Cowan makes an intriguing case for a fundamental limit in the number of chunks that can be stored in short term memory (STM). Chunks are collections of concepts that have strong associations to one another and much weaker associations to other chunks. A translation of this definition for the visual domain would be that a visual chunk is a collection of features that belong to the same perceptual group (see also Mahoney & Ullman 1988). Here, we will first address the neuronal mechanisms that may demarcate visual chunks. Then we critically evaluate to what extent these mechanisms might be responsible for the limit on the number of chunks that can be held in STM. We conclude that the clarity with which the psychophysical data point to the number four is not matched by a similarly clear limit imposed by physiological mechanisms.