Psychophysical Increment Threshold Research Articles

Visual suppression and its effect upon color and luminance sensitivity

Psychophysical increment thresholds were compared for periods of phenomenological dominance or suppression produced by different stimulation of the two eyes. Three experimental procedures were used; binocular rivalry, permanent suppression and flash suppression. The amount of suppression produced by each procedure was evaluated under conditions intended to accentuate color or luminance system contribution to the detection of a spectral flash. All three procedures resulted in a different pattern of color and luminance suppression. Binocular rivalry suppressed color sensitivity more than luminance and within color, blue (439 nm) sensitivity was more suppressed than red (613 nm). Permanent suppression resulted in a similar pattern of suppression but only blue color sensitivity was reliably more suppressed than luminance sensitivity. Flash suppression produced distinctly different results such that blue color sensitivity was reliably less suppressed than luminance or red color sensitivity, which were not different from each other. Taken together these results provide clues as to where and when the physiological processes mediating visual suppression may be found in the nervous system.

Red-green interactions in the spectral sensitivity of primates as derived from ERG and behavioral data

Different techniques were used to manipulate the inhibitory interaction between the red and green photoreceptors (R and G cones) of rhesus and human primates. The response techniques that were used were the corneal electroretinogram (ERG) and psychophysical increment-threshold spectral sensitivity functions. Red-green opponency, as measured by the depth of the notch at 580 nm, is removed by intravitreal injection of bicuculline but not by strychnine. Therefore, red-green opponency is mediated by GABA and not glycine. The depth of the notch is dependent upon stimulus size. Between 30' and 15' test light diameters, this sign of red-green opponency disappears. Psychophysical increment thresholds are shown to produce the notch while decrements do not and intravitreal APB is shown to reduce the notch, evidence that red-green opponency is carried by the "ON" and not the "OFF" bipolar pathways of the retina. Red and green annuli are shown to selectively reduce red and green inhibition, as though there were selective reduction of the surround response in center-surround organized red-green receptive fields.

The lateral effect of oscillation of peripheral luminance gratings: Test of various hypotheses

Foveal, psychophysical increment thresholds measured on a background of variable luminance are elevated by a peripheral grating of constant luminance oscillating with a frequency of 4 Hz. There is an optimal luminance of the background for which this reduction of threshold visibility is maximum. This finding leads to rejection of the facilitation, excitation, stray light, and “noise” hypotheses as possible explanations of the lateral effect of the oscillations. The dependency of threshold elevation on background luminance is accounted for by a formal model which allows the construction of response curves of a hypothetical threshold detecting mechanism. According to this model, the effect of a jerking, as opposed to a stationary peripheral pattern, is to decrease the sensitivity and to increase the operating luminance range of the underlying mechanism. This result is related to the analogous suppression of vision during saccadic eye movements for which a possible cortical transient-on-sustained neuron inhibition is suggested.

Colour coding of pattern responses in man investigated by evoked potential feedback and direct plot techniques

Visual sensitivity can be directly measured by arranging that evoked potential (EP) amplitude controls stimulus intensity (method of evoked potential feedback). A second technique is to directly plot graphs of EP amplitude vs the required stimulus parameter: this method gives more precise estimates of the shapes of EP plots than the conventional procedure of measuring individual points. The practical details, advantages and drawbacks of these two rapid techniques are discussed. Pattern EPs are mediated via colour channels, whereas EPs to rapidly flickering lights are mediated by pooled colour signals. The red pattern channel's peak sensitivity falls at a wavelength no shorter than 580–610 nm compared with the 555 nm peak for flicker sensitivity. An objective analogue of the psychophysical increment threshold procedure is made possible by means of EP feedback. Stimulus pattern intensity is approximately proportional to adapting intensity down to a pattern intensity of 5–10 td. below which EP amplitude attenuates rapidly.

The saturation of rod receptors.

If it is assumed that the excitable tissue immediately associated with a rod receptor may generate impulses when light quanta are absorbed, then it follows that there is an upper limit to the number of impulses generated by each rod of ∼1/τ, where τ is the refractory period of the excitable tissue. Two possible modes of excitation are investigated. In the first, analogous to a Geiger counter of dead time τ, it is assumed that quanta absorbed during the refractory period delay recovery for a further time τ and this leads to a situation in which an increase in stimulus reduces the output of the rod beyond saturation. In the second, where the dead time is not prolonged by absorption during the refractory period, the output approaches asymptotically to the limit of 1/ τ as the stimulus is increased. Psychophysical increment threshold curves cannot distinguish between the two models but it is shown that ERG results in monkey and man can be fitted using the invariant refractory period model if equilibrium bleac...