Multiple-domain dissociation between impaired visual perception and preserved mental imagery in a patient with bilateral extrastriate lesions

Abstract

A brain-damaged patient is described whose pattern of performance provides insight into both the functional mechanisms and the neural structures involved in visual mental imagery. The patient became severely agnosic, alexic, achromatopsic and prosopagnosic following bilateral brain lesions in the temporo-occipital cortex. However, her mental imagery for the same visual entities that she could not perceive was perfectly preserved. This clear-cut dissociation held across all the major domains of high-level vision: object recognition, reading, colour and face processing. Our findings, together with other reports on domain-specific dissociations and functional brain imaging studies, provide evidence to support the view that visual perception and visual mental imagery are subserved by independent functional mechanisms, which do not share the same cortical implementation. In particular, our results suggest that mental imagery abilities need not be mediated by early visual cortices.

Visually- and motor-based knowledge of letters: evidence from a pure alexic patient

“…Our present results concur with other abundant evidence in suggesting that damage to downstream visual areas located in the temporal lobes is, instead, both necessary and sufficient to produce visual imagery deficits [13,19,43,49,52,54]. In the domain of pure alexia, we see as particularly suggestive the contrast between the patterns of performance of Madame D [3], whose lesions affected BA 18, 19 and 37, and whose visual mental imagery for letters and words was entirely spared (despite her profound alexia), and of VSB, whose lesion extended much more anteriorly into the left temporal lobe, as well as into the parietal lobe, and who showed severely impaired 4 Also, several neuroimaging studies [reviewed in 44] found no evidence of an increased activity in early visual cortices during visual mental imagery.…”

“…Why VSB did not spontaneously do so remains an open question (note, however, that after a few testing sessions he did began to spontaneously trace letters and words in the air or on the table, when questioned about their visual form). 3 A reviewer of the present article pointed out that the validity of this patient"s symptoms has been doubted.…”

“…Neuropsychological evidence suggests that each of these imagery domains may be affected independently of the others [32]. The patterns of co-occurrence of different imagery and perceptual deficits may ultimately shed light on the mechanisms of mental imagery abilities and on their relationships with perceptual processes [3,23]. VSB claimed to be able to visually imagine nonorthographic items, and indeed showed normal or near-normal performance in the experimental tasks involving these items.…”

“…Madame D, a pure alexic patient whose residual reading capacities were disrupted by a second, right hemisphere lesion [5], showed a similar pattern of dissociation between severely impaired visual identification of letters, words and digits, and intact visual mental imagery for these same visual entities [3].…”

Mechanisms of Pure Alexia: Spatially Based Impairment, Letter Identification Deficit, or Both?

“…We did not have the opportunity to test the patient's knowledge of the visual form of letters in this study, but we did that after her second stroke, and found that Madame D could perfectly conjure up and manipulate mental images of letters (Bartolomeo et al, 1998a). This suggests that her letter identification deficit occurred at a perceptual level of impairment.…”