Local and global forms of inhibition control directionally selective ganglion cells (DSGCs) in the mammalian retina. Specifically, local inhibition arising from GABAergic starburst amacrine cells (SACs) strongly contributes to direction selectivity. In this thesis, I demonstrate that increasing ambient illumination leads to the recruitment of GABAergic wide-field amacrine cells (WACs) endowing the DS circuit with an additional feature: size selectivity. Using a combination of electrophysiology, pharmacology and light/electron microscopy, I demonstrate that WACs predominantly contact presynaptic bipolar cells, which drive direct excitation and feed-forward inhibition (through SACs) to DSGCs, therefore maintaining the appropriate balance of inhibition/excitation required for generating DS. This circuit arrangement permits high-fidelity direction coding over a range of ambient light levels, over which size selectivity is adjusted. Together, these results provide novel insights into the anatomical and functional arrangement of multiple inhibitory interneurons within a single computational module in the retina. / Graduate

Thesis (M. A. Agricultural Management (Horticulture)) -- University of Limpopo, 2003 / Refer to document

Uppsatsen undersöker hur de olika panoreringsteknikerna, direkt/ambient och direkt-överallt, för 5.1 surround uppfattas utav lyssnare. Enligt direkt/ambient ska alla direkta källor panoreras till frontkanalerna (L, C, R) och endast ambiens finnas i surroundkanalerna (Ls, Rs). Enligt direkt-överallt kan direkta källor panoreras till alla kanaler. Ljudstimuli i tre genrer har utav undertecknad framställts, rock/hårdrock, jazz/fusion och hip hop. Ett exempel enligt direkt/ambient och ett enligt direkt-överallt framställdes i varje genre. Vidare har två lyssningsundersökningar ägt rum, en med ”vana” lyssnare i ett kontrollrum samt en med ”vanliga” lyssnare i hemmiljö. Undersökningarna visade att de flesta föredrog direkt-överallt-produktionerna och dessa uppfattades även som mest omslutande vilket har visat sig vara en viktig egenskap för en musikproduktion i surround.

Thesis (M.S. in Modeling, Virtual Environments, and Simulation)--Naval Postgraduate School, September 2003. / Thesis advisor(s): Russell D. Shilling, Rudolph P. Darken. Includes bibliographical references (p. 169-172). Also available online.

Thesis (Ph.D)--Psychology, Georgia Institute of Technology, 2010. / Committee Chair: Walker, Bruce N.; Committee Member: Corballis, Paul M.; Committee Member: Corso, Gregory M.; Committee Member: Folds, Dennis J.; Committee Member: Houtsma, Adrianus J. M. Part of the SMARTech Electronic Thesis and Dissertation Collection.

Sensitivity to differential motion components, shearing and compressive (opposed) motion, was examined. The hypothesis that the visual system contains local mechanisms specifically sensitive to these types of motion was tested. Stimuli consisted of two moving sinusoidal gratings. Sensitivity to shear and compression was compared with sensitivity for linear motion. Lower thresholds of motion and contrast sensitivities were obtained. Subjects were more sensitive to opposed than to non-opposed motion for a range of grating orientations and different grating spatial frequencies. However sensitivity for opposed motion decreased in the presence of a second added linear motion. The hypothesis of local shear and compression mechanisms was rejected in favour of antagonistic (opponent) interactions between local motion mechanisms. Motion capture was examined. Stimuli were made up of a circular test grating surrounded by another grating. Subjects were required to judge the direction of motion of the test grating. Experiments examined the effects on motion capture of: centre grating size; orientation of surround; relative contrast of centre and surround; plaids in the surround. Conditions favouring motion capture were: with the smallest centre grating; with surround and centre orientations within thirty degrees; with surround had higher contrast than the centre; and only when a plaid surround contained a component of similar orientation as the centre. For conditions of motion capture relative to those of no-capture, increased velocity thresholds for judging the centre direction were found. This was associated with a shift in the bias point between opposed directions with no change in overall sensitivity to motion. It is suggested that a cooperative network of local motion mechanisms featuring centre-surround opponency can account for all the results of this study.

The requirement for multi-channel surround sound in audio production applications is growing rapidly. Audio processing in these applications can be costly, particularly in multi-channel systems. A distributed approach is proposed for the development of a realtime spatialization system for surround sound music production, using Ambisonic surround sound methods. The latency in the system is analyzed, with a focus on the audio processing and network delays, in order to ascertain the feasibility of an enhanced, distributed real-time spatialization system.

No description available.

Audio signal processing systems for converting two-channel (stereo) recordings to four or five channels are increasingly relevant. These audio upmixers can be used with conventional stereo sound recordings and reproduced with multichannel home theatre or automotive loudspeaker audio systems to create a more engaging and natural-sounding listening experience. This dissertation discusses existing approaches to audio upmixing for recordings of musical performances and presents specific design criteria for a system to enhance spatial sound quality. A new upmixing system is proposed and evaluated according to these criteria and a theoretical model for its behavior is validated using empirical measurements. / The new system removes short-term correlated components from two electronic audio signals using a pair of adaptive filters, updated according to a frequency domain implementation of the normalized-least-means-square algorithm. The major difference of the new system with all extant audio upmixers is that unsupervised time-alignment of the input signals (typically, by up to +/-10 ms) as a function of frequency (typically, using a 1024-band equalizer) is accomplished due to the non-minimum phase adaptive filter. Two new signals are created from the weighted difference of the inputs, and are then radiated with two loudspeakers behind the listener. According to the consensus in the literature on the effect of interaural correlation on auditory image formation, the self-orthogonalizing properties of the algorithm ensure minimal distortion of the frontal source imagery and natural-sounding, enveloping reverberance (ambiance) imagery. / Performance evaluation of the new upmix system was accomplished in two ways: Firstly, using empirical electroacoustic measurements which validate a theoretical model of the system; and secondly, with formal listening tests which investigated auditory spatial imagery with a graphical mapping tool and a preference experiment. Both electroacoustic and subjective methods investigated system performance with a variety of test stimuli for solo musical performances reproduced using a loudspeaker in an orchestral concert-hall and recorded using different microphone techniques. / The objective and subjective evaluations combined with a comparative study with two commercial systems demonstrate that the proposed system provides a new, computationally practical, high sound quality solution to upmixing.

When recording music for surround audio engineers sometimes face limitations in time, ideal microphone positions or a noisy audience. If this cannot be dealt with at the location, artificial reverbs are often used in the mixing to “fill in the blanks”. In this study, three instruments were recorded separately with two 5.1 surround microphone setups. Two audio engineer students created artificial reverbs that replaced the back channels of each microphone setup. A listening test was conducted where test subjects compared the 5.1 real recording to the two other stimuli with artificial back channels in terms of realism, envelopment and preference. The result showed that the real recording and the artificial back channels were interchangeable, but that the artificial back channels pointed towards more envelopment, and that the real recording pointed towards more realism.