Relativistic compression and expansion of experiential time in the left and right space

Time, space and numbers are closely linked in the physical world. However, the relativistic-like effects on time perception of spatial and magnitude factors remain poorly investigated. Here we wanted to investigate whether duration judgments of digit visual stimuli are biased depending on the side of space where the stimuli are presented and on the magnitude of the stimulus itself. Different groups of healthy subjects performed duration judgment tasks on various types of visual stimuli. In the first two experiments visual stimuli were constituted by digit pairs (1 and 9), presented in the centre of the screen or in the right and left space. In a third experiment visual stimuli were constituted by black circles. The duration of the reference stimulus was fixed at 300 ms. Subjects had to indicate the relative duration of the test stimulus compared with the reference one. The main results showed that, regardless of digit magnitude, duration of stimuli presented in the left hemispace is underestimated and that of stimuli presented in the right hemispace is overestimated. On the other hand, in midline position, duration judgments are affected by the numerical magnitude of the presented stimulus, with time underestimation of stimuli of low magnitude and time overestimation of stimuli of high magnitude. These results argue for the presence of strict interactions between space, time and magnitude representation on the human brain.     

Lateralization of perception of short time intervals and cortical evoked activity in man

Recognition of short time intervals (10, 60, and 180 ms) between visual stimuli presented to the left or right hemisphere was studied in adult healthy people. The interval of 180 ms is recognized better than that of 10 or 60 ms. Learning with repeated tests with 180 ms intervals proceeds better than that with short intervals. The predominance of the left hemisphere has been revealed only for perception of 10 ms interval. The other time intervals asymmetry is not observed. It is suggested that the left hemisphere is predominant in estimation of short (less than 60 ms) time intervals. In formation of time nervous model a significant role is played by local activation of the cortical zone where the standard stimulus is addressed.     

Amplitude-temporal parameters of the evoked potentials of the visual and motor areas in the visual perception and mental representation of an image

Amplitude-temporal analysis was carried out of the EP components of the visual and motor areas elicited by neutral (diffuse light) and structural (checker board pattern) stimuli in different situations, defined by instruction. Interserial comparisons showed that at any instruction, the latency of the first EP component of the motor areas is reduced; as a result it can appear here simultaneously with the EP of the visual areas. At the instruction involving the subject in the process of active change of perception, activation of the right hemisphere, including the motor area, is manifest by EP parameters, while the right occipital area is activated in response to the structural stimulus, and the left one--in response to the neutral stimulus. At complication of the stimulus or instruction, the period is prolonged when the latency of EP components of the motor area is shorter than the latency of the isopolar components of the visual area--from 120 to 150 ms in response to the neutral stimuli and the neutral with their counting; from 90 to 150 ms in response to the structural stimuli; from 80 to 210 ms in response to the neutral stimuli with mental representation of the structural one.     

Movement in the normal visual hemifield induces a percept in the 'blind' hemifield of a human hemianope.

We have investigated visual responses to moving stimuli presented to the normal hemifield of a hemianope, GY, who exhibits residual visual function in his right, ''blind'' hemifield. Preliminary experiments established that his perception of moving stimuli localized in his ''blind'' hemifield is retained when a similar stimulus is presented simultaneously in the normal hemifield. In response to a grating stimulus moving horizontally towards fixation in the non-foveal region of the normal, left hemifield, he perceives in addition to a normal motion percept in the left hemifield, a sensation of movement localized in the right hemifield. Qualitatively, this latter is indistinguishable from responses elicited by direct stimulation localized within his ''blind'' hemifield by moving stimuli. We have investigated the characteristics of the mechanisms which induce the ''blind'' field component of GY''s responses to stimulation of the normal hemifield. We show that GY''s sensitivity for detection of movement localized within his ''blind'' hemifield is dependent on the direction of movement, the contrast and the velocity of a grating presented to the normal hemifield. No induced effects were recorded in response to colour or to non-moving, flickering stimuli. We examine the possible contribution of scattered light to our observations, and eliminate this factor by consideration of our experimental results. We discuss the neural mechanisms which may be involved in this response.     

A model of binocular brightness and binaural loudness perception in humans with general applications to nonlinear summation of sensory inputs

A single neural model is proposed to account for how responses of the two eyes and two ears combine to form the perception of binocular brightness and binaural loudness respectively. It involves nonlinear reciprocal feedback inhibition between left and right channels, followed by linear summation between the channels. Local circuit synaptic interactions are an important source of nonlinearity. The model combines inputs in a manner that approximates vector magnitude models in general. This suggests that the model can be applied to a variety of circumstances beyond the visual and auditory data discussed here.     

MEG responses during rhythmic finger tapping in humans to phasic stimulation and their interpretation based on neural mechanisms

 The phase-resetting experiment was applied to human periodic finger tapping to understand how its rhythm is controlled by the internal neural clock that is assumed to exist. In the experiment, the right periodic tapping movement was disturbed transiently by a series of left finger taps in response to impulsive auditory cues presented randomly at various phases within the tapping cycle. After each left finger tap, the original periodic tapping was reestablished within several tapping cycles. Influences of the disturbance on the periodic right finger tapping varied depending on the phase of the periodic right finger tapping at which each left finger tap was made. It was confirmed that the periodic tapping was disturbed not by the auditory cues but by the left finger taps. Based on this fact, in this paper each single left tap was considered as the stimulus, and the phase of the periodic tapping of the right index finger when the left tap was executed as the phase of the stimulus. Responses of the neural activities (magnetoencephalography, MEG), the tapping movement, and the corresponding muscle activities (electromyography) were simultaneously measured. Phase-resetting curves (PRCs) representing the degree of phase reset as a function of the phase of the stimulus were obtained both for the left sensorimotor cortex MEG response and for the right index finger tapping response. The shapes of both PRCs were similar, suggesting that the phase reset of the left sensorimotor cortex activities and that of the finger tapping rhythm were the same. Four out of eight subjects showed type-0 reset in Winfree's definition, and the others showed type-1 reset. For general limit-cycle oscillators, type-0 reset is obtained for relatively strong perturbations and type 1 for weak perturbations. It was shown that the transient response of MEG to the single left tap stimuli in type-0 subjects, where the phase was progressively reset, were different from those in type-1 subjects. Based on detailed analysis of the differences, a neural network model for the phase reset of the tapping rhythm is proposed. Received: 10 February 2000 / Accepted in revised form: 15 January 2002     

Effects of long-term visual experience on responses of distinct classes of single units in inferior temporal cortex

As a precursor to the selection of a stimulus for gaze and attention, a midbrain network categorizes stimuli into "strongest" and "others." The categorization tracks flexibly, in real time, the absolute strength of the strongest stimulus. In this study, we take a first-principles approach to computations that are essential for such categorization. We demonstrate that classical feedforward lateral inhibition cannot produce flexible categorization. However, circuits in which the strength of lateral inhibition varies with the relative strength of competing stimuli categorize successfully. One particular implementation--reciprocal inhibition of feedforward lateral inhibition--is structurally the simplest, and it outperforms others in flexibly categorizing rapidly and reliably. Strong predictions of this anatomically supported circuit model are validated by neural responses measured in the owl midbrain. The results demonstrate the extraordinary power of a remarkably simple, neurally grounded circuit motif in producing flexible categorization, a computation fundamental to attention, perception, and decision making.     

Visibility reflects dynamic changes of effective connectivity between V1 and fusiform cortex

Identifying the neural basis of visibility is central to understanding conscious visual perception. Visibility of basic features such as brightness is often thought to reflect activity in just early visual cortex. But here we show under metacontrast masking that fMRI activity in stimulus-driven areas of early visual cortex did not reflect parametric changes in the visibility of a brightness stimulus. The psychometric visibility function was instead correlated with activity in later visual regions plus parieto-frontal areas, and surprisingly, in representations of the unstimulated stimulus surround for primary visual cortex. Critically, decreased stimulus visibility was associated with a regionally-specific decoupling between early visual cortex and higher visual areas. This provides evidence that dynamic changes in effective connectivity can closely reflect visual perception.     

Visual Features Underlying Perceived Brightness as Revealed by Classification Images

Along with physical luminance, the perceived brightness is known to depend on the spatial structure of the stimulus. Often it is assumed that neural computation of the brightness is based on the analysis of luminance borders of the stimulus. However, this has not been tested directly. We introduce a new variant of the psychophysical reverse-correlation or classification image method to estimate and localize the physical features of the stimuli which correlate with the perceived brightness, using a brightness-matching task. We derive classification images for the illusory Craik-O''Brien-Cornsweet stimulus and a “real” uniform step stimulus. For both stimuli, classification images reveal a positive peak at the stimulus border, along with a negative peak at the background, but are flat at the center of the stimulus, suggesting that brightness is determined solely by the border information. Features in the perceptually completed area in the Craik-O''Brien-Cornsweet do not contribute to its brightness, nor could we see low-frequency boosting, which has been offered as an explanation for the illusion. Tuning of the classification image profiles changes remarkably little with stimulus size. This supports the idea that only certain spatial scales are used for computing the brightness of a surface.     

Tactile rivalry demonstrated with an ambiguous apparent-motion quartet

When observers view ambiguous visual stimuli, their perception will often alternate between the possible interpretations, a phenomenon termed perceptual rivalry [1]. To induce perceptual rivalry in the tactile domain, we developed a new tactile illusion, based on the visual apparent-motion quartet [2]. Pairs of 200 ms vibrotactile stimuli were applied to the finger pad at intervals separated by 300 ms. The location of each successive stimulus pair alternated between the opposing diagonal corners of the approximately 1 cm(2) stimulation array. This stimulation sequence led all participants to report switches between the perception of motion traveling either up and down or left and right across their fingertip. Adaptation to tactile stimulation biased toward one direction caused subsequent ambiguous stimulation to be experienced in the opposing direction. In contrast, when consecutive trials of ambiguous stimulation were presented, motion was generally perceived in the direction consistent with the motion reported in the previous trial. Voluntary eye movements induced shifts in the tactile perception toward a motion axis aligned along a world-centered coordinate frame. Because the tactile quartet results in switching perceptual states despite unvaried sensory input, it is ideally suited to future studies of the neural processes associated with conscious tactile perception.     

Hemispheric specialization for global and local processing: the effect of stimulus category.

Neuropsychological evidence indicates that the global aspect of complex visual scenes is preferentially processed by the right hemisphere, and local aspects are preferentially processed by the left hemisphere. Using letter-based hierarchical stimuli (Navon figures), we recently demonstrated, in a directed-attention task, lateralized neural activity (assessed by positron emission tomography) in the left prestriate cortex during local processing, and in the right prestriate cortex during global processing. Furthermore, temporal-parietal cortex was critically activated bilaterally in a divided-attention task that involved varying the number of target switches between local and global levels of letter-based hierarchical stimuli. Little is known about whether such stimulus categories influence such hemispheric lateralization. We now present data on brain activity, derived from positron emission tomography, in normal subjects scanned during either local or global processing of object-based hierarchical stimuli. We again observe attentional modulation of neural activity in prestriate cortex. There is now greater right-sided activation for local processing and greater left-sided activation for global processing, which is the opposite of that seen with letter-based stimuli. The results suggest that the relative differential hemispheric activations in the prestriate areas during global and local processing are modified by stimulus category.     

Representation of Time-Varying Stimuli by a Network Exhibiting Oscillations on a Faster Time Scale

Sensory processing is associated with gamma frequency oscillations (30–80 Hz) in sensory cortices. This raises the question whether gamma oscillations can be directly involved in the representation of time-varying stimuli, including stimuli whose time scale is longer than a gamma cycle. We are interested in the ability of the system to reliably distinguish different stimuli while being robust to stimulus variations such as uniform time-warp. We address this issue with a dynamical model of spiking neurons and study the response to an asymmetric sawtooth input current over a range of shape parameters. These parameters describe how fast the input current rises and falls in time. Our network consists of inhibitory and excitatory populations that are sufficient for generating oscillations in the gamma range. The oscillations period is about one-third of the stimulus duration. Embedded in this network is a subpopulation of excitatory cells that respond to the sawtooth stimulus and a subpopulation of cells that respond to an onset cue. The intrinsic gamma oscillations generate a temporally sparse code for the external stimuli. In this code, an excitatory cell may fire a single spike during a gamma cycle, depending on its tuning properties and on the temporal structure of the specific input; the identity of the stimulus is coded by the list of excitatory cells that fire during each cycle. We quantify the properties of this representation in a series of simulations and show that the sparseness of the code makes it robust to uniform warping of the time scale. We find that resetting of the oscillation phase at stimulus onset is important for a reliable representation of the stimulus and that there is a tradeoff between the resolution of the neural representation of the stimulus and robustness to time-warp.     

Timing Rhythms: Perceived Duration Increases with a Predictable Temporal Structure of Short Interval Fillers

Variations in the temporal structure of an interval can lead to remarkable differences in perceived duration. For example, it has previously been shown that isochronous intervals, that is, intervals filled with temporally regular stimuli, are perceived to last longer than intervals left empty or filled with randomly timed stimuli. Characterizing the extent of such distortions is crucial to understanding how duration perception works. One account to explain effects of temporal structure is a non-linear accumulator-counter mechanism reset at the beginning of every subinterval. An alternative explanation based on entrainment to regular stimulation posits that the neural response to each filler stimulus in an isochronous sequence is amplified and a higher neural response may lead to an overestimation of duration. If entrainment is the key that generates response amplification and the distortions in perceived duration, then any form of predictability in the temporal structure of interval fillers should lead to the perception of an interval that lasts longer than a randomly filled one. The present experiments confirm that intervals filled with fully predictable rhythmically grouped stimuli lead to longer perceived duration than anisochronous intervals. No general over- or underestimation is registered for rhythmically grouped compared to isochronous intervals. However, we find that the number of stimuli in each group composing the rhythm also influences perceived duration. Implications of these findings for a non-linear clock model as well as a neural response magnitude account of perceived duration are discussed.     

Reflection of the Emotional Significance of Visual Stimuli in the Characteristics of Evoked EEG Potentials

Healthy subjects (n = 88) were asked to passively visualize positive and passive emotiogenic visual stimuli and also stimuli with a neutral emotional content. Images of the International Affective Picture System (IAPS) were used. Amplitude/time characteristics of the components of evoked EEG potentials (EPs), P1, N1, P2, N2, and P3 and topographic distribution of the latter components were analyzed. The latencies, amplitudes, and topography of the EP waves induced by presentation of positive and negative stimuli were found to be different from the respective values for the EPs induced by neutral stimuli. The level and pattern of these differences typical of different EP components were dissimilar and depended on the sign of the emotions. Specificities related to the valency of an identified stimulus were observed within nearly all stages of processing of visual signals, for the negative stimuli, beginning from an early stage of sensory analysis corresponding to the development of wave Р1. The latencies of components Р1 in the case of presentation of emotiogenic negative stimuli and those of components N1, N2, and Р3 in the case of presentation of the stimuli of both valencies were shorter than the latencies observed at neutral stimuli. The amplitude of component N2 at perception of positive stimuli was, on average, lower, while the Р3 amplitude at perception of all emotiogenic stimuli was higher than in the case of presentation of neutral stimuli. The time dynamics of topographic peculiarities of processing of emotiogenic information were complicated. Activation of the left hemisphere was observed during the earliest stages of perception, while the right hemisphere was activated within the intermediate stages. Generalized activation of the cortex after the action of negative signals and dominance of the left hemisphere under conditions of presentation of positive stimuli were observed only within the final stages. As is supposed, emotiogenic stimuli possess a greater biological significance than neutral ones, and this is why the former attract visual attention first; they more intensely activate the respective cortical zones, and the corresponding visual information is processed more rapidly. The observed effects were more clearly expressed in the case of action of negative stimuli; these effects involved more extensive cortical zones. These facts are indicative of the higher intensity of activating influences of negative emotiogenic stimuli on neutral systems of the higher CNS structures.     

Level dependent signal flow in the light pupil reflex

Latency of pupillary responses to light stimuli are smaller for larger steps of light, and larger for smaller steps of light (Alpern 1954; Lowenstein et al. 1964; Lee et al. 1969; Terdiman et al. 1969; Cibis et al. 1977; and many others). Miller and Thompson (1978), however, reported negligible change in pupil cycle time (period of high gain instability oscillations) with increased mean brightness. Sandberg and Stark (1968) reportd a negligible reduction in phase lag of pupillary responses to sinusoidal light stimuli as the modulation coefficient (m) increased. To resolve the inconsistency between the well-documented dependence of latency upon brightness, and the apparent absence of level dependence in the phase characteristics (as reflected directly in the responses to sinusoidal stimuli and indirectly in pupil cycle time experiments) we measured: 1. Latency to step stimuli of light, 2. Phase of responses to sinusoidal light stimuli and 3. Period (pupil cycle time) of high gain instability oscillations. The dependence of pupillary latency upon stimulus level (both light and accommodation) and the interaction between accommodation and light responses were investigated. We show that most of the level dependence of light-pupil latency resides in the afferent path. In the companion papers, we demonstrate that: 1. Phase of pupillary response to sinusoidal light stimuli is reduced by increased mean light level, but is independent of pupil size and accommodative stimulus level; and 2. The period of high gain oscillations is shown to decrease with increased mean light level. Taken together, these results imply the existence of a Level Dependent Signal Flow (LDSF) operator that resides in the light-pupil pathway, but not in the accommodation-pupil pathway. We propose a systems model of this operator in which the neural signals controlling pupil size are treated as waves whose phase velocity increases in response to brighter stimuli, and decreases in response to dimmer stimuli. When parameters of the model are adjusted to fit measured pupillary latency over a range of light levels, the model exhibits reduced phase lag in response to increased mean light level in the sinusoidal paradigm, and it exhibits reduced pupil cycle time in the high-gain oscillation paradigm. The model exhibits saturation of the LDSF effect in all paradigms at high light levels, as do experimental results. It simulates directional asymmetry of pupillary response to positive and negative steps of light, with constriction more rapid than dilatation. Finally, it simulates tonic pupillary constriction in response to modulation of a light simulus without changing average light level (Varju 1964; Troelstra 1968). All of these stimulated results are in accord with experimental observation.     

Color and luminance influence, but can not explain, binocular rivalry onset bias

When an observer is presented with dissimilar images to the right and left eye, the images will alternate every few seconds in a phenomenon known as binocular rivalry. During sustained viewing, the timing of these switches appears to be unpredictable. Recent research has suggested that the initial 'onset' period of rivalry is not random and may be different in its neural mechanism than subsequent dominance periods. It is known that differences in luminance and contrast have a significant influence on the average dominance during sustained rivalry and that perception of luminance can vary between individuals and across the visual field. We therefore investigated whether perception of luminance contrast plays a role in onset rivalry. Observers viewed rival targets of equal brightness for brief presentations in eight locations of the near periphery and reported the color that was first dominant in each location. Results show that minimizing differences in brightness and contrast yields a stronger pattern of onset dominance bias and reveals evidence of monocular dominance. The results suggest that both contrast and monocular dominance play a role in onset dominance, though neither can fully explain the effect.     

Discrimination of a Right Angle with Monocular and Cyclopic Perception

Psychophysical tests with monocular and cyclopic perception were carried out to evaluate the accuracy of discrimination of right, acute, and obtuse angles. Tests with monocular perception were carried out with stimuli made by light line segments, spots, or elements of the Oppel-Kundt and Muller-Lyer figures. In tests with cyclopic perception, pairs of V-shaped stimuli with an identical orientation in the visual field and equal length of the sides but different divergence angles were presented to the different eyes of subjects. The test data demonstrated features of the perception of a right angle, namely, a high accuracy of reproduction, periodicity of errors as a function of the general orientation of a stimulus, similar characteristics of the manifestation of geometric illusions in angle reproduction and length comparison, and the manifestation of Hering’s law in cyclopic perception. These results agree with the multilocal hypothesis, which explains the perception of right and other angles on the basis of the information about the coordinates of stimulus parts.__________Translated from Fiziologiya Cheloveka, Vol. 31, No. 4, 2005, pp. 14–26.Original Russian Text Copyright © 2005 by Bulatov, Bertulis, Bulatova.     

Point Me in the Right Direction: Same and Cross Category Visual Aftereffects to Directional Cues

Of the many hand gestures that we use in communication pointing is one of the most common and powerful in its role as a visual referent that directs joint attention. While numerous studies have examined the developmental trajectory of pointing production and comprehension, very little consideration has been given to adult visual perception of hand pointing gestures. Across two studies, we use a visual adaptation paradigm to explore the mechanisms underlying the perception of proto-declarative hand pointing. Twenty eight participants judged whether 3D modeled hands pointed, in depth, at or to the left or right of a target (test angles of 0°, 0.75° and 1.5° left and right) before and after adapting to either hands or arrows which pointed 10° to the right or left of the target. After adaptation, the perception of the pointing direction of the test hands shifted with respect to the adapted direction, revealing separate mechanisms for coding right and leftward pointing directions. While there were subtle yet significant differences in the strength of adaptation to hands and arrows, both cues gave rise to a similar pattern of aftereffects. The considerable cross category adaptation found when arrows were used as adapting stimuli and the asymmetry in aftereffects to left and right hands suggests that the adaptation aftereffects are likely driven by simple orientation cues, inherent in the morphological structure of the hand, and not dependent on the biological status of the hand pointing cue. This finding provides evidence in support of a common neural mechanism that processes these directional social cues, a mechanism that may be blind to the biological status of the stimulus category.