Navigation in AD and MC

Navigational deficits are more pronounced in AD patients (Lithfous et al., 2013) but similar deficits are also present in individuals with a MCI (Cushman et al.,

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2008). Since navigational and spatial deficits are present in MCI, which is believed to be a prodromal state of AD (Petersen et al., 1999), testing this particular memory domain could improve the earlier diagnosis of AD. Furthermore, those MCI participants with navigational difficulties seem to be at an elevated risk of conversion to an AD diagnosis (Laczó et al., 2011) compared with MCI participants with intact spatial memory. This has important implications for the treatment and subsequent monitoring of these individuals. Topographical disorientation (TD) refers to problems orientating oneself to the surrounding environment (Barrash et al., 2000) and produces difficulties navigating to specific locations and recalling routes (Barrash et al., 2000). TD is a common problem in AD resulting in many patients becoming lost or unsure of their surroundings (Cherrier et al, 2001; Lithfous et al., 2013; Serino and Riva, 2013). TD may be due to a failure of the brain areas involved in the encoding of spatial layouts or may reflect a more general deficit in spatial processing (Cherrier et al., 2001). Barrash et al., (2000) comment that TD results when the integrity of the inferior medial occipital, occipitotemporal cortices or the right medial temporal areas are compromised either by lesions or the aging process. Research studies using AD patients have linked TD with other measures of neuropsychological assessment suggesting that numerous brain areas contribute to the manifestation of TD symptoms. For example, Henderson et al., (1989) investigated a group of 28 probable AD patients whose carers had stated that they became lost on a regular basis. Using a regression analysis this study showed that memory performance but not attention, language or disease severity was a significant predictor of spatial disorientation.

Passini et al., (1995) investigated navigational ability in a large hospital setting with 14 mild to moderate AD patients and 28 HE controls. All of the AD patients failed to navigate to a set location without making errors, the participants were recorded along their route and AD patients showed impaired problem solving abilities.

TD could also be linked to problems with optic flow. Optic flow provides a moving individual with information on the stimuli surrounding them. It also aids their navigation and orientation through their environment (Duffy, 2009). Tetewsky and Duffy (1999) and Duffy (2009) report that there is significant

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impairment in measurements of optic flow in AD patients. This implys that there may be deficits in visuospatial processing contributing to the TD experienced in these patients.

TD is one of the earliest symptoms of AD (Lithfous et al., 2013; Cherrier et al., 2001; Morris, 1993; Serino and Riva, 2013). Approximately 25% of AD patients present with initial TD and this increases to 50% within the first three years of the disease being diagnosed (Pengas et al., 2010). These figures are likely to be under represented as some patients often attempt to hide their TD by using new strategies and avoiding new routes or places (Serino and Riva, 2013). As a result, carers often only become aware of spatial difficulties when the patient is away from their home setting such as on holiday, when they fail to remember or learn new places and routes (Pengas et al., 2010).

Whilst some of the HE may experience TD in new surroundings; AD patients are often disorientated in familiar environments such as their home (Cherrier et al., 2001; Cushman et al., 2008; Serino and Riva, 2013). This restricts daily living tasks and results in patients becoming lost when out and about, wandering, and taking the wrong direction on journeys (Head and Isom, 2010). This has the potential for serious consequences and causes anxiety for the carers and families of such individuals.

In Cherrier et al’s., (2001) paper, mild to moderate AD patients (with a mean Dementia Rating Scale score of 107) and a group of elderly control participants learnt an outdoor route whilst accompanied by a researcher. The purpose of this study was to determine whether AD patients experienced problems with spatial memory and orientation rather than simply having an inability to recognise landmarks along the chosen route. AD patients were significantly worse at the route learning test. As the authors predicted, the results confirmed that there were clear deficits in spatial orientation and processing. Strong associations between their route learning test scores and scores on an established route learning task (the Money Road Map test) were noted. These findings were consistent with previous work which has linked TD to the impaired optic flow seen in AD (Tetwesky and Duffy, 1999). All of the AD patients in this sample were impaired at recognising the route, however, they performed almost as well as the control participants in their ability to recall the presence of

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landmarks (Cherrier et al., 2001). These results support the suggestion that spatial orientation is impaired in the disease.

Duffy (2009) suggests that visual information processing is impaired in AD and a number of studies have shown that both AD and MCI populations have problems in visual motion processing, which are thought to underlie their navigational difficulties (Mapstone et al., 2003; Tetewsky and Duffy, 1999; O’Brien et al., 2001; Dubinsky et al., 2000; Uc et al., 2004). Testing navigational deficits using 'real-world' methods can be difficult and requires long administration times (Duffy, 2009). Sheehan et al., (2006) investigated navigational deficits using a real world situation. These authors recruited mild AD patients and healthy controls and accompanied them on a short 30 minute walk close to where they reside. The participants were then asked to complete the route again, the researcher accompanied them but did not correct any wrong turns or errors. Clear deficits were seen for the AD patients and this group experienced significant way finding difficulties leading them to become lost. The results of the study demonstrated the reliance on external cues as many of the patients stated that they knew to look out for certain signs or landmarks to help them return home. This implies that rather than reliance on visuospatial memory, memory for familiar objects seems to play a significant role in route planning and spatial orientation. The authors recognise the limitations to this approach such as the use of subjective recordings by a researcher and the fact that environmental cues may change on a daily basis, Such cues may include the weather, external traffic noise and/or the presence of others and may affect the ability of the participant to remember their route (Sheehan et al., 2006). This technique relies upon the participant being mobile and the experience itself could become stressful and tiresome particularly if the patient becomes disorientated or lost.