The role of brief presentation times and of the spatial-temporal deficit for the recognition of the shape of visual patterns.

The results of a number of experimental works showed that with brief presentation times the visual system can be perceived as a low-frequency spatial filter. In order to verify this hypothesis, we studied the recognition of clear visual patterns (with full spatial-frequency spectrum) and defocused patterns (with optically cut high spatial frequencies), with brief presentation times. We used the method of constant stimuli (simple geometric figures), which were presented tachystoscopically with preceding and subsequent masking. Two experimental subjects took part in the research. The results show that, contrary to our expectations, clear patterns were recognized better after brief presentation times. This shows that when brief presentation times are used, the high spatial frequencies are also accessible for processing by the visual system, contributing at the same time to the better recognition of shapes.

GABAergic control on the intensity-response function of ERG waves under different background illumination.

The effect of GABAergic blockade on the intensity-response function of frog ERG waves was investigated under two levels of background illumination in the mesopic and photopic range. GABAergic blockade by picrotoxin shifted the intensity-response curves to the left along the intensity axis and increased their slope under both backgrounds. The maximal gain (dV/dI) and contrast gain (dV/dlog I) of the mechanisms responsible for the ERG wave generation were also increased after GABAergic blockade. The intensity range in which the amplitude of the response was a linear function of the logarithm of the stimulus intensity was very restricted. GABAergic blockade influenced also the rapid transitory processes, which determined the steady-state amplitude of the ERG b-wave after a considerable change of the stimulus contrast. The adaptational effect of the background illumination on the intensity-response function of the ERG waves was not principally changed after GABAergic blockade. The possible neuronal mechanisms of the observed picrotoxin effects are discussed.

Temporal summation of receptive fields for line orientation in humans.

The contrast threshold for line orientation was studied using two lines with the same orientation under three different experimental conditions (series): (1) the two lines were presented in the same part of the receptive field; (2) they were along the same straight line and separated by 14' visual angle; (3) they were parallel and displaced at 4' of visual angle. The lines were presented for the same duration with an interstimulus interval (ISI) varying between 0 and 300 ms. The threshold for line orientation when only the first or the second line was presented alone was also studied. In all series when the ISI was longer than 100 ms, the line orientation thresholds for two lines (T1,2) and for one line when presented alone (T1 or T2) were the same. At ISI varying from 50 to 100 ms the values for (T1,2) were similar to those expected with probability summation. In the third series at ISI from 0 to 10 ms, the same reciprocal inhibitory interactions prevailed as when the two lines were presented simultaneously. In the first series at the same ISI (0-10 ms), a complete temporal summation was also observed.

Influence of picrotoxin on the retinal sensitivity at different levels of background illumination.

The intensity-response function of the b-wave of the ERG in the retina of the turtle (Emys orbicularis) was investigated at two different background illuminations--0.01 and 10 lx, before and after blockade of the GABA-ergic transmission by 0.4 mmol/l picrotoxin (PT). A shift of the curve to the left along the intensity axis at both backgrounds was observed after PT treatment. The region where the b-wave amplitude was linear function of the logarithm of the stimulus intensity was also considerably narrowed. The registered experimental data fitted in well with the Naka-Rushton equation. The participation of GABA-ergic neurones in the processes of light adaptation and gain control in the retina is discussed.

Participation of GABA in the sensitivity control of the OFF-response in the turtle ERG.

The intensity-response--V/log I, function of the d-wave in the turtle ERG was investigated before and after blockade of the GABAergic transmission by 0.4 mmol/l picrotoxin (PT) under conditions of two adapting background illuminations--0.01 and 10 lx. After PT the d-wave V/log I curves showed a shift to the left along the intensity axis under both adapting conditions, revealing an increase in the gain and the contrast sensitivity of the retinal elements involved in the d-wave generation. These changes were similar to the changes in the b-wave V/log I function described under similar conditions. The relative increase of the b- and d- wave amplitude had approximately equal values at both background levels at higher stimulus intensities. In the range of low stimulus intensity, however, at background illumination of 0.01 lx the relative increase of the d-wave was considerably greater than that of the b-wave. The experimental data obtained were best fitted by the Naka-Rushton equation. The participation of the GABAergic neurons on the d-wave sensitivity control is discussed.

Picrotoxin effects on the frog electroretinogram under different background illumination.

The effect of blockade of the GABA-ergic synapses in the frog retina by picrotoxin (PT) on ERG was studied under two conditions of background illumination (Ib): scotopic Ib with illumination level below 0.01 lx and mesopic Ib with illumination level of 2 lx. Under scotopic Ib PT produced a greater relative increase in the d-wave amplitude as compared to the b-wave. The substitution of scotopic Ib by mesopic one in untreated retinas produced a greater decrease in the b-wave amplitude than in the d-wave amplitude. The presentation of the mesopic Ib in PT treated retinas potentiated to nearly the same extent the PT effect on both b- and d-wave. So the initial shift between the amplitudes of the b- and d-wave produced by the mesopic Ib was not significantly altered by PT. The results obtained show that the GABA-ergic neurons in the mixed frog retina participate in the gain control of the ON and OFF channels in a wider range of background illumination. Their relative significance in this control seems to be greater under mesopic Ib than under scotopic Ib. We suggest that the effect of mesopic Ib on the sensitivity of the ERG b- and d-wave is complex and involves some other mechanisms in addition to the GABA-ergic one.

Studying the stability of clear vision using microcomputer "Pravets-82".

The method of Ferree and Rand assesses the efficiency of the eye and is very sensitive to fatigue in the visual system. A modification of the method is developed using microcomputer "Pravetz-82". The proposed algorithm is realised in Extended Basic. The features of the modified method include: display of the stimulus on the monitor of the microcomputer, option for interactive change of the parameters of the stimulus from test to test, storage of the data on a diskette, measuring of the time intervals by a programme clock, statistical evaluation of the results with graphic display. The modification makes the method more versatile, precise, and easy to perform.

Effects of picrotoxin on the turtle's electroretinogram under different background illumination.

The effect of 0.4 mM solution of picrotoxin (PT) on the electroretinogram (ERG) of isolated eyecup preparations of turtle (Emys orbicularis) was investigated under two different backgrounds (Ib)--scotopic (0.009 lx) and mesopic (9 lx). The test stimulus intensity was 46 lx. An increase of the amplitudes of all ERG waves was observed after PT application. The relative increase of the d-wave was the most pronounced one and was similar under the two Ib. The relative increase of the b-wave, however, was greater in mesopic than in scotopic conditions. The two subcomponents of the a-wave were differentially influenced by PT. The development of a new steady level of b-wave sensitivity was delayed after PT.

Interactions among line orientation detectors.

The thresholds for identifying the orientation of line segments were studied with the following stimuli: a) single line segment (S1); b) two parallel line segments (S2) with or without longitudinal displacement; c) three parallel line segments (S3) with or without longitudinal displacement. The thresholds for the stimuli S2 and S3 are lower in comparison to those for the single line segment S1. When the segments are considerably apart (21') the decrease in the thresholds coincides with the theoretically predicted ones under the assumption for probability summation and could be explained by suggesting and independent processing in the orientation selective units (orientation detectors). With closely located line segments the threshold changes for S2 and S3 deviate from the expected values for probability summation and so an interaction among the detectors could be postulated.