Showing papers on "Summation published in 1981"

Temporal summation of moving images by the human visual system.

Abstract: Measurements of threshold visibility were made as a function of duration of stimulus exposure for small moving dot targets, drifting sinusoidal gratings and moving patches of sinusoidal gratings, to investigate how the human visual nervous system summates over time signals arising from stimuli in motion. At image speeds of less that 16 deg/s, temporal summation is as strong and as extended for moving as for stationary dots (total summation over to about 100 ms). This summation is about twice that which would be expected from separate consideration of the regions of spatial and temporal integration. Measurements with sinusoidal gratings reveal that the nature of the summation depends critically on the spatial frequency of the stimulus: gratings of low spatial frequency summate well when in motion (and only when in motion), whereas those of high spatial frequency summate well only when stationary or in very slow motion. An analogue simulation with electronic filters showed that these psychophysical results are directly predictable from the known transfer characteristics of the human visual system (with the additional assumption of probability summation at threshold). Finally, with small patches of sinusoidal grating, it was established that translation per se across the retina has little effect on temporal summation. This suggests that the results obtained with sinusoidal gratings of large extent are also relevant to small moving stimuli, allowing the summation results obtained with dot stimuli to be discussed in terms of the temporal transfer properties of spatially selective visual detectors. On the basis of these results it is proposed that the extended temporal summation observed for dots in motion results from summation of energy of low spatial frequency present in these stimuli.

Single photon signals in fly photoreceptors and first order interneurones at behavioral threshold.

Abstract: 1. The contrast sensitivity of the optomotor response of the fly Musca domestica was measured using a moving sinusoidal grating as the stimulus. In parallel experiments intracellular recordings were made from photoreceptors and first order visual interneurones to to determine their responses to the same threshold stimuli. Measurements of the spatial modulation transfer function for photoreceptors confirm that the optics of the eye were intact during recordings. 2. At the lowest intensity at which one can obtain an optomotor response, the photoreceptor signal is a train of discrete depolarizations, or bumps. With constant intensity stimuli, the temporal distribution of bumps followed the Poisson distribution with a mean rate of proportional to luminance. The mean bump rate at the threshold intensity for a behavioural response is 1.7 +/- 0.7 s-1 (mean +/- S.D., n = 25). 3. Calibrations and the statistical properties of the bump train indicate that a bump represents one effective photon, implying that the bump : photon ratios are quantum capture efficiencies. 4. At low intensities the first order interneurones (the large monopolar cells or LMCs) show hyperpolarizing bumps each triggered by a receptor bump. Using a point source stimulus, centred in the field of view, the LMC bump rate is six times that in a single receptor viewing the same stimulus, as expected from the known projection of six receptor axons to each LMC. When using an extended stimulus (the grating), the bump rate is 18-20 times that in receptors. Comparison with earlier work suggests that this increased lateral summation of receptor inputs to LMCs only occurs at very low intensities. 5. In both receptor and LMCs the amplitudes and wave forms of bumps depend upon the position of a point source stimulus within the field of view. With the light in the periphery of the field the bumps are smaller and slower than when the light is in the centre. This difference in response suggests that spatial stimulation is brought about by lateral interactions, possibly between receptors. 6. At higher mean intensities the signal-to-noise ratios in receptors responding to the appropriate threshold stimuli increase with intensity. This is suggestive of a decrease in the extent of spatial and/or temporal summation in the optomotor pathway.

Spatial contrast sensitivity of cells in the lateral geniculate nucleus of the rat.

Abstract: 1. The responses to visual stimuli of cells in the dorsal lateral geniculate nucleus of the rat were recorded with micro-electrodes. 2. Maps made with small spots of light showed that most units had concentrically organized receptive fields. Some units gave ‘on-off’ responses to spots flashed anywhere within the receptive field. These units were not directionally selective. 3. By the use of grating patterns as stimuli, units with concentrically organized receptive fields could be divided into groups that showed linear or non-linear spatial summation. Those unit showing linear spatial summation behaved like the ‘X’ cells of the cat, those showing non-linear summation like ‘Y’ cells. 4. ‘On-off’ units showed non-linear spatial summation of a kind that readily distinguished them from Y cells. 5. Measurements of spatial contrast sensitivity made with moving gratings showed, for both X and Y cells, peak sensitivities for spatial frequencies between 0.05 and 0.09 c/deg X and Y cells were not distinguished by their preferred spatial frequencies at any eccentricity.

Spatial and temporal summation in the human dark-adapted retina

Abstract: A coherent set of absolute-threshold data is presented for circular flashes with a diameter of 5–343 min of arc, a flash duration of 32–1000 msec, and at eccentricities between 7 and 50 deg in the temporal retina. A reduction in the flash interval from 4 to 1 sec causes a threshold elevation for eccentricities exceeding 15 deg for all other stimulus parameters. It is shown that local adaptation affects the measurements significantly, especially when long-lasting stimuli and large eccentricities exist. The results can be described with the help of a quanta-coincidence model if adaptational properties are included.

Some psychophysical determinants of discrete Moiré patterns

Abstract: In a series of experiments we have investigated the perception of Moire patterns as a function of spatial density, rotation and temporal display parameters. Results indicate that the local correlation extraction process involved in the perception of these patterns is not feature specific, yet is driven by excitatory (correlated) and inhibitory (uncorrelated) information under a form of spatial summation. These results are comparable with recent results on texture discrimination where texture interpoint distance distributions (dipole statistics) have also been discovered to have excitatory and inhibitory components.

Is there low frequency vibrotactile temporal summation

Abstract: Reaction times of subjects to threshold vibratory stimuli of various durations and frequencies were measured simultaneously with the thresholds in order to test whether more time is needed for the detection of stimuli of long than short duration at threshold intensities. Vibratory stimuli of 20 or 150 Hz frequency and 50, 150, or 300 ms duration were applied to the back of the hand. Lower detection thresholds and longer reaction times were obtained for both 20 and 150 Hz vibration with increase of stimulus duration. The results suggest that temporal summation of high frequencies is due to energy integration, whereas at low frequencies probability summation explains better the threshold decrement.

Limulus psychophysics: Increment threshold

Abstract: Prior temporal summation work had indicated that the sensory code for certain behaviors (in bothLimulus and humans) can be understood if one suggests that the central nervous system analyzes the integral of the photoreceptor potential. An independent test of this suggestion is available because (inLimulus) the physiological increment threshold function obtained from the receptor potential integral is inflected, whereas that obtained from the initial transient peak of the receptor potential is not. The behavioral increment threshold function was measured inLimulus and found to be inflected. Fechner’s scaling assumption (that equally detectable stimulus increments are mediated by sensory signals of equal size) was supported by the fact that a theoretical function, which was calculated from the receptor integral intensity-response function by using Fechner’s scaling assumption, was able to fit the behavioral increment threshold function quite well. Furthermore, the variability of the behavioral data was proportional to the receptor integral variability. A seasonal effect was observed: Fall animals were more sensitive and had a higher criterion than winter animals. These findings permit the specification of the rerceptor potential that mediates this particular behavior at threshold: It has a steady-state amplitude of 7 mV in winter and 16 mV in fall. Taken in conjunction with the results of earlier temporal summation work indicating that the threshold receptor potential is 4.5 sec long, this specification implies that at least some behaviors are mediated by large, long sensory signals, which have properties very different from those of small, short signals, particularly with regard to both linearity and their relative dependence on time vs. energy.

Flicker Fusion and Spatial Summation

Abstract: Flicker fusion frequencies in the central and paracentral retina were studied for liminar and supraliminar stimulations with targets of different size. Statistical analysis of the results obtained in a relatively large series of patients allows a good standardization of ‘normal’ values for healthy people. Furthermore, the evaluation of the results lends insight into the relationships between temporal resolution and spatial summation.

Fully-Photopic and -Scotopic Spatial Summation in Chromatic Perimetry

Abstract: Chromatic static perimetry has in general been limited in the past to investigations involving one stimulus size only (13,17) or different sizes for different chromatic stimuli (16) making the study of spatial summation impossible. In the present study a modified Goldmann perimeter has been used to study spatial summation for chromatic stimuli using Goldmann stimulus sizes I, II, III, and IV (6.8′, 13.6′, 27.2′ and 54.3′). Achromatic thresholds have been obtained for 5 emmetropic normal trichromats for an achromatic and three chromatic stimuli; results are presented here for stimuli of γD = 474 and 617 nm under fully-photopic (250 CD.M−2) and -scotopic conditions. The obtained results are discussed with reference to the earlier work on summation for achromatic stimuli by Fankhauser and Schmidt (6, 7), Sloan (15), and Gougnard (10).

Studies on human visible persistence

Abstract: The visual system has limited temporal resolution, with the intensity of brief stimulus presentations being summed, or integrated over time. This temporal integration may manifest itself in various ways, including timeintensity reciprocity at threshold (Bloch's Law), flicker fusion, some masking phenomena, and visible persistence. The degree of visual temporal resolution varies with a number of conditions including luminance, field size, and the spatial frequency of gratings. Many of these temporal properties may possibly be explained in terms of interactions between two parallel visual mechanisms - "sustained" and "transient" channels. These appear to analyse pattern and movement respectively. This thesis reports investigations into properties of one of these temporal characteristics, visible persistence. It attempts to relate persistence to threshold measures of temporal summation, especially time-intensity reciprocity. It is argued that persistence may occur at more than one level of the visual system and that the more peripheral component may result from processes similar to those responsible for the "critical duration" of time-intensity, reciprocity. Using a separation threshold method, persistence duration was found to increase with both increasing spatial frequency and decreasing contrast. By measuring persistence over several stimulus durations it was possible to show that it consisted of two components. One occurred only at brief stimulus durations and declined linearly with increasing stimulus duration. The other appeared to be present and approximately constant at every stimulus duration. Varying stimulus conditions such as orientation, contrast and spatial frequency affected these components differently. These effects were consistent with the hypothesis that one component is peripherally located and the other more central. It was argued that persistence at both levels is due to the prolonged nature of sustained cell responses, with high frequency gratings stimulating mechanisms with longer response durations than the mechanisms stimulated by low frequency gratings. It was also proposed that response duration may be influenced by the amplitude or latency of transient mechanisms but neither flicker adaptation nor reaction time experiments supported this. It was noted that the conditions which elevate persistence such as the use of higher spatial frequencies, oblique orientations and lower contrast levels would all be expected to produce weaker neural stimulation. It appears therefore that for weaker neural stimulation the response duration is lengthened, as if in compensation.

Spatial Summation in the Foveal and Parafoveal Region

Abstract: Spatial summation effects have been studied in several retinal loci between 0° and 10° from fovea in normal and abnormal subjects using fundus controlled perimetry. Results of spatial summation were as follows: in normal subjects, spatial summation curves have two asymptotes — one shows complete spatial summation and the other shows no summation; in the fovea of amblyopic eye, spatial summation curves represent abnormal gradient with small target; in the eye of optic neuritis patient, spatial summation curves utterly lack the region of complete spatial summation immediately after the visual acuity returned to normal. These results suggest that our new instrument applied to evaluation of visual field in clinical practice is also available for analysis of human abnormal vision function in relation to pathogenesis.