Stimulus detection after interruption of the feedforward response in a backward masking paradigm

In backward masking, a target stimulus is rendered invisible by the presentation of a second stimulus, the mask. When the mask is effective, neural responses to the target are suppressed. Nevertheless, weak target responses sometimes may produce a behavioural response. It remains unclear whether the reduced target response is a purely feedforward response or that it includes recurrent activity. Using a feedforward neural network of biological plausible spiking neurons, we tested whether a transient spike burst is sufficient for face categorization. After training the network, the system achieved face/non-face categorization for sets of grayscale images. In a backward masking paradigm, the transient burst response was cut off thereby reducing the feedforward target response. Despite the suppressed feedforward responses stimulus classification remained robust. Thus according to our model data stimulus detection is possible with purely, suppressed feedforward responses.     

Spontaneous bursting and long-lived local correlation in normal and denervated tectum of goldfish

The formation of fine retinotopic order by growing optic fibers in the goldfish is thought to be mediated by the correlated firing of optic fibers from neighboring retinal ganglion cells. Although the activity of the tectal cells must also be important for this activity-dependent refinement, few studies have analyzed the pattern and local correlation of the intrinsic activity of tectal neurons and the effect of denervation on this activity. To address this issue, spontaneous (nonoptic driven) activity was analyzed and cross-correlograms were computed between individual tectal neurons using single and double electrode extracellular recordings. Recordings were made in normally innervated tectum in which the contribution of optic activity was eliminated by short-term intraocular blockade with tetrodotoxin and in denervated tecta in which the optic nerve had been severed several weeks prior. Several observations were relevant to activitydependent refinement: First, coupling between neighboring tectal cells is weak. Second, the time duration for local correlation is relatively long, as long as 200 ms. Third, tectal neurons exhibit spontaneous bursting. Fourth, denervation increased the level of spontaneous activity in the tectum. The increased spontaneous activity and bursting following denervation implies that tectal neurons are more excitable when optic fibers are beginning to reinnervate the tectum. This could make it possible for optic fibers to drive tectal neurons at a time when their input to individual neurons is severely weakened by a lack of spatial convergence. The weak coupling between tectal cells and the consequent long-time constant for correlated activity implies a constraint on the duration of correlated retinal activity that is used for activitydependent refinement. Since optic fibers likely need to detect the postsynaptic activity of a local group of tectal neurons, rather than that of a single neuron, the long tectal time constant means that retinal activity need not be correlated with precision much better than 200 ms because the postsynaptic circuitry cannot generate shorter correlations. © 1995 John Wiley & Sons, Inc.     

Detection of sonar signals in the presence of pulses of masking noise by the echolocating bat,Eptesicus fuscus

Summary Two big brown bats (Eptesicus fuscus) were trained to report the presence or absence of a virtual sonar target. The bats' sensitivity to transient masking was investigated by adding 5 ms pulses of white noise delayed from 0 to 16 ms relative to the target echo. When signal and masker occurred simultaneously, the bats required a signal energy to noise spectrum level ratio of 35 dB for 50% probability of detection. When the masker was delayed by 2 ms or more there was no significant masking and echo energy could be reduced by 30 dB for the same probability of detection. The average duration of the most energetic sonar signal of each trial was measured to be 1.7 ms and 2.4 ms for the two bats, but a simple relation between detection performance and pulse duration was not found.In a different experiment the masking noise pulses coincided with the echo, and the duration of the masker was varied from 2 to 37.5 ms. The duration of the masker had little or no effect on the probability of detection.The findings are consistent with an aural integration time constant of about 2 ms, which is comparable to the duration of the cries. This is an order of magnitude less than found in backward masking experiments with humans and may be an adaptation to the special constraints of echolocation. The short time of sensitivity to masking may indicate that the broad band clicks of arctiid moths produced as a countermeasure to bat predation are unlikely to function by masking the echo of the moth.Abbreviations SPL sound pressure level - SD standard deviation - SE standard error - BW bandwidth     

On the ability of neurons in the barn owl's inferior colliculus to sense brief appearances of interaural time difference

Summary This paper investigates the ability of neurons in the barn owl's (Tyto alba) inferior colliculus to sense brief appearances of interaural time difference (ITD), the main cue for azimuthal sound localization in this species. In the experiments, ITD-tuning was measured during presentation of a mask-probe-mask sequence. The probe consisted of a noise having a constant ITD, while the mask consisted of binaurally uncorrelated noise. Collicular neurons discriminated between the probe and masking noise by showing rapid changes from untuned to tuned and back to untuned responses.The curve describing the relation between probe duration and the degree of ITD-tuning resembled a leaky-integration process with a time constant of about 2 ms. Many neurons were ITD-tuned when probe duration was below 1 ms. These extremely short effective probe durations are interpreted as evidence for neuronal convergence within the pathway computing ITD. The minimal probe duration necessary for ITD-tuning was independent of the bandwidth of the neurons' frequency tuning and also of the best frequency of a neuron. Many narrowly tuned neurons having different best frequencies converge to form a broad-band neuron. To yield the short effective probe durations the convergence must occur in strong temporal synchronism.Abbreviations ICc central nucleus of the inferior colliculus; - ICx external nucleus of the inferior colliculus; - ITD interaural time difference - LP Likelihood parameter     

Retinal and cortical nonlinearities combine to produce masking in V1 responses to plaids

The visual response of a cell in the primary visual cortex (V1) to a drifting grating stimulus at the cell’s preferred orientation decreases when a second, perpendicular, grating is superimposed. This effect is called masking. To understand the nonlinear masking effect, we model the response of Macaque V1 simple cells in layer 4Cα to input from magnocellular Lateral Geniculate Nucleus (LGN) cells. The cortical model network is a coarse-grained reduction of an integrate-and-fire network with excitation from LGN input and inhibition from other cortical neurons. The input is modeled as a sum of LGN cell responses. Each LGN cell is modeled as the convolution of a spatio-temporal filter with the visual stimulus, normalized by a retinal contrast gain control, and followed by rectification representing the LGN spike threshold. In our model, the experimentally observed masking arises at the level of LGN input to the cortex. The cortical network effectively induces a dynamic threshold that forces the test grating to have high contrast before it can overcome the masking provided by the perpendicular grating. The subcortical nonlinearities and the cortical network together account for the masking effect. Melinda Koelling is formerly from Center for Neural Science and Courant Institute, New York University.     

Modulation of backward pattern masking by focal visual attention

The effect of focal visual attention on backward pattern masking was investigated using an orientation discrimination task. The results show that attention reduces primarily the effect of interruption masking, the later component of pattern masking, which occurs when the delay between the target and mask onset is about 50-150 ms. The strongest spatial cueing effect, i.e. the strongest reduction of the orientation discrimination threshold due to focal attention, was observed at intermediate (approximately 100 ms) target-to-mask stimulus onset asynchrony (SOA). There was a weak effect of cueing at shorter SOAs, and no or a very weak attentional effect was present at longer target-to-mask SOAs, where the pattern masking effect is absent. The dynamics of attentional modulation of backward pattern masking correlates closely with the dynamics of the attentional modulation of neuronal responses in the early visual cortex.     

Temporal Production Signals in Parietal Cortex

We often perform movements and actions on the basis of internal motivations and without any explicit instructions or cues. One common example of such behaviors is our ability to initiate movements solely on the basis of an internally generated sense of the passage of time. In order to isolate the neuronal signals responsible for such timed behaviors, we devised a task that requires nonhuman primates to move their eyes consistently at regular time intervals in the absence of any external stimulus events and without an immediate expectation of reward. Despite the lack of sensory information, we found that animals were remarkably precise and consistent in timed behaviors, with standard deviations on the order of 100 ms. To examine the potential neural basis of this precision, we recorded from single neurons in the lateral intraparietal area (LIP), which has been implicated in the planning and execution of eye movements. In contrast to previous studies that observed a build-up of activity associated with the passage of time, we found that LIP activity decreased at a constant rate between timed movements. Moreover, the magnitude of activity was predictive of the timing of the impending movement. Interestingly, this relationship depended on eye movement direction: activity was negatively correlated with timing when the upcoming saccade was toward the neuron''s response field and positively correlated when the upcoming saccade was directed away from the response field. This suggests that LIP activity encodes timed movements in a push-pull manner by signaling for both saccade initiation towards one target and prolonged fixation for the other target. Thus timed movements in this task appear to reflect the competition between local populations of task relevant neurons rather than a global timing signal.     

Symmetry between the visual B- and D-systems and equivalence of center and surround: Studies of light increment and decrement in retinal and geniculate neurons of the cat

The dual center surround organization of retinal and geniculate neurons in two antagonistic subsystems B and D, having on-center or off-center receptive fields and signalling brightness or darkness respectively, has been studied by local light increments and decrements. Intensity response functions obtained by the introduction and withdrawal of small center spots either brighter or darker than a common homogeneous field are similar in a given neuron, but the phasic responses are stronger in on-center neurons than in off-center neurons. Center size increments and decrements, however, lead to equal excitations in the B- and D-system, respectively, provided that both luminance steps start from the same level and are of equal size on a linear scale. Decrementing and incrementing the surrounding luminance of the same optimal center spots lead to equal surround responses in the two subsystems if the two luminance steps terminate at the same level. This lateral activation is elicited by light decrement in the B-system and by light increment in the D-system. Center and surround responses within a given subsystem are of comparable amplitude, but generally slightly stronger responses are elicited by optimal center increments (decrements) than by the equivalent surround decrements (increments) which lead to the same spatial contrast for B-(D-) neurons. The symmetry relations between the B- and D-system and the equivalence relations between center and surround in each subsystem hold for retinal and geniculate neurons. The difference between center and surround response latencies is about 9 ms in both subsystems at the retinal and 14 ms at the geniculate level. Stimulus response functions of on- and off-center neurons are unified on the basis of linear relative luminance scales.     

Analysis and test of laws for backward (metacontrast) masking

In backward visual masking, it is common to find that the mask has its biggest effect when it follows the target by several tens of milliseconds. Research in the 1960s and 1970s suggested that masking effects were best characterized by the stimulus onset asynchrony (SOA) between the target and mask. In particular, one claim has been that the SOA for which masking is optimal remains fixed, even as target and mask durations varied. Experimental evidence supported this claim, and it was accepted as an SOA law. However, recent modeling (Francis, 1997) and experimental studies (Macknik and Livingstone, 1998) argued for new ISI (interstimulus interval) and STA (stimulus termination asynchrony) laws, respectively. This paper reports a mathematical analysis and experimental tests of the laws. The mathematical analysis demonstrates unsuspected relationships between the laws. The experiments test the predictions of the SOA, ISI, and STA laws. The data favor the ISI law over the SOA and the STA laws.     

Effects of mean luminance changes on human contrast perception: contrast dependence, time-course and spatial specificity

Background

When we are viewing natural scenes, every saccade abruptly changes both the mean luminance and the contrast structure falling on any given retinal location. Thus it would be useful if the two were independently encoded by the visual system, even when they change simultaneously. Recordings from single neurons in the cat visual system have suggested that contrast information may be quite independently represented in neural responses to simultaneous changes in contrast and luminance. Here we test to what extent this is true in human perception.

Methodology/Principal Findings

Small contrast stimuli were presented together with a 7-fold upward or downward step of mean luminance (between 185 and 1295 Td, corresponding to 14 and 98 cd/m²), either simultaneously or with various delays (50–800 ms). The perceived contrast of the target under the different conditions was measured with an adaptive staircase method. Over the contrast range 0.1–0.45, mainly subtractive attenuation was found. Perceived contrast decreased by 0.052±0.021 (N = 3) when target onset was simultaneous with the luminance increase. The attenuation subsided within 400 ms, and even faster after luminance decreases, where the effect was also smaller. The main results were robust against differences in target types and the size of the field over which luminance changed.

Conclusions/Significance

Perceived contrast is attenuated mainly by a subtractive term when coincident with a luminance change. The effect is of ecologically relevant magnitude and duration; in other words, strict contrast constancy must often fail during normal human visual behaviour. Still, the relative robustness of the contrast signal is remarkable in view of the limited dynamic response range of retinal cones. We propose a conceptual model for how early retinal signalling may allow this.     

A model for neural representation of temporal duration

To address how temporal duration is encoded in neural systems, we put forward a simple model for recurrent neural networks. Particular assumptions are only the following two: (1) neuronal bistability and; (2) environmental effects described by a heat bath. The results of Monte Carlo simulation show that population activity triggered at an initial time continues for a prolonged duration, followed by an abrupt self-termination. This time course seems highly suitable for neural representation of temporal duration. The time scale of this prolonged duration is much longer than the time scale of neuronal firing which is of the order of ms. The former time scale implies that of interval timing in cognition and behaviour. Thus, the model provides a possible explanation for a link between these two separated time scales. The Weber law, a hallmark of humans and animals' interval timing, can also be reproduced in our model.     

Partial purification and characterization of masking protein for beta-type transforming growth factor from rat platelets

beta-Transforming growth factor (TGF-beta) is stored in platelets and secreted as a high molecular weight latent form associated with a carrier protein of about 440 KD. This carrier protein could be separated from TGF-beta in 1 N acetic acid and could again mask the activity of TGF-beta under neutral conditions. Therefore, it was named the masking protein of TGF-beta. The masking protein was separated from TGF-beta by gel filtration on a Sephacryl S-300 column or by anion-exchanger FPLC on a Mono Q column in the presence of 6 M urea. Partially purified masking protein from rat platelets neutralized the activity of TGF-beta dose-dependently and was effective at 0.3 microgram/ml. This masking protein could also mask the activity of human TGF-beta, suggesting that it was not species specific. The masking protein was a heat- and acid-stable protein, but was inactivated by treatment with dithiothreitol. The Physiological role of the masking protein in the mechanisms of wound healing and liver regeneration is discussed.     

不同时程弱前掩蔽声对小鼠下丘神经元声反应的选择性抑制

自由声场条件下,以强度为神经元最小阈值阈上5dB,时程分别为40、60、80和100ms的纯音作为前掩蔽声,观察和记录了不同时程弱前掩蔽声对小鼠（Musmusculus Km）下丘神经元发放和声强处理的影响。实验记录到154个神经元,对其中的104个神经元做了不同时程掩蔽声影响的测试。结果发现：掩蔽声对神经元放电率的抑制作用在时间上表现为前抑制（41％）、后抑制（9％）和全抑制（50％）三种类型。改变掩蔽声时程时,大部分神经元（72％）的抑制类型不发生改变,但少部分神经元（28％）随掩蔽声时程的增加,大量的后抑制类型转变为前抑制或全抑制类型。此外,超过一半的神经元（58．06％）其强度．放电率函数曲线随掩蔽声时程的改变而发生转变,主要表现为单调型向饱和型转变及饱和型向非单调型转变,这种转变并不随掩蔽声时程增加表现出规律性的变化。结果表明,前掩蔽作用于下丘神经元声反应的时间域和强度域时具有不均衡性,推测不同时程弱前掩蔽声激活的抑制性输入能分化性调制下丘神经元声反应特性。     

Temporal resolution for calling song signals by female crickets, <Emphasis Type="Italic">Gryllus bimaculatus</Emphasis>

A behavioural gap detection paradigm was used to determine the temporal resolution for song patterns by female crickets, Gryllus bimaculatus. For stimuli with a modulation depth of 100% the critical gap duration was 6–8 ms. A reduction of the modulation depth of gaps to 50% led either to an increase or a decrease of the critical gap duration. In the latter case, the critical gap duration dropped to 3–4 ms indicating a higher sensitivity of auditory processing. The response curve for variation of pulse period was not limited by temporal resolution. However, the reduced response to stimuli with a high duty cycle, and thus short pause durations, was in accordance with the limits of temporal resolution. The critical duration of masking pulses inserted into pauses was 4–6 ms. An analysis of the songs of males revealed that gaps (5.8 ms) and masking pulses (6.9 ms) were at detectable time scales for the auditory pathway of female crickets. However, most of the observed temporal variation of song patterns was tolerated by females. Critical cues such as pulse period and pulse duty cycle provided little basis for inter-individual selection by females.     

Temporal integration of movement: the time-course of motion streaks revealed by masking

Temporal integration in the visual system causes fast-moving objects to leave oriented 'motion streaks' in their wake, which could be used to facilitate motion direction perception. Temporal integration is thought to occur over ≈100 ms in early cortex, although this has never been tested for motion streaks. Here we compare the ability of fast-moving ('streaky') and slow-moving fields of dots to mask briefly flashed gratings either parallel or orthogonal to the motion trajectory. Gratings were presented at various asynchronies relative to motion onset (from -200 to +700 ms) to sample the time-course of the accumulating streaks. Predictions were that masking would be strongest for the fast parallel condition, and would be weak at early asynchronies and strengthen over time as integration rendered the translating dots more streaky and grating-like. The asynchrony where the masking function reached a plateau would correspond to the temporal integration period. As expected, fast-moving dots caused greater masking of parallel gratings than orthogonal gratings, and slow motion produced only modest masking of either grating orientation. Masking strength in the fast, parallel condition increased with time and reached a plateau after 77 ms, providing an estimate of the temporal integration period for mechanisms encoding motion streaks. Interestingly, the greater masking by fast motion of parallel compared with orthogonal gratings first reached significance at 48 ms before motion onset, indicating an effect of backward masking by motion streaks.     

Local circuitry in the IMHV of the domestic chick (Gallus domesticus)

The responses to local stimulation have been recorded from neurons in the intermediate part of the medial hyperstriatum ventrale (IMHV) of the domestic chick, by using an in vitro slice preparation. When the slice is bathed in gassed Krebs' solution, a single stimulus evokes a short-lasting diphasic response. The first phase is negative and lasts some 3 ms, whereas the second, positive phase is often of lower amplitude and usually persists for about 15 ms. The first phase is little altered by perfusion with either Ca2(+)-free Krebs' solution or Krebs' solution containing a high concentration of Mg2+. In contrast, the second phase is abolished by these procedures. The post-synaptic phase is positive when it is recorded anywhere between 0.1-1.25 mm from the stimulated point; however, in the immediate vicinity (0.0-0.1 mm) of the stimulating electrodes, the post-synaptic response is strongly negative. A pair of stimuli has to be separated by at least 10 s to guarantee complete recovery of excitability of the post-synaptic response. The recovery curve for this response shows a refractory period of some 5 ms, a peak of excitability at an interval of about 20 ms, and then a sharp trough of relative inexcitability at about 200 ms. The post-synaptic response is considerably reduced in magnitude and duration by the addition of AP-5 to the perfusion fluid; the remaining post-synaptic response is completely abolished by kynurenic acid. The addition of bicuculline methiodide in concentrations of at least 1 x 10(-6) M increases both the magnitude and duration of the second, positive phase of the response to single stimuli. This extended positive response (which may last from 500-800 ms) is abolished by perfusion with bicuculline dissolved in Ca2(+)-free Krebs' solution. For the entire duration of the extended post-synaptic positive response produced by bicuculline, the irregular discharge of single neurons can be recorded. Like the post-synaptic positive response in Krebs' solution, the much larger response produced by bicuculline shows a very localized negativity beneath the stimulating electrodes and displays an almost identical time-course for the recovery of excitability following a single stimulus. The bicuculline induced positive response is also considerably reduced by the presence of AP-5; the addition of kynurenic acid abolishes the remaining post-synaptic response completely. A post-synaptic response, similar to that produced under bicuculline, can be produced by the addition of a maximally effective dose of d-tubocurarine.(ABSTRACT TRUNCATED AT 400 WORDS)     

Non-linear population firing rates and voltage sensitive dye signals in visual areas 17 and 18 to short duration stimuli

Visual stimuli of short duration seem to persist longer after the stimulus offset than stimuli of longer duration. This visual persistence must have a physiological explanation. In ferrets exposed to stimuli of different durations we measured the relative changes in the membrane potentials with a voltage sensitive dye and the action potentials of populations of neurons in the upper layers of areas 17 and 18. For durations less than 100 ms, the timing and amplitude of the firing and membrane potentials showed several non-linear effects. The ON response became truncated, the OFF response progressively reduced, and the timing of the OFF responses progressively delayed the shorter the stimulus duration. The offset of the stimulus elicited a sudden and strong negativity in the time derivative of the dye signal. All these non-linearities could be explained by the stimulus offset inducing a sudden inhibition in layers II-III as indicated by the strongly negative time derivative of the dye signal. Despite the non-linear behavior of the layer II-III neurons the sum of the action potentials, integrated from the peak of the ON response to the peak of the OFF response, was almost linearly related to the stimulus duration.     

White Noise Masks Some Temporal Parameters of the Auditory Localization of Radially Moving Targets

The temporal parameters of the perception of radially moving sound sources partly masked with broadband internalized noise at an intensity of 40, 46, or 52 dB above the hearing threshold have been studied. The threshold of sound duration necessary for identifying the direction of movement of the sound source (75% correct answers) increases from 135 ms in silence to 285 ms at all intensities of continuous noise studied. The minimum duration of the stimulus beginning with which a subsequent increase in duration does not increase the number of correct responses is the same (385 ms) under all conditions of stimulus presentation. Broadband noise of any intensity increases the time of response to stimuli in the range of durations studied. At a noise of 52 dB, which is close to the threshold of full masking, the reaction time is not increased significantly compared to its estimation at a noise of 46 dB. The minimum duration of the stimulus has proved to be the stablest temporal parameter of the perception of movement of a sound source. Changes in the temporal parameters of sound perception at noise levels close to the threshold of full masking are discussed.     

Response of Human Thalamic Neurons to High-Frequency Stimulation

Thalamic deep brain stimulation (DBS) is an effective treatment for tremor, but the mechanisms of action remain unclear. Previous studies of human thalamic neurons to noted transient rebound bursting activity followed by prolonged inhibition after cessation of high frequency extracellular stimulation, and the present study sought to identify the mechanisms underlying this response. Recordings from 13 thalamic neurons exhibiting low threshold spike (LTS) bursting to brief periods of extracellular stimulation were made during surgeries to implant DBS leads in 6 subjects with Parkinson''s disease. The response immediately after cessation of stimulation included a short epoch of burst activity, followed by a prolonged period of silence before a return to LTS bursting. A computational model of a population of thalamocortical relay neurons and presynaptic axons terminating on the neurons was used to identify cellular mechanisms of the observed responses. The model included the actions of neuromodulators through inhibition of a non-pertussis toxin sensitive K⁺ current (I_KL), activation of a pertussis toxin sensitive K⁺ current (I_KG), and a shift in the activation curve of the hyperpolarization-activated cation current (I_h). The model replicated well the measured responses, and the prolonged inhibition was associated most strongly with changes in I_KG while modulation of I_KL or I_h had minimal effects on post-stimulus inhibition suggesting that neuromodulators released in response to high frequency stimulation are responsible for mediating the post-stimulation bursting and subsequent long duration silence of thalamic neurons. The modeling also indicated that the axons of the model neurons responded robustly to suprathreshold stimulation despite the inhibitory effects on the soma. The findings suggest that during DBS the axons of thalamocortical neurons are activated while the cell bodies are inhibited thus blocking the transmission of pathological signals through the network and replacing them with high frequency regular firing.