NEAR-FIELD VISUAL-ACUITY OF PIGEONS - EFFECTS OF HEAD LOCATION AND STIMULUS LUMINANCE

Two pigeons were trained to discriminate a grating stimulus from a blank stimulus of equivalent luminance in a 3-day chamber. The stimuli and blanks were presented behind a transparent center key. The procedure was a conditional discrimination in which pecks on the left key were reinforced if the blank were present behind the center key, and pecks on the right key were reinforced if the grating were present behind the center key. The spatial frequency of the stimuli was varied in each session from 4-29.5 lines/mm in accordance with a variation of the method of constant stimuli. The number of lines per millimeter that the subjects could discriminate at threshold was determined from psychometric functions. Data were collected at 5 values of stimulus luminance ranging from 0.07-3.29 log cd/m2. The distance from the stimulus to the anterior nodal point of the eye, which was determined from measurements taken from high-speed motion-picture photographs of 3 additional pigeons and published intraocular measurements, was 62.0 mm. This distance and the grating detection thresholds were used to calculate the visual acuity of the birds at each level of luminance. Acuity improved with increasing luminance to a peak value of 0.52, which corresponds to a visual angle of 1.92 min, at a luminance of 2.33 log cd/m2. Further increase in luminance produced a small decline in acuity.