The Psychophysiology of Sex

Discussion Paper

Discussion Paper roy j. levin A problem I have with the study that Cindy has done, and she’s known this for a long time, is that she relies on vaginal pulse amplitude (VPA) as an interpretive method of saying what happens to the genital physiology. The trouble with VPA is that we really don’t know when it increases or exactly where the increase in terms of the peripheral circulation takes place (see also appendix). VPA is the measure of the total peripheral microcirculation ; the signal is obtained from a light shining in and re®ected back from a set of blood vessels; the more blood in the vessels or the more vessels open and ¤lled with blood the bigger the amplitude of the signal. These vessels are arteries, arterioles, met-arterioles, capillaries, venules, and veins. Many are innervated so when you get to the SNS, you don’t really know where the focus of the innervation is that’s creating the changes. On the one hand, it could be a straightforward increase in the ®ow, but on the other hand, it could be causing contractions of the venules or the veins, which may itself create an increase in VPA. Now there is a sympathetic innervation of the veins and the venules, and certainly in the vagina we have no idea at all about the innervation of the venous circulation. I don’t know of any work that’s ever looked at this even in animals where you can restrict the veins and see the effect on VPA. The amazing thing is that there is so little work on VPA in animals. It’s absolutely staggering, especially as there are a large number of “uncontrolled” studies in humans. In fact, I used VPA when I was teaching our honors physiology degree students at Shef¤eld University. We used to do it in the anaesthetized dog. We inserted a photoplethysmograph in the dog’s vagina and injected intraarterially a variety of bioactive agents because we could do close arterial injections through the femoral artery so that the drug went ¤rst to the vagina. When either noradrenaline or adrenaline was injected, the amplitude of the AC signal or VPA (an index of the “blood ®ow”) always decreased (see experiment in Figure 1). There’s no question that it goes down. You can do a similar experiment in humans, which Gorm Wagner and I did in Copenhagen. If you inject noradrenaline or adrenaline subepithelial underneath a heated oxygen electrode (used to monitor vaginal blood ®ow), you ¤nd that the electrode gives a decrease in power consumption and shows a dramatic fall to zero of the surface p02; in other words, the blood ®ow is greatly decreased 129 (see experiment in Figure 2). So, in both dogs and in humans that I’ve had experience of (although a relatively limited number), in these conditions you get a constriction of the blood vessels in the vagina to the neurotransmitters of the sympathetic system. I thus ¤nd it very dif¤cult to say that the SNS is going to give you an increase of the blood ®ow from interpreting the changes in VPA. The amplitude of the VPA under these changed circulatory circumstances is not an artifact, but it is very sensitive to changes in the peripheral circulation. You only have to do a very simple experiment to see effects of changes in the peripheral circulation on the VPA; I’ve talked about this many times. Hold a photoplethysmograph in the palm of your closed hand and then just lift it high above your head; you will see when you do that an increase in the amplitude occurs. Then hold it below the heart and you get a decrease in the amplitude. All you’ve done by these maneuvers is elevate and decrease the hydrostatic pressure on the venous side of the arterioles because the arterial pressure doesn’t change when you lift the arm up. So you can get very big changes in the photoplethysmographic signal (and presumably the VPA) simply by altering the pressure on the venous side of the microcirculation. As a physiologist, that’s my real concern. I think the studies are ¤ne, but I would like to see another independent measure of changes in the hemodynamics as opposed to just using the VPA. So that’s my biggest criticism of the work. I noticed that even Cindy is tending to say, well, there might be other things going on, so...