J.A. Frost

Human Brain Language Areas Identified by Functional Magnetic Resonance Imaging

Functional magnetic resonance imaging (FMRI) was used to identify candidate language processing areas in the intact human brain. Language was defined broadly to include both phonological and lexical–semantic functions and to exclude sensory, motor, and general executive functions. The language activation task required phonetic and semantic analysis of aurally presented words and was compared with a control task involving perceptual analysis of nonlinguistic sounds. Functional maps of the entire brain were obtained from 30 right-handed subjects. These maps were averaged in standard stereotaxic space to produce a robust “average activation map” that proved reliable in a split-half analysis. As predicted from classical models of language organization based on lesion data, cortical activation associated with language processing was strongly lateralized to the left cerebral hemisphere and involved a network of regions in the frontal, temporal, and parietal lobes. Less consistent with classical models were (1) the existence of left hemisphere temporoparietal language areas outside the traditional “Wernicke area,” namely, in the middle temporal, inferior temporal, fusiform, and angular gyri; (2) extensive left prefrontal language areas outside the classical “Broca area”; and (3) clear participation of these left frontal areas in a task emphasizing “receptive” language functions. Although partly in conflict with the classical model of language localization, these findings are generally compatible with reported lesion data and provide additional support for ongoing efforts to refine and extend the classical model.

Conceptual Processing during the Conscious Resting State: A Functional MRI Study

Localized, task-induced decreases in cerebral blood flow are a frequent finding in functional brain imaging research but remain poorly understood. One account of these phenomena postulates processes ongoing during conscious, resting states that are interrupted or inhibited by task performance. Psychological evidence suggests that conscious humans are engaged almost continuously in adaptive processes involving semantic knowledge retrieval, representation in awareness, and directed manipulation of represented knowledge for organization, problem-solving, and planning. If interruption of such conceptual processes accounts for task-induced deactivation, tasks that also engage these conceptual processes should not cause deactivation. Furthermore, comparisons between conceptual and nonconceptual tasks should show activation during conceptual tasks of the same brain areas that are deactivated relative to rest. To test this model, functional magnetic resonance imaging data were acquired during a resting state, a perceptual task, and a semantic retrieval task. A network of left-hemisphere poly-modal cortical regions showed higher signal values during the resting state than during the perceptual task but equal values during the resting and semantic conditions. This result is consistent with the proposal that perceptual tasks interrupt processes ongoing during rest that involve many of the same brain areas engaged during semantic retrieval. As further evidence for this model, the same network of brain areas was activated in two direct comparisons between semantic and perceptual processing tasks. This same conceptual processing network was also identified in several previous studies that contrasted semantic and perceptual tasks or resting and active states. The model proposed here offers a unified account of these findings and may help to explain several unanticipated results from prior studies of semantic processing.

Somatotopic mapping of the human primary motor cortex with functional magnetic resonance imaging

Article abstract—We applied functional magnetic resonance imaging (FMRI) to map the somatotopic organization of the primary motor cortex using voluntary movements of the hand, arm, and foot. Eight right-handed healthy subjects performed self-paced, repetitive, flexiodextension movements of the limbs while undergoing echo-planar imaging. Four subjects performed movements of the right fingers and toes, while the remaining subjects performed movements of the right fingers and elbow joint. There was statistically significant functional activity in the left primary motor cortex in all subjects. The pattern of functional activity followed a topographic representation: finger movements resulted in signal intensity changes over the convexity of the left motor cortex, whereas toe movements produced changes either at the interhemispheric fissure or on the dorsolateral surface adjacent to the interhemispheric fissure. Elbow movements overlapped the more medial signal intensity changes observed with finger movements. Functionally active regions were confined to the cortical ribbon and followed the gyral anatomy closely. These findings indicate that FMRI is capable of generating somatotopic maps of the primary motor cortex in individual subjects.

Effects of stimulus rate on signal response during functional magnetic resonance imaging of auditory cortex

Functional magnetic resonance imaging (FMRI) detects focal MRI signal changes in brain tissue that are believed to result from changes in neuronal activity. We describe the dependence of this response in auditory cortex on the rate of presentation of simple speech stimuli. Speech syllables were presented to five normal subjects at rates ranging from 0.17 to 2.5 Hz, while the subjects performed a phoneme discrimination task. Regions studied with FMRI during this task included the lateral aspect of both temporal lobes. All subjects showed bilateral superior temporal lobe MRI signal increases that were coincident with stimulus presentation and performance of the task. The magnitude of this response increased in a monotonic, non-linear manner with increasing stimulus rate. This rate-response relationship was nearly identical in right and left hemispheres. The relationship may reflect metabolic activity integrated over time and subject to non-linear characteristics of neuronal recovery or blood flow regulation. The dependence of response magnitude on stimulation rate supports the hypothesis that the FMRI phenomenon indirectly reflects neuronal metabolic activity. The measures provided here should assist in the design of optimal activation strategies for the human auditory cortex.

Side of seizure focus predicts left medial temporal lobe activation during verbal encoding

Objective: Functional MRI (FMRI) was used to investigate the effect of medial temporal lobe (MTL) pathology on activation of language encoding areas in patients with temporal lobe epilepsy (TLE). Methods: Whole-brain FMRI was obtained. Twenty-eight patients with either left TLE(LTLE) or right TLE (RTLE) performed a semantic decision task alternating with an auditory perceptual task. Results: Activation of language areas in the frontal and parietal lobes was similar in both groups, with no group differences in the total number of active voxels. However, the RTLE group showed much stronger activation of the left MTL, including the hippocampus, parahippocampal gyrus, and collateral sulcus, than did the LTLE group. Conclusions: Activation of the left MTL during semantic encoding discriminates patients with RTLE and LTLE. This FMRI technique may potentially be of use in determining memory lateralization and for predicting the side of seizure focus in TLE.

Cytochrome oxidase in the human visual cortex: Distribution in the developing and the adult brain

Cytochrome-oxidase (CO) histochemistry has revealed important functional subdivisions, modules, and processing streams in the macaque visual cortex. The present study is aimed at analyzing the development and characteristics of CO patterns in the human visual cortex by means of histochemistry and immunohistochemistry. At 26 weeks of gestation, both the ventricular and subventricular zones had low levels of CO, while the cortical plate had moderate levels of CO. At birth, supragranular CO-rich zones (puffs) were not clearly organized, indicating that the development of puffs in the neonatal striate cortex lags behind that of the macaque monkey, whose puffs appear weeks before birth. Puffs were more clearly discernible in human cortex at postnatal day 24, and became well organized by the fourth postnatal month. Layer IVc alpha in the neonate exhibited a higher level of activity and amount of CO than the central portion of IVc beta, which contained a dense aggregate of small neurons. The base of IVc beta, however, was often as CO reactive as IVc alpha. In contrast, the majority of specimens available to us from the fourth postnatal month and from adults with no known neurological diseases had significantly greater CO reactivity in layer IVc beta than in IVc alpha. Layer VI was moderately reactive for CO throughout development. In V2, stripes with globular zones of high CO activity were sporadically present at birth, suggesting that their development may parallel or precede that of puffs in V1. These stripes with CO-rich globular zones became more prominent in the adult and radiated orthogonally from the V1/V2 border. They were not, however, clearly organized into alternating thick and thin stripes as they are in the squirrel monkey. Visual cortical areas beyond V2 exhibited high CO activity mainly in layers III and IV and moderate levels in VI, suggesting that sites associated with cortico-cortical pathways may be metabolically most active.

A Comparison of Two fMRI Protocols for Eliciting Hippocampal Activation

Summary:  Purpose: Previous research suggests that the hippocampus is modulated both by stimulus novelty and by the extent to which relational processing (formation of associations) occurs during episodic encoding. The aim of this study was to compare hippocampal activation patterns measured by functional magnetic resonance imaging (fMRI) during encoding protocols emphasizing either novelty or relational processing.

Is speech arrest during Wada testing a valid method for determining hemispheric representation of language

BACKGROUND AND OBJECTIVE The intracarotid amobarbital procedure, or Wada test, is the method of choice to determine hemispheric representation of language, and is routinely used in the presurgical evaluation for intractable epilepsy. Some investigators perform comprehensive language assessments, but others base language lateralization solely on speech arrest. This study sought to determine whether speech arrest alone during Wada testing provides valid data regarding language lateralization. METHODS The subjects (previously reported) were 21 patients evaluated for intractable epilepsy, who underwent language lateralization by Wada testing and functional MRI (FMRI). For each patient, language representation was determined by calculating: (1) a Wada laterality index based exclusively on speech arrest; (2) a Wada laterality index based on comprehensive language assessment; and (3) an FMRI laterality quotient. Correlation coefficients and categorical classifications were analyzed. RESULTS There was no significant correlation between the Wada laterality quotient derived from duration of speech arrest and either the comprehensive Wada language laterality score (r =.35, p =.12) or FMRI language laterality score (r =.32, p =.16). Categorical classification as left, right or bilateral language also showed marked discordance between speech arrest and the other two methods. CONCLUSION Duration of speech arrest during Wada testing is not a valid measure of language dominance.

The human primary motor cortex

Reply from the Authors : We appreciate the insightful comments of Drs. Foundas, Hong, Leonard, and Heilman regarding our recent paper. 1 Our response to each is as follows: We measured the distance between the center of mass of the activated region and the midline for both the finger and elbow movements. Consistent with the study by Yousry et a1., 3 the mean distance was 37.7 mm …

Function of the left planum temporale in auditory and linguistic processing

Previous research suggests that the human left planum temporale (PT) plays an important role in language. To test this hypothesis, functional MRI (fMRI) data were collected from 12 normal right-handed subjects during passive and active listening to words and tone sequences. Several left hemisphere areas, including the superior temporal sulcus, middle temporal gyrus, angular gyrus and lateral frontal lobe showed stronger activation during the word conditions. This was not true of the PT, which responded equally to tones and words during passive listening and more strongly to tones during active listening. The PT is likely to be involved in early auditory processing, while specifically linguistic functions are mediated by multimodal association areas distributed elsewhere in the left hemisphere.