Neural correlates for roughness choice in monkey second somatosensory cortex (SII)

Vision of haptics tunes the somatosensory threshold

The interaction between different sensory modalities represents a crucial issue in the neuroscience of consciousness: when the processing of one modality is deficient, the concomitant presentation of stimuli of other spared modalities may sustain the restoring of the damaged sensory functions. In this regard, visual enhancement of touch may represent a viable tool in the rehabilitation from tactile disorders, yet the specific visual features mostly modulating the somatosensory experience remain unsettled. In this study, healthy subjects underwent a tactile detection task during the observation of videos displaying different contents, including static gratings, meaningless motions, natural or point-lights reach-to-grasp-and-manipulate actions. Concurrently, near-threshold stimuli were delivered to the median nerve at different time-points. Subjective report was collected after each trial; the sensory detection rate was computed and compared across video conditions. Our results indicate that the specific presence of haptic contents (i.e., vision of manipulation), either fully displayed or implied by point-lights, magnifies tactile sensitivity. The notion that such stimuli prompt an aware tactile experience opens to novel rehabilitation approaches for tactile consciousness disorders.

Somatotopic Map and Inter- and Intra-Digit Distance in Brodmann Area 2 by Pressure Stimulation

The somatotopic representation of the tactile stimulation on the finger in the brain is an essential part of understanding the human somatosensory system as well as rehabilitation and other clinical therapies. Many studies have used vibrotactile stimulations and reported finger somatotopic representations in the Brodmann area 3 (BA 3). On the contrary, few studies investigated finger somatotopic representation using pressure stimulations. Therefore, the present study aimed to find a comprehensive somatotopic representation (somatotopic map and inter- and intra-digit distance) within BA 2 of humans that could describe tactile stimulations on different joints across the fingers by applying pressure stimulation to three joints-the first (p1), second (p2), and third (p3) joints-of four fingers (index, middle, ring, and little finger). Significant differences were observed in the inter-digit distance between the first joints (p1) of the index and little fingers, and between the third joints (p3) of the index and little fingers. In addition, a significant difference was observed in the intra-digit distance between p1 and p3 of the little finger. This study suggests that a somatotopic map and inter- and intra-digit distance could be found in BA 2 in response to pressure stimulation on finger joints.

Predictive coding accounts of shared representations in parieto-insular networks

The discovery of mirror neurons in the ventral premotor cortex (area F5) and inferior parietal cortex (area PFG) in the macaque monkey brain has provided the physiological evidence for direct matching of the intrinsic motor representations of the self and the visual image of the actions of others. The existence of mirror neurons implies that the brain has mechanisms reflecting shared self and other action representations. This may further imply that the neural basis self-body representations may also incorporate components that are shared with other-body representations. It is likely that such a mechanism is also involved in predicting other's touch sensations and emotions. However, the neural basis of shared body representations has remained unclear. Here, we propose a neural basis of body representation of the self and of others in both human and non-human primates. We review a series of behavioral and physiological findings which together paint a picture that the systems underlying such shared representations require integration of conscious exteroception and interoception subserved by a cortical sensory-motor network involving parieto-inner perisylvian circuits (the ventral intraparietal area [VIP]/inferior parietal area [PFG]-secondary somatosensory cortex [SII]/posterior insular cortex [pIC]/anterior insular cortex [aIC]). Based on these findings, we propose a computational mechanism of the shared body representation in the predictive coding (PC) framework. Our mechanism proposes that processes emerging from generative models embedded in these specific neuronal circuits play a pivotal role in distinguishing a self-specific body representation from a shared one. The model successfully accounts for normal and abnormal shared body phenomena such as mirror-touch synesthesia and somatoparaphrenia. In addition, it generates a set of testable experimental predictions.

Somato-motor haptic processing in posterior inner perisylvian region (SII/pIC) of the macaque monkey

The posterior inner perisylvian region including the secondary somatosensory cortex (area SII) and the adjacent region of posterior insular cortex (pIC) has been implicated in haptic processing by integrating somato-motor information during hand-manipulation, both in humans and in non-human primates. However, motor-related properties during hand-manipulation are still largely unknown. To investigate a motor-related activity in the hand region of SII/pIC, two macaque monkeys were trained to perform a hand-manipulation task, requiring 3 different grip types (precision grip, finger exploration, side grip) both in light and in dark conditions. Our results showed that 70% (n = 33/48) of task related neurons within SII/pIC were only activated during monkeys’ active hand-manipulation. Of those 33 neurons, 15 (45%) began to discharge before hand-target contact, while the remaining neurons were tonically active after contact. Thirty-percent (n = 15/48) of studied neurons responded to both passive somatosensory stimulation and to the motor task. A consistent percentage of task-related neurons in SII/pIC was selectively activated during finger exploration (FE) and precision grasping (PG) execution, suggesting they play a pivotal role in control skilled finger movements. Furthermore, hand-manipulation-related neurons also responded when visual feedback was absent in the dark. Altogether, our results suggest that somato-motor neurons in SII/pIC likely contribute to haptic processing from the initial to the final phase of grasping and object manipulation. Such motor-related activity could also provide the somato-motor binding principle enabling the translation of diachronic somatosensory inputs into a coherent image of the explored object.

Different timescales for the neural coding of consonant and vowel sounds

Psychophysical, clinical, and imaging evidence suggests that consonant and vowel sounds have distinct neural representations. This study tests the hypothesis that consonant and vowel sounds are represented on different timescales within the same population of neurons by comparing behavioral discrimination with neural discrimination based on activity recorded in rat inferior colliculus and primary auditory cortex. Performance on 9 vowel discrimination tasks was highly correlated with neural discrimination based on spike count and was not correlated when spike timing was preserved. In contrast, performance on 11 consonant discrimination tasks was highly correlated with neural discrimination when spike timing was preserved and not when spike timing was eliminated. These results suggest that in the early stages of auditory processing, spike count encodes vowel sounds and spike timing encodes consonant sounds. These distinct coding strategies likely contribute to the robust nature of speech sound representations and may help explain some aspects of developmental and acquired speech processing disorders.

Perceptual constancy of texture roughness in the tactile system

Our tactual perception of roughness is independent of the manner in which we touch the surface. A brick surface feels rough no matter how slowly or how rapidly we move our fingers, despite the fluctuating sensory inputs that are transmitted to the finger. Current theories of roughness perception rely solely on inputs from the cutaneous afferents, which are highly affected by scan velocity and force. The question then is: how is roughness constancy achieved? To this end, we characterized the subject's perceived roughness in six scanning conditions. These included two modes of touch: direct touch, where the finger is in contact with the surface, and indirect touch, where the surface is scanned with a hand-held probe; and three scanning modes: active (moving the hand across a stationary surface), passive (moving the surface across a stationary hand), and pseudo-passive (subject's hand is moved by the experimenter across a stationary surface). Here, we show that roughness constancy is preserved during active but not passive scanning, indicating that the hand movement is necessary for roughness constancy in both direct and indirect touch. Roughness constancy is also preserved during pseudo-passive scanning, which stresses the importance of proprioceptive input. The results show that cutaneous input provides the signals necessary for roughness perception and that proprioceptive input resulting from hand movement-rather than a motor efference copy-is necessary to achieve roughness constancy. These findings have important implications in providing realistic sensory feedback for prosthetic-hand users.

Differential modulation of corticospinal excitability during haptic sensing of 2-D patterns vs. textures

BACKGROUND

Recently, we showed a selective enhancement in corticospinal excitability when participants actively discriminated raised 2-D symbols with the index finger. This extra-facilitation likely reflected activation in the premotor and dorsal prefrontal cortices modulating motor cortical activity during attention to haptic sensing. However, this parieto-frontal network appears to be finely modulated depending upon whether haptic sensing is directed towards material or geometric properties. To examine this issue, we contrasted changes in corticospinal excitability when young adults (n = 18) were engaged in either a roughness discrimination on two gratings with different spatial periods, or a 2-D pattern discrimination of the relative offset in the alignment of a row of small circles in the upward or downward direction.

RESULTS

A significant effect of task conditions was detected on motor evoked potential amplitudes, reflecting the observation that corticospinal facilitation was, on average, ~18% greater in the pattern discrimination than in the roughness discrimination.

CONCLUSIONS

This differential modulation of corticospinal excitability during haptic sensing of 2-D patterns vs. roughness is consistent with the existence of preferred activation of a visuo-haptic cortical dorsal stream network including frontal motor areas during spatial vs. intensive processing of surface properties in the haptic system.

Neural correlates of auditory temporal-interval discrimination in cats

Auditory temporal cues are very important in the perception of speech; nevertheless, the neural correlates underlying even a simple temporal task, such as interval discrimination, remain unclear, mainly due to the lack of comparable data of psychophysical and electrophysiological experiments in the same species. To address this, we measured both behavioral and neural responses in cats. Cats' ability to discriminate differences in sound intervals was tested by presenting two identical pure tone markers interrupted by intervals ranging from 10 to 320ms duration. All three subjects could accurately discriminate tones of 80-320ms interval from those of 10ms interval (correct rate>75%), but could not discriminate 20-40ms interval from 10ms interval. Neural responses to the same stimuli were recorded in the primary auditory cortex (A1) of three others awake cats. Consistent with previous studies, we found that the majority of A1 neurons showed a suppressed response to the second tone, and the amount of suppression generally increased with the decrease of intertone interval. Neurometric analysis revealed that neural responses could be used to discriminate the intertone interval, while discrimination performance was dependent on temporal precision to read the neural information. When spike activities were analyzed by 100ms bin size, 80% of neurometric functions matched the cats' psychometric function, suggesting a possible correlation between A1 activities and interval perception.

Comparing Tactile Pattern and Vibrotactile Frequency Discrimination: A Human fMRI Study

We investigated to which extent the discrimination of tactile patterns and vibrotactile frequencies share common cortical areas. An adaptation paradigm has been used to identify cortical areas specific for processing particular features of tactile stimuli. Healthy right-handed subjects performed a delayed-match-to-sample (DMTS) task discriminating between pairs of tactile patterns or vibrotactile frequencies in separate functional MRI sessions. The tactile stimuli were presented to the right middle fingertip sequentially with a 5.5 s delay. Regions of interest (ROIs) were defined by cortical areas commonly activated in both tasks and those that showed differential activation between both tasks. Results showed recruitment of many common brain regions along the sensory motor pathway (such as bilateral somatosensory, premotor areas, and anterior insula) in both tasks. Three cortical areas, the right intraparietal sulcus (IPS), supramarginal gyrus (SMG)/parietal operculum (PO), and PO, were significantly more activated during the pattern than in the frequency task. Further BOLD time course analysis was performed in the ROIs. Significant BOLD adaptation was found in bilateral IPS, right anterior insula, and SMG/PO in the pattern task, whereas there was no significant BOLD adaptation found in the frequency task. In addition, the right hemisphere was found to be more dominant in the pattern than in the frequency task, which could be attributed to the differences between spatial (pattern) and temporal (frequency) processing. From the different spatio-temporal characteristics of BOLD activation in the pattern and frequency tasks, we concluded that different neuronal mechanisms are underlying the tactile spatial and temporal processing.

Perceived intensity of somatosensory cortical electrical stimulation

Artificial sensations can be produced by direct brain stimulation of sensory areas through implanted microelectrodes, but the perceptual psychophysics of such artificial sensations are not well understood. Based on prior work in cortical stimulation, we hypothesized that perceived intensity of electrical stimulation may be explained by the population response of the neurons affected by the stimulus train. To explore this hypothesis, we modeled perceived intensity of a stimulation pulse train with a leaky neural integrator. We then conducted a series of two-alternative forced choice behavioral experiments in which we systematically tested the ability of rats to discriminate frequency, amplitude, and duration of electrical pulse trains delivered to the whisker barrel somatosensory cortex. We found that the model was able to predict the performance of the animals, supporting the notion that perceived intensity can be largely accounted for by spatiotemporal integration of the action potentials evoked by the stimulus train.

An investigation of auditory dimensional interaction in a bivariate bilateral conditioning paradigm in the rabbit

The current study adapted the Garner paradigm for diagnosing separable versus integral perceptual dimensions to the eye-blink classical conditioning paradigm using rabbits. Specifically, this study examined the ability of rabbits to categorize stimuli based on one auditory dimension while ignoring a second, irrelevant dimension by displaying an appropriate eye-blink for bilaterally conditioned discriminative responses. Tones used in training varied along two dimensions, starting frequency and magnitude of frequency sweep upwards from the start. Rabbits first learned to categorize along a single dimension (blinking one eye for one category response and the other eye for the other response) and then continued to categorize tones in a second phase in which the irrelevant dimension was varied. The variation of the irrelevant dimension did not disrupt performance, indicating that rabbits perceive these dimensions as separable.

Instructed Delay Discharge in Primary and Secondary Somatosensory Cortex Within the Context of a Selective Attention Task

The neuronal mechanisms that contribute to tactile perception were studied using single-unit recordings from the cutaneous hand representation of primate primary (S1) and secondary (S2) somatosensory cortex. This study followed up on our recent observation that S1 and S2 neurons developed a sustained change in discharge during the instruction period of a directed-attention task. We determined the extent to which the symbolic light cues, which signaled the modality (tactile, visual) to attend and discriminate, elicited changes in discharge rate during the instructed delay (ID) period of the attention task and the functional importance of this discharge. ID responses, consisting of a sustained increase or decrease in discharge during the 2-s instruction period, were present in about 40% of the neurons in S1 and S2. ID responses in both cortical regions were very similar in most respects (frequency, sign, latency, amplitude), suggesting a common source. A major difference, however, was related to attentional modulation during the ID period: attentional influences were almost entirely restricted to S2 and these effects were always superimposed on the ID response (additive effect). These findings suggest that the underlying mechanisms for ID discharge and attention are independent. ID discharge significantly modified the initial response to the standard stimuli (competing texture and visual stimuli), usually enhancing responsiveness. We also showed that tactile detection in humans is enhanced during the ID period. Together, the results suggest that ID discharge represents a priming mechanism that prepares cortical areas to receive and process sensory inputs.

Multisensory contributions to the perception of vibrotactile events

We argue that audio-tactile interactions during vibrotactile processing provide a promising, albeit largely neglected, benchmark for the systematic study multisensory integration. This article reviews and discusses current evidence for multisensory contributions to the perception of vibratory events, and proposes a framework to address a number of relevant questions. First, we highlight some of the features that characterize the senses of hearing and touch in terms of vibratory information processing, and which allow for potential cross-modal interactions at multiple levels along the functional architecture of the sensory systems. Second, we briefly review empirical evidence for interactions between hearing and touch in the domain of vibroactile perception and related stimulus properties, covering behavioural, electrophysiological and neuroimaging studies in humans and animals. Third, we discuss the vibrotactile discrimination task, which has been successfully applied in the study of perception and decision processes in psychophysical and physiological research. We argue that this approach, complemented with computational modeling using biophysically realistic neural networks, may be a convenient framework to address auditory contributions to vibrotactile processing in the somatosensory system. Finally, we comment on a series of particular issues which are relevant in multisensory research and potentially addressable within the proposed framework.

Cortical activity patterns predict speech discrimination ability

Neural activity in the cerebral cortex can explain many aspects of sensory perception. Extensive psychophysical and neurophysiological studies of visual motion and vibrotactile processing show that the firing rate of cortical neurons averaged across 50-500 ms is well correlated with discrimination ability. In this study, we tested the hypothesis that primary auditory cortex (A1) neurons use temporal precision on the order of 1-10 ms to represent speech sounds shifted into the rat hearing range. Neural discrimination was highly correlated with behavioral performance on 11 consonant-discrimination tasks when spike timing was preserved and was not correlated when spike timing was eliminated. This result suggests that spike timing contributes to the auditory cortex representation of consonant sounds.

The involvement of ipsilateral temporoparietal cortex in tactile pattern working memory as reflected in beta event-related desynchronization

Cortical oscillatory activity in various frequency bands has been shown to reflect working memory processes operating on visual and auditory stimulus information. Here we use magnetoencephalography to investigate cortical oscillatory activity related to working memory for tactile patterns. Right-handed subjects made same-different judgements on two dot patterns sequentially applied with a 3-s delay to the right middle fingertip. Spectral analysis revealed beta desynchronization (17+/-2.5 Hz) at contralateral postcentral and ipsilateral temporoparietal regions preceding and during the presentation of both tactile stimuli as well as during the early and late delay periods. Whereas contralateral beta desynchronization preceding tactile stimulation may reflect anticipation of incoming stimuli, ipsilateral beta desynchronization may underlie working memory maintenance of tactile patterns. The later hypothesis is supported by a significant positive correlation between subjects' performance and the amplitude of ipsilateral beta desynchronization 800 ms to 500 ms before the onset of the second pattern stimulus. Thus, our results suggest that ipsilateral temporoparietal cortex contributes to the maintenance of tactile pattern information in working memory.

Neural mechanisms of detecting and orienting attention toward unattended threatening somatosensory target stimuli. II. Intensity effects

Negative potentials evoked by painful electrical stimulation of the sural nerve that occur at 100-180 ms poststimulus over the contralateral temporal scalp (CTN100-180) and at 130-200 ms over the fronto-central scalp (FCN130-200) exhibit unusual attention effects. That is, their amplitudes are larger when the painful evoking stimulus is unattended than when it is attended. In this experiment, I show that attention has no effect on the CTN100-180 evoked by a weak, nonthreatening sural nerve electrical stimulus. These data suggest that the generators of the CTN100-180, which include the somatosensory association areas in the parietal operculum, are specifically involved in detecting threatening somatosensory stimuli. The FCN130-200 showed a small increase in the unattended condition, which is consistent with the role of its medial prefrontal cortex generators in monitoring any situation that might require a change in attentional control.

Centrifugal regulation of human cortical responses to a task-relevant somatosensory signal triggering voluntary movement

Many studies have reported a movement-related modulation of response in the primary and secondary somatosensory cortices (SI and SII) to a task-irrelevant stimulation in primates. In the present study, magnetoencephalography (MEG) was used to examine the top-down centrifugal regulation of neural responses in the human SI and SII to a task-relevant somatosensory signal triggering a voluntary movement. Nine healthy adults participated in the study. A visual warning signal was followed 2 s later by a somatosensory imperative signal delivered to the right median nerve at the wrist. Three kinds of warning signal informed the participants of the reaction which should be executed on presentation of the imperative signal (rest or extension of the right index finger, extension of the left index finger). The somatosensory stimulation was used to both generate neural responses and trigger voluntary movement and therefore was regarded as a task-relevant signal. The responses were recorded using a whole-head MEG system. The P35m response around the SI was reduced in magnitude without alteration of the primary SI response, N20m, when the signal triggered a voluntary movement compared to the control condition, whereas bilateral SII responses peaking at 70-100 ms were enhanced and the peak latency was shortened. The peak latency of the responses in the SI and SII preceded the onset of the earliest voluntary muscle activation in each subject. Later bilateral perisylvian responses were also enhanced with movement. In conclusion, neural activities in the SI and SII evoked by task-relevant somatosensory signals are regulated differently by motor-related neural activities before the afferent inputs. The present findings indicate a difference in function between the SI and SII in somatosensory-motor regulation.

Receptive field (RF) properties of the macaque second somatosensory cortex: RF size, shape, and somatotopic organization

The detailed structure of multidigit receptive fields (RFs) in somatosensory cortical areas such as the SII region has not been investigated previously using systematically controlled stimuli. Recently (Fitzgerald et al., 2004), we showed that the SII region comprises three adjoining fields: posterior, central, and anterior. Here we characterize the RF structures of the 928 neurons that were reported in that study using a motorized oriented bar that was indented into the 12 finger pads of digits 2-5. Most (81%) of the neurons were responsive to the oriented bar stimuli, and 81% of those neurons had RFs that spanned multiple digits. Furthermore, the RFs varied greatly in size, shape, and complexity. Some RFs contained only excitatory finger pads, some contained only inhibitory pads, and some contained both types of pads. A subset of the neurons (23%) showed orientation tuning within one or more pads. The RFs spread across different digits more than within individual digits, and the responsive finger pads for a given neuron tended to cluster together within the hand. Distal and lateral finger pads were better represented than proximal and medial finger pads. Furthermore, neurons in the posterior, central, and anterior SII region fields contained different proportions of RF types. These results collectively indicate that most SII region neurons are selective for different stimulus forms either within single finger pads or across multiple pads. We hypothesize that these RFs represent the kernels underlying the representation of tactile shape.

Receptive field properties of the macaque second somatosensory cortex: representation of orientation on different finger pads

Orientation tuning has been studied extensively in the visual system, but little is known about it in the somatosensory system. Here we investigate tuning in the second somatosensory (SII) region using a motorized stimulator that presented a small oriented bar to the 12 finger pads of digits 2-5 (D2-D5) of the macaque monkey. A subset (23%; n = 218) of the 928 SII region neurons [the same 928 neurons studied by Fitzgerald et al. (2004, 2006)] exhibited tuning, and most of these were tuned on one or two finger pads. All eight 22.5 degrees separated orientations were represented as the preferred orientation of multiple neurons, although not necessarily in equal numbers. A measure of bandwidth indicated that tuning in the SII region is sharp and is similar to the tuning observed in visual cortical areas. In addition, two-dimensional Gaussians that were fit to the tuning curves had very high r2 values, indicating that most tuning curves are both unimodal and symmetrical with respect to their preferred orientation. Most tuned neurons had additional untuned pads, although the responsiveness of these pads tended to be less than the responsiveness of tuned pads. Neurons with multiple tuned pads tended to have similar preferred orientations on their tuned pads, which can be interpreted as evidence for integration of information across fingers or as a form of positional invariance. Finally, comparison of the tuning properties showed that there are small but significant differences between the posterior, central, and anterior fields of the SII region.

Modality maps within primate somatosensory cortex

The sensations of pressure, flutter, and vibration are psychophysically distinct tactile modalities produced by frequency-specific vibrotactile stimulation of different mechanoreceptors in the skin. The information coded by the different low-threshold mechanoreceptors are carried by anatomically and electrophysiologically distinct pathways that remain separate at least up to and including the input stage of primary somatosensory cortex (SI) in primates, area 3b. Little is known about the functional organization of tactile representation beyond that stage. By using intrinsic optical imaging methods to record from area 1, the second processing stage of SI, we present evidence that pressure, flutter, and vibratory stimuli activate spatially distinct cortical domains in area 1, further strengthening the foundation for modality-specific processing streams in SI. These modality domains exhibit an organization that is unlike the discontinuous modality maps in visual area V2 but more like the continuous visual orientation maps in V1. The results demonstrate that psychophysically distinct sensory modalities can have fundamentally different modes of cortical representation.

Correlated neuronal discharges that increase coding efficiency during perceptual discrimination

During a sensory discrimination task, the responses of multiple sensory neurons must be combined to generate a choice. The optimal combination of responses is determined both by their dependence on the sensory stimulus and by their cofluctuations across trials-that is, the noise correlations. Positively correlated noise is considered deleterious, because it limits the coding accuracy of populations of similarly tuned neurons. However, positively correlated fluctuations between differently tuned neurons actually increase coding accuracy, because they allow the common noise to be subtracted without signal loss. This is demonstrated with data recorded from the secondary somatosensory cortex of monkeys performing a vibrotactile discrimination task. The results indicate that positive correlations are not always harmful and may be exploited by cortical networks to enhance the neural representation of features to be discriminated.

Flutter discrimination: neural codes, perception, memory and decision making

Recent studies combining psychophysical and neurophysiological experiments in behaving monkeys have provided new insights into how several cortical areas integrate efforts to solve a vibrotactile discrimination task. In particular, these studies have addressed how neural codes are related to perception, working memory and decision making in this model. The primary somatosensory cortex drives higher cortical areas where past and current sensory information are combined, such that a comparison of the two evolves into a behavioural decision. These and other observations in visual tasks indicate that decisions emerge from highly-distributed processes in which the details of a scheduled motor plan are gradually specified by sensory information.

Analysing neuronal correlates of the comparison of two sequentially presented sensory stimuli

In a typical sequential sensory discrimination task, subjects are required to make a decision based on comparing a sensory stimulus against the memory trace left by a previous stimulus. What is the neuronal substrate for such comparisons and the resulting decisions? This question was studied by recording neuronal responses in a variety of cortical areas of awake monkeys (Macaca mulatta), trained to carry out a vibrotactile sequential discrimination task. We describe methods to analyse responses obtained during the comparison and decision phases of the task, and describe the resulting findings from recordings in secondary somatosensory cortical area (S2). A subset of neurons in S2 become highly correlated with the monkey's decision in the task.

Neuronal correlates of decision-making in secondary somatosensory cortex

The ability to discriminate between two sequential stimuli requires evaluation of current sensory information in reference to stored information. Where and how does this evaluation occur? We trained monkeys to compare two mechanical vibrations applied sequentially to the fingertips and to report which of the two had the higher frequency. We recorded single neurons in secondary somatosensory cortex (S2) while the monkeys performed the task. During the first stimulus period, the firing rate of S2 neurons encoded the stimulus frequency. During the second stimulus period, however, some S2 neurons did not merely encode the stimulus frequency. The responses of these neurons were a function of both the remembered (first) and current (second) stimulus. Moreover, a few hundred milliseconds after the presentation of the second stimulus, these responses were correlated with the monkey's decision. This suggests that some S2 neurons may combine past and present sensory information for decision-making.