Biomedical subjects

S L Guth

Publications and source records attributed to S L Guth.

At least 19 recordsLinked to original sources

PubMed1991

Model for color vision and light adaptation.

A multizone color model is described. It has nonlinear receptor gain control, two postreceptor opponent-colors processing stages, and neural compression late in the visual pathway. It is assumed that gain control can be activated by receptor responses from a test light itself (self-adaptation) and (or) by receptor responses from other adapting fields. Apparent brightnesses and visual discriminations are mediated by the first processing stage, and apparent hues and saturations are mediated by the second stage. The model accounts for a wide range of data, including nonlinear hue shifts in the color solid, various apparent brightness effects, visual discriminations for achromatic and chromatic lights under various adaptation conditions, and effects of chromatic adaptation on color appearances.

Adaptation, Ocular↗

PubMed1990

Effects of divalent cations on the frequency of spontaneous action potentials from the lateral line organ of Xenopus laevis.

The effect of superfusion of the internal surface of the skin of Xenopus laevis with saline containing Co2+, Ca2+, Mg2+, or Ba2+, on the frequency of spontaneous action potentials of the lateral line nerve, was studied to investigate the role of extracellular Ca2+ in spontaneous neural activity. Addition of divalent cations to frog saline, either singly or as a mixture of two different ions, produced concentration-dependent suppression of spontaneous rate. The rank order of potency for suppression by each ion, perfused alone, was Co2+ greater than Ca2+ greater than Mg2+ greater than Ba2+. Suppression by combinations of Mg2+ and Co2+, or of Ca2+ and Co2+, was approximated by the sum of the suppressive effects of each cation. Ca2+ was more suppressive than Mg2+ when each of these ions was paired with the same amount of Co2+, while Ca2+ was approximately as suppressive as Co2+ when similarly paired with Mg2+. One interpretation of the suppression by Ca2+ invokes the hypothesis that divalent cations suppress spontaneous activity by charge screening of voltage-sensitive Na+ channels on afferent dendrites and that release of neurotransmitter by the influx of extracellular Ca2+ through voltage-sensitive Ca2+ channels of hair cells may not be the sole mechanism for generation of spontaneous activity in the lateral line. These results quantify the relative suppressive potency of common divalent cations in the lateral line, and serve as a caveat to investigators who interpret a blockade of action potentials by high concentrations of Co2+ or Mg2+ as sufficient evidence for dependence of neurotransmission upon extracellular Ca2+, particularly in acousticolateralis systems.

Action Potentials↗

PubMed1990

Argiotoxin-636 blocks effects of N-methyl-D-aspartate on lateral line of Xenopus laevis at concentrations which do not alter spontaneous or evoked neural activity.

Activity of primary afferent neurons in acousticolateralis organs can be modulated by excitatory amino acids (EAA) and their antagonists, and EAA are among the better candidates for the transmitter(s) between hair cells and afferent neurons. Argiotoxin-636 (ATX) is a spider venom toxin that is a selective antagonist of the N-methyl-D-aspartate (NMDA) subtype of excitatory amino acid receptors in vertebrates. In the present study, the effects of NMDA alone and in combination with ATX on resting firing rate (spontaneous activity) and mechanically stimulated activity of lateral line afferent neurons of post-metamorphic Xenopus laevis were compared. Perfusion of NMDA (100 or 200 microM) typically produced a biphasic effect on spontaneous activity consisting of a transient increase followed by a decrease in firing rate. Mechanical stimulation applied after the initial response to NMDA produced an increase in firing activity that was not significantly different from the increase in firing activity evoked in the absence of NMDA. ATX at 1-2 microM did not alter either spontaneous or evoked activity, but blocked in a reversible manner both the excitatory and inhibitory actions of NMDA on spontaneous activity. These results suggest the presence of an NMDA receptor in the lateral line but do not support the hypothesis that the generation of action potentials by the afferent transmitter is solely dependent on activation of postsynaptic NMDA receptor-ion channels.

Afferent Pathways↗

PubMed1989

Dichoptic opponent hue cancellations.

Relative to monoptic opponent hue cancellations, dichoptic cancellations, which were possible over most of the visible spectrum, generally required more blue or less yellow to cancel the opponent yellow or blue response and less green to cancel the opponent red response. Differences between dichoptic and monoptic red-to-cancel-green functions were wavelength dependent. These data were approximately predicted by a model in which monoptic cancellations are based at the receptor level, whereas dichoptic cancellations are based upon cancellations of postreceptor opponent hue responses at a central level in the visual system. The model also makes reasonable predictions of other dichoptic hue-matching data.

Color Perception↗

PubMed1985

Computer analysis of vestibular responses to putative neurotransmitters.

An inexpensive microcomputer (Commodore 64K) based system was developed for the analysis of neural spike trains. The trains were recorded from single ampullary units in response to mechanical stimulation of the isolated semicircular canal of the bullfrog (Rana catesbeiana). A BASIC program provided a number of options while machine language subroutines generated interstimulus interval (ISI) and peristimulus time (PST) histograms. Up to thirty 5-s spike trains could be combined for analysis (0.1 ms resolution ISI, 100 ms bin width PST). Histograms and summary statistics were saved on floppy disks. The cost of adding this computer system to an existing neurophysiology laboratory is less than US $600 (printed, tape, and disk versions of these programs are available). The system was used to measure vestibular responses to putative vestibular neurotransmitters such as carbachol (an acetylcholine mimic) (Rossi et al., 1980) and gamma-aminobutyric acid (GABA) (Flock and Lam, 1974).

Animals↗

PubMed1984

Pharmacology of the isolated semicircular canal: effect of GABA and picrotoxin.

We have investigated the proposal that gamma aminobutyric acid (GABA) is the afferent transmitter in the vestibular system using the isolated posterior semicircular canal of the bullfrog (Rana catesbeiana). This was done by examining the effects of GABA and picrotoxin on the evoked response of neural units in the posterior ampullar nerve. GABA had no effect on evoked response up to 1 mM, and inhibited the evoked response at 10 mM. Picrotoxin, a GABA antagonist, had no effect on evoked response between 5 microM and 100 microM. These results indicate that GABA is not the afferent transmitter in the vestibular system.

Animals↗

PubMed1982

Post-receptor chromatic mechanisms revealed by flickering vs fused adaptation.

For both normals and dichromats, adaptation to certain pairs of alternating monochromatic caused reduced sensitivity for detection of some test wavelengths when compared to sensitivity losses caused by adaptation to the fused (supra-CFF) sums of the same lights. Since the two adaptation conditions caused equivalent photopigment depletion, and since sensitivities to some wavelengths did not differ between conditions, the effects are ascribed to post-receptor adaptation. Such post-receptor effects were not obtained when adapting wavelengths did not straddle the presumed "crosspoints" of opponent-colors mechanisms, and, when only one opponent mechanism was adapted, effects were absent for test wavelengths at the crosspoint of that mechanism. For the red vs green system, increasing the intensities of adapting wavelengths from 2.2 to 5.5 log td did not appreciably increase the magnitudes of postreceptor effects. Quantitative accounts for the results are derived from a vector model for color vision. The results provide general support for opponent-colors interpretations of normal and dichromatic vision and suggest that the flicker/fused adaptation paradigm is a useful tool for probing postreceptor mechanisms of vision.

Adaptation, Ocular↗

PubMed1976