Entire Stimulus Research Articles

Harmonic analysis of the cone flicker ERG of rabbit

Harmonic analysis was used to characterize the rabbit flicker ERG elicited by sinusoidally modulated full-field stimuli under light-adapted conditions. The frequency-response function for fundamental amplitude, derived from Fourier analysis of the ERG waveforms, exhibited two limbs, with an amplitude minimum at approximately 30 Hz, and a high-frequency region peaking at around 45 Hz and extending to more than 100 Hz at higher adapting levels. At low frequencies (<20 Hz), the fundamental response amplitude was independent of mean luminance (Weber law behavior), whereas the response amplitude at high stimulus frequencies varied nonlinearly with mean luminance. At low frequencies, intravitreal administration of L-AP4, which blocks ON-pathway activity, reduced the fundamental response amplitude and produced a phase shift. On the other hand, PDA, which reduces OFF-pathway activity, had a minimal effect on both the response amplitude and phase at low frequencies. At high frequencies, L-AP4 increased the fundamental response amplitude at low mean luminances, whereas PDA had only a small effect on amplitude and phase. Both pharmacologic agents removed the minimum in the amplitude-frequency function as well as the abrupt change in phase at stimulus frequencies near 30 Hz. The results suggest that there is a nonlinear interaction between ON- and OFF-pathway activity over the entire stimulus frequency range examined in this study. These findings provide a basis for formulating protocols to evaluate the effect of pharmacologic agents and/or disease on the cone flicker ERG of rabbit.

Featural selective attention, exemplar representation, and the inverse base-rate effect

Selective attention plays a central role in theories of category learning and representation. In exemplar theory, selective attention has typically been formalized as operating uniformly across entire stimulus dimensions. Selective featural attention operating within dimensions has been recognized as a conceptual possibility, but relatively little research has focused on evaluating it. In the present research, we explored the usefulness of selective featural attention in the context of exemplar representation. We report the results of embedding the feature-to-category relations typically associated with the inverse base-rate effect--a classic and paradoxical category-learning result--within a perceptual category-learning task using a category structure with three multivalued feature dimensions. An exemplar model incorporating featural selective attention accurately accounted for the inverse base-rate effect that occurred but failed to do so with only dimensional attention.

Lateralization of noise-burst trains based on onset and ongoing interaural delays

The lateralization of 250-ms trains of brief noise bursts was measured using an acoustic pointing technique. Stimuli were designed to assess the contribution of the interaural time delay (ITD) of the onset binaural burst relative to that of the ITDs in the ongoing part of the train. Lateralization was measured by listeners' adjustments of the ITD of a pointer stimulus, a 50-ms burst of noise, to match the lateral position of the target train. Results confirmed previous reports of lateralization dominance by the onset burst under conditions in which the train is composed of frozen tokens and the ongoing part contains multiple ambiguous interaural delays. In contrast, lateralization of ongoing trains in which fresh noise tokens were used for each set of two alternating (left-leading/right-leading) binaural pairs followed the ITD of the first pair in each set, regardless of the ITD of the onset burst of the entire stimulus and even when the onset burst was removed by gradual gating. This clear lateralization of a long-duration stimulus with ambiguous interaural delay cues suggests precedence mechanisms that involve not only the interaural cues at the beginning of a sound, but also the pattern of cues within an ongoing sound.

Two Components of Nocturnal Locomotor Suppression by Light

In nocturnal rodents, millisecond light ("flash") stimuli can induce both a large circadian rhythm phase shift and an associated state change from highly active to quiescence followed by behavioral sleep. Suppression of locomotion ("negative masking") is an easily measured correlate of the state change. The present mouse studies used both flashes and longer light stimuli ("pulses") to distinguish initiation from maintenance effects of light on locomotor suppression and to determine whether the locomotor suppression exhibits temporal integration as is thought to be characteristic of phase shift responses to pulse, but not flash, stimuli. In experiment 1, locomotor suppression increased with irradiance (0.01-100 microW/cm( 2)), in accordance with previous reports. It also increased with stimulus duration (3-3000 sec), but interpretation of this result is complicated by the ability of light to both initiate and maintain locomotor suppression. In experiment 2, an irradiance response curve was determined using a stimulus series of 10 flashes, 2 msec each, with total flash energy varying from 0.0025 to 110.0 J/m(2). This included a test for temporal integration in which the effects of two equal energy series of flashes that differed in the number of flashes per series (10 vs 100), were compared. The 10 flash series more effectively elicited locomotor suppression than the 100 flash series, a result consistent with prior observations involving flash-induced phase shifts. In experiment 3, exposure of mice to an 11-h light stimulus yielded irradiance-dependent locomotor suppression that was maintained for the entire stimulus duration by a 100-microW/cm(2) stimulus. Light has the ability to initiate a time-limited (30-40 min) interval of locomotor suppression (initiation effect) that can be extended by additional light (maintenance effect). Temporal integration resembling that seen in phase-shifting responses to light does not exist for either phase shift or locomotor suppression responses to flashes or for locomotor suppression responses to light pulses. The authors present an alternative interpretation of data thought to demonstrate temporal integration in the regulation of phase shift responses to light pulses.

The discrimination of correlated and anti-correlated motion in the human visual system

&#x0D; &#x0D; &#x0D; Introduction: how the brain integrates spatial and temporal information is not known. This issue is referred to as the “binding problem” of visual perception. It has been proposed that groups of neurons which correspond to the same elements of an image become synchronous in order to form a coherent neural representation; however, direct experimental evidence supporting this role for neural synchrony is highly controversial. as our perceptual capabilities are limited by the neural mechanism that supports them, an alternative approach to understanding neural synchrony is to instead characterize our ability to perceive synchrony. Thus, our aim was to demonstrate how correlated (i.e., synchronous) motion is perceived by the brain and how its discrimination can be enhanced or impaired. Methods: In this study, we used human psychophysics experiments to characterize the ability of subjects to discriminate synchrony in a moving visual stimulus. results: By varying stimulus length, motion speed, and direction we found that humans were less than optimal in their ability to discriminate correlated motion when compared to an ideal mathematical model. In addition, we found that the length of the entire stimulus was not an important factor, but the length of individual motion pulses which made up the stimulus was crucial to performance. discussion: overall these results suggest that neural synchrony is likely used by the brain, but its resolution is highly limited compared to an ideal model.&#x0D; &#x0D; &#x0D;

Frequency-dependent neural activity, CBF, and BOLD fMRI to somatosensory stimuli in isoflurane-anesthetized rats

Inhalation anesthetics (e.g. isoflurane) are preferable for longitudinal fMRI experiments in the same animals. We previously implemented isoflurane anesthesia for rodent forepaw stimulation studies, and optimized the stimulus parameters with short stimuli (1–3-s long stimulation with ten electric pulses). These parameters, however, may not be applicable for long periods of stimulation because repetitive stimuli induce neural adaptation. Here we evaluated frequency-dependent responses (pulse width of 1.0ms and current of 1.5mA) for 30-s long stimulation under 1.3–1.5% isoflurane anesthesia. The cerebral blood flow (CBF) response (using laser Doppler flowmetry: CBFLDF) and field potential (FP) changes were simultaneously measured for nine stimulus frequencies (1–24Hz). CBF (using arterial spin labeling: CBFASL) and blood oxygenation level dependent (BOLD) fMRI responses were measured at 9.4T for four stimulus frequencies (1.5–12Hz). Higher stimulus frequencies (12–24Hz) produced a larger FP per unit time initially, but decreased more rapidly later due to neural adaptation effects. On the other hand, lower stimulus frequencies (1–3Hz) induced smaller, but sustained FP activities over the entire stimulus period. Similar frequency-dependencies were observed in CBFLDF, CBFASL and BOLD responses. A linear relationship between FP and CBFLDF was observed for all stimulus frequencies. Stimulation frequency for the maximal cumulative neural and hemodynamic changes is dependent on stimulus duration; 8–12Hz for short stimulus durations (<10s) and 6–8Hz for 30-s stimulation. Our findings suggest that neural adaptation should be considered in determining the somatosensory stimulation frequency and duration under isoflurane anesthesia.

Input-dependent switching of inhibitory configurations in neural networks

Event Abstract Back to Event Input-dependent switching of inhibitory configurations in neural networks Alex D. Reyes1* 1 New York University , United States The responses of neurons during a stimulus depend on the summed excitatory (E) and inhibitory (I) synaptic inputs. Shifts in the E-I balance cause both qualitative and quantitative changes in the neuronal firing patterns. How the E and I inputs scale with stimuli is yet unclear. An important determinant is the network architecture, broadly classified either as the lateral inhibitory network (LIN) where the I inputs to a neuron are more broadly tuned than the E inputs, or the co-tuned network (CON) where the E and I inputs co-vary for the entire stimulus range. E and I scale with input differently in each configuration to produce complementary sets of responses, suggesting that both may be needed to account for the diverse stimulus-evoked firing behavior. Here I show that a single E-I network transitions seamlessly between LIN and CON when the amplitude and spatial extent of the input to the network changes. Simulations were performed with a 2D network (10000 E cells; 2000 I cells) of Adaptive exponential integrate and fire neurons adjusted to reproduce the firing of pyramidal cells, and fast spiking and low threshold spiking interneurons. The patterns of connections between E and I cells were based on experimental data obtained from paired recordings in an in vitro slice preparation. Synaptic barrages (200 ms duration) were delivered to each cell; the number of barrages was adjusted so that the mean input was Gaussian distributed (parameterized by amplitude A and standard deviation S) in space. When the input was narrow (small S), the network was configured as LIN. E cells tended to fire tonically, and exhibited side-band inhibition. As the input broadened, the network switched to CON. The neurons tended to fire phasically and exhibited no sideband inhibition. To characterize quantitatively the variation of E-I balance and firing in the A-S space, mean field techniques were used to calculate the population activity of the neurons in the network. This yielded relations that describe how the widths and amplitude of excitatory and inhibitory inputs and the associated firing patterns change with the magnitude and spatial distribution of the input. Physiologically, the Gaussian input may represent tuning to stimuli such as tone frequency in the auditory system or disc location in the visual system; A may vary with stimulus intensity while S with bandwidth or disc diameter. The changes in firing that occur with increasing the bandwidth/diameter of auditory/visual stimulus are reproduced by tracing appropriate trajectories in the A and S space. That the LIN and CON configurations are not hardwired into the network has important implications. The ability to switch between configurations provides potentially a mechanism for modulating the response of the network to a variety of inputs and behavioral states. Additionally, many of the heterogeneous firing and receptive field properties that had been postulated to arise from different network configurations may in fact be due to transitions within a single network that are triggered perhaps by changes in the stimulus characteristics or state of the animal. Conference: Computational and Systems Neuroscience 2010, Salt Lake City, UT, United States, 25 Feb - 2 Mar, 2010. Presentation Type: Oral Presentation Topic: Oral presentations Citation: Reyes AD (2010). Input-dependent switching of inhibitory configurations in neural networks. Front. Neurosci. Conference Abstract: Computational and Systems Neuroscience 2010. doi: 10.3389/conf.fnins.2010.03.00003 Copyright: The abstracts in this collection have not been subject to any Frontiers peer review or checks, and are not endorsed by Frontiers. They are made available through the Frontiers publishing platform as a service to conference organizers and presenters. The copyright in the individual abstracts is owned by the author of each abstract or his/her employer unless otherwise stated. Each abstract, as well as the collection of abstracts, are published under a Creative Commons CC-BY 4.0 (attribution) licence (https://creativecommons.org/licenses/by/4.0/) and may thus be reproduced, translated, adapted and be the subject of derivative works provided the authors and Frontiers are attributed. For Frontiers’ terms and conditions please see https://www.frontiersin.org/legal/terms-and-conditions. Received: 17 Feb 2010; Published Online: 17 Feb 2010. * Correspondence: Alex D Reyes, New York University, New York, United States, reyes@cns.nyu.edu Login Required This action requires you to be registered with Frontiers and logged in. To register or login click here. Abstract Info Abstract The Authors in Frontiers Alex D Reyes Google Alex D Reyes Google Scholar Alex D Reyes PubMed Alex D Reyes Related Article in Frontiers Google Scholar PubMed Abstract Close Back to top Javascript is disabled. Please enable Javascript in your browser settings in order to see all the content on this page.

Synaptic and Network Mechanisms of Sparse and Reliable Visual Cortical Activity during Nonclassical Receptive Field Stimulation

During natural vision, the entire visual field is stimulated by images rich in spatiotemporal structure. Although many visual system studies restrict stimuli to the classical receptive field (CRF), it is known that costimulation of the CRF and the surrounding nonclassical receptive field (nCRF) increases neuronal response sparseness. The cellular and network mechanisms underlying increased response sparseness remain largely unexplored. Here we show that combined CRF + nCRF stimulation increases the sparseness, reliability, and precision of spiking and membrane potential responses in classical regular spiking (RS(C)) pyramidal neurons of cat primary visual cortex. Conversely, fast-spiking interneurons exhibit increased activity and decreased selectivity during CRF + nCRF stimulation. The increased sparseness and reliability of RS(C) neuron spiking is associated with increased inhibitory barrages and narrower visually evoked synaptic potentials. Our experimental observations were replicated with a simple computational model, suggesting that network interactions among neuronal subtypes ultimately sharpen recurrent excitation, producing specific and reliable visual responses.

Neural activity-induced modulation of BOLD poststimulus undershoot independent of the positive signal

Despite intense research on the blood oxygenation level-dependent (BOLD) signal underlying functional magnetic resonance imaging, our understanding of its physiological basis is far from complete. In this study, it was investigated whether the so-called poststimulus BOLD signal undershoot is solely a passive vascular effect or actively induced by neural responses. Prolonged static and flickering black-white checkerboard stimulation with isoluminant grey screen as baseline condition were employed on eight human subjects. Within the same region of interest, the positive BOLD time courses for static and flickering stimuli were identical over the entire stimulus duration. In contrast, the static stimuli exhibited no poststimulus BOLD signal undershoot, whereas the flickering stimuli caused a strong BOLD poststimulus undershoot. To ease the interpretation, we performed an additional study measuring both BOLD signal and cerebral blood flow (CBF) using arterial spin labeling. Also for CBF, a difference in the poststimulus period was found for the two stimuli. Thus, a passive blood volume effect as the only contributor to the poststimulus undershoot comes short in explaining the BOLD poststimulus undershoot phenomenon for this particular experiment. Rather, an additional active neuronal activation or deactivation can strongly modulate the BOLD poststimulus behavior. In summary, the poststimulus time course of BOLD signal could potentially be used to differentiate neuronal activity patterns that are otherwise indistinguishable using the positive evoked response.

Convergence of Submodality-Specific Input Onto Neurons in Primary Somatosensory Cortex

At the somatosensory periphery, slowly adapting type 1 (SA1) and rapidly adapting (RA) afferents respond very differently to step indentations: SA1 afferents respond throughout the entire stimulus interval (sustained response), whereas RA afferents respond only at stimulus onset (on response) and offset (off response). We recorded the responses of cortical neurons to step indentations and found many neurons in areas 3b and 1 to exhibit properties that are intermediate between these two extremes: These neurons responded during the sustained portion of the stimulus and also at the offset of the stimulus. Several lines of evidence indicate that these neurons, which exist in large proportions even at these early stages of somatosensory cortical processing, receive input from both populations of afferents. First, we show that many cortical neurons have both a significant sustained response and a significant off response. Second, the strength of the off response is uncorrelated with that of the sustained response, which is to be expected if sustained and off responses stem from different populations of afferent fibers. Third, the bulk of the variance in cortical responses to step indentations can be accounted for using a linear combination of both SA1 and RA responses. Finally, we show that the off response in cortical neurons does not reflect rebound from inhibition. We conclude that the convergence of modality specific input onto individual neurons is common in primary somatosensory cortex and discuss how this conclusion might be reconciled with previous findings.

Quality Time: Representation of a Multidimensional Sensory Domain through Temporal Coding

Receptive fields of sensory neurons in the brain are usually restricted to a portion of the entire stimulus domain. At all levels of the gustatory neuraxis, however, there are many cells that are broadly tuned, i.e., they respond well to each of the basic taste qualities (sweet, sour, salty, and bitter). Although it might seem that this broad tuning precludes a major role for these cells in representing taste space, here we show the opposite--namely, that the tastant-specific temporal aspects (firing rate envelope and spike timing) of their responses enable each cell to represent the entire stimulus domain. Specifically, we recorded the response patterns of cells in the nucleus of the solitary tract (NTS) to representatives of four basic taste qualities and their binary mixtures. We analyzed the temporal aspects of these responses, and used their similarities and differences to construct the taste space represented by each neuron. We found that for the more broadly tuned neurons in the NTS, the taste space is a systematic representation of the entire taste domain. That is, the taste space of these broadly tuned neurons is three dimensional, with basic taste qualities widely separated and binary mixtures placed close to their components. Further, the way that taste quality is represented by the firing rate envelope is consistent across the population of cells. Thus, the temporal characteristics of responses in the population of NTS neurons, especially those that are more broadly tuned, produce a comprehensive and logical representation of the taste world.

Interaction between Attention and Bottom-Up Saliency Mediates the Representation of Foreground and Background in an Auditory Scene

Author SummaryAttention is the cognitive process underlying our ability to focus on specific aspects of our environment while ignoring others. By its very definition, attention plays a key role in differentiating foreground (the object of attention) from unattended clutter, or background. We investigate the neural basis of this phenomenon by engaging listeners to attend to different components of a complex acoustic scene. We present a spectrally and dynamically rich, but highly controlled, stimulus while participants perform two complementary tasks: to attend either to a repeating target note in the midst of random interferers (“maskers”), or to the background maskers themselves. Simultaneously, the participants' neural responses are recorded using the technique of magnetoencephalography (MEG). We hold all physical parameters of the stimulus fixed across the two tasks while manipulating one free parameter: the attentional state of listeners. The experimental findings reveal that auditory attention strongly modulates the sustained neural representation of the target signals in the direction of boosting foreground perception, much like known effects of visual attention. This enhancement originates in auditory cortex, and occurs exclusively at the frequency of the target rhythm. The results show a strong interaction between the neural representation of the attended target with the behavioral task demands, the bottom-up saliency of the target, and its perceptual detectability over time.

Predicting stimulus‐rate sensitivity of human somatosensory fMRI signals with MEG

With increasing stimulus rate (SR), cortical EEG and MEG responses typically decrease in amplitude whereas BOLD fMRI signals increase. To address this discrepancy, we predicted BOLD responses with squared MEG waveforms using a recently proposed energy-density model. Tactile stimuli were delivered to finger tips at SRs of 1, 4, or 10 Hz in successive 25-s blocks, and brain signals were detected from area 3b of the primary somatosensory cortex of nine healthy adults using a 306-channel whole-scalp neuromagnetometer and a 3-T fMRI magnet. The main MEG deflections decreased in amplitude as a function of SR, whereas the BOLD signals increased from 1- to 4-Hz SR, with no further change at 10 Hz. MEG energy densities, obtained over the whole stimulus train and convolved with different hemodynamic response functions, predicted both the shape and amplitude of the BOLD signals well, and incorporation of nonlinear terms into the model did not offer any further advantage. Thus, squared MEG waveforms obtained over the entire stimulus train provided an appropriate estimate of area 3b neuronal activity associated with the BOLD signal.

Accurate Sound Localization in Reverberant Environments Is Mediated by Robust Encoding of Spatial Cues in the Auditory Midbrain

In reverberant environments, acoustic reflections interfere with the direct sound arriving at a listener's ears, distorting the spatial cues for sound localization. Yet, human listeners have little difficulty localizing sounds in most settings. Because reverberant energy builds up over time, the source location is represented relatively faithfully during the early portion of a sound, but this representation becomes increasingly degraded later in the stimulus. We show that the directional sensitivity of single neurons in the auditory midbrain of anesthetized cats follows a similar time course, although onset dominance in temporal response patterns results in more robust directional sensitivity than expected, suggesting a simple mechanism for improving directional sensitivity in reverberation. In parallel behavioral experiments, we demonstrate that human lateralization judgments are consistent with predictions from a population rate model decoding the observed midbrain responses, suggesting a subcortical origin for robust sound localization in reverberant environments.

Stimulus sampling as an exploration mechanism for fast reinforcement learning

Reinforcement learning in neural networks requires a mechanism for exploring new network states in response to a single, nonspecific reward signal. Existing models have introduced synaptic or neuronal noise to drive this exploration. However, those types of noise tend to almost average out-precluding or significantly hindering learning -when coding in neuronal populations or by mean firing rates is considered. Furthermore, careful tuning is required to find the elusive balance between the often conflicting demands of speed and reliability of learning. Here we show that there is in fact no need to rely on intrinsic noise. Instead, ongoing synaptic plasticity triggered by the naturally occurring online sampling of a stimulus out of an entire stimulus set produces enough fluctuations in the synaptic efficacies for successful learning. By combining stimulus sampling with reward attenuation, we demonstrate that a simple Hebbian-like learning rule yields the performance that is very close to that of primates on visuomotor association tasks. In contrast, learning rules based on intrinsic noise (node and weight perturbation) are markedly slower. Furthermore, the performance advantage of our approach persists for more complex tasks and network architectures. We suggest that stimulus sampling and reward attenuation are two key components of a framework by which any single-cell supervised learning rule can be converted into a reinforcement learning rule for networks without requiring any intrinsic noise source.

Uncertainty reveals surround modulation of shape

Noisy estimations of shape can be partially resolved by incorporating relevant information from the context. The effect of surround stimuli on shape perception becomes clear in illusions of shape contrast and assimilation. In this study, we answer the question how a surround-induced bias depends on the reliability of shape signals. This way, we assess the processes by which an observer incorporates relevant data from the context into the shape estimate. We selectively added visual noise to the center and surround and compared a bias in shape perception with a control condition where no noise was added. In the conditions where shape and surround stimuli were well defined, we found a shape-contrast bias. When the surround stimuli were degraded, this contrast bias decreased. Most interestingly, when the central shape was degraded, an assimilation bias was observed. This bias was larger when the entire stimulus was degraded compared to when only the central shape was degraded. This suggests that shape contrast is the result of inference processes relying on local representations in early visual areas whereas assimilation is related to inference processes by global representations in higher visual areas.

The mfERG response topography with scaled stimuli: effect of the stretch factor

In multifocal electroretinogram recordings, the stimulus array is usually scaled with eccentricity to compensate for cone density changes. The strength of this scaling is parameterized by the so-called "stretch factor" (SF). In this study, we determined the quantitative influence of the SF striving for equal response densities over the entire stimulus area. VERIS 4.8 software was used to record multifocal ERGs from 11 normals with 61 hexagons and a 60 degrees stimulus diameter. Six recordings were obtained per subject with SFs 0, 12, 23, 25, 27, and 29. For analysis, we calculated the mean hexagon area per eccentricity and the average amplitude for elements of equal eccentricity (rings 1-5 = R1-R5), normalized to R2. As expected, recordings with SF = 0 showed the steepest amplitude drop-off with eccentricity (relative amplitudes of 1.90 (R1/R2), 1.0 (R2/R2), 0.67 (R3/R2), 0.51 (R4/R2), 0.38 (R5/R2). Using the preset SF = 12 in our set-up resulted in R1-amplitudes 1.7x larger than the peripheral amplitudes (relative amplitudes 1.39, 1.0, 0.88, 0.87, 0.82). SF = 23 compensated best for eccentricity, yielding relative amplitudes of 1.2, 1.0, 1.02, 1.08, and 1.11. Deviations increased again with higher SFs. To minimize amplitude variability over the stimulated retinal area, the optimal SF needs to be adjusted to the individual recording set-up.

The detection of multiple global directions: Capacity limits with spatially segregated and transparent-motion signals

An important constraint on motion processing is the maximum number of directions that can be perceived at the same time. When transparent-motion stimuli are constructed based solely on direction differences, prior studies demonstrate that no more than two directions are seen simultaneously. However, this limit has been extended to three when signal directions drive independent speed- or disparity-tuned global-motion systems. The present study sought to determine whether this three-direction capacity reflects the specific mechanisms of transparent-motion detection or a more general restriction on global-motion processing. Using both transparent and spatially segregated stimuli, observers indicated which of the two intervals contained the most directions, with simultaneous processing ensured through brief durations and n vs. n + 1 signal comparisons. When spatially segregated directions were interleaved in patches, no more than two were seen, as with direction-defined transparent motion. In contrast, separating these directions into distinct spatial regions allowed the detection of up to three. Signal-detection thresholds did not vary across these signal arrangements, suggesting that the two-direction capacity results from signal-to-noise pooling across the entire stimulus, with the higher capacity for spatially distinct directions arising from independent pooling within each region. Together, these results provide further evidence for an upper capacity of three directions within the global-motion stage.

Context-Dependent Changes in Functional Circuitry in Visual Area MT

Animals can flexibly change their behavior in response to a particular sensory stimulus; the mapping between sensory and motor representations in the brain must therefore be flexible as well. Changes in the correlated firing of pairs of neurons may provide a metric of changes in functional circuitry during behavior. We studied dynamic changes in functional circuitry by analyzing the noise correlations of simultaneously recorded MT neurons in two behavioral contexts: one that promotes cooperative interactions between the two neurons and another that promotes competitive interactions. We found that identical visual stimuli give rise to differences in noise correlation in the two contexts, suggesting that MT neurons receive inputs of central origin whose strength changes with the task structure. The data are consistent with a mixed feature-based attentional strategy model in which the animal sometimes alternates attention between opposite directions of motion and sometimes attends to the two directions simultaneously.

Hemispheric asymmetries for temporal information processing: Transient detection versus sustained monitoring

This study investigated functional differences in the processing of visual temporal information between the left and right hemispheres (LH and RH). Participants indicated whether or not a checkerboard pattern contained a temporal gap lasting between 10 and 40 ms. When the stimulus contained a temporal signal (i.e. a gap), responses were more accurate for the right visual field-left hemisphere (RVF-LH) than for the left visual field-right hemisphere (LVF-RH). This RVF-LH advantage was larger for the shorter gap durations (Experiments 1 and 2), suggesting that the LH has finer temporal resolution than the RH, and is efficient for transient detection. In contrast, for noise trials (i.e. trial without temporal signals), there was a LVF-RH advantage. This LVF-RH advantage was observed when the entire stimulus duration was long (240 ms, Experiment 1), but was eliminated when the duration was short (120 ms, Experiment 2). In Experiment 3, where the gap was placed toward the end of the stimulus presentation, a LVF-RH advantage was found for noise trials whereas the RVF-LH advantage was eliminated for signal trials. It is likely that participants needed to monitor the stimulus for a longer period of time when the gap was absent (i.e. noise trials) or was placed toward the end of the presentation. The RH may therefore be more efficient in the sustained monitoring of visual temporal information whereas the LH is more efficient for transient detection.