Leo E. Lipetz

A new method for determining peak absorbance of dense pigment samples and its application to the cone oil droplets ofEmydoidea blandingii

Abstract A new method measures the spectral absorption of small volume samples having peak optical densities ( D p ) too high (up to 50 and higher) to be measured by standard direct or indirect methods. This method specifically corrects for the effect on the microspectrophotometer (MSP) of wavelength-independent light which bypasses the sample. The method involves: (1) measuring by MSP the long wavelength cutoff of the dense samples and the absorption spectrum of thinned samples of the same pigment, (2) calculating D p from these. The methods application is illustrated for the colored oil droplets in the retinal cones of the turtle, Emydoidea blandingii .

Application of an invariant spectral form to the visual pigments of crustaceans: Implications regarding the binding of the chromophore

Visual pigment absorption spectra were measured in single photoreceptors of a stomatopod, a crayfish, a hermit crab, and five species of brachyuran crab. All fitted a Mansfield (1985) invariant form for visual pigment, the form also fitted by vertebrate retinal-based visual pigments. This is consistent with a theoretical model based on the structure of visual pigment molecules (Greenberg et al., 1975; Honig et al., 1976) which predicts that spectral bandwidth decreases as lambda max increases. The conformation to the invariant form implies that for any given chromophore bandwidth times lambda max is a constant.

A Mechanism of Light Adaptation

In the isolated retina of the bullfrog (Rana catesbiana) illumination of one part of a ganglion cells receptive field increased the light threshold (for response by that cell) not only in the illuminated part but also in the unilluminated parts of the field. Scattered light is insufficient to account for the effect. Apparently it depends on changes in the efficiency of excitation transmission along the neural pathways from photoreceptors to ganglion cell.

The transfer functions of sensory intensity in the nervous system.

Abstract Instructions are given for testing whether the input-output relation (transfer function) of a portion of any sensory system matches the tanh log relation. This relation holds for a cell membrane whose response to stimulation is an increase in permeability which shunts the source of the membranes voltage i.e. a “self-shunting” transducer mechanism. The tanh log relation is shown to fit data for certain mechanoreceptors, photoreceptors, postsynaptic neurons, and to fit data for the responses to light by horizontal cells and C-type S-potential sites of fish, by certain ganglion cells of the frog, and for the mammalian b-waves and x-waves. The psychophysical relation between stimulus strength and magnitude estimates of sensation is fitted by a power law relation. This relation is predicted by MacKays model which relates physiological excitation to sensation. The model is applied to psychophysical data to infer: (a) changes of lateral inhibition from photoreceptors for changes in area illuminated, (b) changes of transmission effectiveness of the pathway from the sensation generator, “the organizer”, with adaptation state or wavelength of illumination, (c) possible forms of the transfer function between the tanh log transfer function of receptors and a “comparator” stage of the model, (d) use of the Fechnerian jnd curve together with electrophysiological data to locate in the nervous system the comparator stage for a given sensory modality, (e) application of this to crudely locate the comparator and organizer stages of the mammalian visual system, (f) generalization of the model to sensory systems having linear or power law relations between stimulus strength and neural response.

Effect of Tactile Stimulation Pulse Characteristics on Sensation Threshold and Power Consumption

The psychophysical responses of human subjects to vibratory tactile stimulation of the skin were investigated experimentally. The parameters, of the waveform important to the minimization of power consumed by the tactile array of electromechanical vibrators and the maximization of the skin sensitivity to the stimulus were explored to develop optimum stimulation. Parameters investigated included the amplitude, frequency, and duty cycle of the current waveform used to drive the vibrators as well as the number of pulses per stimulating burst and the recovery time between bursts. Graphical techniques were used to determine, the optimal combination of the parameters which gave a stimulus that excited the skin to above tactile threshold while maintaining at a relative minimum the power required for the stimulus. The optimal stimulation waveform contains a burst of 10 rectangular pulses of 4% duty cycle separated by a period of nonstimulation of 2 s. Such a waveform can elicit a sensitivity of 29.4 mA−1 consuming only 55 μW of power.

Receptive field properties of the photopic luminosity horizontal cell of carp retina

The receptive field of the LEHC of the carps retina is different when tested with red versus green stimuli. The sensitivities to 706 nm vs 519 nm flashes were compared for various size spots centered on the receptive field. Full summation (area times intensity equaling a constant at threshold) and greater than full summation were found to occur up to larger diameters of spots with red illumination than with green. A further test was made of the effect of constant background green (502 nm) illumination on the sensitivity to red vs green flashes. At all background intensities the sensitivity to red vs green flashes. At all background intensities the sensitivity to green flashes was reduced, but over an optimal range of background intensities the sensitivity to red flashes was increased. These findings are explained in terms of a previously proposed model of cone-LEHC connections.

Glial Control of Neuronal Activity

The concept that the activity of neurons is both passively and actively modified by the surrounding glial and other non-neuronal cells is found in this review to be supported, but not conclusively demonstrated, by recent and previously unreported experiments. The evidence makes such non-neuronal control seem highly likely in the vertebrate retina. In particular, the non-neuronal horizontal cells of the retina were found to sunumate, and apparently to transmit, changes in their membrane potentials (the L-response type of S-potential) over distances many times the span of a single such cell. Available evidence is consistent with the idea that these potential changes can affect the neuronal transmission of excitation from the photoreceptors to the ganglion cells. This is proposed as the basis for a mechanism accounting for the low-luminance portion of light adaptation and for certain retinal functions of spatial summation and movement detection.

A Model of Function at the Outer Plexiform Layer of the Cyprinid Retina

This talk concerns behavior. The behavior of animals is thought to depend on activity of their nervous systems. Certainly, if the nervous system is altered by damage, the behavior is altered. Nevertheless, we would feel more confident of that dependence of behavior on the nervous system and would feel we had a better understanding of the nervous system if the actions of the nervous system could be described cell by cell and shown to lead to the observed behavior. Many scientists are working toward this end.

Investigation of significant interface parameters involved in the electromechanical transfer of tactile information

The authors describe work toward the implementation of a tactile vision information system (TVIS). The prototype system consists of a video camera for image acquisition, a microcomputer for image processing, and a 16*16 vibro-tactile array for image transfer to the skin. The characteristics of the TVIS are discussed and proposed modifications are analyzed to determine the optimum configuration. The mechanical impedance of both vibrator and selected skin sites are discussed and related to the image-transfer process. This is combined with the published properties of the tactile sensors to determine the overall transfer function of electromechanical-physiological interface.<<ETX>>

Microcontrolled system to transform visual images into vibrational images

Details are presented of a system configuration to convert a prestored 256-element image into a vibrational image. The purpose of the system is to interface with the human tactile sensory system to transfer visual information. A vibratory array is driven by pulses with amplitudes proportional to the amount of light in the corresponding spatial position of the image, using fours gray levels. The array is scanned ten times in 4 ms, giving the sensation of continuous stimulation. A recovery time of 1 s is allowed every ten pulses (one burst) to minimize adaptation. Parameters such as number of pulses per burst, duty cycle, pulse shape and modality of scanning are programmable. The flexibility of the system makes it possible to study the best combination of parameters to transfer maximum information using relatively low power.<<ETX>>