Stream segregation of concurrent speech and the verbal transformation effect: Influence of fundamental frequency and lateralization cues.

Repeating a recorded word produces verbal transformations (VTs); perceptual regrouping of acoustic-phonetic elements may contribute to this effect. The influence of fundamental frequency (F0) and lateralization grouping cues was explored by presenting two concurrent sequences of the same word resynthesized on different F0s (100 and 178 Hz). In experiment 1, listeners monitored both sequences simultaneously, reporting for each any change in stimulus identity. Three lateralization conditions were used - diotic, ±680-µs interaural time difference, and dichotic. Results were similar for the first two conditions, but fewer forms and later initial transformations were reported in the dichotic condition. This suggests that large lateralization differences per se have little effect - rather, there are more possibilities for regrouping when each ear receives both sequences. In the dichotic condition, VTs reported for one sequence were also more independent of those reported for the other. Experiment 2 used diotic stimuli and explored the effect of the number of sequences presented and monitored. The most forms and earliest transformations were reported when two sequences were presented but only one was monitored, indicating that high task demands decreased reporting of VTs for concurrent sequences. Overall, these findings support the idea that perceptual regrouping contributes to the VT effect.

The verbal transformation effect and the perceptual organization of speech: influence of formant transitions and F0-contour continuity.

This study explored the role of formant transitions and F0-contour continuity in binding together speech sounds into a coherent stream. Listening to a repeating recorded word produces verbal transformations to different forms; stream segregation contributes to this effect and so it can be used to measure changes in perceptual coherence. In experiment 1, monosyllables with strong formant transitions between the initial consonant and following vowel were monotonized; each monosyllable was paired with a weak-transitions counterpart. Further stimuli were derived by replacing the consonant-vowel transitions with samples from adjacent steady portions. Each stimulus was concatenated into a 3-min-long sequence. Listeners only reported more forms in the transitions-removed condition for strong-transitions words, for which formant-frequency discontinuities were substantial. In experiment 2, the F0 contour of all-voiced monosyllables was shaped to follow a rising or falling pattern, spanning one octave. Consecutive tokens either had the same contour, giving an abrupt F0 change between each token, or alternated, giving a continuous contour. Discontinuous sequences caused more transformations and forms, and shorter times to the first transformation. Overall, these findings support the notion that continuity cues provided by formant transitions and the F0 contour play an important role in maintaining the perceptual coherence of speech.

Re-evaluating split-fovea processing in word recognition: effects of word length.

Several studies have claimed that, when fixating a word, letters to the left and right of fixation project to different hemispheres and are consequently subjected to different processes. In support of this claim, Lavidor M, et al. (2001; hereafter LES&B) report that lexical decisions were affected by increasing the number of letters to the left of fixation but not to the right, and that this indicates divided hemispheric access at the point of fixation to length-sensitive processes in the right hemisphere (RH). We re-evaluated these claims in Experiment 1 using Lavidor et al.'s original stimuli and procedure of merely instructing participants where to fixate. In contrast to the earlier study, increases in the number of letters to the left and right of the designated fixation location produced near-identical effects on reaction time, and increases to the left actually improved response accuracy and increases to the right impaired it. When larger stimuli were used to improve stimulus perceptibility and an eye-tracker monitored fixation accuracy (Experiment 2), left and right increases in the number of letters again produced near-identical effects on reaction time (and accuracy), but frequent and substantial fixation errors were revealed. When an eye-tracker ensured accurate fixations (Experiment 3), left and right increases in the number of letters again produced near-identical effects on reaction time and accuracy. Thus, the findings of all three experiments provide no support for the findings of LES&B (2001) and no evidence of split-fovea processing. The findings also indicate the dangers of assuming fixation of precisely-specified locations within words, both in experiments designed to reveal split-foveal processing and hemispheric asymmetry and in more normal circumstances of word perception.

Re-evaluating split-fovea processing in word recognition: effects of retinal eccentricity on hemispheric dominance.

Several studies have claimed that hemispheric asymmetries affect word recognition right up to the point of fixation because each fovea is split precisely at its vertical midline and information presented either side of this midline projects unilaterally to different, contralateral hemispheres. To investigate this claim, four-letter words were presented to the left or right of fixation, either close to fixation entirely in foveal vision (0.15, 0.25, and 0.35 degrees from fixation) or further from fixation entirely in extrafoveal vision (2.00, 2.10, and 2.20 degrees from fixation). Fixation location and stimulus presentation were controlled using an eye-tracker linked to a fixation-contingent display and performance was assessed using a forced-choice task to suppress confounding effects of guesswork. A left hemisphere advantage was observed for words presented in extrafoveal locations but no hemisphere advantage (left or right) was observed for words presented in any foveal location. These findings support the well-established view that words encountered outside foveal vision project to different, contralateral hemispheres but indicate that this division for word recognition occurs only outside the fovea and provide no support for the claim that a functional split in hemispheric processing exists at the point of fixation.