The period in which rhythmic notation first appeared in western music, and concepts of formal structure, such as proportion, texture, and architectural effect were developed, was:

Medieval Period Music or High Medieval Music Period (1150-1300)

Explanation:

The Medieval period music is often referred to as the period in which the western music started to adapt rhythmic notation and formal structures. This is specifically found in the High Medieval Music Period starting from 1150 to 1300. Formal structure like proportion, texture, and architectural effect were developed. Music composers moved towards a more structured use of notes and developed music that was focused and based on rhythms.