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.
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!
Schenk (2012a): Patient DF and haptic feedback
The classic result from DF that inspired the Goodale and Milner account has two parts (a dissociation). When DF is asked to reach for an object, she can do so well and she shows appropriate pre-shaping of her hand which scales correctly with the size of the object. This suggests she has intact visual perception-for-action. However, if you ask her to judge the size of the object without reaching for it, she cannot do it; she has selectively impaired visual perception. Schenk ran the following experiments on DF and to demonstrate that her unimpaired reaching is not because of intact visual perception-for-action, but rather because of haptic feedback from real objects.
'Perception': these are the tasks DF typically fails due to her visual form agnosia, and she fails them here too.
- Size discrimination: Choose the largest of two objects
- Manual estimation: Judge the size of an object by shaping your hand correctly
- Standard grasping: Reach to grasp an object in the mirror and there is actually an object there (vision + haptic information match)
- Grasping without haptic feedback: Reach for the reflection but there is no actual object there.
- Grasping with intermittent haptic feedback: there was an object present on half the trials; these were scattered randomly throughout the session and a light cued participants when the object would be present.
- Grasping with dissociated positions: participants saw an object in the middle location, were asked to reach for an object at the far position, and there was a real object there.
|Figure 1. Grip performance for controls (open circles) and DF (red diamonds).|
Milner, Ganel & Goodale (2012)
Unsurprisingly, Milner et al (2012) do not agree that these data cast doubt on the perception-action hypothesis about the function of the dorsal and ventral streams. They make the following criticisms:
- They suggest that "so-called 'haptic feedback'" (to quote the paper) from trial n could only inform a reach on trial n+1 if the objects were the same size in both trials; object size was randomised across trials, however.
- They then claim that Schenk's interpretation means he thinks DF's reaches are prepared on the basis of previous haptic, rather than current visual information. Therefore, they suggest, Schenk must make 'the inescapable prediction' that a reach on trial n+1 should be appropriate for what happened on trial n, regardless of what is presented on trial n+1.They allow that there may be some 'minor intrusion' of haptic information from previous trials.
Again unsurprisingly, Schenk (2012b) does not agree with Milner et al's evaluation.
- He claims that the Milner et al critique assumes that prehension requires the visual computation of an object's size. He then cites recent work by Smeets & Brenner (1999) who claimed to show that prehension involves the independent targeting of the thumb and forefinger, and thus you don't need object size.
- He then suggests that DF is generally able to reach successfully because she has access to the necessary egocentric information (in hand-centred coordinates) about the location of object edges. This information requires regular calibration (Bingham et al, 2007) to remain accurate.
- He therefore predicts that if DF has egocentric information about the object, and this information has been calibrated recently, she can reach successfully, otherwise she fails. His 2012a data then support this pattern.
- Regarding the pantomime problem: Schenk tested this in Task 5 with the trials with no objects. DF knew there would be no object on these trials (the light cue) but still produced normal reaches because of the calibration on other trials.
There is a lot that is weird about the replies. Milner et al make some odd claims, and Schenk goes to strange places in his defence. Let's address those first.
1. "So-called 'haptic feedback'"
Milner et al want to keep claiming that DF reaches on the basis of current visual information, and not on the basis of previous haptic information. But there's a problem for them - this is the claim Schenk's data actually refutes! So they make an odd move, and simply claim that earlier haptic information does not affect reaches, and that even if it could, it won't here because the size changes from trial to trial.
However, Coats, Bingham & Mon-Williams (2008) have demonstrated (using a mirror rig similar to Schenk's) that if you systematically change the size of the grasped object while leaving the visual object the same size, people happily recalibrate their reach actions and change their grip apertures. Bingham et al (2007) have also shown that even occasional calibration allows stable reach behaviour to persist; calibration lasts some time. So even when the visual size remains unchanged, people's grip behaviour reflects the haptic calibration of the visual perception of size and if this calibration changes, so does grip.
Milner et al's second point - that haptic feedback can't help because the object size changes randomly - is actually addressed by Schenk's control data, which shows that neurologically intact people can happily scale their grips appropriately under these conditions (albiet slightly more noisily).
2. Reaching and the need for visual size
Schenk centres his reply on the idea that Milner et al assume you need to compute (or perceive) the size of objects in order to scale your grasp. He then cites Smeets & Brenner (1999) who claim that instead, you simply control your thumb and forefinger independently and bring them into alignment with the edges of the object.
The problem here is that Smeets & Brenner's work is highly controversial, and in fact more recent work from Mon-Williams & Bingham (2011) tested the predictions of this account in great detail and found no support for this claim. Instead, they showed that the unit of control is an opposition axis (Iberall, Bingham & Arbib, 1986). This is the space between the thumb and forefinger, and Mon-Williams & Bingham (2011) demonstrated that prehension is about aligning this space with the object. You do still therefore need to perceive object size, specifically the maximum object extent. I'll blog this paper in more detail sometime, it is a master class in affordance research.
I think Schenk had it basically right in the first paper; the explanation for his data is that in tasks 3 and 5, DF has sufficient access to haptic information about the object's size to allow her to bypass her visual perceptual deficit. She can therefore successfully reach to grasp. In all other tasks, she can't go round the problem and she fails. This suggests that her visual deficit is not simply restricted to 'perception'; the visual system involves both anatomical streams working in concert and these are not functionally independent of each other. What Schenk needs to do is treat haptic information as perceptual information for size in it's own right, not simply feedback or an 'egocentric cue'. DF has unimpaired access to this information and when it's available, she can reach-to-grasp.
Bingham, G. P., Coats, R., & Mon-Williams, M. (2007). Natural prehension in trials without haptic feedback but only when calibration is allowed. Neuropsychologia, 45, 288 –294. Download
Coats, R., Bingham, G.P. & Mon-Williams, M. (2008). Calibrating grasp size and reach distance: Interactions reveal integral organization in reaching-to-grasp movements. Experimental Brain Research, 189, 211-220. Download
Goodale, M. A., Jakobson, L. S., & Keillor, J.M. (1994). Differences in the visualcontrol of pantomimed and natural grasping movements. Neuropsychologia, 32(1), 1159-1178.
Goodale, M. A., & Milner, A. D. (1992). Separate visual pathways for perception and action. Trends in Neuroscience, 15(1), 20 –25. Download
Iberall, T., Bingham, G. P., & Arbib, M. A. (1986). Opposition space as a structuring concept for the analysis of skilled hand movements. In: Heuer H, From C (eds) Experimental Brain Research Series 15. Springer, Berlin, pp 158–173. Download
James, T.W., Culham, J., Humphrey, G.K., Milner, A.D, & Goodale, M.A. (2003). Ventral occipital lesions impair object recognition but not object-directed grasping: an fMRI study. Brain 126, 2463–2475. Download
Milner, A., Ganel, T., & Goodale, M. (2012). Does grasping in patient D.F. depend on vision? Trends in Cognitive Sciences DOI: 10.1016/j.tics.2012.03.004
Mon-Williams, M. & Bingham, G.P. (2011). Discovering affordances that determine the spatial structure of reach-to-grasp movements. Experimental Brain Research, 211(1), 145-160. Download
Schenk, T. (2012a). No Dissociation between Perception and Action in Patient DF When Haptic Feedback is Withdrawn. Journal of Neuroscience, 32 (6), 2013-2017 DOI: 10.1523/JNEUROSCI.3413-11.2012
Schenk, T. (2012b). Response to Milner et al.: Grasping uses vision and haptic feedback Trends in Cognitive Sciences DOI: 10.1016/j.tics.2012.03.006
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