Why are visual summation fields larger when stimulus contrast is low?

In the study of visual physiology, the dimensions of a neuron's summation field—the region over which sensory stimuli can be integrated to influence a cell's activity—are not fixed and can vary based on stimulus conditions, such as contrast.1

The observation that summation fields are larger when stimulus contrast is low is linked to the way neurons integrate synaptic inputs to reach a threshold for firing. At low contrast, individual stimuli provide weaker signals, necessitating the integration of inputs from a broader spatial area to elicit a detectable response. By expanding the summation area, the neuron can collect sufficient excitatory input from a wider region to overcome the background activity level and generate an action potential. Conversely, when stimulus contrast is high, the signal is strong enough to trigger a response within a more restricted, central region, making the larger, peripheral summation area less critical for activation. This dynamic adjustment allows the visual system to maintain sensitivity across varying environmental conditions.1