Temporal summation and reaction times for detecting gratings.

The effects of the duration of presentation of stimuli on the reaction time for detection of gratings of different spatial frequencies were studied. The product of contrast and duration (the contrast "energy") was kept constant for each duration. These experiments showed that at near-threshold "energies," reaction times did not change with increases in the duration to 15 and 30 msec at low and high spatial frequencies respectively. At high contrast "energies," this critical duration was identical (15 msec) for both low and high spatial frequencies. The effects of the duration of a linearly increasing front for grating presentation on detection reaction times were also studied. Increases in the duration of the front (to 60 msec) significantly lengthened the reaction time at low spatial frequencies at all contrast levels studied. Conversely, at high spatial frequencies, a significant increase in the reaction time was seen only at high contrast levels. These results suggest that reaction time depends on two types of mechanisms (phasic and tonic) at near-threshold contrast and one type (phasic) at high contrast.

[Temporal summation and reaction time for grid detection]. / Vremennia summatsiia i vremia reaktsii dlia obnaruzheniia reshetok.

We studied the stimulus-duration effect on the reaction time (RT) to gratings with different spatial frequencies. At each duration value, the product of the contrast and the duration, i.e. the contrast "energy" was kept constant. We found that at near-threshold "energy" levels the RT was constant for up to 15 ms at lower spatial frequencies and up to 30 ms at higher spatial frequencies. At higher "energy" levels, this critical interval was the same (up to 15 ms) for both lower and higher spatial frequencies. The effect of the duration of gradual onset of the stimulus on RT was also studied. An increase of onset duration (up to 60 ms) at low spatial frequencies substantially delayed reaction time for the contrasts under study. Conversely, at high spatial frequencies, this effect was present only for gratings with a high contrast. These results suggest that reaction time is determined by two types of mechanisms (transient and sustained) at a near threshold contrast, and by one type (transient) mechanism at higher contrasts.

Temporal characteristics of line orientation identification.

The hypothesis that identification of line orientation is based on different mechanisms--a detector mechanism at large orientation differences and a computational one at small orientation differences--was tested in three experiments. The first two experiments compared reaction time and time of complete temporal summation (tc) in two tasks, line detection and line orientation identification. Identification at orientation differences 15 degrees or more was similar to detection in several respects, suggesting that it was accomplished according to the principle of "labeled lines." In agreement with the initial hypothesis, identification at differences smaller than 15 degrees had a slower time course and could not be explained by the "labeled lines" principle. Experiment 3 explored the orientation acuity as a function of exposure duration and stimulus energy. Energy could not completely substitute for time in providing high orientation acuity, a result suggesting the involvement of neurophysiological mechanisms of large time constants.

Grating detection and identification dissociated by pattern adaptation.

It two gratings differ in a higher harmonic only, grating identification is expected to depend on that harmonic detection. Yet the comparison of data from several studies suggests that pattern adaptation might differently affect detection and identification. In order to verify this dissociation, a study was made of the effects of adaptation to complex gratings and their harmonics on the detection of the harmonics and on the identification of the complex gratings. Square-wave, first-plus-third-harmonic and missing-fundamental complex gratings were used. The threshold was measured either by the two-interval forced-choice method or by the one-interval method of constant stimuli. For all types of stimuli and procedures, the detection threshold was mostly elevated after adaptation to the corresponding harmonic and the identification threshold was mostly elevated after adaptation to the complex stimulus. This dissociation could only partly be related to the presence of higher harmonics or to the psychophysical procedure. Analysis of threshold changes by adaptation and of phenomenal stimulus appearance suggests that both local features and more global features might contribute to grating identification.

Computer-controlled attenuator.

Various possibilities for applying electronic computer-controlled attenuators for the automation of physiological experiments are considered. A detailed description is given of the design of a 4-channel computer-controlled attenuator, in two of the channels of which the output signal can change by a linear step, in the other two channels--by a logarithmic step. This, together with the existence of additional programmable timers, allows to automate a wide range of studies in different spheres of physiology and psychophysics, including vision and hearing.

Hemisphere asymmetry of the visually evoked potentials elicited by gratings of varying spatial frequency.

Visually evoked potentials (VEP) were recorded upon hemifield stimulation with sinusoidal gratings of varying spatial frequency (SF). Recording was bipolar from 0.1-0.2. The gratings were presented randomly in the left, in the right or in both visual hemifields. No systematic VEP asymmetry was observed at low SF. At SFs above 1.5-2 cpd, however, the early wave peaking at about 100 msec after grating onset was usually of greater amplitude when the grating was presented in the left visual field. In previous experiments of our, contrast sensitivity was almost the same in both hemifields. Thus, the VEP data suggest right-hemisphere specialization in processing high SFs and their comparison with the contrast sensitivity data suggests that this specialization occurs at a level higher than stimulus detection or is evident at suprathreshold contrast levels only.

Hemisphere asymmetry in the detection of gratings of varying spatial frequency.

Visual field symmetry in contrast sensitivity was investigated under conditions of spatial uncertainty. The stimuli were sinusoidal gratings of varying spatial frequency (SF). They were presented randomly in the left, right or in both visual hemifields. The percent detected gratings were slightly higher than the gratings in the left visual field except for gratings of low SF (0.24-0.48 c/deg). The results suggest the existence of a slight left-field (right-hemisphere) superiority in detecting stimuli of medium and high SF. The asymmetry is too slight to allow any significant difference in the SF-content of the neural representation of the left and right halves of the images; neither might it provide an explanation of some recent data on hemispheric specialization, in particular visual tasks.

On the processing rate of gratings superimposed on gratings.

Psychophysical and electrophysiological experiments suggest that visual image processing is effectuated successively as regard as its spatial frequency (SF) content, low SF-information being processed faster than high SF-information. In an investigation of the effect of exposure time on the detection threshold of a test grating superimposed on a high-contrast masking grating Mezrich (1978) has obtained data which he interprets as suggesting the reverse spatio-temporal relationship in processing of suprathreshold masking stimuli, namely, that processing latency decreases with increasing SF. The experiments described in the present paper confirmed his data. It was, however, shown that his results are not related to the processing rate of the masking stimulus and can be explained by an impaired temporal summation due to reduced signal-to-noise ratio or to interchannel interaction.

Application of the equiweight analysis method for the evaluation of the origin of the N1 and N2 waves of visual evoked potentials.

The applicability of the equiweight analysis method was studied with a view to evaluating the position in depth of the two dominating waves of the visual evoked potentials, recorded from the scalp of human subjects upon the appearance of a sinusoidal grating. The stimulus spatial frequency was 2, 4, 8 or 16 c/deg and its contrast was 0.2 or 0.3. The data of the topographic studies confirm earlier observations that the N1 wave with latency of about 100 ms was localized medially with maximum in Oz, while the N2 wave with latency of about 150 ms was spread over a wider area of the scalp. The results of the equiweight analysis corresponded to the evidence of cortical origin of N1. The wide propagation of the N2 wave proved to be an obstacle for the applicability of the method, although it seems promising when the source of the visual evoked potential is limited in area.

Reaction time and critical duration for lines' detection and identification of their orientation.

Reaction time (RT) and duration of complete temporal summation (tc) were studied for both detection of lines and identification of their orientation. The RT was studied as a function of stimulus intensity, number of orientations and angular difference between them. The temporal summation was studied for detection and identification at several angular differences. It was found that the intensity-dependent part of RT for identification at angular difference of 22 degrees equaled that for detection but considerably lengthened at smaller angular differences (10 degrees). The intensity-independent part of RT was minimal for detection and maximal for identification with eight alternatives. The tc increased from 30 msec for detection and identification with the accuracy of 22 degrees to 100 msec for identification with the accuracy of 5 degrees. The results support the conclusion that detection and coarse identification differ only in their later stages and that accurate identification involves additional processes with longer time-constants compared to coarse identification.

Spatial frequency and the pattern onset-offset response.

Visually evoked responses (VERs) were recorded with vertical sinusoidal gratings presented in the on-off mode at rates of 0.5 and 1 Hz. Stimulus spatial frequency ranged from 0.5 to 16 c/deg and its contrast varied from near-threshold to the value of 0.3. At low spatial frequencies, 0.5 and 1 c/deg, the onset and offset VERs were of similar shape, magnitude and contrast dependence. At spatial frequencies higher than 2 c/deg the onset VERs were usually larger and persisted at lower contrast levels than the offset VERs. The results are consistent with the hypothesis that a gradual transition from the operation of transient channels to the operation of sustained channels takes place on increasing stimulus spatial frequency and that within a wide spatial frequency range (at least from 2 to 8 c/deg) suprathreshold gratings effectively stimulate both types of channels. It is confirmed that both onset and offset VERs are delayed at high spatial frequencies. In addition, a longer delay of the late onset waves was found in comparison with the delay of the early onset waves.

Time estimation of visually presented increment and decrement stimuli: the filled-duration illusion.

Experiments were carried out to estimate the duration of time intervals in the 100-1100 ms range, using the direct scaling method. Time intervals were filled: (a) by increment visual stimuli on a light background, (b) by decrement visual stimuli on a light background, (c) only by background illumination. The results obtained show that increment stimuli are perceived as equal in duration to the decrement stimuli. Both increment and decrement stimuli are perceived to be longer than the intervals during which only the background was presented. These results make it possible to generalize the concept of filled and unfilled intervals. A hypothesis is raised that the filled-duration illusion is due not to the properties of the on- and off-effects in the visual system but to processes in the iconic memory connected with the presence of an image (increment or decrement) in the filled intervals.