Limits of visual acuity in the frontal field of the rock pigeon (Columba livia).

The eye of the rock pigeon is typical of a granivorous lateral-eyed bird, in that it has both a laterally projecting central fovea and a second high-density cellular area in peripheral retina (area dorsalis) which projects to the binocular frontal field below the beak. Such a dual system is faced with potentially different optical restraints arising from central and peripheral vision. We asked whether the frontal axis can support high resolution vision from a refractive resting position (predicted to be 25-33 cm; Fitzke et al, 1985 Journal of Physiology 369 33-44) to some near point of accommodation. We measured the visual acuity on the frontal axis in five pigeons using an operant discrimination of high-contrast square-wave gratings at a series of distances from 7 to 80 cm from the eye. The peak average acuity was 11.04 cycles deg(-1), which occurred 10 cm from the eye. The average of the maximum acuity of each bird at 10 cm was 12.8 +/- 1.1 cycles deg(-1), a value equal to the Nyquist frequency calculated from the peak ganglion cell density of the area dorsalis. However, this maximum acuity was restricted to a narrow depth in space, located around 10 cm from the eye, and at greater distances fell exponentially such that acuity was 50% of its maximum at 35 cm and less than 1 cycle deg(-1) at 100 cm. We propose that the range of high-acuity vision is limited in the frontal field by either increased refractive power and/or inaccuracy in frontal accommodation, and is optimized for a preferred far point located 10 cm from the eye.