid-off waveform. Varying the contrast of the surround affected observers' sensitivity to the polarity of the sawtooth stimulus to the extent that the pattern of sensitivity with the equiluminant surround was the opposite of that with the dark surround. To examine temporal factors, we measured thresholds for slow (500-ms ramps) and fast (8.3-ms pulses) test stimuli. The adaptation effect was preserved with the ramp stimuli but not with the pulse stimuli. Blurring the edge between the test and surround fields in the equiluminant surround condition raised thresholds for all sawtooth test stimuli, suggesting that spatiotemporal channels sensitive to high spatial frequencies and low temporal frequencies facilitate detection in that condition. These findings suggest that adaptation to sawtooth modulation can differentially effect the sensitivity of ON and OFF pathways, but the relative desensitization of each pathway depends on an interaction with the adaptation state of spatiotemporal channels that are involved in detection.

Flashed stimuli and the suppression of flicker response from long-wavelength-sensitive cones: integrating two separate approaches

The selective suppression of flicker response from LWS cones has been investigated with two approaches. One approach has emphasized the use of light-adaptation conditions, and the other has emphasized the use of dark-adaptation conditions. In both cases, stimuli are arranged to restrict or exceed the ability of adaptation processes to maintain an afferent flicker response, and long-wavelength stimuli are used to overload spectrally opponent processes. By integrating these two approaches, this study shows that diverse manifestations of flicker response suppression can be closely related mechanistically. For instance, the steep flicker TVI slopes that resulted from superimposing temporally modulated (100% contrast) test stimuli on flashed backgrounds corresponded to the disappearance of flicker that resulted from increasing the time-averaged illuminance of temporally modulated stimuli (contrast x < 100%) that were flashed alone in an otherwise dark field. For the stimulus parameters of this study, flicker response suppression was more evident for small (19' diameter) than for large (1 degree diameter) stimuli. However, flicker response suppression was elicited reliably for both sizes by adding a spatially coincident short-wavelength stimulus to the interstimulus interval between presentations of the long-wavelength stimuli. By showing that temporal contrast can be treated as an independent variable for an important set of test/background stimulus combinations, the results of this study make it possible to investigate the means by which changes of contrast gain help to maintain flicker response as assessed in a conventional flicker TVI paradigm. The reduced degree of suppression for relatively large stimuli probably is related to the increased action of spatially extensive contrast gain-control processes. These contrast gain-control processes might not act independently of spectrally opponent processes.