Spatial Suppression Research Articles

Level dependence of spatial processing in the primate auditory cortex

Sound localization in both humans and monkeys is tolerant to changes in sound levels. The underlying neural mechanism, however, is not well understood. This study reports the level dependence of individual neurons' spatial receptive fields (SRFs) in the primary auditory cortex (A1) and the adjacent caudal field in awake marmoset monkeys. We found that most neurons' excitatory SRF components were spatially confined in response to broadband noise stimuli delivered from the upper frontal sound field. Approximately half the recorded neurons exhibited little change in spatial tuning width over a ~20-dB change in sound level, whereas the remaining neurons showed either expansion or contraction in their tuning widths. Increased sound levels did not alter the percent distribution of tuning width for neurons collected in either cortical field. The population-averaged responses remained tuned between 30- and 80-dB sound pressure levels for neuronal groups preferring contralateral, midline, and ipsilateral locations. We further investigated the spatial extent and level dependence of the suppressive component of SRFs using a pair of sequentially presented stimuli. Forward suppression was observed when the stimuli were delivered from "far" locations, distant to the excitatory center of an SRF. In contrast to spatially confined excitation, the strength of suppression typically increased with stimulus level at both the excitatory center and far regions of an SRF. These findings indicate that although the spatial tuning of individual neurons varied with stimulus levels, their ensemble responses were level tolerant. Widespread spatial suppression may play an important role in limiting the sizes of SRFs at high sound levels in the auditory cortex.

Orientation and Spatial Frequency Selective Surround Suppression Impairment in High Autistic tendency

Orientation and Spatial Frequency Selective Surround Suppression Impairment in High Autistic tendency

Mitigation of Loopback Self-Interference in Full-Duplex MIMO Relays

Full-duplex relaying is more spectrally efficient than half-duplex relaying as only one channel use is needed per two hops. However, it is crucial to minimize relay self-interference to render full duplex feasible. For this purpose, we analyze a broad range of multiple-input multiple-output (MIMO) mitigation schemes: natural isolation, time-domain cancellation, and spatial suppression. Cancellation subtracts replicated interference signal from the relay input while suppression reserves spatial dimensions for receive and transmit filtering. Spatial suppression can be achieved by antenna subset selection, null-space projection, i.e., receiving and transmitting in orthogonal subspaces, or joint transmit and receive beam selection to support more spatial streams by choosing the minimum eigenmodes for overlapping subspaces. In addition, minimum mean square error (MMSE) filtering can be employed to maintain the desired signal quality, which is inherent for cancellation, and the combination of time- and spatial-domain processing may be better than either alone. Targeting at minimal interference power, we solve optimal filters for each scheme in the cases of joint, separate and independent design. The performance of mitigation schemes is evaluated and compared by simulations. The results confirm that self-interference can be mitigated effectively also in the presence of imperfect side information.

Increasing stimulus size impairs first- but not second-order motion perception

As stimulus size increases, the direction of high-contrast moving stimuli becomes increasingly difficult to perceive. This counterintuitive effect, termed spatial suppression, is believed to reflect antagonistic center-surround interactions--mechanisms that play key roles in tasks requiring sensitivity to relative motion. It is unknown, however, whether second-order motion also exhibits spatial suppression. To test this hypothesis, we measured direction discrimination thresholds for first- and second-order stimuli of varying sizes. The results revealed increasing thresholds with increasing size for first-order stimuli but demonstrated no spatial suppression of second-order motion. This selective impairment of first-order motion predicts increasing predominance of second-order cues as stimulus size increases. We confirmed this prediction by utilizing compound stimuli that contain first- and second-order information moving in opposite directions. Specifically, we found that for large stimuli, motion perception becomes increasingly determined by the direction of second-order cues. Overall, our findings show a lack of spatial suppression for second-order stimuli, suggesting that the second-order system may have distinct functional roles, roles that do not require high sensitivity to relative motion.

Spatial characteristics of motion-sensitive mechanisms change with age and stimulus spatial frequency

Contrast-dependent interactions between classical (CRF) and non-classical regions (nCRF) of visual neuron receptive fields are well documented in primate visual cortex. Physiological models that describe CRF and nCRF interactions in single neurons have recently been applied to psychophysical measures of spatial summation and suppression in motion perception of young adults ( Tadin & Lappin, 2005). We wished to determine whether such models could account for the reduction in spatial suppression that occurs in normal aging ( Betts et al., 2005). We applied three models to duration thresholds obtained in a simple motion discrimination task using drifting Gabor stimuli that ranged in spatial frequency from 0.5 to 4 c/deg. We found that a model in which the center CRF and surrounding nCRF are represented as spatially-overlapping excitatory and inhibitory 2D Gaussians with independent contrast response functions, which we call the Gain model, could account for the effects of aging simply by increasing the spatial extent of the CRF. Two additional models were evaluated. The Size model, which varied the size of the CRF as a function of contrast, produced CRF and nCRF size constants that departed significantly from physiological estimates of receptive field sizes. The Drive model, which yoked the activation of the suppressive nCRF to the CRF response, yielded reasonable fits to the data and suggested an age-related decline in the strength of suppression from the nCRF. However, the Drive model estimated the CRF size parameter to be equal to, or even slightly larger than, the nCRF size parameter, which is inconsistent with the physiological literature. Our findings therefore suggest that the Gain model provides the most plausible estimates of receptive field sizes. Based on this model, age-related increases in the size of central excitatory receptive fields relative to the inhibitory surrounds may contribute to behavioral measures of reduced spatial suppression found in older observers.

Asymmetric Effects of Spatial Suppression on First- and Second-Order Motion

Asymmetric Effects of Spatial Suppression on First- and Second-Order Motion

Spatial Suppression of the Perceived Contrast is Polarity Selective

The apparent contrast of a texture is reduced when it is surrounded by another texture with high contrast. This contrast-contrast illusion has been thought to be a result of spatial interactions between visual channels that encode contrast energy. In the present study, we show that the contrast-contrast is selective to the luminance polarity by using texture patterns composed of sparse elongated blobs. The apparent contrast of a texture of bright (dark) elements was substantially reduced only when it was surrounded by a texture of elements with the same polarity. This polarity specificity was not found for textures of dense elements, which was similar to those used in previous studies, probably because such stimuli should inevitably activate both on- and off-type sensors. The polarity selective suppression decreased as the difference in orientation between the centre and surround elements increased, but still remained as for orthogonally-oriented elements. These results suggest that the contrast-contrast illusion largely depends on spatial interactions between visual channels that are selective to the luminance polarity and partially to the orientation.

Atlas‐based automated positioning of outer volume suppression slices in short‐echo time 3D MR spectroscopic imaging of the human brain

Spatial suppression of peripheral lipid-containing regions in volumetric MR spectroscopic imaging of the human brain requires placing large numbers of outer volume suppression (OVS) slices, which is time-consuming, prone to operator error and may introduce subject-dependent variability in volume coverage. We developed a novel, computationally efficient atlas-based approach for automated positioning of up to 16 OVS slices and the MR spectroscopic imaging slab. Standardized positions in Montreal Neurological Institute atlas space were established offline using a recently developed iterative optimization procedure. During the scanning session, positions in subject space were computed using affine transformation of standardized positions in Montreal Neurological Institute space. Offline analysis using magnetization prepared rapid gradient echo scans from 11 subjects demonstrated reliable OVS placement, comparable with but faster than iterative placement in subject space. This atlas-based method was further validated in 14 subjects using 3D short-echo time proton-echo-planar-spectroscopic-imaging at 3 T. Comparison of manual and automatic placement using 8 OVS slices demonstrated consistent MR spectroscopic imaging volume selection and comparable spectral quality with similar degree of lipid suppression and number of usable voxels. Automated positioning of 16 OVS slices enabled larger volume coverage, while maintaining similar spectral quality and lipid suppression. Atlas-based automatic prescription of short echo time MR spectroscopic imaging is expected to be advantageous for longitudinal and cross-sectional studies.

Improved Motion Perception and Impaired Spatial Suppression following Disruption of Cortical Area MT/V5

As stimulus size increases, motion direction of high-contrast patterns becomes increasingly harder to perceive. This counterintuitive behavioral result, termed "spatial suppression," is hypothesized to reflect center-surround antagonism-a receptive field property ubiquitous in sensory systems. Prior research proposed that spatial suppression of motion signals is a direct correlate of center-surround antagonism within cortical area MT. Here, we investigated whether human MT/V5 is indeed causally involved in spatial suppression of motion signals. The key assumption is that a disruption of neural mechanisms that play a critical role in spatial suppression could allow these normally suppressed motion signals to reach perceptual awareness. Thus, our hypothesis was that a disruption of MT/V5 should weaken spatial suppression and, consequently, improve motion perception of large, moving patterns. To disrupt MT/V5, we used offline 1 Hz transcranial magnetic stimulation (TMS)-a method that temporarily attenuates normal functioning of the targeted cortex. Early visual areas were also targeted as a control site. The results supported our hypotheses and showed that disruption of MT/V5 improved motion discrimination of large, moving stimuli, presumably by weakening surround suppression strength. This effect was specific to MT/V5 stimulation and contralaterally presented stimuli. Evidently, the critical neural constraints limiting motion perception of large, high-contrast stimuli involve MT/V5. Additionally, our findings mimic spatial suppression deficits that are observed in several patient populations and implicate impaired MT/V5 processes as likely neural correlates for the reported perceptual abnormalities in the elderly, patients with schizophrenia and those with a history of depression.

Spatial characteristics of center-surround antagonism in motion discrimination

In young observers, the stimulus duration required to correctly discriminate the direction of low contrast moving targets decreases (i.e., performance improves) as stimulus area increases (spatial summation). However, at high contrast stimuli, duration thresholds increase with increasing stimulus size (spatial suppression), a result thought to be linked to center-surround antagonism in non-classical receptive fields (Tadin et al., 2003). Previous research from our lab suggests that such center-surround antagonism changes across the life span, as inhibitory mechanisms in visual cortex decline (Betts et al., 2005). Here we examine the extent to which spatial summation and suppression, and age-related changes in both, depend on stimulus spatial frequency. Stimulus duration thresholds for 0.5, 1, 2, and 4 c/deg Gabors drifting at 2 deg/s were measured over a range of contrasts (2.8% – 46%) and sizes (0.7–5 visible cycles) for younger (19–30 years) and older (63–75 years) observers. In younger observers, spatial summation and suppression occurred in all spatial frequency conditions, with summation occurring over a larger range of cycles as frequency increased for low contrast stimuli, and similar levels of suppression across all frequencies for high contrast stimuli. Older observers showed a similar pattern of results at low contrast, but as contrast increased, suppression was reduced compared to younger observers across all spatial frequencies. Our results suggest that patterns of spatial summation and suppression are preserved across low and medium spatial frequencies, and that the age-related reduction in spatial suppression is independent of stimulus spatial frequency.

Automatic placement of outer volume suppression slices in MR spectroscopic imaging of the human brain

Spatial suppression of peripheral regions (outer volume suppression) is used in MR spectroscopic imaging to reduce contamination from strong lipid and water signals. The manual placement of outer volume suppression slices requires significant operator interaction, which is time consuming and introduces variability in volume coverage. Placing a large number of outer volume saturation bands for volumetric MR spectroscopic imaging studies is particularly challenging and time consuming and becomes unmanageable as the number of suppression bands increases. In this study, a method is presented that automatically segments a high-resolution MR image in order to identify the peripheral lipid-containing regions. This method computes an optimized placement of suppression bands in three dimensions and is based on the maximization of a criterion function. This criterion function maximizes coverage of peripheral lipid-containing areas and minimizes suppression of cortical brain regions and regions outside of the head. Computer simulation demonstrates automatic placement of 16 suppression slices to form a convex hull that covers peripheral lipid-containing regions above the base of the brain. In vivo metabolite mapping obtained with short echo time proton-echo-planar spectroscopic imaging shows that the automatic method yields a placement of suppression slices that is very similar to that of a skilled human operator in terms of lipid suppression and usable brain voxels.

Low-level mechanisms do not explain paradoxical motion percepts.

Classic psychophysical studies have shown that increasing the size of low-contrast moving stimuli increases their discriminability, indicating spatial summation mechanisms. More recently, a number of studies have reported that for moderate and high contrasts, size increases yield substantial deteriorations of motion perception-a result described as psychophysical spatial suppression. While this result resembles known characteristics of suppressive center-surround neural mechanisms, a recent study (C. R. Aaen-Stockdale, B. Thompson, P. C. Huang, & R. F. Hess, 2009) argued that observed size-dependent changes in motion perception might be explained by differences in contrast sensitivity for stimuli of different sizes. Here, we tested this hypothesis using duration threshold measurements-an experimental approach used in several spatial suppression studies. The results replicated previous reports by demonstrating spatial suppression at a fixed, high contrast. Importantly, we observed strong spatial suppression even when stimuli were normalized relative to their contrast thresholds. While the exact mechanisms underlying spatial suppression still need to be adequately characterized, this study demonstrates that a low-level explanation proposed by Aaen-Stockdale et al. (2009) cannot account for spatial suppression results.

Enhanced Visual Motion Perception in Major Depressive Disorder

Major depressive disorder (MDD) is a mood disorder that is not traditionally considered to affect the visual system. However, recent findings have reported decreased cortical levels of the inhibitory neurotransmitter GABA in occipital cortex. To explore possible functional consequences of MDD on visual processing, we applied a psychophysical visual motion processing task in which healthy young adults typically exhibit impaired perceptual discrimination of large high-contrast stimuli. It has been suggested that this phenomenon, spatial suppression, is mediated by GABAergic center-surround antagonism in visual pathways. Based on previous findings linking MDD to occipital GABA dysfunction, we hypothesized that MDD patients would exhibit decreased spatial suppression, leading to the counterintuitive hypothesis of better psychophysical performance. Indeed, motion perception for typically suppressed stimuli was enhanced in patients with MDD compared with age-matched controls. Furthermore, the degree of spatial suppression correlated with an individual's illness load; patients with greater lifetime duration of depression exhibited the least spatial suppression and performed the best in the high-contrast motion discrimination task. Notably, this decrease in spatial suppression persisted beyond recovery and without the confound of acute illness or treatment; all patients had been clinically recovered and unmedicated for several months at the time of testing, suggesting that depression has ubiquitous consequences that may persist long after mood symptoms have receded. This finding raises the possibility that spatial suppression may represent a sensitive endophenotypic marker of trait vulnerability in MDD.

Spatial characteristics of center-surround antagonism in younger and older adults

Sensitivity to motion direction is affected by stimulus size, contrast (D. Tadin & J. S. Lappin, 2005; D. Tadin, J. S. Lappin, L. A. Gilroy, & R. Blake, 2003), and observer age (L. R. Betts, C. P. Taylor, A. B. Sekuler, & P. J. Bennett, 2005). Here, we investigated the effect of spatial frequency on motion discrimination and how sensitivity changes in older adulthood. We measured stimulus duration thresholds for younger (18-30 years) and older (60-75 years) observers using drifting Gabor gratings that differed in size, spatial frequency, and contrast. A simple model characterized age differences in the threshold-vs.-size functions. The model parameter fits were consistent with an age-related decrease in the strength of spatial suppression. The model also provided good fits to the thresholds when plotted as a function of the total stimulus contrast energy, which suggests that age-related changes in summation and suppression can be modeled as reduced sensitivity to contrast energy. The summation parameter scaled according to stimulus spatial frequency in younger observers only. Suppression strength decreased as a function of spatial frequency in both age groups. Our findings provide additional evidence for age-related changes in summation and suppression mechanisms and suggest that the total contrast energy in the stimulus plays an important role in determining sensitivity to motion direction in both younger and older adults.

Influence of a secondary, parallel, low Reynolds number, round jet on a turbulent axisymmetric jet

The influence of a secondary, parallel, low Reynolds number, round jet on the development of a turbulent axisymmetric jet with Re 5500 is studied experimentally by means of 2 D laser Doppler anemometry. Two weakly turbulent jets of Reynolds numbers, 1200 and 1800, are considered. In both test cases the secondary jet is fully absorbed by the primary one. The mean and turbulent flow structure of the standalone primary jet is compared to previous studies referring to the jet’s development before and inside the self-similarity region. The flow field of interacting jets is then analyzed and discussed. Characteristic profiles are presented at locations within the merging and the mixing zones. In the early stages of development, the patterns of both jets can be identified. Within the merging region, besides the absorption of the secondary jet, the measurements indicate a spatial suppression of primary jet’s characteristics. Further downstream, the profiles resemble to those of a standalone jet. Higher values of the mean and turbulent terms are observed while the profiles extend over a wider region, in accordance with the secondary jet’s Reynolds number. Results of the present study demonstrate that skewness and flatness factors can be used as indicators of small scale mixing.

Joint bandwidth allocation, element assignment and scheduling for wireless mesh networks with MIMO links

With the unique features of spatial multiplexing and interference suppression, Multiple Input Multiple Output (MIMO) techniques have great potential in the improvement of network capacity over conventional antenna technologies. In order to exploit the benefit of simultaneous transmissions provided by MIMO, researchers have proposed a number of cross-layer optimizations and MAC layer designs to increase the throughput of wireless mesh or ad hoc networks, where the number of elements in the antenna arrays are pre-allocated or evenly assigned to the routers. In this paper, we argue that using the same number of elements in each antenna array in all routers is not a necessary condition for the improvement of system performance. This is because the requirement for the number of elements is quite different for each router. Especially at those critical routers that have huge aggregate traffic toward the gateway, more elements are needed not only for the traffic relay but also for the interference suppression. Based on this observation, we define the joint problem of bandwidth allocation, element assignment and scheduling to characterize the throughput benefits of cross-layer optimizations. We propose a Cost-Aware Element Assignment (CAEA) technique to minimize the total number of the antenna elements when still achieving the optimal bandwidth allocation. In addition, to verify the efficiency of the CAEA assignment, a heuristic Traffic-aware Stream-controlled Link Scheduling (TSLS) algorithm is proposed to provide a schedulable bandwidth allocation. We demonstrate through extensive simulations that our solutions (CAEA, TSLS) not only effectively save the total cost on antenna elements but also perform close to optimal on the average.

Evaluation of the interference rejection capability of a uniform circular array

This paper deals with the interference rejection capability of circular antenna arrays, as base stations of wireless communications systems. In general, this figure of merit depends strongly on the array configuration and is a function of the direction of arrival of the desired signal, the number of antenna elements, and their spacing. The main goal and contribution of this paper is to provide a detailed presentation of the behavior of the interference rejection capability of uniform circular arrays. The effect of varying the number of elements and inter‐element spacing on the SIR (signal‐to‐interference ratio) performance is considered. In addition, the effect of mutual coupling is taken into account. Analytical results, for the total average of spatial interference suppression coefficient for circular arrays, as a function of inter‐element spacing when mutual coupling is known and without mutual coupling are provided.

EFFECT OF MUTUAL COUPLING AND CONFIGURATION OF CONCENTRIC CIRCULAR ARRAY ANTENNA ON THE SIGNAL-TO-INTERFERENCE PERFORMANCE IN CDMA SYSTEMS

The suitable use of an array antenna at the base station of a wireless communication system can result in improvement in signalto-interference ratio (SIR). In the present work, we consider circular array (CA) and concentric circular antenna array (CCAA) that are used for smart antenna systems. The performance criteria for SIR improvement is employed in this paper, is the spatial interference suppression coefficient. We first develop the expression of this figure of merit for CCAA and then analyze and compare the SIR performance for various configurations of eight and nine elements of CA and CCAA, with and without the element in the center by using circular patch antenna are provided. In addition, the effect of mutual coupling (MC) is taken into account.

Effects of Dominant Other-Sector Interference on Dual Rx-Antenna HSDPA Performance

Link level SINR simulation results and network level sector throughput simulation results that quantify the benefit of dual antenna MMSE reception in a macrocellular WCDMA/HSDPA system are provided. Dual antenna RAKE receiver performance serves as baseline reference. Link-level simulation results are accompanied by a novel analytical expression that in flat Rayleigh fading and for uncorrelated rx-antenna branches describes spatial interference suppression mean SINR gain as function of a dominant other-sector interference ratio (DIR). It is shown that the MMSE receiver's spatial interference suppression gain heavily depends on the amount of experienced DIR. The higher the DIR the higher the SINR gain. Nevertheless, seen on network level the SINR gain turns into moderate sector throughput gain, well below 50%. This is due to the fact that high DIR situations are rare in the investigated macrocellular scenario. Moreover, the logarithmic relation between SINR and capacity hinders translation of the full SINR gain into HSDPA sector throughput.

Direct neurophysiological evidence for spatial suppression surrounding the focus of attention in vision

The spatial focus of attention has traditionally been envisioned as a simple spatial gradient of enhanced activity that falls off monotonically with increasing distance. Here, we show with high-density magnetoencephalographic recordings in human observers that the focus of attention is not a simple monotonic gradient but instead contains an excitatory peak surrounded by a narrow inhibitory region. To demonstrate this center-surround profile, we asked subjects to focus attention onto a color pop-out target and then presented probe stimuli at various distances from the target. We observed that the electromagnetic response to the probe was enhanced when the probe was presented at the location of the target, but the probe response was suppressed in a narrow zone surrounding the target and then recovered at more distant locations. Withdrawing attention from the pop-out target by engaging observers in a demanding foveal task eliminated this pattern, confirming a truly attention-driven effect. These results indicate that neural enhancement and suppression coexist in a spatially structured manner that is optimal to attenuate the most deleterious noise during visual object identification.