Biomedical subjects
Publications and source records attributed to J A Foley-Fisher.
Eye movements have been recorded during a free visual search task both with and without an artificial foveal scotoma. Results of the eye movement measurements show that neither fixation durations nor mean saccade amplitudes are significantly modified by the loss of foveal vision.
One-dimensional visual lobes representing the detectability of a small bright target on an unstructured background were measured at several contrast levels, both with and without an artificial foveal scotoma. The effect of the scotoma on the lobe is shown to be restricted to the area of the retina covered by the scotoma, and detection ability is unaffected outside this area.
Experiments have been carried out to determine free visual search capabilities when an observer uses only peripheral vision. Foveal vision was eliminated by simulating a full foveal scotoma by means of a stabilized retinal image. Observers were asked to search an unstructured field for a single static target. Results from these experiments established that foveal vision plays a more significant role than is generally assumed in even the most basic of search tasks where the primary mechanism for target acquisition is assumed to be detection only. Results are discussed in terms of the visual lobes for the targets of search.
Two methods are described for simulating a retinal scotoma by means of a stabilized retinal image, hence allowing some aspects of the impairment of visual function associated with scotomas to be studied in observers with normal visual fields. In the first method, an optical lever arrangement is used to generate a small artificial scotoma at a controllable position with 15 degrees field of view. The second, direct attachment, method uses a tight-fitting scleral contact lens, to which is attached a very light, aluminium-alloy tube. This carries a small piece of lightweight black card, the stabilized image of this card generating the scotoma. Although precise positioning of the scotoma is more difficult with the second method, the induced scotoma is absolute and the method is relatively cheap and easy to implement.
A stabilized image, including a straight-line boundary between two areas of different colour was displayed. Movements normal to itself were imposed on this boundary while other boundaries remained stabilized. Broad band filters giving red, yellow, green and blue were used in various pairs. Step movements, pulse movements and oscillatory movements (including sinusoidal, triangular and square wave forms) were applied in turn. Threshold movements to give perception of hue were measured for steps and pulses and signal/photon-noise ratios were calculated for a red/blue boundary. The ratio was found to be constant at a value of about 20 when the retinal illuminance (ER) varied between 5 td and 60 td. When ER was increased (up to 2000 td) the ratio increased rapidly. None of the colour appearances produced by moving the boundary exactly matched the corresponding appearances in an unstabilized image, though the nearest matches were obtained with oscillatory movements of frequency 2 Hz and displacement greater than 5'.
There is strong experimental evidence that colour discrimination depends upon signals originating at colour boundaries. Controlled movements were imposed on a boundary between an illuminated coloured area and a dark area in a previously stabilized image. Red, yellow, green and blue fields were used. Step-movements of amplitude M min arc and pulse-movements of amplitude M min arc and pulse width tau s were studied. The movement M50 to produce 50% positive responses for perception of hue was measured as a function of retinal illuminance, boundary length and speed for step-movements and pulse-width (tau) for pulse movements. Signal/photon-noise ratios were calculated.
A stabilized image of a coloured square seen against a black background was produced. After the image had faded, oscillatory movements were applied in a direction parallel to a diagonal so that all boundaries had an equal component of motion normal to themselves. Movements of different frequency, amplitude and wave-form were studied. Most of the experiments were carried out at fairly high retinal illuminance (80-500 td). The objective was to find the amplitude and frequency required for perception of hue and saturation. It was found that these movements had (i) a minimum amplitude of about 5' (10' peak-to-peak), (ii) a minimum velocity of 15'(-1) and (iii) a frequency range from about 1 Hz to about 4 Hz. The movements required are larger than those required for perception of the shape of the target and also larger than the normal movements which remain when a subject fixates a well-defined mark. Also the movements associated with the perception of hue and saturation are restricted to a narrower frequency range. These features probably imply that it is necessary to excite numbers of cones covering a minimum area to give perception of hue and saturation.
Two experiments are described in which visual information is derived from a single moving boundary in the retinal image, all other boundaries being stabilized so that they contribute no visual information. In this situation the subject perceives the same colours on the two sides of the boundary as would be perceived in normal vision when, owing to eye-movements, all boundaries in the retinal image are moving relative to the retina. Thus complete information leading to perception of hue and saturation as well as brightness, and not merely differences of hue etc. on the two sides, can be derived from a single boundary.
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