The act of looking at an object is a complex process, so too is the act of reaching out.
There is a phenomenon known as ‘proprioception’ – basically the brain maintains a mental image of the body in space. In the moment before we physically reach out for an object the brain imagines or, perhaps more accurately, rehearses the process of reaching out. According to Dijkerman et al. (2009), “neuroimaging studies investigating the neural basis of mental simulation of movements suggest that they engage largely similar cortical areas to actual performance of movements.” In this case Dijkerman and co-authors are interested in the therapeutic value of deliberately imagining physical action as a part of rehabilitation following motor impairments (ibid). The idea can be extrapolated to normal acts of reaching out: the brain rehearses the act of reaching out using the bits of the brain that will actually control the reaching out when it happens. Again, this neural activity occurs below the level of conscious awareness.
When we reach out though, we are generally reaching to/for someone or something (okay – excepting martial arts, dance and various forms of symbolic gesturing). I’m interesting in reaching out for objects. So, we’ve seen the object and we reach out for it – it seems that the seeing is crucial. Ietswaart et al. (2001) were interested in better understanding that reaching out, through studying patients with limb apraxia. The model that Ietswaart used was one derived from Milner & Goodale – that there are two ‘streams’ or ‘pathways’, ventral and dorsal, for dealing with visual data. Ietswaart writes about the “stored perceptual representations from the ventral stream of the appearance of specific actions and their associated meanings” and “the sensorimotor control processes of the dorsal stream, and associated motor cortices of the frontal lobes.” Put more simply we have both a memory of learned movements, which we can draw on, and we can carry out acts we have no memory for through on-line (visuo-motor) feedback control of our arms and hands. Ietswaarts findings (and the other research she cites) suggests that the act of reaching out requires communication between these two streams.
This contains another interesting concept for the role of objects in re-evoking memory. Ietswaart & co. cite other authors to talk about ‘motor memories’ or, more accurately, “spatio-temporal representations of learned movements”. It may be interesting to pursue those references to find out how those spatio-temporal representations work and where they are stored.
Anyway, we reach out for an object – reaching out is a familiar action and (up to the point we get hold of the object) relies on learned, motor memories. (This is not always the case, Dijkerman et al. point out that “recover of motor function after brain damage often involves a redistribution of activity within the cortical network [with early stage attempts involving] widespread activation in a bilateral cortical network” (2009).
At the last moment we have to grab the object – this is where the reference from Cavina-Pratesi et al. (2009) comes in. Cavina-Pratesi points to earlier work indicating that while the reaching out element is based on spatial information, the shaping of the hand is “guided principally by the geometric properties of the object” – both are based on different elements of the available visual information. Personal experience suggests that, in addition to this, the person reaching out makes judgements about the weight and feel of the object (based on visual information and prior experience of similar objects) to prepare the muscles in the arm(s) to carry the presumed weight. (Hence the surprised reactions when an object is much lighter or heavier than expected, e.g. cast glass or hollow ceramic objects.) Cavina-Pratesi states that, “although it is accepted that the two components must be somehow mutually coordinated, there is now extensive evidence that the transport component and the grip component are each controlled online by dedicated visuomotor networks within the posterior parietal cortex, in association with linked systems in the premotor cortex.”
(The 3 references were very kindly sent to me by Ietswaart, who is based at Northumbria University (Newcastle-upon-Tyne, UK), and who is co-author on all 3 papers.)
Cavina-Pratesi, C. et al. (2009) ‘Impaired grasping in a patient with optic ataxia: Primary visuomotor deficit or secondary consequence of misreaching?’ Neuropsychologica, 48, 226-234
Dijkerman, H.C., Ietswaart, M. & Johnston, M. (2009) ‘Motor imagery and the rehabilitation of movement disorders: an overview’ (book chapter, in preparation)
Ietswaart, M. et al. (2001) ‘Memory-driven movements in limb apraxia: is there evidence for impaired communication between the dorsal and ventral streams?’ Neuropsychologica, 39, 950-961