Biomedical subjects

M J Lankheet

Publications and source records attributed to M J Lankheet.

25 records · Page 2Linked to original sources

PubMed1990

Spatial properties of horizontal cell responses in the cat retina.

The spatial properties of horizontal cells in the cat retina have been studied by means of intracellular recordings in the optically intact, in situ, eye. The spread of potentials in the horizontal cell layer and the spatial summation properties have been investigated using "white light" stimuli in several different configurations. Area-response curves were measured with flashing spots carefully centered on the receptive field. The lateral spread of potentials was studied using long, narrow slits of light and circular spots of different sizes at different positions in the receptive field. Two-dimensional receptive field profiles showed that the receptive field structure was, both on a large scale and at higher resolutions, relatively homogeneous and isotropic. The size and shape of the receptive fields have been characterized by applying a simplified version of the model proposed by Naka and Rushton (1967) of electrical coupling in the horizontal cells layer in fish. Results show that the model describes the variation of response amplitude reasonably well both as a function of spot size and spot or slit position. However, deviations were found for area-response curves measured at higher light intensities and for receptive field profiles measured with relatively small spots. Furthermore, the estimated length constants resulting from the different experiments were not in agreement. Different model parameters were needed in order to describe the spatial summation properties and the lateral spread of potentials respectively. It is concluded that passive electrical spread and linear summation of potentials cannot account for the observed spatial properties.

Action Potentials↗

PubMed1989

The spike generating mechanism of cat retinal ganglion cells.

The spike generating mechanism (SGM) of sustained and transient-type ganglion cells has been investigated from intracellular recordings in the cat retina. The relationship between the generator potential and the impulse pattern, which are respectively the input and the output of the SGM, has been studied after separating one component from the other. Comparison of averaged generator potentials and the corresponding PSTHs showed that the spike generator is highly sensitive to changes of the generator potential. It proved to be relatively indifferent to the prevailing average levels of depolarization. During sustained parts of the responses the SGM exhibits a stochastic nature. At higher light flicker frequencies, during spike bursts, on the other hand spike generation is very regular and phase locked to the stimulus. The averaged generator potentials were also used to develop and test a set of minimal, computer simulated models of the spike generator. A slow threshold adaptation (time constant about 50 msec) is absolutely necessary in addition to the faster refractory recovery in order to produce spike patterns similar to those measured during the corresponding response periods. The developed model accounts completely for the observed characteristics of ganglion cell spike generation under a wide variety of light stimulus regimes and both for the sustained and for the transient type of ganglion cell.

Action Potentials↗

PubMed1989

Frequency transfer properties of the spike generating mechanism of cat retinal ganglion cells.

The dynamic properties of the spike generating (SG) mechanism of retinal ganglion cells have been studied from intracellular recordings in the cat eye. Intracellularly recorded light flicker responses were separated by computer into spike trains and corresponding generator potentials. Both the spike train and the generator potential responses to temporally modulated light spots were analysed in terms of amplitude and phase plots. The differences in dynamic properties between the two response measures reveal that the SG-mechanism affects the temporal frequency transfer properties of retinal ganglion cells to a considerable extent. With respect to the transfer of the amplitude of the first harmonic the SG-mechanism has differentiating (or high-pass) properties. This means that the responses to high temporal stimulus frequencies are amplified relatively much more than are the responses to lower frequencies. Furthermore, the SG-mechanism causes a phase lead of the spike train response relative to the generator potential by, on average, 37 degrees. The measured frequency responses, among other things, have been used to verify and to quantify the SG-model that we proposed in a previous paper (Lankheet, Molenaar & van de Grind, 1989). With this model it proved possible to reproduce the spike train responses as model output from the corresponding measured generator potentials as model input. A good qualitative and quantitative correspondence between model output and the measured spike trains was obtained for a wide range of stimulus frequencies and with fixed values of the model parameters. With parameter values that optimized this correspondence the model allowed us to investigate the dynamic behaviour of the SG-mechanism in more detail. It also provides a reliable and validated method to predict the shape of the generator potential from the spike train (the "inversion problem").

Action Potentials↗

PubMed1987

Influence of stimulus intensity on the categories of single-unit responses recorded from olfactory bulb neurons in awake freely-breathing rabbits.

This study analyzes the influence of increasing odor intensity on the single-unit activity of olfactory bulb neurons recorded in awake freely-breathing rabbits. Five odorants were delivered at four concentrations over a range of 2 log units. Inspiration-and expiration-related firing activities were analyzed separately to categorize the odor-evoked responses into excitation, suppression and no response. Increasing the odor concentration caused more neurons to show excitatory responses during the inspiratory phase of the respiratory cycle and/or suppressive responses during the expiratory phase. The highest concentration of each odor also caused several units to give suppressive responses during inspiratory phases or excitatory responses during expiratory phases. However, increasing odor concentration caused few responses (less than 2%) to change from excitation to suppression or vice versa. How these results relate to those from studies performed in anaesthetized animals and the implications of the results for olfactory coding will be discussed.

Animals↗

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↗

PubMedDate not supplied

Temporal-chromatic interactions in LGN P-cells.

We studied the interaction between the chromatic and temporal properties of parvocellular (P) neurons in the lateral geniculate nucleus (LGN) of macaque monkeys. We measured the amplitudes and phases of responses to stimulation by spatially uniform fields modulated sinusoidally about a white point in a three-dimensional color space, at a range of temporal frequencies between 1 and 25 Hz. Below about 4 Hz, temporal frequency had relatively little effect on chromatic tuning. At higher frequencies chromatic opponency was weakened in almost all cells. The complex interactions between temporal and chromatic properties are represented by a linear filter model that describes response amplitude and phase as a function of temporal frequency and direction in color space along which stimuli are modulated. The model stipulates the cone inputs to center and surround, their temporal properties, and the linear combination of center and surround signals. It predicts the amplitudes and phases of responses of P-cells, and the change of chromatic properties with temporal frequency. We used the model to investigate whether or not the chromatic signature of the surround in a red-green cell could be estimated from the change in the cell's chromatic properties with temporal frequency. Our findings could be equally well described by mixed cone surrounds as by pure cone surrounds, and we conclude that, with regard to temporal properties, there is no benefit to be gained by segregating cone classes in center and surround.

Animals↗

PubMedDate not supplied

Distinctive characteristics of subclasses of red-green P-cells in LGN of macaque.

We characterized the chromatic and temporal properties of a sample of 177 red-green parvocellular neurons in the LGN of Macaca nemestrina, using large-field stimuli modulated along different directions through a white point in color space. We examined differences among the properties of the four subclasses of red-green P-cells (on- and off-center, red and green center). The responses of off-center cells lag the stimulus more than do those of on-center cells. At low temporal frequencies, this causes the phase difference between responses of the two kinds of cells to be considerably less than 180 deg. For isoluminant modulations the phases of on- and off-responses were more nearly 180 deg apart. A cell's temporal characteristics did not depend on the class of cone driving its center. Red center and green center cells have characteristically different chromatic properties, expressed either as preferred elevations in color space, or as weights with which cells combine inputs from L- and M-cones. Red center cells are relatively more responsive to achromatic modulation, and attach relatively more weight to input from the cones driving the center. Off-center cells also attach relatively more weight than do on-center cells to input from the class of cone driving the center.

Action Potentials↗