A study that appeared in Science magazine on Sept 6, 2013 entitled Topographic Representation of Numerosity in the Human Parietal Cortex discusses the possibility that the ability to understand the quantity of a small group of visual objects without counting is an additional sense. The article was written by B. M. Harvey, B. P. Klein, N. Petridou, S. O. Dumoulin and its abstract reads:
Numerosity, the set size of a group of items, is processed by the association cortex, but certain aspects mirror the properties of primary senses. Sensory cortices contain topographic maps reflecting the structure of sensory organs. Are the cortical representation and processing of numerosity organized topographically, even though no sensory organ has a numerical structure? Using high-field functional magnetic resonance imaging (at a field strength of 7 teslas), we described neural populations tuned to small numerosities in the human parietal cortex. They are organized topographically, forming a numerosity map that is robust to changes in low-level stimulus features. The cortical surface area devoted to specific numerosities decreases with increasing numerosity, and the tuning width increases with preferred numerosity. These organizational properties extend topographic principles to the representation of higher-order abstract features in the association cortex.
To better understand the basis for this research, consider the following example. Consider the photo to the right. At first glance, can you immediately tell me how many highlighters I have (without first counting)? There are five highlighters. This is usually the number at which most humans’ numerosity begins to decrease. That is, when trying to estimate the number of objects in a group of six or greater, their accuracy begins to decrease.
Doucleff  brings to our attention, the common pop culture reference to numerosity, Rain Man.
Lewis  mentions a previous study conducted on monkeys that found similar activity in the brains of monkeys with regard to numerosity; however, these studies had no findings of a map.
Notice that this study is showing a link to numerosity and operation of the five senses. It seems to me that the senses (sight, hearing, taste, smell, and touch) are innate. These are all perceptions that we have at the time of birth. This leads to my question of, do we actually understand numbers at the time of birth? I would think we do not; however, Castro  has some evidence in a related discussion that seems to the contrary. A quote from his article by Kristy vanMarle, assistant professor of psychological sciences at University of Missouri reads,
We believe that infants are born with expectations about the objects around them, even though that knowledge is a skill that’s never been taught…As the child develops, this knowledge is refined and eventually leads to the abilities we use as adults.
These researchers are reporting that the infants can begin to understand intuitive physics as early as two months of age. This them makes me wonder if it is actually a sense since it takes at least two months to develop. Shiobara  brings up the question of what is a sense.
What do you think? Could numerosity be the sixth sense? What exactly is a sense?
 Doucleff, Michaeleen. Scientists Put A ‘Sixth Sense’ For Numbers On Brain Map. NPR Health News: Sept 9, 2013. (http://www.npr.org/blogs/health/2013/09/06/219246974/scientists-put-a-sixth-sense-for-numbers-on-brain-map).