A method for determining the spectral sensitivity of the different color mechanisms of the human eye uses the pattern of color names applied to small, brief, dim, monochromatic flashes. Such responses are often due to the activation of single neural units. Preliminary spectral sensitivity curves for two color mechanisms have been obtained.
Stochastic resonance refers to the enhancement of a signal by the addition of a small amount of noise and its degradation by a larger amount of noise. It occurs in a variety of physical systems including neuronal systems. However, its demonstration in neuronal systems has so far been limited to single-dimensional systems such as a single mechanoreceptor. We report here the existence of stochastic resonance in the visually evoked potential, a very high-dimensional neuronal system.
SUMMARY1. The temperature dependence of the relative spectral sensitivity of the excised lateral eye of Limnulus was examined using its electrical response to light.2. At wave-lengths longer than 625 m,u lowering the temperature from 27 to 70 C reduced the relative spectral sensitivity, while no effect was measurable at shorter wave-lengths.3. The reduction in relative sensitivity increased linearly with decreasing wave number.4. The observations support the hypothesis that a critical amount of energy (activation energy) must be supplied to the photopigment molecule in order that it excite the photoreceptor. Quanta with energy lower than the activation energy are effective only if the thermal energy of the molecule can supply the deficit.5. The lower limit of the activation energy for the photoreceptor of the lateral eye of Limulus determined from the results of this studv is 44 kcal mole-1.
SUMMARY1. Discrete, transient depolarization (discrete waves) of the ventral photoreceptor of the horseshoe crab, Limulu8, occur spontaneously in the dark adapted photoreceptor and are also evoked by light. Thiey form the basic events which comprise the receptor potential. A brief, low energy flash of light evokes variable numbers of discrete waves. which have variable latencies. Evidence suggesting that discrete wave latency reflects the kinetics of the chemical reactions of phototransduction is reviewed.2. The concentration of extracellular Ca influences both the average discrete wave latency and its variability. Lowering extracellular Ca prolongs the latency and increases its variability. Increasing extracellular Ca has the opposite effect.3. Changes in discrete wave latency caused by changes in extracellular Ca require 10-15 min to become fully manifest, whereas when the concentration of extracellular K is increased the photoreceptor achieves a steadystate depolarization in 10-15 sec.4. Iontophoresis of the Ca-chelating agent EGTA into the photoreceptor increases both the average discrete wave latency and its variability. Iontophoresis ofCa-EGTA mixtures may either increase or decrease discrete wave latency and its variability depending upon the proportion of Ca mixed with EGTA.5. It is suggested that the concentration of intracellular rather than extracellular ionized Ca is the prime factor influencing discrete wave latency. The effects of changing extracellular Ca can be explained if the photoreceptor is permeable to Ca in the dark and if it maintains a low intracellular Ca concentration by virtue of active metabolic processes (a pump-leak system).6. Lowering the temperature of the photoreceptor also has the dual effect of increasing discrete wave latency and its variability. However, IX PHIY 26i
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