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.     

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.     

Feedback contributions to visual awareness in human occipital cortex

It has traditionally been assumed that processing within the visual system proceeds in a bottom-up, feedforward manner from retina to higher cortical areas. In addition to feedforward processing, it is now clear that there are also important contributions to sensory encoding that rely upon top-down, feedback (reentrant) projections from higher visual areas to lower ones. By utilizing transcranial magnetic stimulation (TMS) in a metacontrast masking paradigm, we addressed whether feedback processes in early visual cortex play a role in visual awareness. We show that TMS of visual cortex, when timed to produce visual suppression of an annulus serving as a metacontrast mask, induces recovery of an otherwise imperceptible disk. In addition to producing disk recovery, TMS suppression of an annulus was greater when a disk preceded it than when an annulus was presented alone. This latter result suggests that there are effects of the disk on the perceptibility of the subsequent mask that are additive and are revealed with TMS of the visual cortex. These results demonstrate spatial and temporal interactions of conscious vision in visual cortex and suggest that a prior visual stimulus can influence subsequent perception at early stages of visual encoding via feedback projections.     

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.     

耳蜗电位的持续反应与其中单个脉冲的反应

研究了豚鼠耳蜗电位中持续反应与其中的单个脉冲反应的关系。由于听学系统存在着非线性,因此仅仅知道由单个短声诱发的耳蜗电位脉冲反应还无法预测一串等间隔重复短声诱发的持续反应。然而,等间隔重复短声串中第5个以后的每个短声受前面所有短声的掩蔽都相同,诱发的反应都相同,因此持续反应的稳态部分可以由掩蔽作用达到饱和时单个短声的反应通过延时相加得到。本文在时间域和频率域上定量地证明了这点。     

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.     

Instructive effect of visual experience in mouse visual cortex

We describe a form of experience-dependent response enhancement in the visual cortex of awake mice. Repeated presentations of grating stimuli of a single orientation result in a persistent enhancement of responses evoked by the test stimulus. Response potentiation is specific to the orientation of the test stimulus, develops gradually over the course of several training sessions, and occurs in both juvenile and adult mice. The stimulus-selective response potentiation (SRP) can mask deprivation-induced response depression in adult mice. SRP requires NMDA receptor activation and is prevented by viral delivery of a peptide that interferes with AMPA receptor trafficking. SRP may reveal the mechanisms involved in certain forms of perceptual learning.     

Psychoneural models of the auditory masking process.

Two mathematical models are presented which describe perception of the loudness of a tone masked by noise. The designation “psychoneural” indicates that the models are consistent with accepted findings in the fields of neuroanatomy, neurophysiology, and psychophysics. Each of the models consists of two channels, one which responds only to noise and another which responds to a weighted sum of tone and noise. Each channel is functionally equivalent to the model proposed by MacKay to explain intensity perception. In the first of the two masking models, it is assumed that the noise decreases the perceived loudness of the tone by lateral inhibition at a central location. In the second model, it is assumed that the noise decreases the loudness of the tone by efferent inhibition which acts at the periphery. The quantitative predictions of both models are identical and, with the appropriate adjustment of certain parameters, can be brought into close agreement with psychophysical masking data. The relative merits and handicaps of the models are discussed in the context of a more complex representation of the masking process in which a separate channel is assigned to each critical frequency band.     

A fast and symmetric DUST implementation to mask low-complexity DNA sequences.

The DUST module has been used within BLAST for many years to mask low-complexity sequences. In this paper, we present a new implementation of the DUST module that uses the same function to assign a complexity score to a sequence, but uses a different rule by which high-scoring sequences are masked. The new rule masks every nucleotide masked by the old rule and occasionally masks more. The new masking rule corrects two related deficiencies with the old rule. First, the new rule is symmetric with respect to reversing the sequence. Second, the new rule is not context sensitive; the decision to mask a subsequence does not depend on what sequences flank it. The new implementation is at least four times faster than the old on the human genome. We show that both the percentage of additional bases masked and the effect on MegaBLAST outputs are very small.     

Recognition of forward-masked complex and simple images

The accuracy and time of motor response in tests for visual recognition of simple (sets of horizontal and vertical lines, angles, and crosses) and complex (contour images of animate and inanimate objects) images forward-masked with stimuli of varying complexity were estimated in psychophysiological experiments. It was found that forward masking impaired recognition, the mask whose complexity was close to that of the given test image being the most efficient in each particular case. The masking effect was the strongest in the experiments where the sets of test and masking images coincided. Recognition of complex images proved to be more resistant to masking than recognition of simple images. The observed effects of forward masking are assumed to be determined by lateral inhibition and changes in the excitability cycle. The relative contributions of these mechanisms depend on the degree of similarity between the test and masking stimuli, as well as the hierarchical level of the visual cortex at which the test stimulus is “targeted.”     

Reduced variability of ongoing and evoked cortical activity leads to improved behavioral performance

Sensory responses of the brain are known to be highly variable, but the origin and functional relevance of this variability have long remained enigmatic. Using the variable foreperiod of a visual discrimination task to assess variability in the primate cerebral cortex, we report that visual evoked response variability is not only tied to variability in ongoing cortical activity, but also predicts mean response time. We used cortical local field potentials, simultaneously recorded from widespread cortical areas, to gauge both ongoing and visually evoked activity. Trial-to-trial variability of sensory evoked responses was strongly modulated by foreperiod duration and correlated both with the cortical variability before stimulus onset as well as with response times. In a separate set of experiments we probed the relation between small saccadic eye movements, foreperiod duration and manual response times. The rate of eye movements was modulated by foreperiod duration and eye position variability was positively correlated with response times. Our results indicate that when the time of a sensory stimulus is predictable, reduction in cortical variability before the stimulus can improve normal behavioral function that depends on the stimulus.     

Modulation of activity in human visual area V1 during memory masking

Neurons in the primary visual cortex, V1, are specialized for the processing of elemental features of the visual stimulus, such as orientation and spatial frequency. Recent fMRI evidence suggest that V1 neurons are also recruited in visual perceptual memory; a number of studies using multi-voxel pattern analysis have successfully decoded stimulus-specific information from V1 activity patterns during the delay phase in memory tasks. However, consistent fMRI signal modulations reflecting the memory process have not yet been demonstrated. Here, we report evidence, from three subjects, that the low V1 BOLD activity during retention of low-level visual features is caused by competing interactions between neural populations coding for different values along the spectrum of the dimension remembered. We applied a memory masking paradigm in which the memory representation of a masker stimulus interferes with a delayed spatial frequency discrimination task when its frequency differs from the discriminanda with ±1 octave and found that impaired behavioral performance due to masking is reflected in weaker V1 BOLD signals. This cross-channel inhibition in V1 only occurs with retinotopic overlap between the masker and the sample stimulus of the discrimination task. The results suggest that memory for spatial frequency is a local process in the retinotopically organized visual cortex.     

Interweaving and overlapping of evoked potentials

The refractory effect of one stimulus upon the response to a closely following stimulus in a different modality is much less than upon the response to a stimulus in the same modality. It is therefore far more efficient to record responses to stimuli in different modalities concurrently than to record each one separately. We evaluated 2 techniques for concurrent recording. Interweaving involves recording the response to one stimulus in the intervals between recording responses to other stimuli. Overlapping occurs when two or more responses are at times being simulateneously recorded. Interweaving and overlapping reduced the time required to record auditory brain-stem responses, short-latency somatosensory evoked potentials and pattern-reversal visual evoked potentials by a factor of 3 over the time required to record each response separately. Overlapping caused no significant change in the evoked potentials. Depending upon the actual timing schedule, interweaving may distort the evoked potentials if later parts of the response to one stimulus override the evoked potential to a following stimulus. Filtering and randomization of stimulus timing may attenuate the effects of these overriding potentials.     

Integrated mismatch negativity (MMNi): a noise-free representation of evoked responses allowing single-point distribution-free statistical tests

If the repeated presentation of a single (standard) auditory stimulus is randomly interspersed with a second acoustically different (deviant) stimulus, the cortical activity evoked by the deviant stimulus can contain a negative component known as the mismatch negativity (MMN). The MMN is derived by subtracting the averaged response evoked by the standard stimulus from that evoked by the deviant stimulus. When the magnitude of the response is small or the signal-to-noise ratio is poor, it is difficult to judge the presence or absence of the MMN simply by visual inspection, and statistical detection techniques become necessary. A method of analysis is proposed to quantify the magnitude and statistically evaluate the presence of the MMN based on time-integrated evoked responses. This paper demonstrates the use of this integrated mismatch negativity (MMN_i) analysis to detect the MMN evoked by stimulus contrasts near the perceptual threshold of two subjects. The MMN_i, by virtue of being equivalent to a low-pass filtered response, presents an almost noise-free estimate of MMN magnitude. A single measure of the integrated evoked response at a fixed time point is used in a distribution-free statistic that compares the magnitude of the averaged response evoked by the deviant stimulus with a magnitude distribution derived from 200 subaveraged responses to the standard stimulus (with the number of sweeps per average equal to that of the deviant stimulus). This allows a calculation of the exact probability for the null hypothesis that the negative magnitude of the response evoked by the deviant stimulus is drawn from the magnitude distribution of responses evoked by the standard stimulus. Rejection of this hypothesis provides objective evidence of the presence of the MMN.     

Recovery cycles of mass evoked potentials of different levels of the visual system following electrical stimulation of the optic nerve

Recovery cycles of mass evoked potentials of the optic tract, lateral geniculate body, optic radiation, and primary projection area of the visual cortex were investigated in amytal-anesthetized cats following electrical stimulation of the optic nerve (in some experiments, optic radiation) by paired short stimuli of supraliminal intensity. The recovery of the amplitude and time characteristics of all components of the mass responses to a testing stimulus applied to the nerve at different intervals after an identical conditioning stimulus was studied. The responses of all portions of the visual system (except the retina) were recorded simultaneously, which made it possible to compare the changes of their reactivity arising after the first stimulus under rigorously identical conditions. It was shown that at the upper levels of the visual system the exaltation phase of the recovery cycle and the depression phase following it become increasingly more pronounced and protracted. Depression is especially pronounced in the visual cortex, which maximally affects the recovery of the late components of the mass response. Under the experimental conditions the processes of successive inhibition in the cortex were more pronounced than at the lower levels. There are grounds to assume that the depression of the cortical responses is partially due to pre- and partially postsynaptic inhibition.Institute of Higher Nervous Activity and Neurophysiology, Academy of Sciences of the USSR, Moscow. Translated from Neirofiziologiya, Vol. 2, No. 3, pp. 251–259, May–June, 1970.     

Selectivity of visual cortical neurons of rabbits to magnitude of moving stimuli

Unit responses of the rabbit visual cortex were investigated in relation to size of visual stimuli moving in their receptive field. With an increase in size of the stimulus in a direction perpendicular to the direction of movement ("width" of the stimulus) an initial increase in the intensity of the unit response through spatial summation of excitory effects is followed by a decrease through lateral inhibition. This inhibition is observed between zones of the receptive field which behave as activating when tested by a stimulus of small size. Each neuron has its own "preferred" size of stimuli evoking its maximal activation. No direct correlation is found between the "preferred" stimulus size and the size of the receptive field. With a change in stimulus size in the direction of movement ("length" of the stimulus) the responses to stimuli of optimal size may be potentiated through mutual facilitation of the effects evoked by the leading and trailing edges of the stimulus and weakened in response to stimuli of large size. The selective behavior of the neurons with respect to stimulus size is intensified in the case of coordinated changes in their length and width. It is postulated that the series of neurons responding to stimuli of different "preferred" dimensions may constitute a system classifying stimuli by their size.A. N. Severtsov Institute of Evolutionary Morphology and Ecology of Animals, Academy of Sciences of the USSR, Moscow. Translated from Neirofiziologiya, Vol. 4, No. 6, pp. 636–644, November–December, 1972.     

In Praise of Masking: Behavioural Responses of Retinally Degenerate Mice to Dim Light

There are two ways in which animals compartmentalize their activities into day or night: they can have an endogenous clock which is synchronized each day by a light-dark cycle or there can be a direct response to light such as the decrease in activity by a nocturnal rodent. In the first case activity is said to be entrained by light. In the second case activity is said to be masked by light. Any demonstration of entrainment by periodic presentation of a stimulus must show that activity occurring in phase with that stimulus is not simply a direct response to the stimulus but represents control of phase by an endogenous clock. Thus masking has come to be something to be avoided and excluded in experiments on circadian rhythms. This has led chronobiologists into displaying a lack of interest or a negative attitude toward masking, though there are some exceptions (e.g., refs. 1-5).     

In Praise of Masking: Behavioural Responses of Retinally Degenerate Mice to Dim Light

There are two ways in which animals compartmentalize their activities into day or night: they can have an endogenous clock which is synchronized each day by a light-dark cycle or there can be a direct response to light such as the decrease in activity by a nocturnal rodent. In the first case activity is said to be entrained by light. In the second case activity is said to be masked by light. Any demonstration of entrainment by periodic presentation of a stimulus must show that activity occurring in phase with that stimulus is not simply a direct response to the stimulus but represents control of phase by an endogenous clock. Thus masking has come to be something to be avoided and excluded in experiments on circadian rhythms. This has led chronobiologists into displaying a lack of interest or a negative attitude toward masking, though there are some exceptions (e.g., refs. 1-5).     

Demonstration of interactions between visual and splanchnic afferences at the level of the cerebral cortex in cats]

In the chloralose-anesthetized cats, evoked potentials (E.P) were recorded from cerebral cortex with the use of 1) macroelectrodes in two overlapping zones between splanchnic and visual areas : Medial Lateral and Anterior Suprasylvian (S II) gyri ; 2) microelectrodes in Medial Lateral gyrus. Our results show : 1) a strong increase of the primary visual E.P. in Medial Laternal gyrus by splanchnic stimulus ; 2) a strong inhibition of the primary splanchnic E.P. in S II by visual stimulus ; 3) an inhibition of some cortical neurons in Medial Lateral gyrus by splanchnic stimulus ; 4) an inhibition of associative visual E.P. in S II by splanchnic stimulus, and of associative splanchnic E.P. in Medial Lateral gyrus by visual stimulus. In the other hand splanchnic and visual E.P. were recorded from N. reticularis and Corpus Geniculatum Mediale with the use of concentric electrode. Some hypothesis concerning interrelations mechanisms are discussed.     

Opponent inhibition: a developmental model of layer 4 of the neocortical circuit.

We model the development of the functional circuit of layer 4 (the input-recipient layer) of cat primary visual cortex. The observed thalamocortical and intracortical circuitry codevelop under Hebb-like synaptic plasticity. Hebbian development yields opponent inhibition: inhibition evoked by stimuli anticorrelated with those that excite a cell. Strong opponent inhibition enables recognition of stimulus orientation in a manner invariant to stimulus contrast. These principles may apply to cortex more generally: Hebb-like plasticity can guide layer 4 of any piece of cortex to create opposition between anticorrelated stimulus pairs, and this enables recognition of specific stimulus patterns in a manner invariant to stimulus magnitude. Properties that are invariant across a cortical column are predicted to be those shared by opponent stimulus pairs; this contrasts with the common idea that a column represents cells with similar response properties.