The Impact of Pitch and Timbre Cues on Auditory Grouping and Stream Segregation

In multi-talker listening environments, the culmination of different voice streams may lead to the distortion of each source’s individual message, causing deficits in comprehension. Voice characteristics, such as pitch and timbre, are major dimensions of auditory perception and play a vital role in grouping and segregating incoming sounds based on their acoustic properties. The current study investigated how pitch and timbre cues (determined by fundamental frequency, notated as F0, and spectral slope, respectively) can affect perceptual integration and segregation of complex-tone sequences within an auditory streaming paradigm. Twenty normal-hearing listeners participated in a traditional auditory streaming experiment using two alternating sequences of harmonic tone complexes A and B with manipulating F0 and spectral slope. Grouping ranges, the F0/spectral slope ranges over which auditory grouping occurs, were measured with various F0/spectral slope differences between tones A and B. Results demonstrated that the grouping ranges were maximized in the absence of the F0/spectral slope differences between tones A and B and decreased by 2 times as their differences increased to ±1-semitone F0 and ±1-dB/octave spectral slope. In other words, increased differences in either F0 or spectral slope allowed listeners to more easily distinguish between harmonic stimuli, and thus group them together less. These findings suggest that pitch/timbre difference cues play an important role in how we perceive harmonic sounds in an auditory stream, representing our ability to group or segregate human voices in a multi-talker listening environment.

Perceptual Integration and Visual Illusions

In my first post I argued that inconsistencies in visual space reflect a conflict between visual experience and perceptual judgement. In this second post I argue that the same approach can be applied to (a) the integration of depth cues, and (b) illusions of visual space, to show that they too operate at the level of cognition rather than perception.

Selective modulation of interhemispheric connectivity by transcranial alternating current stimulation influences binaural integration

Brain connectivity plays a major role in the encoding, transfer, and integration of sensory information. Interregional synchronization of neural oscillations in the γ-frequency band has been suggested as a key mechanism underlying perceptual integration. In a recent study, we found evidence for this hypothesis showing that the modulation of interhemispheric oscillatory synchrony by means of bihemispheric high-density transcranial alternating current stimulation (HD-TACS) affects binaural integration of dichotic acoustic features. Here, we aimed to establish a direct link between oscillatory synchrony, effective brain connectivity, and binaural integration. We experimentally manipulated oscillatory synchrony (using bihemispheric γ-TACS with different interhemispheric phase lags) and assessed the effect on effective brain connectivity and binaural integration (as measured with functional MRI and a dichotic listening task, respectively). We found that TACS reduced intrahemispheric connectivity within the auditory cortices and antiphase (interhemispheric phase lag 180°) TACS modulated connectivity between the two auditory cortices. Importantly, the changes in intra- and interhemispheric connectivity induced by TACS were correlated with changes in perceptual integration. Our results indicate that γ-band synchronization between the two auditory cortices plays a functional role in binaural integration, supporting the proposed role of interregional oscillatory synchrony in perceptual integration.

Low-Intensity Steady Background Noise Enhances Pitch Fusion Across the Ears in Normal-Hearing Listeners

Binaural pitch fusion is the perceptual integration of stimuli that evoke different pitches between the ears into a single auditory image. This study was designed to investigate how steady background noise can influence binaural pitch fusion. The binaural fusion ranges, the frequency ranges over which binaural pitch fusion occurred, were measured with three signal-to-noise ratios (+15, +5, and −5dB SNR) of the pink noise and compared with those measured in quiet. The preliminary results show that addition of an appropriate amount of noise can reduce binaural fusion ranges, an effect called stochastic resonance. This finding increases the understanding of how specific noise levels can sharpen binaural pitch fusion in normal hearing individuals. Furthermore, it elicits more pathways for research to explore how this benefit can practically be used to help improve binaural auditory perception.

Horizontal orthography versus vertical orthography: The effects of writing direction and syllabic format on visual word recognition in Korean Hangul

The Korean writing system has the flexibility of writing horizontally and vertically as well as two syllabic formats that cannot be found in any other alphabetic script. Consolidating these two characteristics, this study investigated the differential extractions of visual information from the mutilated stimuli of the two syllabic formats of CVC syllables and two writing directions using a lexical decision task. In Experiment 1, the two syllabic formats (i.e., balanced syllables, [Formula: see text] and vertical syllables, [Formula: see text]) were mutilated downwards and upwards (i.e., [Formula: see text] [Formula: see text] respectively), along with whole stimuli, in horizontal writing direction. In Experiment 2, the same stimuli and syllabic formats were mutilated rightwards and leftwards (i.e., [Formula: see text] [Formula: see text] respectively) in vertical writing direction. Linear mixed effects models showed significant syllabic format effects and writing direction effects, indicating an upper-part superiority in horizontal writing and a right-part advantage in vertical writing. In particular, the right-part superiority in vertical writing is different from a left-part advantage found in Chinese characters. While the upper-part superiority is script-universal, the leftward or rightward bias seems to be script-specific, as readers’ perceptual integration depends on the nature of orthography.

Dissociating conscious and unconscious influences on visual detection effects

The scope of unconscious processing is highly debated, with recent studies showing that even high-level functions such as perceptual integration and category-based attention occur unconsciously. For example, upright faces that are suppressed from awareness through interocular suppression break into awareness more quickly than inverted faces. Similarly, verbal object cues boost otherwise invisible objects into awareness. Here, we replicate these findings, but find that they reflect a general difference in detectability not specific to interocular suppression. To dissociate conscious and unconscious influences on visual detection effects, we use an additional discrimination task to rule out conscious processes as a cause for these differences. Results from this detection-discrimination dissociation paradigm reveal that while face orientation is processed unconsciously, category-based attention requires awareness. These findings provide insights into the function of conscious perception and offer an experimental approach for mapping out the scope and limits of unconscious processing.

Importance of the hippocampus for the learning of route fidelity in homing pigeons

The avian hippocampal formation (HF) is thought to regulate map-like memory representations of visual landmarks/landscape features and has more recently been suggested to be similarly important for the perceptual integration of landmarks/landscapes. Aspects of spatial memory and perception likely combine to support the now well-documented ability of homing pigeons to learn to retrace the same route when homing from familiar locations, leading to the prediction that damage to the HF would result in a diminished ability to repeatedly fly a similar route home. HF-lesioned homing pigeons were repeatedly released from three sites to assess the importance of the hippocampus as pigeons gradually learn a familiar route home guided by familiar landmark and landscape features. As expected, control pigeons displayed increasing fidelity to a familiar route home, and by inference, successful perceptual and memory processing of familiar landmarks/landscape features. By contrast, the impoverished route fidelity of the HF-lesioned pigeons indicated an impaired sensitivity to the same landmark/landscape features.