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The article considers the sensitivity of the mechanisms for perceiving simultaneity and temporal order. A method similar to the psychophysical "staircase" method is used for determining the thresholds of the perception of simultaneity and temporal order. The simultaneity and temporal order thresholds are measured for two levels of visual adaptation of the subjects. Two clearly distinguishable thresholds are established: simultaneity threshold and temporal order threshold. The threshold values for simultaneity and temporal order are preserved, irrespective of the adaptation level, which suggests that the general illumination does not affect the mechanisms for perceiving simultaneity and temporal order.

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The perception of simultaneity and temporal order of a pair of light stimuli is studied in dependence on their localization in the visual field. Four different modes of presenting the light stimuli on the screen were used: (1) both stimuli in the middle of the screen, immediately above the fixation point; (2) both stimuli in the left visual half-field, 8 deg. of arc to the left of the fixation point; (3) both stimuli in the right visual half-field, at the same distance from the fixation point; (4) one stimulus in the left and the other one in the right visual half-field, accordingly at 8 deg. of arc to the left or to the right of the fixation point. The duration of each of the pair of simultaneous or consecutive light stimuli was 100 ms. The percentage of correct responses to the presentation of simultaneous or consecutive pairs of light flashes was estimated. The experimental data suggest that the perception of simultaneity of two visual events depends on the spatial position of these events in the visual field.

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Temporally overlapping, spatially separated visual stimuli were used for studying perception of simultaneity and temporal order. Pairs of flashes each of 100 ms duration were presented with stimulus onset asynchronies of 0, 30, 50, and 70 ms. Three spatial arrangements of flash presentation were tested: both flashes were presented foveally; one flash was presented foveally and the other at 8 deg in the left visual hemifield; one flash was presented foveally and the other at 8 deg in the right visual hemifield. Onset asynchronies of 30 and 50 ms were not sufficient for correct identification of temporal order although the flashes were not perceived as simultaneous. Analysis of the response distributions suggests the existence of two-independent mechanisms for evaluating temporal interrelations: one for detecting simultaneity and the other for identifying temporal order. A better detection of simultaneity was found when synchronous flashes were presented together with pairs of flashes separated by larger onset asynchronies. Reading habits may explain only part of the left-right asymmetries of the response distributions. The possible lateralization of the two suggested mechanisms within the cerebral hemispheres is discussed.

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The perception of temporal order is investigated. Suitable localization of the signals is also used to study the functional asymmetry of the cerebral hemispheres during perception of temporal order. The experimental data obtained correlate well with earlier assumptions of ours, namely: (a) separate mechanism for perception of temporal order; (b) the right hemisphere is dominant for perception of the order of events in time; (e) dependence of the subjective threshold for ordering of events in time on different physical characteristics of the stimuli.

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The perception of duration, visually presented, has been studied depending on the visual half-field in which the light stimulations have been presented. The pair of intervals of equal duration, presented for comparison in the 200-1000 ms range, was studied in three different series, in each of which the beginning of the second interval followed accordingly: (a) 10 ms after the beginning of the first interval; (b) after 1/2 of the duration of the first interval; (c) the end of the first interval is at the same time the beginning of the second one. It has been established that two light stimuli of equal duration, presented consecutively in the left and right visual half-fields, are perceived as being different due to their temporal order, provided that the duration of the first interval has elapsed either prior to or at the moment of the onset of the second interval. The temporal order does not affect the subjective duration of overlapping light stimuli. It is possible to assume some kind of hemispheric asymmetry in the duration perception, provided some critical time has elapsed between the end of the first and the beginning of the second intervals. Such an asymmetry cannot be detected in the case of the overlapping intervals studied in the present work.

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The perception of visually presented duration is studied depending on the visual half-field in which the light stimulation has been presented. Light stimuli of the order of 200-1000 ms, of equal duration, divided by a 10 ms interval, were presented in random order, one in the left, the other in the right visual half-field. The subjects' task was to determine which of the pair of stimuli seemed longer. The response "equal in duration" was not allowed. It was found that two light stimuli of equal duration, consecutively presented in the left and right visual half-fields, are perceived as different owing to their time order. This difference reveals the effect of hemispheric differences in duration perception--the second of the pair of stimuli was reported by the subjects as longer in 70% of the cases when it was presented in the left visual half-field, while the second--in the right visual half-field was reported as longer in 60% of the cases.

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A definition is given of filled and unfilled short time intervals presented visually by light and dark stimuli. A reply is given to the question whether the existence of events in a given time interval influences the estimation of the duration of this interval. The mean values of the estimations of filled intervals are lower compared with the mean values of the estimations of single unfilled intervals of the same duration. The influence of the complexity of the task on the mean values of the estimations is discussed. The results obtained are explained in terms of the motor theory of perception.

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A complex experiment is carried out in order to study the human ability to estimate and reproduce short time intervals in the 100-1000 ms range presented with light stimuli. The estimation task consists in verbal nomination of the time intervals presented to the subject in random order after preliminary training. In the production task the subject is expected to press a telegraph key simultaneously with the end of the light interval presented the duration of which is announced to him in advance. In the reproduction task the subject is to press the key a given time after the onset of the stimulus which duration is of 2 sec. The results show that in spite of the relatively good achievements in tasks involving verbal estimation of the duration of the intervals, the reproduction tasks are performed with systematic errors. In the production task the mean values of the response times decrease upon increasing of the required interval, while in the reproduction task they increase parallel with the increase of the required interval. Consequently, one and the same short time interval is perceived differently depending on the task to be performed, namely: verbal nomination, coincidence between hand movement and the end of the interval, or reproduction after verbal nomination.

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The article reports the results of experiments on human ability to estimate time intervals separately or in groups of two intervals as elements of a three-interval array presented by two light signals separated by a dark interval. The first light interval is shown to be systematically overestimated compared with the second one. Moreover, the subjects usually ascribe a constant value to the dark interval. The double interval (light + dark or dark + light) proves to be underestimated compared with the case when estimation of the same intervals was required without estimation of a third interval. The additional task changes the estimations of the subjects. It may be claimed that the complexity of the problem is connected with definite changes in the average estimations of the subjects.

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It is shown that when a human subject estimates verbally the duration of an array consisting of three intervals - two light intervals divided by a dark one - considerable underestimation is observed compared with estimates of the same duration in the control experiment. In case of separate estimation of the three intervals composing the array the sum of the estimates is much more precise even compared with estimates in the control. Perhaps in this case the system for time intervals estimation operates with two "internal clocks", each of which reads the duration of the dark and of the light intervals. In estimating the array of three intervals the subjects estimate the total array and they do not summate their separate estimations. It may be assumed that the system uses only one "clock" for measuring the duration of the total array.

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