Читать книгу Essentials of Veterinary Ophthalmology - Kirk N. Gelatt - Страница 132

Photoreceptors can respond to changes in levels of background luminance by processes of adaptation and this results in an extended operating range, allowing the eye optimal performance at a given illumination level. A decrease in background illumination to below 0.03 cd/m² will deactivate the cone system, resulting in increased light sensitivity (i.e., lower threshold) and scotopic rod vision. An increase in background illumination, to 0.03–3 cd/m², will lead to mesopic vision in which both the rod and cone systems are active, for example, before dawn or after sunset. Further increase in background illumination above 3 cd/m², to photopic levels, will result in rod saturation. In such an environment, cones will continue to function, albeit with a higher threshold, or with lower sensitivity.

Figure 2.15 A considerable amount of processing of data from the photoreceptors is performed already in the neuroretina. Left‐hand panels briefly describe the purpose of the computations performed, and the right‐hand column illustrates important elements of the underlying circuits schematically (triangle – neuron; A – amacrine cell; B – bipolar cell; G – RGC; P – photoreceptor; rectangle – temporal filter function; oval – instantaneous rectifier; closed/open circle – sign‐preserving/sign‐inverting synapse. (a) The rod‐to‐rod pathway detects single photons. The output of each rod (noisy tracings) is sent through a bandpass temporal filter followed by a thresholding operation. Signals from several rods are then pooled to and summed by one rod bipolar cell, which shows distinct activations (tracings without noise). (b) The Y‐RGC is activated by texture motion in either direction over its receptive field (red circle). Each movement elicits either transient ON or OFF responses in the bipolar cells, but only the depolarized bipolar cells signal to the ganglion cell that fires transiently to each shift in the grating. (c) An RGC sensitive to local motion fires when the object in its central receptive field moves in different direction that from the background, thus detecting differential motion. This RGC is silent when the object in the center moves in the same direction as the background because the excitatory input in the center is counteracted by inhibitory input from the surround via the amacrine cell. (d) A RGC responds strongly (several spikes) to an approaching dark object, but only weakly to lateral motion. More OFF bipolar cells are excited when a larger part of the receptive field is dark. When the object only moves laterally, the RGC receives both excitatory signals from the OFF bipolar cells and inhibitory signals from amacrine cells activated by ON bipolar cells. (e) Specific RGCs use differences in spike latencies to rapidly encode the structure of an image. RGCs with receptive fields (circles) in the dark part of the image have short latencies, and those in the light part have long latencies. RGCs with receptive fields containing both dark and light areas fire in between, thus indicating the position of the border. Here, signals from both ON and OFF bipolar cells are individually rectified, and the timing difference follows from a delay (Δt) in the ON pathway. (f) Wide‐field amacrine cells (A₁) are activated during rapid shifts of the image in the retinal periphery, which suppresses the OFF bipolar cell signal and disinhibits the ON bipolar cell through a local amacrine cell (A₂). Hence, this circuit acts like a switch, in this case enabling a signal in the more central part of the retina.