Bias in magnitude estimation following left hemisphere injury

Abstract

There is a growing interest both in identifying the neural mechanisms of magnitude estimation and in identifying forms of bias that can explain aspects of behavioral syndromes like unilateral neglect. Magnitude estimation is associated with activation of temporo-parietal cortex in both cerebral hemispheres of normal subjects; however, it is unclear if and how left hemisphere lesions bias magnitude estimation because the infrequency of neglect and the presence of aphasia in these subjects confound examination. In contrast, we examined magnitude estimation using 12 different types of sensory stimuli that spanned five sensory domains in two patients with very different clinical presentations following unilateral left hemisphere stroke. One patient had neglect subacutely without aphasia. The other had aphasia chronically after a temporo-parietal lesion but not neglect. The neglect patient was re-examined 48 hours after being treated with modafinil (Provigil) for decreased arousal. Both patients demonstrated bias in magnitude estimation relative to normal subjects (n=83). Alertness improved in the neglect patient after taking modafinil. His neglect also resolved and his magnitude estimates more closely resembled those of normal subjects. This is the first evidence, to our knowledge, that the left hemisphere injury can bias magnitude estimation in a manner similar but not identical to that associated with right hemisphere injury.

The parietal cortex and the representation of time, space, number and other magnitudes

“…One issue that may be developed more fully in ATOM is that of hemispheric asymmetries (see Woods et al 2006;Niemeier et al 2007). Owing to the association of numerical mapping with space, there is an emphasis on right hemisphere functions, but it is clear that explicit numerical operations and motor action selection and preparation rely on left parietal cortex mechanisms.…”

Distractor removal amplifies spatial frequency-specific crossover of the attentional bias: a psychophysical and Monte Carlo simulation study

“…Woods et al 2006;for reviews: Walsh 2003;Bueti and Walsh 2009) can be associated with left or right-hemisphere structures, and dominances can switch depending on a variety of factors, for example dexterous actions such as finger counting might be more closely associated with magnitude processes in the left hemisphere (Bueti and Walsh 2009), thus giving rise to TMS-induced symptoms similar to Gerstmann's syndrome (Rusconi et al 2005). A different example is the SNARC effect, so the observation that responses in left and right space speed up judgements about small and large numbers, respectively (Dehaene et al 1993), with a possible link to biases and crossover observed for the bisection of mental number lines (Zorzi et al 2002).…”

Effects of Psychostimulants on Alertness and Spatial Bias in Healthy Participants

“…Indeed, several studies have investigated the ability of pharmacological agents to ameliorate spatial bias in patients with neglect (Müller, 2008;Malhotra et al, 2006;Woods et al, 2006;Mukand et al, 2001;Geminiani et al, 1998;Grujic et al, 1998;Hurford et al, 1998;Fleet et al, 1987), and have, with the exception of one study (Grujic et al, 1998), found generally encouraging results. However, most of these studies have tested either a single patient or a very small sample of patients, making it difficult to generalize results.…”

“…There have been a number of promising attempts to investigate the potential benefits of different stimulants in patients with left spatial neglect (Malhotra, Parton, Greenwood, & Husain, 2006;Woods et al, 2006;Mukand et al, 2001;Geminiani, Bottini, & Sterzi, 1998;Grujic et al, 1998;Hurford, Stringer, & Jann, 1998;Fleet, Valenstein, Watson, & Heilman, 1987). However, the work with patients raises the same important caveats that applied to behavioral manipulations of alertness, that is, is this a general effect of symptom reduction or a spatially specific effect?…”