Threshold versus intensity functions in two‐colour automated perimetry

Abstract

Purpose Two‐colour computerised perimetry is a technique developed for assessing cone‐ and rod‐function at fixed background luminances in retinal disease. However, the state of adaptation during testing is unknown but crucial in the interpretation of results. We therefore aimed to determine the adaptational state of rod‐ and cone‐mechanisms in two‐colour perimetry. Methods Sensitivity to 480 nm (blue) and 640 nm (red) Goldmann size V targets was determined for 10 normal subjects aged 16 to 46 years at 17 locations in the central 60 degrees of the visual field under scotopic conditions and then from −1.5 log cd m−2 to 2 log cd m−2 (white background) in 0.5 log unit steps. Data were fitted with threshold versus intensity (tvi) functions of the form logT = logT0 + log ((A + A0)/A0)n. Results No clear rod‐cone break was observed for 640 nm stimuli. For 480 nm stimuli, transition from rod‐detection to cone‐detection occurred at mesopic illumination levels, where rod adaptation approached Weber behaviour. Cone detection mechanisms did not display Weber‐like adaptation until the background luminance approached 1 log cd.m−2. Diseases resulting in a “filter effect” ‐ including disorders of the photoreceptors ‐ are therefore predicted to affect sensitivity when rod function is probed with short‐wavelength targets under scotopic conditions, but less so under mesopic conditions. Filter effects are similarly anticipated to affect cone function measured using long‐wavelength targets under mesopic conditions (e.g., during microperimetry), but less so under photopic conditions. Conclusions Asymmetries in adaptation in automated two‐colour perimetry are predicted to artefactually favour the detection of losses in rod sensitivity under scotopic conditions and cones under mesopic conditions.

Perifoveal Cone- and Rod-Mediated Temporal Contrast Sensitivities in Stargardt Disease/Fundus Flavimaculatus

“… 43 Second, we measured rods and cones with identical retinal adaptation, and asymmetrical adaptation can also explain seemingly predominant loss of rod function. 44 In fact, Collison et al 45 found that, when both rods and cones were measured at absolute threshold, cone function losses even exceeded rod function losses.…”

Structural and Functional Characteristics of Color Vision Changes in Choroideremia

“…Furthermore, changes in receptoral function could be determined by assessing threshold-vs. intensity functions pre-, and post-surgery, to elucidate changes in the S-, M-, and L-cones as well as the rods. Such testing may also help to identify the site of losses of sensitivity (i.e., d 1/2 , or receptoral vs. d 3 , or post-receptoral), as previously outlined (Seiple et al, 1993;Simunovic et al, 2021).…”

“…Such a hypothesis cannot be invoked to explain our data, however, as neutral adaptation conditions were used. However, adaptation asymmetries have also been identified for neutral/white backgrounds (Kalloniatis and Harwerth, 1989;Simunovic, 2016;Simunovic et al, 2021). In particular, the S-cone mechanism is more likely to be probed under conditions where it does not display Weber-like adaptation.…”

“…In particular, the S-cone mechanism is more likely to be probed under conditions where it does not display Weber-like adaptation. This may lead to apparent selective pathology through adaptational asymmetry (Kalloniatis and Harwerth, 1989;Simunovic, 2016;Simunovic et al, 2021); this asymmetry may account for at least part of the apparent vulnerability of the S-cones. Another hypothesisthe so-called "vulnerable photoreceptor" hypothesis-supposes that the S-cones are inherently more susceptible to pathological processes.…”

Luminance Thresholds and Their Correlation With Retinal Structure in X-Linked Retinoschisis

“…Threshold data obtained at the different background illuminance levels were modeled using a standard equation 21 – 23 : where T is threshold, T 0 is the absolute dark-adapted threshold, A is background intensity, A 0 is a constant “dark-light” value, and n is a gain (slope) constant.…”