Spatio-temporal Rayleigh matches

Rayleigh Matches are identical whether the red plus green primaries are presented steadily or in counterphase sinusoidal flicker above fusion. Below fusion, however, the color appearance of the heterochromatic field is a function of flicker frequency such that at lower frequencies subjects selected shorter wavelengths as a match to the heterochromatic field. When Rayleigh Matches were made at color fusion threshold, the wavelength selected as a match was invariant with frequency. At several modulation amplitudes above color fusion threshold Rayleigh Matches were both frequency and modulation amplitude dependent.Given a standard wavelength 5–20 nm shorter than the static Rayleigh Match monochromatic standard, subjects were able to adjust the modulation amplitude of the flickering red and green primaries to achieve a color match at various flicker frequencies. The modulation amplitude/frequency/wavelength relationship adequately describes a three dimensional Rayleigh Match space.     

Quality of retinal image stabilization during small natural and artificial body rotations in man

Two-dimensional head rotations recorded from 2 subjects sitting still, without artificial head support, showed appreciable movement over the frequency range d.c. to 7 Hz. Capacity of vestibuloocular reflex and visually guided eye movements to null motion over this dynamic range was examined by simultaneously recording 2-dimensional head and eye rotations while sinusoidally rotating subjects over the frequency range 0.1 to 15 Hz using small amplitudes. At best, oculomotor compensation removed about 90% of head motion from eye motion in space. Representative compensation was poorer. Compensation for natural motions of unsupported heads while sitting and standing was also incomplete resulting in substantially more eye motion in space than was observed with head supported.These observations, coupled with recent demonstrations of plasticity of the vestibulo-ocular reflex, led us to suggest that the degree of compensatory oculomotor response is actively adjusted downwards so as to guarantee sufficient retinal image motion to prevent perceptual fading when the body is relatively stationary and is actively adjusted upwards, so as to guarantee sufficient retinal stability to prevent perceptual blurring when the body moves actively. Seen this way. the goal of oculomotor compensation is not retinal image stabilization, but rather controlled retinal image motion adjusted so as to be optimal for visual processing over the full range of natural motions of the body.     

Influence of stimulus parameters on visual sensitivity during saccadic eye movement

Visual threshold for stroboscope test flashes was measured during saccadic eye movements over various backgrounds and compared with measures obtained during eye fixation when the same backgrounds were “saccadically” displaced. Amount and time course of threshold change in the two situations compared well, suggesting no necessity for corollary discharge or other oculomotor interference with primary visual processes during eye movement. No significant threshold rise took place during saccades in the dark. Diffuse test flashes and small well focused flashes were affected differently by specific background conditions. Diffuse flashes were perceived with more difficulty during a saccade over a contour-free background than well focused, punctate stimuli. On the other hand, contours in the background raised saccadic thresholds for small stimuli much more than for diffuse test flashes. All threshold changes occurring during saccades were accentuated by increasing the background luminance.     

Voluntary selection of the target for smooth eye movement in the presence of superimposed, full-field stationary and moving stimuli

Prior work has shown that smooth eye movements in the presence of both stationary and moving stimuli are determined, at least in part, by the voluntary selection of either the stationary or the moving stimulus as the target for smooth eye movements. The effectiveness of voluntary selection in eliminating the influence of the stimuli not selected (i.e. backgrounds) on smooth eye movement is not known because prior studies used targets and backgrounds with different physical characteristics. Thus, effects of voluntary selection were confounded with the relative strength of target and background as stimuli for smooth eye movements. We measured eye movements (resolution 1') of two highly-experienced eye movement subjects with a target and background with the same physical characteristics: two, identical, full-field, superimposed patterns of randomly-positioned dots (1 dot/deg2 or 8 dots/deg2). One field was stationary and the other moved at 70.2 minarc/sec. The effect of the moving background on smooth eye movements when the stationary field was the target, and the effect of the stationary background on smooth eye movements when the moving field was the target was negligible (0-4% for one subject; 0-2% for the other). The influence of the background on smooth eye movements was affected by a six-fold reduction in the intensity of either the target or background, but effects of such intensity changes were small and different for each subject. Taken together, these results show that the effectiveness of voluntary selection in eliminating the influence of background stimuli on smooth eye movements can be virtually complete. Any observed influence of the background--however small--can be attributed to voluntary factors (e.g. subjects' failure to apply sufficient effort or attention) rather than to the operation of an involuntary mechanism that automatically integrates velocity information from target and background. The attention and effort required to ensure that voluntary selection is perfect may impair the accuracy of psychophysical judgments made about the background.     

Ocular localization of circulating bacterial lipopolysaccharide

A bacterial lipopolysaccharide (LPS) extracted from a rough strain of Salmonella minnesota ( Re595 ) containing primarily lipid A was used to study tissue distribution following its intravenous injection in rabbits. For this purpose, the Re595 was either labelled with 125iodine (125I) and localization quantitated by gamma radiation spectrometry and radioautography of different tissues or a fluorescein labelled antibody to Re595 was employed. Localization of [125I]- Re595 or LPS was looked for at 30 min and 2 hr after intravenous injection. [131I]-albumin was employed either to measure intravascular protein or total tissue protein and, in some studies, [131I]-albumin and [125I]-fibrinogen were used to measure protein after unlabelled Re595 . Both fractions of isotopes injected and estimates of tissue-bound Re595 were made in the whole eye, liver, spleen, kidney and lung and in isolated iris-ciliary processes, aqueous, lens, vitreous, and posterior segment (retina, choroid, and sclera). Aqueous and iris-ciliary processes, but no other tissues, showed a marked extravazation of [131I]-albumin and [125I]-fibrinogen. [125I]- Re595 was found to localize primarily in liver and spleen. At 30 min, [125I]- Re595 was found in nanogram quantities within the eye and at 2 h greater amounts of LPS were found in iris-ciliary processes and aqueous than in the posterior segment. Neither radioautography nor fluorescein-labelled antibody to Re595 showed evidence of histologic localization of LPS in the iris-ciliary processes. These results indicate that although LPS does not localize preferentially in the eye, it does accumulate in quantities sufficient to have an effect locally.     

Sustained and transient mechanisms in human vision: Temporal and spatial properties

In two experiments, properties ofsustained andtransient mechanisms were studied psychophysically. In the first contrast thresholds were measured for 6sinewave gratings ranging from 0.375 to 12.0c/deg at 10 durations ranging from 18 to 3000msec. Thresholds were measured in the presence and absence of high contrast 20msec gratings whichmasked the onsets and offsets of the signals. At 1.5 c/deg and above, the unmasked thresholds decreased as power functions of duration in two stages, reaching an asymptotic level near 1000msec. Below l.5c/deg, the unmasked threshold became independent of duration beyond 100 msec. At all frequencies, the masked thresholds decreased as power functions of duration to 1000msec or more, but the curves for 0.375 and 0.75c/deg never reached the unmasked asymptotic level. In the second experiment,spatial frequency bandwidths were obtained for sinewave gratings ranging from 0.375 to 12.0c/deg. by measuring threshold elevation as a function of the spatial frequency of masking gratings. At 3.0, 6.0 and 12.0 c/deg, the bandwidth functions peaked at the signal frequencies and showed medium bandwidth frequency selectivity. Below 1.5 c/deg, the bandwidth functions exhibited broader spatial frequency tuning, were of higher magnitude, and there was a shift in peak masking to frequencies near 1.0–1.5 c/deg which were above the signal frequencies. The results of both experiments are discussed in terms of the sustained/transient dichotomy.     

Subthreshold addition of real and apparent motion

The subthreshold interaction of real and apparent motion has been investigated by measurement of threshold-illumination levels for detection of a continuously moving target in the presence of subthreshold apparent movement. The results of these experiments show that the subthreshold illumination level of the apparent motion stimulus configuration adds almost linearly to the threshold illumination level for detection of a continuously moving target, provided that the direction of apparent motion movement corresponds to that of the test target. The maximum interaction is observed for test target speeds centred around the “equivalent” target speed of the apparent motion stimulus configuration, and falls gradually as the relative direction of the two motion stimuli is made increasingly dissimilar. Subthreshold-apparent motion in the opposite direction to that of the test target fails to cause any significant change in threshold illumination levels for detection of the continuously moving target over the whole range of test target speeds. These results suggest that the extraction of movement information in response to continuous target displacement on the retina involves visual mechanisms which also contribute to the extraction of apparent movement.