Which brain mechanism cannot count beyond four?

Cowan makes an intriguing case for a fundamental limit in the number of chu nks that can be stored in short term memory (STM). Chunks are collections o f concepts that have strong associations to one another and much weaker ass ociations 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 wi ll first address the neuronal mechanisms that may demarcate visual chunks. Then we critically evaluate to what extent these, mechanisms might be respo nsible for the limit on the number of chunks that can be held in STM. We co nclude that the clarity with which the psychophysical data point to the num ber four is not matched by a similarly clear limit imposed by physiological mechanisms.