Binocular vision is the use of the two eyes in a coordinated manner so as to produce a single mental impression. Normal binocular vision implies binocular single vision (fusion) and a high level of stereoacuity.
In this laboratory we will examine several phenomena that cannot be appreciated monocularly. Not all are necessarily an advantage of binocular vision as far as visual function is concerned, but all have stimulated thought as to the mechanisms of binocular vision. We will also learn about the threshold for stereopsis and utilize several tests for assessing stereoacuity.
Binocular vision entails the integration of two monocular images into a single binocular percept. The integration occurs at cortical levels and is therefore essentially a perception. The formation of two suitable monocular sensations requires:
1. overlapping of the visual fields of the two eyes so that the same object can be seen by both eyes
2. the simultaneous stimulation of corresponding retinal areas
3. the ability of the extraocular muscles to adjust the visual axes so that the corresponding retinal areas are placed in a position to receive simultaneous stimulation by the same object.
Given that these requirements are met, the uniquely binocular phenomena of physiological diplopia, Pulfrich effect, retinal rivalry and luster, Fechner's paradox and stereopsis can be appreciated.
When two images of an object fall on corresponding retinal areas, single vision is attained. The extent of corresponding areas allows a certain amount of error; however, if the images fall on widely disparate areas, diplopia results. Physiological diplopia is inevitable and is always present. The double images are not noticed in everyday activities; we apparently see only one view of the external world as if we possessed only one centrally placed eye.
The Pulfrich effect is the phenomenon whereby a moving object binocularly perceived with unequal retinal illuminances appears to be displaced from it’s objective location. Swinging a pendulum in a frontal plane and viewing it binocularly with a neutral density filter placed before one eye usually demonstrates this effect. The resultant percept is of the pendulum moving about an elliptical path. This effect has also been used to create stereoscopic effects in broadcast television programs (Superbowl half-time show of 1989).
The Pulfrich effect occurs because the transmission time of signals from the retina to the cortex depends inversely on retinal illuminance. At photopic levels, reducing retinal illuminance by one log unit (a factor of ten) increases the transmission time by 15-20 milliseconds. If an image is moving across both retinae, a delay in one eye results in an effective shift of the image in that eye relative to the other. If the motion is horizontal, this effective disparity is perceived as depth when the signals from each eye are combined in the cortex. The amount of depth depends on both the velocity of the moving image and on the amount of delay produced by the filter.
Note that unequal retinal illuminances may also be produced by unilateral mydriasis or by disease processes that effect neural transmission of the visual signal; a person in this condition may experience potentially dangerous stereo percepts while driving.
Figure 1. Pulfrich Effect
Illustration of how a swinging pendulum appears to follow an elliptical path when viewed with a neutral density filter over one eye. The delay in visual processing due to the filter causes the central nervous system to fuse images which originated at different times, resulting in an effective disparity for moving objects.