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

Symmetry recognition by pigeons: Generalized or not?

This note looks into the reasons why earlier reports may have arrived at differing conclusions about pigeons’ capacity to categorize bilaterally symmetric and asymmetric visual patterns. Attention is drawn to pigeons’ comparatively superior visual flicker resolution and superior visual linear acuity by reporting results of two ad-hoc experiments. This circumstance turns out to constrain conclusions drawn by earlier symmetry–asymmetry studies that used computer-generated patterns displayed on cathode ray tube monitors as these suffered from pictorial distortions. Additionally one of the studies involved patterns of inconsistent symmetry at global and local levels. A smaller-scale experiment using slide-projected unequivocal symmetric and asymmetric patterns yielded results compatible with the supposition that pigeons are capable of a symmetry–asymmetry categorization. The possibility that an artfactual cue may have inadvertently accentuated this capability in an earlier own experiment is considered.

Visual acuity in the cathemeral strepsirrhine Eulemur macaco flavifrons

Studies of visual acuity in primates have shown that diurnal haplorhines have higher acuity (30-75 cycles per degree (c/deg)) than most other mammals. However, relatively little is known about visual acuity in non-haplorhine primates, and published estimates are only available for four strepsirrhine genera (Microcebus, Otolemur, Galago, and Lemur). We present here the first measurements of visual acuity in a cathemeral strepsirrhine species, the blue-eyed black lemur (Eulemur macaco flavifrons). Acuity in two subjects, a 3-year-old male and a 16-year-old female, was assessed behaviorally using a two-alternative forced choice discrimination task. Visual stimuli consisted of high contrast square wave gratings of seven spatial frequencies. Acuity threshold was determined using a 70% correct response criterion. Results indicate a maximum visual acuity of 5.1 c/deg for the female (1718 trials) and 3.8 c/deg for the male (846 trials). These values for E. macaco are slightly lower than those reported for diurnal Lemur catta, and are generally comparable to those reported for nocturnal Microcebus murinus and Otolemur crassicaudatus. To examine ecological sources of variation in primate visual acuity, we also calculated maximum theoretical acuity for Cheirogaleus medius (2.8 c/deg) and Tarsius syrichta (8.9 c/deg) using published data on retinal ganglion cell density and eye morphology. These data suggest that visual acuity in primates may be influenced by activity pattern, diet, and phylogenetic history. In particular, the relatively high acuity of T. syrichta and Galago senegalensis suggests that visual predation may be an important selective factor favoring high visual acuity in primates.

Limits of intraocular and interocular transfer in pigeons

The retina of the pigeon has two areas of enhanced vision: the red field looking into the frontal binocular field and the yellow field projecting into the lateral monocular field. The entire retina projects to the tectofugal pathway, whereas the monocular areas mainly project to the thalamofugal pathway. In the present study we examine how the information received in different retinal areas and hemispheres is integrated within the pigeon brain. The pigeons' task was to discriminate between two shapes by pecking on one of the two keys located at one end of an experimental alley, while walking back and forth between two feeders. Intraocular transfer between the red and the yellow field was tested by moving the stimulus from the frontal to the lateral visual field in consecutive steps and vice versa. When the stimuli were perceived among the edge between the red and the yellow field, the pigeons showed a drastic decrease of performance that we interpret to result from a switch from the tectofugal to the thalamofugal system. There were virtually no traces of intraocular transfer of information from the tectofugal to the thalamofugal pathway, although, in a second experiment a weak intraocular transfer of information from the thalamofugal to the tectofugal system was observed. In a third experiment, interocular transfer of information between the yellow fields of the two eyes was tested. In eight out of nine birds, no interocular transfer was found. In addition, pigeons showed more difficulties to learn the task in the monocular right visual field than in the monocular left visual field, suggesting the existence of an asymmetric organization of the thalamofugal system in the pigeon brain.

Prey detection by great cormorant (Phalacrocorax carbo sinensis)in clear and in turbid water

The scattering and absorption of light by water molecules and by suspended and dissolved matter (turbidity) degrade image transmission and, thus,underwater perception. We tested the effects on visual detection of prey size and distance (affecting apparent prey size) and of low-level water turbidity in hand-reared great cormorants (Phalacrocorax carbo sinensis) diving for natural prey (fish) in a forced-choice situation. The cormorants'detection of underwater prey relied on vision. The minimal tested subtending visual angle of the prey at detection ranged between ∼34.2′ (prey size constant; distance varied) and 9.5′ (distance constant; prey size varied). For all tested distances (0.8–3.1 m) the mean detection success was significantly higher than the chance level. The probability of a correct choice declined significantly with increased distance, with Detection success=–0.034D+1.021 (where D is distance, r2=0.5, N=70, P<0.001). The combined effect of turbidity and distance on the probability of detection success was significant, with both variables having a negative effect: Detection success=–0.286D–0.224Tu+1.691 (where Tu is turbidity, r2=0.68, N=144, P<0.001). At prey detection threshold, the relationship between distance and turbidity was: D=3.79e–4.55Tu. It is concluded that (i) the subtending angle of natural prey at detection was lower than that of resolution of square-wave, high-contrast grating and (ii) turbidity, at levels significantly lower than commonly used in behavioural experiments, had a pronounced effect on visually mediated behaviour patterns.