August 1967 L E T T E R S T O T H E E D I T O R 1059

Invariance of Detection Thresholds Across a Light-Dark Boundary Using

Dichopic Presentation STANLEY NOVAK

Hunter College of the City University of New York 10021 (Received 14 March 1967)

INDEX HEADING: Vision.

IN a previous study,1 the luminance increment necessary for the monocular, peripheral detection of a small test field

positioned against a larger background field was measured as a function of different locations of a "black bar" in the background field. Detection threshold was most elevated when the bar was adjacent to the test field. This observation is consistent with, data derived from both electro-physiologicai studies on lateral in­hibitory mechanisms2,3 and psycho-physical investigations.4-9

The present report is concerned with changes of foveal detection threshold in the center of a background field in one eye as a func­tion oî different locations of a black bar in an identical background field in the other eye (dichopic presentation). Were no interocular effect to be found, the data would lend additional support to a peripheral, i.e., retinal, locus for sensitivity changes occurring near a light-dark boundary.

Stimuli were presented using a modified version of a previously described tachistoscope10 employing fluorescent light sources (Sylvania F14T12/CWSX). The instrument was adapted for dichopic stimulation by addition of a central dividing septum and binocular eyepiece. Each eyepiece contained a circular artificial pupil having a diameter of 4.5 mm. Viewing distance between the artificial pupil and stimuli was 129.3 cm. Figure 1 presents the stimulus configuration, and spatial, temporal, and

FIG. 1. Stimulus configurations of the separate fields and their appearance after fusion. A, background field (14°40', 2.40 log mL); B contralateral fixation field (0o48′, just suprathreshold); C, ipsilateral test field (0°48' wide×2°11′ high, variable luminance, 100 msec duration); D, ipsilateral fixation field (0°48′, just suprathreshold); E, Black bar (2°16' wide, arbitrarily shown at upper left in contralateral +positìon).

luminance specifications are given in the caption. The stimuli presented to the observer's left eye (contralateral eye) consisted of a circular, white, background field containing a circular, red, fixation field. The stimuli presented to the right eye (ipsilateral eye) consisted of an identical background field containing a centrally positioned, rectangular, test field (100 msec in duration, variable in luminance) and also contained a fixation field. When the separate fields were fused, the center of the contralateral fixation field appeared 3° above the center of the test field, and the center of the ipsilateral fixation field appeared 3° below the center of the test field. The spectral characteristics of the fixation fields were obtained by interposing Kodak Wratten No. 26 filters in the light path. The system also contained a black bar (dark vertical stripe) which could be positioned at any location in either the contralateral or ipsilateral background field. The luminance of the black bar was 5.5 log units below that of the given field in which it was located. The displacement of the bar relative to the test field after fusion was expressed in terms of the visual angle between the centers of the bar and test field. Exact symmetrical superimposition of the bar over the test field indicated zero displacement. Positive and negative signs indicate the displace­ment of the bar to the right and left of the test field, respectively, as viewed by the observer.

In Experiment 1, detection threshold was determined by a modified method of limits, using only increasing luminance increments. The mid-point of the luminance interval occurring before two successive detections was taken as a measure of detec­tion threshold. In each session, following 5 min of adaptation to the fused fields, four threshold determinations were made, with­out the bar in either of the fields. This procedure was repeated at the end of each session. A comparison of these no-bar determina­tions before and after the session indicated any systematic thresh­old shifts which may have occurred during the session. In any given session, four threshold determinations for two different locations of the bar were made in an ABBA order in the ipsilateral eye followed by an identical procedure using the corresponding locations of the bar in the contralateral eye. Ac important aspect of the method was that at each luminance increment, the observer presented himself with the test field only when both fixation fields were present. This assured that visual information was present from both eyes in the final image and offset the effects of binocular suppression of one or the other of the background fields, which occasionally occurred.

1060 L E T T E R S T O T H E E D I T O R Vol. 57

FIG. 2. Exp. 1. Mean-detection threshold luminance in the ipsilateral eye as a function of bar displacement in degrees of visual angle in both the ipsilateral-● (solid circle) and contralateral-○ (open circle) eye. Dashed horizontal line indicates detection threshold with no bar presented to either eye. Bar and test field are shown in zero displacement position for comparison. Exp. 2. Mean detection threshold for monocular viewing of ipsilateral field alone with no bar present (arrow on ordinate). Exp. 3. Mean detection threshold for monocular viewing of ipsilateral field alone with bar superimposed over test field in zero displacement position (arrow on ordinate).

Typical data are reported for a 28-year-old male observer. All findings reported were verified with additional observers. All data-points in Fig. 2 are based on an average obtained from 20 threshold determinations with the exception of the no-bar con­dition (dashed horizontal line). The threshold value for the no-bar condition was based on 40 determinations obtained by pooling the presession and postsession determinations which were previously shown not to be significantly different (p>0.20) with a standard t test. A similar test was used to analyze the rest of the data. As shown in Fig. 2, different locations of the bar in the ipsilateral background field resulted in a function similar to that found previously using monocular, peripheral, stimulation.1 Elevation of the threshold was maximum when the bar was adjacent to the test field. Reduction of the threshold was maximum when the bar was centrally superimposed on the test field. Both maximum and minimum threshold values are significantly different from detection thresholds with no bar (p <0.01). Different locations of the bar in the contralateral background field failed to affect detection threshold in the ipsilateral eye even in the contralateral superimposed zero position. There was no significant difference between any of the contralateral bar thresholds and the detec­tion threshold with no bar in either of the fields (p>0.50).

The findings reported above suggested that complete removal of the contralateral background field would also not affect detec­tion threshold in the ipsilateral eye. In Experiment 2, conditions were identical with those described above, except that the ipsi­lateral background field without the bar was viewed with the right eye alone. The mean threshold value obtained under these conditions is indicated on the ordinate in Fig. 2. This value is not significantly different (p>0.50) from the no-bar value obtained under fusion with the contralateral background field. Hence, the presence or absence of the contralateral background field did not affect detection threshold in the ipsilateral eye.

Experiment 3 was a repetition of Experiment 2 except that the bar was superimposed over the test field in the ipsilateral eye. The mean threshold value obtained under this condition is indicated on the ordinate in Fig. 2. This value is not significantly different p>0.50) from that found for this condition when the contralateral background field was present. Hence, detection threshold was invariant even when measured in the darkened area of the bar in the same eye with or without the contralateral background field.

I t should be noted that the stimulus configuration did have one particular disadvantage. Increasing the distance between the bar and the test field does result in a lessening of the area of the bar and a change of its shape where it contacts the edge of the back­ground field. Such changes may have contributed to the shape of the threshold function obtained in the ipsilateral eye. However, the precise shape of the function was not the primary concern of the study and the stimulus configuration did meet certain requirements which appeared to outweigh this disadvantage. Pre­liminary observations indicated that this particular configuration resulted in a lower frequency of binocular suppression and loss of fusion than occurred with background fields containing different types of rectangular bars which maintained constant area and shape in different locations in the contralateral background field. Any stimulus configuration resulting in frequent momentary losses of fusion could have upset the steady-state adapted condition produced by the bar and resulted in transient stimulation. Such transient stimulation might well have affected spuriously the outcome of the study; therefore, the present stimulus configuration was deemed preferable for the purpose of the study.

The stimulus conditions used in the present study may be interpreted as a special case of a visual-masking paradigm, in which the effects of a continuously presented light-decrement conditioning stimulus (bar) on the visibility of a test flash (test field) are studied. The results obtained in the present study are consistent with results obtained in dichopic masking experiments by Kandel (1958) reported by Boynton.11 The present results support Boynton's conclusions derived from KandePs data that, except for transient stimulation, the left- and right-eyed com­ponents of the visual system are largely independent with respect to sensitivity under certain steady-state adapted conditions of stimulation.

The author wishes to express his appreciation to John Van Esen who participated in all phases of the investigation.

1 S. Novak and G. Sperling, Opt. Acta 10, 187 (1963). 2 F . Ratliff and H. K. Hartline, J. Gen. Physiol. 42, 1241 (1959). 3 F . Ratliff, in Sensory Communication, W. R. Rosenblith, Ed. (John Wiley & Sons, Inc., New York, 1961), p. 183.

4 L. Kruger and J. R. Boname, J. Exp. Psychol. 49, 220 (1955). 5 A. Fiorentini, M. Jeanne, and G. Toraldo di Francia, Opt. Acta 1, 192 (1955).

6 H. Harmes and E. Aulhorn, Graefes Arch. Opthal. 157, 3 (1955). 7A. Fiorentini, Opt. Acta 5, 71 (1958). 8 J. P. Thomas, J. Opt. Soc. Am. 55, 521 (1965). 9 M. L. Matthews, J. Opt. Soc. Am. 56, 1401 (1966). 10 G. Sperling, J. Opt. Soc. Am. 55, 541 (1965). 11 R. M. Boynton, in Sensory Communication, W. R. Rosenblith, Ed.

(John Wiley & Sons, Inc., New York, 1961), p. 748.