Often times traditional vision charts only measure general parts of normal vision, such as acuity and contrast. However, many times everyday life scenarios place these two aspects in unique situations. For example, acuity is very important in vision. Testing acuity normally involves a standard eye test. However, when reading the newspaper, the eyes are challenged by small print, continuous text, and several close contact lines. This basic living situation may cause problems for patients with scotomata and may not be detected on conventional acuity charts. Continuous text charts give one more possibility during vision testing.
D’ is the standard viewing distance (usually 6 metres) and D is the distance at which each letter of this line subtends 5 minutes of arc (each stroke of the letter subtending 1 minute
The image above is an example of contrast magnitude and contrast polarity. The 14 lines of this sonnet are rendered with 7 contrast levels for each of the two contrast polarities. The numbers on the left refer to the Michelson contrast between the letters and the background. Michelson contrast ranges from 0 to 1.0 and is often expressed as a percentage: 0-100%Peak-to-Peak Constrast (Michelson Contrast, Modulation) measures the relation between the spread and the sum of the two luminances. This definition is typically used in signal processing theory, to determine the quality of a signal relative to its noise level. In the context of vision, such noise could be caused by scattered light introduced into the view path by a translucent element partly obscuring the scene behind it.Two advantages: Contrast values range from 0-1.0 for both polarities of textWidely used for sine-wave grating stimuli, simplifying comparisons to the grating literature.
Several studies have examined how both reading speed and the size of the visual span depend on stimulus attributes of text. High correlations have been found between reading speed and the size of the visual span for variations in character size and contrast (Legge et al., 2007), character spacing (Yu et al., 2007), text orientation, i.e., vertical text compared with conventional horizontal text (Yu et al., 2010), and the retinal eccentricity of text presentation (Legge et al., 2001). These high correlations mean that when the visual span gets small, reading speed slows down.Summary: The “visual span” is defined as the number of letters, in a line of text, that one can identify without moving one’s eyes. While reading, people advance their eyes by a distance roughly equal to the visual span, five times per secondTrigrams (random strings of 3 letters) were flashed brieflyat varying letter positions (-5 to +5) left and right of thefixation point (the trigram position is defined by the positionof the central letter of the trigram). Two print sizes (0.25°,1.0°) and two exposure times (100ms, 200ms) were used.
Reification is the constructive or generative aspect of perception, by which the experienced percept contains more explicit spatial information than the sensory stimulus on which it is based.For instance, a triangle will be perceived in picture A, although no triangle has actually been drawn. In pictures B and D the eye will recognize disparate shapes as "belonging" to a single shape, in C a complete three-dimensional shape is seen, where in actuality no such thing is drawn.
Multistability (or multistable perception) is the tendency of ambiguous perceptual experiences to pop back and forth unstably between two or more alternative interpretations. This is seen for example in the Necker cube, and in Rubin's Figure/Vase illusion shown here.
Invariance is the property of perception whereby simple geometrical objects are recognized independent of rotation, translation, and scale; as well as several other variations such as elastic deformations, different lighting, and different component features. For example, the objects in A in the figure are all immediately recognized as the same basic shape, which are immediately distinguishable from the forms in B. They are even recognized despite perspective and elastic deformations as in C, and when depicted using different graphic elements as in D. Computational theories of vision, such as those by David Marr, have had more success in explaining how objects are classified.
G-ScalE Psychophysics for Interface Avoiding Scalure
Issues in Scaling Research• The use of objects and icons are memory-based.• We can test and use principles of photo-object identification, such as shading, edge detection, etc.• However, we may be better off using trigrams text-type features, as these are more complex and recruit the same features as icon and image processing without the reliance on prior knowledge.
Visual Mechanisms• Contrast Coding – Contrast refers to luminance differences between the target and the background rather than overall luminance levels. – Contrast Signals and are extracted from retinal signals for visual encoding.• Size Coding • Size refers to a target’s angular size, proportional to retinal image size. • Target size is an important variable
Reading Acuity Charts • Reading Acuity: • The smallest print that a person can read without making significant errors.• Critical Print Size: • The smallest print that an individual can read with maximum speed.
Angular character size:– The corresponding visual angle subtended at the observer’s eye, and depends upon the physical size and the viewing distance. • Angular size in degrees = 57.3 x angular size in radians – (57.3 x physical size/viewing distance) – Measured in angular degrees or minutes of arc
40 cm 12/60° E 12 min arcCritical Print Size (CPS)Range = 0.15° to 0.3°Across studies, consensus for normally sighted readers = 0.2° (12 min-arc)Huey (1908/1968 X-height 1.5 mm, character size 2°, reading distance of 40cm
I min arc 12/60° E 12 min arc 40 cmCritical Print Size (CPS)• This angle is also used to specify visual acuity. It is called the minimum angle of resolution (MAR) and can also be given in log10 form, abbreviated as logMAR.• Snellan letters are constructed so that the size of the critical detail (stroke width and gap width) subtends 1/5th of the overall height.• Visual acuity (VA) in Snellen is given by the relation: VA = D’D
VA = D’/DVisual AcuityTarget resolution thresholds are usually expressed as the smallest angular sizeat which subjects can discriminate the separation between critical elements ofa stimulus pattern such as a pair of dots, a grating or a checkerboard
Visual SpanA key consideration for thearrangement of text may be theinclusion of visual span into designconsiderations. The Visual Span wasconstructed using trigrams, a non-textobject for letter recognition. It isconstructed of random strings of 3letters. 3 letters are used because thisrepresents a key feature of text:letters are flanked on both sides.human readers have a visual span of 7-11 letters. Fine and Rubin (1999)for high-contrast 1° letters the visualspan is 10.6 letters. Legge, Ahn, Klitz,and Luebker (1997a)
Understand Visual Processing The visual system is a complex network of modules and pathways, allspecializing in different tasks to contribute to our eventual impression of the world
Take home• Reification, multistability, and invariance are not necessarily separable modules to be modeled individually, but they could be different aspects of a single unified dynamic mechanism.• We are pattern recognition machines.
Memory in Perception• Experience in the world leads to memory.• As we interact with objects, we identify causation, concept, and variation.• This is useful for prediction and planning.• Common design features utilize this, often described as skeuomorphism:
Prior Knowledge• When we describe memory in image identification, we are often referring to the process called working memory.• Working memory success is based upon recency, frequency, and congruence.