Color vision, originating with opponent processing of spectrally distinct photoreceptor signals, plays important roles in animal behavior.1–4 Surprisingly, however, comparatively little is understood ...about color processing in the brain, including in widely used laboratory mammals such as mice. The retinal gradient in S- and M-cone opsin (co-)expression has traditionally been considered an impediment to mouse color vision.5–8 However, recent data indicate that mice exhibit robust chromatic discrimination within the central-upper visual field.9 Retinal color opponency has been reported to emerge from superimposing inhibitory surround receptive fields on the cone opsin expression gradient, and by introducing opponent rod signals in retinal regions with sparse M-cone opsin expression.10–13 The relative importance of these proposed mechanisms in determining the properties of neurons at higher visual processing stages remains unknown. We address these questions using multielectrode recordings from the lateral geniculate nucleus (LGN) in mice with altered M-cone spectral sensitivity (Opn1mwR) and multispectral stimuli that allow selective modulation of signaling by individual opsin classes. Remarkably, we find many (∼25%) LGN cells are color opponent, that such cells are localized to a distinct medial LGN zone and that their properties cannot simply be explained by the proposed retinal opponent mechanisms. Opponent responses in LGN can be driven solely by cones, independent of cone-opsin expression gradients and rod input, with many cells exhibiting spatially congruent antagonistic receptive fields. Our data therefore suggest previously unidentified mechanisms may support extensive and sophisticated color processing in the mouse LGN.
•Color opponency is common among neurons within a chromatic zone in the mouse LGN•Cone inputs to LGN drive color opponency across mesopic to photopic light levels•Cone inputs to opponent neurons derive from central and upper visual field•Subsets of LGN neurons display opponency for small or large stimuli only
Mouland et al. examine color processing in the mouse visual thalamus. They show that color opponency is widespread among cells within a specific zone of the lateral geniculate nucleus and that cones drive such responses, independent of cone-opsin expression gradients and rod input, providing a robust capacity for color discrimination in mice.
Intrinsically photosensitive retinal ganglion cells convey intrinsic, melanopsin‐based, photoreceptive signals alongside those produced by rods and cones to the suprachiasmatic nucleus (SCN) ...circadian clock. To date, experimental data suggest that melanopsin plays a more significant role in measuring ambient light intensity than cone photoreception. Such studies have overwhelmingly used diffuse light stimuli, whereas light intensity in the world around us varies across space and time. Here, we investigated the extent to which melanopsin or cone signals support circadian irradiance measurements in the presence of naturalistic spatiotemporal variations in light intensity. To address this, we first presented high‐ and low‐contrast movies to anaesthetised mice whilst recording extracellular electrophysiological activity from the SCN. Using a mouse line with altered cone sensitivity (Opn1mwR mice) and multispectral light sources we then selectively varied irradiance of the movies for specific photoreceptor classes. We found that steps in melanopic irradiance largely account for the light induced‐changes in SCN activity over a range of starting light intensities and in the presence of spatiotemporal modulation. By contrast, cone‐directed changes in irradiance only influenced SCN activity when spatiotemporal contrast was low. Consistent with these findings, under housing conditions where we could independently adjust irradiance for melanopsin versus cones, the period lengthening effects of constant light on circadian rhythms in behaviour were reliably determined by melanopic irradiance, regardless of irradiance for cones. These data add to the growing evidence that modulating effective irradiance for melanopsin is an effective strategy for controlling the circadian impact of light.
Twilight is characterised by changes in both quantity ("irradiance") and quality ("colour") of light. Animals use the variation in irradiance to adjust their internal circadian clocks, aligning their ...behaviour and physiology with the solar cycle. However, it is currently unknown whether changes in colour also contribute to this entrainment process. Using environmental measurements, we show here that mammalian blue-yellow colour discrimination provides a more reliable method of tracking twilight progression than simply measuring irradiance. We next use electrophysiological recordings to demonstrate that neurons in the mouse suprachiasmatic circadian clock display the cone-dependent spectral opponency required to make use of this information. Thus, our data show that some clock neurons are highly sensitive to changes in spectral composition occurring over twilight and that this input dictates their response to changes in irradiance. Finally, using mice housed under photoperiods with simulated dawn/dusk transitions, we confirm that spectral changes occurring during twilight are required for appropriate circadian alignment under natural conditions. Together, these data reveal a new sensory mechanism for telling time of day that would be available to any mammalian species capable of chromatic vision.
The daily variation in background light intensity (irradiance) can entrain the endogenous clock in the suprachiasmatic nucleus (SCN) to the external environment. The only source of this photic ...information in mammals is the eye, which is primarily a visual organ. It is therefore highly specialised to detect high frequency spatiotemporal modulations. This together with the adaptation which occurs within the retina could be present difficulties when encoding global irradiance. This raises the question of whether spatial patterns, which are present in our everyday viewing, might affect the ability of the SCN to receive 'true irradiance' signals and entrain to the external environment. My first approach was to determine whether individual SCN cells might receive a 'true irradiance' signal. To this end I mapped and characterised the receptive field properties of SCN neurons using in vivo electrophysiology. Indeed a handful of neurons (full field cells) responded to light anywhere in the visual scene and thus may act as 'irradiance detectors'. However, the vast majority of cells only sampled local radiance from a limited area of the visual scene. Having mapped the receptive field properties it became clear that cells which sampled from a limited area of the visual scene would be sensitive to spatial contrast (patterns). To examine the effect of spatiotemporal contrast on the SCN I examined two SCN outputs: locomotor activity and neuronal firing rates. Although spatiotemporal modulation in light intensity could induce large amplitude oscillations in neuronal activity; the time averaged firing rate and locomotor activity, which are believed to be determined by irradiance, were largely unaffected by spatial patterns. This led to the conclusion that the SCN can multiplex photic information into information regarding irradiance, and spatial information by encoding them under different timescales. Melanopsin has been heralded as the key photopigment for encoding irradiance and entraining the SCN. However such experiments have been only performed using diffuse light stimuli. Here I investigated the role of melanopsin under natural viewing conditions which incorporated spatial patterns. Under such stimuli the SCN response can be almost entirely accounted for by the melanopic irradiance of the stimuli.
The daily variation in background light intensity (irradiance) can entrain the endogenous clock in the suprachiasmatic nucleus (SCN) to the external environment. The only source of this photic ...information in mammals is the eye, which is primarily a visual organ. It is therefore highly specialised to detect high frequency spatiotemporal modulations. This together with the adaptation which occurs within the retina could be present difficulties when encoding global irradiance. This raises the question of whether spatial patterns, which are present in our everyday viewing, might affect the ability of the SCN to receive 'true irradiance' signals and entrain to the external environment.My first approach was to determine whether individual SCN cells might receive a 'true irradiance' signal. To this end I mapped and characterised the receptive field properties of SCN neurons using in vivo electrophysiology. Indeed a handful of neurons (full field cells) responded to light anywhere in the visual scene and thus may act as 'irradiance detectors'. However, the vast majority of cells only sampled local radiance from a limited area of the visual scene.Having mapped the receptive field properties it became clear that cells which sampled from a limited area of the visual scene would be sensitive to spatial contrast (patterns). To examine the effect of spatiotemporal contrast on the SCN I examined two SCN outputs: locomotor activity and neuronal firing rates. Although spatiotemporal modulation in light intensity could induce large amplitude oscillations in neuronal activity; the time averaged firing rate and locomotor activity, which are believed to be determined by irradiance, were largely unaffected by spatial patterns. This led to the conclusion that the SCN can multiplex photic information into information regarding irradiance, and spatial information by encoding them under different timescales.Melanopsin has been heralded as the key photopigment for encoding irradiance and entraining the SCN. However such experiments have been only performed using diffuse light stimuli. Here I investigated the role of melanopsin under natural viewing conditions which incorporated spatial patterns. Under such stimuli the SCN response can be almost entirely accounted for by the melanopic irradiance of the stimuli.
Twilight is characterised by changes in both quantity ("irradiance") and quality ("colour") of light. Animals use the variation in irradiance to adjust their internal circadian clocks, aligning their ...behaviour and physiology with the solar cycle. However, it is currently unknown whether changes in colour also contribute to this entrainment process. Using environmental measurements, we show here that mammalian blue-yellow colour discrimination provides a more reliable method of tracking twilight progression than simply measuring irradiance. We next use electrophysiological recordings to demonstrate that neurons in the mouse suprachiasmatic circadian clock display the cone-dependent spectral opponency required to make use of this information. Thus, our data show that some clock neurons are highly sensitive to changes in spectral composition occurring over twilight and that this input dictates their response to changes in irradiance. Finally, using mice housed under photoperiods with simulated dawn/dusk transitions, we confirm that spectral changes occurring during twilight are required for appropriate circadian alignment under natural conditions. Together, these data reveal a new sensory mechanism for telling time of day that would be available to any mammalian species capable of chromatic vision.
Carolinian habitats on Middle Island in the western basin of Lake Erie have recently experienced a dramatic rise in nesting Double-crested Cormorants (Phalacrocorax auritus). Nesting cormorants on ...the 0.3 × 1.1 km island increased from three pairs in 1987 to 4,690 pairs in 2006. The physical attributes of individual trees and poles were assessed using common indices of tree health to determine whether forest damage increased with cormorant nesting densities. Crown density, branch damage, foliage transparency and decay were measured at 54 sampling stations along twelve transects in June of 2004, 2005 and 2006. All damage indices except for tree crown density and pole decay class increased over time, with trees more damaged than poles. Nests were more than four times more likely to be found in superstory trees than overstory trees and were virtually absent from understory trees, suggesting that Double-crested Cormorants prefer larger trees as nesting sites. However, despite greater cormorant preference for large trees, understory and open canopy trees had significantly greater levels of foliage transparency compared to overstory trees. The spatial distribution of damage varied across the island. Stem damage was lower in the center of the island compared to the western and eastern sections. Branch damage and foliage transparency were also greater on the eastern edge of the island than in the center suggesting that impacts were not yet concentrated within the forest interior. Densities of Double-crested Cormorant nests per station were significantly related to all damage indices except decay class. Stations with high numbers of cormorant nests were more likely to have lower crown densities, more transparent foliage and greater branch damage than stations with fewer cormorant nests. The data suggested that the distributional variation in damage could be used to better target specific areas of the island for management such as deployment of sonic deterrent systems, egg oiling or culling.
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