Photopic negative response
Component of electroretinogram reflecting retinal ganglion cell function
From Wikipedia, the free encyclopedia
The photopic negative response (PhNR) is a component of the electroretinogram (ERG) that reflects the function of retinal ganglion cells and their axons. It appears as a slow negative deflection following the b-wave in light-adapted (photopic) ERG recordings.[1][2][3]
| Photopic negative response | |
|---|---|
| Purpose | Assessment of retinal ganglion cell function |
The PhNR is used in research and clinical investigations of diseases affecting the inner retina, particularly those involving ganglion cell dysfunction such as glaucoma.[4]
Physiology
The PhNR originates primarily from retinal ganglion cells and their associated pathways. Pharmacological and experimental studies have shown that selective damage or suppression of ganglion cells leads to a marked reduction in PhNR amplitude.[1][2]
Additional studies have demonstrated contributions from amacrine cells and inner retinal circuitry, though ganglion cells are considered the dominant source.[5]
Recording and measurement
The PhNR is recorded under photopic conditions using full-field ERG. A commonly used stimulus is a red flash presented on a blue background to enhance cone-mediated responses and improve isolation of the PhNR.[6]
Other stimulus conditions and recording paradigms have been explored to optimize reproducibility and sensitivity.[7]
Parameters
Common PhNR metrics include:
- Amplitude: measured from baseline or b-wave peak to the negative trough
- Implicit time: time from stimulus onset to the trough
- PhNR/b-wave ratio: used to normalize variability between individuals
The PhNR is typically a low-amplitude signal (approximately 5–30 μV), making it sensitive to noise and recording conditions.[4]
Clinical significance
Glaucoma
Reduction in PhNR amplitude has been shown to correlate with retinal ganglion cell loss in glaucoma and may detect functional deficits earlier than visual field loss.[1][7][8]
Optic neuropathies
The PhNR is reduced in various optic neuropathies, including ischemic, inflammatory, and compressive conditions affecting the optic nerve.[4][9]
Other retinal diseases
Alterations in the PhNR have also been reported in:
- Diabetic retinopathy[4][10]
- Retinal vascular occlusions
- Inner retinal ischemia
Advantages and limitations
Advantages
- Provides a non-invasive functional measure of retinal ganglion cells
- Complements structural imaging such as optical coherence tomography
- Can be incorporated into standard ERG protocols
Limitations
- Low signal amplitude increases susceptibility to noise
- Sensitive to electrode placement and recording conditions
- Less standardized compared to a-wave and b-wave measurements
Integration with clinical ERG systems
Recent developments in electroretinography systems, including handheld devices such as RETeval and established tabletop systems such as the Espion system and UTAS system, have enabled broader clinical use of full-field ERG in both clinical and research settings. [11]
Because the PhNR is derived from the photopic ERG waveform, it has been investigated as a potential parameter that may be derived from recordings obtained with such systems. However, accurate measurement may be limited by signal-to-noise ratio, electrode type, and stimulus characteristics, and remains an area of ongoing research.
See also
- Electroretinography
- Pattern electroretinography
- Multifocal electroretinography
- Glaucoma
- Optical coherence tomography