I would be interested in knowing how long it takes between the moment something touches our skin and the moment something is activated in the brain. Also how long does it take in total until we subjectively perceive something? And I would like to know whether this is dependent on which part of the body was stimulated (same latency for foot and neck?), or if it is dependent on the nature of stimuli (heat or mechanical). I would also be interested in that latency in the auditory and visual system. I tried searching for that but could not find what I needed due to the lack of the correct terminology.
Given your questions taken together and your comment
It would be interesting to know how much delay the brain can handle in understanding causality and associating events I think you are interested in intersensory asynchrony. A well-known example where two stimulus modalities are perceived as separate while they are in fact coming from the same event is a thunder heard after lightning. This is caused by the fact that sound travels at a speed much slower than light, and hence a thunder can lag lightning by seconds.
In many instances, however, one event generating perceptions across stimulus modalities are actually perceived as synchronous while they are in fact offset in time due to differences in physical characteristics of the stimuli. Taking the storm as an example - when it is far away, the thunder is perceived as separate because the thunder lags by seconds. But when the thunderstorm is close enough, the auditory crack and visual lightning are actually perceived as synchronous, while in fact they are still offset because of the sound travelling so much slower than light.
So the question becomes, as you rightfully ask, what are the margins in which stimuli across modalities can be asynchronous while perceived as one event? In other words, what is the window of integration?
A review by Vroomen and Keetels (2010) describes various psychophysical studies that have examined what the minimal asynchrony is between two stimuli of different modality that lead to a synchronized percept. The following values are for simple stimuli, such as tactile taps, auditory beeps and visual flashes.
- Auditory and tactile stimuli: 80 ms
- Auditory beeps and visual flashes: 25 - 50 ms
- Visual and tactile stimili: 35 - 65 ms
Note that these lag differences are relatively large, given the speed of peripheral neural transduction (see the answer of poka.nandor). Moreover, the window of integration between more complex stimuli can be much greater. For example, the window for speech and visual information can be as large as 203 ms. Such large windows of integration point toward higher processes playing a role in the brain. Note that only temporal lags below 20 msec are expected to go unnoticed because of hard-wired limitations on the resolution power of the individual senses,
Hence, Vroomen and Keetels (2010) argue there must be higher processes at work in the brain that actively synchronize percepts that are offset in time, but seem to belong to one and the same event. One such mechanism is referred to as temporal ventriloquism, which means that one perceptual modality is actively shifted in time to match it with another. This effect is most pronounced in visual stimuli, in that a visual percept is actively adjusted in time to match a sound or tactile stimulus. Likely visual percepts are shifted preferably by the brain, because the visual system is the slowest of all the senses.
This is a really interesting set of questions and I'll try to answer all yet keep it compact. So first of all let's see the nerve classes and conduction speed from this and this wikipedia pages:
Peripherial nerves can be classified into three groups A, B, and C (based on their diameter).
Group A are the thickest (largest in diameter), are myelinated and have high conduction velocity.
Group A can be divided to subclasses:
- 'A' alpha nerves are 13-20um (micrometer) thick and have 80-120m/s conduction velocity (this equals 288-432km/h (meter/s * 3,6) or 180-270mph (khm/h / 1,6)). These are associated with proprioception (this subconscious sensation is responsible for us knowing how our body is positioned in 3D and where are our limbs compared to our body)
This means that in a fairly average 180 cm (approx 6 feet) tall human a stimulus on such nerve can travel the whole body of the person in 0,018s (with an average of 100m/s conduction velocity) How ever this is not the real case, I'll discuss that later.
- 'A' beta nerves are about half as thick as 'A' alpha (6-12um) and conduct stimulus with 33-75m/s. Among others these are linked to cutaneous mechano receptors
Alpha and beta fibers can be both afferent (sensory) or efferent (motor)
'A' delta nerves are thinly myelinated and far slower than the previous nerves with 3-30m/s velocity. These type of nerves are linked to nociception, cold thermoreceptors and make up the free nerve endings for touch and pressure.
'A' gamma fibers are efferent fibers and with 4-34m/s velocity.
Group B nerves are preganglionic fibers (ganglions are hubs of neurons of the autonomic nervous system) and conduct stimuli with 3-15m/s velocity. Thes are 1-5um thick and myelinated.
Group C are post ganglionic fibers (leading to the organs) and belong to nociceptors and warmth receptors They're thin (0.2 - 1.5um) and non myelinated. Their conduction speed is 0.5-2m/s.
So different sensory types have different nerves associated to them, and this results in different conduction velocity of that stimuli. Thus the answer is yes there is difference in sensation lag times to different types of stimuli.
In this paper scientist tested reaction lag time to heat caused pain and found the following:
Researchers stimulated two different parts of the hand of test subjects, the thenar eminence - that is the base of the thumb, and the volar forearm - that is the same side of the forearm as the palm ( the lower part of your forearm if you imagine your hands palms down). As you can see on the graph the volar forearm has lower lag times that the base of the thumb after a certain level of stimulus. These authors also made a control experiment to:
estimate the sum total of time necessary for central sensory processing, the decision making plus motor execution times were measured in one subject for responses to suprathreshold auditory stimuli. Auditory stimuli were chosen because primary afferent conduction time is negligible. Latencies were measured in the same way as with the thermal stimuli. The tenth percentile (point at which 10 ~ of the latencies were faster and 90 ~ were slower) was near 0.19 s.
So response to sound is pretty fast, but this as with the heat experiment includes the measurable response of the test subject, thus the value is higher than the actual sensation.
For the heat-pain experiment the authors concluded :
The conduction distance from the area stimulated to the spinal cord was 0.78 m in this subject. If receptor utilization time, rise time, and conduction time in the cervical spinal cord are ignored, then primary afferents that signal first pain must have a conduction velocity of at least 0.78 m/ (0.33 --0.19) s, or 6 m/s.
The 6/ms velocity correlates well with the A-delta nerve's conduction speed.
Also here's another diagram on the distribution of lag times to different level of stimulus:
The difference of lag times can result from the activation of differrent receptors. Temperatures around 40 degrees Celsius are more likely to activate warmth receptors only that have lower conduction velocity. The high numbers of slow (1-1,5s) response to these stimuli correlate well with the group C fibers for warmth reception. On the other hand, responses to higher and potentially more damaging temperatures, nociceptors are more likely to be activated and faster conduction of these stimuli result in lower latencies. Also the transition from high latency to low is quite smooth with increase of the temperature.
Difference in nerve conduction values are not the only thing that occurs lag in sensation. These stimuli do not travel directly to the brain from receptors, but have to be transmitted to a sensory neuron, then these have connections in the spinal cord before stimulus can be transmitted to the adequate part of the brain, and then these neurons of the brain still have to process the signal. It is important to note that lag times are different from individual to individual. Authors of this paper conducted a great series of measurements to find out effects of age, sex and height on nerve conduction velocity and it turns out that for example:
Controlling for age and temperature, the 0.17 m/s decrease in sural conduction velocity per centimeter increase in height (0.44 m/s per inch) was somewhat larger than previously reported.
Also this last article has many good comparison tables of different nerves and conditions.