Michael J. Zigler

Dark Adaptation Level and Size of Testfield

Data for dark adaptation curves were obtained for four square fields, subtending respectively 2.5, 5, 10, and 20 degrees at the eye, (a) when the subject indicated when light could just be perceived and (b) when parallel fine lines could just be detected. The levels attained in both the cone and in the rod segments of this function were found to vary when the area of the testfield exposed to the eye was changed, being higher for a smaller than for a larger testfield. This denotes that sensitivity is greater for a larger than for a smaller field of retinal stimulation. These findings indicate that the limit between so called photopic and scotopic vision is not to be regarded as fixed and definite but as functionally variable.

Location of the Break in the Dark Adaptation Curve in Relation to Pre-Exposure Brightness and Pre-Exposure Time

Dark adaptation curves are obtained by presenting a 2° square test field, subdivided by vertical bars, 6° in the lateral parafovea. When pre-exposure time is constant (10 minutes), and pre-exposure brightness is progressively decreased, the cone plateau gradually drops to lower brightness levels, while the final rod level remains constant. However, when pre-exposure brightness is constant (1510 millilamberts), and pre-exposure time is varied, the cone plateau and the final rod level remain unchanged. The break in the duplex curve shifts to earlier times with decreasing pre-exposure brightness as well as with decreasing preexposure time. Linear relationships exist between time of appearance of the break (minutes) and log preexposure brightness, or log pre-exposure time. The slope constants are different, however, indicating that pre-exposure brightness and pre-exposure time are not reciprocally interchangeable as far as the location of the break is concerned.

Tone Shapes: A Novel Type of Synaesthesia

Cet article est une enquěte de deux cas de synesthesie ou les tons de divers instruments musicaux sont accompagnes de formes de trois dimensions. Bien que ces formes ne sont pas les měme chez les d...

Course of dark adaptation under various conditions of pre-exposure and testing.

Dark adaptation curves are obtained by presenting a 2° square testfield 6° below the center of the fovea for both eyes tested separately and for both eyes tested alternately. When the pre-exposure luminances for the two eyes differ, the independently recorded dark adaptation curves for the two eyes show differences in correspondence with the difference in radiant energy delivered to the eyes during pre-exposure. However, if both eyes are pre-exposed simultaneously to their respective luminances, and are alternately tested, the dark adaptation curves are not identical with those previously found. They move closer together, or coincide, i.e., the differences in threshold sensitivity become smaller, or disappear. When pre-exposure of the two eyes differs merely in admitting the near ultraviolet (285–400 mμ) to one eye, and screening it from the other, the curves independently recorded for the two eyes have different threshold levels, but in alternate testing no difference is found. It is also shown that an influence of the near ultraviolet upon visual thresholds is readily found when white and blue testlights are employed; it is barely evident when a green testlight is used and is not found with a red testlight.

Effects of Uniocular and Binocular Excitation of the Peripheral Retina with Test Fields of Various Shapes on Binocular Summation

Square test fields subtending a visual angle of 1° on a side were presented at 28 positions in the field of vision on a circle with a radius of 10° around fixation. Visual thresholds for the right eye, the left eye, and for both eyes were determined on two observers. In the lateral periphery the binocular thresholds are lower than the monocular thresholds (binocular summation); near the vertical meridian above and below fixation, however, binocular sensitivity is not greater than monocular sensitivity. When, instead of squares, rectangular test fields of 12°×2° and 14°×4° are presented in horizontal or vertical position, it is found that on the median line no binocular summation occurs when the test fields are presented vertically, but summation prevails when the test fields are presented horizontally. It appears that the amount of overlap of a test field into the lateral periphery and the conveying of impulses from both retinas to the same cerebral hemisphere is important for binocular summation.

Dark Adaptation Level and Duration of Testflash

Using a testfield subtending a visual angle of 5 degrees, dark adaptation functions were obtained with six test-exposures (1.0, 0.5, 0.1, 0.04, 0.02, and 0.01 sec) both from the central retina, and from the temporal parafovea. The functions for both retinal locations were found to vary systematically in relation to duration of testflash. With increasing test-exposure time, the final cone levels as well as the final rod levels are at lower threshold brightnesses. The sensitivity for a given test-exposure time is greater in the central retina than in the parafovea for the photopic range of vision, whereas the sensitivity for the scotopic range is greater in the parafovea than in the central retina.

The Influence of Surround Brightness and Short Wave Components of Radiation on Dark Adaptation

Dark adaptation functions were determined by presenting a 2° square test field in the temporal parafovea, or a 1.3° field in the fovea, against backgrounds or surround luminances varying in brightness from complete darkness to 40 millilamberts. The test field was composed of parallel, equally spaced dark and light vertical bars, and presented in the form of flashes of 0.04 sec. As surround luminance is systematically increased the total extent of adaptation, the relative participation of rods and cones, and the transition from cone to rod function vary as a function of background luminance. Pre-exposure to a light source emitting radiation from 2850 to 4100 angstroms in addition to white light has an inhibitory effect on the sensitivity of the retinal receptors which is evident in higher excitation thresholds as compared with those obtained when the ultraviolet is screened from the pre-exposure source. The ultraviolet action which previously was only shown for the rods is also found in foveal and parafoveal cones, when, owing to the presence of a surround luminance, the rods are prevented from entering into action, and the range of adaptation is limited to levels above threshold for cone excitation.

Excitation of the Peripheral Retina with Coincident and Disparate Test Fields

Rectangular test fields 12°×2°, with long dimension horizontal and vertical, were presented 10° from fixation on the horizontal and vertical meridian of the visual field. Threshold luminances were determined for each eye singly, and for both eyes when the retinal images were coincident or disparate. In the lateral visual field, binocular thresholds for coincident images were lower than when they were disparate. On the vertical meridian, binocular summation of coincident images is insignificant or totally absent. If, however, in this retinal location the images are disparate and the image in one eye falls to one side of the median line, summation occurs. It is thought that binocular summation is a function of the transmission of impulses from both eyes to the same cortical area of the same cerebral hemisphere.