The responses of neurons in the primary visual cortex (V1) to an optimally-oriented grating are suppressed when a non-optimal grating is superimposed. Although cross-orientation suppression is thought to reflect mechanisms that maintain a distributed code for orientation, the impact of superimposed gratings upon V1 populations is unknown. Using intrinsic signal optical imaging, we find that patterns of tree shrew V1 activity evoked by superimposed equal-contrast gratings were predicted by the mean of patterns evoked by individual component gratings. This prediction held across contrasts, for summed sinusoidal gratings or non-summing square-wave gratings, and was reflected in single-unit extracellular recordings. Intracellular recordings revealed consistent levels of suppression throughout the evolution of subthreshold responses. These results indicate that divisive suppression powerfully governs population responses to multiple orientations. Moreover, the specific form of suppression we observe appears to support independent population codes for stimulus orientation and strength, and calls for a reassessment of mechanisms that underlie cross-orientation suppression.

Large-scale changes in luminance are known to exert a significant suppressive or masking effect on visual perception, but the neural substrate for this effect remains unclear. In this report, we describe the results of experiments using in vivo intracellular recording to explore the impact of luminance transients on the responses of orientation-selective neurons in layer 2/3 of tree shrew primary visual cortex. By measuring changes in excitatory and inhibitory conductances, we find that instantaneous changes in luminance evoke strong cortical inhibition. When combined with visual stimuli that would otherwise yield strong excitatory responses, luminance transients produce significant reductions in excitation as well as increases in inhibition. As a result, luminance transients significantly delay the emergence of orientation tuned cortical responses, and virtually eliminate ongoing responses to effective stimuli. We conclude that cortical inhibition is a critical factor in luminance-evoked cortical suppression and the likely substrate for luminance-induced visual masking phenomenon.

The horizontal network in visual cortex layer 2/3 is implicated in numerous psychophysical and physiological properties. To investigate the spatial and temporal distribution of excitation and inhibition evoked by this network, we used voltage-sensitive dyes to image the responses to focal electrical stimulation in tangential slices of ferret visual cortex layer 2/3. The resulting optical patterns included a diffuse zone of activation near the stimulation site and numerous ovoid domains throughout the slice. In contrast to the fixed anatomy of the horizontal connections, substantial shifts in both space and time were evident in the distribution of population-based neuronal activity during stimulus trains. Both of these shifts relied on inhibitory synaptic potentials, suggesting that inhibition driven by horizontal connections sculpts the distribution of activity in this cortical network.

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This paper details an investigation of the variation in the electronic transition moment with internuclear separation for the NO(B2Π–X2Π) transition. Measurements of the relative intensities of a number of NO B–X vibronic transitions having a common upper level were used to construct a relative transition-moment function between 1.27 and 1.60 Å. After normalizing this relative function by experimentally determined radiative lifetimes, the transition-moment function was extended down to 1.23 Å by incorporating data from oscillator strength measurements. In contrast to empirical transition-moment functions that have been proposed previously, the function in this paper decreases with increasing internuclear separation. Unlike these other functions, however, this one is consistent with theoretical predictions, with most available oscillator strength data, and with the observed trend in B-state radiative lifetimes as a function of vibrational level.

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The visual system is thought to represent the direction of moving objects in the relative activity of large populations of cortical neurons that are broadly tuned to the direction of stimulus motion; but how changes in the direction of a moving stimulus are represented in the population response remains poorly understood. Here we take advantage of the orderly mapping of direction selectivity in ferret primary visual cortex (V1) to explore how abrupt changes in the direction of a moving stimulus are encoded in population activity using voltage-sensitive dye (VSD) imaging. For stimuli moving in a constant direction, the peak of the V1 population response accurately represented the direction of stimulus motion; but following abrupt changes in motion direction, the peak transiently departed from the direction of stimulus motion in a fashion that varied with the direction offset angle and was well predicted from the response to the component directions. We conclude that cortical dynamics and population coding mechanisms combine to place constraints on the accuracy with which abrupt changes in direction of motion can be represented by cortical circuits.

Mechanical response and thermal coupling measurements are reported for aluminum and titanium targets exposed to high-intensity 1.06-μ laser radiation. Measurements are made in air and vacuum for pulse lengths from 1 to 100 μsec, providing incident fluences of between 106 and 108 W/cm2. Total momentum delivered to the target and time-resolved pressure developed over the target surface were measured at irradiances spanning the threshold for laser-supported detonation (LSD) wave ignition. The slope of the impulse/energy ratio shows a marked discontinuity at LSD threshold intensity. Peak target surface pressure is found to increase as the 2/3 power of the beam intensity in agreement with the hydrodynamic model of LSD wave propagation. Thermal coupling coefficients α for Al and Ti drop continuously from ∼0.3 to 0.07 over the intensity range examined. This behavior is consistent with the presence of an optically absorbing plasma at the target surface. The decrease in α is attributed to an increase in the plasma propagation velocity with intensity.