Each receptor is sensitive to a form of physical energy, e.g. mechanical, chemical, electromagnetic energy. Receptors convert energy into electrical activity. Receptors transfer their stimulation to the sensory areas of the brain through several neuronal connections.
Neurons monitor the world by parceling it into small regions of space, known as receptive fields.
RFs are defined in terms of single neurons. RF location is related to stimulus modality. Due to the inhomogenous receptor density in the retina, only a small spot around fixation is seen sharply. Peripheral blur is counterbalanced here by enlarged letters. Consult the book.