Robert K. Moore

Contrast sensitivity during saccadic eye movements

Abstract The experiment measured contrast sensitivity of three human observers to sinusoidal gratings presented in 10 msec exposures. The gratings were presented to the steadily fixating eye and during 6° horizontal saccades. Experimental conditions of viewing in a Ganzfeld reduced possible effects of contour masking. The use of horizontal gratings minimized retinal smear. Results showed a significant suppression of sensitivity (more than 0.6 log unit of contrast) to low spatial frequency gratings presented during saccades. The magnitude of saccadic suppression decreased as spatial frequency of the gratings increased. We conclude that optical and neural effects combine in normal viewing to produce saccadic suppression. Minimizing the optically originating effects of contour masking and retinal image smear failed to eliminate the considerable impairment of vision that occurs during a saccadic eye movement.

Measurements of visual suppression during opening, closing and blinking of the eyes

We have previously shown that the sensitivity of human vision, as measured with a stimulus that bypasses the eyelids, is briefly impaired at the time of an eyeblink. We now find that the visual loss is almost equally extensive during eye closure if the eyes then remained closed. But little impairment occurs during eye opening when the eyes then remain open. We have previously concluded that, in blinking, visual suppression is associated with an inhibitory signal sent out by the brain. We now conclude that this signal arises primarily as a corollary to the efferent discharge that closes the eyelids.

Suppression of the blackout due to blinks

Abstract An eyeblink causes an almost complete occlusion of light entering the eye. Yet people scarcely notice their blinks and the subjective visual world remains continuous and stable. We first duplicated the optical effect of a blink with an appropriate decrement in the illumination of a Ganzfeld. Viewed with eyes open, this momentary dimming seemed much stronger than the physically equal one produced by lid closure during a real blink. We then found that subjective equality with the real blink could be attained when the open eyes viewed a decrement of reduced magnitude and duration. We infer that a voluntary blink is accompanied by a suppression that fills in the blackout that would otherwise be perceived.

Perception of suprathreshold stimuli during saccadic eye movement

We have investigated the extent to which a graded series of suprathreshold stimuli are perceptually weakened by saccadic suppression. Stimuli were brief decrements of variable amplitude in the illumination of a Ganzfeld. For each of several decrements presented during fixation, we determined a matching amplitude of decrement presented during saccades. The measure of saccadic suppression was the ratio of these two amplitudes. The matches made by three observers exhibit ratios as high as 6:1 (0.8 log unit of suppression) near the threshold. Suppression decreases systematically as the strength of the stimulus increases, reaching 0.1 log unit or less for stimuli 2 log units above threshold.

Temporal properties of the human visual nervous system

Abstract An image stabilisation system was used to study basic temporal properties which are normally obscured by eye movements. The time course of visual response was measured for three types of presentation of a near-threshold line—sudden presentation (on-response), switching it off again (which generates a negative afterimage) and a sudden small lateral displacement. A general model can be used to fit all these data, indicating that the responses to all three stimuli are generated by similar mechanisms. The responses are similar to that of a capacitor-coupled amplifier with a time constant of about 2.5 sec; the corresponding prediction that the visual frequency response should extend down to about 1/6 Hz was confirmed by measuring contrast thresholds for a line oscillated sinusoidally through a small amplitude. The relation of these observations to the process of light adaptation is discussed.

A method for determining threshold from single-unit neural activity

A computationally straightforward method is described for determining the latency, duration and magnitude of stimulus-evoked single-unit neural activity. A unique feature of the method is its ability to define the neural response without reference to stimulus parameters. First, the temporal component of the spike train that represents the response is located and then that component is analyzed to determine the magnitude of the response. Intensity-response functions can then be constructed, using the number of extra spikes above baseline activity as a measure of response magnitude. Threshold can be defined as any point on the intensity-response function.

A practical method of measuring the human temporal contrast sensitivity function

One of the more significant indicators of neural age-related loss and disease is reduced temporal processing speed. It would, therefore, be useful to have an accurate and practical device that measures the full range of an individuals temporal processing abilities (characterized as the temporal contrast sensitivity function, TCSF). 70 subjects (15-84 yrs) were tested. A small tabletop device utilizing electronic control of light-emitting diodes (LEDs) was constructed that delivered a 1-degree, 660 nm test (the modulation depth of which could be adjusted directly by the subject) centered within a 10-degree 660 nm surround. The method provided a TCSF that had a shape consistent with past studies (peaking around 8 Hz). Also consistent with past work, the largest age-decline was found at the highest frequencies and for the central fovea (r = 0.47, p<0.0001, ~2 Hz per decade). Psychophysical assessment of temporal vision offers an easy and dynamic measure of central visual function.

Changes in the human visually evoked cortical potential in response to chromatic modulation of a sinusoidal grating.

Human visually evoked cortical potentials (VECPs) were recorded from 4 subjects in response to the counterphase alternation of an equal brightness chromatic grating pattern. The pattern was constructed from 2 monochromatic sinusoidal gratings registered 180 deg out of phase. Eleven wavelengths from 450 to 650 nm (at 20 nm intervals) were used. Each wavelength was paired in the grating stimulus with every other to produce a total of 55 different chromatic gratings. The chromatic modulation depth (contrast) of each grating was varied and resulting VECPs were recorded. VECP amplitude was found to vary linearly with log chromatic modulation depth. VECP threshold values were inferred by extrapolation of linear regression lines to zero VECP amplitude. Chromatic modulation sensitivity functions were derived and a multidimensional scaling analysis of the data for each subject was performed. The data were adequately described by a two-dimensional geometric configuration of the 11 wavelengths used. The configurations were similar in shape to those obtained psychophysically by Butler and Riggs (1978) Vision Res. 18, 1407-1416, who used a similar stimulus pattern. Their shapes are consistent with an opponent-color model of color vision.

A retinal smear technique for varying grating contrast

We present here a technique for modulating contrast in gratings seen in Maxwellian-view or on a projection screen. The method can be used with achromatic or chromatic grating stimuli. It is based on a high frequency oscillatory displacement of the grating image, which in turn produces a smearing of the image on the observers retina. The waveform of the displacement and its amplitude determine the appearance and contrast of the image produced on the retina.