A modified silent substitution electroretinography protocol to separate photoreceptor subclass function in lightly sedated dogs

Abstract

Objective A previously published study successfully isolated photoreceptor responses from canine rods, long/medium‐wavelength (L/M) cones, and short‐wavelength (S) cones using silent substitution electroretinography (ERG) performed under general anesthesia. We hypothesized that responses would be similar in dogs under sedation and that a curtailed protocol suitable for use in clinical patients could effectively isolate responses from all three photoreceptor subtypes. Animals studied Three normal adult purpose‐bred beagles (2 females and 1 male). Methods Dogs were dark‐adapted for 1 hour. Sine wave color stimuli were delivered using LEDs in a Ganzfeld dome. The ERG protocol under anesthesia was performed as previously published; sedated ERG protocols were performed after a 3‐day washout period. Intravenous sedation (dexmedetomidine 1.25 mcg/kg, butorphanol 0.1 mg/kg) was administered for sedation. Statistical analysis was performed using two‐way repeated‐measures ANOVA and linear regression. Results In both anesthetized and sedated animals, rod‐derived responses peaked at low frequency (4‐12 Hz), L/M‐cone responses peaked at high frequency (32‐38 Hz), and S‐cone responses peaked at low frequency (4‐12 Hz). The frequencies eliciting maximal responses were similar in sedated and anesthetized protocols, although rod amplitudes were significantly higher in the sedated protocols compared with anesthetized (P < .001). Conclusion We present a clinically applicable method to consistently isolate rod and cone subclass function in sedated dogs. This may allow detailed evaluation of photoreceptor function in clinical patients with rod or cone subclass deficits without the need for general anesthesia or protracted adaptation times.

Cone-Driven, Geniculocortical Responses in Canine Models of Outer Retinal Disease

“…Although the S-cones are relatively sparse in the canine retina, 46 dogs are known to be able to distinguish luminance and blue–yellow chrominance. 47 Our study used a relatively slow, 4 Hz L–S flicker stimulus, based on the low-pass characteristic of the S-cone ERG response 24 and by analogy to the temporal sensitivity of human vision for isolated S-cone modulations. 48 Visual cortex responses may have been reduced by habituation to this slow modulation.…”

“…The flicker frequency of these stimuli was 32 Hz, 4 Hz, and 16 Hz, respectively, selected based on the temporal sensitivity of canine vision. 23 , 24 The set of three acquisitions was collected for one eye, then the stimulus was switched to the other eye, and the acquisition set was repeated. A total of 18 acquisitions were acquired in a given scanning session.…”

Cone-driven, geniculo-cortical responses in canine models of outer retinal disease

“…While the S-cones are relatively sparse in the canine retina 46 , dogs are known to be able to distinguish luminance and “blue-yellow” chrominance 47 . Our study used a relatively slow, 4 Hz L–S flicker stimulus, based upon the low-pass characteristic of the S-cone ERG response 24 and by analogy to the temporal sensitivity of human vision for isolated S-cone modulations 48 . Visual cortex responses may have been reduced by habituation to this slow modulation.…”

PySilSub: An open-source Python toolbox for implementing the method of silent substitution in vision and non-visual photoreception research

“…Such lights can be used to stimulate pathways contributing to "nonvisual" responses to light, including melatonin suppression (Allen et al, 2018;Blume et al, 2022;Souman et al, 2018), sleep (Blume et al, 2022;Schöllhorn et al, 2023), and other neuroendocrine and circadian functions (Zandi et al, 2021). The method of silent substitution can also be used to investigate the contribution of melanopsin signaling to canonical visual processing (Allen et al, 2019;Brown et al, 2012;DeLawyer et al, 2020;Spitschan et al, 2017;Uprety et al, 2022;Vincent et al, 2021), and its potential as a diagnostic tool for retinal disease has garnered attention in recent years (Kuze et al, 2017;Wise et al, 2021).…”

“…The ColorimetricObserver class stores the photoreceptor action spectra in a pandas DataFrame (McKinney, 2010) which interacts with other parts of the toolbox as required. An advantage of this approach is that it enables the software to flexibly adapt to scenarios that may require alternative action spectra, such as targeting cones in the shadow of retinal blood vessels (Figure 5, C: Adams & Horton, 2002;Spitschan et al, 2015), performing silent substitution with non-human animals (Figure 5, D: e.g., Allen & Lucas, 2016;Mouland et al, 2019Mouland et al, , 2021Wise et al, 2021) ). Note that the effects of macular pigment absorption are considered negligible for rhodopsin and melanopsin, as rods are absent from the fovea and ipRGCs sit above the retinal pigment layer (Trieschmann et al, 2008).…”