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Easton Myers
Easton Myers


The term "fetal heartbeat," as used in the anti-abortion law in Texas, is misleading and not based on science, say physicians who specialize in reproductive health. What the ultrasound machine detects in an embryo at six weeks of pregnancy is actually just electrical activity from cells that aren't yet a heart. And the sound that you "hear" is actually manufactured by the ultrasound machine. Scott Olson/Getty Images hide caption


The Texas abortion law that went into effect last fall reads: "A physician may not knowingly perform or induce an abortion on a pregnant woman if the physician detected a fetal heartbeat for the unborn child."

The law defines "fetal heartbeat" as "cardiac activity or the steady and repetitive rhythmic contraction of the fetal heart within the gestational sac" and claims that a pregnant woman could use that signal to determine "the likelihood of her unborn child surviving to full-term birth."

"When I use a stethoscope to listen to an [adult] patient's heart, the sound that I'm hearing is caused by the opening and closing of the cardiac valves," says Dr. Nisha Verma, an OB-GYN who specializes in abortion care and works at the American College of Obstetricians and Gynecologists.

Later in a pregnancy is when a clinician might use the term "fetal heartbeat," after the sound of the heart valves can be heard, she says. That sound "usually can't be heard with our Doppler machines until about 10 weeks."

The term "fetal heartbeat" has been used in laws restricting access to abortion for years. According to the Guttmacher Institute, which tracks reproductive health policy, the first such law was passed in North Dakota in 2013, but it was struck down in the courts. Since then, over a dozen states have passed similar laws, but Texas' is the first to go into effect.

The text of the Texas law claims that "fetal heartbeat has become a key medical predictor that an unborn child will reach live birth" and continues, "the pregnant woman has a compelling interest in knowing the likelihood of her unborn child surviving to full-term birth based on the presence of cardiac activity."

Functional near-infrared spectroscopy (fNIRS) is a non-invasive brain imaging technique that measures changes in oxygenated and de-oxygenated hemoglobin concentration and can provide a measure of brain activity. In addition to neural activity, fNIRS signals contain components that can be used to extract physiological information such as cardiac measures. Previous studies have shown changes in cardiac activity in response to different sounds. This study investigated whether cardiac responses collected using fNIRS differ for different loudness of sounds. fNIRS data were collected from 28 normal hearing participants. Cardiac response measures evoked by broadband, amplitude-modulated sounds were extracted for four sound intensities ranging from near-threshold to comfortably loud levels (15, 40, 65 and 90 dB Sound Pressure Level (SPL)). Following onset of the noise stimulus, heart rate initially decreased for sounds of 15 and 40 dB SPL, reaching a significantly lower rate at 15 dB SPL. For sounds at 65 and 90 dB SPL, increases in heart rate were seen. To quantify the timing of significant changes, inter-beat intervals were assessed. For sounds at 40 dB SPL, an immediate significant change in the first two inter-beat intervals following sound onset was found. At other levels, the most significant change appeared later (beats 3 to 5 following sound onset). In conclusion, changes in heart rate were associated with the level of sound with a clear difference in response to near-threshold sounds compared to comfortably loud sounds. These findings may be used alone or in conjunction with other measures such as fNIRS brain activity for evaluation of hearing ability.

For the remaining channels, the original (unfiltered) data was converted to optical density and motion artefacts corrected using Homer2 function hmrMotionCorrectWavelet (function parameter set to 1.5) which uses wavelet coefficient distributions to remove artefacts [18, 20]. Signals were then band-pass filtered between 0.5 and 1.5 Hz (corresponding to heart rates between 30 and 90 beats per minute). For each channel, the heart rate was then extracted from the 760 nm wavelength optical density signals based on the method described by Purdue et al. [1]. Briefly, heart rate and inter-beat intervals were calculated for each channel, epoched around time of stimulus and averaged across channels and the 10 repeated trials. Details are described below.

Percentage change in heart rate and inter-beat intervals compared to pre-stimulus baseline at different sound intensity levels were compared using mixed linear models with subject treated as a random factor and time (pre and post stimulus) and intensity levels treated as fixed factors. Pre-stimulus measures were calculated as mean heart rate across 5 seconds before stimulus onset and 5 averaged pre-stimulus inter-beat intervals. Post-stimulus heart rate was calculated as the mean heart rate change from 0 to 8 seconds after onset. Post-stimulus inter-beat intervals were averaged across the first 2 inter-beat intervals and across intervals 3 to 5 (see Results for further details). Tukey post-hoc comparisons were used to determine changes in cardiac measures compared to baseline and differences between measures at different stimulus levels. Assumption of data normality was tested using residual normal probability plots. Statistical analyses were performed using Matlab 2016b (Mathworks, USA). A value of p

Fig 2A shows the percentage change in heart rate following stimulus onset, averaged across participants. At 15 and 40 dB SPL, an initial drop and subsequent rise in heart rate was seen following stimulus onset. At 15 dB SPL (near the perceptual threshold for this sound for most participants) there was an average drop in heart rate of 2.8% at 3.6 s and at 40 dB SPL, an average drop of 1.1% at1.6 seconds was seen. At the higher intensity levels of 65 and 90 dB, average heart rate increased by approximately 4% and 5% respectively following stimulus onset.

To quantify this change in heart rate with sound onset, the mean heart rate change was calculated over the interval from 0 to 8 seconds post-stimulus onset (Fig 2B). This period was chosen to cover the first peak in heart rate change seen after sound onset in the grand averaged data. Heart rate changes were modelled using a linear mixed model. The change from baseline varied significantly between different stimulus levels (level x pre-post interaction) (F(3,182) = 59.77, p

To quantify the time at which changes in heart rate first occurred, inter-beat intervals were assessed. Fig 3 shows inter-beat intervals for one participant during a seven minute recording. Shorter inter-beat intervals (corresponding to increased heart rate) following the 65 and 90dB SPL levels are clearly seen.

To quantify the immediate change in inter-beat intervals following sound onset, the percentage change in post-stimulus inter-beat intervals relative to baseline (defined as the averaged five inter-beat intervals before stimulus onset), was calculated. Values averaged across all participants are shown in Fig 4. Post-stimulus inter-beat intervals were averaged across two ranges: 1) the first two intervals and 2) across intervals three to five. These choices were driven by the initial reduction and subsequent rise in average heart rate seen at the two lower stimulus levels (Fig 2A).

For each range of inter-beat intervals, a significant effect of stimulus level on the change in inter-beat interval relative to baseline (level x pre/post interaction) was found (post-stimulus averaged beats 1 and 2: F(3,182) = 3.16, p = 0.024, post-stimulus beats three to five: F(3,182) = 48.45, p

A review by Graham et al. suggested that heart rate deceleration with sound onset appeared to indicate an orienting response [14]. At 15 dB SPL (an intensity near the perceptual threshold), our results show a decrease in heart rate following stimulus onset that is consistent with this proposal. This change from baseline began immediately following sound onset and was statistically significant when averaged across inter-beat intervals three to five relative to stimulus onset. At 40 dB SPL (still a relatively low level), a brief but significant increase in averaged inter-beat intervals was seen before the intervals returned to baseline (Fig 4). This pattern is consistent with the orienting response being weakened at medium intensities [13, 22]. Our data also shows an initial decrease in heart rate following stimulus offset, which is also consistent with the orienting response occurring to termination of a stimulus. Decreases in heart rate at near-threshold levels could provide an indication of internal processing or attention to stimuli [23], in this case potentially showing that a near-threshold sound was perceived. 041b061a72


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