Binocular Measurements Research Articles

Retinal irradiation and aniseikonia.

Thependulum bobappears tomake rotatory excursions, previously described asellipses. Trincker (1953a) has shownthat thepathisnotreally anellipse butisflattened ontheside facing theobserver, and, moreimportant still, that theprincipal axis ofthis figure issoinclined astoputoneendnearer totheunobstructed eye.Hecalled this theasymmetry phenomenon. Thestereo-effect wasexplained byPulfrich (1922) interms ofthedifference inthelatent periods ofthedimmedandunobstructed eyesrespectively. Inits original form, theexplanation will account neither forthefact that the effect ishardly observable whenthependulum bobisfixated norforthe asymmetry phenomenon. Recent binocular measurements (Arden and Weale, 1954) haveshownthattheintensity variation ofthevisual latent period issmall inthefovea butlarge outside it, andthatthis isconsistent withthevirtual disappearance oftheeffect whenthebobisfixated. The asymmetry phenomenon, however, isobserved also withstationary targets, suchasvertical rods.Whenthree rods areviewed binocularly (one eyebeing again obstructed withaneutral density filter) andanobserver isasked to arrange theminafronto-parallel plane, thesubjective will differ fromthe objective plane. Trincker (1953b) concludes fromthis that theasymmetry phenomenon found inconjunction withthePulfrich effect cannot haveanything todowiththelatent period ofvision. Inanattempt toexplain this effect, itshould beemphasized that theperceptual size ofaretinal image depends notonly ontheobject size, diffraction, andaberration effects, butalso onits luminance. Oftwoequally large objects, thebrighter will appear tobethelarger. Using aspectrometric slit ofwidth 2mm.,itisfound ataluminance level of800e.f.c. that theleast contrast producing this illusion is0-16logunit andthat theextent oftheangular increase isabout1'ofarc, i.e. iscomparable withtheaccepted value for visual acuity. A suitable arrangement makesitpossible todetect retinal irradiation also binocularly. Thusiftwoequal slits areequally illuminated

THEORY AND MEASUREMENT OF VISUAL MECHANISMS

Comparison of monocular and binocular critical flash intensities for recognition of flicker, using a centrally fixated square image subtending ca. 6.13 degrees on a side (white light), shows that for the cone segment of the response contour the inflection point of the probability integral correlating flash frequency F (for symmetrical flicker) and log mean critical flash intensity I(m) is with the binocular measurements exactly intermediate between those for each eye separately. This does not mean that in general the data are intermediate; they are not; the binocular asymptotic F(max.) agrees with or lies above the greater one of the two uniocular curves. The entire contour must be considered for valid intercomparisons, as is also true if homologous curves for different observers are to be compared. For the measurements in the predominantly rod region the binocular data are more or less intermediate. The rod curves result, however, from the integrative interplay of rod and cone effects for which the intrinsic curves overlap. The resultant rod curve as measured is determined by the partial inhibition of rod effects by cone effects, and by the summation of the remaining rod contributions with those labelled cone in origin. It is pointed out that in this respect, as in others, it is desirable to consider the rôles of retinal area, and location, from the standpoint of integration of neural effects. These phenomena are essentially independent of the light-time fraction and of the spectral (lambda) quality of the light used. For binocular, as for uniocular excitation, the normal probability summation provides an efficient general description, under diverse conditions of size and location of retinal image, wave-length composition of light, light-time cycle-fraction, and kind of animal. It is pointed out that this is the only function abstractly likely to exhibit this kind of efficiency. That a summation of veritable effects independently generated by simultaneous, symmetrical uniocular excitation does occur in the recognition of flicker is specifically demonstrated by the fact that for a given mean critical flash intensity the associated variation is lower for binocular than for either or the average of the single-eyed presentations,-and in the ratio not statistically different from 1:1.41; the relative scatter of the binocular indices of dispersion is also reduced below the uniocular. Since the mean variation of the critical intensity is statistically in a constant ratio to I(m), in appropriately homogeneous series, independent for example of the brightness level and of the light-time fraction, this signifies an essential doubling of the effectiveness (potential) of each of the elements concerned in the discrimination of flicker when binocular excitation is concerned, although the total number of these elements is only slightly or not at all affected. The potential in question is not exclusively correlated with subjective brightness-at-fusion, which is, however, increased with binocular regard.

The foveal light adaptation process

This paper is the continuation of one written by the author in 1934, in which the results of binocular measurements of adaptation phenomena were reported. In the method of observation used, a patch of light seen in the right eye was compared with a second patch observed in the left eye; the right eye was then subjected to a period of light adaptation while the left was maintained in a state of constant (dark) adaptation, after which the two patches were compared again. The left eye was thus used as a reference standard against which changes induced in the right eye could be measured. The author’s trichromatic colorimeter (Wright 1929) was used in the experiments and it was arranged so that the test patch (the right eye) could be illuminated by any selected monochromatic radiation, while in the left eye a mixture of the three instrument primaries was viewed and these could be controlled to produce a match both in colour and intensity between the two patches. A further series of observations has now been made and is reported here. Some of the earlier experiments have been repeated and a great many new observations have been made, but in all the work some important improvements in the method of observation have led to a greatly increased accuracy and significance in the results and have added materially to the information that can be obtained by this method of experiment.

A STEREOSCOPIC FIXATION ATTACHMENT FOR THE PERIMETER

Not infrequently the ophthalmologist desires to make an accurate study of the peripheral fields of an eye which has lost central vision. Charting of the central and paracentral fields is accurately done with stereoscopic instruments. Without binocular fixation field measurements are made with great difficulty and questionable precision when the patient cannot fix the eye under test. Several devices have been developed for use in fixing the eye with a central scotoma, but they are, to my knowledge, hard to adjust, and the results are inaccurate. The old time method of placing the end of the index finger of the right hand (if the patient is right-handed) on the point of fixation and then instructing the patient to direct the eye to that point is still in use. This simple method, which seems to me to be as accurate as any of the mechanical devices and attachments, is still open