Quantifying the visual information sourced from melanopsin photoreceptors in mouse LGN field responses

Abstract

Melanopsin photoreceptors make a third type of photoreceptor along with rods and cones in human and mouse.Until recently melanopsin was thought to participate only in subconscious responses to light (such as pupillaryreflexes and regulating the circadian rhythm) and not in image-forming visual responses. Recently, it has beenshown that melanopsin derived signals are also widespread in image-forming visual pathways of mice [1]. The goalof this study is to quantify their contribution in the visual pathway. We used information theoretic measures [2,3] onfield potentials [3] to quantify the amount of information that is originated in melanopsin photoreceptors. Thecontinuous field potentials were recorded from lateral geniculate nucleus (LGN) of transgenic mice (Opn1mwR) [1]using multichannel electrodes.The role of melanopsin was quantified by estimating the information [2] found in LGN responses, about theintensity of constant blue (460nm) and red (655nm) light (7 levels of irradiance were used). The intensities of thesetwo lights were carefully matched to provide equal stimulation of the red-sensitive cones of Opn1mwR animals andto control for the influence of rod photoreceptors. Since the long wavelength (red) light was essentially invisible tomelanopsin, subtracting the amount of information in 655nm stimuli from the 460nm should reveal the visualinformation that is sourced from melanopsin photoreceptors in each recording channel. To investigate the localcontinuous response signals, the power and phase of recorded field signals were examined at various frequencybands and time points. For each spectrotemporal component of the responses, information about the intensity oflight was estimated [1,3] separately for blue and red lights for each recording channel.The main conclusion is that in different spectrotemporal components of field responses the presence of melanopsininformation was confirmed and quantified for both power and phase. Information in response to blue light wassignificantly higher than red (Fig. 1). Melanopsin influence on the phase of field oscillations was stronger than fieldpower. The results were confirmed in mice that genetically lacked melanopsin photoreceptors [1] (Fig. 1). Thisconfirms that LGN receives visual content through functional pathways that are specifically sourced frommelanopsin photoreceptors.