Electrophysiologic studies of the auditory cortex in the awake monkey

Neuronal activity in primate auditory cortex during the performance of audiovisual tasks

This study aimed at a deeper understanding of which cognitive and motivational aspects of tasks affect auditory cortical activity. To this end we trained two macaque monkeys to perform two different tasks on the same audiovisual stimulus and to do this with two different sizes of water rewards. The monkeys had to touch a bar after a tone had been turned on together with an LED, and to hold the bar until either the tone (auditory task) or the LED (visual task) was turned off. In 399 multiunits recorded from core fields of auditory cortex we confirmed that during task engagement neurons responded to auditory and non-auditory stimuli that were task-relevant, such as light and water. We also confirmed that firing rates slowly increased or decreased for several seconds during various phases of the tasks. Responses to non-auditory stimuli and slow firing changes were observed during both the auditory and the visual task, with some differences between them. There was also a weak task-dependent modulation of the responses to auditory stimuli. In contrast to these cognitive aspects, motivational aspects of the tasks were not reflected in the firing, except during delivery of the water reward. In conclusion, the present study supports our previous proposal that there are two response types in the auditory cortex that represent the timing and the type of auditory and non-auditory elements of a auditory tasks as well the association between elements.

Behavioral dependence of auditory cortical responses

Neural responses in the auditory cortex have historically been measured from either anesthetized or awake but non-behaving animals. A growing body of work has begun to focus instead on recording from auditory cortex of animals actively engaged in behavior tasks. These studies have shown that auditory cortical responses are dependent upon the behavioral state of the animal. The longer ascending subcortical pathway of the auditory system and unique characteristics of auditory processing suggest that such dependencies may have a more profound influence on cortical processing in the auditory system compared to other sensory systems. It is important to understand the nature of these dependencies and their functional implications. In this article, we review the literature on this topic pertaining to cortical processing of sounds.

Does attention play a role in dynamic receptive field adaptation to changing acoustic salience in A1?

Acoustic filter properties of A1 neurons can dynamically adapt to stimulus statistics, classical conditioning, instrumental learning and the changing auditory attentional focus. We have recently developed an experimental paradigm that allows us to view cortical receptive field plasticity on-line as the animal meets different behavioral challenges by attending to salient acoustic cues and changing its cortical filters to enhance performance. We propose that attention is the key trigger that initiates a cascade of events leading to the dynamic receptive field changes that we observe. In our paradigm, ferrets were initially trained, using conditioned avoidance training techniques, to discriminate between background noise stimuli (temporally orthogonal ripple combinations) and foreground tonal target stimuli. They learned to generalize the task for a wide variety of distinct background and foreground target stimuli. We recorded cortical activity in the awake behaving animal and computed on-line spectrotemporal receptive fields (STRFs) of single neurons in A1. We observed clear, predictable task-related changes in STRF shape while the animal performed spectral tasks (including single tone and multi-tone detection, and two-tone discrimination) with different tonal targets. A different set of task-related changes occurred when the animal performed temporal tasks (including gap detection and click-rate discrimination). Distinctive cortical STRF changes may constitute a "task-specific signature". These spectral and temporal changes in cortical filters occur quite rapidly, within 2min of task onset, and fade just as quickly after task completion, or in some cases, persisted for hours. The same cell could multiplex by differentially changing its receptive field in different task conditions. On-line dynamic task-related changes, as well as persistent plastic changes, were observed at a single-unit, multi-unit and population level. Auditory attention is likely to be pivotal in mediating these task-related changes since the magnitude of STRF changes correlated with behavioral performance on tasks with novel targets. Overall, these results suggest the presence of an attention-triggered plasticity algorithm in A1 that can swiftly change STRF shape by transforming receptive fields to enhance figure/ground separation, by using a contrast matched filter to filter out the background, while simultaneously enhancing the salient acoustic target in the foreground. These results favor the view of a nimble, dynamic, attentive and adaptive brain that can quickly reshape its sensory filter properties and sensori-motor links on a moment-to-moment basis, depending upon the current challenges the animal faces. In this review, we summarize our results in the context of a broader survey of the field of auditory attention, and then consider neuronal networks that could give rise to this phenomenon of attention-driven receptive field plasticity in A1.

Toward the mechanisms of auditory attention

Since the earliest studies of auditory cortex, it has been clear that an animal's behavioral or attentional state can play a crucial role in shaping the response characteristics of single neurons. Much of what has been learned about attention has been made using human and animal models, but little is known about the cellular and synaptic mechanisms by which attentional modulation of neuronal responses occurs. The use of rodent experimental models allows us to exploit the full armamentarium of modern cellular and molecular neuroscience techniques. Here we present our program for studying auditory attention, specifically for development of rodent models of attention and finding the neural correlates of attention.

Auditory cortical responses elicited in awake primates by random spectrum stimuli

Contrary to findings in subcortical auditory nuclei, auditory cortex neurons have traditionally been described as spiking only at the onsets of simple sounds such as pure tones or bandpass noise and to acoustic transients in complex sounds. Furthermore, primary auditory cortex (A1) has traditionally been described as mostly tone responsive and the lateral belt area of primates as mostly noise responsive. The present study was designed to unify the study of these two cortical areas using random spectrum stimuli (RSS), a new class of parametric, wideband, stationary acoustic stimuli. We found that 60% of all neurons encountered in A1 and the lateral belt of awake marmoset monkeys (Callithrix jacchus) showed significant changes in firing rates in response to RSS. Of these, 89% showed sustained spiking in response to one or more individual RSS, a substantially greater percentage than would be expected from traditional studies, indicating that RSS are well suited for studying these two cortical areas. When firing rates elicited by RSS were used to construct linear estimates of frequency tuning for these sustained responders, the shape of the estimate function remained relatively constant throughout the stimulus interval and across the stimulus properties of mean sound level, spectral density, and spectral contrast. This finding indicates that frequency tuning computed from RSS reflects a robust estimate of the actual tuning of a neuron. Use of this estimate to predict rate responses to other RSS, however, yielded poor results, implying that auditory cortex neurons integrate information across frequency nonlinearly. No systematic difference in prediction quality between A1 and the lateral belt could be detected.

Central auditory onset responses, and temporal asymmetries in auditory perception

Historically, central auditory responses have been studied for their sensitivity to various parameters of tone and noise burst stimulation, with response rate plotted as a function of the stimulus variable. The responses themselves are often quite brief, and locked in time to stimulus onset. In the stimulus amplitude domain, it has recently become clear that these responses are actually driven by properties of the stimulus' onset transient, and this has had important implications for how we interpret responses to manipulations of tone (or noise) burst plateau level. That finding was important in its own right, but a more general scrutiny of the available neurophysiological and psychophysical evidence reveals that there is a significant asymmetry in the neurophysiological and perceptual processing of stimulus onsets and offsets: sound onsets have a more elaborate neurophysiological representation, and receive a greater perceptual weighting. Hypotheses about origins of the asymmetries, derived independently from psychophysics and from neurophysiology, have in common a response threshold mechanism which adaptively tracks the ongoing level of stimulation.

Spatial processing in the auditory cortex of the macaque monkey

The patterns of cortico-cortical and cortico-thalamic connections of auditory cortical areas in the rhesus monkey have led to the hypothesis that acoustic information is processed in series and in parallel in the primate auditory cortex. Recent physiological experiments in the behaving monkey indicate that the response properties of neurons in different cortical areas are both functionally distinct from each other, which is indicative of parallel processing, and functionally similar to each other, which is indicative of serial processing. Thus, auditory cortical processing may be similar to the serial and parallel "what" and "where" processing by the primate visual cortex. If "where" information is serially processed in the primate auditory cortex, neurons in cortical areas along this pathway should have progressively better spatial tuning properties. This prediction is supported by recent experiments that have shown that neurons in the caudomedial field have better spatial tuning properties than neurons in the primary auditory cortex. Neurons in the caudomedial field are also better than primary auditory cortex neurons at predicting the sound localization ability across different stimulus frequencies and bandwidths in both azimuth and elevation. These data support the hypothesis that the primate auditory cortex processes acoustic information in a serial and parallel manner and suggest that this may be a general cortical mechanism for sensory perception.

Correlation between the activity of single auditory cortical neurons and sound-localization behavior in the macaque monkey

Lesion studies have indicated that the auditory cortex is crucial for the perception of acoustic space, yet it remains unclear how these neurons participate in this perception. To investigate this, we studied the responses of single neurons in the primary auditory cortex (AI) and the caudomedial field (CM) of two monkeys while they performed a sound-localization task. Regression analysis indicated that the responses of approximately 80% of neurons in both cortical areas were significantly correlated with the azimuth or elevation of the stimulus, or both, which we term "spatially sensitive." The proportion of spatially sensitive neurons was greater for stimulus azimuth compared with stimulus elevation, and elevation sensitivity was primarily restricted to neurons that were tested using stimuli that the monkeys also could localize in elevation. Most neurons responded best to contralateral speaker locations, but we also encountered neurons that responded best to ipsilateral locations and neurons that had their greatest responses restricted to a circumscribed region within the central 60 degrees of frontal space. Comparing the spatially sensitive neurons with those that were not spatially sensitive indicated that these two populations could not be distinguished based on either the firing rate, the rate/level functions, or on their topographic location within AI. Direct comparisons between the responses of individual neurons and the behaviorally measured sound-localization ability indicated that proportionally more neurons in CM had spatial sensitivity that was consistent with the behavioral performance compared with AI neurons. Pooling the responses across neurons strengthened the relationship between the neuronal and psychophysical data and indicated that the responses pooled across relatively few CM neurons contain enough information to account for sound-localization ability. These data support the hypothesis that auditory space is processed in a serial manner from AI to CM in the primate cerebral cortex.

Frequency and intensity response properties of single neurons in the auditory cortex of the behaving macaque monkey

Response properties of auditory cortical neurons measured in anesthetized preparations have provided important information on the physiological differences between neurons in different auditory cortical areas. Studies in the awake animal, however, have been much less common, and the physiological differences noted may reflect differences in the influence of anesthetics on neurons in different cortical areas. Because the behaving monkey is gaining popularity as an animal model in studies exploring auditory cortical function, it has become critical to physiologically define the response properties of auditory cortical neurons in this preparation. This study documents the response properties of single cortical neurons in the primary and surrounding auditory cortical fields in monkeys performing an auditory discrimination task. We found that neurons with the shortest latencies were located in the primary auditory cortex (AI). Neurons in the rostral field had the longest latencies and the narrowest intensity and frequency tuning, neurons in the caudomedial field had the broadest frequency tuning, and neurons in the lateral field had the most monotonic rate/level functions of the four cortical areas studied. These trends were revealed by comparing response properties across the population of studied neurons, but there was considerable variability between neurons for each response parameter other than characteristic frequency (CF) in each cortical area. Although the neuronal CFs showed a systematic spatial organization across AI, no such systematic organization was apparent for any other response property in AI or the adjacent cortical areas. The results of this study indicate that there are physiological differences between auditory cortical fields in the behaving monkey consistent with previous studies in the anesthetized animal and provide insights into the functional role of these cortical areas in processing acoustic information.

Neuronal activity in visual, auditory and polysensory areas in the monkey temporal cortex during visual fixation task

The activity of 252 neurons in the inferotemporal visual area TEO, the superior temporal auditory area (AA), and the superior temporal polysensory area (STP) during the performance of a visual spot-fixation task and two variations, blink and tone tests, was examined in two behaving monkeys. A considerable number of not only TEO cells (45%) but also AA (29%) and STP (34%) cells were activated during the spot-fixation task, but unresponsive to the blanking of the spot during the fixation stage in the blink test. In addition, it was found that the activity of a third of the TEO, AA and STP cells which fired during the task-start stage in the spot-fixation task was modulated by cross-interaction between spot and tone simultaneously presented in the tone test: among these, the spot-induced activity of all TEO cells was enhanced by the tone, whereas the spot-induced activity of all AA and STP cells was suppressed by the tone. These findings are discussed in relation to the process of attending selectively to a fixation-spot.

Neural substrates for tone-conditioned bradycardia demonstrated with 2-deoxyglucose. II. Auditory cortex plasticity

The 2-deoxyglucose (2-DG) method was used to map the metabolic activity of the auditory cortex (AC) during and after conditioning. Using separate groups of animals, the effects of both paired and unpaired presentations of a 4-5 kHz FM tone (CS) and midbrain reticular stimulation (US) were compared for acquisition, extinction and sensitization training. Rats with cardiac deceleration conditioned to the FM tone showed a pattern of AC metabolic activity distinctly different from that seen in control animals. The tonotopic pattern of 2-DG labeling consisted of two contiguous spindle-shaped bands corresponding to the location of neurons with best frequency response in the 4-5 kHz band width. Reticular stimulation alone or combined with the tones produced a widespread increase of 2-DG uptake. At least two types of modulatory effects appeared to interact with the tonotopic pattern. The first involved a selective enhancement of evoked activity in the AC region of convergence of CS-US effects. This effect may be related to learning because it was restricted to the conditioning group. The second effect involved a general increase in background uptake of 2-DG in AC. This effect may be related to reticular sensitization because it was common to all groups subjected to reticular stimulation. The present findings are the first anatomical demonstration of the modulatory effects of auditory learning on AC metabolic activity.

Neural substrates for tone-conditioned bradycardia demonstrated with 2-deoxyglucose. I. Activation of auditory nuclei

The 2-deoxyglucose (2-DG) autoradiographic method was used to map the metabolic activity of auditory nuclei before, during and after conditioning. The experiment involved freely behaving rats in a Pavlovian conditioning paradigm in which a 4-5 kHz frequency modulated tone (CS) was paired with aversive electrical stimulation of the midbrain reticular formation (US). The unconditioned response was a rapid decrease in heart rate evoked by the US. Eight groups of rats were subjected to: (1) the tone CS before conditioning; (2) the US alone; (3) the paired CS-US (acquisition); (4) the tone CS after conditioning (extinction); (5) the US prior to the CS (sensitization); (6) the unpaired CS-US (pseudoconditioning); (7) the CS after pseudoconditioning; and (8) no stimulation. The major finding was the differential effect produced by the same tone before and after conditioning. The results showed that: (a) reticular mechanisms interact with incoming acoustic stimuli and modulate the response of auditory nuclei; (b) within each auditory nucleus the region of overlap of the spatial representations of CS and US developed an enhanced metabolic response during conditioning; and (c) the CS representation within the neuronal space of the tonotopic maps in all auditory nuclei, with the exception of the medial geniculate, reflected the learned behavioral value of the CS. The changes revealed by the 2-DG method represent the first anatomical demonstration of the activating effects of reticular sensitization and conditioning on a sensory system. The observations support the conclusion that auditory responses are dependent on the physical as well as on the behavioral parameters of a stimulus.