Although the innervation of the cochlea is very complex, the interconnections in the central auditory brain pathways are even more intricate, and are responsible for the coding of sound. Normally frequencies are coded by temporal and place codes. With the temporal code (top) the cells fire in time or phase with the sine wave, and with the place code (centre) it is the place of stimulation that is important. The inner ear filters sounds so that high and low frequencies excite different ends of the cochlea. There is an orderly arrangement of ascending pathways to cells throughout the brain centres, and this preserves a frequency scale (centre). Studies by Clark demonstrated that with electrical stimulation the temporal code could not be used to convey frequency above approximately 500 pulses/s. Therefore, a place code would be required for the perception of speech frequencies and thus multiple–channel stimulation (bottom).