The standard rheobase definition assumes that a given cell does not spike when a current is not injected. However, some cells are spontaneously spiking (e.g. Cardiac pacemaker cells). For such cells, a "negative" (inhibitory) current will quiet them, while a slightly less negative current will result in action potentials. In such cases, stimulation protocols that utilize the rheobase and assume that spiking rates are proportional to the rheobase will produce nonsense results (e.g. spike rate in response to 2X rheobase will not be greater than at 1.5X rheobase).