By idealism I mean where the mind is all there is. How do they take into account the fact that all the neuroscience that is there?
This is not a problem at all for idealists. Of course, idealism can be defined in various ways, from Plato to Berkeley to Fichte. But it is never just dreamland where "everything is in the mind" in the sense of no limits or correlations.
To take Berkeley, for example, he denies that there are, or that we can speak coherently about, "mind independent" objects. Obviously, we perceive things. But how would we know that they also have some other imperceptible existence that materialists call "real," beyond or behind what we perceive? Aren't the so-called materialists the real "idealists"?
In the case of "brain scans" we are once again just dealing with something we are perceiving. We may observe correlation, but why assume that this is also causation, the why and wherefore of consciousness? I don't feel, see, perceive, or experience these scans immediately or intuitively. Why think of them as "more real" than my interior ideas or feelings?
In fact, we could say that the widely adopted Copenhagen interpretation of quantum physics is a sort of idealism that agrees we cannot really talk validly about "observer independent" observations.
I find it quite hard, but also quite enjoyable, to convolute my modern, ordinary thinking into classic, idealist positions. But one way to start is the grapple with the unsatisfactory chicken-and-egg paradox we are left with by neuroscience: the brain is in the world and the world is in the brain.
Neuroscience may well have pragmatic and useful outcomes, but these too can be accommodated within some "idealist" framework. I hope this is helpful, but it is also quite simplified and everything I say above can easily be washed away in a storm of refutations.
Apart from the classic famous Leibniz's Mill Argument, Searle's Chinese Room Argument, and modern Tegmark's quantum factorization problem. One of my arguments for my understanding of idealism is as follows, which I call it Mirror Argument. With purely matter existence under reductive materialism (ie, science, or more specifically physics), it sounds absurd if a sensible person claims he or she "understands" anything outside his or her brain. Since brain is only a tiny part of the physical world, how can a hard cold mechanical or electrical part understand the whole? What's composing the brain cortex's "integrated information" are only reflections of the outside world, as in optics, the mirror's reflection will obey certain physical laws, but the mirror can only passively reflect, no vis-viva (a kind of true "active energy") present to apply this reflection to predict or judge another reflection in an intelligent way.
To put it in a more vivid and straight way, anyone holding reductive materialism who tries to defeat my argument is meaningless and also impossible, since matter alone without intentional vis-viva to try to argue certain statement sounds absurd...
Emergentism or Eliminativism (non-reductive materialism) are usually brought up to account for the apparent agency of matter (ie, brain), since neuroscience shows correlations of the brain to the mind. Emergentism was heavily criticized by modern realization of the reduction possibility of chemistry from quantum physics without added emergent laws. I personally prefer and held Panpsychic Idealism. Rational reasoning is the common ground between materialism and most idealism, a major difference lies in the bottom-up or top-down direction. In Leibniz's Panpyschic Idealism with Pre-determnined Harmony, matter/body is not denied its important agency attribute for its associated monad, actually body/matter is an indispensable part of the complete attributes of most monad types, excluding perhaps the ultimate simplest monad - the omni-monad which created all other monads. A body in this view is composed of infinite monads (u can imagine neurons) with a dominant soul monad, each was created and pre-programmed and "coordinates" according to its own laws about all of its internal attributes, much like object-oriented programming in software design. So in explaining the "seemingly apparent causality" within neuroscience (the kingdom of efficient cause), there's essentially no difference between above philosophical views, except ontologically (in the kingdom of final cause) materialism or dualism truly believe there're "interactions" or "influences" between mind and body while idealism truly believe monads' ontological existence as real simple substance and matter/body are just illusional phenomena of aggregates. Another philosophical difference manifested lies in the goal of life, non-idealism normally sets its goal to explain and thus control all perceived phenomena in an analytical reduction manner, while idealism usually strives to train one's mind to get rewired to gain evolved experiences to see the world more clearly as a whole like an artist. Modern physics QM shows matter are nothing but metric field, and GR with its famous "Hole Argument" teaches us spacetime is also an illusory base manifold quotiented from mass and affine connection metric fields, spacetime should not be viewed as real substance like a container, however, should be viewed like an external relational illusion via various ways of fields bundling . So what's really a "metric" as the quiddity of any field? It's essentially a kind of perceptible valuations, which seems support panpsychic idealsim. Also due to the possibility of gauge transformations, various different valuations (metrics) can be consistent with their same common underlying physical phenomenon.
In summary from my panpsychic rational idealism regarding relation of mind and brain (neurology), human mind is composed of many different types of perceptible, conscious, and sub-conscious monads, together with a soul monad providing ultimate self-awareness and vitality. So certain part of brain damage will affect certain types of "mind", like memory/language verb missing, personality change, etc. All these changes will have the effect to cloud and distort the soul monad, such that the original person seems like a different one. In some rare cases a person even can develop 2 conflict "minds" like schizophrenia. Under materialism or dualism, such person may be regarded as truly "having" 2 minds, while my idealism will still regard such person as having one monadic mind, since the soul exists stably as a whole like a diamond...
There are multiple views that label themselves "idealism", and it is important to clarify what they are. Classic 19th century idealism noted the centrality of the observer in all observations -- such that the existence of matter and the materiel world is only an inference, while awareness and apprehension are indisputable. Modern materialists try to obscure the nature of data and observation, as they undercut the coherence of materialism, but this sort of "matter may be real, but ideas are inseparable from it" idealism isn't what you asked about.
Another form of idealism, popular today with mathematicians and some theoretical physicists, is to extend physical reductionism further to math, such that matter is just a manifestation of quantum math. Math has no mass or location, and is not plausibly material, so this math reductionism is an abstraction/platonic form of idealism. No math reductionist would question the causal effect of neurological incisions on the mind, as opposed to the mind having causal effect on the incisions, so this too is not the form of idealism you are concerned with.
Yet another view labeled idealism by many of its adherents is pan-psychic neutral monism. In this view, the coupling between matter and mind is mandated by some fundamental other substance, hence mind and matter are forced to work in parallel, and there is no actual causation in either direction from matter to mind, or mind to matter.
What you are asking about is the mind-over-matter idealism of Christian Science, the New Age movement, and Qi-Gong. All of these movements, as well as the Perennial Philosophy, hold that mind controls matter, and the direction of causation in neurosurgery is the opposite of that assumed by materialists.
If one examines the views of these idealists, one pretty quickly discovers that most of them do NOT think that the universe is infinitely malleable to a specific individual's mind. Instead, most agree that our universe is stubborn, and not easily persuaded by mental causation. There are two explanations for this I have found among idealists:
It is not OUR minds that form the universe, but a godlike mind, a Mind-at-Large. An individual may be able to minorly influence the local manifestation of the Mind-at-Large, hence spiritual and miracle cures to ailments, and occasional feats of miraculous strength occur at times in extremis. A disciplined mind who practices can achieve greater control -- hence yogi and qi-gong masters. Part otf this theory holds that the info of the Mind-at-Large is accessible to us at all times, and our brains actually act as an information filter to prevent information overload, NOT as sensors. This set of views can match most of the brain damage etc neurology data. The Mind at Large in this approach need not have personality nor agency -- Taoism holds by an underlying force that has neither.
Collaboratively created reality -- the second main view I have encountered is that our universe is created by an accumulation of all of our unconscious expectations. Therefore, while an individual may have the ability to affect some minor local phenomena with mental agency, this is a rarity. In this view, to change the world requires changing the expectations of a critical mass of individuals. This view is held by many of the New Age Movement self-help advocates. I also find it to be held by many pagans -- who extend the "created by mass expectation" thesis to their Gods as well.
Under either option, a world created by a Mind-at-large, or by collaborative expectations, could very reasonably show an apparent dependence of an individuals mental function on neuro-surgical procedures. It would only be at a more fundamental level that their idealism would manifest.
However, it is only for MOST neurology/mind interactions that this filter model works. In it, damage to the brain should cause damage to expressed thoughts and actions, but one's fundamental mind should not be affected by neurology, only the info coming in to to it, and its output. However, the effects of mind altering drugs to degrade one's thinking, and cause unconsciousness, and the consequences of neural damage to one's personality, or fundamental mental skills such as recognizing verbs, or processing the left side of one's visual OR mental field -- these show neural effects extend beyond the information/control consequences that this model predicts. Pure causal idealism IS in conflict with some aspects of the neurological data we have on minds.