Mitsuo Ikeda

Influence of foveal load on the functional visual field

The functional visual field defined in terms of a discrimination task of a target presented peripherally among ambiguous background patterns was investigated for various foveal loads which were to be recognized at the central retina. Foveal loads were numbers, letters, place names, traffic signs, and other figures to simulate commonplace situations for foveal information processing, and grouped into three in order of recognition difficulty based on daily experience. Boundaries of the functional visual field were obtained for simple fixation and for certain foveal loads. Comparison of these boundaries clearly showed shrinkage of the functional visual field size with the foveal loads of greater recognition difficulty.

Temporal Summation of Positive and Negative Flashes in the Visual System

Temporal summation characteristics of the human eye have been studied by various authors by measuring the increment threshold when two test stimuli are presented successively. Among other things, some have observed an inhibition between the effects of two flashes for a certain inter-flash interval and others have not. Here a similar experiment is carried out with a red test stimulus superposed upon a red adapting field. Inhibition is observed at an interval of 52 or 70 msec, depending on the adapting level. Such an inhibition is also found when two stimuli are both negative. The introduction of a negative test stimulus into the double-flash, increment-threshold technique is a new aspect of the present work. Some new phenomena are observed, particularly, that a positive and a negative flash summate with each other at the interval where double positive or double negative flashes yield inhibition. The luminance ratio of the two stimuli (positive or negative) was freely adjusted and new information concerning the linearity of the summation was obtained. Based on these findings, hypothetical response-potential functions have been derived, which are assumed to be responses in the visual system at some peripheral level.

Temporal properties of information extraction in reading studied by a text-mask replacement technique

A text was replaced with a visual mask for a fixed duration every time the reader made a saccade. The threshold duration was measured when the mask was presented at the beginning of a saccade or a fixation, or at a certain delay after the onset of a fixation. The effect of the mask on reading time, as well as on the subjective legibility of the text, was also investigated. It was shown that saccadic suppression exists in reading; subjects are not affected by a mask as long as it is inserted during the saccade. The visual sensitivity is recovered only partially at the initial part of fixation and is recovered fully approximately 70 msec after the beginning of the fixation. The visual information can be extracted during a later part of the fixation period as efficiently as or even more efficiently than during the early part of the fixation.

Useful visual field size for pattern perception.

The useful visual field size at each fixation in a pattern was investigated by artificially supplying various visual field sizes on a TV display. The degree of pattern perception was measured in terms of recognition memory for pictures, and the speed of processing pictures was determined as a function of field size. A serious deterioration in the perception of pictures occurred as the visual field was limited to a small area around the fovea (about 3.3° × 3.3°), processing speed becoming extremely slow. Speed increased gradually as visual field size became larger, to reach a certain level beyond which no further increase was observed. The visual field size at this asymptotic speed was called the useful visual field and was found to be about 50% of the entire pattern size. Analysis of eye-movement records demonstrated that in terms of the useful visual field, the scanning characteristics of the eye over the pattern occurred in a heavily overlapping manner to assure good perception of the pattern.

Span of recognition in reading

A question may arise when reading as to the width of the span of recognition at fixation points. A new technique was introduced to determine the size directly in a real reading situation. The visual field size was artificially narrowed, other visual functions being kept uncontrolled, and the deterioration of the reading task was investigated. An increase of the visual field size resulted in an increase of the reading rate. But no increase of the reading rate was observed beyond a certain size of the visual field, which we call the critical visual field size,Sc The value ofsc varied among subjects, ranging from 10 to 17 letters, but it was always much larger than the mean size of saccades. By regarding the critical visual field size as the span of recognition we may conclude that the reading mechanism, with its span of recognition, scans a text in a heavily overlapping manner suggesting the pre-processing of letters at the outer edge of the critical span of recognition.

Useful resolution for picture perception as a function of eccentricity

Observers inspected for different lengths of time pictures which contained high-resolution information within an eye-contingent viewing window and low-resolution information outside the area of that window. A recognition test followed in which the pictures inspected were presented together with other, distractor, pictures. The time required to reach 75% picture recognition (the criterion study time) was determined as a function of window size and degree of completeness of video sampling of information outside the window. For each level of information sampling density, criterion study time decreased as window size increased up to a critical size, and then remained approximately constant beyond this size. From these critical-window sizes, a function which describes the resolution actually used by the visual system at each eccentricity (the useful-resolution function) was obtained and this decreased monotonically with eccentricity. The useful resolution at each eccentricity was coarser than the resolution available at that eccentricity as determined by visual acuity, suggesting that useful resolution is not limited by visual acuity. The relationship between useful resolution and saccade length was also analyzed.

Mesopic luminous-efficiency functions

In order to establish the luminous-efficiency function for mesopic vision of the standard observer, luminous efficiency functions were measured with the direct heterochromatic brightness-matching method at retinal illuminance in the range of -2-2 log photopic trolands in order to cover the scotopic, mesopic, and photopic conditions. A steady visual field of 10 degrees arc was presented foveally. The functions underwent the usual complicated change from a rod type of luminous-efficiency function close to V(lambda) to a cone function that had a wide and almost double-peaked shape. A simple formula to represent the mesopic luminous-efficiency functions was derived in which log sensitivities of scotopic and photopic vision were linearly added after being multiplied by coefficients that were dependent on the luminance level. Saturation functions were also obtained at the various luminance levels for which luminous efficiency was investigated. In spite of a great variation of luminance level, the saturation function remained more or less the same, which indicates that the chromatic channels retain their contributions to the brightness sensation at low retinal illumination.

Temporal impulse response.

A visa stamped in my old passport (Fig. 1) reminds me that, on the night of 21 August 1961, I was on a ship that had sailed from Harwich, England. Inside my bag were some experimental data on temporal summation obtained in collaboration with Professor R. M. Boynton at Dr W. S. Stiles’ laboratory at the National Physical Laboratory (NPL). The purpose of my trip was to attempt to solve one of two serious problems that bothered me at that time. Early the next morning, I landed at the Hook of Holland and, after some difficulty locating the place, went immediately to the Institute for Perception in Soesterberg, hoping to find Dr van den Brink.

STUDY OF INTERRELATIONS BETWEEN MECHANISMS AT THRESHOLD.

A new method for the study of color vision is introduced where two test flashes are used in an increment-threshold experiment. Summation index is defined as the logarithm of the factor by which threshold is reduced when the two light flashes are presented together in the ratio of their individual threshold radiances. When the wavelength of one test flash is fixed at 630 mμ and the wavelength of the other flash is varied, a summation-index curve can be drawn as a function of the wavelength of the second flash. This curve is assumed to reveal the interrelations between the underlying mechanisms for color discrimination which are stimulated by the first test flash and the second test flash, respectively. Two different durations of the test flash are employed and two different summation-index curves are obtained. It is found that under certain conditions the summation index becomes smaller than that predicted by probability summation, which suggests an inhibition between two mechanisms. With the aid of the experimental spectral sensitivity curves obtained under the same adapting conditions as above, the summation-index curves are interpreted with the help of conceptions of opponent-colors theory.

Temporal deterioration of wavelength discrimination with successive comparison method

Abstract The wavelength discrimination threshold (Δλ) was measured with a successive comparison method, in which two stimuli of different wavelengths were presented in the left and right half of 2.2° or 2.7° bipartite field, respectively, with a certain stimulus onset asynchrony (SOA). The stimulus duration was kept constant at 110 msec. The Δλ value for five different wavelengths, 430, 470, 520, 570 and 610 nm, was found to be constant up to SOA= 60 msec and increased gradually till SOA= 190 msec. The wavelength discrimination functions with SOA of 0 and 550 msec were also obtained for 430 through 650 nm in 10 nm steps. At all wavelengths the values with SOA of 550 msec were about twice as large as those with SOA of 0 msec. The present results indicate that the wavelength discrimination deterioration is complete within a relatively short period of time.