Stimulus Range Research Articles

Firing-rate models for neurons with a broad repertoire of spiking behaviors

Instantaneous firing rates are commonly used to describe either the compound spiking activity of neuron ensembles (population rate) or the trial-averaged response of individual neurons to multiple repetitions of the same stimulus. The dynamics of firing rates is often studied by means of population or firing-rate models. The main motivation for using such models rather than spiking neuron models is to reduce the dimensionality and complexity of the microscopic dynamics to allow analytical tractability, efficient simulation, and intuitive understanding. In most cases, population models have to be treated as purely phenomenological descriptions. Only under simplifying assumptions can they be derived or extracted from the single-neuron dynamics. Previous studies in this field were mainly restricted to homogeneous populations of simple integrate-and-fire neurons which can, to some extent, mimic the rather stereotypical behaviour of regularly firing neurons (e.g., pyramidal cells in the neocortex). Electrophysiological studies have revealed a large diversity of neuron types with different dynamical behaviours, ranging from regular spiking to bursting cells, from accommodating to non-accommodating cells, from integrators to resonators, from depolarization- to hyperpolarization-activated cells, etc. So far, the effect of this neuron-type diversity on the dynamics of neural populations is unclear. Here, we investigate the rate-response characteristics for a variety of neuron types in simulation experiments. To this end, we extend previous work on simple integrate-and-fire neurons [1,2] to the multi-timescale adaptive threshold (MAT) [3] and the Izhikevich model [4]. The model neurons are probed by spike trains modeled as realizations of inhomogeneous Poisson or Gamma point processes with sinusoidally modulated intensity. Average rates, modulation amplitudes and phases of the period-averaged spike responses are measured for a broad range of stimulus, neuron and synapse parameters. The stationary and nonstationary response properties are parameterized by means of nonlinear activation functions and linear filter kernels, respectively, serving as ingredients for simple linear-nonlinear firing rate models. We show that linear-nonlinear rate models can accurately predict the population responses to novel test stimuli for a broad range of parameters and neuron types.

Magno- and Parvocellular Contrast Responses in Varying Degrees of Autistic Trait

Autistic tendency has been associated with altered visual perception, especially impaired visual motion sensitivity and global/local integration, as well as enhanced visual search and local shape recognition. However, the neurophysiological mechanisms underlying these abnormalities remain poorly defined. The current study recruited 29 young adults displaying low, middle or high autistic trait as measured by Baron-Cohen's Autism spectrum Quotient (AQ), and measured motion coherence thresholds psychophysically, with manipulation of dot lifetime and stimulus contrast, as well as nonlinear cortical visual evoked potentials (VEPs) over a range of temporal luminance contrast levels from 10% to 95%. Contrast response functions extracted from the major first order and second order Wiener kernel peaks of the VEPs showed consistent variation with AQ group, and Naka-Rushton fits enabled contrast gain and semi-saturation contrasts to be elicited for each peak. A short latency second order response (previously associated with magnocellular processing) with high contrast gain and a saturating contrast response function showed higher amplitude for the High AQ (compared with Mid and Low groups) indicating poorer neural recovery after rapid stimulation. A non-linearity evoked at longer interaction times (previously associated with parvocellular processing) with no evidence of contrast saturation and lower contrast gain showed no difference between autism quotient groups across the full range of stimulus contrasts. In addition, the short latency first order response and a small, early second order second slice response showed gain and semi-saturation parameters indicative of magnocellular origin, while the longer latency first order response probably reflects a mixture of inputs (including feedback from higher cortical areas). Significant motion coherence (AQ group) * (dot lifetime) interactions with higher coherence threshold for limited dot lifetime stimuli is consistent with atypical magnocellular functioning, however psychophysical performance for those with High AQ is not explained fully, suggesting that other factors may be involved.

Effect of hip flexion angle on hamstring optimum length after a single set of concentric contractions

The eccentric contraction mode was proposed to be the primary stimulus for optimum angle (angle at which peak torque occurs) shift. However, the training range of motion (or muscle excursion range) could be a stimulus as important. The aim of this study was to assess the influence of the training range of motion stimulus on the hamstring optimum length. It was hypothesised that performing a single set of concentric contractions beyond optimal length (seated at 80° of hip flexion) would lead to an immediate shift of the optimum angle to longer muscle length while performing it below (supine at 0° of hip flexion) would not provide any shift. Eleven male participants were assessed on an isokinetic dynamometer. In both positions, the test consisted of 30 consecutive knee flexions at 4.19 rad · s−1. The optimum angle was significantly shifted by ∼15° in the direction of longer muscle length after the contractions at 80° of hip flexion, while a non-significant shift of 3° was found at 0°. The hamstring fatigability was not influenced by the hip position. It was concluded that the training range of motion seems to be a relevant stimulus for shifting the optimum angle to longer muscle length. Moreover, fatigue appears as a mechanism partly responsible for the observed shift.

Using Forced Choice Discrimination to Measure the Perceptual Response to Light of Different Characteristics

Forced choice discrimination—one of the fundamental procedures for comparing two or more stimuli—has been used in past studies to compare the relative spatial brightness of scenes illuminated by light sources of different spectra. This article—which focuses on spatial brightness perception but is relevant to other perceptual dimensions—suggests procedural steps for offsetting bias that can be expected in the discrimination task. The first step is to counterbalance the spatial locations of the two stimuli to offset position bias in simultaneous evaluations, or similarly to counterbalance the temporal locations to offset interval bias in sequential evaluations. The second step is to use an all-possible pairs approach when comparing the range of stimuli to offset stimulus range bias. We also suggest that multiple tests be carried out in a random order and that reports of the work present sufficient data to understand the procedure and to interpret the results. These procedural steps can be employed to improve practice in future work and can be considered as tools for screening past studies. Of 12 past studies that employed a discrimination procedure to investigate spatial brightness and preference under lighting of different spectral power distribution, only five were found to meet these criteria.

Functional specializations of the ventral intraparietal area for multisensory heading discrimination.

The ventral intraparietal area (VIP) of the macaque brain is a multimodal cortical region with directionally selective responses to visual and vestibular stimuli. To explore how these signals contribute to self-motion perception, neural activity in VIP was monitored while macaques performed a fine heading discrimination task based on vestibular, visual, or multisensory cues. For neurons with congruent visual and vestibular heading tuning, discrimination thresholds improved during multisensory stimulation, suggesting that VIP (like the medial superior temporal area; MSTd) may contribute to the improved perceptual discrimination seen during cue combination. Unlike MSTd, however, few VIP neurons showed opposite visual/vestibular tuning over the range of headings relevant to behavior, and those few cells showed reduced sensitivity under cue combination. Our data suggest that the heading tuning of some VIP neurons may be locally remodeled to increase the proportion of cells with congruent tuning over the behaviorally relevant stimulus range. VIP neurons also showed much stronger trial-by-trial correlations with perceptual decisions (choice probabilities; CPs) than MSTd neurons. While this may suggest that VIP neurons are more strongly linked to heading perception, we also find that correlated noise is much stronger among pairs of VIP neurons, with noise correlations averaging 0.14 in VIP as compared with 0.04 in MSTd. Thus, the large CPs in VIP could be a consequence of strong interneuronal correlations. Together, our findings suggest that VIP neurons show specializations that may make them well equipped to play a role in multisensory integration for heading perception.

Stability of simple/complex classification with contrast and extraclassical receptive field modulation in macaque V1

A key property of neurons in primary visual cortex (V1) is the distinction between simple and complex cells. Recent reports in cat visual cortex indicate the categorization of simple and complex can change depending on stimulus conditions. We investigated the stability of the simple/complex classification with changes in drive produced by either contrast or modulation by the extraclassical receptive field (eCRF). These two conditions were reported to increase the proportion of simple cells in cat cortex. The ratio of the modulation depth of the response (F1) to the elevation of response (F0) to a drifting grating (F1/F0 ratio) was used as the measure of simple/complex. The majority of V1 complex cells remained classified as complex with decreasing contrast. Near contrast threshold, an equal proportion of simple and complex cells changed their classification. The F1/F0 ratio was stable between optimal and large stimulus areas even for those neurons that showed strong eCRF suppression. There was no discernible overall effect of surrounding spatial context on the F1/F0 ratio. Simple/complex cell classification is relatively stable across a range of stimulus drives, produced by either contrast or eCRF suppression.

Magno and parvocellular contributions to the cortical visually evoked response – time-frequency and source considerations

Event Abstract Back to Event Magno and parvocellular contributions to the cortical visually evoked response – time-frequency and source considerations David P. Crewther1*, Laila Hugrass1, Alyse Brown1 and Callum Hollis1 1 Swinburne University of Technology, Centre for Human Psychopharmacology, Australia Purpose: The search for cortical evoked responses identifiable as magnocellular (M) or parvocellular (P) in origin is heightened by claims for abnormal magno processing in disorders as disparate as Schizophrenia, Autism, Dyslexia and Dyscalculia. However, in human, the readily available techniques of EEG, MEG, fMRI measure different signals with different technology. Nonlinear Wiener kernel analysis of visual evoked cortical potentials (VEP) and magnetic evoked fields (MEF) offer some distinct advantages in isolating M and P generated activity. Methods: Contrast responses of VEP kernels recorded with 29 normal young adults were compared with Magnetoencephalographic evoked fields (MEF) generated using a similar pseudorandom stimulus developed using a VPixx/DataPIXX combination recorded through the 306 SQUID sensors of an Elekta TRIUX magnetoencephalograph on a (different) population of 15 normal young adults. Results: Contrast response functions were extracted from the major first and second order Wiener kernel peaks of the VEP. Naka-Rushton fits enabled contrast gain and semi-saturation contrasts to be elicited for each peak. A short latency second order response with high contrast gain and a saturating contrast response function mimicking primate magnocellular (M) responses was found. A longer latency non-linearity (P) with lower contrast gain and no evidence of contrast saturation across the full range of stimulus contrasts was also found. MEG nonlinear analysis yielded similar responses, with the added source localization capability indicating that the earliest cortical responses (~50ms) occurred simultaneously over the occipital pole and an extrastriate location (MT/V5). Time-Frequency MEG analysis differentiates the slices of the second order response, with the putative magno response of the first slice of the second order response showing heightened gamma band activity, while the second slice (K2.2 - putative parvocellular response) shows strong beta band activation but little gamma. Conclusion: The cortical VEP and MEF first and second order responses can be decomposed into those with high contrast gain and contrast saturation (M-like) versus those with lower contrast gain and little evidence of saturation (P-like). TF analysis demonstrates higher gamma power in the magno components. The latency of the M-like components is approximately 25 ms less than those of the P-like components. Comparison of MEF with cortical VEP results gives strong confirmation of the M and P origins of the first and second slice major peaks respectively. Acknowledgements National Health and Medical Research Council NHMRC (APP1004740) Keywords: Magnocellular, Parvocellular pathway, nonlinearity, Magnetoencephalography (MEG), visual evoked potential (VEP) Conference: ACNS-2013 Australasian Cognitive Neuroscience Society Conference, Clayton, Melbourne, Australia, 28 Nov - 1 Dec, 2013. Presentation Type: Poster Topic: Sensation and Perception Citation: Crewther DP, Hugrass L, Brown A and Hollis C (2013). Magno and parvocellular contributions to the cortical visually evoked response – time-frequency and source considerations. Conference Abstract: ACNS-2013 Australasian Cognitive Neuroscience Society Conference. doi: 10.3389/conf.fnhum.2013.212.00007 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: 15 Oct 2013; Published Online: 25 Nov 2013. * Correspondence: Prof. David P Crewther, Swinburne University of Technology, Centre for Human Psychopharmacology, Hawthorn, VIC, 3122, Australia, dcrewther@swin.edu.au 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 David P Crewther Laila Hugrass Alyse Brown Callum Hollis Google David P Crewther Laila Hugrass Alyse Brown Callum Hollis Google Scholar David P Crewther Laila Hugrass Alyse Brown Callum Hollis PubMed David P Crewther Laila Hugrass Alyse Brown Callum Hollis 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.

A Retinal Source of Spatial Contrast Gain Control

Sensory cortex is able to encode a broad range of stimulus features despite a great variation in signal strength. In cat primary visual cortex (V1), for example, neurons are able to extract stimulus features like orientation or spatial configuration over a wide range of stimulus contrasts. The contrast-invariant spatial tuning found in V1 neuron responses has been modeled as a gain control mechanism, but at which stage of the visual pathway it emerges has remained unclear. Here we describe our findings that contrast-invariant spatial tuning occurs not only in the responses of lateral geniculate nucleus (LGN) relay cells but also in their afferent retinal input. Our evidence suggests that a similar contrast-invariant mechanism is found throughout the stages of the early visual pathway, and that the contrast-invariant spatial selectivity is evident in both retinal ganglion cell and LGN cell responses.

Eye movements and attention: The role of pre-saccadic shifts of attention in perception, memory and the control of saccades

Saccadic eye movements and perceptual attention work in a coordinated fashion to allow selection of the objects, features or regions with the greatest momentary need for limited visual processing resources. This study investigates perceptual characteristics of pre-saccadic shifts of attention during a sequence of saccades using the visual manipulations employed to study mechanisms of attention during maintained fixation. The first part of this paper reviews studies of the connections between saccades and attention, and their significance for both saccadic control and perception. The second part presents three experiments that examine the effects of pre-saccadic shifts of attention on vision during sequences of saccades. Perceptual enhancements at the saccadic goal location relative to non-goal locations were found across a range of stimulus contrasts, with either perceptual discrimination or detection tasks, with either single or multiple perceptual targets, and regardless of the presence of external noise. The results show that the preparation of saccades can evoke a variety of attentional effects, including attentionally-mediated changes in the strength of perceptual representations, selection of targets for encoding in visual memory, exclusion of external noise, or changes in the levels of internal visual noise. The visual changes evoked by saccadic planning make it possible for the visual system to effectively use saccadic eye movements to explore the visual environment.

Critical role for cochlear hair cell BK channels for coding the temporal structure and dynamic range of auditory information for central auditory processing

Large conductance, voltage- and Ca2+-activated K+ (BK) channels in inner hair cells (IHCs) of the cochlea are essential for hearing. However, germline deletion of BKα, the pore-forming subunit KCNMA1 of the BK channel, surprisingly did not affect hearing thresholds in the first postnatal weeks, even though altered IHC membrane time constants, decreased IHC receptor potential alternating current/direct current ratio, and impaired spike timing of auditory fibers were reported in these mice. To investigate the role of IHC BK channels for central auditory processing, we generated a conditional mouse model with hair cell-specific deletion of BKα from postnatal day 10 onward. This had an unexpected effect on temporal coding in the central auditory system: neuronal single and multiunit responses in the inferior colliculus showed higher excitability and greater precision of temporal coding that may be linked to the improved discrimination of temporally modulated sounds observed in behavioral training. The higher precision of temporal coding, however, was restricted to slower modulations of sound and reduced stimulus-driven activity. This suggests a diminished dynamic range of stimulus coding that is expected to impair signal detection in noise. Thus, BK channels in IHCs are crucial for central coding of the temporal fine structure of sound and for detection of signals in a noisy environment.—Kurt, S., Sausbier, M., Rüttiger, L., Brandt, N., Moeller, C. K., Kindler, J., Sausbier, U., Zimmermann, U., van Straaten, H., Neuhuber, W., Engel, J., Knipper, M., Ruth, P., Schulze, H. Critical role for cochlear hair cell BK channels for coding the temporal structure and dynamic range of auditory information for central auditory processing.

The interrelation between acoustic context effects and available response categories in speech sound categorization

In an investigation of contextual influences on sound categorization, 64 Peruvian Spanish listeners categorized vowels on an /i/ to /e/ continuum. First, to measure the influence of the stimulus range (broad acoustic context) and the preceding stimuli (local acoustic context), listeners were presented with different subsets of the Spanish /i/-/e/ continuum in separate blocks. Second, the influence of the number of response categories was measured by presenting half of the participants with /i/ and /e/ as responses, and the other half with /i/, /e/, /a/, /o/, and /u/. The results showed that the perceptual category boundary between /i/ and /e/ shifted depending on the stimulus range and that the formant values of locally preceding items had a contrastive influence. Categorization was less susceptible to broad and local acoustic context effects, however, when listeners were presented with five rather than two response options. Vowel categorization depends not only on the acoustic properties of the target stimulus, but also on its broad and local acoustic context. The influence of such context is in turn affected by the number of internal referents that are available to the listener in a task.

Detection and resolution of vanishing optotype letters in central and peripheral vision

Previous studies of peripheral vision have shown that detection acuity is superior to resolution acuity for gratings over a range of contrasts, which is attributed to different limiting mechanisms (contrast insufficiency and neural undersampling) for the two tasks. To extend the analysis to letters in a way that avoided luminance cues, we used “vanishing optotype” characters, conveying second-order information, and constructed from tripole strokes having the same mean luminance as the surround. We measured the minimum letter size for detection and identification tasks for two different pairs of vanishing optotype characters (O vs. + and orthogonally oriented Landolt-C’s) as a function of contrast in central and peripheral vision. Foveally there was no significant difference between detection acuity and resolution acuity for either pair of letters over a range of stimulus contrasts from 20% to 100%, indicating performance is contrast-limited for both tasks. The same result was obtained at 30° eccentricity in the peripheral field for the O vs. + letters, again indicating performance is contrast-limited for both tasks. However, resolution acuity for the Landolt-C letters was significantly worse than detection acuity in the periphery over the same range of contrasts, which suggests performance is limited by neural undersampling for these letters. All of our experimental results are explained by a model of neural sampling in which detection acuity is determined by the size of neural receptive fields relative to the dimensions of the tripole responsible for spatial contrast, whereas resolution acuity is determined by the spacing of receptive fields relative to the spacing between strokes responsible for letter form.

Combining Symbolic Cues with Sensory Input and Prior Experience in an Iterative Bayesian Framework

Perception and action are the result of an integration of various sources of information, such as current sensory input, prior experience, or the context in which a stimulus occurs. Often, the interpretation is not trivial hence needs to be learned from the co-occurrence of stimuli. Yet, how do we combine such diverse information to guide our action? Here we use a distance production-reproduction task to investigate the influence of auxiliary, symbolic cues, sensory input, and prior experience on human performance under three different conditions that vary in the information provided. Our results indicate that subjects can (1) learn the mapping of a verbal, symbolic cue onto the stimulus dimension and (2) integrate symbolic information and prior experience into their estimate of displacements. The behavioral results are explained by to two distinct generative models that represent different structural approaches of how a Bayesian observer would combine prior experience, sensory input, and symbolic cue information into a single estimate of displacement. The first model interprets the symbolic cue in the context of categorization, assuming that it reflects information about a distinct underlying stimulus range (categorical model). The second model applies a multi-modal integration approach and treats the symbolic cue as additional sensory input to the system, which is combined with the current sensory measurement and the subjects’ prior experience (cue-combination model). Notably, both models account equally well for the observed behavior despite their different structural assumptions. The present work thus provides evidence that humans can interpret abstract symbolic information and combine it with other types of information such as sensory input and prior experience. The similar explanatory power of the two models further suggest that issues such as categorization and cue-combination could be explained by alternative probabilistic approaches.

Patients with Sickle Cell Disease Have Increased Sensitivity to Cold and Heat Stimuli, 2

Abstract 2116Sickle Cell Disease (SCD) pain is associated with colder temperatures, touch, and increased wind speed and barometric pressure. The specific array of associated factors suggests hypersensitivity to tactile stimuli, a characteristic of neuropathic pain. Sickle mice exhibit hypersensitivity at baseline compared to controls to cold, heat, and mechanical stimuli via a TRPV1 mediated pathway. However, it is not known whether humans experience this same hypersensitivity. Thus, the objective of this study was to quantify sensitivity differences to thermal (heat, cold) and mechanical stimuli between SCD patients and healthy African American controls. We hypothesized SCD patients will exhibit hypersensitivity to thermal and mechanical stimuli compared to controls and this hypersensitivity will worsen with age and frequency of pain.We conducted a cross-sectional study of SCD patients in baseline health and race-matched controls age ≥ 7 yrs. Our primary outcome was detection of hypersensitivity to thermal and mechanical stimuli. We excluded those with a pain phenotype other than SCD, overt stroke, analgesics within 24 hrs of testing, or acute SCD pain event within 2 weeks of testing. Subjects underwent quantitative sensory testing (QST) to thermal and mechanical stimuli. QST evaluates the somatosensory system detecting sensory loss (hyposensitivity) or gain (hypersensitivity). Thermal stimulation was performed with a Thermal Sensory Analyzer (Medoc;Israel), an FDA approved computer-assisted device that delivers cold and warm stimuli via a thermode attached to the skin (baseline temperature, 32°C; stimulus range, 0–50°C). Mechanical testing was performed using graded vonFrey monofilaments (force range 0.255 mN to 1078.731 mN). Testing was done on the thenar eminence of the non-dominant hand. Primary outcomes included: 1) Cold Pain Threshold (°C), 2) Heat Pain Threshold (°C), 3) Mechanical Pain Threshold (mN) as reported by “method of limits” where subjects pushed a button (thermal) or spoke (mechanical) when the progressive stimulus was painful. The final outcome was the computed mean of 3 tests (thermal) and 5 tests (mechanical).Independent samples t-tests were used to compare outcomes between SCD patients and controls. Linear regression was used to evaluate the impact of age and gender on pain thresholds in both groups and the impact of lifetime history of pain, defined as total number of emergency department visits or hospitalizations for pain, on pain thresholds in SCD patients.55 SCD patients and 57 controls were recruited (Jan 2010-June 2011). There were no differences in mean age (15.4 yrs vs.16.3 yrs; p=0.59, t-test) or gender (SCD=60% female vs. Controls=56% female; p=0.70, Pearson Chi-Square). SCD genotypes were 67% (n=37) HbSS, 18% (n=10) HbSC, 11% (n=6), HbSβ+thal, and 4% (n=2) other. SCD patients had significantly lower cold pain thresholds (p=0.008) and heat pain thresholds (p=0.04) compared to controls (Table 1). There were no differences in mechanical pain thresholds (p=0.38) (Table 1). Older age was associated with lower cold pain thresholds (parameter estimate=0.19°C; p=0.05), lower heat pain thresholds (parameter estimate=0.13°C; p=0.0069), and lower mechanical pain thresholds (parameter estimate=0.11mN; p=0.02) in both groups. Gender had no effect on the outcomes (cold pain threshold, p=0.15; heat pain threshold, p=0.07; mechanical pain threshold, p=0.29). Total number of lifetime SCD pain events had no effect on the outcomes (cold pain threshold, p=0.91; heat pain threshold, p=0.65, mechanical pain threshold, p=0.77).SCD patients in baseline health experience increased sensitivity to cold and heat stimuli compared to race-matched controls and this sensitivity worsens with older age. These findings suggest peripheral sensitization may exist in SCD. Further research into how cold and heat sensing receptors and pathways contribute to SCD pain is warranted. Ultimately, this may lead to the development of novel therapeutics targeted to the specific underlying neurobiology of SCD pain that aids in the treatment or prevention of SCD pain.Table 1Pain Thresholds of SCD Patients and Healthy Controls.OutcomeSCD Patients (Mean ± SD)Controls (Mean ± SD)P-valueCold Pain Threshold (°C)18.5 (7.7)14.1 (9.4)0.008Heat Pain Threshold (°C)42.5 (4.4)44.3 (4.6)0.04Mechanical Pain Threshold (mN)303.9 (409.6)375.6 (451.2)0.38 Disclosures:No relevant conflicts of interest to declare.

The effect of luminance on simulated driving speed

Perceived speed is modulated by a range of stimulus attributes such as contrast, luminance and adaptation duration. It has been suggested that such changes in perceived speed may influence driving behaviour. In order to evaluate the effect of luminance on driving speed we have measured subjects’ driving speed in a driving simulator for a range of luminance and speed over time. The results indicate that reducing luminance results in a decrease in driving speed for all speeds measured. This reduction in driving speed at low luminance is consistent with previous findings that perceived speed increases at low luminance. However, the results also indicated that driving speed remained stable over a 30 s period. The stability of driving speed over time is inconsistent with previous findings that perceived speed reduces exponentially as a function of adaptation duration. The results are suggestive of a scheme whereby driving speed is consistent with the known effects of luminance upon perceived speed but may also be modulated by higher order processes that serve to maintain a constant speed over time.

Force transformation in spider strain sensors: white light interferometry.

Scanning white light interferometry and micro-force measurements were applied to analyse stimulus transformation in strain sensors in the spider exoskeleton. Two compound or 'lyriform' organs consisting of arrays of closely neighbouring, roughly parallel sensory slits of different lengths were examined. Forces applied to the exoskeleton entail strains in the cuticle, which compress and thereby stimulate the individual slits of the lyriform organs. (i) For the proprioreceptive lyriform organ HS-8 close to the distal joint of the tibia, the compression of the slits at the sensory threshold was as small as 1.4 nm and hardly more than 30 nm, depending on the slit in the array. The corresponding stimulus forces were as small as 0.01 mN. The linearity of the loading curve seems reasonable considering the sensor's relatively narrow biological intensity range of operation. The slits' mechanical sensitivity (slit compression/force) ranged from 106 down to 13 nm mN(-1), and gradually decreased with decreasing slit length. (ii) Remarkably, in the vibration-sensitive lyriform organ HS-10 on the metatarsus, the loading curve was exponential. The organ is thus adapted to the detection of a wide range of vibration amplitudes, as they are found under natural conditions. The mechanical sensitivities of the two slits examined in this organ in detail differed roughly threefold (522 and 195 nm mN(-1)) in the biologically most relevant range, again reflecting stimulus range fractionation among the slits composing the array.

Rapid Dynamics of Contrast Responses in the Cat Primary Visual Cortex

The visual information we receive during natural vision changes rapidly and continuously. The visual system must adapt to the spatiotemporal contents of the environment in order to efficiently process the dynamic signals. However, neuronal responses to luminance contrast are usually measured using drifting or stationary gratings presented for a prolonged duration. Since motion in our visual field is continuous, the signals received by the visual system contain an abundance of transient components in the contrast domain. Here using a modified reverse correlation method, we studied the properties of responses of neurons in the cat primary visual cortex to different contrasts of grating stimuli presented statically and transiently for 40 ms, and showed that neurons can effectively discriminate the rapidly changing contrasts. The change in the contrast response function (CRF) over time mainly consisted of an increment in contrast gain (CRF shifts to left) in the developing phase of temporal responses and a decrement in response gain (CRF shifts downward) in the decay phase. When the distribution range of stimulus contrasts was increased, neurons demonstrated decrement in contrast gain and response gain. Our results suggest that contrast gain control (contrast adaptation) and response gain control mechanisms are well established during the first tens of milliseconds after stimulus onset and may cooperatively mediate the rapid dynamic responses of visual cortical neurons to the continuously changing contrast. This fast contrast adaptation may play a role in detecting contrast contours in the context of visual scenes that are varying rapidly.

Investigating the use of an adjustment task to set preferred colour of ambient illumination

AbstractAn experiment was carried out to examine the method of adjustment when determining user preferences for the colour appearance of ambient lighting. A booth was lit using luminaires containing an array of white and coloured light emitting diodes (LEDs), allowing continuous variation of correlated colour temperature (CCT) between 2736 and 4014 K. It was predicted that the preferred CCT would be influenced by the available stimulus range, the anchor (initial setting before adjustment), adaptation time before onset of adjustment action and type of control dial. Preference was estimated for three different CCT stimulus ranges within the available range. All three ranges led to significantly different results for preferred CCT: 3288, 3490 and 3671 K. The experimental results confirmed that stimulus range, anchor and adaptation time have significant effect on the preferred CCT determined using the adjustment task, and thus, the importance of considering and reporting this information in studies that employ the method of adjustment. © 2011 Wiley Periodicals, Inc. Col Res Appl, 2013

Perceptual improvement following repetitive sensory stimulation depends monotonically on stimulation intensity

Electrical repetitive sensory stimulation (rSS) is a direct and effective means of inducing plasticity processes in human beings, and is increasingly being used as a therapeutic intervention. Suprathreshold intensities induce beneficial effects on tactile perception and sensorimotor abilities. However, it is not known whether there is an optimal range of stimulus intensity. We investigated the effect of varied intensities (low, 1.19 ± 0.07 mA; intermediate, 3.33 ± 0.27 mA; and high, 4.42 ± 0.56 mA) on the outcome of a 30-minute electrical rSS applied to the index finger (intermittent high-frequency stimulation, 20 Hz and interburst interval, 5 seconds) in three groups (n = 10 each) of participants. As a marker of perceptual changes, we measured tactile spatial two-point discrimination on the stimulated finger and on the heel of the hand before and after the rSS. rSS improved discrimination performance, with the gain being the highest in the high-intensity group and the lowest in the low-intensity group. Measurements on the heel of the hand revealed small improvements in the high-intensity group, indicative of recruitment processes. rSS of maximal intensity induced the strongest effects, indicative of a monotonic intensity-gain characteristic with no U-shaped dependency.

Rate dynamics of the retina-LGN connection

Firing-rate models provide a practical tool for studying the dynamics of trial- or population-averaged neuronal signals. The derivation or extraction of such models through investigation of the firing-rate response characteristics of ensembles of neurons has been the subject of several studies (see references in [1]). The majority of these focused on neurons that receive input spikes at a high rate through weak synapses (diffusion approximation). For many neural systems, however, this assumption cannot be justified. One example in the early visual system is the lateral geniculate nucleus (LGN), where synapses between retinal ganglion cells and relay cells are so strong that single retinal spikes can initiate action potentials in the thalamic targets. Using a comprehensive numerical approach, we recently studied the firing-rate response properties of leaky integrate-and-fire (LIF) neurons receiving current input through strong synapses [1]. Input spike trains were modeled as inhomogeneous Poisson point processes with sinusoidally modulated rate. Average rates, modulation amplitudes, and phases of the period-averaged spike responses were measured for a broad range of stimulus, synapse, and neuron parameters, cf Fig. ​Fig.1.1. We found that the resulting responses could be described well by a linear first-order low-pass filter over a wide range of model parameters. Combining this filter with the nonlinear response characteristic for stationary inputs, we constructed a linear-nonlinear firing-rate model, which accurately predicted the population response for a variety of non-sinusoidal test stimuli. Figure 1 A neuron model (denoted by large circle) receives, through a synapse (small circle), spike trains generated by a Poisson point process with sinusoidal rate of mean a0 and modulation amplitude a1. The response firing rate is characterized by its mean r ... In the present study, we use the same approach to investigate whether linear-nonlinear firing-rate models can capture equally well the firing rate properties of LGN relay neuron models that have been fitted to experimental data [2,3]. Models investigated include more “realistic” ones with conductance-based synaptic and after-hyperpolarizing currents [2] as well as more abstract spike-response models [3,4].