Acoustic space

Abstract: Cortical representational plasticity has been well documented after peripheral and central injuries or improvements in perceptual and motor abilities This has led to inferences that the changes in cortical representations parallel and account for the improvement in performance during the period of skill acquisition There have also been several examples of rapidly induced changes in cortical neuronal response properties, for example, by intracortical microstimulation or by classical conditioning paradigms This report describes similar rapidly induced changes in a cortically mediated perception in human subjects, the ventriloquism aftereffect, which presumably reflects a corresponding change in the cortical representation of acoustic space The ventriloquism aftereffect describes an enduring shift in the perception of the spatial location of acoustic stimuli after a period of exposure of spatially disparate and simultaneously presented acoustic and visual stimuli Exposure of a mismatch of 8° for 20–30 min is sufficient to shift the perception of acoustic space by approximately the same amount across subjects and acoustic frequencies Given that the cerebral cortex is necessary for the perception of acoustic space, it is likely that the ventriloquism aftereffect reflects a change in the cortical representation of acoustic space Comparisons between the responses of single cortical neurons in the behaving macaque monkey and the stimulus parameters that give rise to the ventriloquism aftereffect suggest that the changes in the cortical representation of acoustic space may begin as early as the primary auditory cortex

Abstract: This book is the proceedings of the NATO Advanced Study Institute held at Lueneburg, Germany, July 30-August 10, 1984. As such it is part of the NATO Advanced Study Institute Series, published by an international board of publishers in conjunction with the NATO Scientific Affairs Division. Specifically, the book is Volume 151 of Series C: Mathematical and Physical Sciences. There are 63 papers published in this volume. The authors represent 13 different countries. There are 17 papers from the United States, plus two shared with Australia; nine from France; seven each from England and Germany; four each from Canada and Italy (including La Spezia); three each from Norway and Australia (not counting the two shared with the United States); two each from Portugal and South Africa; and one each from Denmark, Japan, and Turkey. According to the editor, a “major effort was made to obtain a commensurate contribution of tutorial and advanced research papers.” It is his hope “that the material in this volume may be equally well suited for students getting an introduction to some of the basic problems in underwater signal processing and for the professionals who may obtain an upto-date overview of the present state of the art.” In this reviewer’s judgment, he has succeeded. The subject matter of the book is divided into three categories (throughout the book, advanced research papers appear side-by-side with tutorial papers and are not specifically identified as one or the other). Part 1, devoted to the acoustical background of signal processing, contains 14 papers. The first paper in this category is entitled “Twenty Years of Signal Processing,” and is by J. W. R. Griffiths. This is a great historical paper which explains to the uninitiated reader where this book “is coming from.“ There are papers on ambient noise, propagation, and target characteristics. Part 2, theoretical and practical aspects of signal processing, is the longest of the book, with 36 papers. There are papers on time-delay estimation, spectrum analysis, and adaptive array processing. Part 3, techniques and applications, has 13 papers. There is an interesting mixture of new technologies here: digital signal processing, optical signal processing, expert systems (part of the field of artificial intelligence), ocean tomography, spaceborne remote sensing, and passive synthetic aperture. In addition, five workshops were held and their summaries published: environmental modeling and signal processing, test tanks and measuring facilities, nonlinear filtering, signal processing, and expert systems. AI1 of these topics are exciting’ and current, and many of the authors are leaders in their fields. This reviewer finds the inclusion of expert systems material, to choose an example, to be quite interesting. “Expert Systems for Ship Noise Interpretation,” by Maksym is a good tutorial paper. The next paper, “Comparison of the Statistical and the Expert System Approach for ‘the Interpretation of Ship Noise,” by Bendig and Wittig is an application of these ideas. (Of course, as is befitting a pioneer discipline like this one, there is room for disagreement with specific points in the treatment of the subject). After eading these two papers, it is interesting to read “New Advances Toward Ocean, Acoustics and Space Integration,” by Scully-Power and Stevenson, particularly the following quotation (referring to the question of manned versus unmanned space research): “While there is no question that computers can search out the “gems from the pebbles,” there is little evidence to assure us that the computer can distinguish the ‘diamond from the zircon.’ ” This sounds like the makings of a great debate, especially in the wake of the Challenger tragedy. It is difficult to single out papers for discussion-they are all so good. One that caught this reviewer’s eye was “Passive Synthetic Aperture Sonar-An Analysis of the Beamforming Process,” by Pusone and Lloyd. Curiously, one has always associated synthetic aperture ideas with active sonar! Another paper that provoked considerable discussion was “Detection and Classification Phenomena of Biological Systems,” by Altes. To quote one sentence from the conclusion, “Our appreciation of biological sensory systems is limited by our own knowledge, but we can perceive trends and signal representations in animals that could improve our own technology.” Interesting! In conclusion, the purchase of this book is recommended for those who could not attend the study institute-and that includes this reviewer. The book is certainly expensive, but, for most of us, it is less expensive than a trip to Germany!

Abstract: Several algorithms are presented that increase the robustness of SPHINX, the CMU (Carnegie Mellon University) continuous-speech speaker-independent recognition systems, by normalizing the acoustic space via minimization of the overall VQ distortion. The authors propose an affine transformation of the cepstrum in which a matrix multiplication perform frequency normalization and a vector addition attempts environment normalization. The algorithms for environment normalization are efficient and improve the recognition accuracy when the system is tested on a microphone other than the one on which it was trained. The frequency normalization algorithm applies a different warping on the frequency axis to different speakers and it achieves a 10% decrease in error rate. >