<p>Biological signals are analogues and hence continuous. Early EEG systems simply recorded the analogue signals and displayed it as a continuous signal graphically as wiggly lines written by little pens on a roll of paper (Fig. 1). Only after analogue-to-digital converters became available, could EEG signals and the likes be digitally sampled. </p>

<p><a href="https://i.sstatic.net/1N0x5.jpg" rel="nofollow noreferrer"><img src="https://i.sstatic.net/1N0x5.jpg" alt="analogue EEG signal"></a><br>
<sup>Fig. 1. Analogue EEG signal. source: <a href="https://www.ncbi.nlm.nih.gov/books/NBK390356/figure/f39/" rel="nofollow noreferrer">Britton <em>et al.</em>, 2016</a>)</sup>  </p>

<p><sub><strong>Reference</strong><br>
<strong>-</strong> <a href="https://www.ncbi.nlm.nih.gov/books/NBK390356/figure/f39/" rel="nofollow noreferrer">Britton <em>et al.</em>, Electroencephalography (EEG): An Introductory Text and Atlas of Normal and Abnormal Findings in Adults, Children, and Infants, 2016. Chicago: American Epilepsy Society</a></sub></p>

CC BY-SA 4.0
2019-07-30T07:48:07.657Z
i understand that it looks continuous but logically if you see that each point is a potential -> Discrete.
roaibrain
CC BY-SA 4.0
2019-07-30T08:15:29.327Z
Each point is a potential yes, but why would it be discrete? The original signal has no points, there are only discrete data points if you create them.
AliceD
CC BY-SA 4.0
2019-08-03T19:32:17.730Z
@AliceD the potential comes from ions flowing through the membrane. These ions each have a discrete charge.
StrongBad
CC BY-SA 4.0
2019-08-04T21:31:37.140Z
@StrongBad yes, but for a potential in the order of microvolts to be recorded in your average EEG signal, say, millions of electrons flow. With Coulomb's law you can calculate how much. Gross potentials recorded with superficial electrodes measure thousands of neurons, and even within a single neuron your supposedly fractal ion flow is anything but when recorded with an electrode.
AliceD
CC BY-SA 4.0
2019-07-30T07:00:54.817Z
2019-08-02T21:29:38.620Z
2019-08-02T21:29:38.620Z
AliceD
<p>To add to the answer provided by @AliceD, pure digital waveforms are square waveforms as they represent steps between 1s and 0s and are therefore not continuous.</p>

<p><a href="https://i.sstatic.net/l5YQu.png" rel="nofollow noreferrer"><img src="https://i.sstatic.net/l5YQu.png" alt="Digital waveform"></a></p>

<p>Analogue waveforms are not.  They are smooth continuous waves and can represent many voltage points at each millisecond, microsecond or nanosecond between the peaks and troughs.</p>

<p><a href="https://i.sstatic.net/m6p5X.jpg" rel="nofollow noreferrer"><img src="https://i.sstatic.net/m6p5X.jpg" alt="analogue waveform"></a></p>

<p>Outputs from ADCs (Analogue to Digital Converters) will have steps in them representing the various voltage ranges at their relative timings. In a sense they are digital outputs but are really a digitized version (a representation) of an analogue waveform through digital sampling of the wave voltage at the relevant millisecond, microsecond or nanosecond depending on the ADC timing (see the image below from <a href="https://www.allaboutcircuits.com/textbook/digital/chpt-13/practical-considerations-adc-circuits/" rel="nofollow noreferrer">https://www.allaboutcircuits.com/textbook/digital/chpt-13/practical-considerations-adc-circuits/</a>).</p>

<p><a href="https://i.sstatic.net/cJAHu.png" rel="nofollow noreferrer"><img src="https://i.sstatic.net/cJAHu.png" alt="digitized version of an analogue signal"></a></p>

CC BY-SA 4.0
2019-08-02T05:01:47.807Z
What's the point of having a digital ouput? Why can't we just take the sample times/points on the signal and use them for filtering/other operations?
roaibrain
CC BY-SA 4.0
2019-08-02T15:06:48.507Z
@Roshan Computers are digital. Hardware can do filtering before digitization and typically does when recording these sorts of signals to make them appropriate to digitize.
Bryan Krause
CC BY-SA 4.0
2019-08-02T16:16:34.180Z
I am not a neurologist so therefore I don’t interact with EEGs etc. but as Bryan points out, computers are digital and cannot deal with analogue signals.  If VGA/HDMI/DVI monitors are used, the signal will need digitizing and processed in order for the monitor to display it.
Chris Rogers
CC BY-SA 4.0
2019-07-30T13:16:54.137Z
2019-07-30T13:16:54.137Z
Chris Rogers
<p>I disagree with the other answers, to an extent. Biological signals are generally created by ion passing through a cell membrane. Each of these ions cause a tiny current/magnetic field which is too small to measure in most cases. Each cell action potential is made up of hundreds (???) of ions passing through the membrane at a variety of positions. Most measures of biological signals measure potentials from many cells. These cells are distributed in a non homogeneous extracellular matrix. At the limit the biological signals are discrete and made up of individual ions, but by the time we measure them, they are so mixed up that they are continuous.</p>

CC BY-SA 4.0
2019-08-03T23:25:07.357Z
Like I commented to your other comment, charged ions do not exist in a binary inside/outside configuration, they can also be partly between. Unless space is discrete, voltage is continuous. In any case, digitization is making a signal far more discrete than originally measured.
Bryan Krause
CC BY-SA 4.0
2019-08-04T10:51:46.827Z
Do you have any references to back your claims @StrongBad?
Chris Rogers
CC BY-SA 4.0
2019-08-03T19:29:04.257Z
2019-08-03T19:29:04.257Z
StrongBad
<p>Whether we talk about EEG, ECG or EKG aren't all these signals discrete as we are reading the voltages from the electrodes that are due to certain bio-chemical or bio-electrical activities. I do not get why people say that EEG is continuous and hence we need to convert it to digital when it is already digital.</p>

CC BY-SA 4.0
2019-08-16T20:17:48.217Z
There's actually a whole hierarchy of continuous and discrete signals here: EEGs etc. are spatially-averaged continuous signals, composed by summing discrete spike trains (which are sometimes recorded as such, as times-of-spike), which are summaries of continuous membrane potential timeseries, which are smoothed sums of discrete ion counts (or individual neurotransmitter molecule receptions).
tsbertalan
CC BY-SA 4.0
2019-08-16T20:19:46.907Z
The "membrane potential" -> "spike train" summarization in particular is useful for the same reason that digital "square waves" can be meaningfully be digitized by looking for rising and falling edges within well-defined voltage ranges, though, in contrast with digital computation, small MP oscillations that cannot be called spiking might still be computationally meaningful.
tsbertalan
CC BY-SA 4.0
2019-08-01T22:00:08.540Z
I don't understand why "bioelectric activities" can't take on a continuous range of values.  The membrane potential of a neuron could be -70.123456789... That seems like it would be continuous to me.
Chuck Sherrington
CC BY-SA 4.0
2019-08-02T05:00:00.423Z
So can a continuous signal be considered as a discrete signal with number of sample points tending to infinity?
roaibrain
CC BY-SA 4.0
2019-08-02T21:14:25.317Z
A continuous signal has a (countably) infinite number of points in it by nature.  It's more that a discrete signal can be one with a finite number of samples of a continuous signal.  When a cell depolarizes, it doesn't go from -70.002 to -70.001 by steps of 0.0001 or something, the voltage changes continuously.
Chuck Sherrington
CC BY-SA 4.0
2019-08-03T19:30:57.320Z
@ChuckSherrington at the limit it changes by the total charge of all the ions that pass through the membrane in a unit time.
StrongBad
CC BY-SA 4.0
2019-08-03T23:21:27.167Z
@StrongBad Not exactly. Ions can also be in different locations relative to the membrane. Unless space is also discrete it is truly continuous.
Bryan Krause
CC BY-SA 4.0
2019-08-03T23:26:17.407Z
@BryanKrause yes. I try and explain that in my answer. At the ionic level things are really complicated. My point it that we teach students that action potentials are discrete events and that they are governed by the flow of ions, both may have lead the OP to ask the question.
StrongBad
CC BY-SA 4.0
2019-08-05T02:07:03.793Z
@BryanKrause Is space considered to be discrete or continuous?
roaibrain
CC BY-SA 4.0
2019-08-05T02:56:27.640Z
@Roshan No one has been able to measure space as discrete. It could be but so far there is no evidence. As far as neural signals, it also matters how far away you are. Especially in something like an EEG you are measuring the distance weighted average, even if the signals are all discrete spikes, which they are definitely not.
Bryan Krause
<eeg><signal-processing><ecg>
Why are biosignals from the body said to be continuous?
roaibrain
# Why are biosignals from the body said to be continuous?

(Asked by: username_0 on 2019-07-30)

Whether we talk about EEG, ECG or EKG aren't all these signals discrete as we are reading the voltages from the electrodes that are due to certain bio-chemical or bio-electrical activities. I do not get why people say that EEG is continuous and hence we need to convert it to digital when it is already digital.

**Tags:** eeg, signal-processing, ecg

> **Comments:**
> **username_2**: I don't understand why "bioelectric activities" can't take on a continuous range of values.  The membrane potential of a neuron could be -70.123456789... That seems like it would be continuous to me.  
> **username_0**: So can a continuous signal be considered as a discrete signal with number of sample points tending to infinity?  
> **username_2**: A continuous signal has a (countably) infinite number of points in it by nature.  It's more that a discrete signal can be one with a finite number of samples of a continuous signal.  When a cell depolarizes, it doesn't go from -70.002 to -70.001 by steps of 0.0001 or something, the voltage changes continuously.  
> **username_3**: @ChuckSherrington at the limit it changes by the total charge of all the ions that pass through the membrane in a unit time.  
> **username_4**: @username_3 Not exactly. Ions can also be in different locations relative to the membrane. Unless space is also discrete it is truly continuous.  
> **username_3**: @BryanKrause yes. I try and explain that in my answer. At the ionic level things are really complicated. My point it that we teach students that action potentials are discrete events and that they are governed by the flow of ions, both may have lead the OP to ask the question.  
> **username_0**: @BryanKrause Is space considered to be discrete or continuous?  
> **username_4**: @Roshan No one has been able to measure space as discrete. It could be but so far there is no evidence. As far as neural signals, it also matters how far away you are. Especially in something like an EEG you are measuring the distance weighted average, even if the signals are all discrete spikes, which they are definitely not.  
> **username_1**: There's actually a whole hierarchy of continuous and discrete signals here: EEGs etc. are spatially-averaged continuous signals, composed by summing discrete spike trains (which are sometimes recorded as such, as times-of-spike), which are summaries of continuous membrane potential timeseries, which are smoothed sums of discrete ion counts (or individual neurotransmitter molecule receptions).  
> **username_1**: The "membrane potential" -> "spike train" summarization in particular is useful for the same reason that digital "square waves" can be meaningfully be digitized by looking for rising and falling edges within well-defined voltage ranges, though, in contrast with digital computation, small MP oscillations that cannot be called spiking might still be computationally meaningful.  

---

## 3 answers

### Answer by username_5 on 2019-07-30. Score: 4

Biological signals are analogues and hence continuous. Early EEG systems simply recorded the analogue signals and displayed it as a continuous signal graphically as wiggly lines written by little pens on a roll of paper (Fig. 1). Only after analogue-to-digital converters became available, could EEG signals and the likes be digitally sampled.

[![analogue EEG signal](https://i.sstatic.net/1N0x5.jpg)](https://i.sstatic.net/1N0x5.jpg)  
Fig. 1. Analogue EEG signal. source: [Britton *et al.*, 2016](https://www.ncbi.nlm.nih.gov/books/NBK390356/figure/f39/))

**Reference**  
**-** [Britton *et al.*, Electroencephalography (EEG): An Introductory Text and Atlas of Normal and Abnormal Findings in Adults, Children, and Infants, 2016. Chicago: American Epilepsy Society](https://www.ncbi.nlm.nih.gov/books/NBK390356/figure/f39/)

> **Comments:**
> **username_0**: i understand that it looks continuous but logically if you see that each point is a potential -> Discrete.  
> **username_5**: Each point is a potential yes, but why would it be discrete? The original signal has no points, there are only discrete data points if you create them.  
> **username_3**: @username_5 the potential comes from ions flowing through the membrane. These ions each have a discrete charge.  
> **username_5**: @username_3 yes, but for a potential in the order of microvolts to be recorded in your average EEG signal, say, millions of electrons flow. With Coulomb's law you can calculate how much. Gross potentials recorded with superficial electrodes measure thousands of neurons, and even within a single neuron your supposedly fractal ion flow is anything but when recorded with an electrode.  

---

### Answer by username_6 on 2019-07-30. Score: 1

To add to the answer provided by @username_5, pure digital waveforms are square waveforms as they represent steps between 1s and 0s and are therefore not continuous.

[![Digital waveform](https://i.sstatic.net/l5YQu.png)](https://i.sstatic.net/l5YQu.png)

Analogue waveforms are not. They are smooth continuous waves and can represent many voltage points at each millisecond, microsecond or nanosecond between the peaks and troughs.

[![analogue waveform](https://i.sstatic.net/m6p5X.jpg)](https://i.sstatic.net/m6p5X.jpg)

Outputs from ADCs (Analogue to Digital Converters) will have steps in them representing the various voltage ranges at their relative timings. In a sense they are digital outputs but are really a digitized version (a representation) of an analogue waveform through digital sampling of the wave voltage at the relevant millisecond, microsecond or nanosecond depending on the ADC timing (see the image below from <https://www.allaboutcircuits.com/textbook/digital/chpt-13/practical-considerations-adc-circuits/>).

[![digitized version of an analogue signal](https://i.sstatic.net/cJAHu.png)](https://i.sstatic.net/cJAHu.png)

> **Comments:**
> **username_0**: What's the point of having a digital ouput? Why can't we just take the sample times/points on the signal and use them for filtering/other operations?  
> **username_4**: @Roshan Computers are digital. Hardware can do filtering before digitization and typically does when recording these sorts of signals to make them appropriate to digitize.  
> **username_6**: I am not a neurologist so therefore I don’t interact with EEGs etc. but as Bryan points out, computers are digital and cannot deal with analogue signals.  If VGA/HDMI/DVI monitors are used, the signal will need digitizing and processed in order for the monitor to display it.  

---

### Answer by username_3 on 2019-08-03. Score: -1

I disagree with the other answers, to an extent. Biological signals are generally created by ion passing through a cell membrane. Each of these ions cause a tiny current/magnetic field which is too small to measure in most cases. Each cell action potential is made up of hundreds (???) of ions passing through the membrane at a variety of positions. Most measures of biological signals measure potentials from many cells. These cells are distributed in a non homogeneous extracellular matrix. At the limit the biological signals are discrete and made up of individual ions, but by the time we measure them, they are so mixed up that they are continuous.

> **Comments:**
> **username_4**: Like I commented to your other comment, charged ions do not exist in a binary inside/outside configuration, they can also be partly between. Unless space is discrete, voltage is continuous. In any case, digitization is making a signal far more discrete than originally measured.  
> **username_6**: Do you have any references to back your claims @username_3?  

---

