Frequency-independent Peak Research Articles

Binocular responses of neurons in the barn owl's visual Wulst

Binocular responses have been recorded extra-cellularly at 58 sites in the barn owl's (Tyto alba) visual Wulst. Neurons showed disparity tuning to stimulation with moving bars, moving sinewave gratings and a moving visual-noise stimulus. Responses to sinewave gratings as a function of disparity were cyclic, with the period of a cycle of the response being correlated to one cycle of the stimulus. Cyclic responses were also found when bars or noise were used as a stimulus, but, especially in response to visual noise, one response peak, the main peak, was different from the other peaks, the sidepeaks: usually, the main peak was either higher or narrower or both higher and narrower than the sidepeaks. When the responses to different spatial frequencies were compared, response maxima coincided at the main peak, but not at the other peaks. In analogy to auditory physiology the disparity at which the frequency-independent peak occurs is termed ‘characteristic disparity’. Spatial-frequency tuning revealed broad tuning, ranging from 1 to more than 3 octaves at 50% of the maximal response. Disparity tuning was broad at the onset of the response and sharpened later. The data are discussed within the framework of a model for the neural representation of visual disparity that was derived from a model proposed earlier for the representation of interaural time difference, the main cue for encoding sound-source azimuths in the barn owl.

Application of Q-band electron spin echo spectrometer to investigation of glasses doped with rare earth ions

The Q-band electron spin echo (ESE) spectrometer which was created using modern microwave components is described. This simple incoherent apparatus was used with the X-band one for the study of phosphate and silicate glasses doped with non-Kramers rare earth Tb3+ ions. the EPR spectra measured by the ESE method have frequency independent peaks. The experimental results presumably show the existence of several types of paramagnetic centers in studied systems.

Low-frequency grazing propagation over periodic steep-sloped rigid roughness elements

Extensive experimental studies of the coherently forward scattered sound at grazing incidence to low roughness rigid surfaces with periodic steep-sloped elements have confirmed several of the theoretical predictions of large boundary wave amplitude and subsonic dispersion [Tolstoy, J. Acoust. Soc. Am. 75, 1–22 (1984)]. For example, at short ranges the boundary wave amplitude diverges cylindrically and is proportional to εk3/2r−1/2, where ε is the scattering parameter, k is the wavenumber, and r is the range; thereby, at sufficient ranges and frequencies it exceeds the amplitude of the spherically diverging direct wave. The dispersion is subsonic and goes as Ak2ε2, where A depends on the roughness element. The experiments have also revealed an attenuation factor exp (−δr), where δ=αk6 due to incoherent scatter up to critical range kr=π/(Ak2ε2) beyond which an attenuation of form exp (− 1/2 A2ε4k6r2) becomes dominant. This leads to frequency-independent peak amplitudes two to six times the direct wave amplitude for the surfaces studied. Beyond this critical, frequency-dependent, range the boundary wave catastrophically self-destructs due to interference. The amplitude, dispersion, and attenuation of the boundary wave is a function of the packing, heights, and slopes or shapes of the scattering elements. Results are presented for spheres, spaced and packed circular cylinders, and several wedge corrugated roughness elements. The boundary wave is also shown to diffract over a ridge in the same manner as the direct wave from a point source.