Mach bands

The Mach bands illusion. The image consists of two regions with uniform lightness separated by a narrower strip with a uniform lightness gradient. Notice the dark band that appears immediately to the right and the light band that appears immediately to the left of the middle strip.

Mach bands are independent of orientation.

Mach bands is an optical illusion named after the physicist Ernst Mach. The illusion consists of light or dark stripes that are perceived next to the boundary between two regions of an image that have different lightness gradients (even if the lightness itself is the same on both sides of the boundary).

Explanation

The Mach bands effect is due to the spatial high-boost filtering performed by the human visual system on the luminance channel of the image captured by the retina. This filtering is largely performed in the retina itself, by lateral inhibition among its neurons.

The effect is independent of the orientation of the boundary.

Mach bands in radiology

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An extreme example of Mach bands in dental radiography due to the high density of the metal orthodontic wires as projected against the teeth.

This visual phenomenon is important to keep in mind when evaluating dental radiographs for evidence of decay, in which grayscale images of teeth and bone are analyzed for abnormal variances of intensity. A false-positive radiological diagnosis of dental caries can easily arise if the practitioner doesn't take into account the likelihood of this illusion. Not only do Mach bands manifest adjacent to metal restorations or appliances, as in the radiograph at right, but they can also present at the boundary between enamel and dentin.^[1] Mach bands may also result in the misdiagnosis of horizontal root fractures because of the differing radiographic intensities of tooth and bone.^[2]. Mach effect can also lead to an erroneous diagnosis of pneumothorax by creating a dark line at the lung periphery (whereas a true pneumothorax will have a white pleural line). ^[3]

See also

• Acutance
• Cornsweet illusion
• Hermann grid illusion
• Lateral inhibition

References

Further reading

• Lotto RB, Williams SM, Purves D (1999). "Mach bands as empirically derived associations". Proceedings of the National Academy of Sciences 96 (9): 5245–50. Bibcode 1999PNAS...96.5245L. doi:10.1073/pnas.96.9.5245. PMC 21849. PMID 10220451. http://www.pubmedcentral.nih.gov/articlerender.fcgi?tool=pmcentrez&artid=21849. 
• Eagleman, DM (2001) "Visual Illusions and Neurobiology." Nature Reviews Neuroscience. 2(12): 920-6.

External links

• Demonstration of the Mach band
• Laboratory of Dale Purves, MD