Response of the retina at low temporal frequencies

Chirped flicker optoretinography for in vivo characterization of human photoreceptors' frequency response to light

Flicker electroretinography (ERG) has served as a valuable noninvasive objective tool for investigating retinal physiological function through the measurement of electrical signals originating from retinal neurons in response to temporally modulated light stimulation. Deficits in the response at certain frequencies can be used as effective biomarkers of cone-pathway dysfunction. In this Letter, we present the progress we made on its optical counterpart-photopic flicker optoretinography (f-ORG). Specifically, we focus on the measurement of the response of light-adapted retinal photoreceptors to a flicker stimulus with chirped frequency modulation. In contrast to measurements performed at discrete frequencies, this technique enables a significantly accelerated characterization of photoreceptor outer segment optical path length modulation amplitudes in the nanometer range as a function of stimulus frequency, enabling the acquisition of the characteristic frequency response in less than 2 sec.

Multifocal Electroretinography in the Presence of Temporal and Spatial Correlations and Eye Movements

Releasing patients from the fixation task, and permitting them to view natural stimuli such as movies, would provide increased comfort, and potentially additional signs of retinal function, when recording multifocal electroretinograms (mfERGs). Techniques must be developed to handle the difficulties that arise from these alternative stimulation strategies. Multifocal stimuli were presented to volunteer human subjects with and without fixation. Retinocentric analyses were performed to deal with shifts of the stimulus across the retina in the presence of eye movements. Artificial scotomas that moved with the eyes to simulate local retinal defects were presented to assess whether such defects might be detectable in the presence of eye movements. Temporal and spatial correlations in the stimulus can be discounted, permitting retinal kernels to be measured in response to natural stimuli. Responses to temporally natural stimuli tend to have slightly stronger amplitudes because of the presence of low temporal frequencies in these stimuli. The effects of eye movement artifacts can be reduced, permitting similar kernels to be obtained in the absence and presence of eye movements. Convergence to stable kernels took slightly longer in the presence of temporal correlations or eye movements. Artificial scotomas can be localized with these methods. It may be possible to perform multifocal ERG recordings in the clinic using more flexible, natural techniques. However, work is needed to achieve results comparable to those routinely obtained with conventional methods.

Visual sensitivity and cortical response to the temporal envelope of amplitude-modulated flicker

To investigate the perception of a temporal envelope of flickering light is important for understanding nonlinear temporal processing in the visual system. The influence of the frequency components of a flickering light on the perception of the envelope remains unclear, with few studies having investigated the cortical activities for the envelope. We investigated the detection thresholds, brightness, and magnetoencephalographic responses related to amplitude-modulated (AM) flickering lights. The results showed that the sensitivity to flicker at the envelope periodicity of the AM flickering light was lower for a high-frequency carrier (40 Hz) than for a lower-frequency one (10, 20, or 30 Hz). Also, the primary visual cortex responded at the frequency corresponding to the envelope periodicity of the AM flickering light, and the strength of the cortical response reflected the brightness of the flicker at the envelope periodicity.

Origin of deficits in the flicker electroretinogram of the cone system in X-linked retinoschisis as derived from response nonlinearities

The aim of this study was to identify the origin of a high-frequency attenuation in the flicker electroretinogram (ERG) of patients with X-linked retinoschisis (XLRS) through an analysis of nonlinearities in the ERG response. The ERGs of six patients with XLRS and six age-similar control subjects were recorded in response to stimuli that consisted of pairs of sinusoids that had varying temporal frequencies and that differed by either 8 or 16 Hz. Compared with the control subjects, the patients with XLRS showed a significant reduction in the amplitude of the difference frequency to high-frequency stimuli that paralleled the high-frequency attenuation of their ERG response fundamental. This result indicates that a response attenuation at an initial linear filter, most likely photoreceptoral, was a major determinant of the reduced ERG amplitude of the XLRS patients at high temporal frequencies. Additional analyses of nonlinearities in the ERG responses provided evidence of a postreceptoral component to the flicker ERG deficits of the XLRS patients, as well.

Effects of pattern, spatial frequency, number, and rate of stimulus presentation on the accuracy of detection

Brief trains of pulsed stimuli were used to assess whether magnocellular or parvocellular visual pathways could be differentiated perceptually. Trains of either one to four sine-wave, square-wave, or checkerboard gratings were presented at three temporal and two spatial frequencies to six observers. The task of the observer was to report the perceived number of stimuli (gratings) in a train. The difference between actual number and perceived number of gratings was recorded as an error score. It was found that neither the pattern nor the spatial frequency of the gratings significantly affected perceptual accuracy. On the other hand, the number of gratings in a train and the interstimulus interval between gratings produced significant differences. Perceptual accuracy was greater when lower numbers of gratings in a train were presented with longer interstimulus intervals. The observers typically reported fewer stimuli than were presented. The source of the discrepancy is discussed in terms of a light adaptive process initiated in the retina.

Color from invisible flicker: a failure of the Talbot-Plateau law caused by an early 'hard' saturating nonlinearity used to partition the human short-wave cone pathway

The Talbot-Plateau law fails for flicker detected by the short-wavelength-sensitive (S) cones: a 30-40 Hz target, flickering too fast for the flicker to be resolved, looks more yellow than a steady target of the same average intensity. The color change, which is produced by distortion at an early compressive nonlinearity, was used to reveal a slightly bandpass S-cone temporal response before the distortion site and a lowpass response after it. The nonlinearity is probably a 'hard' nonlinearity that arises because the S-cone signal is limited by a response ceiling, which the mean signal level approaches and exceeds as the S-cone adaptation level increases. The nonlinearity precedes the combination of flicker signals from all three cone types.