Abstract
Purpose:
The emerging role of melanopsin and intrinsically photosensitive retinal ganglion cells (ipRGCs) in perceptual vision demands a more quantitative description of melanopsin’s input to these pathways in visually intact animals. Particularly lacking is a description of the spatial and temporal information conveyed by ipRGCs that arises exclusively from melanopsin’s photon capture, and what functional impact this may have on image forming vision.
Methods:
We have modified a commercially available projection system so that each of the R, G and B channels is instead a combination of up to five, independently controlled wavelengths (λmax 405, 455, 525, 561, 630nm). This allows us to present patterned stimuli that only present spatial/temporal contrast for particular photopigments. We have used multi-channel recording electrodes to record light-evoked activity in the dorsal lateral geniculate nucleus (dLGN) of urethane anaesthetised Opn1mwR mice; these mice display a long-wavelength shifted spectral sensitivity of green cones, which allows us to achieve maximal contrast for each photopigment.
Results:
We have recorded evoked activity in the dLGN in response to spatially structured stimuli, that originate from melanopsin in isolation (49% Michelson contrast); rod opsin & cone opsins (33% Michelson contrast); or melanopsin, rod and cone opsins (49% and 33%, respectively). We have measured the spatial receptive fields in each of these conditions; when measurable, isolated melanopsin receptive fields tend to be large, extending beyond the size of rod/cone driven receptive fields recorded in the same dLGN cell. The temporal properties of these responses are also distinct from rod/cone evoked responses. Moreover, comparisons of the responses evoked by rod/cone stimuli presented with or without concurrent melanopsin contrast reveal a defined input of melanopsin that is quite distinct from rod and cone-evoked responses.
Conclusions:
Visual responses arising exclusively with melanopsin are detectible at the level of the dLGN at relatively modest contrasts, and on a physiologically relevant spatial and temporal scale. Although these signals are outside the range required for high spatiotemporal acuity vision, they instead occupy a distinct sensory niche that may still be useful for pattern vision.