Approximate Distribution of Rods and Cones across the Retina

The figure above presents an approximate distribution of rods and cones across the retina. Cone distribution is signified by the line with the circle markers and the rod distribution is signified by the square markers. Note the absence of both rods and cones between -15 and -19 degrees of visual angle. This is the region of the bind spot where there are no cones or rods and the collected ganglion axons exit the retina as the optic nerve.

The figure below presents a schematic drawing of rod and cone distribution from a posterior (rear) view of the retina.

The receptive characteristics of rods and cones differ markedly.

Rods
low intensity threshold; sensitive to low levels of illumination

detect movement

detect direction of movement

achromatic; one channel responding only to intensity and not variation in wavelength (black, white, shades of gray response)

relatively greater neural convergence than cone neural feeds

Cones
higher intensity threshold than rods; more intense light necessary for a response; less sensitive

Three types of cones with best responsiveness to short, medium and long wavelengths of electromagnetic radiation in the range of approximately 400 to 700 nM.

The output signals from the three cone types feed into four chromatic color channels

Dense cone packing in the fovea and lesser convergence of cone feeds promotes higher acuity vision from cones than rods

Did you ever see a star disappear when you looked at it? What would explain your experience?

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