Biomedical subjects

L Spillmann

Publications and source records attributed to L Spillmann.

At least 37 records · Page 2Linked to original sources

PubMed1995

Assimilation: asymmetry between brightness and darkness?

A pincushion formed by four arcs on a gray background looks darker when the arcs are black, and lighter when the arcs are white. Yet, a matching-experiment shows that this difference is relative. Whereas the apparently darker pincushion requires a matching luminance that is lower than the background luminance (i.e. assimilation), the apparently lighter pincushion curiously is also matched to a darker-than-background value (i.e. simultaneous contrast). A change-over in direction of a higher luminance occurs only at the lowest contrast. The size of the decrement required for matching the brightness of the pincushions increases with increasing contrast of the inducing stimulus, as well as with viewing distance. Assimilation is found also in the domain of color, however, only when the luminance of the colored inducers is below that of the background. Analogous asymmetries in the perception of darkness and lightness are discussed.

Color Perception↗

PubMed1995

Phenomena of illusory form: can we bridge the gap between levels of explanation?

The study of illusory brightness and contour phenomena has become an important tool in modern brain research. Gestalt, cognitive, neural, and computational approaches are reviewed and their explanatory powers are discussed in the light of empirical data. Two well-known phenomena of illusory form are dealt with, the Ehrenstein illusion and the Kanizsa triangle. It is argued that the gap between the different levels of explanation, bottom-up versus top-down, creates scientific barriers which have all too often engendered unnecessary debate about who is right and who is wrong. In this review of the literature we favour an integrative approach to the question of how illusory form is derived from stimulus configuration which provide the visual system with seemingly incomplete information. The processes that can explain the emergence of these phenomena range from local feature detection to global strategies of perceptual organisation. These processes may be similar to those that help us restore partially occluded objects in everyday vision. To understand better the Ehrenstein and Kanizsa illusions, it is proposed that different levels of analysis and explanation are not mutually exclusive, but complementary. Theories of illusory contour and form perception must, therefore, take into account the underlying neurophysiological mechanisms and their possible interactions with cognitive and attentional processes.

Arousal↗

PubMed1994

The Hermann grid illusion: a tool for studying human perspective field organization.

Psychophysical research on the Hermann grid illusion is reviewed and possible neurophysiological mechanisms are discussed. The illusion is most plausibly explained by lateral inhibition within the concentric receptive fields of retinal and/or geniculate ganglion cells, with contributions by the binocular orientation-specific cortical cells. Results may be summarized as follows: (a) For a strong Hermann grid illusion to be seen bar width must be matched to the mean size of receptive-field centers at any given retinal eccentricity. (b) With the use of this rationale, the diameter of foveal perceptive-field centers (the psychophysical correlate of receptive-field centers) has been found to be in the order of 4-5 min arc and that of total fields (centers plus surrounds) 18 min arc. These small diameters explain why the illusion tends to be absent in foveal vision. (c) With increasing distance from the fovea, perceptive-field centers increase to 1.7 deg at 15 deg eccentricity and then to 3.4 deg at 60 deg eccentricity. This doubling in diameter agrees with the change in size of retinal receptive-field centers in the monkey. (d) The Hermann grid illusion is diminished with dark adaptation. This finding is consistent with the reduction of the center-surround antagonism in retinal receptive fields. (e) The illusion is also weakened when the grid is presented diagonally, which suggests a contribution by the orientation-sensitive cells in the lateral geniculate nucleus and visual cortex. (f) Strong induction effects, similar to the bright and dark spots in the Hermann grid illusion, may be elicited by grids made of various shades of grey; and by grids varying only in chroma or hue. Not accounted for are: the illusory spots occurring in an outline grid ie with hollow squares, and the absence of an illusion when extra bars are added to the grid. Alternative explanations are discussed for the spurious lines connecting the illusory spots along the diagonals and the fuzzy dark bands traversing the rhombi in modified Hermann grids.

Animals↗

PubMed1993

The perception of movement and depth in moiré patterns.

Moiré patterns can produce striking movement effects and in more complex stimuli can induce vivid stereoscopic depth. The physical rules underlying these phenomena are reviewed and their relationship to psychophysics is discussed. First, it is shown how moirés in 'optical line interference' patterns are created by superimposing periodic visual stimuli, eg gratings, and shifting them relative to each other. When two gratings are presented in this manner, small differences in spatial frequency, orientation, and speed are magnified. This magnification has prompted the use of moiré patterns both in industry and in art where their enhanced sensitivity to misalignment and spatial distortion has been widely exploited. Next, it is demonstrated how enhanced depth in 'stereoscopic interference' patterns is produced by presenting grating stimuli in two (or more) depth planes. The perceived depth effect in the resulting moiré pattern can be elicited similarly by binocular disparity and motion parallax. Finally, it is described how perceived movements occurring in different directions and at different depths are the basis for the perceptual 'irritations' that fascinate observers in complex moiré patterns. The use of moirés for the noninvasive examination of the human retina by aliasing is discussed.

Contrast Sensitivity↗

PubMed1992

Dynamic noise backgrounds facilitate target fading.

With strict fixation, a small uniform target of medium contrast, placed at 10 deg eccentricity, faded much faster when presented on a dynamic random noise background than on either a static random noise background or a uniform background of the same luminance. Time to first disappearance was between 10 and 16 sec when the background was dynamic, 26 sec when it was static, and 57 sec when it was uniform. Times were shortest for temporal noise frequencies of the background between 3.5 and 15 Hz. These findings are unexpected: the frequent change of pixel contrast at the edge of the target should perceptually enhance the border, make it less susceptible to local adaptation, and prevent fading. Instead, dynamic random noise facilitates, rather than suppresses fading. Three potential mechanisms are discussed: edge perturbation, jerk effect and surround induction.

Adaptation, Ocular↗

PubMed1992

Reduced spatial sensitization on nonuniform backgrounds.

Spatial sensitivity (Westheimer) functions, when measured on nonuniform backgrounds made up of light dots of 12 min arc, were found to differ in shape, depending on the polarity of the central area on which the test spot was placed. When thresholds were measured on the dark centre between light dots, ie on the adapting-field illumination, the resulting curve was similar to the control curve, measured on a uniform background equated for flux. In comparison, thresholds measured on a central light dot, serving as a pedestal, peaked at larger background diameters and showed much less sensitization compared to the control function.

Adaptation, Ocular↗

PubMed1991

Temporal phase response of the short-wave cone signal for color and luminance.

A chromatic discrimination paradigm was used to measure the temporal phase of the S (short-wave cone) signal relative to the L--M (long-wave cone minus middle-wave cone) signal. Suprathreshold equiluminant red-green flicker that stimulates the L--M mechanism was presented on a steady, intense yellow-green adapting field. Violet flicker that stimulates the S cones was added to the red-green flicker at different temporal phase angles, and the violet modulation depth was varied to achieve a chromatic discrimination threshold. A template was fitted to the data relating thresholds to phase: the location of the template symmetry axis showed that the S signal lagged L--M by about 75-90 degrees at 10 Hz. This is about one half the phase lag obtained for luminance or motion discrimination. The phase discrepancy shows that there are separate luminance and chromatic mechanisms receiving S cone inputs. The hue of the flicker in the present study varied strongly with phase angle, with the positive and negative excursions of the S cone signal producing a reddish-blue and greenish-yellow, respectively, and these colors combined with the reddish and greenish hues produced by the L--M signal. The observed phase shift, and measured color appearance of the combined flicker, account for the colors seen on a radially segmented disk of Munsell hues when rotated: the colors differ strikingly depending on the direction of rotation.

Adaptation, Ocular↗

PubMed1989

UV-absorbing intraocular lenses: safety, efficacy, and consequences for the cataract patient.

The crystalline lens absorbs most of the incident UV radiation between 300 and 400 nm and thereby protects the retina from a significant, potential source of photochemical damage. This protection is lost when the lens is removed by cataract surgery, but can be restored by the implantation of an intraocular lens (IOL) that has UV-absorbing chromophores incorporated into a polymethylmethacrylate (PMMA) substrate. Spectrophotometric data show that the various, commercially available, UV-absorbing IOLs are not equally effective in absorbing UV radiation; thus, a standard, quantitative metric for comparing their performance is proposed. Cytotoxicity and biocompatibility studies have failed to demonstrate that UV-absorbing IOLs are unsafe, even when damaged by Nd:YAG lasers used for photodiscission posterior capsulotomy. There are positive consequences for the pseudophakic patient with a UV-absorbing IOL, in that it may restore normal spectral sensitivity, reduce erythropsia and cystoid macular edema, and stabilize the blood-vitreous barrier.

Cataract Extraction↗

PubMed1987

Duration of visual afterimages on modulated backgrounds: postreceptoral processes.

A foveal afterimage produced by a small photoflash increases in duration when the luminance of a 5.8 degrees diameter background on which it is seen is temporally modulated. At a modulation frequency of 1 Hz and a depth of modulation of 52%, the duration of the afterimage is prolonged by 335% compared to the duration obtained on a steady background. This increase has been attributed to the functional border resulting from the difference in excitability between bleached and unbleached photoreceptors. Afterimage duration is also prolonged, although only by 20%, when the luminance of the background is kept constant, while the luminance of an annular surround is modulated. This finding suggests a weak effect of neural lateral interaction (via area contrast). If the background luminance is modulated only in the contralateral eye (dichoptic presentation), afterimage duration increases by as much as 54% compared to the unmodulated state. This result indicates that afterimages, in part, are sustained by processes mediated by the visual cortex.

Adult↗

PubMed1987

Flicker adaptation in the peripheral retina.

With strict fixation, a flickering light spot smaller than 3 deg presented to the peripheral retina will rapidly appear to lose contrast and stop flickering within 35 s, before fading away completely. The time required for this adaptation to occur decreases with: decreasing depth of modulation (97-9%); decreasing stimulus diameter (2 deg-7 min arc); increasing retinal eccentricity (20-50 deg); and increasing flicker frequency (1-7 Hz). Interestingly, the effect does not depend upon the regularity of the flickering stimulus, and it occurs twice as fast for stimuli presented to the temporal retina as for stimuli presented to the nasal retina. When changes in retinal eccentricity are compensated for by taking into account the cortical magnification factor, the time needed for perceived flicker to disappear remains constant at all eccentricities. With dichoptic stimulation interocular transfer is about 35%, suggesting a cortical contribution to flicker adaptation. The results indicate that the visual system adapts rather easily to peripheral flickering stimuli. Similarities as well as differences to motion adaptation are discussed.

Adaptation, Ocular↗

PubMed1987

A comparison of perceptive and receptive fields in man and monkey.

We have measured the perceptive field, the psychophysical correlate of the physiologically determined receptive field, in man and monkey. Measurements were made using the Hermann grid illusion and the Westheimer paradigm. The following results were found: First, in both man and monkey, the size of perceptive fields and field centers increases from the fovea to the periphery. As with receptive fields, this increase is first rapid and then more gradual; and it is more pronounced on the temporal than on the nasal side of the retina. Second, monkey and human perceptive field centers are approximately the same size. But total perceptive fields (i.e., centers plus surrounds) tend to be smaller in monkeys. Third, in monkey, psychophysically measured perceptive field centers are about the same size as neurophysiologically measured receptive field centers. And as these, they are larger, by a factor 1.3-2, than histologically measured dendritic fields. These findings strongly indicate that in monkey all three measurements refer to the same underlying retinal mechanism. The same relationship is assumed to hold in man.

Animals↗

PubMed1984

Movement adaptation in the peripheral retina.

With strict fixation, the eye quickly adapts to moving periodic stimuli presented to the peripheral retina. A slowly spinning sector disk, 7 degrees in diameter, will rapidly appear to slow down and come to a standstill (within 5-25 sec). The time required for this full motion adaptation decreases with (a) increasing retinal eccentricity (30-70 degrees); (b) increasing number of sectors (16-60); and (c) decreasing speed of rotation (0.3-0.5 rev/sec). After the standstill, the disk fades from view in much the same way as a stationary stimulus (Troxler effect). A spinning disk presented to the temporal retina appears to stop about 2.5 times faster than a disk presented to the nasal side. Adapting one eye reduces the time of adaptation for the other eye by 70%. If an aperiodic sector disk is used, no standstill is perceived.

Adaptation, Ocular↗

PubMed1984

Colored neon flanks and line gap enhancement.

When a colored line connects two black (or differently colored) lines across a gap, colored neon flanks are seen on either side of it. These flanks extend over gap sizes of 50 min arc foveally and are not explained by Bezold-type assimilation. They may be elicited by black lines as short as 6 min arc adjoining the colored line at each end. To maximize these flanks, the black and colored lines must appear linearly continuous. Nonaligned junctions weaken the effect and an angular tilt of more than 40 dog destroys it. In this and other respects, (local) neon flanks are similar to van Tuijl's (global) neon color spreading (1975). Both phenomena have analogs in brightness perception. We propose that neon spreading is a lateral extension of neon flanks across the empty space between them, and discuss similarities of these effects with other brightness illusions (Schumann, Prandtl, Ehrenstein). For this group of illusions the term "line gap enhancement" is introduced to imply perceptual enhancement of changes in brightness and/or color along lines. Correspondences between the psychophysical properties and structural prerequisites for line gap enhancement on one hand and neuronal response properties of end-zone inhibited (hypercomplex) cortical cells on the other are discussed.

Color Perception↗

PubMed1984

Brightness matching, brightness cancellation, and increment threshold in the Ehrenstein illusion.

Matching and cancellation techniques were used to measure the relative strength of the Ehrenstein illusion in dark figures on a light background (negative contrast) and light figures on a dark background (positive contrast). Brightness enhancement on the former was shown to be maximally 0.28 log unit (relative to the detection threshold), and darkness enhancement on the latter 0.43 log unit. Values differed little with figure-ground contrast (down to a minimum of +/- 0.5), but decreased with decreasing level of illumination. The luminance increment (decrement) needed to match the illusory brightness (darkness) was similar in size to the luminance decrement (increment) needed to cancel the illusion. The increment threshold for a small test flash measured in three locations relative to the subjective contour delineating the illusion did not differ systematically. The results are compatible with a neurophysiological explanation of the Ehrenstein illusion in terms of line-induced lateral interaction in hypercomplex receptive fields.

Fixation, Ocular↗

PubMed1984

Change in hue of spectral colors by dilution with white light (Abney effect).

Monochromatic light, when mixed with white light, not only becomes desaturated but also changes in hue ( Abney effect). This effect was studied in three observers by using three unique hues (blue, green, and yellow) and four compound (intermediate) hues. The whites used for desaturation ( desaturants ) included Abney 's white (3890 K), two bluish whites (10,000 and 20,000 K), and each observer's own, perceptually neutral white (6200-6980 K). Test stimuli of 0.5 degree diameter were presented to the dark-adapted fovea for 1 sec in a dark surround. Abney 's results were confirmed, except in the shortwave and middle-wave parts of the spectrum. At short wavelengths we always observed a hue shift toward increasing redness, whereas Abney reported a shift toward blue. At middle wave-lengths (500-556 nm), we found smaller effects than did Abney . Here Abney 's white produced an increase in perceived yellow, whereas all other desaturants produced an increase in perceived green. Two colors, blue-green and yellow, changed least. In general, the hue shifts increased with decreasing colorimetric purity (from 1.0 to 0.5). The results are discussed in relation to color additivity, constant-hue loci, and the Benzold -Br ucke effect.

Adult↗

PubMed1983

Time thresholds for increments and decrements in luminance.

Time thresholds, i.e., the minimal durations necessary to just detect a change in brightness, were measured for light increments and decrements of a 1 degree test spot centered on a background of 20 degrees. Background luminance varied from -1 to 3 log td and retinal eccentricity from 0 degree to 50 degrees. Step size ranged from 0.04 to 1.5 log units and was the same in absolute units for both directions. Two types of stimuli were used: Type A, in which increments and decrements emerge from the same uniform background, and Type B, in which increments are the same as in Type A but decrements consist of a brief interruption of the test spot. Type A stimulation resulted in similar time thresholds for increments and decrements or, under some conditions, slightly shorter decrement thresholds. Type B stimulation resulted in similar thresholds for foveal vision. However, with increasing step size, decreasing background luminance, and increasing eccentricity, the time threshold for the decrement progressively exceeded that for the increment (up to 80 msec). This difference is attributed to different rise and fall times of the photoreceptor response as well as to Troxler's effect.

Adult↗

PubMed1982

Dark adaptation with interposed white adapting fields.

It is proposed that dark adaptation following a moderate pigment bleach may nearly as well be carried out (and more conveniently) under low room lighting conditions as in complete darkness. To test this idea, dark adaptation curves were determined either immediately after the termination of a 3 min, 4.1 log td white pre-exposure field, or following 10 or 15 min of additional exposure to one of three low-level photopic (2.9, 2.4, 1.8 log td) backgrounds of white light. Dark thresholds measured after the additional exposure fell rapidly and reached the rod plateau of the normal dark adaptation curve with a maximal delay of 1.5 min (for the 10 min backgrounds) or 6.5 min (for the 15 min backgrounds). For the time to be spent in the dark, this meant a savings of 8.5 min. At smaller delays savings were even greater. The difference between savings and delay indicates whether or not an interposed background is feasible.

Dark Adaptation↗

PubMed1982

Low spatial-frequency channels in human vision: adaptation and masking.

Previous work showed that adapting to low spatial frequency gratings (below 1.5 cycles/degree) may cause maximal spatial adaptation at a significantly higher spatial frequency. It has been suggested that there are no adaptable spatial-frequency channels tuned to below 1.5 c/deg. Contrary to this view, we found that adaptation and masking with low spatial frequencies (0.12-1.0 c/deg) produced maximal threshold elevations when the test patterns were the same spatial frequency as the adapting or masking pattern. These results were obtained using test patterns that turned on and off gradually or sharply. The results suggest that there are form mechanisms optimally sensitive to very low spatial frequencies. Adaptation was selective to position (phase) and orientation at low spatial frequencies; masking was observed to be selective to orientation at a spatial frequency as low as 0.2 c/deg. A clear dichotomy between transient, motion channels and sustained, form channels at low spatial and temporal frequencies may represent an unrealistic simplification. There may exist directionally-selective motion mechanisms sensitive to very slow motion, and these may play a role in the discrimination of form. The discussion considers the bandwidths of the low spatial frequency mechanisms.

Adaptation, Ocular↗