In phototransduction, spatial summation results when what sequence occurs?

• A. multiple photoreceptors activate bipolar cells, and multiple bipolar cells activate ganglion cells
• B. multiple photoreceptors activate one bipolar cell
• C. single photoreceptor activates a ganglion cell
• D. multiple photoreceptors activate multiple bipolar cells

Answer: (A)

Spatial summation in vision happens when signals from many photoreceptors are pooled as they pass through the retinal circuitry to reach a single output neuron. In the retina, input from numerous photoreceptors converges onto several bipolar cells, and those bipolar cells converge onto ganglion cells. The combined activity from many photoreceptors, flowing through multiple bipolar cells and into a ganglion cell, increases the likelihood and strength of the ganglion cell’s response. This convergence across space makes it possible to detect faint or spread-out light by summing inputs, which is why this pathway—many photoreceptors feeding many bipolar cells, which in turn feed many ganglion cells—is the best description of spatial summation. The other scenarios either involve too little convergence or miss the final step of integration onto a single ganglion cell.