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W S Geisler

Publications and source records attributed to W S Geisler.

29 records · Page 2Linked to original sources
PubMed1984

Physical limits of acuity and hyperacuity.

An ideal detector is derived for the discrimination of arbitrary stimuli in the two-alternative forced-choice paradigm. The ideal detector's performance is assumed to be limited only by quantal fluctuations, the optics of the eye, and the size and spacing of the receptors in the retinal mosaic. Detailed predictions are presented for two-point acuity and hyperacuity tasks. The ideal detector's two-point resolution, over a wide range of luminances, is approximately 10 times worse than its two-point vernier acuity or separation discrimination. Furthermore, two-point resolution is shown to vary in proportion to the -1/4 power of spot intensity, but vernier acuity and separation discrimination vary in proportion to the -1/2 power of spot intensity. It is shown that this ideal detector can be implemented by the use of appropriately shaped receptive fields. The derivation provides a simple way to determine the shapes of these optimal receptive fields for arbitrary stimuli. The sensitivities of real (human) and ideal detectors are compared.

Discrimination, Psychological↗
PubMed1983

Mechanisms of visual sensitivity: backgrounds and early dark adaptation.

There is substantial physiological and psychophysical evidence for an adaptation mechanism whose effect, under many circumstances, is equivalent to placing a neutral density filter in front of the eye. Furthermore, this mechanism is of sufficient strength to predict the generalized Weber's law for increment thresholds on steady backgrounds. However, it was shown that an additional transient mechanism (with a time-course of around 100 msec) is also needed to account for the increment-threshold results. The effect of this mechanism on increment thresholds during early dark adaptation was parametrically examined. Several models for the transient mechanism were considered. The one best able to account for the results consists of a subtractive inhibitory stage operating prior to a saturating nonlinearity.

Dark Adaptation↗
PubMed1981

Effects of bleaching and backgrounds on the flash response of the cone system.

1. Increment-threshold functions for flashed backgrounds were measured in the human fovea under several conditions: (1) during dark adaptation following full bleaches, (2) in the presence of steady adapting backgrounds and (3) 500 msec following extinction of adapting backgrounds.2. To prevent the intense flashed backgrounds from interfering with the course of dark adaptation the inter-trial interval was continuously increased during dark adaptation. This technique may prove generally useful for presenting suprathreshold stimuli during dark adaptation.3. All the increment-threshold functions measured during dark adaptation were found to be roughly shape invariant and continuously accelerating when plotted in log-log co-ordinates. Furthermore, in order to translate a function obtained at any given time into coincidence with a function obtained at any other time, it had to be translated vertically and horizontally the same number of log units. This is equivalent to adding or removing neutral density filters from in front of the eye.4. The increment-threshold functions obtained with steady adapting backgrounds were also continuously accelerating, but could not be brought into coincidence by equal vertical and horizontal translation. However, this became possible again if the adapting background was extinguished during presentation of the flashed background.5. These results contradict the equivalent-background hypothesis. None the less, they suggest that under present conditions the effects of bleaches and backgrounds may be similar except that steady backgrounds provide additional quanta which drive the visual system part of the way up its intensity-response function.6. The conclusions above were supported by applying a simple model based on the equation R = R(max). I(n) / (I(n) + I(1) (n)), which has frequently been used to describe the peak responses of retinal neurones to flashed stimuli. Virtually all of the data reported here were fitted by this simple model with R(max) held constant.7. The parameters estimated from the model imply that the flash responses measured in the present experiments differ in at least one fundamental way from receptor responses. Even after taking into account changes in the half saturation constant I(1), steady backgrounds were found to be much less effective than flashed backgrounds in driving the visual system up its intensity-response function. A subtractive inhibitory network prior to the non-linear stages responsible for threshold saturation could explain this result.

Dark Adaptation↗
PubMed1979

Initial-image and afterimage discrimination in the human rod and cone systems.

1. The rod-isolation technique of Aguilar & Stiles (1954) was used to obtain scotopic increment-threshold functions in the dark-adapted eye. Increment-threshold functions were obtained for background durations of 50 to 500 msec, but the onset of the background and increment fields was always simultaneous. In all conditions the duration of the increment field was 50 msec. 2. The pattern of results obtained is the same as that reported earlier for the cone system (Geisler, 1978). For background durations greater than that of the increment field, the increment-threshold functions have two distinct branches. It was shown, by measuring action spectra, that both branches reflect the sensitivity of the rod system. 3. When the increment thresholds are plotted as a function of background retinal illuminance, all the lower branches superimpose. This implies that those thresholds are dependent only on the number of background quanta absorbed during presentation of the increment field. On the other hand, when the increment thresholds are plotted as a function of background energy, all the upper branches superimpose, implying that those thresholds are determined by the total number of background quanta absorbed. 4. For the thresholds falling on the lower branches observes reported that the increment field was detected in the initital image of the background and increment fields when they were flashed. For the upper branches, the increment field was detected in a short-term afterimage that appeared after the background was extinguished. The higher the background intensity the longer was the latency until the increment appeared in the afterimage. 5. All of the above findings appear to be consistent with the known properties of the electrical responses of vertebrate photoreceptors. A model based on Penn & Hagins' (1972) model for the photocurrent in rat rods predicts, fairly accurately, the rod and cone increment-threshold results. The parameters estimated by fitting the model support the hypothesis that the short-term rod and cone afterimages are due to the relatively slow decay of internal transmitter, but they suggest that post-receptor mechanisms are responsible for the threshold saturation observed with flashed backgrounds.

Afterimage↗
PubMedDate not supplied

Visual cortex neurons in monkeys and cats: detection, discrimination, and identification.

A descriptive function method was used to measure the detection, discrimination, and identification performance of a large population of single neurons recorded from within the primary visual cortex of the monkey and the cat, along six stimulus dimensions: contrast, spatial position, orientation, spatial frequency, temporal frequency, and direction of motion. First, the responses of single neurons were measured along each stimulus dimension, using analysis intervals comparable to a normal fixation interval (200 ms). Second, the measured responses of each neuron were fitted with simple descriptive functions, containing a few free parameters, for each stimulus dimension. These functions were found to account for approximately 90% of the variance in the measured response means and response standard deviations. (A detailed analysis of the relationship between the mean and the variance showed that the variance is proportional to the mean.) Third, the parameters of the best-fitting descriptive functions were utilized in conjunction with Bayesian (optimal) decision theory to determine the detection, discrimination, and identification performance for each neuron, along each stimulus dimension. For some of the cells in monkey, discrimination performance was comparable to behavioral performance; for most of the cells in cat, discrimination performance was better than behavioral performance. The behavioral contrast and spatial-frequency discrimination functions were similar in shape to the envelope of the most sensitive cells; they were also similar to the discrimination functions obtained by optimal pooling of the entire population of cells. The statistics which summarize the parameters of the descriptive functions were used to estimate the response of the visual cortex as a whole to a complex natural image. The analysis suggests that individual cortical neurons can reliably signal precise information about the location, size, and orientation of local image features.

Animals↗
PubMedDate not supplied

Motion direction signals in the primary visual cortex of cat and monkey.

When an image feature moves with sufficient speed it should become smeared across space, due to temporal integration in the visual system, effectively creating a spatial motion pattern that is oriented in the direction of the motion. Recent psychophysical evidence shows that such "motion streak signals" exist in the human visual system. In this study, we report neurophysiological evidence that these motion streak signals also exist in the primary visual cortex of cat and monkey. Single neuron responses were recorded for two kinds of moving stimuli: single spots presented at different velocities and drifting plaid patterns presented at different spatial and temporal frequencies. Measurements were made for motion perpendicular to the spatial orientation of the receptive field ("perpendicular motion") and for motion parallel to the spatial orientation of the receptive field ("parallel motion"). For moving spot stimuli, as the speed increases, the ratio of the responses to parallel versus perpendicular motion increases, and above some critical speed, the response to parallel motion exceeds the response to perpendicular motion. For moving plaid patterns, the average temporal tuning function is approximately the same for both parallel motion and perpendicular motion; in contrast, the spatial tuning function is quite different for parallel motion and perpendicular motion (band pass for the former and low pass for the latter). In general, the responses to spots and plaids are consistent with the conventional model of cortical neurons with one rather surprising exception: Many cortical neurons appear to be direction selective for parallel motion. We propose a simple explanation for "parallel motion direction selectivity" and discuss its implications for the motion streak hypothesis. Taken as a whole, we find that the measured response properties of cortical neurons to moving spot and plaid patterns agree with the recent psychophysics and support the hypothesis that motion streak signals are present in V1.

Animals↗