Dorsal and ventral streams: a framework for understanding aspects of the functional anatomy of language

Despite intensive work on language-brain relations, and a fairly impressive accumulation of knowledge over the last several decades, there has been little progress in developing large-scale models of the functional anatomy of language that integrate neuropsychological, neuroimaging, and psycholinguistic data. Drawing on relatively recent developments in the cortical organization of vision, and on data from a variety of sources, we propose a new framework for understanding aspects of the functional anatomy of language which moves towards remedying this situation. The framework posits that early cortical stages of speech perception involve auditory fields in the superior temporal gyrus bilaterally (although asymmetrically). This cortical processing system then diverges into two broad processing streams, a ventral stream, which is involved in mapping sound onto meaning, and a dorsal stream, which is involved in mapping sound onto articulatory-based representations. The ventral stream projects ventro-laterally toward inferior posterior temporal cortex (posterior middle temporal gyrus) which serves as an interface between sound-based representations of speech in the superior temporal gyrus (again bilaterally) and widely distributed conceptual representations. The dorsal stream projects dorso-posteriorly involving a region in the posterior Sylvian fissure at the parietal-temporal boundary (area Spt), and ultimately projecting to frontal regions. This network provides a mechanism for the development and maintenance of "parity" between auditory and motor representations of speech. Although the proposed dorsal stream represents a very tight connection between processes involved in speech perception and speech production, it does not appear to be a critical component of the speech perception process under normal (ecologically natural) listening conditions, that is, when speech input is mapped onto a conceptual representation. We also propose some degree of bi-directionality in both the dorsal and ventral pathways. We discuss some recent empirical tests of this framework that utilize a range of methods. We also show how damage to different components of this framework can account for the major symptom clusters of the fluent aphasias, and discuss some recent evidence concerning how sentence-level processing might be integrated into the framework.     

The neural bases of difficult speech comprehension and speech production: Two Activation Likelihood Estimation (ALE) meta-analyses

The role of speech production mechanisms in difficult speech comprehension is the subject of on-going debate in speech science. Two Activation Likelihood Estimation (ALE) analyses were conducted on neuroimaging studies investigating difficult speech comprehension or speech production. Meta-analysis 1 included 10 studies contrasting comprehension of less intelligible/distorted speech with more intelligible speech. Meta-analysis 2 (21 studies) identified areas associated with speech production. The results indicate that difficult comprehension involves increased reliance of cortical regions in which comprehension and production overlapped (bilateral anterior Superior Temporal Sulcus (STS) and anterior Supplementary Motor Area (pre-SMA)) and in an area associated with intelligibility processing (left posterior MTG), and second involves increased reliance on cortical areas associated with general executive processes (bilateral anterior insulae). Comprehension of distorted speech may be supported by a hybrid neural mechanism combining increased involvement of areas associated with general executive processing and areas shared between comprehension and production.     

Cortical bases of speech perception: evidence from functional lesion studies

Functional lesion studies have yielded new information about the cortical organization of speech perception in the human brain. We will review a number of recent findings, focusing on studies of speech perception that use the techniques of electrocortical mapping by cortical stimulation and hemispheric anesthetization by intracarotid amobarbital. Implications for recent developments in neuroimaging studies of speech perception will be discussed. This discussion will provide the framework for a developing model of the cortical circuitry critical for speech perception.     

A cortical circuit for voluntary laryngeal control: Implications for the evolution language

The development of voluntary laryngeal control has been argued to be a key innovation in the evolution of language. Part of the evidence for this hypothesis comes from neuroscience. For example, comparative research has shown that humans have direct cortical innervation of motor neurons controlling the larynx, whereas nonhuman primates do not. Research on cortical motor control circuits has shown that the frontal lobe cortical motor system does not work alone; it is dependent on sensory feedback control circuits. Thus, the human brain must have evolved not only the required efferent motor pathway but also the cortical circuit for controlling those efferent signals. To fill this gap, I propose a link between the evolution of laryngeal control and neuroscience research on the human dorsal auditory-motor speech stream. Specifically, I argue that the dorsal stream Spt (Sylvian parietal-temporal) circuit evolved in step with the direct cortico-laryngeal control pathway and together represented a key advance in the evolution of speech. I suggest that a cortical laryngeal control circuit may play an important role in language by providing a prosodic frame for speech planning.     

Structure and function of auditory cortex: music and speech

We examine the evidence that speech and musical sounds exploit different acoustic cues: speech is highly dependent on rapidly changing broadband sounds, whereas tonal patterns tend to be slower, although small and precise changes in frequency are important. We argue that the auditory cortices in the two hemispheres are relatively specialized, such that temporal resolution is better in left auditory cortical areas and spectral resolution is better in right auditory cortical areas. We propose that cortical asymmetries might have developed as a general solution to the need to optimize processing of the acoustic environment in both temporal and frequency domains.     

Successful treatment of stuttering following closed head injury

Based on a review of the literature, sudden onset of stuttering following a closed head injury has generally had a prognosis of poor recovery. The present case study of an adult male reviews the relevant neurologic data, psychologic information, and subsequent speech therapy methods used to facilitate an essentially complete recovery of normal speech within a few months. A possible neurologic explanation for the stuttering behavior and associated factors in recovery is discussed.     

The postnatal development of the motor speech area: a preliminary study

This preliminary study follows the maturation of motor speech areas and their adjacent orofacial motor zones in the right and left hemispheres of the human infant from 3 months to 72 months of age. Quantitative studies of basilar dendrite patterns of layer 5 cortical pyramids are reported in 17 age-graded subjects. The sequence of developmental changes is characterized by early (3 month) prominence of orofacial motor zones over motor speech areas and of the right hemisphere over the left as expressed in total basilar dendritic length and the length of proximal order dendritic segments. The complex series of changes which follow appear to involve increase in number and marked increase in length of the developing distal order segments and possible shortening of the proximal segments. During a variable sequence of apparent growth spurts and resorptions, dendrite arborizations of the motor speech areas overtake and exceed those of orofacial musculature and the total length of dendrite systems on the left finally exceeds those on the right, although even in the most mature group, (42-72 months), distal segment length in the right motor speech area still exceeds that of the left. The early structural primacy of right-sided dendrite systems and the progressive shift to left-sided primacy is considered in the light of the presence of a long phylogenetic and ontogenetic history of gross anatomical asymmetry favoring the left side. The possibility that developmental patterns of the cortical macroenvironment and microenvironment may be dissociated with the microenvironment depending heavily on epigenetic factors receives support from recent data suggesting the existence of a time window or critical period during which primary language proficiency must be attained if normal lateralization of language function is to occur.     

Visual information processing in the left and right hemispheres during unilateral tachistoscopic stimulation of stutterers

Tachistoscopy was used to investigate 42 stutterers and a corresponding control group for their speech dominance. A bilateral cortical responsibility for speech was equally frequent in the two groups. However, 2.5 times as many stutterers showed a superiority in the information processing in the right hemisphere than did nonstutterers.     

Neural modeling and imaging of the cortical interactions underlying syllable production

This paper describes a neural model of speech acquisition and production that accounts for a wide range of acoustic, kinematic, and neuroimaging data concerning the control of speech movements. The model is a neural network whose components correspond to regions of the cerebral cortex and cerebellum, including premotor, motor, auditory, and somatosensory cortical areas. Computer simulations of the model verify its ability to account for compensation to lip and jaw perturbations during speech. Specific anatomical locations of the model's components are estimated, and these estimates are used to simulate fMRI experiments of simple syllable production.     

Language in a congenitally acallosal brain

A case of complete callosal agenesis shows two limitations of cognitive development. The first, a discrete loss of syntactic-pragmatic function within the language domain, may or may not be associated with anomalous cerebral lateralization of speech and language; its neural basis is unknown. The second deficit, an inability to identify free-field or dichotic speech sounds with overlapping temporal-acoustic properties, appears due to a nonlinguistic limitation of the acallosal brain, the inability to suppress ipsilateral information.     

Multidisciplinary assessment and diagnosis of conversion disorder in a patient with foreign accent syndrome

Multiple reports have described patients with disordered articulation and prosody, often following acute aphasia, dysarthria, or apraxia of speech, which results in the perception by listeners of a foreign-like accent. These features led to the term foreign accent syndrome (FAS), a speech disorder with perceptual features that suggest an indistinct, non-native speaking accent. Also correctly known as psuedoforeign accent, the speech does not typically match a specific foreign accent, but is rather a constellation of speech features that result in the perception of a foreign accent by listeners. The primary etiologies of FAS are cerebrovascular accidents or traumatic brain injuries which affect cortical and subcortical regions critical to expressive speech and language production. Far fewer cases of FAS associated with psychiatric conditions have been reported. We will present the clinical history, neurological examination, neuropsychological assessment, cognitive-behavioral and biofeedback assessments, and motor speech examination of a patient with FAS without a known vascular, traumatic, or infectious precipitant. Repeated multidisciplinary examinations of this patient provided convergent evidence in support of FAS secondary to conversion disorder. We discuss these findings and their implications for evaluation and treatment of rare neurological and psychiatric conditions.     

Developmental stuttering and the role of the supplementary motor cortex

Developmental stuttering is a frequent neurodevelopmental disorder with a complex neurobiological basis. Robust neural markers of stuttering include imbalanced activity of speech and motor related brain regions, and their impaired structural connectivity. The dynamic interaction of cortical regions is regulated by the cortico-basal ganglia-thalamo-cortical system with the supplementary motor area constituting a crucial cortical site. The SMA integrates information from different neural circuits, and manages information about motor programs such as self-initiated movements, motor sequences, and motor learning. Abnormal functioning of SMA is increasingly reported in stuttering, and has been recently indicated as an additional “neural marker” of DS: anatomical and functional data have documented abnormal structure and activity of the SMA, especially in motor and speech networks. Its connectivity is often impaired, especially when considering networks of the left hemisphere. Compatibly, recent data suggest that, in DS, SMA is part of a poorly synchronized neural network, thus resulting in a likely substrate for the appearance of DS symptoms. However, as evident when considering neural models of stuttering, the role of SMA has not been fully clarified. Herein, the available evidence is reviewed, which highlights the role of the SMA in DS as a neural “hub”, receiving and conveying altered information, thus “gating” the release of correct or abnormal motor plans.     

Development of speech and language following bilateral frontal lesions

Language and speech were studied in a young child with perinatally acquired bifrontal lesions. Bilateral frontal pathology seriously interfered with the development of intelligible speech and resulted in a persistent expressive aphasia. Analysis of the neuropsychological profile indicated impairments in intelligence and language comprehension. These deficits, however, were considered secondary to the profound speech programming disorder. The findings indicate that, despite the plasticity of the immature central nervous system, bilateral frontal injury sustained at an early age precludes the development of intelligible speech. Furthermore, structurally intact cortical regions outside the territories of the speech zones fail to mediate normal speech and language development.     

An interpretation of research of feedback interruption in speech.

The controversial question of the scope of sensory control in the voluntary motor patterns involved in speech is examined by reviewing studies in which the auditory, tactile, and proprioceptive feedback channels have been distorted or interrupted. The author makes a case for open loop control of well-learned speech patterns under normal circumstances. The concept of internal feedback is introduced as a possible control system of skilled speech, whereas response feedback and external feedback are viewed as necessary for children developing speech or adults learning new speech patterns.     

Evaluation of the efficacy of a stutter modification program with particular reference to two new measures of secondary behaviors and control of stuttering

This study aimed to assess the changes in speech behaviors and attitudes of a group of stutterers who attended an intensive stutter modification program, using measures specifically designed to reflect the aims of the program. These included a measure of “clean” stuttering to quantify overt secondary behaviors and a quantitative measure of speech control to assess changes in control over stuttering moments using the taught handling techniques. High interjudge and intrajudge agreement was obtained on the two new speech measures. In addition, the program’s own Attitude Scale was used to assess changes in attitudes toward communication. The five subjects were assessed immediately after and 2 years after the program. Each subject was analyzed as a single case study, and there was great individual variation in response to therapy. The general trend was a decline in the maintenance of speech behaviors, with good maintenance of attitude gains 2 years postworkshop. The poorest maintenance was linked to lack of attendance at refresher therapy and initial severity of stuttering. The two new measures used to evaluate change in clean stuttering and control may be useful tools in the assessment protocol of other Stutter Modification programs.     

Neuroelectrical correlates of categorical speech perception in adults

Auditory evoked potentials were recorded from the left and right hemispheres of 16 adults during a phoneme identification task. The use of multivariate statistics enabled researchers to identify a number of cortical processes related to categorical speech perception which were common to both hemispheres, as well as several which disinguished between the two hemispheres.     

An interpretation of research on feedback interruption in speech

The controversial question of the scope of sensory control in the voluntary motor patterns involved in speech is examined by reviewing studies in which the auditory, tactile, and proprioceptive feedback channels have been distorted or interrupted. The author makes a case for open loop control of well-learned speech patterns under normal circumstances. The concept of internal feedback is introduced as a possible control system of skilled speech, whereas response feedback and external feedback are viewed as necessary for children developing speech or adults learning new speech patterns.     

早期听觉剥夺后的大脑可塑性：来自先天性听力障碍群体的证据

皮层功能的正常发展依赖于充分的外部感觉信息的输入。先天性听力障碍群体由于经历早期听觉剥夺, 皮层功能往往出现异常。具体表现为初级听皮层功能退化, 初级、次级听皮层的功能连接变弱, 次级听皮层出现跨通道功能重组; 在后天听力重建后听皮层功能重组仍然存在, 言语加工需要更多高级认知资源的补偿。已有研究在探讨听力重建后皮层的长期可塑性机制、复杂声学环境下言语加工机制、汉语言加工独特性等方面尚不深入, 值得进一步研究。     

Stuttering following brain damage

This paper describes the neurological, language, and speech status of seven brain-damaged patients with symptoms of cortical stuttering. We compare data from our seven patients to data presented on other brain-damaged stutterers, review theories of cortical stuttering's etiology, advance alternative hypotheses about mechanisms underlying such dy sfluencies, and compare cortical stuttering to stuttering in childhood.