although there is not enough data to reliably compare the strength of the two kinds of stimuli. One study suggests that there is a difference in mental processing level due to innate differences between visual and tactile stimuli representations. In this study, subjects were presented with an object in both visual and tactile form (a subject is shown a sphere but cannot touch it, and later is given a similar sphere to only hold and not view). Subjects had more trouble identifying size difference in visual fields than using tactile feedback. A suggestion for the lower level of size processing in visual fields is that it results from the high variance in viewed object size due to perspective and distance. Smell Odor memory is weaker than visual memory, achieving a successful identification rate of only 70-80% of visual memory. Levels-of-processing effects have been found within odor memory if subjects are asked to "visualize" smells and associate them with a particular picture. Subjects who perform this task have a different recall value on explicit memory tests than subjects who memorize smells using self-chosen methods. The difference in recall value, however, depends on the subject, and the subject's ability to form images from odors. Attributing verbal attributes to odors has similar effects. Semantic processing of odors (e.g. attributing the "mud" odor to "smell like a puddle") has found to have the most positive effects on recall. Neural evidence Several brain imaging studies using positron emission tomography and functional magnetic resonance imaging techniques have shown that higher levels of processing correlate with more brain activity and activity in different parts of the brain than lower levels. For example, in a lexical analysis task, subjects showed activity in the left inferior prefrontal cortex only when identifying whether the word represented a living or nonliving object, and not when identifying whether or not the word contained an "a". Similarly, an auditory analysis task showed increased activation in the left inferior prefrontal cortex when subjects performed increasingly semantic word manipulations. Synaptic aspects of word recognition have been correlated with the left frontal operculum and the cortex lining the junction of the inferior frontal and inferior precentral sulcus. The self-reference effect also has neural correlates with a region of the medial prefrontal cortex, which was activated in an experiment where subjects analyzed the relevance of data to themselves. Specificity of processing is explained on a neurological basis