Electrical stimulation, where anelectric current is used to stimulate the surface of the cortex using anelectrode, has long been a tool used by neuroscientists to study the structureand function of the brain. The goals of early studies, such as mapping thehomunculus of the somatosensory cortex, seem simple when compared to the goalsof today’s microstimulation studies, which stimulate pools of neurons toinvestigate whether differences in frequency can be perceived, whethersubthreshold stimulations can be detected and whether the presence/absence ofstimulation can guide behavioral actions (such as go, no-go decisions). Whilethere is no doubt that electrical microstimulation is sufficient to generatesensation, questions remain on whether the sensations elicited from electricalmicrostimulation can perfectly emulate natural, mechanical stimuli. In animalstudies, subjects cannot verbally describe the qualitative feelings of elicitedsensation, so whether electrical stimulation can perfectly substitute fornatural stimuli is left to speculation. In human studies, where subjects canspeak, many people report sensations that feel natural.
In fact, in certainstudies on humans who suffer from seizures, microstimulation sometimes producedpercepts so convincing that subjects were unable to distinguish betweenmicrostimulation and actual sensory experience. However, other subjects inhuman microstimulation studies reported sensations that were only somewhatnatural, or highly unnatural. Overall, it is unclear whether microstimulationcan serve as a substitute for natural stimuli. More human experiments that gobeyond the somatosensory cortex and include subjects other than seizurepatients are needed before the question of whether microstimulation cansubstitute for natural stimuli can be answered. Evidence that microstimulation can substitute for natural stimuli:Numerous studies on monkeys, mice andmore recently, humans have been conducted to try to uncover the relationshipbetween electrical stimulation and the nature of elicited sensations. Themajority of studies discussed here focus on stimulation of the somatosensorycortex (S1), as the somatosensory cortex is a classically studied area inmicrostimulation studies. The fact that animals perform so well onmicrostimulation detection tasks and the fact that humans describe a subset of sensationsas natural support the idea that microstimulation can substitute for naturalstimuli.
In a study by Romo et al. (2000),monkeys successfully discriminated (with above 75% accuracy) between twodifferent frequencies of electrical stimulation applied to the somatosensory cortexin the 3b area corresponding to their fingertips. Monkeys must have sensed somesort of percept, or else they would fail this discrimination task. In anotheranimal study by Connor et al. (2013), mice were much more likely to report thepresence of a virtual pole (created by infrared lasers) when their vibrissalsomatosensory cortex (vS1) was photostimulated in an area that corresponded toa specific whisker. Given that stimulation evoked so many “yes” responses(licking indicated “yes”) and mice were trained to initiate a lick specificallywhen they sensed a pole, it seems that microstimulation was a good substitutefor natural stimuli. One theory that might explain why animals perform so wellon tactile discrimination tasks is that animals cannot tell the differencebetween sensations evoked by microstimulation and sensations evoked by naturalstimuli.
Perhaps they lack the sophistication to discriminate betweenmicrostimulation and mechanical stimulation. However, there is no directevidence supporting this theory because again, animals cannot talk. Humanstudies provide further insight into the nature of elicited sensations. Hiremath et al. (2017) recentlyconducted an experiment very similar to Romo et al, except using a human witharm paralysis. They found that the subject could successfully discriminatebetween different frequencies applied to S1. Sensations evoked in the hand weredescribed as “wind running down the hand”, while sensations evoked in thefinger were described as “muffled, or as if something was wrapped around thefinger” and sensations on the lip were described as “a light rub or a lightbuzz”. This experiment was limited in that only one patient was studied, but itprovides strong evidence that manipulating the parameters of stimulation on thesomatosensory cortex can change the intensity, type and location of sensationsin a way that parallels the natural world.
Ohara et al. (2003) electricallystimulated 124 human thalami during surgery meant to treat movement disorders.During stimulation, subjects were asked to categorize the evoked sensations astouch, pressure, sharp, vibration or movement across the skin. They alsocharacterized the sensation according to a variety of other parameters,including surface/deep, non-painful/painful, temperature and natural/unnatural.The “touch” category was predominantly characterized as “something you mightencounter in everyday life” (that is, natural).
Researchers also noted that asensation was more likely to be described as natural when a neuronal soma(rather than the axon or an afferent axon) was stimulated. In both human studies described above,the stimuli were very naturalistic at times, but subjects could nonetheless knewthat elicited sensationswere from microstimulation. Interestingly, there are certain conditions where humans cannot differentiate betweenmicrostimulation and actual sensory experience. Penfield and Perot (1963) foundthat by electrically stimulating the grey matter of the temporal lobe in humanseizure patients, they could evoke hallucinations that normally occurred duringspontaneous seizures. Hallucinations ranging from listening to music to observinga baseball game to being suddenly approached from robbers were reported. Whilethis study was not done on the somatosensory cortex like most other studiesdescribed, it is significant in that evoked “sensations” were completelyrealistic and indistinguishable from real life. It is evidence thatmicrostimulation has the potential to perfectly substitute for natural stimuliin subjects other than seizure patients.
Evidence that microstimulation cannot substitute for natural stimuli:Though studies where microstimulationis indistinguishable from natural stimuli exist, these results have only beenfound in seizure patients. In both animal and human studies, doubts concerningwhether microstimulation could replace natural stimulation remain.In animal microstimulation studies, onetheory is that animals are able to perform vibrotactile discrimination tasks notby feeling sensations in their body, but by some other method that doesn’t relyon actual sensory perception in the body. Perhaps they are able to directlycompare neuronal firing rates in the cortex. Romo et al. (2000) added animportant qualifier to the end of their paper, saying that it is unknownwhether electrically stimulating S1 actually elicits a vibrating, “flutter”sensation in the monkeys’ fingertips. A similar study by De Lafuente and Romo(2005) theorized that microstimulation doesn’t produce somatic sensation butsimply activates a task rule, such as ‘a stimulus is present’, which couldexplain the almost identical neurometric and psychometric curves in the stimulidetection task. Similarly, perhaps the mice in the Connor et al.
(2013) study knewwhat microstimulation meant in terms of go, go-no behavioral choices, butdidn’t actually perceive a sensation on their whiskers. If this is true,microstimulation is not simply a poor substitute for natural stimulation. It cannotsubstitute for natural stimuli.
However, we know this theory is not true forhumans, because humans can communicate through language that they feel microstimulationin their body.A second theory is that electricalmicrostimulation elicits somatic, bodily sensations, but that these sensationsare not completely natural. In human studies, where subjects can verballydescribe the nature of stimulation, most elicited sensations fall into the grayarea between natural and unnatural. The tetraplegia patient studied by Flesheret al. (2016) described 93% of sensations evoked by stimulating S1 area 3b as”possibly natural”. He elaborated on the sensations, saying, “It’s almost likeif you pushed there, but I didn’t quite feel…the touch”. In comparison,mechanically stimulating the forearm skin with the blunt end of a cotton swabfelt “totally natural”. Additionally, in the Ohara et al.
(2003) study, thevast majority of sensations categorized as evoking “movement” (as opposedto other categories of sensation) were characterized as “unnatural”. These twopapers show that in human studies, where subjects can verbally describe qualityof a sensation, microstimulation cannot perfectly substitute for naturalstimuli.As far as microstimulation isconcerned, there are three main hypotheses: 1) Microstimulation cannotsubstitute for natural percept, 2) Microstimulation is indistinguishablefrom natural percept, 3) Microstimulation is a poor substitute for naturalpercept. Based on current research, I support the third hypothesis. While thehallucinations reported by Penfield and Perot (1963) are tantalizing evidencefor the second hypothesis, all their patients suffered from temporal lobeepilepsy, an extraordinary condition that no doubt made it easier to elicitlife-like hallucinations.
Most microstimulation studies do not elicitsensations anywhere near that level of realism and even in studies wheresubjects do report “natural” sensations, the results are too inconsistent.Animal studies are helpful in testing the methods of microstimulation butunfortunately, we will never know the nature of evoked sensations in animals. Wecan only observe when microstimulation produces the same behavioral reactions asnatural stimulation and speculate from there. We must conduct more rigorous humanstudies to better understand the complicated mechanisms behindmicrostimulation. Borchers et al (2012) warned that stimulationcan have downstream effects that aren’t measured and in some cases, could leadto reduced sensation (such as deafness and numbness).
de Lafuente and Romo (2006)studied some of these downstream effects, observing that the correlationbetween neuronal activity and perceptual judgements increases as activitytravels from the somatosensory cortex to the frontal lobe. Without a betterunderstanding of electrical microstimulation mechanisms and without details about the exact stimulation threshold needed and the preciseelectrode location required, we cannot work towards increasing therealism of elicited sensations. If scientists are ever able to useelectrical microstimulation to elicit 100% natural sensations, it would sparkmany philosophical questions. Perhaps the idea of existing as a “brain in avat” is not as far away as we once thought. Additionally, if electricalstimulation has the power to evoke the same sudden feelings of familiarity orflashes of past experiences that epilepsy patients experience during seizures,this would have implications for understanding why non-epileptic humanssometimes experience déjà vu. However, at this stage, most sensations elicitedby electrical microstimulation fall into a different category than that of everydaystimuli.
Electrical microstimulation still has a while to go before it can perfectly substitutefor natural, mechanical stimuli.