Optimality of Human Contour Integration

For processing and segmenting visual scenes, the brain is required to combine a multitude of features and sensory channels. It is neither known if these complex tasks involve optimal integration of information, nor according to which objectives computations might be performed. Here, we investigate if optimal inference can explain contour integration in human subjects. We performed experiments where observers detected contours of curvilinearly aligned edge configurations embedded into randomly oriented distractors. The key feature of our framework is to use a generative process for creating the contours, for which it is possible to derive a class of ideal detection models. This allowed us to compare human detection for contours with different statistical properties to the corresponding ideal detection models for the same stimuli. We then subjected the detection models to realistic constraints and required them to reproduce human decisions for every stimulus as well as possible. By independently varying the four model parameters, we identify a single detection model which quantitatively captures all correlations of human decision behaviour for more than 2000 stimuli from 42 contour ensembles with greatly varying statistical properties. This model reveals specific interactions between edges closely matching independent findings from physiology and psychophysics. These interactions imply a statistics of contours for which edge stimuli are indeed optimally integrated by the visual system, with the objective of inferring the presence of contours in cluttered scenes. The recurrent algorithm of our model makes testable predictions about the temporal dynamics of neuronal populations engaged in contour integration, and it suggests a strong directionality of the underlying functional anatomy

Spike-Triggered Covariance Analysis Reveals Phenomenological Diversity of Contrast Adaptation in the Retina

When visual contrast changes, retinal ganglion cells adapt by adjusting their sensitivity as well as their temporal filtering characteristics. The latter has classically been described by contrast-induced gain changes that depend on temporal frequency. Here, we explored a new perspective on contrast-induced changes in temporal filtering by using spike-triggered covariance analysis to extract multiple parallel temporal filters for individual ganglion cells. Based on multielectrode-array recordings from ganglion cells in the isolated salamander retina, we found that contrast adaptation of temporal filtering can largely be captured by contrast-invariant sets of filters with contrast-dependent weights. Moreover, differences among the ganglion cells in the filter sets and their contrast-dependent contributions allowed us to phenomenologically distinguish three types of filter changes. The first type is characterized by newly emerging features at higher contrast, which can be reproduced by computational models that contain response-triggered gain-control mechanisms. The second type follows from stronger adaptation in the Off pathway as compared to the On pathway in On-Off-type ganglion cells. Finally, we found that, in a subset of neurons, contrast-induced filter changes are governed by particularly strong spike-timing dynamics, in particular by pronounced stimulus-dependent latency shifts that can be observed in these cells. Together, our results show that the contrast dependence of temporal filtering in retinal ganglion cells has a multifaceted phenomenology and that a multi-filter analysis can provide a useful basis for capturing the underlying signal-processing dynamics

Audio quality evaluation in MUSHRA tests - influences between loop setting and a listener's ratings

In many listening tests for audio quality evaluation the participants have the possibility to set loops, meaning they can focus on smaller parts of the audio signal and listen to those parts repeatedly. In previous papers, we already showed that experienced listeners set more loops than untrained ones and that learning to set loops in-creases the ability of a listener to perceive artifacts. Now we analyze to what extent the chosen loops vary from listener to listener and whether the ratings are influenced by the choice of loops. We show that - depending on the stimulus - listeners who set different loops may also give significantly different ratings

Generation and evaluation of isolated audio coding artifacts

Many existing perceptual audio codec standards define only the bit stream syntax and associated decoder algorithms, but leave many degrees of freedom to the encoder design. For a systematic optimization of encoder parameters as well as for education and training of experienced test listeners, it is instrumental to provoke and subsequently assess individual coding artifact types in an isolated fashion with controllable strength. The approach presented in this paper consists of a pre-selection of suitable test audio content in combination with forcing a specially modified encoder into non-common operation modes to willingly generate controlled coding artifacts. In conclusion, subjective listening tests were conducted to assess the subjective quality for different parameters and test content

Energy aware modelling of inter-channel level difference distortion impact on spatial audio perception

In spatial audio processing, Inter-aural Level Difference Distortions (ILDD) between reference and coded signals play an important role in the perception of quality degradation. In order to reduce costs, there are efforts to develop algorithms that automatically predict the perceptual quality of multichannel/spatial audio processing operations relative to the unimpaired original without requiring extensive listening tests. Correct modelling of perceived ILDD has a great influence in the prediction performance of automated measurements. We propose an energy aware model of ILDD perception that contemplates a dependency of energy content in different spectral regions of the involved signal. Model parameters are fitted to subjective results obtained from listening test data over a synthetically generated audio database with arbitrarily induced ILDD at different intensities, frequency regions and energy levels. Finally, we compare the performance of our proposed model over two extensive databases of real coded signals along with two state-of-the-art ILDD models

On the effect of inter-channel level difference distortions on the perceived subjective quality of stereo signals

Perceptual audio coding at low bitrates and stereo enhancement algorithms can affect perceived quality of stereo audio signals. Besides changes in timbre, also the spatial sound image can be altered, resulting in quality degradations compared to an original reference. While effects of timbre degradation on quality are well-understood, effects of spatial distortions are not sufficiently known. This paper presents a study designed to quantify the effect of Inter-Channel Level Difference (ICLD) errors on perceived audio quality. Results show systematic effects of ICLD errors on quality: bigger ICLD errors led to greater quality degradation. Spectral portions containing relatively higher energy were affected more strongly

Perception of Coding Artifacts by Nonnative Speakers : A Study with Mandarin Chinese and German Speaking Listeners

Using a standard protocol and sample audio cases to enhance reproducibility, tests of coding quality are often performed jointly by laboratories around the world. Multiple Stimuli with Hidden Reference and Anchor (MUSHRA) is one such standard protocol. The same audio samples are used in all labs and as a result, listeners inevitably are judging quality in either their native language or one that they do not understand. It is not clear if a lack of understanding the language and its phonemes can influence the listener’s perception and his or her quality ratings during the test. This study used German and Mandarin Chinese speaking listeners, as well as test material in these two languages. The authors analyzed how ratings and listening times were affected by the foreign language. When results were pooled over all conditions, no significant differences between the ratings were found. However, for items of high audio quality, it was observed that listeners needed more time to evaluate samples that were not their native language, and it took more effort to compare different audio signals. As in MUSHRA tests – contrary to ITU-T P.800 tests - listeners can compensate for any difficulty they may have in perceiving artifacts by more effort and longer listening times, it seems to be no problem to include nonnative listeners in these tests at the expense of making them slightly less efficient

Is it harder to perceive coding artefact in foreign language items? - A study with Mandarin Chinese and German speaking listeners

MUSHRA listening tests for the evaluation of audio coded material often also include speech stimuli in a language the listener does not understand. However, it is not clear to what extent the lack of understanding and the unfamiliarity with that language and its phonemes may influence the listener's behavior during the test and his or her quality ratings. In a study containing German and Mandarin Chinese speaking listeners as well as items of these two languages we analyze how ratings and listening times are affected by the foreign language. Pooled over all conditions we find no significant differences in the ratings. However, for high quality items we find that compared to native listeners, non-native listeners need longer listening times and compare more between items

On the influence of cultural differences on the perception of audio coding artifacts in music

Modern audio codecs are used all over the world, reaching listeners with many different cultures and languages. This study investigates if and how cultural background influences the perception and preference of different audio coding artifacts, focusing on musical content. A subjective listening test was designed to directly compare different types of audio coding and was performed with Mandarin Chinese and German speaking listeners. Overall comparison showed largely consistent results, affirming the validity of the proposed test method. Differential comparison indicates preferences for certain artifacts in different listener groups, e.g., Chinese listeners tended to grade tonality mismatch higher and pre-echoes worse compared to German listeners, and musicians preferred bandwidth limitation over tonality mismatch when compared to non-musicians