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.

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.

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.

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.

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.

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.

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.

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.

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.

On tuning and amplification by lateral inhibition.

Lateral inhibition in a neural network generally attenuates the amplitudes of the responses to sinusoidal stimuli-both spatial and temporal. For an inhibitory influence with an abrupt onset and an exponential decay in time, and with a Gaussian distribution in space (the forms often assumed in theoretical calculations), the attenuation is greatest at low temporal and spatial frequencies. The attenuation diminishes with increasing frequencies until ultimately the amplitudes of inhibited responses become equal to, but never exceed, the amplitudes of the uninhibited.

Haldan keffer hartline.

By the methods of comparative physiology, or of experimental biology, by the choice of a suitable organ, tissue or process, in some animal far removed in evolution, we may often throw light upon some function or process in the higher animals, or in man.

Ragnar granit.

If it shall be demanded, then, when a man begins to have any ideas? I think the true answer is, when he first has any sensation. For since there appear not to be any ideas in the mind before the senses have conveyed any in, I conceive that ideas in the understanding are coeval with sensation. .

Nobel prize: 3 named for medicine, physiology award (George Wald, Ragnar Granit and Haldan Keffer Hartline).

Three scientists, George Wald, Ragnar Granit, and Haldan Keffer Hartline, were named last week to share the 1967 Nobel prize in medicine or physiology. Wald is professor of biology at Harvard University. Granit is retired director of the Neurophysiological Institute of the Royal Medical School in Stockholm; at present he is serving as a visiting professor at St.