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Biomedical subjects

F Ratliff

Publications and source records attributed to F Ratliff.

At least 19 recordsLinked to original sources
PubMed1984

Intermodulation components of the visual evoked potential: responses to lateral and superimposed stimuli.

Nonlinear interactions in the human visual system were studied using visual evoked potentials (VEPs). In one experiment (superimposed condition), all segments of a dartboard pattern were contrast reversed in time by a sum of two sinusoidal signals. In a second experiment (lateral condition), segments in some regions of the dartboard pattern were contrast reversed by a single sinusoid of one frequency, while segments in other (contiguous) regions of the pattern were contrast reversed by a single sinusoid of another frequency. An identical set of ten frequency pairs was used in each experiment. The frequency pairs were chosen such that the difference between frequencies in each pair was 2Hz. Amplitudes and phases of the sum and difference frequency components of the VEP (intermodulation terms) were retrieved by Fourier analysis and served as measures of nonlinear interactions. The use of input pairs with a fixed separation in frequency enabled the estimation of the temporal characteristics of the visual pathways prior to a second linear stage. The use of superimposed and lateral conditions revealed antagonistic contributions to the VEP, possibly reflecting direct-through excitatory and lateral inhibitory pathways, respectively.

Evoked Potentials, Visual↗
PubMed1984

Why Mach bands are not seen at the edges of a step.

UNLABELLED: Mach bands are seen at the two ends of a ramp in luminance from one uniform level to another. Narrow, sharp-edged stimuli centered on the ramp attenuate both Mach bands simultaneously. CONCLUSION: Mach bands are not seen at the edges of an abrupt step change in luminance because the sharp edges actively suppress them.

Form Perception↗
PubMed1983

Attenuation of Mach bands by adjacent stimuli.

Pronounced bright and dark bands are seen at the bright and dark edges of half-shadows and similar distributions of illumination. These are the so-called Mach bands. A pair of vertical Mach bands was generated with a ramp pattern in the central strip of a horizontal tripartite oscilloscope display. This pattern consisted of two uniform fields (one of low luminance, one of high luminance) joined by a gradient of uniform slope. The upper and lower strips were uniform throughout. A coupled pair of pointers could be displayed in these two strips and adjusted by the observer to match the apparent location and width of either of the Mach bands in the central strip. Insertion of a vertical bar in the central strip nearby and on either side of the ramp attenuates the corresponding Mach band. The closer the bar is to the Mach band, the stronger the attenuation. The attenuation is nearly independent of the sign of the contrast of the bar, but it does depend upon the magnitude and sharpness of the contrast. Also, the attenuation is independent of the width of the bar; a narrow line is as effective as a broad bar of the same contrast. No net luminance change is required; a bipolar stimulus with equal parts above and below the mean is as effective as a monopolar stimulus. These results point to two competing physiological mechanisms with different spatial sensitivities--one that generates Mach bands and one that attenuates them.

Darkness↗
PubMed1982

Visual evoked potentials: evidence for lateral interactions.

Electrical potentials evoked in the human brain by visual stimulation can easily be recorded by using electrodes attached to the scalp. It is difficult, however, to relate these visual evoked potentials (VEPs) to specific neural processes: scalp electrodes, far removed from the brain, sum potentials from large areas of cortex. We improved identification and localization of lateral interactions by differentially modulating small neighboring parts of a "windmill-dartboard" stimulus pattern-a central disc surrounded by three contiguous annuli, all radially divided into light and dark segments. With temporal contrast reversal of all segments in the pattern, the major component of the VEP is at the second harmonic of the frequency of modulation--as expected. Temporal contrast reversal of the segments in the central disc and second annulus, with contrast of segments held constant in the first and third annuli, unexpectedly amplifies the VEP at the fundamental frequency of modulation and attenuates it at the second harmonic. Slight spatial separation of static and dynamic zones reduces both the amplification of the fundamental and the attenuation of the second harmonic. Thus, both phenomena appear to result from strong lateral interactions over relatively short distances. Nevertheless, different neural mechanisms must be involved; fundamental and second-harmonic components of the VEP are different functions of spatial separation and relative contrast of the segments in contiguous static and dynamic zones.

Animals↗
PubMed1980

Bicuculline enhances a negative component and diminishes a positive component of the visual evoked cortical potential in the cat.

Visual evoked potentials (VEPs), elicited by modulation of luminance of homogeneous fields of light, were recorded from the scalp and from the surface of the visual cortex of cats before and after topical application of bicuculline to the cortex. The application of this drug drastically altered the VEP: the amplitude of a normally small negative component was increased greatly, and a normally prominent late positive component was diminished. Bicuculline is known to block the action of gamma-aminobutyric acid, which is thought to be the primary inhibitory neurotransmitter in the visual cortex. We suggest, therefore, that the affected negative wave reflects an excitatory process in the visual cortex and that the affected late positive wave reflects an intracortical inhibitory process.

Animals↗
PubMed1978

The response of the Limulus retina to moving stimuli: a prediction by Fourier synthesis.

The Limulus retina responds as a linear system to light stimuli which vary moderately about a mean level. The dynamics of such a system may conveniently be summarized by means of a spatiotemporal transfer function, which describes the response of the system to moving sinusoidal gratings. The response of the system to an arbitrary stimulus may then be calculated by adding together the system's responses to suitably weighted sinusoidal stimuli. We have measured such a spatiotemporal transfer function for the Limulus eye. We have then accurately predicted, in a parameter-free calculation, the eye's response to various stimulus patterns which move across it at several different velocities.

Action Potentials↗
PubMed1978

The spatiotemporal transfer function of the Limulus lateral eye.

The dynamics of the Limulus retina may be well described by the spatiotemporal transfer function, which measures the response of the eye to moving sinusoidal gratings. We consider a model for this system, which incorporates an excitatory generator potential, and self- and lateral inhibitory processes. Procedures are described which allow estimation of parameters for the model consistent with the empirical transfer function data. Transfer functions calculated from the model show good agreement with laboratory measurements, and may be used to predict accurately the response of the eye to arbitrary moving stimuli. The model allows convenient interpretation of the transfer function measurements in terms of physiological processes which underly the response of the Limulus retina.

Animals↗
PubMed1976

Georg von Békésy.

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History, Modern 1601-↗
PubMed1973

Georg von Békésy.

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History, 20th Century↗
PubMed1973

Facilitation of inhibition in the compound lateral eye of limulus.

In the compound eye of Limulus the inhibitory effect of a burst of impulses from one group of ommatidia on the response of a neighboring ommatidium is greater when that burst is preceded by another burst of impulses. This facilitation of inhibition decays slowly, with a time constant of several seconds. Facilitation of inhibition accumulates as the number of impulses in the first burst increases, but there is a maximum that it cannot exceed. The facilitation is localized; one group of ommatidia does not facilitate the inhibition exerted by another group. The mechanism of this facilitation may be similar to that which has been postulated for facilitation of excitatory influences at the neuromuscular junction.

Journal Article↗
PubMed1970

Superposition of excitatory and inhibitory influences in the retina of Limulus: effect of delayed inhibition.

In an optic nerve fiber of the compound eye of the horseshoe crab, Limulus, the time course of a train of nerve impulses discharged in response to illumination reflects the interplay of excitatory and inhibitory influences. Responses to sinusoidally modulated excitation and inhibition, as a function of frequency, were measured separately and in combination. A simple linear superposition of the separate frequency responses properly accounts for the composite frequency response for both synchronous and asynchronous modulation of the excitatory and inhibitory influences. In general, the effect on the frequency response of increasing the delay of the inhibitory influence is progressively to shift the maximum amplitude to lower frequencies and gradually to produce pronounced maxima and minima in both the amplitude and phase.

Animals↗