Models of retinal signal processing at high luminances

A new blind color watermarking based on a psychovisual model

In this paper, we address the problem of the use of a human visual system (HVS) model to improve watermark invisibility. We propose a new color watermarking algorithm based on the minimization of the perception of color differences. This algorithm is based on a psychovisual model of the dynamics of cone photoreceptors. We used this model to determine the discrimination power of the human for a particular color and thus the best strategy to modify color pixels. Results were obtained on a color version of the lattice quantization index modulation (LQIM) method and showed improvements on psychovisual invisibility and robustness against several image distortions.

Some renewal process models for single neuron discharge

Leslie (1969) obtained the Laplace transform for the recurrence time of clusters of Poisson processes, which can be thought of as yielding the interspike interval distribution for a neuron that receives Poisson excitatory inputs subject to decay. Here, several extensions of this model are derived, each including Poisson inhibitory inputs. Expressions for the mean and variance are derived for each model, and the results for the different models are compared.

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.

Simulation of movement detection by direction-selective ganglion cells in the rabbit and squirrel retina

A veto-gate model of movement detection by direction-selective ganglion cells in the vertebrate retina, first proposed by Barlow and Levick (1965), provides the basis for a model described in this study. The model is a simple network consisting basically of (1) two subunits that have receptive fields with a center-surround organization and an adaptational gain control, (2) a lateral inhibitory pathway, (3) a site of nonlinear interaction, followed by (4) a leaky temporal integrator. The model is tested by comparing its basic properties to those reported in the physiological literature on rabbit and squirrel direction-selective retinal ganglion cells. It is shown that the physiological findings on sensitivity to flashes, moving spots or slits, and phi-movement stimuli, can be mimicked quite well by our model. Similarities between the component processes of the subunits and known retinal processes are pointed out. The simulation studies shed a new light on some of the known properties and suggest several new, more revealing, physiological experiments. Such experiments are necessary to develop a full specification of this type of model and to fix more parameter values than is possible at present. Results of some critical experiments are predicted to enable physiologists to falsify or corroborate the model. The simulation studies also help to distinguish use from abuse of this type of model in explanations of psychophysical findings. For example, neither the most complete Barlow-Levick detector nor any stripped-down versions that retain a temporally extended lateral inhibition (which is essential to mimick the physiological findings), respond well to moving random-pixel arrays.