Biomedical subjects

R J van Wezel

Publications and source records attributed to R J van Wezel.

10 recordsLinked to original sources

PubMed1998

Spatial asymmetries in cat retinal ganglion cell responses.

Enroth-Cugell and Robson (1966) first proposed a classification of retinal ganglion cells into X cells, which exhibit approximate linear spatial summation and largely sustained responses, and Y cells, which exhibit nonlinearities and transient responses. Gaudiano (1992a, 1992b, 1994) has suggested that the dominant characteristics of both X and Y cells can be simulated with a single model simply by changing receptive field profiles to match those of the anatomical counterparts of X and Y cells. He also proposed that a significant component of the spatial nonlinearities observed in Y (and sometimes X) cells can result from photoreceptor nonlinearities coupled with push-pull bipolar connections. Specifically, an asymmetry was predicted in the ganglion cell response to rectangular gratings presented at different locations in the receptive field under two conditions: introduction/withdrawal (on-off) or contrast reversal. When measuring the response to these patterns as a function of spatial phase, the standard difference-of-Gaussians model predicts symmetrical responses about the receptive field center, while the push-pull model predicts slight but significant asymmetry in the on-off case only. To test this hypothesis, we have recorded ganglion cell responses from the optic tract fibers of anesthetized cat. The mean and standard deviations of responses to on-off and contrast-reversed patterns were compared. We found that all but one of the cells that yielded statistically significant data confirmed the hypothesis. These results largely support the theoretical prediction.

Animals↗

PubMed1998

Electrical microstimulation of cortical area MST biases heading perception in monkeys.

As we move through the environment, the pattern of visual motion on the retina provides rich information about our movement through the scene. Human subjects can use this information, often termed "optic flow", to accurately estimate their direction of self movement (heading) from relatively sparse displays. Physiological observations on the motion-sensitive areas of monkey visual cortex suggest that the medial superior temporal area (MST) is well suited for the analysis of optic flow information. To test whether MST is involved in extracting heading from optic flow, we perturbed its activity in monkeys trained on a heading discrimination task. Electrical microstimulation of MST frequently biased the monkeys' decisions about their heading, and these induced biases were often quite large. This result suggests that MST has a direct role in the perception of heading from optic flow.

Animals↗

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

Directional motion sensitivity under transparent motion conditions.

We measured directional sensitivity to a foreground pattern while an orthogonally directed background pattern was present under transparent motion conditions. For both foreground and background pattern, the speed was varied between 0.5 and 28 deg sec-1. A multi-step paradigm was employed which results in a better estimation of the suppressive or facilitatory effects than previously applied single-step methods (e.g. measuring Dmax or Dmin). Moreover, our method gives insight into the interactions for a wide range of speed and not just the extreme motion thresholds (the D-values). We found that high background speeds have an inhibitory effect on the detection of a range of high foreground speeds and low background speeds have an inhibitory effect on a range of low foreground speeds. Intermediate background pattern speeds inhibit the detection of both low and high foreground pattern speeds and do so in a systemic manner.

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

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↗

PubMed1994

Spatial integration in coherent motion detection and in the movement aftereffect.

Sensitivity characteristics and spatial integration properties of the motion-detection system are compared with those of the system responsible for the movement aftereffect (MAE), elicited by the same stimulus. This provides new information about the mechanisms involved in MAE generation. A screen was divided into a chequerboard where the squares were filled with random-pixel arrays moving in opposite directions. Changing the size of the squares produced drastic changes in the percept during the adaptation phase and in the MAE during the test phase. One striking new phenomenon that is described is 'structure from MAE.' The results indicate that the receptive fields of units involved in eliciting the MAE are larger than the receptive-field sizes of units involved in detection and segregation of motion components in the stimulus. Furthermore, the results suggest that the receptive fields contributing to the MAE are involved in complex interactions in which different local motion directions are integrated in pattern-specific ways.

Humans↗

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↗

PubMedDate not supplied

Horizontal cell sensitivity in the cat retina during prolonged dark adaptation.

The effects of dark adaptation on the response properties of ganglion cells have been documented extensively in the cat retina. To pinpoint the different retinal mechanisms that underlie these effects, we studied the response characteristics of cat horizontal (H) cells during prolonged dark adaptation. H-cell responses were recorded intracellularly in the optically intact, in vivo eye. To disentangle rod and cone contributions, sensitivity changes during dark adaptation were tracked with white light and with monochromatic lights that favored either rod or cone excitation. Stable, long-lasting recordings allowed us to measure changes of sensitivity for adaptation periods up to 45 min. Thresholds for white light and 503-nm monochromatic light decreased steadily and in parallel. The maximum increase of sensitivity, after extinguishing a photopic adaptation light, was 1.8 log units only, reached after about 35 min. Sensitivity for 581-nm lights also increased steadily, but at a shallower slope. The steady increase of sensitivity was concomitant with a linear shift in resting membrane potential and with an increase in relative rod contribution to the threshold responses. Even though small-amplitude responses were rod dominated after prolonged dark adaptation, sensitivity to rod signals remained relatively low, compared to sensitivity of cone responses or to the absolute sensitivity of ganglion cells. This suggests that the cone-H-cell pathway plays no role in the dark-adapted cat retina.

Adaptation, Ocular↗