verbal

Barbara Clark described the situation with regard to the brain quite well.

The human brain is organized into four major systems with radically different structures and chemistry. Educationally, this organization presents some important considerations. Two of the four brain systems have no network for verbal communication. Because the integration of total brain function is the basis for intelligence, a test that measures primarily verbal communication as its sampling of intelligence may be seen as limited.

In other words, the standardized testing used in American schools, being so heavily reliant on verbal communication is a poor means of measuring overall intelligence.