Visual Abilities in Individuals with Profound Deafness A Critical Review

After more than 30 years of systematic research conducted mainly on the visual abilities of profoundly deaf individuals, it is apparent that the long-standing debate as to whether perceptual and cognitive functions of deaf individuals are deficient or supranormal is far from being settled. Several reviews of this literature (e.g., Parasnis 1983; Bavelier et al. 2006; Mitchell and Maslin 2007) clearly indicate that deaf and hearing individuals perform comparably on a number of perceptual tasks. As we shall see later (see Section 22.2.1), this conclusion is strongly supported by tasks involving basic perceptual thresholds. Instead, other studies have revealed a differential performance in the two groups, either in the direction of deficient abilities in deaf than hearing participants (e.g., Quittner et al. 2004; Parasnis et al. 2003), or in the direction of supranormal abilities for the deaf population (e.g., Bottari et al. 2010; Loke and Song 1991; Neville and Lawson 1987). In this context, it should perhaps be emphasized that in the absence of clear behavioral differences between deaf and hearing participants, even the most striking differences between the two groups observed at the neural level cannot disentangle between the perceptual deficit hypothesis and the sensory compensation hypotheses. For instance, much of the renewed interest in the study of visual abilities in deaf individuals has been motivated by the seminal work of Neville et al. (1983). In that study, visual evoked potentials (VEPs) recorded from the scalp of eight congenitally deaf adults were significantly larger over both auditory and visual cortices, with respect to those of eight hearing controls, specifically for visual stimuli occurring in the periphery of the visual field (8.3°). Although this pioneering work implies that the lack of auditory experience from an early age can influence the organization of the human brain for visual processing [a finding that was later confirmed and extended by many other studies using different methodologies for the recording of brain responses; e.g., electroencephalogram (EEG): Neville and Lawson 1987; magnetoencephalography: Finney et al. 2003; functional magnetic resonance imaging: Bavelier et al. 2000, 2001], in the absence of a behavioral difference between the two groups it remains potentially ambiguous whether modifications at the neural level are an index of deficiency or compensation. In other words, even if one assumes that larger visual evoked components (e.g., Neville et al. 1983; Neville and Lawson 1987) or stronger bold responses (e.g., Bavelier et al. 2000;2001) indicate enhanced processing of the incoming input, if this is not accompanied by behavioral enhancement it is difficult to conclude that it really serves some adaptive functional role. Unfortunately, the current evidence in the literature lacks this explicative power. With the only exception of the work by Neville and Lawson (1987), all other neuroimaging studies focused on measures of brain response alone, instead of combined measures of brain response and behavior. Furthermore, conclusive evidence that cortical reorganization serves a functional role can only originate from the observation that interfering with the reorganized brain response [e.g., using transcranial magnetic stimulation (TMS)] impairs the supranormal behavioral performance in the sensory-deprived participants (e.g., see Cohen et al. 1997 for an example of abolished supranormal tactile discrimination in the blind, following disruption of occipital lobe function using TMS).