Electrical neurons to investigate whether differences in frequency

Electrical stimulation, where an
electric current is used to stimulate the surface of the cortex using an
electrode, has long been a tool used by neuroscientists to study the structure
and function of the brain. The goals of early studies, such as mapping the
homunculus of the somatosensory cortex, seem simple when compared to the goals
of today’s microstimulation studies, which stimulate pools of neurons to
investigate whether differences in frequency can be perceived, whether
subthreshold stimulations can be detected and whether the presence/absence of
stimulation can guide behavioral actions (such as go, no-go decisions). While
there is no doubt that electrical microstimulation is sufficient to generate
sensation, questions remain on whether the sensations elicited from electrical
microstimulation can perfectly emulate natural, mechanical stimuli. In animal
studies, subjects cannot verbally describe the qualitative feelings of elicited
sensation, so whether electrical stimulation can perfectly substitute for
natural stimuli is left to speculation. In human studies, where subjects can
speak, many people report sensations that feel natural. In fact, in certain
studies on humans who suffer from seizures, microstimulation sometimes produced
percepts so convincing that subjects were unable to distinguish between
microstimulation and actual sensory experience. However, other subjects in
human microstimulation studies reported sensations that were only somewhat
natural, or highly unnatural. Overall, it is unclear whether microstimulation
can serve as a substitute for natural stimuli. More human experiments that go
beyond the somatosensory cortex and include subjects other than seizure
patients are needed before the question of whether microstimulation can
substitute for natural stimuli can be answered.

Evidence that microstimulation can substitute for natural stimuli:

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Numerous studies on monkeys, mice and
more recently, humans have been conducted to try to uncover the relationship
between electrical stimulation and the nature of elicited sensations. The
majority of studies discussed here focus on stimulation of the somatosensory
cortex (S1), as the somatosensory cortex is a classically studied area in
microstimulation studies. The fact that animals perform so well on
microstimulation detection tasks and the fact that humans describe a subset of sensations
as natural support the idea that microstimulation can substitute for natural
stimuli.

In a study by Romo et al. (2000),
monkeys successfully discriminated (with above 75% accuracy) between two
different frequencies of electrical stimulation applied to the somatosensory cortex
in the 3b area corresponding to their fingertips. Monkeys must have sensed some
sort of percept, or else they would fail this discrimination task. In another
animal study by Connor et al. (2013), mice were much more likely to report the
presence of a virtual pole (created by infrared lasers) when their vibrissal
somatosensory cortex (vS1) was photostimulated in an area that corresponded to
a specific whisker. Given that stimulation evoked so many “yes” responses
(licking indicated “yes”) and mice were trained to initiate a lick specifically
when they sensed a pole, it seems that microstimulation was a good substitute
for natural stimuli. One theory that might explain why animals perform so well
on tactile discrimination tasks is that animals cannot tell the difference
between sensations evoked by microstimulation and sensations evoked by natural
stimuli. Perhaps they lack the sophistication to discriminate between
microstimulation and mechanical stimulation. However, there is no direct
evidence supporting this theory because again, animals cannot talk. Human
studies provide further insight into the nature of elicited sensations.

Hiremath et al. (2017) recently
conducted an experiment very similar to Romo et al, except using a human with
arm paralysis. They found that the subject could successfully discriminate
between different frequencies applied to S1. Sensations evoked in the hand were
described as “wind running down the hand”, while sensations evoked in the
finger were described as “muffled, or as if something was wrapped around the
finger” and sensations on the lip were described as “a light rub or a light
buzz”. This experiment was limited in that only one patient was studied, but it
provides strong evidence that manipulating the parameters of stimulation on the
somatosensory cortex can change the intensity, type and location of sensations
in a way that parallels the natural world.

Ohara et al. (2003) electrically
stimulated 124 human thalami during surgery meant to treat movement disorders.
During stimulation, subjects were asked to categorize the evoked sensations as
touch, pressure, sharp, vibration or movement across the skin. They also
characterized 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 might
encounter in everyday life” (that is, natural). Researchers also noted that a
sensation 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 knew
that elicited sensations
were from microstimulation. Interestingly, there are certain conditions where humans cannot differentiate between
microstimulation and actual sensory experience. Penfield and Perot (1963) found
that by electrically stimulating the grey matter of the temporal lobe in human
seizure patients, they could evoke hallucinations that normally occurred during
spontaneous seizures. Hallucinations ranging from listening to music to observing
a baseball game to being suddenly approached from robbers were reported. While
this study was not done on the somatosensory cortex like most other studies
described, it is significant in that evoked “sensations” were completely
realistic and indistinguishable from real life. It is evidence that
microstimulation has the potential to perfectly substitute for natural stimuli
in subjects other than seizure patients.  

Evidence that microstimulation cannot substitute for natural stimuli:

Though studies where microstimulation
is indistinguishable from natural stimuli exist, these results have only been
found in seizure patients. In both animal and human studies, doubts concerning
whether microstimulation could replace natural stimulation remain.

In animal microstimulation studies, one
theory is that animals are able to perform vibrotactile discrimination tasks not
by feeling sensations in their body, but by some other method that doesn’t rely
on actual sensory perception in the body. Perhaps they are able to directly
compare neuronal firing rates in the cortex. Romo et al. (2000) added an
important qualifier to the end of their paper, saying that it is unknown
whether 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 but
simply activates a task rule, such as ‘a stimulus is present’, which could
explain the almost identical neurometric and psychometric curves in the stimuli
detection task. Similarly, perhaps the mice in the Connor et al. (2013) study knew
what microstimulation meant in terms of go, go-no behavioral choices, but
didn’t actually perceive a sensation on their whiskers. If this is true,
microstimulation is not simply a poor substitute for natural stimulation. It cannot
substitute for natural stimuli. However, we know this theory is not true for
humans, because humans can communicate through language that they feel microstimulation
in their body.

A second theory is that electrical
microstimulation elicits somatic, bodily sensations, but that these sensations
are not completely natural.  In human studies, where subjects can verbally
describe the nature of stimulation, most elicited sensations fall into the gray
area between natural and unnatural. The tetraplegia patient studied by Flesher
et al. (2016) described 93% of sensations evoked by stimulating S1 area 3b as
“possibly natural”. He elaborated on the sensations, saying, “It’s almost like
if 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 swab
felt “totally natural”. Additionally, in the Ohara et al. (2003) study, the
vast majority of sensations categorized as evoking “movement” (as opposed
to other categories of sensation) were characterized as “unnatural”. These two
papers show that in human studies, where subjects can verbally describe quality
of a sensation, microstimulation cannot perfectly substitute for natural
stimuli.

As far as microstimulation is
concerned, there are three main hypotheses: 1) Microstimulation cannot
substitute for natural percept,  2) Microstimulation is indistinguishable
from natural percept, 3) Microstimulation is a poor substitute for natural
percept. Based on current research, I support the third hypothesis. While the
hallucinations reported by Penfield and Perot (1963) are tantalizing evidence
for the second hypothesis, all their patients suffered from temporal lobe
epilepsy, an extraordinary condition that no doubt made it easier to elicit
life-like hallucinations. Most microstimulation studies do not elicit
sensations anywhere near that level of realism and even in studies where
subjects do report “natural” sensations, the results are too inconsistent.
Animal studies are helpful in testing the methods of microstimulation but
unfortunately, we will never know the nature of evoked sensations in animals. We
can only observe when microstimulation produces the same behavioral reactions as
natural stimulation and speculate from there.

We must conduct more rigorous human
studies to better understand the complicated mechanisms behind
microstimulation. Borchers et al (2012) warned that stimulation
can have downstream effects that aren’t measured and in some cases, could lead
to reduced sensation (such as deafness and numbness). de Lafuente and Romo (2006)
studied some of these downstream effects, observing that the correlation
between neuronal activity and perceptual judgements increases as activity
travels from the somatosensory cortex to the frontal lobe. Without a better
understanding of electrical microstimulation mechanisms and without details about the exact stimulation threshold needed and the precise
electrode location required, we cannot work towards increasing the
realism of elicited sensations. 

If scientists are ever able to use
electrical microstimulation to elicit 100% natural sensations, it would spark
many philosophical questions. Perhaps the idea of existing as a “brain in a
vat” is not as far away as we once thought. Additionally, if electrical
stimulation has the power to evoke the same sudden feelings of familiarity or
flashes of past experiences that epilepsy patients experience during seizures,
this would have implications for understanding why non-epileptic humans
sometimes experience déjà vu. However, at this stage, most sensations elicited
by electrical microstimulation fall into a different category than that of everyday
stimuli. Electrical microstimulation still has a while to go before it can perfectly substitute
for natural, mechanical stimuli.