Previous experiments about goal-directed reaching mainly focused on the space in close proximity to the body, and therefore within reach distance. One aim of this thesis is to extend the current research about movement planning and execution beyond the reachable space. There is evidence that neural processes might be categorically different between the space near and far from the body.
Patients with unilateral lesions in the middle cerebral artery territory can suffer from a visual neglect for which they have deficits in attending the contralesional side of the visual space relative to an object or to the own body (Li & Malhotra, 2015). A typical experiment to identify unilateral spatial neglect is the line bisection task in which patients are asked to mark the middle of a horizontal line. Patients will considerably misjudge the middle in the opposite direction of the side which is affected by the neglect. For example, a patient with a lesion on the left hemisphere would show a right-sided neglect. That means, the patient can not properly attend to the right part of the line and mark the middle too far to the left. Halligan and Marshall reported a patient with lesions in the right parietal cortex, temporal cortex and some subcortical structures (cerebral peduncle, pons and internal capsule) after a stroke that revealed a left-sided visual neglect in the line bisection task (Halligan & Marshall, 1991). However, when performing the task with a laser pointer at a distance of 2.44m, the patient performed significantly better. This indicates that the brain lesion impacted neural circuitry responsible for spatial attention in near but not far space. Another patient with lesions in the right temporal and occipital cortex showed a left-sided visual neglect at a distance of 3.5m but not for tasks on a desk in front of (Vuilleumier et al., 1998). This suggests that different brain areas encode the space near or far from the body. Yet, another study reported a patient with a near-space specific left-sided visual neglect resulting from a widespread lesion in the right hemisphere affecting frontal, temporal, parietal and occipital cortex and subcortical structures (basal ganglia and insula) (Berti & Frassinetti, 2000). While the patient performed the line bisection task with a laser pointer at 1m distance with little displacement error, the error was higher when performing the task at 0.5m distance.
However, when using a stick in contrast to a laser pointer, the error was high at the far distance as well. This suggests, that this “near space” does not represent a defined distance from the body but rather reflects the space we can interact with which enlarges when using a tool such as a stick. Since lesions resulting from a stroke involve multiple areas, it is difficult to identify
1.1 Neural encoding of near and far space 5
the brain areas involved in far or near space processing. One study used repetitive transcranial magnetic stimulation on different cortical areas in the right hemisphere of healthy humans to induce far-space specific visual neglect (Bjoertomt et al., 2002). The researchers could induce a near-space visual neglect when stimulating the posterior parietal cortex and a far-space visual neglect when stimulating the ventral occipital lobe suggesting a dorsal-ventral segregation for near and far space processing. And again, another group showed that near-space neglect induced by repetitive transcranial magnetic stimulation can extent to far-space when using a tool (Giglia et al., 2015).
Further evidence that the space near the body is differently processed than the space far away comes from early electrophysiological studies with non-human primates. They found in the ventral premotor cortex (Fogassi et al., 1996; Graziano & Gross, 1994; Rizzolatti et al., 1981) and intraparietal sulcus (Graziano & Gross, 1994) neurons with tactile and visual receptive fields for which the visual receptive fields are anchored around their tactile receptive field on a part of the body. Those receptive fields cover a space anchored to the body that does not exceed the reachable space. This space is termed peripersonal space (Rizzolatti et al., 1981). Similar to the neglect studies, researchers could show that tool-use extends the receptive field of such neurons covering the enlarged space the monkey is now able to interact with (Iriki et al., 1996;
Maravita & Iriki, 2004). The tool can be seen as a functional part of the body and in this respect the peripersonal space is considered to reflect a representation of the own body (Blanke et al., 2015; Maravita & Iriki, 2004). This view is supported by the rubber-hand illusion experiment which was originally studied with humans (Botvinick & Cohen, 1998). A fake arm is placed on top of the subject’s occluded arm. When the fake arm and the occluded real arm received tactile stimulation simultaneously, the subjects reported that they felt the touch on the rubber arm as if it was their real arm. An experiment with monkeys showed that neurons in area 5 of the parietal cortex encode the visual location of a fake but realistic looking arm (Graziano et al., 2000). The researchers also tested a few neurons which did not respond to the fake arm in an experiment like the human study. And indeed, after simultaneous tactile stimulation most of the neurons responded to the location of the fake arm. The hand, body, real hand or fake hand do not relate to a single peripersonal space. Rather is the peripersonal space body-part specific. At least three different peripersonal spaces are known relative to respective body parts (Figure 1.1A): the peri-hand, peri-trunk and peri-head space (Blanke et al., 2015; Cléry et al., 2015).
Given that the peripersonal space is originally defined by the extent of multimodal recep-tive fields, the interaction of different sensory modalities (crossmodal interaction) within the peripersonal space should be higher than outside of. One way to test this in healthy subjects is by asking participants to discriminate a stimulus, often tactile, as fast as possible in one of two locations while ignoring a second stimulus, often visual (Spence et al., 2004b). Participants react faster if the visual distractor is congruent, i.e. at the location of the tactile stimulation, than if it is incongruent, i.e. at the other location. This is called the crossmodal congruency effect (CCE).
In relation to the peri-hand space, tactile stimuli are delivered on index finger or thumb, and
hand PPS
head PPS trunk PPS
extrapersonal space
hand PPS
A B
Figure 1.1:Peripersonal space. A) The peripersonal space covers our immediate surrounding. At least three different body-part centered peripersonal spaces exist: hand (peri-hand space), head (per-head space) and trunk (peri-trunk space). The space beyond the peripersonal space is called extrapersonal space. B) The peripersonal space can change with goal-directed reaching. When reaching to an object, the peri-hand space (red) expands to the object with onset of the movement.
when the distractors are placed further away from the hand, the crossmodal interaction decreases (Spence et al., 2004a). Usually, tactile stimuli are applied to both hands but the visual distractor is only placed at one hand. The hand without distractor serves as a baseline. This way it was shown that the CCE “follows” the hand when crossing arms (Spence et al., 2004a). This is in accordance with the electrophysiological results of bimodal hand centered receptive fields in the macaque brain. Further similarities were found when the CCE was tested on a tool (Holmes, 2012; Maravita et al., 2002) or a rubber hand (Maravita et al., 2003; Pavani et al., 2000). This suggests that the crossmodal congruency effect is a valid indicator for the extent of the peri-hand space.
Apart from the view that the peripersonal space reflects a representation of our body, there is a second (not opposing) view. Since the peripersonal space is constrained to the reachable space but extents with tool use, it is considered to be related to the encoding of interactions with objects in our environment (Brozzoli et al., 2011; Rizzolatti et al., 1997). However, very little is known about how the peripersonal space is modulated during goal-directed reaching. One study investigated the CCE during goal-directed reaching and grasping, and found an increase with onset of the hand movement (Brozzoli et al., 2010, 2009). According to their interpretation, the peri-hand space expands towards the reach goal (Figure 1.1B) (Brozzoli et al., 2014).
Those results and the fact that tool use extends the peripersonal space suggest that the peripersonal space reflects the space which we can interact with. That the brain contains such a representation is supported by electrophysiological studies investigating mirror neurons in the monkey ventral premotor cortex PMv (Bonini et al., 2014; Caggiano et al., 2009). Mirror