Hans-Jochen Heinze

Phonetic perception and the temporal cortex.

Recent functional neuroimaging studies have emphasized the role of the different areas within the left superior temporal sulcus (STS) for the perception of various speech stimuli. We report here the results of three independent studies additionally demonstrating hemodynamic responses in the vicinity of the planum temporale (PT). In these studies we used consonant-vowel (CV) syllables, tones, white noise, and vowels as acoustic stimuli in the context of whole-head functional magnetic resonance imaging, applying a long TR to attenuate possible masking effects by the scanner noise. To summarize, we obtained the following results for the contrasts comparing hemodynamic responses obtained during the perception of CV syllables compared to tones or white noise: (i) stronger activation in the vicinity of the left PT with two distinct foci of activation, one in a lateral position and the other more medial in the vicinity of Heschls sulcus; (ii) stronger activation in the vicinity of the right PT; and (iii) stronger bilateral activation within the mid-STS. Further contrasts revealed the following findings: (iv) stronger bilateral activation to CV syllables than to vowels in the medial PT, (v) stronger left-sided activation to CV syllables than to vowels in the mid-STS, and (vi) stronger activation to CV syllables with voiceless initial consonants than to CV syllables with voiced initial consonants in the left medial PT. The results are compatible with the hypothesis that the STS contains neurons specialized for speech perception. However, these results also emphasize the role of the PT in the analysis of phonetic features, namely the voice-onset-time. Yet this does not mean that the PT is solely specialized for phonetic analysis. We hypothesize rather that the PT contains neurons specialized for the analysis of rapidly changing cues as was suggested by P. Tallal et al. (1993, Ann. N. Y. Acad. Sci. 682: 27-47).

Delayed Striate Cortical Activation during Spatial Attention

Recordings of event-related potentials (ERPs) and event-related magnetic fields (ERMFs) were combined with functional magnetic resonance imaging (fMRI) to study visual cortical activity in humans during spatial attention. While subjects attended selectively to stimulus arrays in one visual field, fMRI revealed stimulus-related activations in the contralateral primary visual cortex and in multiple extrastriate areas. ERP and ERMF recordings showed that attention did not affect the initial evoked response at 60-90 ms poststimulus that was localized to primary cortex, but a similarly localized late response at 140-250 ms was enhanced to attended stimuli. These findings provide evidence that the primary visual cortex participates in the selective processing of attended stimuli by means of delayed feedback from higher visual-cortical areas.

Brain potential and functional MRI evidence for how to handle two languages with one brain

Bilingual individuals need effective mechanisms to prevent interference from one language while processing material in the other. Here we show, using event-related brain potentials and functional magnetic resonance imaging (fMRI), that words from the non-target language are rejected at an early stage before semantic analysis in bilinguals. Bilingual Spanish/Catalan and monolingual Spanish subjects were instructed to press a button when presented with words in one language, while ignoring words in the other language and pseudowords. The brain potentials of bilingual subjects in response to words of the non-target language were not sensitive to word frequency, indicating that the meaning of non-target words was not accessed in bilinguals. The fMRI activation patterns of bilinguals included a number of areas previously implicated in phonological and pseudoword processing, suggesting that bilinguals use an indirect phonological access route to the lexicon of the target language to avoid interference.

Selective Activation of a Parietofrontal Circuit during Implicitly Imagined Prehension

It is generally held that motor imagery is the internal simulation of movements involving ones own body in the absence of overt execution. Consistent with this hypothesis, results from numerous functional neuroimaging studies indicate that motor imagery activates a large variety of motor-related brain regions. However, it is unclear precisely which of these areas are involved in motor imagery per se as opposed to other planning processes that do not involve movement simulation. In an attempt to resolve this issue, we employed event-related fMRI to separate activations related to hand preparation-a task component that does not demand imagining movements-from grip selection-a component previously shown to require the internal simulation of reaching movements. Our results show that in contrast to preparation of overt actions, preparation of either hand for covert movement simulation activates a large network of motor-related areas located primarily within the left cerebral and right cerebellar hemispheres. By contrast, imagined grip selection activates a distinct parietofrontal circuit that includes the bilateral dorsal premotor cortex, contralateral intraparietal sulcus, and right superior parietal lobule. Because these areas are highly consistent with the frontoparietal reach circuit identified in monkeys, we conclude that motor imagery involves action-specific motor representations computed in parietofrontal circuits.

Attention to features precedes attention to locations in visual search: evidence from electromagnetic brain responses in humans

Single-unit recordings in macaque extrastriate cortex have shown that attentional selection of nonspatial features can operate in a location-independent manner. Here, we investigated analogous neural correlates at the neural population level in human observers by using simultaneous event-related potential (ERP) and event-related magnetic field (ERMF) recordings. The goals were to determine (1) whether task-relevant features are selected before attention is allocated to the location of the target, and (2) whether this selection reflects the locations of the relevant features. A visual search task was used in which the spatial distribution of nontarget items with attended feature values was varied independently of the location of the target. The presence of task-relevant features in a given location led to a change in ERP/ERMF activity beginning ∼140 msec after stimulus onset, with a neural origin in the ventral occipito-temporal cortex. This effect was independent of the location of the actual target. This effect was followed by lateralized activity reflecting the allocation of attention to the location of the target (the well known N2pc component), which began at ∼170 msec poststimulus. Current source localization indicated that the allocation of attention to the location of the target originated in more anterior regions of occipito-temporal cortex anterior than the feature-related effects. These findings suggest that target detection in visual search begins with the detection of task-relevant features, which then allows spatial attention to be allocated to the location of a likely target, which in turn allows the target to be positively identified.

A multivariate, spatiotemporal analysis of electromagnetic time-frequency data of recognition memory

Electromagnetic indices of fast (above 12 Hz) oscillating brain activity are much more likely to be considerably attenuated by time-averaging across multiple trials than slow (below 12 Hz) oscillating brain activity. To the extent that both types of oscillations represent the activity of temporally and topographically separable neural populations, time averaging can cause a loss of brain activity information that is important both conceptually and for multimodal integration with hemodynamic techniques. To address this issue for recognition memory, simultaneous electroencephalography (EEG) and whole-head magnetoencephalography (MEG) recordings of explicit word recognition from 11 healthy subjects were analyzed in two different ways. First, the time course of neural oscillations ranging from theta (4.5 Hz) to gamma (42 Hz) frequencies were identified using single-trial continuous wavelet transforms. Second, traditional analyses of amplitude variations of time-averaged EEG and MEG signals, event-related potentials (ERPs), and fields (ERFs) were performed and submitted to distributed source analyses. To identify data patterns that covaried with the difference between correctly recognized studied (old) words and correctly rejected nonstudied (new) words, a multivariate statistical tool, partial least squares (PLS), was applied to both types of analyses. The results show that ERPs and ERFs are mainly displaying those neural indices of recognition memory that oscillate in the theta (4.5-7.5 Hz), alpha (8-11.5), and to some extent in the beta1 (12-19.5 Hz) frequency range. The sources of the ERPs/ERFs were in good agreement with the topography of theta/alpha/beta 1 oscillations in being confined to the anterior temporal lobe at 400 ms and being distributed across temporal, parietal, and occipital areas between 500 and 700 ms. Gamma oscillations covaried either positively or negatively with theta/alpha/beta1 oscillations. A positive covariance, for instance, was detected over left anterior temporal sensors as early as 200-350 ms and is compatible with studies in rodents showing that gamma and theta oscillations emerge together out of the interaction of the hippocampus and the entorhinal and perirhinal cortices. Fast beta oscillations (20-29.5 Hz), on the other hand, did not strongly covary with slow oscillations and were likely to arise from neural populations not adequately represented in ERPs/ERFs. In summary, by providing a more comprehensive description of electromagnetic signals, time-frequency data are of potential benefit for integrating electrophysiological and hemodynamic indices of brain activity and also for integrating human and animal electrophysiology.

Dynamics of feature binding during object-selective attention

Objects in the environment may be attended selectively and perceived as unified ensembles of their constituent features. To investigate the timing and cortical localization of feature-integration mechanisms in object-based attention, recordings of event-related potentials and magnetic fields were combined with functional MRI while subjects attended to one of two superimposed transparent surfaces formed by arrays of dots moving in opposite directions. A spatiotemporal analysis revealed evidence for a rapid increase in neural activity localized to a color-selective region of the fusiform gyrus when the surface moving in the attended direction displayed an irrelevant color feature. These data provide support for the “integrated-competition” model of object-selective attention and point to a dynamic neural substrate for the rapid binding process that links relevant and irrelevant features to form a unified perceptual object.

Differentiation of idiopathic Parkinson's disease, multiple system atrophy, progressive supranuclear palsy, and healthy controls using magnetization transfer imaging

The differentiation of multiple system atrophy (MSA) and progressive supranuclear palsy (PSP) from idiopathic Parkinsons disease (IPD) is difficult. Magnetization transfer imaging (MTI), a measure that correlates with myelination and axonal density, was employed in this study in the attempt to distinguish between these disorders. Measurements were carried out in 15 patients with IPD, 12 patients with MSA, 10 patients with PSP, and in 20 aged-matched healthy control subjects. The main finding was a change in the magnetization transfer ratio in the globus pallidus, putamen, caudate nucleus, substantia nigra, and white matter in IPD, MSA, and PSP patients, matching the pathological features of the underlying disorder. Furthermore, stepwise linear discriminant analysis provided a good classification of the individual patients into the different disease groups. All IPD patients and control subjects were correctly separated from the MSA and PSP cohort, and all PSP patients and 11 of 12 MSA patients were correctly separated from the IPD and control cohort. There was also a fairly good discrimination of IPD patients from control subjects and of MSA from PSP patients. In conclusion, MTI revealed degenerative changes in patients with different parkinsonian syndromes matching the underlying pathological features of the different diseases, underlining the high potential of this method in distinguishing MSA and PSP from IPD.

Functional magnetic resonance tomography correlates of taste perception in the human primary taste cortex

The present study investigated the functional magnetic resonance tomography correlates of taste perception in the human primary taste cortex. There is conflicting evidence in the literature about chemotopical organization in this brain region. The topography of hemodynamic activity elicited by five taste stimuli (sweet, sour, salty, bitter and umami) was analyzed on the flattened cortical surfaces of six single subjects. A high inter-individual topographical variability had to be noted. The results showed different patterns of hemodynamic activity for the investigated tastes with some considerable overlap. However, the taste specific patterns were stable over time in each subject. Such an individual taste specific pattern was also found for the umami taste within the primary taste cortex of each subject. These results suggest that input from glutamate receptors on the tongue might be processed in an exclusive way in the primary taste cortex rather than as a combination of inputs from the classical taste receptors.

Asymmetric hemodynamic responses of the human auditory cortex to monaural and binaural stimulation

Applying whole-head functional magnetic resonance imaging (fMRI) in 11 neurologically intact subjects, hemodynamic responses to mon- or binaurally presented auditory stimuli were measured. To expand on previous studies in this research area, we used tones and consonant-vowel (CV) syllables. In one group of subjects (n=6) the perceived loudness of the monaurally presented stimuli were adjusted so that they matched the loudness of the binaurally presented stimuli. In a second group (n=5) no loudness adjustment was performed, thus the monaural stimuli were perceived less loud ( approximately 10 dB) than the binaural stimuli. These extensions allowed us to test whether CV syllables and tones produce different contralaterality effects (stronger hemodynamic responses in the auditory cortex contralateral to the stimulated ear) and whether binaural stimulation results in stronger activations in the auditory areas than during both monaural stimulation conditions (binaural summation) independent of loudness influences. In summary, we obtained the following findings: (1) strong contralaterality effects during monaural acoustic stimulation in the posterior superior temporal gyrus (STG) comprising the planum temporale and the dorsal bank of the superior temporal sulcus to CV syllables and tones; (2) the hemodynamic responses to contralaterally presented stimuli (during the monaural conditions) were mostly stronger than those to binaurally presented CV syllables; (3) there was no interaction between stimulus type and the size of the contralaterality effect; (4) there was no indication of binaural summation, rather we found stronger hemodynamic responses to the sum of both monaural stimulations (right and left ear) than to binaural stimulation in all auditory areas; (5) there were generally stronger hemodynamic responses to CV syllables than to tones in the posterior STG, while the hemodynamic responses to tones were stronger in the anterior part of the STG (temporal pole); and finally (6) there was no general difference in terms of hemodynamic response in the auditory cortex between the two groups when receiving either loudness-matched or non-loudness-matched monaural stimulation. These findings are discussed in the context of the underlying neurophysiological mechanisms, the peculiarities of functional fMRI, and the direct access and callosal relay models of hemispheric lateralization.