Susana Bloch

Limits of the pigeon's binocular field and direction for best binocular viewing

The divergence of the optic axes has been used as a crude index of binocular field width in animals (Hughes, 1977); however, such estimates do not account for refraction in the cornea. More direct estimates have been based on transcleral illumination (Rochon-Duvigneaud, 1922) or upon ophthalmological examination in animals which possess well defined fixation areas (Hughes, 1977). In granivorous species, such as the pigeon, the binocular field has been estimated as 24” wide, with a total field of 340”-342” (Walls, 1967). In the course of a study of depth perception in the pigeon, we needed to have a more accurate information on the binocular overlap in the bird’s visual field to know where to place our testing cues. It occurred to us that the binocular field might be determined by analizing the shape of the animal’s pupils as seen by a photographic camera from different visual field directions. This method would take into account both the position of the eyes and the refraction of light at the air-cornea interface. If the pigeon’s eye possesses rotational symmetry, this method could also serve to detect the best direction for binocular viewing as considered from an optical point of view. Lateral-eyed species such as the pigeon have almost separated uniocular fields, the binocular overlap being defined as the intersection of the nasal-in this case peripheral-portions of both uniocular fields, frontal head features playing a minor role. Rays coming from different parts of the binocular overlap always form large angles with the axes of the eyes. For this reason a continuous and strong variation of light and the degree of optical aberrations may be expected in the binocular field. The transition from binocular to monocular viewing in these lateral-eyed species would therefore be gradual, so that a qualification of binocularity seemed pertinent. Good optical conditions for binocular fusion are present all along the saggital plane. Nevertheless there is only one direction in this plane where the quantity of light is maximal and the degree of optical aberrations minimal: the direction defined by the intersection of the sagittal plane and the plane containing both optical axes. It is this direction which we shall refer to as the direction of “best binocularity”.

Effector patterns of basic emotions: a psychophysiological method for training actors

Abstract The contention that the execution of the effector pattern of an emotion initiates the corresponding subjective activation (feeling), presumably by a feedback mechanism, had been previously proposed ( Bloch & Santibanez-H, 1972 ). In the present study, we report a method for expressing emotions, based on our findings. Actors were trained to perform the respiratory-postural-facial effector patterns of the basic emotions of happiness, sadness, anger, fear, eroticism and tenderness with different degrees of intensity and in different combinations. The learning and systematic repetition of these patterns allowed the actors to bypass the subjective activation which occurs at the beginning. Since the trained patterns correspond to the actual effectors involved during ‘real-life’ emotions, their correct execution is sufficient to convey the corresponding emotional meaning to the observer. A precise technique for ‘stepping out’ of an emotion was developed, which contributes to the subjects psychological balance and may find application in psychotherapy. The entire procedure results in a powerful method for training the expression of emotional behaviours at will in a controlled, graded and organic manner. It turns out to be particularly useful for actors.

Comparing frontal and lateral viewing in the pigeon. II. Velocity thresholds for movement discrimination

Pigeons have been described as poor movement detectors when tested in the frontal binocular field. Retinal organization and behaviour suggest that motion sensitivity may be better in the lateral field. Pigeons were trained to discriminate the direction of moving square gratings (0.3 cyc/deg) appearing briefly (250 msec) contingent upon pecking a key (behavioural fixation). Stimuli were presented at isoacuity distance (40 cm) 25 degrees below the beak for frontal and 80 degrees back from the beak for lateral viewing. The animal had to discriminate the direction of movement for decreasing angular velocities. Results show that lateral motion sensitivity in the pigeon is 3 times better than frontal motion sensitivity. The fovea centralis, looking laterally, seems to be adapted for motion detection and may play a special role in the recognition of moving predators.

Comparing frontal and lateral viewing in the pigeon. I. Tachistoscopic visual acuity as a function of distance

Pigeons visual acuity has mainly been tested in free viewing conditions so that the direction of gaze could not be controlled. In order to be able to compare the resolving power of the two retinal areas of higher cellular density--the area dorsalis in the red field with frontal binocular projection and the fovea centralis with lateral monocular projection--a method of behavioural fixation was used. This method consists in a forced pecking schedule and a tachistocopic presentation of the stimulus. The pigeon has to discriminate the orientation (vertical, positive; horizontal, negative) of square gratings of increasing spatial frequency. Tests were done with the stimuli appearing 25 degrees below the beak for frontal and 80 degrees back from the beak for lateral viewing, at distances of 10, 20, 40 and 80 cm for each direction. Results show that while frontal acuity decreases with distance, lateral acuity increases with distance. These psychophysical data confirm previous dioptric measurements done on frozen eyes, showing that the pigeon is myopic in the frontal field and hyperopic in the lateral field. Pigeons seem to be well adapted for visually guided frontal tasks at near distances (feeding, landing) and for visually guided lateral tasks at far distances (warning).

Comparing frontal and lateral viewing in the pigeon. III. Different patterns of eye movements for binocular and monocular fixation.

The presence in the pigeons retina of two areas of higher cellular density which we have shown mediate different visual functions, suggests the existence of two modes of fixation: a lateral monocular and a frontal binocular one. The participation of eye movements in these modes of fixation remained unexplored. We analyzed oculomotor behaviour in awake head-restrained pigeons by means of EOG and video film. Orienting saccades attaining up to 17 degrees from the resting positions could be elicited by presenting stimuli in different parts of the visual field. Two typical ocular patterns were consistently observed to the sudden presentation of large and novel stimuli: coordinated vergence of both eyes (even with one eye occluded) to stimulation within the frontal binocular field, and uncoordinated ipsilateral saccades to stimuli moving in one lateral field. Results point towards two different and reciprocally exclusive mechanisms of oculomotor control in the pigeon. The relevance of a trident mode of vision correlated to retinal organization and living praxis of some lateral-eyed vertebrates is discussed.

Specialization of Visual Functions for Different Retinal Areas in the Pigeon

The pigeon’s field of view is largely panoramic. The eyes, placed laterally cover about 320deg, each eye surveying more than 180deg of visual angle. In spite of the large separation of the optic axes, a 30deg binocular overlap has been recently confirmed (Martinoya et al., 1981). Pigeons seem to look at near objects with their frontal binocular field and at distant objects with their lateral monocular fields. This would correspond to a “trident model of vision” as proposed by Rochon-Duvigneaud (1943).

Depth resolution in the pigeon

SummaryPigeons possess a binocular visual field and a retinal region of higher cellular density pointing to the center of this overlap. These features and the precision of pecking behavior suggest that in this lateral-eyed bird cues other than monocular ones might participate in depth judgements.Pigeons were trained with an operant procedure to discriminate between luminous points differing in depth which appeared to the observer as floating in the dark. The accuracy of depth judgements was found to be a function of the ratio between the interstimulus distance and the mean eyes-to-stimulus distance. In a first test (experiment I) no external binocular disparity cues were available, the animal only seeing one luminous point at a time (near or far). In a second test (experiment II) where binocular disparity cues were available, the animal having this time to discriminate a pair of points placed at equal depth from a pair placed at unequal depths, only one pair being visible at a time, depth resolution did not improve. This suggests that, at least within the range of distances explored, the pigeon has no stereoscopic vision. Notwithstanding this, binocular cues do play a role, since when tests were done comparing binocular with monocular viewing (experiment III), monocular depth resolution was significantly worse.

VISUAL ACUITY AS A FUNCTION OF DISTANCE FOR FRONTAL AND LATERAL VIEWING IN THE PIGEON

Publisher Summary Pigeons are described as looking at nearby objects with frontal binocular regard and at distant objects with lateral monocular regard. Their eyes, placed laterally, cover a field of over 320˚, allowing the pigeons to simultaneously have a view of the entire horizon while frontally looking at the nearby ground. The densely packed predominant cone retina seems well adapted for such panoramic viewing and shows two neatly defined areas. The first,area is the post-erodorsal region that is thick and rich in cellular elements and is red in colour because of a massive concentration of red oildroplets. The second area is thinner, poorer in cellular elements, and yellowish because of the higher proportion of yellow oil droplets. Both these areas possess restricted portions of higher cellular density: the area dorsalis in the red field and the fovea centralis in the yellow field. Recordings in the optic tectum show that these two loci have a relatively higher degree of spatial representation as compared to surrounding regions.

COORDINATED VERGENCE FOR FRONTAL FIXATION, BUT INDEPENDENT EYE MOVEMENTS FOR LATERAL VIEWING, IN THE PIGEON

The pigeons retina has two specialized areas projecting respectively into the frontal binocular and into the lateral monocular fields. We analysed (EOG and video recording) the eye movements accompanying these two modes of vision. Two different oculomotor strategies occur as a function of the retinal area sollicited: an object of interest presented in the near frontal field always elicits a graded–saccadic convergence coordinated for both eyes, while a stimulus appearing in the lateral viewing field mostly triggers independent orienting saccades of the ipsilateral eye. The existence of two types of oculomotor control, for frontal and for lateral fixation, is suggested.

DEPTH PERCEPTION IN THE PIGEON: LOOKING FOR THE PARTICIPATION OF BINOCULAR CUES

Publisher Summary The finest mechanisms of depth perception in man and other higher vertebrates are binocular in nature. In spite of their laterally placed eyes, pigeons possess a frontal binocular overlap. The knowledge of how the pigeon perceives the three-dimensional space could help to disclose the functional significance of the peculiarities of its retina and could contribute to a better understanding of its entire visual system. Visual depth perception has not received special attention in the case of birds. In other non-mammalian vertebrates, some behavioral studies have been reported. In studying depth perception in the pigeon, it would be desirable to exploit its well known manipulable pecking behaviour, a choice that may limit the kind of cues to be studied to mainly those for near distance judgements. The direction of space from which both the eyes simultaneously receive the largest amount of light lies at 25° below the beak. The fact that this direction coincides with the projection of the area dorsalis of the red field in the retina, characterized by a higher cellular density, provides a good optical and anatomical basis to presume that this region is concerned with stereopsis, a function that could well profit of fine binocular analysis.