Age differences in the effect of epidermal hydration on electrodermal activity

The effect of epidermal hydration on skin potential and conductance measurements was investigated in young and old men. The condition of least hydration used a 0.5% KCl glycol electrolyte. Two conditions used a 0.5% aqueous KCl electrolyte differing in that the most hydrated site received a 15-min pretreatment of soaking in distilled water whereas the intermediate hydration site received no pretreatment. These hydration conditions were used in recording three channels of skin potential and three channels of skin conductance during three tasks: 1) tone presentations after rest, 2) simple reaction time, and 3) choice reaction time. There were no significant age differences in the effect of electrolyte on skin conductance level and response. There were age differences in the effect of electrolyte on skin potential level (SPL) and response (SPR). Young adult SPR was monotonically related to hydration with the largest response magnitude occurring with the least hydration. Electrolyte did not significantly affect SPR magnitude of the aged. For the young subjects, SPL was monotonically related to hydration with the most negative SPL occurring with the least hydration. For the aged subjects, the least negative SPL occurred in the condition of least hydration. We postulate that this reversal in the hydration/SPL relationship in old age reflects a reversal in the relative magnitudes of sweat gland and epidermal potentials: in old subjects the epidermal potential is greater than the sweat gland potential.