Contrast modulated maskers: test of a late nonlinearity hypothesis

The contrast threshold for detecting a low frequency test grating in the presence of a contrast-modulated high frequency masker is heavily dependent upon the phase relations between the gratings: test stimuli in + or - cosine phase are much more detectable than those in + or -sine phase. The present study tests a new hypothesis of these and related phenomena. It is based on the report by Derrington (1987) that recordings from cat geniculate on-center and off-center X-cells exhibit significant point nonlinearities. The major empirical finding of this study is that for highly practiced observers, sine-phase test stimuli and cosine-phase test stimuli are similar in two important respects: (1) there exist phase uncertainty effects; that is, detection thresholds for test stimuli differing in phase by 180 degrees are elevated when they are intermixed within the same block of trials, rather than being presented in separate blocks; (2) phase identification thresholds for test stimuli differing in phase by 180 degrees are the same as their detection thresholds. The new nonlinearity hypothesis cannot account for the results obtained with sine-phase test stimuli, though it gives a better account of the results with cosine-phase stimuli than does the early nonlinearity hypothesis which was tested and rejected by Nachmias and Rogowitz (1983).     

Summation and discrimination of gratings moving in opposite directions

We have measured the amount of summation occurring at threshold between gratings which move in opposite directions. The small amount of summation observed at low spatial and high temporal frequencies is approximately consistent with the action of direction-selective mechanisms, as proposed by Levinson and Sekuler (1975), provided that probability summation between such mechanisms is taken into account. However, at high spatial and low temporal frequencies much more summation is found, an amount approximately consistent with detection by directionally non-selective mechanisms.We have also measured thresholds for identifying the direction of a moving grating. For those gratings which show little summation, direction of motion is judged correctly at the detection threshold, while for those gratings which show the most summation, the identification threshold is considerably above the detection threshold.     

Isolation of a 5-kilodalton actin-sequestering peptide from human blood platelets.

Resting human platelets contain approximately 0.3 mM unpolymerized actin. When freshly drawn and washed platelets are treated with saponin, 85-90% of the unpolymerized actin diffuses out. Analysis by polyacrylamide gel electrophoresis under nondenaturing conditions shows that the bulk of this unpolymerized actin migrates with a higher mobility than does pure G-actin, profilactin, or actin-gelsolin complex. When muscle G-actin is added to fresh or boiled saponin extract, the added muscle actin is shifted to the high-mobility form. The saponin extract contains an acidic peptide having a molecular mass in the range of 5 kDa, which has been purified to homogeneity by reverse-phase HPLC. This peptide also shifts muscle actin to the high-mobility form. Addition of either boiled saponin extract or the purified peptide to muscle G-actin also strongly and stoichiometrically inhibits salt-induced polymerization, as assayed by falling-ball viscometry and by sedimentation. We conclude that this peptide binds to the bulk of the unpolymerized actin in platelets and prevents it from polymerizing.     

Detection of grating patterns containing two spatial frequencies: a comparison of single-channel and multiple-channels models

Contrast thresholds were measured for gratings containing two superimposed sinusoidal components. The frequency of one component was always three times that of the other, but the phase between components and the ratio of their contrasts took on several values. Two models of pattern vision were tested

(1)

a single-channel model in which pattern vision is a function of a single neural network and     

Adaptation to square-wave gratings: In search of the elusive third harmonic

Blakemore andCampbell (1969) found that a square-wave adapting grating, unlike a sinewave grating of the same period, elevates threshold contrast for test gratings whose frequency is in the vicinity of the third harmonic in the square-wave. Because of its important bearing on the frequency-specificity of adaptation to gratings, we attempted to replicate this finding. Our attempt was successful with one of two observers when the method of adjustment was used to determine thresholds. With the temporal two alternative, forced-choice method, individual differences in this regard disappeared. For all three observers a 3 c/deg adapting square-wave and a 3 c/deg sinewave were equally ineffective in raising the contrast threshold for a 9 c/deg test grating, whereas a 9 c/deg adapting grating, matched to the 3rd harmonic in the square-wave, raised the test grating's threshold by 0.2 log units. Our failure to find consistent evidence that the third harmonic in a square-wave does any adapting whatever casts doubt on the frequency-specificity of adaptation to gratings.     

Patterns of temporal interaction in the detection of gratings

Threshold contrasts were determined for pairs of brief, temporally separated gratings of the same frequency, same or opposite phase, and various contrast ratios. For low spatial frequencies, a range of separations is found that results in facilitation between opposite-phased pairs and inhibition between same-phased pairs. For higher spatial frequencies, this range is absent or nearly absent. The relation between the contrasts of the two gratings in a threshold pair of fixed separation suggests either a detector operating on the squared, integrated output of a linear temporal filter, or probability summation over time of the noise-perturbed output of such a filter.     

Polarized distribution of integrin and fibronectin in retinal pigment epithelium

We have examined the presence and distribution of integrin and fibronectin in the retinas of 21-day chick embryos and adult rats, with particular emphasis on the question of localization in the retinal pigment epithelium (RPE). Isolated sheets of RPE solubilized and separated by gel electrophoresis contain integrin, as indicated by immunoblotting with polyclonal and monoclonal antibodies to the complex. By the same technique, antibodies to fibronectin reacted with a single protein in the isolated RPE. In both chick and rat, integrin and fibronectin were localized by indirect immunofluorescence exclusively to the basement membrane of the RPE, the choriocapillaris and the retinal-vitreal border. When isolated RPE cells from chick retinas were examined, integrin was seen to be present along the basolateral surfaces of the cells as well. Similarly, in the intact rat retina, staining for integrin could be seen along the lateral surfaces of some of the RPE cells. Neither integrin nor fibronectin were present along the apical surfaces of the RPE in either rat or chick. The close similarity between the location of integrin and fibronectin supports the idea that the RPE adheres to the basal lamina at least in part via integrin-fibronectin linkages. A clear implication of our results is that the adhesion between RPE and retina requires a different set of linkage proteins.     

Contrast discrimination with and without spatial uncertainty.

The main purpose of these experiments was to examine in detail how successfully the uncertainty reduction explanation accounts for the effects of spatial cues in a temporal forced-choice contrast discrimination task in which any one of four well-separated Gabor patches is incremented. In preliminary experiments, it was shown that in the absence of uncorrelated contrast jitter, observers could use either of at least two different decision strategies, whereas in the presence of random contrast jitter, their response was based on the largest contrast value in the two intervals of a trial.Independent spatial cues were used in each interval to minimize the likelihood of eye movement artifacts. Cues either preceded each stimulus presentation by 100 ms, or were coincident with it. Precues were only slightly more effective than simultaneous cues, and either of them improved performance nearly as much as could be expected from the uncertainty reduction account of spatial attention.