Final answer:
Merkel discs, also known as Merkel cells, are responsible for sensing light touch, shapes, and textures, but they do not sense pain or temperature variations.
Step-by-step explanation:
The tactile modality that involves the activation of tactile (Merkel) discs is the sensation of light touch. Merkel cells are mechanoreceptors located in the upper layers of skin near the base of the epidermis and are responsible for sensing light touch, shapes, and textures. These discs are slowly-adapting, encapsulated nerve endings with small, well-defined receptive fields, which make them particularly sensitive to edges and fine details, such as those experienced when typing on a keyboard. They do not, however, sense pain or temperature variations; those sensations are perceived by free nerve endings. Merkel's disks are not involved in sensing pain or temperature, which are instead sensed by free nerve endings.