Low-contrast Stimuli Research Articles

Apparent motion speed dependence on contrast and orientation: Evidence from MEG

The perceived speed of fast (>40°/s.) apparent motion of a Gabor patch whose orientation is aligned with the motion axis is much faster than when it is orthogonal to it. Electrophysiological recordings in cat and modeling (Lorenceau & al., 2002) suggest that this perceptual bias is explained by the phase advance in the firing of V1 neurons resulting from synaptic summation of horizontal and feed-forward inputs, whose responses are read-out by MT speed selective cells. Using the same paradigm in humans, we present additional psychophysical and MEG experiments showing that: 1. At high physical speeds, perceived speed is faster at low (20%) than at high (50%) Gabor contrast, a result at odd with previous reports of lower perceived speed at low contrasts (Thompson, 1982). 2. MEG data show that: i. the latencies of the first peak responses (∼90 ms.) are longer (by ∼20 ms.) at low as compared to high contrast. At these latencies, response amplitude is independent of Gabor orientation; ii. At longer latencies (∼160 ms.) and low stimulus contrast, the peak response amplitude is larger for a Gabor aligned to the motion axis than for a Gabor orthogonal to it, an effect not seen at high contrast. As predicted by our model, perceived speed of fast apparent motion sequences is biased in a contrast dependent way by the orientation of a target relative to its motion path, a finding well correlated to the recorded MEG activity.

Reduced contrast does not reduce visual crowding

Background: Previous studies have reported reduced crowding for low contrast stimuli (Kothe & Regan, 1990: Optom Vis Sci; Simmers et al., 1999: Ophthal Physiol Opt). Both defined crowding as a change in acuity for single vs. flanked letters, and used foveal viewing. This study sought to confirm these findings using a more traditional crowding task (i.e. comparison of single and flanked letter identification at different retinal eccentricities). Methods: 10 observers (18–34 yo; VA 20/14) identified 0.34 deg letters and letters flanked by an ‘x’ on either side (xax) at fixation and ±1, 3, 5, and 7 letter spaces (±0.34, ±1.02, ±1.70, and ±2.38 deg) from fixation on the horizontal meridian. All 25 letters (other than x) were presented at each location; letter and position were randomly selected from trial-to-trial. In separate blocks, the stimuli were ∼100, 30, and 10% contrast. Crowding was defined as the difference between single letter and flanked letter performance. Results: There was little difference in crowding for the three contrasts. Averaged across position, there was 14±0.3, 15±0.1, and 19±0.2% crowding for the 100, 30, and 10% contrasts, respectively [F(2, 18) = 2.3, p = 0.127]. There was little change from 100% contrast for single or flanked letters presented with 30% contrast, but with 10% contrast, performance was significantly reduced for both stimulus types. Accuracy was similar across positions for single letters presented with 10% contrast and flanked letters presented with 100% contrast. Conclusions: Unlike previous reports, these data show no change in crowding with reduced contrast stimuli. It is possible that 10% is not sufficiently reduced for crowding to disappear, but Kothe and Regan showed reduced crowding for 11% contrast stimuli. The data suggest that the effect of flanking letters may be to reduce the effective contrast of the target letters.

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.

A gain-control theory of binocular combination

In binocular combination, light images on the two retinas are combined to form a single “cyclopean” perceptual image, in contrast to binocular rivalry which occurs when the two eyes have incompatible (“rivalrous”) inputs and only one eye`s stimulus is perceived. We propose a computational theory for binocular combination with two basic principles of interaction: in every spatial neighborhood, each eye (i) exerts gain control on the other eye's signal in proportion to the contrast energy of its own input and (ii) additionally exerts gain control on the other eye's gain control. For stimuli of ordinary contrast, when either eye is stimulated alone, the predicted cyclopean image is the same as when both eyes are stimulated equally, coinciding with an easily observed property of natural vision. The gain-control theory is contrast dependent: Very low-contrast stimuli to the left- and right-eye add linearly to form the predicted cyclopean image. The intrinsic nonlinearity manifests itself only as contrast increases. To test the theory more precisely, a horizontal sine wave grating of 0.68 cycles per degree is presented to each eye. The gratings differ in contrast and phase. The predicted (and perceived) cyclopean grating also is a sine wave; its apparent phase indicates the relative contribution of the two eyes to the cyclopean image. For 48 measured combinations of phase and contrast, the theory with only one estimated parameter accounts for 95% of the variance of the data. Therefore, a simple, robust, physiologically plausible gain-control theory accurately describes an early stage of binocular combination.

Lapsing when sleep deprived: Neural activation characteristics of resistant and vulnerable individuals

Lapses of attention, in the form of delayed responses to salient stimuli, increase in frequency for some but not all persons after sleep deprivation (SD). To identify patterns of task-related brain activation that might explain differences in vulnerability to SD, we performed fMRI on participants during a visual, selective attention task. We analyzed the correct responses in a trial-by-trial fashion to model the effects of response time. Stimulus contrast was varied to modulate perceptual difficulty. Attentional lapses and low-contrast stimuli were independently associated with increased signal in fronto-parietal regions associated with biasing attention. Sleep-deprived vulnerable individuals showed reduced top down fronto-parietal signal across all levels of image contrast and this reduction was particularly significant during lapses. There was concurrent reduction in extrastriate cortex and thalamus activation. Non-vulnerable persons showed a trend towards higher top-down biasing of attention and preserved visual cortex activation during SD lapses. A major contributor to performance degradation in SD appears to be a reduction in top-down biasing of attention that is independent of task difficulty.

Diversity of efficient coding solutions for a population of noisy linear neurons

Methods E cient coding posits that neural resources (cost) should e ciently be utilized for signal representation (objective). (5, 14) • Dependency on the objective function has been well examined. (13, 6, 8) • Dependency on the cost function: For a single neuron: output range, ring rate, or variance constraints. (11, 4, 14) For multiple neurons: less studied. We examine the e ects of costs that are likely to be relevant in neural populations.

Temporal auditory capture does not affect the time course of saccadic mislocalization of visual stimuli

Irrelevant sounds can "capture" visual stimuli to change their apparent timing, a phenomenon sometimes termed "temporal ventriloquism". Here we ask whether this auditory capture can alter the time course of spatial mislocalization of visual stimuli during saccades. We first show that during saccades, sounds affect the apparent timing of visual flashes, even more strongly than during fixation. However, this capture does not affect the dynamics of perisaccadic visual distortions. Sounds presented 50 ms before or after a visual bar (that change perceived timing of the bars by more than 40 ms) had no measurable effect on the time courses of spatial mislocalization of the bars, in four subjects. Control studies showed that with barely visible, low-contrast stimuli, leading, but not trailing, sounds can have a small effect on mislocalization, most likely attributable to attentional effects rather than auditory capture. These findings support previous studies showing that integration of multisensory information occurs at a relatively late stage of sensory processing, after visual representations have undergone the distortions induced by saccades.

Fayez Nazeer Botros

The ability to predict the timing of forthcoming events, known as temporal expectation, has a strong impact on human information processing. Although there is growing consensus that temporal expectations enhance...

Evaluation of optic neuropathy in multiple sclerosis using low-contrast visual evoked potentials

Contrast acuity (identification of low-contrast letters on a white background) is frequently reduced in patients with demyelinating optic neuropathy associated with multiple sclerosis (MS), even when high-contrast (Snellen) visual acuity is normal. Since visual evoked potentials (VEPs) induced with high-contrast pattern-reversal stimuli are typically increased in latency in demyelinating optic neuropathy, we asked if VEPs induced with low-contrast stimuli would be more prolonged and thus helpful in identifying demyelinating optic neuropathy in MS. We studied 15 patients with clinically definite MS and 15 age-matched normal controls. All subjects underwent a neuro-ophthalmologic assessment, including measurement of high-contrast visual acuity and low-contrast acuities with 25%, 10%, 5%, 2.5%, and 1.25% contrast Sloan charts. In patients with MS, peripapillary retinal nerve fiber layer (RNFL) thickness was determined using optical coherence tomography. Monocular VEPs were induced using pattern-reversal checkerboard stimuli with 100% and 10% contrast between checks, at 5 spatial frequencies (8-130 minutes of arc). VEP latencies were significantly increased in response to low- compared with high-contrast stimuli in both groups. VEP latencies were significantly greater in patients with MS than controls for both high- and low-contrast stimuli. VEP latencies correlated with high- and low-contrast visual acuities and RNFL thickness. VEPs were less likely to be induced with low- than with high-contrast stimuli in eyes with severe residual visual loss. Visual evoked potentials obtained in patients with multiple sclerosis using low-contrast stimuli are increased in latency or absent when compared with those obtained using high-contrast stimuli and, thus, may prove to be helpful in identifying demyelinating optic neuropathy.

Impairments of Contrast Discrimination and Contrast Adaptation in Glaucoma

Contrast detection is commonly measured clinically; however, discrimination between contrasts is also important for natural vision. Furthermore, optimal performance requires the visual system to adapt to ambient contrast conditions. Recent studies of primate neurophysiology demonstrate significant retinal involvement in contrast adaptation. This study was conducted to investigate whether glaucoma alters contrast adaptation. Both detection and discrimination task performance were examined. Psychophysical contrast detection and discrimination thresholds were measured in central vision, for a vertically oriented D6 centered on 3 cyc/deg. Thresholds were measured with and without adaptation to low (15%)- and high (70%)-contrast, vertically oriented, 3-cyc/deg sinusoidal gratings. Fifteen people with glaucoma, and 15 age-similar control subjects participated. Full-contrast discrimination (dipper) functions were measured for a subset (three patients with glaucoma and three control subjects). On average, the glaucoma group showed elevated detection and discrimination thresholds relative to control subjects (detection: t(28) = 2.42; P = 0.02; discrimination: F(1,28) = 6.157, P = 0.02). For the subset of additionally tested participants, normalized contrast discrimination functions were similarly shaped for all observers. Glaucoma group thresholds were less influenced by contrast adaptation than were control subjects, for discrimination (F(1,28) = 10.89, P < 0.01) but not detection (F(1,28) = 2.28; P = 0.11). Differences between groups were greatest for low-contrast stimuli (significant interaction between contrast and group: P < 0.01). Glaucoma alters the effect of contrast adaptation on discrimination performance, particularly at low contrast. The study of suprathreshold aspects of vision may reveal new insights into the pathophysiology of glaucoma and possibly relate better to real-world visual performance than detection measures.

Modulation of the Contrast Response Function by Electrical Microstimulation of the Macaque Frontal Eye Field

Spatial attention influences representations in visual cortical areas as well as perception. Some models predict a contrast gain, whereas others a response or activity gain when attention is directed to a contrast-varying stimulus. Recent evidence has indicated that microstimulating the frontal eye field (FEF) can produce modulations of cortical area V4 neuronal firing rates that resemble spatial attention-like effects, and we have shown similar modulations of functional magnetic resonance imaging (fMRI) activity throughout the visual system. Here, we used fMRI in awake, fixating monkeys to first measure the response in 12 visual cortical areas to stimuli of varying luminance contrast. Next, we simultaneously microstimulated subregions of the FEF with movement fields that overlapped the stimulus locations and measured how microstimulation modulated these contrast response functions (CRFs) throughout visual cortex. In general, we found evidence for a nonproportional scaling of the CRF under these conditions, resembling a contrast gain effect. Representations of low-contrast stimuli were enhanced by stimulation of the FEF below the threshold needed to evoke saccades, whereas high-contrast stimuli were unaffected or in some areas even suppressed. Furthermore, we measured a characteristic spatial pattern of enhancement and suppression across the cortical surface, from which we propose a simple schematic of this contrast-dependent fMRI response.

65. VEPs elicited by sinusoidal grating stimuli with low spatial frequency

Objective: VEPs elicited by low spatial frequency and low contrast stimuli were recorded to investigate the function of the magnocellular system in normal subjects. Methods: Healthy six adults and five children participated in this study. Black and white sinusoidal gratings with spatial frequency of 0.28 c/deg and reversal rate of 15 rev/ sec were presented through CRT monitor to each subject. Results: Substantial VEP responses were recorded at Oz with 30% contrast stimuli in most subjects. Peak to trough amplitudes were decreased linearly as the decreasing of the contrast from 30% to 10%. Contrast thresholds which are obtained by extrapolating the regression lines to 0 lV, were 2.6% to 3.1% for adults and 2.7% to 4.2% for children. Conclusion: Low contrast, low spatial frequency stimuli were reported to be optimal to activate magnocellular system. Therefore, VEPs in the present study may reflect the function of such specific visual system in control subjects. Further research is needed to record more stable responses, especially in children, to estimate normal contrast thresholds.

216 Réponse corticale de la sommation binoculaire : une étude IRMf

But Psychophysical and electrophysiological studies have shown that the cortical response related to binocular summation increases with low contrast stimuli. In this study, we investigated this effect using functional MRI (fMRI). Since this technique records signals related to blood flow, it is important to optimise the parameters of stimulation in order to maximise the measurement of the cortical response. Presentation time, and spatial and temporal frequency were varied in order to explore the phenomenon in depth. Objectives and Methods Three normal healthy volunteers participated in this study. Their binocular function was defined by >60 seconds of stereopsis using the TNO test. The data were acquired with a 3T clinical scanner using 2mm 3 isovoxel for the functional images and 1mm 3 .for the high-resolution anatomical images. In order to weight the magno or parvocellular layer response, two different checkerboard stimuli were used. The high temporal frequency checkerboard flickered at 14Hz and had a spatial frequency of 50min of arc. The low temporal frequency checkerboard flickered at 4Hz and had a spatial frequency of 5min of arc. The contrast was 20% for all checkerboard stimuli. Each cycle was of 24 seconds and consisted of one stimulation and a rest condition. The duration of the stimulation was 6, 9, or 12 seconds. Binocular and monocular conditions were repeated every 2 cycles. The data were analyzed with the Brain Voyager software. Results The fMRI signal increased in all subjects with the increase in presentation time. For all stimulations, the signal was greater in the binocular condition compared to the monocular condition. Moreover, the binocular/monocular ratio was different depending on the location of the cortical response. Discussion These results suggest the existence of different cortical processings underlying the phenomenon of binocular summation. Conclusion Efforts are being done to make this method capable to evaluate abnormal binocular function.

Attention Alters Visual Plasticity during Exposure-Based Learning

It is generally believed that attention enhances the processing of sensory information during perception and learning. Here we report that, contrary to common belief, attention limits the degree of plasticity induced by repeated exposure to image features. Specifically, daily exposure to oriented stimuli that are not linked to a specific task causes an orientation-specific improvement in perceptual performance along the "exposed" axes. This effect is modulated by attention: human subjects showed a larger improvement in orientation discrimination when attention is directed toward the location where stimuli are presented. However, the capacity to perform discriminations away from the exposed orientation is enhanced when the exposure stimuli are unattended. Importantly, the improvement in orientation discrimination at the unattended location leads to a robust enhancement in the discrimination of complex stimuli, such as natural texture images, with orientation components along the exposed axes, whereas the improvement in orientation discrimination at the attended location exhibits only weak transfer to complex stimuli. These results indicate that sensory adaptation by passive stimulus exposure should be viewed as a form of perceptual learning that is complementary to practice-based learning in that it reduces constraints on generalization.

Attentional Modulation of Visual Responses by Flexible Input Gain

Although it is clear that sensory responses in the cortex can be strongly modulated by stimuli outside of classical receptive fields as well as by extraretinal signals such as attention and anticipation, the exact rules governing the neuronal integration of sensory and behavioral signals remain unclear. For example, most experiments studying sensory interactions have not explored attention, while most studies of attention have relied on the responses to relatively limited sets of stimuli. However, a recent study of V4 responses, in which location, orientation, and spatial attention were systematically varied, suggests that attention can both facilitate and suppress specific sensory inputs to a neuron according to behavioral relevance. To explore the implications of such input gain, we modeled the effects of a center-surround organization of attentional modulation using existing receptive field models of sensory integration. The model is consistent with behavioral measurements of a suppressive effect that surrounds the facilitatory locus of spatial attention. When this center-surround modulation is incorporated into realistic models of sensory integration, it is able to explain seemingly disparate observations of attentional effects in the neurophysiological literature, including spatial shifts in receptive field position and the preferential modulation of low contrast stimuli. The model is also consistent with recent formulations of attention to features in which gain is variably applied among cells with different receptive field properties. Consistent with functional imaging results, the model predicts that spatial attention effects will vary between different visual areas and suggests that attention may act through a common mechanism of selective and flexible gain throughout the visual system.

MMN as an index of premorbid function in schizophrenia

Event Abstract Back to Event MMN as an index of premorbid function in schizophrenia Daniel C. Javitt1*, G. Bar2 and M. Weiser2 1 Nathan Kline Institute for Psychiatric Research,New York University School of Medicine, United States 2 Sheba Medical Center, Tel Hashomer, Israel Mismatch negativity (MMN) is among the best established neurophysiological markers of neurocognitive dysfunction in schizophrenia. MMN abnormalities are reproduced by antagonists of N-methyl-D-aspartate (NMDA)-type glutamate receptors. As such, MMN may serve as an index of impaired glutamatergic function in schizophrenia. To date, literature regarding MMN utility as an endophenotype have been conflicting, with some studies showing deficits in family members of schizophrenia probands and in individuals at high symptomatic risk for schizophrenia but other studies showing normal MMN at illness onset and in unaffected family members. Umbricht et al. (Biol Psychiatry, 59:762, 2006) did not find deficits in first-episode patients as a group, but significant reduction in MMN in first episode subjects with poor premorbid function as indexed by premorbid educational achievement. The present study provides further evaluation of the relationship between premorbid function and MMN generation in a group of 31 chronic patients with good (n=19) vs. poor premorbid (n=12) patients vs. controls (n=20) assessed using the premorbid adjustment scale. Patients were assessed on auditory MMN to pitch and duration deviants, and on visual P1 to low contrast stimuli. As predicted, patients with poor premorbid function showed significant reductions in MMN to pitch deviants relative to controls, whereas good premorbids did not. Neither group showed a significant deficit in duration MMN generation. Furthermore, deficits in MMN generation to pitch deviants correlated significantly with impaired general premorbid function. By contrast to MMN findings, both good and poor premorbid patients showed reduced visual P1 generation. These findings support the concept that MMN may, at least in part, index a subgroup of schizophrenia individuals with poor premorbid function even during chronic illness phases, whereas visual P1 deficits may index more general aspects of the schizophrenia diathesis. Poor premorbid patients, in general, show worse outcome and may therefore be overrepresented in patient samples drawn from chronic care settings. Furthermore, such patients correspond most closely to the NMDA dysfunction phenotype, and may therefore be enriched in patients with NMDA dysfunction as a core pathological feature of their illness. Rather than serving as a general endophenotype, MMN may serve as an endophenotype for poor premorbid patients. Family studies differentiating probands by premorbid functional level are required. Conference: MMN 09 Fifth Conference on Mismatch Negativity (MMN) and its Clinical and Scientific Applications, Budapest, Hungary, 4 Apr - 7 Apr, 2009. Presentation Type: Oral Presentation Topic: Symposium 1: MMN as a potential endophenotype in schizophrenia Citation: Javitt DC, Bar G and Weiser M (2009). MMN as an index of premorbid function in schizophrenia. Conference Abstract: MMN 09 Fifth Conference on Mismatch Negativity (MMN) and its Clinical and Scientific Applications. doi: 10.3389/conf.neuro.09.2009.05.034 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: 20 Mar 2009; Published Online: 20 Mar 2009. * Correspondence: Daniel C Javitt, Nathan Kline Institute for Psychiatric Research,New York University School of Medicine, New York, United States, javitt@nki.rfmh.org 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 Daniel C Javitt G. Bar M. Weiser Google Daniel C Javitt G. Bar M. Weiser Google Scholar Daniel C Javitt G. Bar M. Weiser PubMed Daniel C Javitt G. Bar M. Weiser 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.

Effects of GABAa somatic inhibition on orientation tuning and contrast sensitivity in visual cortex

Event Abstract Back to Event Effects of GABAa somatic inhibition on orientation tuning and contrast sensitivity in visual cortex The selectivity of neurons in sensory cortex is thought to be at least partially shaped by inhibitory connections. In primary visual cortex (V1) many studies have examined how orientation tuning is affected by application of the GABAa antagonist bicuculline. These studies, however, have reported inconsistent results: some have seen a loss of selectivity and others only minor effects. A possible reason lies in the broad spectrum of effects of bicuculline; much more selective blockage of GABAa receptors is achieved with the potent antagonist gabazine (SR95531). We examined the effect of local gabazine iontophoresis on orientation tuning in V1 of anesthetized cats. Using extracellular recordings, we measured orientation tuning with drifting gratings. Gabazine was iontophoresed immediately above the recording electrode. An array of 96 electrodes was placed ~0.5 mm away to examine the spatial spread of gabazine. If gabazine did not affect responses measured with the array, we assumed that its effects were mostly restricted to the soma and proximal dendrites of neurons at the site of iontophoresis. Blocking GABAa-receptor mediated, somatic inhibition increased responses to all orientations by 150%, broadened tuning width by 22%, and reduced direction selectivity by 32% (medians, n=23). The resulting tuning curves resembled responses typically measured in subthreshold membrane potential, suggesting that GABAa provides untuned inhibition, keeping responses to most orientations below threshold. We tested this hypothesis using a simple rectification model. The model relies on the finding that the shape of orientation tuning curves for firing rates can be predicted by applying an appropriate threshold and gain to noisy membrane potential responses. Given these parameters, we sought to determine two hypothetical membrane potential tuning curves, which were allowed to differ only in their mean responses across orientations, to predict the tuning curve for firing rates in both the control and gabazine condition. This rectification model captured the observed effects of blocking GABAa-receptor mediated, somatic inhibition, i.e. increase in responses, broadening of orientation tuning, and decrease of direction selectivity. The success of the model suggests that inhibition hyperpolarizes the membrane potential by a constant amount, resulting in fewer threshold crossings, and therefore decreased firing rates and increased selectivity. Next, we asked whether inhibition depended on overall contrast. We measured contrast response functions with drifting gratings of optimal orientation before, during, and after gabazine administration. Blocking inhibition led to increases in firing rates that were stronger for high-contrast than for low-contrast stimuli. These effects are consistent with the notion that inhibition grows with contrast. We conclude that GABAa-receptor mediated, somatic inhibition plays an important role in orientation and direction selectivity by keeping membrane potential responses to most stimulus orientations below threshold, but not necessarily by shaping the tuning curve itself. Our results are consistent with the view that somatic inhibition provides a constant hyperpolarization, whose magnitude depends on stimulus contrast, but not on orientation. Support: NIH EY017396. Conference: Computational and systems neuroscience 2009, Salt Lake City, UT, United States, 26 Feb - 3 Mar, 2009. Presentation Type: Poster Presentation Topic: Poster Presentations Citation: (2009). Effects of GABAa somatic inhibition on orientation tuning and contrast sensitivity in visual cortex. Front. Syst. Neurosci. Conference Abstract: Computational and systems neuroscience 2009. doi: 10.3389/conf.neuro.06.2009.03.203 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: 03 Feb 2009; Published Online: 03 Feb 2009. 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 Google Google Scholar PubMed 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.

The dog’s meow: asymmetrical interaction in cross-modal object recognition

Little is known on cross-modal interaction in complex object recognition. The factors influencing this interaction were investigated using simultaneous presentation of pictures and vocalizations of animals. In separate blocks, the task was to identify either the visual or the auditory stimulus, ignoring the other modality. The pictures and the sounds were congruent (same animal), incongruent (different animals) or neutral (animal with meaningless stimulus). Performance in congruent trials was better than in incongruent trials, regardless of whether subjects attended the visual or the auditory stimuli, but the effect was larger in the latter case. This asymmetry persisted with addition of a long delay after the stimulus and before the response. Thus, the asymmetry cannot be explained by a lack of processing time for the auditory stimulus. However, the asymmetry was eliminated when low-contrast visual stimuli were used. These findings suggest that when visual stimulation is highly informative, it affects auditory recognition more than auditory stimulation affects visual recognition. Nevertheless, this modality dominance is not rigid; it is highly influenced by the quality of the presented information.

Visual evoked spread spectrum analysis (VESPA) responses to stimuli biased towards magnocellular and parvocellular pathways

Stimulus contrast fluctuations were controlled while the properties of a novel ERP known as the VESPA (Visual Evoked Spread Spectrum Analysis) were examined using data from 8 healthy human subjects. Substantial differences were seen between the morphologies of VESPAs obtained using low contrast stimuli when compared with those obtained using high contrast and full contrast range stimuli. Topographic distributions for both responses are compared and the findings are considered in terms of the response characteristics of the magnocellular and parvocellular visual pathways and the VESPA method itself.

Parvocellular and Magnocellular Contributions to the Initial Generators of the Visual Evoked Potential: High-Density Electrical Mapping of the “C1” Component

The C1 component of the VEP is considered to index initial afference of retinotopic regions of human visual cortex (V1 and V2). C1 onsets over central parieto-occipital scalp between 45 and 60 ms, peaks between 70 and 100 ms, and then resolves into the following P1 component. By exploiting isoluminant and low-contrast luminance stimuli, we assessed the relative contributions of the Magnocellular (M) and Parvocellular (P) pathways to generation of C1. C1 was maximal at 88 ms in a 100% luminance contrast condition (which stimulates both P and M pathways) and at 115 ms in an isoluminant chromatic condition (which isolates contributions of the P pathway). However, in a 4% luminance contrast condition (which isolates the M pathway), where the stimuli were still clearly perceived, C1 was completely absent. Absence of C1 in this low contrast condition is unlikely to be attributable to lack of stimulus energy since a robust P1-N1 complex was evoked. These data therefore imply that C1 may be primarily parvocellular in origin. The data do not, however, rule out some contribution from the M system at higher contrast levels. Nonetheless, that the amplitude of C1 to P-isolating isoluminant chromatic stimuli is equivalent to that evoked by 100% contrast stimuli suggests that even at high contrast levels, the P system is the largest contributor. These data are related to intracranial recordings in macaque monkeys that have also suggested that the initial current sink in layer IV may not propagate effectively to the scalp surface when M-biased stimuli are used. We also discuss how this finding has implications for a long tradition of attention research that has used C1 as a metric of initial V1 afference in humans. C1 has been repeatedly interrogated for potential selective attentional modulations, particularly in spatial attentional designs, under the premise that modulation of this component, or lack thereof, would be evidence for or against selection at the initial inputs to visual cortex. Given the findings here, we would urge that in interpreting C1 effects, a consideration of the dominant cellular contributions will be necessary. For example, it is plausible that spatial attention mechanisms could operate primarily through the M system and that as such C1 may not always represent an adequate dependent measure in such studies.