An epidemic of "Necessary and Sufficient" neurons

A great deal of neuroscience has become “circuit cracking.”
— Alex Gomez-Marin

A miniaturized holy grail of neuroscience is discovering that activation or inhibition of a specific population of neurons (e.g., prefrontal parvalbumin interneurons) or neural circuit (e.g., basolateral amygdala → nucleus accumbens) is “necessary and sufficient” (N&S) to produce a given behavior.

from: Optogenetics, Sex, and Violence in the Brain: Implications for Psychiatry ¹ 


In the last year or so, it has become acceptable to question the dominant systems/circuit paradigm of “manipulate and measure” as THE method to gain insight into how the brain produces behavior (Krakauer et al., 2017; Gomez-Marin, 2017). Detailed analysis of an organism's natural behavior is indispensable for progress in understanding brain-behavior relationships. Claims that optogenetic and other manipulations of a neuronal population can demonstrate that it is “N&S” for a complex behavior have also been challenged. Gomez-Marin (2017) pulled no punches and stated:

I argue that to upgrade intervention to explanation is prone to logical fallacies, interpretational leaps and carries a weak explanatory force, thus settling and maintaining low standards for intelligibility in neuroscience. To claim that behavior is explained by a “necessary and sufficient” neural circuit is, at best, misleading.

The latest entry into this fault-fest goes further, indicating that most N&S claims in biology violate the principles of formal logic and should be called ‘misapplied-N&S’ (Yoshihara & Yoshihara, 2018). They say the use of “necessary and sufficient” terminology should be banned and replaced with “indispensable and inducing” (except for a handful of instances). ²

modified from Fig. 1A (Yoshihara & Yoshihara, 2018). The relationship between squares and rectangles as a typical example of true necessary (being a rectangle; pale green) and sufficient condition (being a square; magenta) in formal logic.


N&S claims are very popular in optogenetics, which has become a crucial technique in neuroscience. But demonstrating true N&S is nearly impossible, because the terminology disregards: activity in the rest of the brain, whether all the activated neurons are “necessary” (instead of only a subset), what actually happens under natural conditions (rather than artificially induced), the requirement of equivalence, etc. Yoshihara & Yoshihara (2018) are especially disturbed by the incorrect use of “sufficient”, which leads to results being overstated and misinterpreted:

The main problem comes from the word ‘sufficient,’ which is often used to emphasize that artificial expression of only a single gene or activation of only a single neuron can cause a substantial and presumably relevant effect on the whole process of interest. Although it may be sufficient as an experimental manipulation for triggering the effect, it is not actually sufficient for executing the whole effect itself.

And for optogenetics:

Rather, the importance of ‘sufficiency’ experiments lies in demonstrating a causal link through optogenetic activation of neurons... Thus, words such as triggers, promotes, induces, switches, or initiates may better reflect or express the desired nuance without creating such confusion.

Y & Y (2018) aren't shy about naming names in their Commentary, and even say that misapplied-N&S has generated unproductive and misleading studies that offer no scientific insight whatsoever. Although one could say that N&S has a different meaning in biology, or is merely a figure of speech, such strong statements have consequences for the future directions of a field.

Thanks to BoOrg Lab for the link to Gomez-Marin.

Footnotes

¹ “...neurons necessary and sufficient for inter-male aggression are located within the ventrolateral subdivision of the ventromedial hypothalamic nucleus (VMHvl)...”

² One of the instances uses the old discredited “command neuron” concept of Ikeda & Wiersma (1964). They call it A‘Witch Hunt’ of Command Neurons and note that only three command neurons meet the true N&S criteria (one each in lobster, Aplysia, and Drosophila).


References

Gomez-Marin A. (2017). Causal circuit explanations of behavior: Are necessity and sufficiency necessary and sufficient? In: Decoding Neural Circuit Structure and Function (pp. 283-306). Springer, Cham.  {PDF}

Krakauer JW, Ghazanfar AA, Gomez-Marin A, MacIver MA, Poeppel D. (2017). Neuroscience Needs Behavior: Correcting a Reductionist Bias. Neuron. 93(3):480-490.

Yoshihara M, Yoshihara M. (2018). 'Necessary and sufficient' in biology is not necessarily necessary - confusions and erroneous conclusions resulting from misapplied logic in the field of biology, especially neuroscience. J Neurogenet. 32(2):53-64.