Athanassios Protopapas

Speech comprehension is correlated with temporal response patterns recorded from auditory cortex

Speech comprehension depends on the integrity of both the spectral content and temporal envelope of the speech signal. Although neural processing underlying spectral analysis has been intensively studied, less is known about the processing of temporal information. Most of speech information conveyed by the temporal envelope is confined to frequencies below 16 Hz, frequencies that roughly match spontaneous and evoked modulation rates of primary auditory cortex neurons. To test the importance of cortical modulation rates for speech processing, we manipulated the frequency of the temporal envelope of speech sentences and tested the effect on both speech comprehension and cortical activity. Magnetoencephalographic signals from the auditory cortices of human subjects were recorded while they were performing a speech comprehension task. The test sentences used in this task were compressed in time. Speech comprehension was degraded when sentence stimuli were presented in more rapid (more compressed) forms. We found that the average comprehension level, at each compression, correlated with (i) the similarity between the frequencies of the temporal envelopes of the stimulus and the subjects cortical activity (“stimulus-cortex frequency-matching”) and (ii) the phase-locking (PL) between the two temporal envelopes (“stimulus-cortex PL”). Of these two correlates, PL was significantly more indicative for single-trial success. Our results suggest that the match between the speech rate and the a priori modulation capacities of the auditory cortex is a prerequisite for comprehension. However, this is not sufficient: stimulus-cortex PL should be achieved during actual sentence presentation.

Relations between the Neural Bases of Dynamic Auditory Processing and Phonological Processing: Evidence from fMRI

Functional magnetic resonance imaging (fMRI) was used to examine how the brain responds to temporal compression of speech and to determine whether the same regions are also involved in phonological processes associated with reading. Recorded speech was temporally compressed to varying degrees and presented in a sentence verification task. Regions involved in phonological processing were identified in a separate scan using a rhyming judgment task with pseudowords compared to a lettercase judgment task. The left inferior frontal and left superior temporal regions (Brocas and Wernickes areas), along with the right inferior frontal cortex, demonstrated a convex response to speech compression; their activity increased as compression increased, but then decreased when speech became incomprehensible. Other regions exhibited linear increases in activity as compression increased, including the middle frontal gyri bilaterally. The auditory cortices exhibited compression-related decreases bilaterally, primarily reflecting a decrease in activity when speech became incomprehensible. Rhyme judgments engaged two left inferior frontal gyrus regions (pars triangularis and pars opercularis), of which only the pars triangularis region exhibited significant compression-related activity. These results directly demonstrate that a subset of the left inferior frontal regions involved in phonological processing is also sensitive to transient acoustic features within the range of comprehensible speech.

Check Vocal: A program to facilitate checking the accuracy and response time of vocal responses from DMDX

CheckVocal is a Windows application that facilitates checking the accuracy and response time of recorded vocal responses in naming and other experimental tasks using the DMDX display and response collection software. CheckVocal handles all keeping-track and presents each recorded response audiovisually (as waveform, spectrogram, and sound played out) along with the corresponding printed correct response and registered response time. The user simply decides whether the response was correct, wrong, or missing, with a single mouse click advancing to the next response. Response time correction can be done manually or automatically (retriggering by a power threshold). Data safety and integrity is ensured by cross-checking and status saving, so that interrupted sessions can be resumed later. Check Vocal is freely available to the DMDX community via a dedicated Web page.

Success and failure in teaching the (r)-(l) contrast to Japanese adults: Tests of a Hebbian model of plasticity and stabilization in spoken language perception

A Hebbian model of learning predicts that adults may be able to acquire a nonnative speech contrast if they are trained with stimuli that are exaggerated to make them perceptually distinct. To test these ideas, we asked Japanese adults to identify contrasting [r]-[l] stimuli (e.g., rock-lock) in two training conditions. In the adaptive condition, the [r]-[l] contrast was exaggerated at first and then adjusted to maintain accurate identification. In the fixed condition, a fixed pair of stimuli were used that were distinguishable by native English speakers but difficult for the Japanese learners to discriminate. To examine whether feedback contributes to learning, we ran separate groups with and without feedback in the fixed and the adaptive conditions. Without feedback, 3 days of adaptive training produced substantial improvements, but 3 days of fixed training produced no benefit relative to control, consistent with the Hebbian account. With feedback, both fixed and adaptive training led to robust improvements, and the benefit of training transferred to a second continuum (e.g., road-load). The results are consistent with Hebbian models that are augmented to be sensitive to feedback.

Selective speech motor, syntax and cognitive deficits associated with bilateral damage to the putamen and the head of the caudate nucleus: a case study

Deficits in speech production, sentence comprehension and abstract reasoning occurred in a subject having profound bilateral damage to the putamen and the caudate nucleus. Acoustic analyses indicated that the subjects speech was degraded due to inappropriate sequencing of articulatory gestures that involve different articulatory structures. Transitions between sounds were slow and often did not achieve target configurations. The subject had a 14% error rate comprehending distinctions in meaning conveyed by syntax in English sentences; normal controls make virtually no errors in this test. Cognitive deficits involving impaired sequencing occurred: the subject had a 70% error rate on the Odd Man Out test when making decisions within a single category. Cognitive perseveration occurred when the subject was asked to shift categories. In contrast, performance was within normal ranges in tests of lexical access and memory. The pattern of deficits provides evidence for basal ganglia involvement in the regulation of sequencing across modalities.

Fundamental frequency of phonation and perceived emotional stress

Nonlinguistic information about the speakers emotional state is conveyed in spoken utterances by means of several acoustic characteristics and listeners can often reliably identify such information. In this study we investigated the effect of short- and long-term F0 measures on perceived emotional stress using stimuli synthesized with the LPC coefficients of a steady vowel and varying F0 tracks. The original F0 tracks were taken from naturally occurring speech in highly stressful (contingent on terror) and nonstressful conditions. Stimuli with more jitter were rated as sounding more hoarse but not more stressed, i.e., a demonstrably perceptible amount of jitter did not seem to play a role in perceived emotional stress. Reversing the temporal pattern of F0 did not affect the stress ratings, suggesting that the directionality of variations in F0 does not convey emotional stress information. Mean and maximum F0 within an utterance correlated highly with stress ratings, but the range of F0 did not correlate significantly with the stress ratings, especially after the effect of maximum F0 was removed in stepwise regression. It is concluded that the range of F0 per se does not contribute to the perception of emotional stress, whereas maximum F0 constitutes the primary indicator. The observed effects held across several voices that were found to sound natural (three male voices and one of two female ones). An effect of the formant frequencies was also observed in the stimuli with the lowest F0; it is hypothesized that formant frequency structure dominated the F0 effect in the one voice that gave discrepant results.

Reading ability is negatively related to Stroop interference

Stroop interference is often taken as evidence for reading automaticity even though young and poor readers, who presumably lack reading automaticity, present strong interference. Here the relationship between reading skills and Stroop interference was studied in a 7th-grade sample. Greater interference was observed in children diagnosed with reading disability (dyslexia) than in unimpaired children. Moreover, poorer reading skills were found to correlate with greater Stroop interference in the general school population. In correlation and regression analyses, interference was primarily associated with reading speed, with an additional unique contribution of reading accuracy. Color naming errors were few and not comparably related to reading skills. The relation of reading skill to Stroop interference was examined in computational modeling simulations. The production model of Roelofs [Roelofs, A. (2003). Goal-referenced selection of verbal action: modeling attentional control in the Stroop task. Psychological Review, 110, 88-125], in which interference is primarily due to word stimuli having direct access to word form encoding whereas color naming must pass through concept activation and lemma selection, was found to account well for the human data after imposing covariation constraints on parameters controlling word processing and blocking latency, in modifications not affecting the models previous fit to other data. The connectionist model of Cohen, Dunbar, and McClelland [Cohen, J. D., Dunbar, K., & McClelland, J. L. (1990). On the control of automatic processes: a parallel distributed processing account of the Stroop effect. Psychological Review, 97, 332-361], in which interference is caused by differential route strength, implementing an automaticity account, approximated the observed patterns with network-wide parameter manipulations not specific to reading, such as processing speed and response threshold, likely to affect previously optimized performance. On the basis of the empirical and modeling data we argue for a direct link between reading skill and interference, beyond the effects of executive functioning.

Development of Lexical Mediation in the Relation Between Reading Comprehension and Word Reading Skills in Greek

This study focuses on the shared variance between reading comprehension and word-level reading skills in a population of 534 Greek children in Grades 2 through 4. The correlations between measures of word and pseudoword accuracy and fluency, on the one hand, and vocabulary and comprehension skills, on the other, were sizeable and stable or increasing with grade. However, the unique contribution of word reading to comprehension became negligible after vocabulary measures were entered in hierarchical regression analyses, particularly for higher grades, suggesting that any effects of decoding on comprehension may be mediated by the lexicon, consistent with lexical quality hypothesis. Structural modeling with latent variables revealed an invariant path across grades in which vocabulary was defined by its covariation with reading accuracy and fluency and affected comprehension directly. It is argued that skilled word reading influences comprehension by strengthening lexical representations, at least when phonological decoding can be relatively effortless.

A comparative quantitative analysis of Greek orthographic transparency.

Orthographic transparency refers to the systematicity in the mapping between orthographic letter sequences and phonological phoneme sequences in both directions, for reading and spelling. Measures of transparency previously used in the analysis of orthographies of other languages include regularity, consistency, and entropy. However, previous reports have typically been hampered by severe restrictions, such as using only monosyllables or only word-initial phonemes. Greek is sufficiently transparent to allow complete sequential alignment between graphemes and phonemes, therefore permitting full analyses at both letter and grapheme levels, using every word in its entirety. Here, we report multiple alternative measures of transparency, using both type and token counts, and compare these with estimates for other languages. We discuss the problems stemming from restricted analysis sets and the implications for psycholinguistic experimentation and computational modeling of reading and spelling.

The breakdown of functional categories in Greek aphasia: Evidence from agreement, tense, and aspect

Background: Verbal inflectional errors are among the most prominent characteristics of aphasic nonfluent speech. Several studies have shown that such impairment is selective: subject–verb agreement is relatively intact while tense is severely impaired. A number of researchers view the deficit as structural and attribute errors to a breakdown of functional categories and their projections. Agrammatic individuals are thought to produce trees that are intact up to the Tense node and “pruned” from this node up. A partial preliminary report of the data was presented at the “Science of Aphasia V” conference in Potsdam in 16–21 September 2004. An earlier version of this paper was presented at the Departmental Seminars of the Department of Language and Communication Science at City University (February 2005) and appeared at the Reading Working Papers in Linguistics 6 (2005). We thank two anonymous reviewers for comments on an earlier version of this paper as well as the two reviewers of Aphasiology for their useful comments and suggestions. We also thank speech pathologists M. Diamanti, A. Xofillis, and M. Moudouris for referring the patients. Note: All co‐authors have contributed equally to this article. Aims: The present study investigates (a) the relative sensitivity of functional categories related to verbal inflection in Greek aphasia and the systematicity thereof; and (b) the relation between patterns of impairment in production and grammaticality judgements. Method & Procedures: We present results from a sentence completion and a grammaticality judgement task with seven Greek‐speaking aphasic individuals and seven control participants matched for age and education. Materials were constructed to assess three functional categories: subject–verb agreement, tense, and aspect. Eight verbs were used, balancing estimated familiarity and regularity of aspectual conjugation. Outcomes & Results: A great variability was observed among participants in overall performance but the pattern of performance was quite systematic. The results indicated that inflectional morphemes are not all impaired to the same degree in Greek aphasia. In both tasks, as a group, patients made more errors in aspect than in agreement. The group differences between tense and the other two conditions did not reach statistical significance. Moreover, a comparison of individual aphasic performance in the three functional categories indicated that in every case in which statistically significant differences were observed among the three functional categories, agreement was found to be less impaired than tense, aspect, or both. Conclusions: These findings do not support a global impairment of inflectional morphemes in aphasia but support a selective one and, in particular, a dissociation between agreement, on the one hand, and tense and/or aspect, on the other hand. Moreover, our findings do not support a hierarchical account along the lines of Friedmann and Grodzinsky (1997) but are compatible with Chomskys (2000) Minimalist Program and with Wenzlaff and Clahsens (2004) tense underspecification theory.