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M Alpern

Publications and source records attributed to M Alpern.

At least 73 records · Page 4Linked to original sources
PubMed1970

Signals from cones.

1. We have studied red and green cones by contrast flash inhibition and found them both to be very similar to rods in their response to flash energy, except that all light quantities must be some 100-fold greater in cones for the same effect.2. Using the methods of a previous paper (A.R.T. a) where no backgrounds were employed, we plotted log test flash lambda against log surround flash varphi with criterion that lambda should just be detected. The experiment was repeated with a ;windmill stop' interposed in the varphi flash which reduced its area symmetrically to (1/8). From these two curves it is possible to extract the relation between N, the inhibitory signal, and varphi, the test flash. It isN = varphi/(varphi+sigma),where sigma, the semi-saturation constant, is about 4.5 log td sec.3. Using the methods of (A.R.T. b) where backgrounds were studied, we measured the increment threshold for the surround flash varphi against its background theta using as criterion not that varphi should just be seen but that it should generate a fixed inhibitory signal N so that the fixed test flash lambda could just be seen.4. This increment threshold curve resembled the Aguilar & Stiles (1954) curve for rods, showed saturation and a complete symmetry about the 45 degrees line through the point with co-ordinates (log theta(D), log sigma).5. These results imply that the cone signal N is related to flash varphi and steady background theta by [Formula: see text], where theta(D) is receptor noise (= eigengrau), and varphi and theta are expressed in units of quantum catch.6. The ordinary increment threshold for cones does not show saturation because a steady saturating background bleaches all the pigment away. When the background is presented for only 100 msec with dark pauses between, no great bleaching occurs and saturation is seen.

Journal Article↗
PubMed1968

The luminosity curve of the protanomalous fovea.

Threshold spectral sensitivities (in the dark, or against bright colored backgrounds) are identical in the red-green range for both protanopes (dichromats) and protanomalous trichromatic color defectives. The latter, however, must have an additional photolabile cone pigment in the red-green range, and its presence is revealed by heterochromatic brightness matching through the spectrum (i.e. luminosity curves). The absorption spectrum of the anomalous cone pigment can be inferred from the protanomalous and protanopic luminosity curve, given reasonable assumptions as to how the different cone mechanisms pool their responses. Depending upon these assumptions, the pigment inferred is either (a) dilute solution of the normal red pigment (assumed density 1.0 for the deuteranope) or (b) similar in its absorption spectrum to the normal green pigment but shifted slightly toward the long wave end of the spectrum. Experimental attempts to choose between these alternatives have so far proved equivocal though (b) seems more likely on the basis of indirect evidence.

Color Perception Tests↗
PubMed1968

The luminosity curve of the deuteranomalous fovea.

Analogous to protans, the two types of deutan color-defectives-the dichromats (deuteranopes) and the anomalous trichromats (deuteranomalous)-do not differ in spectral sensitivity in the red-green range at threshold (either in the dark or against bright colored backgrounds). However, luminosity curves obtained by heterochromatic brightness matching show the latter to be slightly more sensitive in the blue-green, and slightly less so in the red, than the former. Experiment proves that these differences are due (at least in part) to contributions of cones containing the deuteranomalous anomalous pigment which are missing from the deuteranope's eye. The absorption spectrum of the anomalous pigment can be inferred with assumptions (analogous to those already made with protanomalous trichromats) about how the different cone mechanisms pool their responses to yield luminosity. Two alternatives thus revealed are (a) the normal red pigment in dilute solution or (b) a spectrum very similar to that of the normal red pigment but shifted slightly toward the short wave end of the spectrum. Since the spectrum inferred by (a) has the same lambda(max) as the normal red pigment, (a) predicts that deuteranomalous observers will require a negative red primary when matching monochromatic lights of wavelengths near the lambda(max). This is not observed.

Color Perception Tests↗
PubMed1968

Prereceptor colour vision distortions in protanomalous trichromacy.

1. Scotopic luminosity and fundus spectral reflexion in the protanomalous fail to confirm predictions made from the hypothesis that protanomalous photopic luminosity loss is due to an inert red-absorbing filter in his ocular media.2. If it were supposed that the luminosity losses were due to a reduced number of normal red cones, the anomaloscope mismatches could result from a prereceptor distortion such as a reduced concentration of macular pigment or a tilt of the foveal cones. Experiments exclude these two possibilities.3. An anomaloscope is described which makes it possible to measure colour-matching properties of the protanomalous eye by transcleral illumination. Such measurements exclude, as a class, hypotheses which attribute protanomalous colour-matching distortions to an inert filter localized anywhere between the cone outer segment and the cornea.4. It is concluded that the absorption spectrum of at least one of the three cone visual pigments of the protanomalous eye must differ from that of the pigments of the normal fovea.

Color Perception↗
PubMed1968

Are there two types of deuteranopes?

1. The colour vision of a Type I deuteranope who fulfils both of Willmer's criteria (normal foveal luminosity curve, two cone mechanisms in the central fovea revealed by a small 10' test flash) has been studied.2. Spectral sensitivity curves (at threshold) on bright red or green backgrounds are identical in the red-green range.3. Heterochromatic brightness-match luminosity curves measured after bright red or green bleaches are identical in the red-green range.4. Study of prereceptor light losses show normal colour of the ocular media; spectral reflexion coefficient measurements reveal no evidence of macular pigment.5. Luminosity curves measured through a filter which artificially replaces the missing macular pigment is identical to the deuteranopic (Type II) curve. Lack of macular pigment explains the ;normal' luminosity curve.6. Red and violet backgrounds raise the thresholds for 10' red and violet tests by different amounts because two cone (the red and the blue) mechanisms are concerned.7. Reducing the size of the test to 4' eliminates the contribution of the blue cone mechanism to threshold. Now only the red mechanism determines the threshold.8. It is concluded that this subject has only a single red-green cone pigment, normal erythrolabe, like other (Type II) deuteranopes.

Color Vision Defects↗
PubMed1967

The electroretinogram evoked by the excitation of human foveal cones.

1. A 2 degrees test stimulus foveally fixed and viewed against a blue background (40 degrees in extent and producing 2.0 x 10(4) scotopic td of retinal illuminance) evokes a small voltage which can be recorded from the human eye with a conventional contact lens electrode if the test stimulus is flashed at a rate of 15 c/s, and the responses to at least several hundred flashes are averaged.2. The action spectrum of the response obtained in this way agrees reasonably well with the observer's psychophysical foveal luminosity curve.3. For the peripheral retina, the action spectrum is similar to that of the fovea when allowance is made for differences in screening macular pigment.4. Such responses diminish when the test stimulus is focused on to the peripheral retina and disappear when the test light is focused on the blind spot.5. Therefore, the response to the test light fixated centrally is the result of the excitation only of cones mainly, if not exclusively, in the fovea.6. When the intensity of the background is reduced by a factor of 10, the action spectrum shows evidence of the effect of excitation of rods in the blue part of the spectrum and of cones in the red. These red and blue responses add linearly when combined together, provided they are adjusted to coincide in phase.

Journal Article↗
PubMed1967

The nature of rise in threshold produced by contrast-flashes.

1. The rod threshold for seeing a flash on a 2(1/2) degrees square is raised by a nearly simultaneous flash that falls on the surround. When this ;contrast-flash' is held fixed in intensity, it raises the log test threshold by a fixed amount no matter how far that threshold has already been raised by light adaptation owing to background or bleaching.2. This is surprising since fixed backgrounds and bleachings raise the log test threshold much more when the eye is dark than when light adapted.3. When the test flash is held at some fixed supra-threshold value, the contrast flash exhibits a ;critical level', above which the test will no longer be seen. If the surround region upon which the contrast-flash falls is adapted by background or bleaching, its efficacy is reduced so that the ;critical level' is raised.4. Surround adaptation raises the log ;critical level' by the same amount that it raises the log threshold for seeing the contrast-flash itself.5. The way that contrast flashes raise the test threshold is thus entirely different from the way that adaptations by bleachings or backgrounds do. Contrast-flash signals appear to inhibit test-flash signals by interaction at some point central to the site where adaptation occurs.6. This permits the effect of adaptation on signals to be measured. A given state of adaptation attenuates all flash signals in the same proportion. And in any state of adaptation a single flash will reach threshold when the attenuated signal has a fixed size.

Accommodation, Ocular↗
PubMed1966

The blue arcs of the retina.

Around a dim light viewed in a dark room can be seen faint blue-gray arcs which occupy that part of the visual field corresponding to the retina where the arcuate nerve fiber bundle passes from macular ganglion cell bodies to the optic nerve. These blue arcs of the retina are an entoptic phenomenon in which action potentials of the arcuate nerve fiber bundle presumably excite adjacent neurons. The experiments here described show that the light stimulus initially evoking the blue arcs excites cones and not rods as has been generally believed until now. Another commonly held idea is that the blue arcs are produced by bioluminescence or fluorescence associated with the action potentials in the arcuate nerve fiber bundle. The experiments described here disprove this hypothesis.

Adaptation, Ocular↗