PubMed · 1138477
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R F Quick, T A Reichert. 1975. Spatial-frequency selectivity in contrast detection.. https://doi.org/10.1016/0042-6989(75)90277-1
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1. Electroretinographic (ERG) methods were used to investigate the effects of background illumination on the responses of mouse rod photoreceptors in vivo. A paired-flash procedure, involving the recording and analysis of the ERG a-wave response to a bright probe flash presented after a brief test flash, was used to derive the rod response to the test flash in steady background light. A related, step-plus-probe procedure was used to derive the step response of the rods to backgrounds of defined strength. 2. Steady background light produced a maintained derived response that was graded with background strength. Determinations of the full time course of the derived weak-flash response in steady background light, and of the effect of background strength on the flash response at fixed post-test-flash times, showed that moderate backgrounds reduce the peak amplitude and duration of the flash response. 3. The response to stepped onset of an approximately half-saturating background (1.2 sc cd m(-2)) exhibited a gradual rise over the first 200-300 ms, and an apparent subsequent relaxation to plateau amplitude within 1 s after background onset. Determinations of normalized amplitudes of the derived response to a test flash presented at 50 or 700 ms after background onset indicated substantial development of background-induced shortening of the test flash response within this 1 s period. These findings indicate a time scale of approximately 1 s or less for the near-completion of light adaptation at this background strength. 4. Properties of the derived response to a stepped background and to test flashes presented in steady background light are in general agreement with photocurrent data obtained from mammalian rods in vitro and suggest that the present results describe, to good approximation, the in vivo desensitization of mouse rods by background light.
1. The a-wave of the electroretinogram was recorded from human subjects with normal vision, using a corneal electrode and ganzfeld stimulation. We applied the paired-flash technique, in which an intense 'probe' flash was delivered at different times after a 'test' flash. The amplitude of the probe-flash response provided a measure of the circulating current remaining at the appropriate time after the test flash. 2. We extended previous methods by measuring not at a fixed time, but at a range of times after the probe flash, and then calculating the ratio of the 'test-plus-probe' response to the 'probe-alone' response, as a function of time. 3. Under dark-adapted conditions the rod response derived by the paired-flash technique (in response to a relatively dim test flash) peaked at ca 120 ms, with a fractional sensitivity at the peak of ca 0.1 Td(-1) s(-1). 4. As reported previously, background illumination reduced the maximal response, reflecting a reduction in rod circulating current. In addition, it shortened the time to peak (to ca 70 ms at an intensity of 170 Td), and reduced the flash sensitivity measured at the peak. The flash sensitivity declined approximately according to Weber's Law, with a 10-fold reduction occurring at an intensity of 100-200 Td. We could not reliably measure responses at significantly higher background intensities because the circulating current became so small. 5. In order to investigate the phototransduction process after correction for response compression, we expressed the derived response as a fraction of the maximal response that could be elicited in the presence of the background. The earliest rising phase of this 'fractional response per unit intensity' was little affected by background illumination, suggesting that the amplification constant of transduction was unaltered by light adaptation.
When an observer moves forward in the environment, the image on his or her retina expands. The rate of this expansion conveys information about the observer's speed and the time to collision. Psychophysical and physiological studies have provided abundant evidence that these expansionary motions are processed by specialized mechanisms in mammalian visual systems. It is commonly assumed that the rate of expansion is estimated from the divergence of the optic-flow field (the two-dimensional field of local translational velocities). But this rate might also be estimated from changes in the size (or scale) of image features. To determine whether human vision uses such scale-change information, we have synthesized stochastic texture stimuli in which the scale of image elements increases gradually over time, while the optic-flow pattern is random. Here we show, using these stimuli, that observers can estimate expansion rates from scale-change information alone, and that pure scale changes can produce motion after-effects. These two findings suggest that the visual system contains mechanisms that are explicitly sensitive to changes in scale.