MACH BANDS! Effect of scenario and experience on interpretation of Mach Bands

What are Mach Bands?? Ramped changes in luminance give the illusion that there is a pair of thin dark and light bands. Lightness profile shows that there are no actual bands. Model ganglion cell output shows presence of bands, suggesting that they arise from this stage of processing.

Some Definitions… Luminance: the amount of light coming from a surface. Illumination: The amount of light falling on the viewed surface. Reflectance: the proportion of incident light that is reflected from the surface. Transmittance: factor affecting intensity of light that actually reaches the eyes. Luminance = Illumination x Reflectance

On convolution The brain has to average a bunch of inputs coming in from receptor mosaics. The process through which it does this is called convolution. The receptors in a given mosaic send their outputs into a second array of elements, which we will call convolution cells. Convolution cells are arranged in convolution arrays, and carry out a consolidation function with visual processing. This replaces the original gray level description with a smoothed gray level description of the image, said to be the convolved image.

The Land-horn Theory This is a computation worked out to explain how convolution could work in the brain using the idea of neurons only having an “on” or “off” level of activity. You start with a reflectance profile and superimpose it onto an illumination profile, represent outputs from receptors in terms of on-center units and off-center units. On-center units will output to “whiteness” arrays, while off- center units will output to “blackness” arrays. The two arrays are then deconvoluted to reconstitute the image and provide something that the brain can understand.

The Land-horn Theory

But why do we care? For the most part, we don’t really recognize or acknowledge Mach Bands in our day-to-day life. We usually just take them for granted or even ignore them. However, one field that this phenomenon is significant in is radiology, where Mach bands could cause errors in image interpretation. The creation of a Mach Band at the intersection of two images of differing radiopacities can be misinterpreted as pathosis in certain situations.

Effect of Scenario and Experience Christen Nielsen did a study about a decade ago that examined this. The study asked 33 fourth-year dental students and 40 dentists to interpret the same radiograph involving a maxillary central incisor under two different hypothetical scenarios.

Effect of Scenario and Experience The presence of this illusion in dental radiology is more prevalent than one might suspect because it often serves to enhance edges of anatomical features that might otherwise be less pronounced. Some people see this illusion more vividly than others. This is somewhat related to the background density effect as well, where the perceived brightness of an object being observed is related to the background.

Effect of Scenario and Experience Instead of just an interface being accentuated, an entire area is emphasized by its background.

Effect of Scenario and Experience Soft tissue borders of the tongue (A) and the nose (B) against the hard tissue of the jaws are enhanced by the presence of mach bands in both cases.

Effect of Scenario and Experience

The possible misdiagnosis of mach bands as fracture lines might be seen wherever abrupt changes in optical densities occur. A student in an Advanced Education in General Dentistry program, realizing the tendency of cast posts to cause vertical root fracture, incorrectly diagnosed what turned out to be simply a mach band in the image as a vertical root fracture. According to Nielsen, the most “disastrous consequence” of misinterpreting mach band illusions comes when a dental student or recent graduate is presented with an emergency situation where a patient has recently received facial trauma.

Effect of Scenario and Experience What is normally recognized as the overlapping of crestal bone and root is now seen as a radiolucent line (mach band) representing a horizontal root fracture. The student/doctor assumes the crown segment is nonsalvagable and broken from the root and proceeds to extract it, only to find out that the root is still attached and healthy. The study compared fourth year dental students with experienced dentists to see if either group interpreted more frequently the junction of crestal bone and root structure as a horizontal root fracture.

Some more x-ray images…

The test image used! Overlapping of margin of crestal bone and root creates an illusory radiolucent line that might be interpreted as a horizontal root fracture of the maxillary left central incisor.

Methods and Results Two scenarios were presented to the dentists and fourth- year dental students: Scenario 1: A 20-year old male presents with no chief complaint, other than desiring all six anterior maxillary teeth bleached. Scenario 2: An 18-year old male received an elbow to the mouth in a basketball game. No complaint of pain. After reading the scenarios, the participants were asked what the most likely explanation of what they were looking at was. Of the 17 students given the bleaching scenario, three interpreted the line as a root fracture, whereas 10 of the 16 students given the trauma scenario saw the line as a root fracture.

Methods and Results Given the bleaching scenario, only 2 of 21 dentists saw the line as a fracture. And given the trauma scenario only 4 of 19 dentists interpreted the line as a fracture line. There was a significant difference between the diagnoses depending on scenario given; both dentists and students were more likely to give a diagnosis of fracture when presented with a trauma scenario. In either scenario, dentists were less likely to diagnose fracture.

Discussion The results of the study show that students are more likely than experienced dentists to interpret the cervical crest of bone as a root fracture. The study shows that both students and dentists are influenced in their diagnosis by the scenario that the patient presents. However, the author believes that experience dentists who did make a tentative diagnosis of root fracture, after reviewing the entire picture (patient history, signs and symptoms, additional radiographs, etc.) would more than likely make the correct diagnosis and not extract the crown segment.

My own thoughts…in conclusion The scenario presented was interesting and enlightening, but the experiment itself was constructed poor. Sample size too small. More similar to an epidemiological study than anything else. But if you were to expand upon this you could perhaps explore whether misdiagnosis can “occur” at a higher level of visual processing or at a lower level.