Two Parts to a Double Dissociation

Two Parts to a Double DissociationOrganisms, at one level, argon apparently collections of parallel systems that are potentially independent, although ordinarily interactive. (Weiskrantz, 1990)The argument of fragmentize optic processing teems is a long and turbulent one, which stems largely from Ungerleider and Mishkins (1982) early work with monkeys. Following this explore, Mishkin, Ungerleider, and Macko (1983) suggested twain well unwraps of processing. They characterised the dorsoventral stream as the what route, used to analyse ocular characteristics of object glasss, and the abaxial stream as the where stream, which calculated the spatial relations of the object. However, in 1992, Milner and Goodale proposed a reinterpretation of the dual pathway model (Goodale Milner, 1992, 2004 Milner Goodale, 1993, 2006). In this new model, the ventral stream was concerned with the optical cognizance and processing of object form and object recognition, to transform optic in formation into a perceptual representation of the world (Goodale Milner, 1992). Importantly, it also encoded spatial relations of objects in an allo-centric sense. This allows us to think about our world, its objects, and the placement of objects. In contrast, the dorsal stream was involved in the controlling actions interacting with the goal object. The dorsal stream calculates spatial relations in an ego-centric view, using accurate and precise measurements. In much general terms, it great deal be stated in short, that this new model suggested that differences between the deuce streams should be evaluated, not in terms of visual inputs, but as the output systems which the two streams serve. Both streams receive the same visual information, but they interpret it in divergent ways (Milner Goodale, 2008).In their study of this model, researchers searched for the ideal complementary double disassociation which would unequivocally hold back Milner and Goodales claim that these s treams are completely separate entities, which receive visual information, and then interpret and react to this information in very different ways. They engraft support for these claims by the flagship double dissociation studies using neuropsychological patients. The key conditions in these case studies and experimental research studies are Optic motor ataxia (OA) and Visual Form Agnosia (VA). These neuropsychological disorders are, to Milner and Goodale, the quintessential case for a double dissociation.The term of double dissociation is an elusive idea in neuropsychology, with clear and condensed dissociations unwieldy to come by. In a single dissociation, damage to a special(a) brain region interrupts one subprogram, but not another function. This implies that these two functions are independent of each other in some form. The to the highest degree commonly referenced single dissociation is the condition VA, in which the patient perceptual abilities are impeded, but not th e visuomotor abilities.Furthering on from single dissociations, interest has turned to finding double dissociations. These double dissociations, as originally described by Teuber (1955) are seen as powerful tools in neuropsychological research, to discover and study the separate functional modules and to strengthen the evidence for a single dissociation. However, double dissociations can be very difficult to prove, as to be a true double dissociation it must be shown that two different external manipulations will hazard two patients differently. That is, the first manipulation will affect patient A, but not B, whereas the second manipulation will affect patient B, but not A. This can be used as a starting block to make inferences of the modular functions of brain areas. The dual visual systems double dissociation of OA and VA, or perception of objects with mis-reaching and inability to perceive with sure-fire clasping became the workhorse of Milner and Goodales model. They based much of their early findings upon studies carried out with patient D.F which showed a single dissociation (James, Culham, Humphrey, Milner, Goodale, 2003 Goodale, Milner, Jakobson, Carey, 1991).Patient D.F. is the most researched neuropsychological patient in the study of dual visual streams, and it is from research carried out with her that led to the fruition of Milner and Goodales model (Goodale et al., 1991). D.F. suffered bilateral lesions of the occipito-temporal cortex, considered to be the ventral stream area, which resulted in a profound case of Visual Form Agnosia (Milner et al, 1991). That is, she was incapable of visually perceiving the form of objects and barely she could accurately make visually guided movements and grasp objects (James et al., 2003 Goodale et al., 1991). It was argued that this research indicated that D.F.s visuomotor skills were left intact, implying firstly, that there was evidence for a single dissociation, and secondly, that D.F. could show pu re visuomotor skill with out the interference of perception. In other words, she could show what the dorsal stream in Milner and Goodales model was capable of achieving (Milner et al., 1991).This original study was quickly followed by a stream of research which investigated the visuomotor capabilities of D.F (Goodale, 1994b Milner et al, 1991) and a second VA patient S.B. (Dijkerman, Le, Demonet, Milner, 2004). The kick upstairs research illustrated that D.F.s visuomotor skills allowed her to scale her becharm and orientate her wrist correctly, similarly to controls (Milner Goodale, 1995). In matching orientation undertakings D.F. failed, appearing to choose orientations at random, yet when asked to reach towards a slot and post an item she performed at a similar level to controls (Goodale et al., 1991). Studies illustrated her ability to use visual information involving the orientation and shape of a particular object for online corrections of hand movements and in an object seizing task for regular shapes, (Dijkerman, Milner, Carey, 1996 Carey, Harvey, Milner, 1996), and irregular shapes (Goodale et al., 1994c). These findings were later replicated with S.B. (Dijkerman, McIntosh, Schindler, Nijboer, Milner, 2009 Dijkerman et al., 2004). The interpretation given to D.F.s visuomotor abilities suggested that the undamaged dorsal stream was controlling the visuomotor abilities, without the input of the damaged ventral stream. This was a powerful argument for Milner and Goodales model as it emphasised the functional dissociation within the visual system.On the opposite side of this dissociation, researchers studied patients (I.G. and A.T.) with Optic Ataxia (OA) a visuomotor disorder. This involves gross mis-reaching for visual targets, usually most severe in the peripheral visual field, can manifest in the contralesional visual field and the contralesional hand (Perenin Vighetto, 1988). However, patients can identify objects formulaly un resembling pa tient D.F., OA patients can discriminate the size, shape, and orientation of objects. However, these patients fill bar in grasping objects correctly or in a functionally correct manner. OA patients will not appropriately scale their grip during reaching they reach their finger grip too wide, and close it once they reach contact with the object (Jeannerod, Decety, Michel, 1994). In bring inition, their reaching duration is increased, their peak velocity is lower than controls, and they put their fingers when they wear to visually guide their hand towards a slit (Gra et al., 2002). Similarly, in reaching tasks with target jumps, both A.T. and I.G. failed to show online adjustment of movement like healthy controls (Pisella et al., 2000 Gra et al., 2002). This indicates a feed forward and feedback deficit in OA. More barely, OA patients do not possess the capabilities to quickly alter their movements they rely on the involvement of slower and later visual and motor feedback.Howe ver, does all this research lead onto the conclusion of a classic double dissociation? Milner and Goodale argue that no clearer evidence could be shown one condition (VA) leads to inability to perceive items, yet can act on these items, and the other condition (OA) shows an inability to grasp an item, and yet they can perceive all their features. The problem is, this case of double dissociation may not be as straight forward and concise as Milner and Goodale assume. There is a new stream of research showing the exceptions and difficulties in the dual visual system assumption.A classic dissociation calls for one function to be within normal performance range and the affected function to be far below normal performance (Shallice, 1988). In relation to D.F.s visuomotor abilities, much recent research has highlighted difficulties in claiming a classic dissociation. Although D.F. does manage to grasp items in most cases, this is not to the level of normal range she makes semantic errors in grasping tools in non-functional ways (Carey, Harvey, Milner, 1996). However, she also fails to grasp neutral laboratory blocks using the most comfortable grasp (Dijkerman, et al., 2009), and she fails to complete visuomotor guiding or grasping tasks with any shapes of significant complexity (Goodale et al., 1994a Carey et al., 1996 Dijkerman et al., 1998 McIntosh, Dijkerman, Mon-Williams, Milner, 2004). In fact, more recent research has found restrictions to D.F.s grasping abilities, showing that she does not automatically select a grip posture which minimises awkward and uncomfortable grasps, like control subjects (Dijkerman et al., 2009).Furthermore, make up in successful completion of simple tasks, D.F. may not use the same visual cues that healthy controls use. When prisms were used to perturb D.F.s flock, it was found that D.F. relies almost exclusively on vergence angle and vertical gaze for establishing object distance in reaching tasks (Mon-Williams, McIntosh, Mil ner, 2001 Mon-Williams, Tresilian, McIntosh, Milner, 2001). In fact there have been reports of the daily difficulty in carrying out actions for VA patients, namely S.B. showing at times greater peripheral misreaching than OA patients (L et al., 2002 Pisella, Binkofski, Lasek, Toni, Rossetti, 2006). VA patients use remuneration techniques such as, moving their head to focus the target in central muckle and slowing their goal directed movements (Rosetti, Vighetto, Pisella, 2003 Pisella et al., 2006). Dijkerman and colleagues found that patient D.F. could perform a grasping task well when she could use binocular viewing, even when her head position was fixed on a chin rest. However, she could not complete the task under monocular viewing unless she could tilt her head to compensate (Dijkerman et al., 1996). Specifically, D.F. needs to use either binocular disparity or motion parallax to recover the depth of an object and successfully carry out a grasping task. The empirical eviden ce illustrates that patients with VA struggle with many visuomotor tasks and in many cases can only complete truthful tasks. Therefore, their performance is far from within the normal range, shown by control tasks with uninjured brains. Firstly, this puts into question the strong single dissociation thought to be illustrated by VA. However, even more importantly and secondly, these findings cast doubts on the pure dorsal abilities, suggesting that even with an uninjured dorsal stream visuomotor skills are affected, which in this case prevents the possibility of a double dissociation.The past research of OA has equally been viewed only through the eyes of the dual processing model, excluding the finer details. For example, clinically, a diagnosis of OA requires for all other perceptual deficits to be excluded. Specifically, issues with visual acuity, visual neglect or injury to the eye itself must be ruled out as explanations for misreaching with visual guidance. However, these diag nostic guidelines have not always been followed, and assessments of such issues have been absent or carried out in approximations (Schenk McIntosh, 2010). Stricter assessments have recurrently shown impaired discrimination of object location or orientation, particularly in the extra-foveal visual field where OA symptoms are most severe (Michel Henaff, 2004 Pisella et al., 2009).It has been argued that in truth, OA is more closely linked to attentional disorders, such as visual neglect or visual extinction (Michel Henaff, 2004 Pisella et al., 2009 Streimer et al., 2007, 2009). A.T.s attentional visual field was described as being narrowed to a functional tunnel vision (Michel Henaff, 2004). The confusion of OAs true origin comes from the fact that misreaching occurs in extra-foveal vision, when patients cannot fixate on the object. The visuomotor abilities of OA patients in central vision show little to no deficits in carrying out visually guided grasping tasks under normal condi tions, unlike the misreaching that is present in the peripheral visual field (Gra et al., 2002 Pisella et al., 2000). More recent studies have suggested that misreaching also affects proprioceptive targets which are not in the direction of gaze (Jackson et al., 2009 Blangero et al., 2007). Jackson et al. (2009) argue that this indicates a difficulty in representing several locations simultaneously, indicating that OA is not simply a visuomotor problem. Similarly, recent papers have shown that perception itself is also impaired in the peripheral visual field (Michel Henaff, 2004 Rosetti et al., 2005). These findings plunge the status of OA as a visuomotor disorder into uncertainty and it unquestionably casts doubts on optic ataxia being considered as evidence of a dissociation of perceptual and motor functions within visual processing. Furthermore, with a growing number of researchers questioning the clarity of OAs strict visuomotor deficits, the argument of a double dissociation lo ses even more conviction.Many years of research have emphasised an impairment of actions in OA, and an impairment of perception in VA. However, are the differences between these two conditions and the empirical evidence strong enough to support a case for a double dissociation? As Pisella and colleagues (2006) highlights, looking over past research on the vision for action studies on OA patients and VA patients it becomes obvious that these sets of patients have not been tested in identical settings. As formerly noted, vision guided grasping movements are impaired in the peripheral vision of OA patients however, these same abilities have only been tested in the central vision for VA patients (Pisella et al., 2006). As indicated earlier, OA patients have been shown to deal with visually guided grasping to a successful level in central vision and ecologically valid conditions (Gra et al., 2002 Pisella et al., 2000). Without empirical evidence to indicate the true abilities of VA pati ents reaching in peripheral vision, it cannot be concluded that their reaching is unaffected. Similarly, OA patients perceptual abilities have not been significantly studied. It is assumed that their perception is at normal levels, however, this same assumption was given to VA reaching until it was more closely studied. Thus, this major fault in the claim for a double dissociation does not take into account the fundamental assumption for double dissociations that examination of the function must be carried out in the same conditions (Teuber, 1955).Given the arguments presented, it seems unlikely that OA and VA are a complementary double dissociation reflecting the inner working of a separate vision for action and vision for perception processing routes. This suggestion is much too simplified. It is much more likely that the vision for perception and vision for action streams interact a great deal, and thus both streams have an effect upon each of these two conditions. We are unclea r of VA patients peripheral visual abilities, and thus they cannot be truly compared to OA patients extra-foveal misreachings. In fact, with doubt mounting about OAs actually link to the vision for action stream, the argument becomes even more clouded. Diagnostically, this must be cleared up before any conclusions of its involvement can be made.In a comparable trend, patients with VA do not perform as well in visually guiding grasping tasks as originally claimed they in fact perform well below normal levels (Goodale et al., 1994a Carey et al., 1996 Dijkerman et al., 1998 McIntosh et al., 2004). Furthermore, under normal conditions and in central vision, OA patients actually perform better than previously claimed, due to their compensatory techniques (Gra et al., 2002 Pisella et al., 2000). This coupled with recent findings of OA patients perceptual difficulties in peripheral vision, (Michel Henaff, 2004 Rosetti et al., 2005), it becomes an extremely difficult task to claim a double dissociation. Although, it may be extreme to claim no interaction between these conditions, they are not completely separate entities either. The fact remains that both conditions allow us to learn a great deal about the visual system under the Milner and Goodale model, and there is sure enough a complementary divergence of symptoms in part. However, the issue lies in attempting to construct these components into a complementary double dissociation the components just do not add up.Hence, it is necessary to move beyond the rudimentary dichotomy of vision for action and vision for perception, and consequently the supposed double dissociation and simplification of OA and VA. Despite previous conventions on the unification of these deficits as one dissociation, as has been shown deeper research is beginning to highlight the cracks in this dissociation. It is necessary to advance the Milner and Goodale model beyond the research opinion on the OA and VA double dissociations. It is imp ortant that the assumptions made of OA and VA being clear and concise indicators of each visual streams abilities is eased. Although individually, patients such as D.F. and S.B., who have perceptual deficits as found in VA, can be useful indicators of the most basic abilities of the dorsal stream, this cannot be guaranteed to indicate workings of the ventral stream. As shown previously, the interaction between the two streams may be greater than previously thought. Thus higher functions of the dorsal stream may fail in patients with VA without the necessary interactive involvement from the ventral stream. Similarly in cases of OA, moreover, this may be in even more doubt with the disagreement of attention deficits playing a vital role in OA symptoms. In essence, the fixation on a double dissociation between OA and VA is hindering future research and the advancement of the dual visual processing model. This simplistic idea of the absolute double dissociation must be abandoned, and a more interactive approach taken to achieve research advancement.