Gregory Hickok

The cortical organization of speech processing

Despite decades of research, the functional neuroanatomy of speech processing has been difficult to characterize. A major impediment to progress may have been the failure to consider task effects when mapping speech-related processing systems. We outline a dual-stream model of speech processing that remedies this situation. In this model, a ventral stream processes speech signals for comprehension, and a dorsal stream maps acoustic speech signals to frontal lobe articulatory networks. The model assumes that the ventral stream is largely bilaterally organized — although there are important computational differences between the left- and right-hemisphere systems — and that the dorsal stream is strongly left-hemisphere dominant.

A neural dissociation within language: Evidence that the mental dictionary is part of declarative memory, and that grammatical rules are processed by the procedural system

Language comprises a lexicon for storing words and a grammar for generating rule-governed forms. Evidence is presented that the lexicon is part of a temporal-parietalhnedial-temporal declarative memory system and that granlmatical rules are processed by a frontamasal-ganglia procedural system. Patients produced past tenses of regular and novel verbs (looked and plagged), which require an -ed-suffixation rule, and irregular verbs (dug), which are retrieved from memory. Word-finding difficulties in posterior aphasia, and the general declarative memory impairment in Alzheimers disease, led to more errors with irregular than regular and novel verbs. Grammatical difficulties in anterior aphasia, and the general impairment of procedures in Parkinsons disease, led to the opposite pattern. In contrast to the Parkinsons patients, who showed sup pressed motor activity and rule use, Huntingtons disease patients showed excess motor activity and rule use, underscoring a role for the basal ganglia in grammatical processing.

Eight problems for the mirror neuron theory of action understanding in monkeys and humans

The discovery of mirror neurons in macaque frontal cortex has sparked a resurgence of interest in motor/embodied theories of cognition. This critical review examines the evidence in support of one of these theories, namely, that mirror neurons provide the basis of action understanding. It is argued that there is no evidence from monkey data that directly tests this theory, and evidence from humans makes a strong case against the position.

Computational neuroanatomy of speech production.

Speech production has been studied predominantly from within two traditions, psycholinguistics and motor control. These traditions have rarely interacted, and the resulting chasm between these approaches seems to reflect a level of analysis difference: whereas motor control is concerned with lower-level articulatory control, psycholinguistics focuses on higher-level linguistic processing. However, closer examination of both approaches reveals a substantial convergence of ideas. The goal of this article is to integrate psycholinguistic and motor control approaches to speech production. The result of this synthesis is a neuroanatomically grounded, hierarchical state feedback control model of speech production.

The functional neuroanatomy of language.

There has been substantial progress over the last several years in understanding aspects of the functional neuroanatomy of language. These advances are summarized in this review. It will be argued that recognizing speech sounds is carried out in the superior temporal lobe bilaterally, that the superior temporal sulcus bilaterally is involved in phonological-level aspects of this process, that the frontal/motor system is not central to speech recognition although it may modulate auditory perception of speech, that conceptual access mechanisms are likely located in the lateral posterior temporal lobe (middle and inferior temporal gyri), that speech production involves sensory-related systems in the posterior superior temporal lobe in the left hemisphere, that the interface between perceptual and motor systems is supported by a sensory-motor circuit for vocal tract actions (not dedicated to speech) that is very similar to sensory-motor circuits found in primate parietal lobe, that verbal short-term memory can be understand as an emergent property of this sensory-motor circuit. These observations are understood within the context of a dual stream model of speech processing in which one pathway supports speech comprehension and the other supports sensory-motor integration. Additional topics of discussion include the functional organization of the planum temporale for spatial hearing and speech-related sensory-motor processes, the anatomical and functional basis of a form of acquired language disorder, conduction aphasia, the neural basis of vocabulary development, and sentence-level/grammatical processing.

Response of anterior temporal cortex to syntactic and prosodic manipulations during sentence processing.

Previous research has implicated a portion of the anterior temporal cortex in sentence‐level processing. This region activates more to sentences than to word‐lists, sentences in an unfamiliar language, and environmental sound sequences. The current study sought to identify the relative contributions of syntactic and prosodic processing to anterior temporal activation. We presented auditory stimuli where the presence of prosodic and syntactic structure was independently manipulated during functional magnetic resonance imaging (fMRI). Three “structural” conditions included normal sentences, sentences with scrambled word order, and lists of content words. These three classes of stimuli were presented either with sentence prosody or with flat supra‐lexical (list‐like) prosody. Sentence stimuli activated a portion of the left anterior temporal cortex in the superior temporal sulcus (STS) and extending into the middle temporal gyrus, independent of prosody, and to a greater extent than any of the other conditions. An interaction between the structural conditions and prosodic conditions was seen in a more dorsal region of the anterior temporal lobe bilaterally along the superior temporal gyrus (STG). A post‐hoc analysis revealed that this region responded either to syntactically structured stimuli or to nonstructured stimuli with sentence‐like prosody. The results suggest a parcellation of anterior temporal cortex into 1) an STG region that is sensitive both to the presence of syntactic information and is modulated by prosodic manipulations (in nonsyntactic stimuli); and 2) a more inferior left STS/MTG region that is more selective for syntactic structure. Hum Brain Mapp, 2005.

Recency preference in the human sentence processing mechanism

Cuetos and Mitchell (1988) observed that in constructions in which a relative clause can attach to one of two possible sites, English speakers prefer the more recent attachment site, but Spanish speakers prefer the least recent attachment site, in violation of the proposed universal principle Late Closure (Recency Preference), which favors attachments to the most recent sites. Based on this evidence, Cuetos and Mitchell concluded that Late Closure is not a universal principle of the human sentence processing mechanism. In this paper, we provide new evidence from Spanish and English self-paced reading experiments on relative clause attachment ambiguities that involve three possible attachment sites. The results of our experiments suggest that a principle like Late Closure is in fact universally operative in the human parser, but that it is modulated by at least one other factor in the processing of relative clause attachment ambiguities. We propose that the second factor involved in the processing of these and related constructions is the principle of Predicate Proximity, according to which attachments are preferred to be as structurally close to the head of a predicate phrase as possible, and we further consider the origins and predictions of the theory combining these two factors.

Conduction aphasia, sensory-motor integration, and phonological short-term memory - An aggregate analysis of lesion and fMRI data

Conduction aphasia is a language disorder characterized by frequent speech errors, impaired verbatim repetition, a deficit in phonological short-term memory, and naming difficulties in the presence of otherwise fluent and grammatical speech output. While traditional models of conduction aphasia have typically implicated white matter pathways, recent advances in lesions reconstruction methodology applied to groups of patients have implicated left temporoparietal zones. Parallel work using functional magnetic resonance imaging (fMRI) has pinpointed a region in the posterior most portion of the left planum temporale, area Spt, which is critical for phonological working memory. Here we show that the region of maximal lesion overlap in a sample of 14 patients with conduction aphasia perfectly circumscribes area Spt, as defined in an aggregate fMRI analysis of 105 subjects performing a phonological working memory task. We provide a review of the evidence supporting the idea that Spt is an interface site for the integration of sensory and vocal tract-related motor representations of complex sound sequences, such as speech and music and show how the symptoms of conduction aphasia can be explained by damage to this system.

Role of anterior temporal cortex in auditory sentence comprehension: an fMRI study.

Recent neuropsychological and functional imaging evidence has suggested a role for anterior temporal cortex in sentence-level comprehension. We explored this hypothesis using event-related fMRI. Subjects were scanned while they listened to either a sequence of environmental sounds describing an event or a corresponding sentence matched as closely as possible in meaning. Both types of stimuli required subjects to integrate auditory information over time to derive a similar meaning, but differ in the processing mechanisms leading to the integration of that information, with speech input requiring syntactic mechanisms and environmental sounds utilizing non-linguistic mechanisms. Consistent with recent claims, sentences produced greater activation than environmental sounds in anterior superior temporal lobe bilaterally. A similar speech > sound activation pattern was noted also in posterior superior temporal regions in the left. Envirornmental sounds produced greater activation than sentences in right inferior frontal gyrus. The results provide support for the view that anterior temporal cortex plays an important role in sentence-level comprehension.

Reflections on mirror neurons and speech perception

The discovery of mirror neurons, a class of neurons that respond when a monkey performs an action and also when the monkey observes others producing the same action, has promoted a renaissance for the Motor Theory (MT) of speech perception. This is because mirror neurons seem to accomplish the same kind of one to one mapping between perception and action that MT theorizes to be the basis of human speech communication. However, this seeming correspondence is superficial, and there are theoretical and empirical reasons to temper enthusiasm about the explanatory role mirror neurons might have for speech perception. In fact, rather than providing support for MT, mirror neurons are actually inconsistent with the central tenets of MT.