Before functional neuroimaging techniques like PET and fMRI became common, what we knew about which parts of the brain did what came from
neuropsychology. This is the study of patients with specific injuries to the brain, and the basic logic of the field is that if you have a patient with a lesion in area A who can't do task 1, then area A is involved in performing task 1. It gets a little more complicated than this, as you search for
double dissociations, etc, but this is essentially it.
A surprising amount of what we think we know about the brain comes from neuropsychology; famous case studies such as
HM have informed theories of memory so that they include short and long term storage, which are separable, and so on. These case studies can have a profound effect on research; my favourite story, though, was about a memory researcher who had a skiing accident and temporarily developed retrograde amnesia - he couldn't remember anything except that there was this guy in Connecticut (HM) who couldn't remember things either!
I always enjoyed classes in neuropsychology; the case studies are always fascinating. But they are deeply limited in what they can actually tell us about the brain. First, they are typically single patient case studies, which restricts how general the conclusions are. Second, they are data from damaged brains; the fairly linear assumption that some localised function has been subtracted out is simply not true, and the damage will have had complex effects on distributed functional networks.Third, the damage is never straight-forward, because these almost all come from accidents or strokes (HM's surgery being a rare example of more detail being known). This has not stopped the field being very excited by these cases, though, and from basing a lot of theory on these patterns of deficits.
In movement research, the most famous neuropsychology case study is
Patient DF She suffered bilateral damage along the
ventral stream of visual processing (James et al, 2003). The effect was visual form agnosia: she is able to control her actions with respect to objects, but cannot describe or recognise these objects verbally. Crucially, her accident did not damage her parietal lobe; specifically, the
dorsal stream of visual processing was left intact. These two streams are well defined anatomical pathways leading out of primary visual cortex, and were first described by Ungerleider & Mishkin, 1982). DF's pattern of deficits led Mel Goodale and David Milner (Goodale & Milner, 1992) to suggest functional roles for these streams. The ventral stream, they suggested, was for
perception - things like object and scene recognition. The dorsal stream, in contrast, was for
perception-for-action, and used visual information for the online control of action. This
perception-action hypothesis has been hugely dominant in the field, and the theory rests heavily on DF's shoulders.
Recently, Thomas Schenk (2012a) published some data which claims to show that DF's visually guided reaching is
not normal if she doesn't have access to haptic feedback about the object. His data suggests that the only reason she succeeds at reaching while failing judgment tasks is that haptic information is only normally available in the former case. If correct, this is actually quite a shot across the bow of the perception vs perception-for-action work; naturally Goodale and Milner don't buy it, and have published a reply to which Schenk has then replied.
An invitation
I like seeing these arguments happen in
the literature; but to be honest, the time scale is too slow. Schenk
publishes, then Milner et al get to reply and Schenk gets right of reply
to that. They may or may not iterate again and it's always left as 'we
agree to disagree'. But these critiques have answers, and I think a blog
comment feed might be the right place to work through the various
cycles of suggestions and rebuttals until the obviously wrong things
have been weeded out. It would also provide a place for other interested
parties to weigh in. So if Schenk, Milner and Goodale
(and anyone else!) feel like using the comments for this post or another made
to purpose to bang around ideas until an obvious experiment or analysis
pops out, please feel free!