O

ne of the first responses following stimulation of the eye is the adjustment of the accommodative mechanism to produce a clear image on the retina. The adequacy of the accommodative response in turn determines the discriminable detail in the retinal image and has consequently been of interest to sensory and perceptual physiologists and psychologists as well as to optometrists and ophthalmologists. Traditionally, the accuracy of accommodation is evaluated by means of subjective as well as objective techniques. In a typical clinical examination, the subject is asked to report the clearness or sharpness of test charts. The objective examinations are usually performed by means of retinoscopy, a relatively objective technique in the hands of an experienced examiner, which is based on evaluation of the direction of moving light reflected from the interior of the subject's eye. More recently, a number of sophisticated objective techniques based on automated retinoscopy (Knoll & Mohrman, 1972), reflected infrared radiation (Cornsweet & Crane, 1970), and evoked potentials (Harter & White, 1968) have become available. From the point of view of behavioral research, there are a number of reasons for knowing to what distance the subject is accommodating. Evaluation of the classical problem of the role of the oculomotor adjustments in space perception (Baird, 1970) requires accurate indicators of the amount of. accommodation (and convergence) in play. The clarity of the retinal image determines the amount of information that can be processed visually. The design of optical instruments, such as microscopes, for which the maximum performance of the instrument-visual system is desirable, requires a reliable evaluation of accommodation response. A persistent difficulty with existing techniques of measuring accommodation is that they may interfere with accommodation. The use of verbal report of clarity requires the subject to shift attention from the task at hand to another stimulus provided by the accommodation-measuring device. Most objective techniques require the subject to view through an optical instrument, often with the head immobilized

This paper was sponsored by Grant MH08061 from the National Institute of Mental Health. The authors wish to express their appreciation to Chris A. Johnson and D. Alfred Owens for assistance in the preparation of the manuscript. Requests for reprints should be sent to Herschel W. Leibowitz, Department of Psychology, 417 Bruce V. Moore Building, Pennsylvania State University, University Park, Pennsylvania 16802.

The Laser Optometer and Some Implications for Behavioral Research H. W. LEIBOWITZ Pennsylvania State University

R. T. HENNESSY Human Factors Research, Goleta, California

by a bite bar, or to observe under low luminance levels, both of which may be expected to have a significant influence on the amplitude of accommodation. The evoked potential technique requires repeated presentation of the same stimulus configuration. The availability of low-cost lasers has led to a method for evaluating accommodation that overcomes these difficulties (Knoll, 1966). The basic optical phenomenon is the scintillation pattern observed when laser light is diverged and reflected from a surface. The appearance of the reflection is not uniform but is modulated in intensity, producing a "speckled" appearance of high and low intensity. The subjective phenomenon is impossible to reproduce photographically and difficult to describe verbally. It is "as if" the bright and dark spots are "suspended" in space. These spots stem from the coherent nature of laser light. For our purposes it is important to note that the spots result from constructive and destructive interference of the coherent beam in the image space. They are not stimuli in the sense that they exist in the fixated object or in the stimulus space. For the present application, their critical characteristic is that if the observer moves or if the surface on which the beam is reflected is moved, the speckles will appear stationary only if the subject is accommodated for the plane of the stimulus. If the subject is accommodated to distances nearer or farther from the focal plane, the speckles will appear to move, the direction of movement dependent on whether there is too much or too little accommodation in force. (For a detailed theoretical analysis, see Ingelstam & Ragnarsson, 1972.) The application of this method to the evaluation of accommodation involves superimposing the speckle pattern, which is reflected from a moving drum, into the field of view. The observer reports the direction of movement, or lack of movement, of the speckle pattern. There are a number of advantages inherent in this technique: 1. Since the speckle pattern is produced by interference at the retina, its sharpness will be independent of the focus of the eye. The speckle pattern per se is not a stimulus for accommodation (Ingelstam & Ragnarsson, 1972). 2. Because of the latency of the accommodative response, about .4 sec, stimulation by the speckle pattern itself does not interfere with the subject's report nor with the inferred level of accommodation. This was established empirically with the aid of 350 • MARCH 1975 • AMERICAN PSYCHOLOGIST

the continuously recording infrared optometer developed by Cornsweet and Crane (1970). The accommodation in one eye was monitored continuously while the other eye was stimulated by the speckle pattern. Data from this study indicate that the speckle pattern, presented for .5 sec, does not interfere with the ongoing state of accommodation (Hennessy &Leibowitz, 1970). 3. The laser optometer makes use of light that has been absorbed by the receptors. Thus, it obviates a potential problem inherent in retinoscopy or any other method that makes use of reflected light. The origin of such reflected light may not be the image plane of the eye. Millodot (1972) suggested that this light originates from in front of the image plane so that estimates of refractive error using reflected light, such as retinoscopy, will produce a systematic error, particularly with small eyes (Glickstein & Millodot, 1970). This is not a problem for the laser technique, since the light energy that initiates the response is processed at the appropriate level. 4. The evaluation of direction of movement is a relatively simple response for the subject and essentially requires no training. 5. If the direction of motion of the drum is reversible, the subject will be unable to relate subjective movement responses to the direction or amplitude of accommodation, thus eliminating the possibility that the subject could intentionally or unintentionally influence the results. • 6. Interference with the ongoing task is minimal. The only requirement is that a means be provided for superimposing the speckle pattern into the subject's visual field. This can be accomplished unobtrusively by viewing through a beam splitter, oriented 45° to the subject's line of sight. The alignment of the subject with the optical axis is not as critical as is the case with a Maxwellian view optical system, which requires precise positioning of the eye. Studies comparing the laser technique with other methods of evaluating accommodative state indicate that it is both reliable and valid (Baldwin & Stover, 1968; Ingelstam & Ragnarsson, 1972). Accuracies of better than .25 diopter are readily obtainable. By combining the laser with the Badal optometer system (Hennessy & Leibowitz, 1972) (see Figure 1), it is possible to obtain a measurement of accommodation in 20-30 sec for a trained subject, and 1 or 2 min for a naive subject. This system is relatively inexpensive. The laser should

Target

Figure 1. Schematic diagram of a laser optometer. (In this arrangement, the eye is at the focal distance of the lens so that the apparent size of the laser speckle pattern remains constant; for details, see Hennessy & Leibowitz, 1972.) be of low power, of the order of from .5 to 2.0 mW,1 which on the current market would cost from $100 for a "student model" to $250 for a portable dc model. A highly satisfactory professional ac model is available for $200. The auxiliary equipment includes an optical bench and holders, a negative diverging lens, a positive field lens, a small reversible motor, a drum (a painted seamless beer can is ideal), and a shutter. In practice, the subject reports whether the speckles have moved and in which direction. The experimenter then changes the optical distance of the stimulus by changing the distance of the drum from the field lens until the opposite response is

1 Laser light at these power levels, when optically diverged and reflected from a surface, is of the same order of magnitude as encountered in everyday environments. If lasers of higher power output are employed, precautions should be taken that the energy levels reaching the eye are not potentially harmful.

obtained. Intermediate positions are then used that either null or bracket the direction of the motion of the speckles. The amount of accommodation is calculated by means of a formula (Ogle, 1968) with a correction for chromatic abberration. This correction is necessary because laser light is monochromatic. Some examples of applications include evaluation of the influence of contextual stimuli on accommodation (Hennessy & Leibowitz, 1971), accommodation in microscopes (Hennessy, 1972), the relationship between the specific distance tendency (Gogel, 1969) and oculomotor adjustments (Owens, 1974), and the role of oculomotor adjustments in size constancy (Leibowitz, Shiina, & Hennessy, 1972). By using lasers of different-output wavelengths, the relationship between wavelength and accommodation can be readily studied. Changes in accommodative amplitude while reading, while controlling a vehicle, or during the performance of AMERICAN PSYCHOLOGIST • MARCH 1975 • 351

any task for which the subject's head is in a relatively fixed position is feasible. The equipment imposes no additional strain on the subject, so that applications to vigilance tasks are not precluded. Since the apparatus can be used in total darkness, it provides a means for evaluating a smooth muscle system, the ciliary body, under conditions that are independent of normal (light) stimulation. Such data are relevant not only to the resting position of accommodation (Schober, 1954) but also to the role of the autonomic nervous system in .determining the amount of accommodation in force. Preliminary studies have indicated a significant influence of "stressful" and "relaxation" stimuli on the resting position of accommodation in total darkness (Costello, 1974). From the point of view of behavior investigations, the principal limitation is that continuous monitoring of accommodation is not possible, However, when evaluation of steady states or slowly changing levels of accommodation is desired, application of this technique is appropriate and uniquely convenient. REFERENCES Baird, J. C. Psychophysical analysis of visual space. London: Pergamon Press, 1970. Baldwin, W. R., & Stover, W. B. Observation of laser standing wave patterns to determine refractive status. American Journal of Optometry, 1968, 45, 143-150. Cornsweet, T. N., & Crane, H. D. Servo-controlled infrared optometer. Journal of the Optical Society of America, 1970, 60, 555-558. Costello, M. The effect of stress and relaxation on autonomic function as measured by the resting point of

352 • MARCH 1975 • AMERICAN PSYCHOLOGIST

accommodation. Unpublished honors thesis, Pennsylvania State University, 1974. Glickstein, M., & Millodot, M. Retinoscopy and eye size. Science, 1970, 168, 605-606. Gogel, W. C. The sensing of retinal size. Vision Research, 1969, o, 1079-1094. Harter, M. R., & White, C. T. Effects of contour sharpness on visual evoked cortical potentials. Vision Research, 1968,5,701-711. Hennessy, R. T. Stimulus and nonstimul-us factors affecting accommodation with particular reference to instrument myopia. Unpublished doctoral dissertation, Pennsylvania State University, 1972. Hennessy, R. T., & Leibowitz, H. W. Subjective measurement of accommodation with laser light. Journal of the Optical Society of America, 1970, 60, 1700-1701. Hennessy, R. T., & Leibowitz, H. W. The effect of a peripheral stimulus on accommodation. Perception & Psychophysics, 1971, 10, 127-132. Hennessy, R. T., & Leibowitz, H. W. Laser optometer incorporating the Bsdal jjrinciple. Behavioral Research Methods and Instrumentation, 1972, 4 ( 5 ) , 237-239. Ingelstam, E., & Ragnarsson, S. I. Eye refraction examined by aid of speckle pattern produced by coherent light. Vision Research, 1972, 12, 411-420. Knoll, H. A. Measuring ametropia with a gas laser. American Journal of Optometry, 1966, 43, 415-418. Knoll, H. A., & Mohrman, R. The Ophthalmetron, principles and operation. American Journal of Optometry and Archives of American Academy of Optometry, 1972, 4 9 ( 2 ) , 122-128. Leibowitz, H. W., Shiina, K., & Hennessy, R. T. Oculomotor adjustments and size constancy. Perception & Psychophysics, 1972, 32, 497-500. Millodot, M. Reflection from the fundus of the eye and its relevance to retinoscopy. Pubblicazioni Dell'istituo Nazionale di Ottica, 1972, 27(1), 31-50.. Ogle, K. W. Optics (2nd ed.). Springfield, Mass.: Charles C Thomas, 1968. Owens, D. A. An investigation of the relationship between the specific distance tendency and the oculomotor system. Unpublished masters thesis, Pennsylvania State University, 1974. Schober, H. t)ber die Akkommodationsruhelage. Optik, 1954, 11, 282-290.

The laser optometer and some implications for behavioral research.

O ne of the first responses following stimulation of the eye is the adjustment of the accommodative mechanism to produce a clear image on the retina...
302KB Sizes 0 Downloads 0 Views