INTRODUCTIONVaginal photoplethysmography is the most commonly used

method for assessing vaginal blood flow. The signal is typically filtered to yield a DC component, referred to as vaginal blood volume (VBV), and an AC component, referred to as vaginal pulse amplitude (VPA). VPA has been shown to be a more sensitive measure of sexual response, so VBV is rarely used.

However, VPA is poorly understood theoretically and many questions remain about the clinical utility of the signal. Although VPA fairly reliably increases when a sexual stimulus is presented, our limited understanding of vaginal physiology and lack of a calibration method constrain its interpretation. VPA tends to correlate with subjective measures of sexual arousal, but it is not known why that relationship can vary as widely as has been reported.

To develop a better understanding of the vaginal photoplethysmograph, it is important to establish first, on a theoretical basis, the best method for processing the output of this instrument. The VPA is typically edited by hand for artifacts by a single, trained editor. To date, no research directly addresses the necessity and impact of signal hand-editing. A possible alternative strategy is to use spectral analyses (e.g., Wouda et al., 1998) to both investigate the exact impact of movement artifacts (e.g., in which frequency spectrum they typically fall) and to process VPA.

Four approaches to the processing of theVaginal Pulse Amplitude (VPA) signal

Prause, N. 1 and Janssen, E., 1,2

1Department of Psychology, Indiana University, Bloomington; 2 Kinsey Institute for Research in Sex, Gender, and Reproduction

ABSTRACT

Vaginal photoplethysmography is the most commonly used method to quantify female physiological sexual arousal. However, laboratories vary in the methods they use to process this signal. We examined four approaches to processing the AC component of the vaginal photoplethysmograph signal, or vaginal pulse amplitude (VPA): Peak amplitude of the raw signal, peak amplitude of a signal hand-edited by a trained rater, peak amplitude of a signal hand-edited by two trained raters and averaged, and the 1-2 Hz spectrum of an Fast Fourier Transform (FFT) of the raw signal. Results indicate that it is necessary to edit artifacts from the signal, but FFT provided no advantage over more traditional methods.

RESULTSCONCLUSIONS

• Using both peak-to-peak and FFT approaches, the conditions were better differentiated if the signal was first cleaned of artifacts by visual inspection. Whether using peak-to-peak or FFT analyses, the VPA signal should first be edited for artifacts.

• Spectral analysis may have filtered out the effects of some artifacts, but the artifacts within the frequency of interest were too large to use FFT in place of hand-editing signals. It is unlikely, although theoretically possible, that a slightly modified frequency spectrum would be more appropriate for this goal. FFT may be used in the future to better identify which artifacts ought to be edited from signals (by identifying the spectrum in which they tend to occur).

• Raters were able to be trained to a high level of agreement in visually editing the artifacts.

Within subjects correlations between self-reported arousal and physiological arousal did not change significantly depending on the analysis method used, and is quite low on average in this study.

WORKS CITEDWouda, J. C., Hartmen, P. M., Bakker, R. M., Bakker, J. O., van de Weil, H. B. M., Schultz, W., et al. (1998). Vaginal plethysmography in women with dyspareunia. Journal of Sex Research, 35(2), 141-147.

METHODSThirty-two women participated in a study examining the

effects of different subjective rating instructions on the concordance between vaginal blood flow (VPA) and subjective sexual arousal (using a continuous lever). Each woman fantasized 3 times for 3 minutes. Only data from the condition in which participants were instructed to use the lever to rate their “level of sexual arousal” were used for analyses.

VPA was derived by band-pass filtering the original signal .5 to 30 Hz sampled at 80 Hz. All signals were binned in 10 second intervals. The 3-minute baseline preceding the fantasy session and the 3-minute fantasy session were each processed using 4 different methods. Undergraduate laboratory assistants were trained up to 70% reliability (successfully maintained through editing), and they were blind to one another’s previous edits.

Processing approaches

Raw signal: The VPA signal was not altered and the peak-to-peak amplitude was used

Edited signal: The VPA signal was hand-edited by a trained editor, connecting the endpoints of any visually-detected movement artifacts

Average, edited signal: The VPA signal was hand-edited by two trained editors and the output was averaged across the two

Fast Fourier Transform (FFT): The raw VPA signal was Fast Fourier Transformed and the power value taken for wavelengths between 1 and 2 Hz (60-120 bpm).

Participants’ VPA was significantly greater on average in the fantasy condition as compared to the baseline condition for analyses using the raw signal (F(1,27)=7.37, p<.05), the signal after artifact editing by one person (F(1,28)=34.27, p<.05), the signal after editing by two assistants averaged (F(1,28)=36.34, p<.05), and the FFT of the signal with artifacts visually removed (F(1,28)=12.21, p<.05). The only approach that did not find a significant difference between the two conditions was the FFT of the raw data.

CorrelationsAverage

Standard deviation

Raw signal .109 .37

Edited signal .121 .45

Average, edited signal .129 .46

Fast Fourier Transform (FFT) .091 .36

Fast Fourier Transform (FFT)

- edited signal.132 .46

Example

4

6

8

10

12

14

16

18

Vag

inal

Pu

lse

Am

plit

ud

e (m

V)

Cleaned peak-to-peak

Baseline Fantasy

4

6

8

10

12

14

16

18

Vag

inal

Pu

lse

Am

plit

ud

e (m

V)

Cleaned, averaged p-p

Baseline Fantasy

0

1000000

2000000

3000000

4000000

5000000

Vag

inal

Pu

lse

Am

plit

ud

e (1

-2 H

z p

ow

er)

Fast Fourier Transform

Baseline Fantasy

0

100000

200000

300000

400000

500000

Vag

inal

Pu

lse

Am

plit

ud

e (1

-2 H

z p

ow

er)

Edited FFT

Baseline Fantasy

4.00

6.00

8.00

10.00

12.00

14.00

16.00

18.00

Vag

inal

Pu

lse

Am

plit

ud

e (m

V)

Raw peak-to-peak

Baseline Fantasy