Five thoughts to think about when thinking about the speed of thought

As inquis­i­tive beings, we are con­stant­ly ques­tion­ing and quan­ti­fy­ing the speed of var­i­ous things. With a fair degree of accu­ra­cy, sci­en­tists have quan­ti­fied the speed of light, the speed of sound, the speed at which the earth revolves around the sun, the speed at which hum­ming­birds beat their wings, the aver­age speed of con­ti­nen­tal drift….

These val­ues are all well-char­ac­ter­ized. But what about the speed of thought? It’s a chal­leng­ing ques­tion that’s not eas­i­ly answer­able – but we can give it a shot.

1. Let’s define “thought”

To quan­ti­fy the speed of any­thing, one needs to iden­ti­fy its begin­ning and end. For our pur­pos­es, a “thought” will be defined as the men­tal activ­i­ties engaged from the moment sen­so­ry infor­ma­tion is received to the moment an action is ini­ti­at­ed. This def­i­n­i­tion nec­es­sar­i­ly excludes many expe­ri­ences and process­es one might con­sid­er to be “thoughts.”

Here, a “thought” includes process­es relat­ed to per­cep­tion (deter­min­ing what is in the envi­ron­ment and where), deci­sion-mak­ing (deter­min­ing what to do) and action-plan­ning (deter­min­ing how to do it). The dis­tinc­tion between, and inde­pen­dence of, each of these process­es is blur­ry. Fur­ther, each of these process­es, and per­haps even their sub-com­po­nents, could be con­sid­ered “thoughts” on their own. But we have to set our start- and end­points some­where to have any hope of tack­ling the question.

Final­ly, try­ing to iden­ti­fy one val­ue for the “speed of thought” is a lit­tle like try­ing to iden­ti­fy one max­i­mum speed for all forms of trans­porta­tion, from bicy­cles to rock­ets. There are many dif­fer­ent kinds of thoughts that can vary great­ly in timescale. Con­sid­er the dif­fer­ences between sim­ple, speedy reac­tions like the sprint­er decid­ing to run after the crack of the start­ing pis­tol (on the order of 150 mil­lisec­onds [ms]), and more com­plex deci­sions like decid­ing when to change lanes while dri­ving on a high­way or fig­ur­ing out the appro­pri­ate strat­e­gy to solve a math prob­lem (on the order of sec­onds to minutes).

2. What exactly should we measure?

Thought is ulti­mate­ly an inter­nal and very indi­vid­u­al­ized process that’s not read­i­ly observ­able. It relies on inter­ac­tions across com­plex net­works of neu­rons dis­trib­uted through­out the periph­er­al and cen­tral ner­vous sys­tems. Researchers can use imag­ing tech­niques, such as func­tion­al mag­net­ic res­o­nance imag­ing and elec­troen­cephalog­ra­phy, to see what areas of the ner­vous sys­tem are active dur­ing dif­fer­ent thought process­es, and how infor­ma­tion flows through the ner­vous sys­tem. We’re still a long way from reli­ably relat­ing these sig­nals to the men­tal events they rep­re­sent, though.

Many sci­en­tists con­sid­er the best proxy mea­sure of the speed or effi­cien­cy of thought process­es to be reac­tion time – the time from the onset of a spe­cif­ic sig­nal to the moment an action is ini­ti­at­ed. Indeed, researchers inter­est­ed in assess­ing how fast infor­ma­tion trav­els through the ner­vous sys­tem have used reac­tion time since the mid-1800s. This approach makes sense because thoughts are ulti­mate­ly expressed through overt actions. Reac­tion time pro­vides an index of how effi­cient­ly some­one receives and inter­prets sen­so­ry infor­ma­tion, decides what to do based on that infor­ma­tion, and plans and ini­ti­ates an action based on that decision.

Purk­in­je neu­ron by Cajal

3. Neural factors involved: distance, myelination, complexity

The time it takes for all thoughts to occur is ulti­mate­ly shaped by the char­ac­ter­is­tics of the neu­rons and the net­works involved. Many things influ­ence the speed at which infor­ma­tion flows through the sys­tem, but three key fac­tors are:

Dis­tance – The far­ther sig­nals need to trav­el, the longer the reac­tion time is going to be. Reac­tion times for move­ments of the foot are longer than for move­ments of the hand, in large part because the sig­nals trav­el­ing to and from the brain have a longer dis­tance to cov­er. This prin­ci­ple is read­i­ly demon­strat­ed through reflex­es (note, how­ev­er, that reflex­es are respons­es that occur with­out “thought” because they do not involve neu­rons that engaged in con­scious thought). The key obser­va­tion for the present pur­pose is that the same reflex­es evoked in taller indi­vid­u­als tend to have longer response times than for short­er indi­vid­u­als. By way of anal­o­gy, if two couri­ers dri­ving to New York leave at the same time and trav­el at exact­ly the same speed, a couri­er leav­ing from Wash­ing­ton, DC will always arrive before one leav­ing from Los Angeles.

Neu­ron char­ac­ter­is­tics – The width of the neu­ron is impor­tant. Sig­nals are car­ried more quick­ly in neu­rons with larg­er diam­e­ters than those that are nar­row­er – a couri­er will gen­er­al­ly trav­el faster on wide mul­ti-lane high­ways than on nar­row coun­try roads. Nerve sig­nals jump between the exposed areas between myelin sheathes. How much myeli­na­tion a neu­ron has is also impor­tant. Some nerve cells have myelin cells that wrap around the neu­ron to pro­vide a type of insu­la­tion sheath. The myelin sheath isn’t com­plete­ly con­tin­u­ous along a neu­ron; there are small gaps in which the nerve cell is exposed. Nerve sig­nals effec­tive­ly jump from exposed sec­tion to exposed sec­tion instead of trav­el­ing the full extent of the neu­ronal sur­face. So sig­nals move much faster in neu­rons that have myelin sheaths than in neu­rons that don’t. The mes­sage will get to New York soon­er if it pass­es from cell­phone tow­er to cell­phone tow­er than if the couri­er dri­ves the mes­sage along each and every inch of the road. In the human con­text, the sig­nals car­ried by the large-diam­e­ter, myeli­nat­ed neu­rons that link the spinal cord to the mus­cles can trav­el at speeds rang­ing from 70–120 meters per sec­ond (m/s) (156–270 miles per hour[mph]), while sig­nals trav­el­ing along the same paths car­ried by the small-diam­e­ter, unmyeli­nat­ed fibers of the pain recep­tors trav­el at speeds rang­ing from 0.5–2 m/s (1.1–4.4 mph). That’s quite a difference!

Com­plex­i­ty – Increas­ing the num­ber of neu­rons involved in a thought means a greater absolute dis­tance the sig­nal needs to trav­el – which nec­es­sar­i­ly means more time. The couri­er from Wash­ing­ton, DC will take less time to get to New York with a direct route than if she trav­els to Chica­go and Boston along the way. Fur­ther, more neu­rons mean more con­nec­tions. Most neu­rons are not in phys­i­cal con­tact with oth­er neu­rons. Instead, most sig­nals are passed via neu­ro­trans­mit­ter mol­e­cules that trav­el across the small spaces between the nerve cells called synaps­es. This process takes more time (at least 0.5 ms per synapse) than if the sig­nal was con­tin­u­al­ly passed with­in the sin­gle neu­ron. The mes­sage car­ried from Wash­ing­ton, DC will take less time to get to New York if one sin­gle couri­er does the whole route than if mul­ti­ple couri­ers are involved, stop­ping and hand­ing over the mes­sage sev­er­al times along the way. In truth, even the “sim­plest” thoughts involve mul­ti­ple struc­tures and hun­dreds of thou­sands of neurons.

4. Is it a thought or an involuntary reflex?

It’s amaz­ing to con­sid­er that a giv­en thought can be gen­er­at­ed and act­ed on in less than 150 ms. Con­sid­er the sprint­er at a start­ing line. The recep­tion and per­cep­tion of the crack of the starter’s gun, the deci­sion to begin run­ning, issu­ing of the move­ment com­mands, and gen­er­at­ing mus­cle force to start run­ning involves a net­work that begins in the inner ear and trav­els through numer­ous struc­tures of the ner­vous sys­tem before reach­ing the mus­cles of the legs. All that can hap­pen in lit­er­al­ly half the time of a blink of an eye.

Although the time to ini­ti­ate a sprint start is extreme­ly short, a vari­ety of fac­tors can influ­ence it. One is the loud­ness of the audi­to­ry “go” sig­nal. Although reac­tion time tends to decrease as the loud­ness of the “go” increas­es, there appears to be a crit­i­cal point in the range of 120–124 deci­bels where an addi­tion­al decrease of approx­i­mate­ly 18 ms can occur. That’s because sounds this loud can gen­er­ate the “star­tle” response and trig­ger a pre-planned sprint­ing response.

Researchers think this trig­gered response emerges through acti­va­tion of neur­al cen­ters in the brain stem. These star­tle-elicit­ed respons­es may be quick­er because they involve a rel­a­tive­ly short­er and less com­plex neur­al sys­tem – one that does not nec­es­sar­i­ly require the sig­nal to trav­el all the way up to the more com­plex struc­tures of the cere­bral cor­tex. A debate could be had here as to whether or not these trig­gered respons­es are “thoughts,” because it can be ques­tioned whether or not a true deci­sion to act was made; but the reac­tion time dif­fer­ences of these respons­es illus­trate the effect of neur­al fac­tors such as dis­tance and com­plex­i­ty. Invol­un­tary reflex­es, too, involve short­er and sim­pler cir­cuit­ry and tend to take less time to exe­cute than vol­un­tary responses.

5. Perception matters

Con­sid­er­ing how quick­ly they do hap­pen, it’s lit­tle won­der we often feel our thoughts and actions are near­ly instan­ta­neous. But it turns out we’re also poor judges of when our actions actu­al­ly occur.

Although we’re aware of our thoughts and the result­ing move­ments, an inter­est­ing dis­so­ci­a­tion has been observed between the time we think we ini­ti­ate a move­ment and when that move­ment actu­al­ly starts. In stud­ies, researchers ask vol­un­teers to watch a sec­ond hand rotate around a clock face and to com­plete a sim­ple rapid fin­ger or wrist move­ment, such as a key press, when­ev­er they liked. After the clock hand had com­plet­ed its rota­tion, the peo­ple were asked to iden­ti­fy where the hand was on the clock face when they start­ed their own movement.

Sur­pris­ing­ly, peo­ple typ­i­cal­ly judge the onset of their move­ment to occur 75–100 ms pri­or to when it actu­al­ly began. This dif­fer­ence can­not be account­ed for sim­ply by the time it takes for the move­ment com­mands to trav­el from the brain to the arm mus­cles (which is on the order of 16–25 ms). It’s unclear exact­ly why this mis­per­cep­tion occurs, but it’s gen­er­al­ly believed that peo­ple base their judg­ment of move­ment onset on the time of the deci­sion to act and the pre­dic­tion of the upcom­ing move­ment, instead of on the move­ment itself. These and oth­er find­ings raise impor­tant ques­tions about the plan­ning and con­trol of action and our sense of agency and con­trol in the world – because our deci­sion to act and our per­cep­tion of when we act appear to be dis­tinct from when we in fact do.

In sum, although quan­ti­fy­ing a sin­gle “speed of thought” may nev­er be pos­si­ble, ana­lyz­ing the time it takes to plan and com­plete actions pro­vides impor­tant insights into how effi­cient­ly the ner­vous sys­tem com­pletes these process­es, and how changes asso­ci­at­ed with move­ment and cog­ni­tive dis­or­ders affect the effi­cien­cy of these men­tal activities.

– Dr. Tim Welsh is Pro­fes­sor of Kine­si­ol­o­gy and Phys­i­cal Edu­ca­tion at the Uni­ver­si­ty of Toron­to, lead­ing research to gain an under­stand­ing of the cog­ni­tive and neur­al mech­a­nisms that under­lie the goal-direct­ed actions of peo­ple from aver­age and spe­cial pop­u­la­tions such as Down syn­drome, autism, and Parkinson’s dis­ease. This arti­cle was orig­i­nal­ly pub­lished on The Con­ver­sa­tion.

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SHARPBRAINS es un think-tank y consultoría independiente proporcionando servicios para la neurociencia aplicada, salud, liderazgo e innovación.

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