Visual Stimuli Quiz

Understanding Sine Waves, Gabor Patches, and Contrast Sensitivity

• Sine waves can be broken down into linear sum of specified spatial frequencies, amplitudes, and phases, as demonstrated by Jean Fourier in 1822.
• Gabor patches, which are sine wave gratings seen through a Gaussian window, have characteristics matching response characteristics of cortical neurons called Receptive Fields, making them useful in research.
• The Spatial Contrast Sensitivity Function (CSF) shows human sensitivity to sine waves of different frequencies and contrasts, with medium spatial frequencies being visible at the lowest contrast.
• The CSF has a characteristic inverted U shape, with a peak around 4 c/deg, and spans about 7 octaves from 0.5 c/deg to 60 c/deg.
• The high frequency cut-off of the CSF is around 40-60 c/deg, equivalent to the resolution acuity limit of vision, with the peak sensitivity around 4 c/deg.
• The CSF shape is influenced by optical factors, such as the blurring of high spatial frequencies by the eye's optics, and neural factors, including the spacing of photoreceptors and neural interactions within receptive fields.
• The CSF is affected by luminance, with overall human sensitivity decreasing as light levels decrease, shifting the peak sensitivity and high frequency cut-off towards lower spatial frequencies.
• Luminance also affects the CSF shape, changing it from a band-pass to a low-pass shape at very low light levels.
• Adapting to high contrast sine-wave gratings can lead to changes in the CSF, with specific losses demonstrating the presence of different populations of neurons tuned to different bands of spatial frequencies.
• The eye's resolution limit and high spatial frequency cut-off are regulated by the sampling density of the retinal cone mosaic, with fewer neurons tuned to lower spatial frequencies and neural factors primarily contributing to the low frequency fall-off in sensitivity.
• The CSF can be altered psychophysically, as demonstrated by changes in the CSF shape after adaptation to high contrast sine-wave gratings.
• Understanding the CSF is crucial for investigating vision and developing visual stimuli, with its shape and characteristics providing insights into the visual system's sensitivity to different spatial frequencies and contrasts.

Visual Acuity and Optotypes: Key Concepts and History

• Visual acuity tests are based on recognition acuity, requiring correct identification of optotypes (letters, symbols, or numbers).
• Optotypes for letter charts are usually based on a grid pattern, with non-serif letters preferred.
• Visual acuity charts can use different letter sets, such as British standard letters (5x4) or Sloan letters (5x5), selected for similar legibility.
• The test distance and size of the letter are specified in a Snellen fraction, which can also be expressed in decimal notation.
• The minimum angle of resolution (MAR) specifies the smallest gap or stroke thickness that can be resolved.
• Logarithmic units, specifically logMAR, are used to express visual acuity measurements, especially when using charts based on logarithmic progression.
• Snellen's chart, proposed in 1862, specified letter size based on stroke width and used serifs on letters.
• Green's chart, proposed in 1867/8, used a geometric (logarithmic) progression and non-serif letters with proportional letter and line spacing.
• Bailey-Lovie logMAR, proposed in 1976, introduced a logarithmic progression of letter size and letter spacing proportional between and within rows.
• ETDRS charts, introduced in 1982, combined the Sloan letter set with Bailey-Lovie layout and became popular for clinical studies.
• Various types of ETDRS and variations have been developed, including language-specific charts like the Hindi logMAR visual acuity chart.
• A conversion table provides Snellen fraction, logMAR, MAR, and decimal notation for different test distances.