Biomedical subjects

M J Lankheet

Publications and source records attributed to M J Lankheet.

At least 19 recordsLinked to original sources

PubMed2000

Fine structure of parvocellular receptive fields in the primate fovea revealed by laser interferometry.

Optical blurring in the eye prevents conventional physiological techniques from revealing the fine structure of the small parvocellular receptive fields in the primate fovea in vivo. We explored the organization of receptive fields in macaque parvocellular lateral geniculate nucleus cells by using sinusoidal interference fringes formed directly on the retina to measure spatial frequency tuning at different orientations. Most parvocellular cells in and near the fovea respond reliably to spatial frequencies up to and beyond 100 cycles/ degrees of visual angle, implying center input arising mainly from a single cone. Temporal frequency and contrast response characteristics were also measured at spatial frequencies up to 130 cycles/degrees. We compared our spatial frequency data with the frequency responses of model receptive fields that estimate the number, configuration, and weights of cones that feed the center and surround. On the basis of these comparisons, we infer possible underlying circuits. Most cells had irregular spatial frequency-response curves that imply center input from more than one cone. The measured responses are consistent with a single cone center together with weak input from nearby cones. By exposing a fine structure that cannot be discerned by conventional techniques, interferometry allows functional measurements of the early neural mechanisms in spatial vision.

Animals↗

PubMed2000

Motion coherence detection as a function of luminance level in human central vision.

We studied the changes and invariances of foveal motion detection upon dark adaptation. It is well-documented that dark adaptation affects both spatial and temporal aspects of visual processing. The question we were interested in is how this alters motion coherence detection for moving random texture. To compare motion sensitivity at different adaptation levels, we adjusted the viewing distance for equal detectability of a stationary pattern. At these viewing distances we then measured velocity tuning curves for moving random pixel arrays (RPAs). Mean luminance levels ranged from 50 down to 0.005 cd m-2. Our main conclusion is that foveal velocity tuning is amazingly close to luminance-invariant, down to a level of 0.05 cd m-2. Because different viewing distances, and hence, retinal image sizes were used, we performed two control experiments to assess variations of these two parameters separately. We examined the effects of retinal inhomogeneities using discs of different size and annuli filled with RPAs. Our conclusion is that the central visual field, including the near periphery is still rather homogeneous for motion detection at 0.05 cd m-2, but the fovea becomes unresponsive at the lowest luminance level. Variations in viewing distance had marked effects on velocity tuning, both at the light adapted level and the 0.05 cd m-2 level. The size and type of these changes indicated the effectiveness of distance scaling, and show that deviations from perfect invariance of motion coherence detection were not due to inaccurate distance scaling.

Adaptation, Ocular↗

PubMed1998

Stereoscopic segregation of transparent surfaces and the effect of motion contrast.

Stereoscopic segregation in depth was studied using two superimposed frontoparallel surfaces displayed in dynamic random dot stereograms. The two patterns were positioned symmetrically in front of and behind a binocular fixation point. They were either stationary, or they could move relative to each other. Sensitivity for segregation was established by adding gaussian distributed disparity noise to the disparities specifying the two planes, and finding the noise amplitude that gave threshold segregation performance. Observers easily segregate the two surfaces for disparity differences between approximately 6 and 30-40 arcmin. Motion contrast, which by itself provides no cue to perform the task, greatly improves sensitivity for segregation. Noise tolerance rises by a factor of two or more when the patterns move at different speeds, or in different (frontoparallel) directions. The effect increases with directional difference, but the optimal directional difference deviated from 180 deg. The optimal speed varies with disparity difference. Thus, motion and disparity must interact in order to resolve the two transparent planes.

Depth Perception↗

PubMed1997

Responses of complex cells in cat area 17 to apparent motion of random pixel arrays.

The characteristics of directionally selective cells in area 17 of the cat are studied using moving random pixel arrays (RPAs) with 50% white and 50% black pixels. The apparent motion stimulus is similar to that used in human psychophysics [Fredericksen et al. (1993). Vision Research, 33, pp. 1193-1205]. We compare motion sensitivity measured with single-step pixel lifetimes and unlimited pixel lifetimes. A motion stimulus with a single-step pixel lifetime contains directional motion energy primarily at one combination of spatial displacement and temporal delay. We recorded the responses of complex cells to different combinations of displacement and delay to describe their spatio-temporal correlation characteristics. The response to motion of RPAs with unlimited lifetime is strongest along the preferred speed line in a delay vs displacement size diagram. When using an RPA with a single-step pixel lifetime, the cells are responsive to a much smaller range of spatial displacements and temporal delays of the stimulus. The maximum displacement that still gives a directionally selective response is larger when the preferred speed of the cell is higher. It is on average about three times smaller than the receptive field size.

Animals↗

PubMed1996

Orthogonal motion after-effect illusion predicted by a model of cortical motion processing.

The motion after-effect occurs after prolonged viewing of motion; a subsequent stationary scene is perceived as moving in the opposite direction. This illusion is thought to arise because motion is represented by the differential activities of populations of cortical neurons tuned to opposite directions; fatigue in one population leads to an imbalance that favours the opposite direction once the stimulus ceases. Following adaptation to multiple directions of motion, the after-effect is unidirectional, indicating that motion signals are integrated across all directions. Yet humans can perceive several directions of motion simultaneously. The question therefore arises as to how the visual system can perform both sharp segregation and global integration of motion signals. Here we show in computer simulations that this can occur if excitatory interactions between different directions are sharply tuned while inhibitory interactions are broadly tuned. Our model predicts that adaptation to simultaneous motion in opposite directions will lead to an orthogonal motion after-effect. This prediction was confirmed in psychophysical experiments. Thus, broadly tuned inhibitory interactions are likely to be important in the integration and segregation of motion signals. These interactions may occur in the cortical area MT, which contains motion-sensitive neurons with properties similar to those required by our model.

Afterimage↗

PubMed1996

On the complex dynamics of intracellular ganglion cell light responses in the cat retina.

We recorded intracellular responses from cat retinal ganglion cells to sinusoidal flickering lights, and compared the response dynamics with a theoretical model based on coupled nonlinear oscillators. Flicker responses for several different spot sizes were separated in a "smooth" generator (G) potential and corresponding spike trains. We have previously shown that the G-potential reveals complex, stimulus-dependent, oscillatory behavior in response to sinusoidally flickering lights. Such behavior could be simulated by a modified van der Pol oscillator. In this paper, we extend the model to account for spike generation as well, by including extended Hodgkin-Huxley equations describing local membrane properties. We quantified spike responses by several parameters describing the mean and standard deviation of spike burst duration, timing (phase shift) of bursts, and the number of spikes in a burst. The dependence of these response parameters on stimulus frequency and spot size could be reproduced in great detail by coupling the van der Pol oscillator and Hodgkin-Huxley equations. The model mimics many experimentally observed response patterns, including non-phase-locked irregular oscillations. Our findings suggest that the information in the ganglion cell spike train reflects both intraretinal processing, simulated by the van der Pol oscillator, and local membrane properties described by Hodgkin-Huxley equations. The interplay between these complex processes can be simulated by changing the coupling coefficients between the two oscillators. Our simulations therefore show that irregularities in spike trains, which normally are considered to be noise, may be interpreted as complex oscillations that might carry information.

Action Potentials↗

PubMed1996

Recovery from adaptation for dynamic and static motion aftereffects: evidence for two mechanisms.

The motion aftereffect (MAE) is an illusory drift of a physically stationary pattern induced by prolonged viewing of a moving pattern. Depending on the nature of the test pattern the MAE can be phenomenally different. This difference in appearance has led to the suggestion that different underlying mechanisms may be responsible and several reports show that this might be the case. Here, we tested whether differences in MAE duration obtained with stationary test patterns and dynamic test patterns can be explained by a single underlying mechanism. We find the results support the existence of (at least) two mechanisms. The two mechanisms show different characteristics: the static MAE (i.e. the MAE tested with a static test pattern) is almost completely stored when the static test is preceded by a dynamic test; in contradistinction, the dynamic MAE is not stored when dynamic testing is preceded by a static test pattern.

Adaptation, Ocular↗

PubMed1996

Spatio-temporal requirements for binocular correlation in stereopsis.

We measured sensitivity to binocular correlation in dynamic random-dot stereograms that defined moving sinusoidal gratings-in-depth. At a range of spatial frequencies and drift rates we established sensitivity by adding Gaussian distributed disparity noise to the modulation of disparity that defined a cyclopean grating, and finding the noise amplitude that rendered the grating just detectable. This permitted correlation thresholds to be measured at a range of suprathreshold disparity amplitudes. Spatial requirements for binocular correlation depend little on temporal frequency, and vice versa. This suggests that binocular correlation mechanisms can be characterized by independent spatial and temporal sensitivity functions. The temporal frequency function has a low pass characteristic. Sensitivity declines above about 1 c/sec, reaching its limit at 4-8 c/sec. The spatial characteristic depends greatly on the amplitude of disparity modulation, changing from band pass at low amplitude to low pass at high amplitude. The maximum resolvable spatial frequency is 4-6 c/deg, but declines sharply for relatively high amplitudes. The interaction between amplitude and spatial frequency cannot be explained by fixed high or low limits on detectable disparity gradients.

Depth Perception↗

PubMed1996

Responses of complex cells in area 17 of the cat to bi-vectorial transparent motion.

We examined the responses to transparent motion of complex cells in cat area 17 which show directional selectivity to moving random pixel arrays (RPAs). The response to an RPA moving in the cell's preferred direction is inhibited when a second RPA is transparently moving in another direction. The inhibition by the second pattern is quantified as a function of its direction. The response to a pattern moving in the preferred direction is never completely suppressed, not even when a second pattern is moving transparently in the opposite direction. To the extent that supra-spontaneous firing rates signal the presence of the optimal velocity vector, these cells therefore still signal the presence of this line-label stimulus despite additional opposing, or otherwise directed, motion components. The results confirm previous suggestions that, for the computation of motion energy in cat area 17 complex cells, a full opponent stage is not plausible. Furthermore, we show that the response to a combination of two motion vectors can be predicted by the average of the responses to the individual components.

Animals↗

PubMed1996

Spatial and temporal properties of cat horizontal cells after prolonged dark adaptation.

We studied the change of spatial and temporal response properties for cat horizontal (H-) cells during prolonged dark adaptation. H-cell responses were recorded intracellularly in the optically intact, in vivo eye. Spatial and temporal properties were first measured for light-adapted H-cells, followed by a period of dark adaptation, after which the same measurements were repeated. During dark adaptation threshold sensitivity was measured at regular intervals. Stable, long lasting recordings allowed us to measure changes of sensitivity and receptive field characteristics for adaptation periods up to 45 min. Although cat H-cells showed no signs of dark suppression or light sensitization, they remained insensitive in the scotopic range, even after prolonged dark adaptation. Absolute thresholds were in the low mesopic range. The sensitization was brought about by a shift from cone to rod input, and by substantial increases of both spatial and temporal integration upon dark adaptation. The length constant in the light-adapted state was on average about 4 deg. After dark adaptation it was up to a factor of three larger, with a median ratio of 1.85. Response delays, latencies and durations for (equal amplitude) threshold flash responses substantially increased during dark adaptation.

Animals↗

PubMed1996

Gain control and hyperpolarization level in cat horizontal cells as a function of light and dark adaptation.

First a model is presented that accurately summarizes the dynamic properties of cat horizontal (H-) cells under photopic conditions as measured in our previous work. The model predicts that asymmetries in response to dark as compared to light flashes are flash-duration dependent. This somewhat surprising prediction is tested and confirmed in intracellular recordings from the optically intact in vivo eye of the cat (Experiment 1). The model implies that the gain of H-cells should be related rather directly to the sustained (baseline) membrane potential. We performed three additional experiments to test this idea. Experiment 2 concerns response vs intensity (R-I-) curves for various flash-diameters and background-sizes with background luminance varying over a 4 log unit range. Results support the assumption of a rather strict coupling between flash sensitivity (gain) and the sustained level of hyperpolarization. In Experiment 3 we investigate this relation for both dark and light flashes given on each of four background light levels. The results suggest that there are fixed minimum and maximum hyperpolarization levels, and that the baseline hyperpolarization for a given illumination thus also sets the available range for dark and light flash-responses. The question then arises whether, or how this changes during dark adaptation, when the rod contribution to H-cell responses gradually increases. The fourth experiment therefore studies the relationship between gain and hyperpolarization level during prolonged dark-adaptation. The results show that the rod contribution increases the polarization range of H-cells, but that the gain and polarization level nevertheless remain directly coupled. H-cell models relying on a close coupling between polarization level and gain thus remain attractive options.

Adaptation, Ocular↗

PubMed1995

Attentional modulation of adaptation to two-component transparent motion.

We have studied the effects of voluntary attention on the induction of motion aftereffects (MAEs). While adapting, observers paid attention to one of two transparently displayed random dot patterns, moving concurrently in opposite directions. Selective attention was found to modulate the susceptibility to motion adaptation very substantially. To measure the strength of the induced MAEs we modulated the signal-to-noise ratio of a real motion signal in a random dot pattern that was used to balance the aftereffect. Results obtained for adapting to single motion vectors show that the MAE can be represented as a shift of the psychometric function for motion direction discrimination. Selective attention to the different components of transparent motion altered the susceptibility to adaptation. Shifting attention from one component to the other caused a large shift of the psychometric curves, about 70-75% of the shift measured for the separate components of the transparent adapting stimulus. We conclude that attention can differentiate between spatially superimposed motion vectors and that attention modulates the activity of motion mechanisms before or at the level where adaptation gives rise to MAEs. The results are discussed in light of the role of attention in visual perception and the physiological site for attentional modulation of MAEs.

Adaptation, Physiological↗

PubMed1993

Nonlinearity and oscillations in X-type ganglion cells of the cat retina.

Intracellularly recorded light-responses of X-type ganglion cells in the cat retina were separated, with the help of a wavelet method, into "slow" membrane ("G")-potentials and the corresponding spike trains. In response to sinusoidally modulated high intensity light spots with different sizes and frequencies, X-type ganglion cells show both oscillations correlated with the stimulus frequency and other, faster, oscillations that were not always locked to the stimulus. A forced van der Pol oscillator model with stimulus-dependent coefficients proved to describe the empirical findings quite well. A linearity-coefficient of the equations indicates strong nonlinearity at a temporal frequency of 8 Hz with spot sizes on the order of 0.5-0.7 deg and decreasing nonlinearity at lower temporal frequencies or smaller spot sizes, while the faster oscillations become more prominent. We could not determine whether the oscillations are intrinsic to the cell-membrane or generated by (or in interaction with) the preganglionic retinal meshwork. The results show that X-cell spike-trains can contain oscillations that are not phase-locked to the stimulus and that are therefore virtually invisible after stimulus synchronous averaging. It is not likely that these retinal oscillations directly induce the well described oscillations in cat visual cortex, since they usually fall in a different frequency range.

Action Potentials↗

PubMed1993

The dynamics of light adaptation in cat horizontal cell responses.

In order to model the dynamic properties of light adaptation processes in cat horizontal (H-) cells, the time course of the gain adjustment following changes in the mean illumination level was studied. H-cell responses were recorded intracellularly in the optically intact, in vivo, eye of the cat. The light stimulus consisted of two spots, a large background spot (8.8 deg diameter) and a concentrically arranged smaller test spot (3.9 deg). The background was either square wave or sine wave modulated in intensity at a frequency of 0.2-1 Hz. The instantaneous value of the response gain was measured with brief flashes (10 msec) of the test spot, generated repetitively at a frequency of 5 or 10 Hz. Modulation of the background intensity, at a contrast of 0.6 and in the photopic range, effectively induces a modulation of the gain. The readjustment of the gain by a stepwise increase or decrease in background illumination is completed within about 200 msec. The amplitude of the gain modulation due to a 0.5 Hz background flicker is quantitatively comparable to that measured between steady illumination levels. Dynamic changes of the gain at low frequency stimuli therefore, have to be taken into account in modelling H-cell responses. For sinusoidal modulations of the background luminance the time course of gain adjustment is quantified by the phase shift of the gain modulation relative to background intensity modulation. The results, together with those described in two preceding papers, are used to test and discuss several light adaptation models that have been proposed previously. It was found that light adaptation in cat H-cells is described more adequately by a de Vries-Rose type of adaptation model than by a Weber type of light adaptation.

Adaptation, Ocular↗

PubMed1993

The lateral spread of light adaptation in cat horizontal cell responses.

To investigate the sites of light adaptation processes in the mammalian distal retina, we studied the lateral spread of adaptation signals in cone-driven cat horizontal (H-) cell responses. The size of the adaptation pool is compared to the receptive field for H-cell responses. H-cell activity was recorded intracellularly in the optically intact, in vivo eye. It is demonstrated that light adaptation as measured in H-cells is not a strictly local process. Background light falling outside a central test region effectively modulates the responses to a small test light, flashed on the receptive field center. The integration area for adaptation signals was quantitatively compared to the H-cell receptive field size by measuring the desensitizing effect of background light on the responses to a small centered test spot, as a function of background spot size. The area-adaptation function is comparable to the area-response function but has a slightly smaller length constant. Light adaptation in H-cell responses, therefore, reveals spread of adaptation over a large distance and is probably mediated through lateral interactions in the H-cell network rather than in the cones.

Adaptation, Ocular↗

PubMed1992

Responses of cat horizontal cells to sinusoidal gratings.

The spatiotemporal properties of cat horizontal (H-) cells were studied by recording the intracellular responses in the optically intact, in vivo, eye to sinusoidal gratings at a photopic mean illumination level. In order to investigate the linearity of spatial summation a "null test" was performed in which the responses to contrast reversal gratings were measured at different positions of the grating relative to the receptive field. Spatial and temporal transfer functions were measured using drifting sinusoidal gratings of variable spatial and temporal frequencies. The amplitudes of cat H-cell responses to contrast reversal gratings modulated with a square wave time-course showed a sinusoidal dependence on spatial phase. When zero crossings of the grating were lined up with the receptive field center, as defined by the maximum of the measured line weighting function, contrast reversal produced no response modulation. This result did not depend on the spatial frequency of the grating or the temporal frequency of contrast modulation over substantial ranges. The response waveform was found not to depend on the spatial phase of the grating. The spatial transfer function of cat H-cells has low-pass characteristics with a cut-off frequency in the range of about 0.4-1.5 c/deg. The shape of the spatial transfer function was roughly the same for temporal frequencies ranging from 3 to 10 Hz. The temporal transfer function exhibited band-pass characteristics with a maximum response amplitude at 3-6 Hz. The amplitude fall-off for low and high temporal frequencies was independent of the spatial frequency of the grating. The results obtained with sine gratings were found not to agree with the receptive field profiles measured with narrow slits flashed at different positions in the receptive field.

Animals↗

PubMed1991

Light adaptation and frequency transfer properties of cat horizontal cells.

The frequency transfer properties of horizontal cells in the cat retina were studied as a function of the mean light intensity level and stimulus contrast. To this end, horizontal cell responses to sinusoidally modulated light stimuli were recorded intracellularly in the optically intact, in vivo eye. The light stimulus consisted of a 3.9 deg dia. spot superimposed on a steady background (8.8 deg dia.). A discrete Fourier analysis was performed in order to describe the amplitude and phase characteristics of the linear response component and in order to specify the nonlinear distortion of the response. The amplitude of the fundamental Fourier component was found to increase linearly with the contrast of the sinusoidal light intensity modulation. Increasing the mean light level while keeping the contrast constant caused a frequency dependent increase in response amplitude. The increase was most pronounced at high temporal frequencies and resulted in a conspicuous increase of the flicker fusion frequency. Steady background illumination caused a reduction of the response amplitudes at the lower temporal frequencies. Responses in the high frequency range, however, were not affected. The phase shifts of the fundamental Fourier components were found to diminish at increasing mean illumination levels. The harmonic distortion of horizontal cell responses to sinewave flicker was studied as a function of stimulus frequency and stimulus contrast. By comparing the data obtained using sinusoidal light intensity modulation with the intensity profiles described in a preceding paper it was investigated to what extent the harmonic distortion can be explained by the nonlinearity expressed in the response vs intensity profiles.

Adaptation, Ocular↗

PubMed1991

Effects of background illumination on cat horizontal cell responses.

The process of light adaptation in cat horizontal cells was studied by means of intracellular recordings in the optically intact, in situ eye. Response vs intensity profiles were measured using increment- as well as decrement-flashes of "white light" on steady backgrounds. The effects of background illumination in the mesopic and photopic range on both purely rod-driven horizontal cells and mixed rod and cone input horizontal cells were investigated. Increasing the background illumination for mixed-input horizontal cells strongly reduced the contribution from the rod system. The rod aftereffect in the responses to high intensity flashes is totally suppressed at higher background levels. Light adaptation resulted in a sustained hyperpolarization without a substantial effect on the total response range of the response vs intensity curve. At higher background intensities more of the response range is made up of depolarizing responses to decrements of light. Increasing the background illumination also shifted the operating curve to higher intensities. Increment threshold functions, measured with a 3.9 deg diameter test spot on a large background (8.8 deg diameter) showed a linear relation between log threshold intensity and log background intensity with a slope of, on average, 0.64. The response vs intensity curve for the rod horizontal cell typically spanned a narrower intensity range and was displaced toward lower intensities as compared to that for the mixed input horizontal cells. Background illumination greatly reduced the total response range for rod horizontal cells. Increment threshold curves for rod horizontal cells clearly indicated loss in sensitivity due to response compression.

Adaptation, Ocular↗