Unit study of monkey frontal cortex: active localization of auditory and of visual stimuli

Orbitofrontal cortex conveys stimulus and task information to the auditory cortex

Auditory cortical neurons modify their response profiles in response to numerous external factors. During task performance, changes in primary auditory cortex (A1) responses are thought to be driven by top-down inputs from the orbitofrontal cortex (OFC), which may lead to response modification on a trial-by-trial basis. While OFC neurons respond to auditory stimuli and project to A1, the function of OFC projections to A1 during auditory tasks is unknown. Here, we observed the activity of putative OFC terminals in A1 in mice by using in vivo two-photon calcium imaging of OFC terminals under passive conditions and during a tone detection task. We found that behavioral activity modulates but is not necessary to evoke OFC terminal responses in A1. OFC terminals in A1 form distinct populations that exclusively respond to either the tone, reward, or error. Using tones against a background of white noise, we found that OFC terminal activity was modulated by the signal-to-noise ratio (SNR) in both the passive and active conditions and that OFC terminal activity varied with SNR, and thus task difficulty in the active condition. Therefore, OFC projections in A1 are heterogeneous in their modulation of auditory encoding and likely contribute to auditory processing under various auditory conditions.

The neurobiology of vocal communication in marmosets

An increasingly popular animal model for studying the neural basis of social behavior, cognition, and communication is the common marmoset (Callithrix jacchus). Interest in this New World primate across neuroscience is now being driven by their proclivity for prosociality across their repertoire, high volubility, and rapid development, as well as their amenability to naturalistic testing paradigms and freely moving neural recording and imaging technologies. The complement of these characteristics set marmosets up to be a powerful model of the primate social brain in the years to come. Here, we focus on vocal communication because it is the area that has both made the most progress and illustrates the prodigious potential of this species. We review the current state of the field with a focus on the various brain areas and networks involved in vocal perception and production, comparing the findings from marmosets to other animals, including humans.

Auditory cues reveal intended movement information in middle frontal gyrus neuronal ensemble activity of a person with tetraplegia

Intracortical brain-computer interfaces (iBCIs) allow people with paralysis to directly control assistive devices using neural activity associated with the intent to move. Realizing the full potential of iBCIs critically depends on continued progress in understanding how different cortical areas contribute to movement control. Here we present the first comparison between neuronal ensemble recordings from the left middle frontal gyrus (MFG) and precentral gyrus (PCG) of a person with tetraplegia using an iBCI. As expected, PCG was more engaged in selecting and generating intended movements than in earlier perceptual stages of action planning. By contrast, MFG displayed movement-related information during the sensorimotor processing steps preceding the appearance of the action plan in PCG, but only when the actions were instructed using auditory cues. These results describe a previously unreported function for neurons in the human left MFG in auditory processing contributing to motor control.

Computational Architecture of the Parieto-Frontal Network Underlying Cognitive-Motor Control in Monkeys

The statistical structure of intrinsic parietal and parieto-frontal connectivity in monkeys was studied through hierarchical cluster analysis. Based on their inputs, parietal and frontal areas were grouped into different clusters, including a variable number of areas that in most instances occupied contiguous architectonic fields. Connectivity tended to be stronger locally: that is, within areas of the same cluster. Distant frontal and parietal areas were targeted through connections that in most instances were reciprocal and often of different strength. These connections linked parietal and frontal clusters formed by areas sharing basic functional properties. This led to five different medio-laterally oriented pillar domains spanning the entire extent of the parieto-frontal system, in the posterior parietal, anterior parietal, cingulate, frontal, and prefrontal cortex. Different information processing streams could be identified thanks to inter-domain connectivity. These streams encode fast hand reaching and its control, complex visuomotor action spaces, hand grasping, action/intention recognition, oculomotor intention and visual attention, behavioral goals and strategies, and reward and decision value outcome. Most of these streams converge on the cingulate domain, the main hub of the system. All of them are embedded within a larger eye–hand coordination network, from which they can be selectively set in motion by task demands.

Auditory Processing in the Posterior Parietal Cortex

Goal-directed behavior can be characterized as a dynamic link between a sensory stimulus and a motor act. Neural correlates of many of the intermediate events of goal-directed behavior are found in the posterior parietal cortex. Although the parietal cortex’s role in guiding visual behaviors has received considerable attention, relatively little is known about its role in mediating auditory behaviors. Here, the authors review recent studies that have focused on how neurons in the lateral intraparietal area (area LIP) differentially process auditory and visual stimuli. These studies suggest that area LIP contains a modality-dependent representation that is highly dependent on behavioral context.

Similar prevalence and magnitude of auditory-evoked and visually evoked activity in the frontal eye fields: implications for multisensory motor control

Saccadic eye movements can be elicited by more than one type of sensory stimulus. This implies substantial transformations of signals originating in different sense organs as they reach a common motor output pathway. In this study, we compared the prevalence and magnitude of auditory- and visually evoked activity in a structure implicated in oculomotor processing, the primate frontal eye fields (FEF). We recorded from 324 single neurons while 2 monkeys performed delayed saccades to visual or auditory targets. We found that 64% of FEF neurons were active on presentation of auditory targets and 87% were active during auditory-guided saccades, compared with 75 and 84% for visual targets and saccades. As saccade onset approached, the average level of population activity in the FEF became indistinguishable on visual and auditory trials. FEF activity was better correlated with the movement vector than with the target location for both modalities. In summary, the large proportion of auditory-responsive neurons in the FEF, the similarity between visual and auditory activity levels at the time of the saccade, and the strong correlation between the activity and the saccade vector suggest that auditory signals undergo tailoring to match roughly the strength of visual signals present in the FEF, facilitating accessing of a common motor output pathway.

Modulation of Frontoparietal Neurovascular Dynamics in Working Memory

Our perception of the world is represented in widespread, overlapping, and interactive neuronal networks of the cerebral cortex. A majority of physiological studies on the subject have focused on oscillatory synchrony as the binding mechanism for representation and transmission of neural information. Little is known, however, about the stability of that synchrony during prolonged cognitive operations that span more than just a few seconds. The present research, in primates, investigated the dynamic patterns of oscillatory synchrony by two complementary recording methods, surface field potentials (SFPs) and near-infrared spectroscopy (NIRS). The signals were first recorded during the resting state to examine intrinsic functional connectivity. The temporal modulation of coactivation was then examined on both signals during performance of working memory (WM) tasks with long delays (memory retention epochs). In both signals, the peristimulus period exhibited characteristic features in frontal and parietal regions. Examination of SFP signals over delays lasting tens of seconds, however, revealed alternations of synchronization and desynchronization. These alternations occurred within the same frequency bands observed in the peristimulus epoch, without a specific correspondence between any definite cognitive process (e.g., WM) and synchrony within a given frequency band. What emerged instead was a correlation between the degree of SFP signal fragmentation (in time, frequency, and brain space) and the complexity and efficiency of the task being performed. In other words, the incidence and extent of SFP transitions between synchronization and desynchronization-rather than the absolute degree of synchrony-augmented in correct task performance compared with incorrect performance or in a control task without WM demand. An opposite relationship was found in NIRS: increasing task complexity induced more uniform, rather than fragmented, NIRS coactivations. These findings indicate that the particular features of neural oscillations cannot be linearly mapped to cognitive functions. Rather, information and the cognitive operations performed on it are primarily reflected in their modulations over time. The increased complexity and fragmentation of electrical frequencies in WM may reflect the activation of hierarchically diverse cognits (cognitive networks) in that condition. Conversely, the homogeneity in coherence of NIRS responses may reflect the cumulative vascular reactions that accompany that neuroelectrical proliferation of frequencies and the longer time constant of the NIRS signal. These findings are directly relevant to the mechanisms mediating cognitive processes and to physiologically based interpretations of functional brain imaging.

Speech preparation in adults with persistent developmental stuttering

Motor efference copy conveys movement information to sensory areas before and during vocalization. We hypothesized speech preparation would modulate auditory processing, via motor efference copy, differently in men who stutter (MWS) vs. fluent adults. Participants (n=12/group) had EEG recorded during a cue-target paradigm with two conditions: speech which allowed for speech preparation, while a control condition did not. Acoustic stimuli probed auditory responsiveness between the cue and target. MWS had longer vocal reaction times (p<0.01) when the cue-target differed (10% of trials), suggesting a difficulty of rapidly updating their speech plans. Acoustic probes elicited a negative slow wave indexing motor efference copy that was smaller in MWS vs. fluent adults (p<0.03). Current density responses in MWS showed smaller left prefrontal responses and auditory responses that were delayed and correlated to stuttering rate. Taken together, the results provide insight into the cortical mechanisms underlying atypical speech planning and dysfluencies in MWS.

Signs of timing in motor cortex during movement preparation and cue anticipation

The capacity to accurately anticipate the timing of predictable events is essential for sensorimotor behavior. Motor cortex holds an established role in movement preparation and execution. In this chapter we review the different ways in which motor cortical activity is modulated by event timing in sensorimotor delay tasks. During movement preparation, both single neuron and population responses reflect the temporal constraints of the task. Anticipatory modulations prior to sensory cues are also observed in motor cortex when the cue timing is predictable. We propose that the motor cortical activity during cue anticipation and movement preparation is embedded in a timing network that facilitates sensorimotor processing. In this context, the pre-cue and post-cue activity may reflect a presetting mechanism, complementing processing during movement execution, while prohibiting premature responses in situations requiring delayed motor output.

From where to what: a neuroanatomically based evolutionary model of the emergence of speech in humans

In the brain of primates, the auditory cortex connects with the frontal lobe via the temporal pole (auditory ventral stream; AVS) and via the inferior parietal lobule (auditory dorsal stream; ADS). The AVS is responsible for sound recognition, and the ADS for sound-localization, voice detection and audio-visual integration. I propose that the primary role of the ADS in monkeys/apes is the perception and response to contact calls. These calls are exchanged between tribe members (e.g., mother-offspring) and are used for monitoring location. Perception of contact calls occurs by the ADS detecting a voice, localizing it, and verifying that the corresponding face is out of sight. The auditory cortex then projects to parieto-frontal visuospatial regions (visual dorsal stream) for searching the caller, and via a series of frontal lobe-brainstem connections, a contact call is produced in return. Because the human ADS processes also speech production and repetition, I further describe a course for the development of speech in humans. I propose that, due to duplication of a parietal region and its frontal projections, and strengthening of direct frontal-brainstem connections, the ADS converted auditory input directly to vocal regions in the frontal lobe, which endowed early Hominans with partial vocal control. This enabled offspring to modify their contact calls with intonations for signaling different distress levels to their mother. Vocal control could then enable question-answer conversations, by offspring emitting a low-level distress call for inquiring about the safety of objects, and mothers responding with high- or low-level distress calls. Gradually, the ADS and the direct frontal-brainstem connections became more robust and vocal control became more volitional. Eventually, individuals were capable of inventing new words and offspring were capable of inquiring about objects in their environment and learning their names via mimicry.

From where to what: a neuroanatomically based evolutionary model of the emergence of speech in humans

In the brain of primates, the auditory cortex connects with the frontal lobe via the temporal pole (auditory ventral stream; AVS) and via the inferior parietal lobe (auditory dorsal stream; ADS). The AVS is responsible for sound recognition, and the ADS for sound-localization, voice detection and integration of calls with faces. I propose that the primary role of the ADS in non-human primates is the detection and response to contact calls. These calls are exchanged between tribe members (e.g., mother-offspring) and are used for monitoring location. Detection of contact calls occurs by the ADS identifying a voice, localizing it, and verifying that the corresponding face is out of sight. Once a contact call is detected, the primate produces a contact call in return via descending connections from the frontal lobe to a network of limbic and brainstem regions. Because the ADS of present day humans also performs speech production, I further propose an evolutionary course for the transition from contact call exchange to an early form of speech. In accordance with this model, structural changes to the ADS endowed early members of the genus Homo with partial vocal control. This development was beneficial as it enabled offspring to modify their contact calls with intonations for signaling high or low levels of distress to their mother. Eventually, individuals were capable of participating in yes-no question-answer conversations. In these conversations the offspring emitted a low-level distress call for inquiring about the safety of objects (e.g., food), and his/her mother responded with a high- or low-level distress call to signal approval or disapproval of the interaction. Gradually, the ADS and its connections with brainstem motor regions became more robust and vocal control became more volitional. Speech emerged once vocal control was sufficient for inventing novel calls.

From where to what: a neuroanatomically based evolutionary model of the emergence of speech in humans

In the brain of primates, the auditory cortex connects with the frontal lobe via the temporal pole (auditory ventral stream; AVS) and via the inferior parietal lobe (auditory dorsal stream; ADS). The AVS is responsible for sound recognition, and the ADS for sound-localization, voice detection and integration of calls with faces. I propose that the primary role of the ADS in non-human primates is the detection and response to contact calls. These calls are exchanged between tribe members (e.g., mother-offspring) and are used for monitoring location. Detection of contact calls occurs by the ADS identifying a voice, localizing it, and verifying that the corresponding face is out of sight. Once a contact call is detected, the primate produces a contact call in return via descending connections from the frontal lobe to a network of limbic and brainstem regions. Because the ADS of present day humans also performs speech production, I further propose an evolutionary course for the transition from contact call exchange to an early form of speech. In accordance with this model, structural changes to the ADS endowed early members of the genus Homo with partial vocal control. This development was beneficial as it enabled offspring to modify their contact calls with intonations for signaling high or low levels of distress to their mother. Eventually, individuals were capable of participating in yes-no question-answer conversations. In these conversations the offspring emitted a low-level distress call for inquiring about the safety of objects (e.g., food), and his/her mother responded with a high- or low-level distress call to signal approval or disapproval of the interaction. Gradually, the ADS and its connections with brainstem motor regions became more robust and vocal control became more volitional. Speech emerged once vocal control was sufficient for inventing novel calls.

Anatomic organization of the auditory cortex

The auditory cortex is a network of areas in the part of the brain that receives inputs from the subcortical auditory pathways in the brainstem and thalamus. Through an elaborate network of intrinsic and extrinsic connections, the auditory cortex is thought to bring about the conscious perception of sound and provide a basis for the comprehension and production of meaningful utterances. In this chapter, the organization of auditory cortex is described with an emphasis on its anatomic features and the flow of information within the network. These features are then used to introduce key neurophysiologic concepts that are being intensively studied in humans and animal models. The discussion is presented in the context of our working model of the primate auditory cortex and extensions to humans. The material is presented in the context of six underlying principles, which reflect distinct, but related, aspects of anatomic and physiologic organization: (1) the division of auditory cortex into regions; (2) the subdivision of regions into areas; (3) tonotopic organization of areas; (4) thalamocortical connections; (5) serial and parallel organization of connections; and (6) topographic relationships between auditory and auditory-related areas. Although the functional roles of the various components of this network remain poorly defined, a more complete understanding is emerging from ongoing studies that link auditory behavior to its anatomic and physiologic substrates.

Auditory-prefrontal axonal connectivity in the macaque cortex: quantitative assessment of processing streams

Primate sensory systems subserve complex neurocomputational functions. Consequently, these systems are organised anatomically in a distributed fashion, commonly linking areas to form specialised processing streams. Each stream is related to a specific function, as evidenced from studies of the visual cortex, which features rather prominent segregation into spatial and non-spatial domains. It has been hypothesised that other sensory systems, including auditory, are organised in a similar way on the cortical level. Recent studies offer rich qualitative evidence for the dual stream hypothesis. Here we provide a new paradigm to quantitatively uncover these patterns in the auditory system, based on an analysis of multiple anatomical studies using multivariate techniques. As a test case, we also apply our assessment techniques to more ubiquitously-explored visual system. Importantly, the introduced framework opens the possibility for these techniques to be applied to other neural systems featuring a dichotomised organisation, such as language or music perception.

Auditory connections and functions of prefrontal cortex

The functional auditory system extends from the ears to the frontal lobes with successively more complex functions occurring as one ascends the hierarchy of the nervous system. Several areas of the frontal lobe receive afferents from both early and late auditory processing regions within the temporal lobe. Afferents from the early part of the cortical auditory system, the auditory belt cortex, which are presumed to carry information regarding auditory features of sounds, project to only a few prefrontal regions and are most dense in the ventrolateral prefrontal cortex (VLPFC). In contrast, projections from the parabelt and the rostral superior temporal gyrus (STG) most likely convey more complex information and target a larger, widespread region of the prefrontal cortex. Neuronal responses reflect these anatomical projections as some prefrontal neurons exhibit responses to features in acoustic stimuli, while other neurons display task-related responses. For example, recording studies in non-human primates indicate that VLPFC is responsive to complex sounds including vocalizations and that VLPFC neurons in area 12/47 respond to sounds with similar acoustic morphology. In contrast, neuronal responses during auditory working memory involve a wider region of the prefrontal cortex. In humans, the frontal lobe is involved in auditory detection, discrimination, and working memory. Past research suggests that dorsal and ventral subregions of the prefrontal cortex process different types of information with dorsal cortex processing spatial/visual information and ventral cortex processing non-spatial/auditory information. While this is apparent in the non-human primate and in some neuroimaging studies, most research in humans indicates that specific task conditions, stimuli or previous experience may bias the recruitment of specific prefrontal regions, suggesting a more flexible role for the frontal lobe during auditory cognition.

Cortical Mechanisms Underlying the Organization of Goal-Directed Actions and Mirror Neuron-Based Action Understanding

Our understanding of the functions of motor system evolved remarkably in the last 20 years. This is the consequence not only of an increase in the amount of data on this system but especially of a paradigm shift in our conceptualization of it. Motor system is not considered anymore just a “producer” of movements, as it was in the past, but a system crucially involved in cognitive functions. In the present study we review the data on the cortical organization underlying goal-directed actions and action understanding. Our review is subdivided into two major parts. In the first part, we review the anatomical and functional organization of the premotor and parietal areas of monkeys and humans. We show that the parietal and frontal areas form circuits devoted to specific motor functions. We discuss, in particular, the visuo-motor transformation necessary for reaching and for grasping. In the second part we show how a specific neural mechanism, the mirror mechanism, is involved in understanding the action and intention of others. This mechanism is located in the same parieto-frontal circuits that mediate goal-directed actions. We conclude by indicating future directions for studies on the mirror mechanism and suggest some major topics for forthcoming research.

Rapid tuning of auditory "what" and "where" pathways by training

Behavioral improvement within the first hour of training is commonly explained as procedural learning (i.e., strategy changes resulting from task familiarization). However, it may additionally reflect a rapid adjustment of the perceptual and/or attentional system in a goal-directed task. In support of this latter hypothesis, we show feature-specific gains in performance for groups of participants briefly trained to use either a spectral or spatial difference between 2 vowels presented simultaneously during a vowel identification task. In both groups, the neuromagnetic activity measured during the vowel identification task following training revealed source activity in auditory cortices, prefrontal, inferior parietal, and motor areas. More importantly, the contrast between the 2 groups revealed a striking double dissociation in which listeners trained on spectral or spatial cues showed higher source activity in ventral ("what") and dorsal ("where") brain areas, respectively. These feature-specific effects indicate that brief training can implicitly bias top-down processing to a trained acoustic cue and induce a rapid recalibration of the ventral and dorsal auditory streams during speech segregation and identification.

The auditory dorsal stream plays a crucial role in projecting hallucinated voices into external space

INTRODUCTION

Verbal auditory hallucinations (VAHs) are experienced as spoken voices which seem to originate in the extracorporeal environment or inside the head. Animal and human research has identified a 'where' pathway for sound processing comprising the planum temporale, the middle frontal gyrus and the inferior parietal lobule. We hypothesize that increased activity of that 'where' pathway mediates the exteriorization of VAHs.

METHODS

The fMRI scans of 52 right-handed psychotic patients experiencing frequent VAHs were compared with the reported location of hallucinations, as rated with the aid of the PSYRATS-AHRS. For each subject, a unique VAH activation model was created based on the VAH timings, and subsequently convolved with a gamma function to model the hemodynamic response. In order to examine the neurofunctional equivalents of perceived VAH location, second-level group effects of subjects experiencing either internal (n = 24) or external (n = 28) VAHs were contrasted within planum temporale, middle frontal gyrus, and inferior parietal lobule regions of interest (ROIs).

RESULTS

Three ROIs were tested for increased activity in relation with the exteriorization of VAHs. The analysis revealed a left-sided medial planum temporale and a right-sided middle frontal gyrus cluster of increased activity. No significant activity was found in the inferior parietal lobule.

CONCLUSIONS

Our study indicates that internal and external VAHs are mediated by a fronto-temporal pattern of neuronal activity while the exteriorization of VAHs stems from additional brain activity in the auditory 'where' pathway, comprising the planum temporale and prefrontal regions.

On the anticipatory precue activity in motor cortex

Motor cortical neurons are activated during movement preparation and execution, and in response to task-relevant visual cues. A few studies also report activation before the expected presentation of cues. Here, we study specifically this anticipatory activity preceding visual cues in motor cortical areas. We recorded the activity of 1215 neurons in the motor cortex of two macaque monkeys while they performed a center-out reaching task, including two consecutive delays of equal duration, known in advance. During the first delay (D1), they had to await the spatial cue and only reach to the cued target after the second delay (D2). Forty-two percent of the neurons displayed anticipatory activity during D1. Among these anticipatory neurons, 59% increased (D1up) their activity and the remaining decreased (D1down) their activity. By classifying the neurons according to these firing rate profiles during D1, we found that the activity during D2 differed in a systematic way. The D1up neurons were more likely to discharge phasically soon after the spatial cue and were less active during movement execution, whereas the D1down neurons showed the opposite pattern. But, regardless of their temporal activity profiles, the two categories seemed equally involved in early and late motor preparation, as reflected in their directional selectivity. This precue activity in motor cortex may reflect two complementary, coexisting processes: the facilitation of incoming spatial information in parallel with the downregulation of corticospinal excitability to prevent a premature response.

Functional connection between posterior superior temporal gyrus and ventrolateral prefrontal cortex in human

The connection between auditory fields of the temporal lobe and prefrontal cortex has been well characterized in nonhuman primates. Little is known of temporofrontal connectivity in humans, however, due largely to the fact that invasive experimental approaches used so successfully to trace anatomical pathways in laboratory animals cannot be used in humans. Instead, we used a functional tract-tracing method in 12 neurosurgical patients with multicontact electrode arrays chronically implanted over the left (n = 7) or right (n = 5) perisylvian temporal auditory cortex (area PLST) and the ventrolateral prefrontal cortex (VLPFC) of the inferior frontal gyrus (IFG) for diagnosis and treatment of medically intractable epilepsy. Area PLST was identified by the distribution of average auditory-evoked potentials obtained in response to simple and complex sounds. The same sounds evoked little if there is any activity in VLPFC. A single bipolar electrical pulse (0.2 ms, charge-balanced) applied between contacts within physiologically identified PLST resulted in polyphasic evoked potentials clustered in VLPFC, with greatest activation being in pars triangularis of the IFG. The average peak latency of the earliest negative deflection of the evoked potential on VLPFC was 13.48 ms (range: 9.0-18.5 ms), providing evidence for a rapidly conducting pathway between area PLST and VLPFC.

Space representation in the prefrontal cortex

The representation of space and its function in the prefrontal cortex have been examined using a variety of behavioral tasks. Among them, since the delayed-response task requires the temporary maintenance of spatial information, this task has been used to examine the mechanisms of spatial representation. In addition, the concept of working memory to explain prefrontal functions has helped us to understand the nature and functions of space representation in the prefrontal cortex. The detailed analysis of delay-period activity observed in spatial working memory tasks has provided important information for understanding space representation in the prefrontal cortex. Directional delay-period activity has been shown to be a neural correlate of the mechanism for temporarily maintaining information and represent spatial information for the visual cue and the saccade. In addition, many task-related prefrontal neurons exhibit spatially selective activities. These neurons are also important components of spatial information processing. In fact, information flow from sensory-related neurons to motor-related neurons has been demonstrated, along with a change in spatial representation as the trial progresses. The dynamic functional interactions among neurons exhibiting different task-related activities and representing different aspects of information could play an essential role in information processing. In addition, information provided from other cortical or subcortical areas might also be necessary for the representation of space in the prefrontal cortex. To better understand the representation of space and its function in the prefrontal cortex, we need to understand the nature of functional interactions between the prefrontal cortex and other cortical and subcortical areas.

Development of multidimensional representations of task phases in the lateral prefrontal cortex

The temporal structuring of multiple events is essential for the purposeful regulation of behavior. We investigated the role of the lateral prefrontal cortex (LPFC) in transforming external signals of multiple sensory modalities into information suitable for monitoring successive events across behavioral phases until an intended action is prompted and then initiated. We trained monkeys to receive a succession of 1 s visual, auditory, or tactile sensory signals separated by variable intervals and to then release a key as soon as the fourth signal appeared. Thus, the animals had to monitor and update information about the progress of the task upon receiving each signal preceding the key release in response to the fourth signal. We found that the initial, short-latency responses of LPFC neurons reflected primarily the sensory modality, rather than the phase or progress of the task. However, a task phase-selective response developed within 500 ms of signal reception, and information about the task phase was maintained throughout the presentation of successive cues. The task phase-selective activity was updated with the appearance of each cue until the planned action was initiated. The phase-selective activity of individual neurons reflected not merely a particular phase of the task but also multiple successive phases. Furthermore, we found combined representations of task phase and sensory modality in the activity of individual LPFC neurons. These properties suggest how information representing multiple phases of behavioral events develops in the LPFC to provide a basis for the temporal regulation of behavior.

Effect of attention on cortical processing of sound motion: an EEG study

The onset of motion in an otherwise continuous sound elicits a prominent auditory evoked potential, the so-called motion onset response (MOR). The MOR has recently been shown to be modulated by stimulus-dependent factors, such as velocity, while the possible role of task-dependent factors has remained unclear. Here, the effect of spatial attention on the MOR was investigated in 19 listeners. In each trial, the subject initially heard a free-field sound, consisting of a stationary period and a subsequent period of motion. Then, two successive stationary test tones were presented that differed in location and pitch. Subjects either judged whether or not the starting and final positions of the preceded motion matched the positions of the two test tones ('motion-focused condition'), or whether or not the test tones were identical in pitch, irrespective of the preceded motion stimulus ('baseline condition'). These two tasks were presented in separate experimental blocks. The performance level in both tasks was similar. However, especially later portions of the MOR were significantly increased in amplitude when auditory motion was task-relevant. Cortical source localization indicated that this extra activation originated in dorsofrontal areas that have been proposed to be part of the dorsal auditory processing stream. These results support the assumption that auditory motion processing is based on a complex interaction of both stimulus-specific and attentional processes.

Cortex and memory: emergence of a new paradigm

Converging evidence from humans and nonhuman primates is obliging us to abandon conventional models in favor of a radically different, distributed-network paradigm of cortical memory. Central to the new paradigm is the concept of memory network or cognit--that is, a memory or an item of knowledge defined by a pattern of connections between neuron populations associated by experience. Cognits are hierarchically organized in terms of semantic abstraction and complexity. Complex cognits link neurons in noncontiguous cortical areas of prefrontal and posterior association cortex. Cognits overlap and interconnect profusely, even across hierarchical levels (heterarchically), whereby a neuron can be part of many memory networks and thus many memories or items of knowledge.

Posterior cingulate and retrosplenial cortex connections of the caudal superior temporal region in the rhesus monkey

The rostral part of the superior temporal gyrus (STG) is known to project to ventral temporal cortex, but analogous paralimbic connections of the caudal STG have received comparatively less attention. The present study of the connections of the STG with medial paralimbic cortex showed that the caudal part of the STG (area Tpt and caudal area paAlt) and adjacent cortex of the upper bank of the superior temporal sulcus (caudal area TPO) have reciprocal connections with the caudal cingulate gyrus (areas 23a, b and c), retrosplenial cortex (area 30), and area 31. By contrast, cortex of the rostral-to-mid STG (areas Ts2, Ts3, and the rostral part of area paAlt) and adjacent upper bank of the STS (mid-area TPO) have few, if any, such interconnections. It is suggested that this connectional pattern of the caudal STG is consistent with its putative role of localizing sounds in space as proposed in recent studies.

The primate cortical auditory system and neural representation of conspecific vocalizations

Over the past decade, renewed interest in the auditory system has resulted in a surge of anatomical and physiological research in the primate auditory cortex and its targets. Anatomical studies have delineated multiple areas in and around primary auditory cortex and demonstrated connectivity among these areas, as well as between these areas and the rest of the cortex, including prefrontal cortex. Physiological recordings of auditory neurons have found that species-specific vocalizations are useful in probing the selectivity and potential functions of acoustic neurons. A number of cortical regions contain neurons that are robustly responsive to vocalizations, and some auditory responsive neurons show more selectivity for vocalizations than for other complex sounds. Demonstration of selectivity for vocalizations has prompted the question of which features are encoded by higher-order auditory neurons. Results based on detailed studies of the structure of these vocalizations, as well as the tuning and information-coding properties of neurons sensitive to these vocalizations, have begun to provide answers to this question. In future studies, these and other methods may help to define the way in which cells, ensembles, and brain regions process communication sounds. Moreover, the discovery that several nonprimary auditory cortical regions may be multisensory and responsive to vocalizations with corresponding facial gestures may change the way in which we view the processing of communication information by the auditory system.

The extreme capsule in humans and rethinking of the language circuitry

Experimental and imaging studies in monkeys have outlined various long association fiber pathways within the fronto-temporo-parietal region. In the present study, the trajectory of the extreme capsule (EmC) fibers has been delineated in five human subjects using DT-MRI tractography. The EmC seems to be a long association fiber pathway, which courses between the inferior frontal region and the superior temporal gyrus extending into the inferior parietal lobule. Comparison of EmC fibers with the adjacent association fiber pathway, the middle longitudinal fascicle (MdLF), in the same subjects reveals that EmC is located in a medial and rostral position relative to MdLF flanking in part the medial wall of the insula. The EmC can also be differentiated from other neighboring fiber pathways such as the external capsule, uncinate fascicle, arcuate fascicle, superior longitudinal fascicles II and III, and the inferior longitudinal fascicle. Given the location of EmC within the language zone, specifically Broca's area in the frontal lobe, and Wernicke's area in the temporal lobe and inferior parietal lobule, it is suggested that the extreme capsule could have a role in language function.

Processing of sound location in human cortex

This functional magnetic resonance imaging study was focused on the neural substrates underlying human auditory space perception. In order to present natural-like sound locations to the subjects, acoustic stimuli convolved with individual head-related transfer functions were used. Activation foci, as revealed by analyses of contrasts and interactions between sound locations, formed a complex network, including anterior and posterior regions of temporal lobe, posterior parietal cortex, dorsolateral prefrontal cortex and inferior frontal cortex. The distinct topography of this network was the result of different patterns of activation and deactivation, depending on sound location, in the respective voxels. These patterns suggested different levels of complexity in processing of auditory spatial information, starting with simple left/right discrimination in the regions surrounding the primary auditory cortex, while the integration of information on hemispace and eccentricity of sound may take place at later stages. Activations were identified as being located in regions assigned to both the dorsal and ventral auditory cortical streams, that are assumed to be preferably concerned with analysis of spatial and non-spatial sound features, respectively. The finding of activations also in the ventral stream could, on the one hand, reflect the well-known functional duality of auditory spectral analysis, that is, the concurrent extraction of information based on location (due to the spectrotemporal distortions caused by head and pinnae) and spectral characteristics of a sound source. On the other hand, this result may suggest the existence of shared neural networks, performing analyses of auditory 'higher-order' cues for both localization and identification of sound sources.

Reflection of the effectiveness of heterosensory integration in measures of event-linked potentials

The phenomenon of bimodal event-linked potential mismatch and summation of unimodal responses were used to study the relationship between mismatch amplitude and the effectiveness of visual-auditory integration. The effectiveness of integration was assessed in terms of the extent to which experimental measures approached the parameters of the "ideal observer." The greater the effectiveness of integration, the greater the focus of mismatch shifted from the occipital to the frontal areas of the cortex. The greatest correlation between mismatch amplitude and the effectiveness of visual-auditory integration was seen in the left frontal cortex.

Auditory-motor and cognitive aspects in area 8B of macaque monkey's frontal cortex: a premotor ear-eye field (PEEF)

In previous reports, we showed the involvement of area 8B neurons in both spontaneous ear and eye movement and in auditory information processing. Audition-related cells responded to complex environmental stimuli, but not to pure tones, and their activity changed during visual fixation as a possible inhibitory expression of the engagement of attention. We observed auditory, auditory-motor and motor cells for both eye and ear movements. This finding suggests that area 8B may be involved in the integration of auditory input with ear and eye motor output. In this paper, we extended these previous studies by examining area 8B activity in relation to auditive orienting behaviour, as well as the ocular orientation (i.e., visual fixation) studied previously. Visual fixation led to inhibition of activity in auditory and auditory-motor cells, which suggests that attention may be involved in both, maintaining the eye position and reducing the response of these cell types. Accordingly, during a given task or natural behaviour, spatial attention seems to affect more than one sensorimotor channel simultaneously. These data add to our understanding of how the neural network, through a two-channel attentive process, accomplishes to switch between two effectors, namely eyes and ears. Considering the functional, anatomical and cytoarchitectonic differences among the frontal eye field (FEF), the supplementary eye field (SEF) and area 8B, we propose to consider area 8B as a separate premotor ear-eye field (PEEF).

Domain-dependent activation during spatial and nonspatial auditory working memory

Visual system has been proposed to be divided into two, the ventral and dorsal, processing streams. The ventral pathway is thought to be involved in object identification whereas the dorsal pathway processes information regarding the spatial locations of objects and the spatial relationships among objects. Several studies on working memory (WM) processing have further suggested that there is a dissociable domain-dependent functional organization within the prefrontal cortex for processing of spatial and nonspatial visual information. Also the auditory system is proposed to be organized into two domain-specific processing streams, similar to that seen in the visual system. Recent studies on auditory WM have further suggested that maintenance of nonspatial and spatial auditory information activates a distributed neural network including temporal, parietal, and frontal regions but the magnitude of activation within these activated areas shows a different functional topography depending on the type of information being maintained. The dorsal prefrontal cortex, specifically an area of the superior frontal sulcus (SFS), has been shown to exhibit greater activity for spatial than for nonspatial auditory tasks. Conversely, ventral frontal regions have been shown to be more recruited by nonspatial than by spatial auditory tasks. It has also been shown that the magnitude of this dissociation is dependent on the cognitive operations required during WM processing. Moreover, there is evidence that within the nonspatial domain in the ventral prefrontal cortex, there is an across-modality dissociation during maintenance of visual and auditory information. Taken together, human neuroimaging results on both visual and auditory sensory systems support the idea that the prefrontal cortex is organized according to the type of information being maintained in WM.

Non-sensory cortical and subcortical connections of the primary auditory cortex in Mongolian gerbils: bottom-up and top-down processing of neuronal information via field AI

In the present study, we will provide further anatomical evidence that the primary auditory cortex (field AI) is not only involved in sensory processing of its own modality, but also in complex bottom-up and top-down processing of multimodal information. We have recently shown that AI in the Mongolian gerbil (Meriones unguiculatus) has substantial connections with non-auditory sensory and multisensory brain structures [Budinger, E., Heil, P., Hess, A., Scheich, H., 2006. Multisensory processing via early cortical stages: Connections of the primary auditory cortical field with other sensory systems. Neuroscience 143, 1065-1083]. Here we will report about the direct connections of AI with non-sensory cortical areas and subcortical structures. We approached this issue by means of the axonal transport of the sensitive bidirectional neuronal tracers fluorescein-labelled (FD) and tetramethylrhodamine-labelled dextran (TMRD), which were simultaneously injected into different frequency regions of the gerbil's AI. Of the total number of retrogradely labelled cell bodies found in non-sensory brain areas, which identify cells of origin of direct projections to AI, approximately 24% were in cortical areas and 76% in subcortical structures. Of the cell bodies in the cortical areas, about 4.4% were located in the orbital, 11.1% in the infralimbic medial prefrontal (areas DPC, IL), 18.2% in the cingulate (3.2% in CG1, 2.9% in CG2, 12.1% in CG3), 9.5% in the frontal association (area Fr2), 12.0% in the insular (areas AI, DI), 10.8% in the retrosplenial, and 34.0% in the perirhinal cortex. The cortical regions with retrogradely labelled cells, as well as the entorhinal cortex, also contained anterogradely labelled axons and their terminations, which means that they are also target areas of direct projections from AI. The laminar pattern of corticocortical connections indicates that AI receives primarily cortical feedback-type inputs and projects in a feedforward manner to its target areas. The high number of double-labelled somata, the non-topographic distribution of single FD- and TMRD-labelled somata, and the overlapping spatial distribution of FD- and TMRD-labelled axonal elements suggest rather non-tonotopic connections between AI and the multimodal cortices. Of the labelled cell bodies in the subcortical structures, about 38.8% were located in the ipsilateral basal forebrain (10.6% in the lateral amygdala LA, 11.5% in the globus pallidus GP, 3.7% in the ventral pallidum VPa, 13.0% in the nucleus basalis NB), 13.1% in the ipsi- and contralateral diencephalon (6.4% in the posterior paraventricular thalamic nuclei, 6.7% in the hypothalamic area), and 48.1% in the midbrain (20.0% in the ipsilateral substantia nigra, 9.8% in the ipsi- and contralateral ventral tegmental area, 5.0% in the ipsi- and contralateral locus coeruleus, 13.3% the ipsi- and contralateral dorsal raphe nuclei). Thus, the majority of subcortical inputs to AI was related to different neurotransmitter systems. Anterograde labelling was only found in some ipsilateral basal forebrain structures, namely, the LA, basolateral amygdala, GP, VPa, and NB. As for the cortex, the proportion and spatial distribution of single FD-, TMRD-, and double-labelled neuronal elements suggests rather non-tonotopic connections between AI and the neuromodulatory subcortical structures.

Orienting auditory spatial attention engages frontal eye fields and medial occipital cortex in congenitally blind humans

What happens in vision-related cortical areas when congenitally blind (CB) individuals orient attention to spatial locations? Previous neuroimaging of sighted individuals has found overlapping activation in a network of frontoparietal areas including frontal eye fields (FEF), during both overt (with eye movement) and covert (without eye movement) shifts of spatial attention. Since voluntary eye movement planning seems irrelevant in CB, their FEF neurons should be recruited for alternative functions if their attentional role in sighted individuals is only due to eye movement planning. Recent neuroimaging of the blind has also reported activation in medial occipital areas, normally associated with visual processing, during a diverse set of non-visual tasks, but their response to attentional shifts remains poorly understood. Here, we used event-related fMRI to explore FEF and medial occipital areas in CB individuals and sighted controls with eyes closed (SC) performing a covert attention orienting task with endogenous verbal cues and spatialized auditory targets. We found robust stimulus-locked FEF activation of all CB subjects, similar to and stronger than in SC, suggesting that FEF plays a role in endogenous orienting of covert spatial attention even in individuals in whom voluntary eye movements are irrelevant. We also found robust activation in bilateral medial occipital cortex in CB but not in SC subjects. The response decreased below baseline following endogenous verbal cues but increased following auditory targets, suggesting that the medial occipital area in CB does not directly engage during cued orienting of attention but may be recruited for processing of spatialized auditory targets.

Processing of auditory and visual location information in the monkey prefrontal cortex

Visuospatial working memory mechanisms have been studied extensively at single cell level in the dorsolateral prefrontal cortex (PFCd) in nonhuman primates. Despite the importance of short-term memory of sound location for behavioral orientation, there are only a few studies on auditory spatial working memory. The purpose of this study was to investigate neuronal mechanisms underlying working memory processing of auditory and visual location information at single cell level in the PFCd. Neuronal activity was recorded in monkeys performing a delayed matching-to-sample task (DMTS). The location of a visual or auditory stimulus was used as a memorandum. The majority of the neurons that were activated during presentation of the cue memorandum were selective either for visual or auditory spatial information. A small group of cue related bimodal neurons were sensitive to the location of the cue regardless of whether the stimulus was visual or auditory, suggesting modality independent processing of spatial information at cellular level in the PFCd. Most neurons that were activated during the delay period were modality specific, responding either during visual or auditory trials. All bimodal delay related neurons that responded during both visual and auditory trials were spatially nonselective. The results of the present study suggest that in addition to the modality specific parallel mechanism, working memory of auditory and visual space also involves modality independent processing at cellular level in the PFCd.

Human brain activity associated with audiovisual perception and attention

Coherent perception of objects in our environment often requires perceptual integration of auditory and visual information. Recent behavioral data suggest that audiovisual integration depends on attention. The current study investigated the neural basis of audiovisual integration using 3-Tesla functional magnetic resonance imaging (fMRI) in 12 healthy volunteers during attention to auditory or visual features, or audiovisual feature combinations of abstract stimuli (simultaneous harmonic sounds and colored circles). Audiovisual attention was found to modulate activity in the same frontal, temporal, parietal and occipital cortical regions as auditory and visual attention. In addition, attention to audiovisual feature combinations produced stronger activity in the superior temporal cortices than attention to only auditory or visual features. These modality-specific areas might be involved in attention-dependent perceptual binding of synchronous auditory and visual events into coherent audiovisual objects. Furthermore, the modality-specific temporal auditory and occipital visual cortical areas showed attention-related modulations during both auditory and visual attention tasks. This result supports the proposal that attention to stimuli in one modality can spread to encompass synchronously presented stimuli in another modality.

Multisensory integration for timing engages different brain networks

How does the brain integrate information from different senses into a unitary percept? What factors influence such multisensory integration? Using a rhythmic behavioral paradigm and functional magnetic resonance imaging, we identified networks of brain regions for perceptions of physically synchronous and asynchronous auditory-visual events. Measures of behavioral performance revealed the existence of three distinct perceptual states. Perception of asynchrony activated a network of the primary sensory, prefrontal, and inferior parietal cortices, perception of synchrony disengaged the inferior parietal cortex and further recruited the superior colliculus, and when no clear percept was established, only the residual areas comprised of prefrontal and sensory areas were active. These results indicate that distinct percepts arise within specific brain sub-networks, the components of which are differentially engaged and disengaged depending on the timing of environmental signals.

Information processing in human parieto-frontal circuits during goal-directed bimanual movements

It is known that, in macaques, movements guided by somatosensory information engage anterior parietal and posterior precentral regions. Movements performed with both visual and somatosensory feedback additionally activate posterior parietal and anterior precentral areas. It remains unclear whether the human parieto-frontal circuits exhibit a similar functional organization. Here, we employed a directional interference task requiring a continuous update of sensory information for the on-line control of movement direction, while brain activity was measured by functional magnetic resonance imaging (fMRI). Directional interference arises when bimanual movements occur along different directions in joint space. Under these circumstances, the presence of visual information does not substantially alter performance, such that we could vary the amount and type of sensory information used during on-line guidance of goal-directed movements without affecting motor output. Our results confirmed that in humans, as in macaques, movements guided by somatosensory information engages anterior parietal and posterior precentral regions, while movements performed with both visual and somatosensory information activate posterior parietal and anterior precentral areas. We provide novel evidence on how the interaction of specific portions of the dorsal parietal and precentral cortex in the right hemisphere might generate spatial representations by integrating different sensory modalities during goal-directed movements.

Brain activity during visuomotor behavior triggered by arbitrary and spatially constrained cues: an fMRI study in humans

Rule-based behavior associating nonspatial visual stimuli with learned responses is called arbitrary visuomotor mapping, an ability that enriches behavioral repertoire. To better understand the underlying neural correlates, the present functional magnetic resonance imaging (fMRI) study explored brain activity during visually informed movement involving two different types of cues and two different effectors. After being trained on the tasks, six healthy subjects performed right or left finger tapping tasks according to either arbitrary cues or spatially constrained cues. An event-related fMRI experiment was conducted on a 3-T MRI. The image data were analyzed with statistical parametric mapping. With the aid of the probabilistic architectonic map in the stereotaxic space, we identified three types of task-related brain activity: cue-selective, effector-selective, and nonselective. The left ventrolateral prefrontal cortex and the rostral part of the right dorsal premotor cortex (PMd) exhibited cue-selective activity, which was greater during the arbitrary condition than the spatially constrained condition. The left ventral prefrontal activity may reflect retrieval of visuomotor association from memory in arbitrary context. The rostral part of the left PMd showed nonselective activity while the caudal part of the PMd on each side showed conspicuous effector-selective activity to the contralateral movement. These findings suggest functional demarcation of the PMd between its rostral and dorsal parts during visuomotor mapping.

Auditory environmental cells and visual fixation effect in area 8B of macaque monkey

Area 8B may be treated as part of either the prefrontal cortex or the premotor cortex. Previous investigations showed an involvement of area 8B in both eye and ear motor control and in auditory perception. In this report, we studied 139 neurons in three macaque monkeys of these, 32 neurons showed an activity related to environmental auditory stimuli. Fifteen cells with auditory characteristics (15/32) presented a firing discharge inhibited during the execution of visual fixation. The remaining 107 units presented complex or indefinable behaviour. The presence of auditory environmental cells which activity is related prevalently to the voice of persons (researchers) suggests that area 8B may be an area involved in auditory cross-modal association, in natural behaviour. The inhibitory effects during visual fixation suggest that area 8B is part of the inhibitory network preventing the gaze shift in relation to an auditory stimulus. This may be the consequence of the engagement of attention during fixation that may affect the auditory perception. Both aspects indicate that area 8B is involved in high cognitive processes in auditory and orienting processes.

Cortical Connections of the Inferior Parietal Cortical Convexity of the Macaque Monkey

We traced the cortical connections of the 4 cytoarchitectonic fields--Opt, PG, PFG, PF--forming the cortical convexity of the macaque inferior parietal lobule (IPL). Each of these fields displayed markedly distinct sets of connections. Although Opt and PG are both targets of dorsal visual stream and temporal visual areas, PG is also target of somatosensory and auditory areas. Primary parietal and frontal connections of Opt include area PGm and eye-related areas. In contrast, major parietal and frontal connections of PG include IPL, caudal superior parietal lobule (SPL), and agranular frontal arm-related areas. PFG is target of somatosensory areas and also of the medial superior temporal area (MST) and temporal visual areas and is connected with IPL, rostral SPL, and ventral premotor arm- and face-related areas. Finally, PF is primarily connected with somatosensory areas and with parietal and frontal face- and arm-related areas. The present data challenge the bipartite subdivision of the IPL convexity into a caudal and a rostral area (7a and 7b, respectively) and provide a new anatomical frame of reference of the macaque IPL convexity that advances our present knowledge on the functional organization of this cortical sector, giving new insight into its possible role in space perception and motor control.

Sounds do-able: auditory-motor transformations and the posterior temporal plane

Accumulating evidence in humans and non-human primates implicates the posterior superior temporal plane (STP) in the processing of both auditory spatial information and vocal sounds. Such evidence is difficult to reconcile with existing accounts of the primate auditory brain. We propose that the posteromedial STP generates sequenced auditory representations by matching incoming auditory information with stored templates. These sequenced auditory representations are subsequently used to constrain motor responses. We argue for a re-assessment of the much-debated dorsal auditory pathway in terms of its generic behavioral role as an auditory "do" pathway.

The role of dorsal premotor area in reaction task: comparing the "virtual lesion" effect of paired pulse or theta burst transcranial magnetic stimulation

We compared the effect on reaction times of transient interference with function of the dorsal premotor cortex (PMd) using a pair (25-ms interval) of transcranial magnetic stimulation (TMS) pulses with long-term interference produced by a new repetitive TMS paradigm known as "theta burst stimulation" (TBS). Pairs of TMS pulses over left PMd increased choice but not simple reaction times of the right hand if given at the onset of the reaction interval. There was no effect of stimulation over right PMd or at a midline parietal control site (Pz). In contrast, TBS over either left or right PMd increased choice RTs of both hands for at least 5-10 min after the end of TBS. Pairs of TMS pulses over left PMd also increased error rates whereas TBS had no effect on error rates despite the effect on RTs. We suggest that TBS leads to widespread changes in activity and more complex effects on behaviour than expected from the paired pulse TMS and conclude that transient and long-term forms of interference with function may influence behavioural tasks in subtly different ways.