Visual Binding of English and Chinese Word Parts is Limited to Low Temporal Frequencies

Effects of Frequency Separation and Diotic/Dichotic Presentations on the Alternation Frequency Limits in Audition Derived from a Temporal Phase Discrimination Task

Temporal phase discrimination is a useful psychophysical task to evaluate how sensory signals, synchronously detected in parallel, are perceptually bound by human observers. In this task two stimulus sequences synchronously alternate between two states (say, A-B-A-B and X-Y-X-Y) in either of two temporal phases (ie A and B are respectively paired with X and Y, or vice versa). The critical alternation frequency beyond which participants cannot discriminate the temporal phase is measured as an index characterizing the temporal property of the underlying binding process. This task has been used to reveal the mechanisms underlying visual and cross-modal bindings. To directly compare these binding mechanisms with those in another modality, this study used the temporal phase discrimination task to reveal the processes underlying auditory bindings. The two sequences were alternations between two pitches. We manipulated the distance between the two sequences by changing intersequence frequency separation, or presentation ears (diotic vs dichotic). Results showed that the alternation frequency limit ranged from 7 to 30 Hz, becoming higher as the intersequence distance decreased, as is the case with vision. However, unlike vision, auditory phase discrimination limits were higher and more variable across participants.

Temporal Integration in Visual Word Recognition

When two displays are presented in close temporal succession at the same location, how does the brain assign them to one versus two conscious percepts? We investigate this issue using a novel reading paradigm in which the odd and even letters of a string are presented alternatively at a variable rate. The results reveal a window of temporal integration during reading, with a nonlinear boundary around ∼80 msec of presentation duration. Below this limit, the oscillating stimulus is easily fused into a single percept, with all characteristics of normal reading. Above this limit, reading times are severely slowed and suffer from a word-length effect. ERPs indicate that, even at the fastest frequency, the oscillating stimulus elicits synchronous oscillations in posterior visual cortices, while late ERP components sensitive to lexical status vanish beyond the fusion threshold. Thus, the fusion/segregation dilemma is not resolved by retinal or subcortical filtering, but at cortical level by at most 300 msec. The results argue against theories of visual word recognition and letter binding that rely on temporal synchrony or other fine temporal codes.

A common perceptual temporal limit of binding synchronous inputs across different sensory attributes and modalities

The human brain processes different aspects of the surrounding environment through multiple sensory modalities, and each modality can be subdivided into multiple attribute-specific channels. When the brain rebinds sensory content information ('what') across different channels, temporal coincidence ('when') along with spatial coincidence ('where') provides a critical clue. It however remains unknown whether neural mechanisms for binding synchronous attributes are specific to each attribute combination, or universal and central. In human psychophysical experiments, we examined how combinations of visual, auditory and tactile attributes affect the temporal frequency limit of synchrony-based binding. The results indicated that the upper limits of cross-attribute binding were lower than those of within-attribute binding, and surprisingly similar for any combination of visual, auditory and tactile attributes (2-3 Hz). They are unlikely to be the limits for judging synchrony, since the temporal limit of a cross-attribute synchrony judgement was higher and varied with the modality combination (4-9 Hz). These findings suggest that cross-attribute temporal binding is mediated by a slow central process that combines separately processed 'what' and 'when' properties of a single event. While the synchrony performance reflects temporal bottlenecks existing in 'when' processing, the binding performance reflects the central temporal limit of integrating 'when' and 'what' properties.