Letter to the Editor |
1 Department of Neurosciences, Royal Free Hospital, London and 2 Department of Psychology, University of Edinburgh, Edinburgh, UK
Correspondence to: E-mail: a.leff{at}fil.ion.ucl.ac.uk
Received January 20, 2006. Accepted January 20, 2006.
Reply
Levine makes an interesting case for explaining why the patients in our study with hemianopic alexia (HA) adopt an inefficient eye-movement strategy when reading text. He argues that the left occipital lesion has caused not only a deficit in the contralesional visual field (a right homonymous hemianopia) but also a separate problem in the ipsilesional field, not a low-level visual deficit like the hemianopia, but a ‘higher-level’ one affecting simultaneous visual processing of multiple objects. If HA patients did initially fixate more to the right as part of a compensatory strategy, then their left parafoveal visual system would be flooded with too much information and reading would collapse. This hypothesis cannot be completely refuted from our data; however, it raises some interesting points both relating to our paper and the classics cited by Levine, two of which we would like to address.
First, our results particularly relate to the rigours of text reading and not single-word reading, which, although mildly impaired in our HA subjects, is not the main cause of their reading impairment. When reading text, our patients' initial landing position on five-letter words on average only allowed them to clearly process the first letter. If this were due to an absolute limit on the intact left field being able to process only one letter at a time, then their single-word reading speeds would reflect this and they would appear to be so-called ‘letter-by-letter’ readers. Although we did not measure eye movements during single-word reading, the vast majority of our patients could read an isolated three-letter word in 1 s or less. If Levine's hypothesis were correct, they would have to produce three eye movements in order to identify the constituent letters of the word, compile these into a unified percept and start the process of vocal articulation all within this time period (we measured single-word reading speeds using a voice key system), which seams unlikely.
Secondly, the seven subjects studied in detail across the Levine and Kinsbourne papers who clearly do show some evidence of simultanagnosia for letters and other visual stimuli in their intact field did not have HA. All but one had a clear right-sided hemianopia (Mr S in Kinsbourne, who was aphasic), but this was associated with either pure alexia (PA) or was part of a generalized language disorder. PA patients often have a hemianopia (Leff et al., 2001
), but this is not the main cause of their reading disorder, which, unlike patients with HA, centres on their inability to easily identify whole words. The neuropsychological ‘lesion’ responsible for PA continues to be a matter of debate. Kinsbourne and Warrington suggest that it may be due to a general visual simultanagnosic deficit in their 1962 paper, and current researchers such as Behrmann continue to support this view (Behrmann et al., 1998). Warrington later published evidence that the ‘lesion’ in PA is not a general one and is limited to words alone, coining the term ‘word-form dyslexia’ (Warrington and Shallice, 1980). As PA and HA are generally caused by damage to adjacent brain regions in the dominant occipitotemporal lobe, it is likely that these disorders form a continuum, and, as such, a degree of impairment of processing items in the intact visual field may help explain why patients with HA are slower than normals when reading single words, but we believe this is unlikely to be the main cause of their persistent text-reading problems.
References
Behrmann M, Nelson J, Sekuler EB. Visual complexity in letter-by-letter reading: ‘pure’ alexia is not pure. Neuropsychologia 1998; 36: 1115–32.[CrossRef][Web of Science][Medline]
Leff AP, Crewes H, Plant GT, Scott SK, Kennard C, Wise RJ. The functional anatomy of single-word reading in patients with hemianopic and pure alexia. Brain 2001; 124: 510–21.
Warrington EK, Shallice T. Word-form dyslexia. Brain 1980; 103: 99–112.
| ||||||||||||||||||||||||||||||||||||||||||||||||||||||