Activity In Human Auditory Cortex Research Articles

Activation of human auditory cortex during speech perception: Effects of monaural, binaural, and dichotic presentation

We used a rapid event-related functional magnetic resonance imaging (fMRI) paradigm to compare cortical activation to speech tokens presented monaurally to each ear, binaurally, and dichotically. Two forms of dichotic conditions were examined: one presented consonant-vowel (CV) syllables simultaneously to each ear while the other paired a CV syllable with a non-speech stimulus (band-limited noise). Right-handed adults were asked to differentially respond to serially presented target and distractor CV syllables. Activations were localized with reference to anatomic segmentation algorithms that allowed us to distinguish between activity in primary (PAC) and non-primary auditory cortex (NPAC). Monaural CV syllables presented to the right ear (CVR) produced highly asymmetric activations in left PAC and NPAC. A similar but reduced left hemisphere (LH) bias was evident in binaural presentation, when monaural syllables were paired with contra-aural noise, and in dichotic CV–CV presentations. However, LH activation was two times larger to CVR than any other condition, while RH activation to CVR was insubstantial. By contrast, a small rightward asymmetry in PAC activation was observed from monaural left ear (CVL) presentation. In all conditions except CVL, magnitude of response favored left PAC and NPAC. CV processing across different listening conditions disclosed complex interactions in activation. Our results confirm the superiority of left NPAC in speech processing and suggest comparable left lateralization in PAC. The findings suggest that monaural CV presentation may be more useful than previously anticipated. The paradigm developed here may hold some promise in investigations where abnormal hemispheric balance of speech processing is suspected.

Attentional modulation of human auditory cortex

Attention powerfully influences auditory perception, but little is understood about the mechanisms whereby attention sharpens responses to unattended sounds. We used high-resolution surface mapping techniques (using functional magnetic resonance imaging, fMRI) to examine activity in human auditory cortex during an intermodal selective attention task. Stimulus-dependent activations (SDAs), evoked by unattended sounds during demanding visual tasks, were maximal over mesial auditory cortex. They were tuned to sound frequency and location, and showed rapid adaptation to repeated sounds. Attention-related modulations (ARMs) were isolated as response enhancements that occurred when subjects performed pitch-discrimination tasks. In contrast to SDAs, ARMs were localized to lateral auditory cortex, showed broad frequency and location tuning, and increased in amplitude with sound repetition. The results suggest a functional dichotomy of auditory cortical fields: stimulus-determined mesial fields that faithfully transmit acoustic information, and attentionally labile lateral fields that analyze acoustic features of behaviorally relevant sounds.

Visual short-term memory load affects sensory processing of irrelevant sounds in human auditory cortex

We used whole-head magnetoencephalography (MEG) to investigate neural activity in human auditory cortex elicited by irrelevant tones while the subjects were engaged in a short-term memory task presented in the visual modality. As compared to a no-memory-task condition, memory load enhanced the amplitude of the auditory N1m response. In addition, the N1m amplitude depended on the phase of the memory task, with larger response amplitudes observed during encoding than retention. Further, these amplitude modulations were accompanied by anterior–posterior shifts in N1m source locations. The results show that a memory task for visually presented stimuli alters sensory processing in human auditory cortex, even when subjects are explicitly instructed to ignore any auditory stimuli. Thus, it appears that task demands requiring attentional allocation and short-term memory result in interaction across visual and auditory brain areas carrying out the processing of stimulus features.

The Influence of Stimulus Properties, Complexity, and Contingency on the Stability and Variability of Ongoing and Evoked Activity in Human Auditory Cortex

The real-time, single-trial activity in the human auditory cortex was extracted from magnetoencephalographic signals. A predictor of single-trial activity was defined as the sum of the average response and a mean-free base level computed over a range of base times. For simple stimuli the residual (predicted–actual) activity had a stimulus-independent oscillatory (10 Hz) component. This component was larger and more durable in trained subjects, reaching saturation only in the most trained of the five subjects studied (S1). Changes in variability and associated reduction of the absolute value and duration of the oscillations were evident in experiments with stimuli loaded with information, saliency, or task contingency. Repetition reintroduces stimulus-independent oscillations very slowly. For S1, after training, the stimulus-independent oscillations were reestablished in the auditory cortices to the level seen for simple stimuli, except for the time periods and in the hemisphere associated with the combination of task demands and stimulus processing.

Magnetic localization of neuronal activity in the human brain.

The performance of a cryogenic system that monitors the extracranial magnetic field simultaneously at 14 positions over the scalp has been evaluated to determine the accuracy with which neuronal activity can be located within the human brain. Initially, measurements were implemented on two model systems, a lucite sphere filled with saline and a model skull. With a magnetic field strength similar to that of a human brain, the measurement and analysis procedures demonstrated a position accuracy better than 3 mm, for a current dipole 3 cm beneath the surface. Subsequently, measurements of the magnetic field pattern appearing 100 ms after the onset of an auditory tone-burst stimulus were obtained in three human subjects. The location of the current dipole representing intracellular ionic current in active neurons of the brain was determined, with 3-mm accuracy, to be within the cortex forming the floor of the Sylvian fissure of the individual subjects, corresponding closely to the Heschl gyrus as determined from magnetic resonance images. With the sensors placed at appropriate positions, the locations of neuronal sources for different tone frequencies could be obtained without moving the recording instrument. Adaptation of activity in human auditory cortex was shown to reveal long-term features with a paradigm that compared response amplitudes for three tones randomly presented.