The simultaneous activation hypothesis: explaining recovery from deep to phonological dyslexia.

Deep dyslexia evolved into phonological dyslexia in one patient. Semantic errors resolved while phonological and derivational errors persisted in reading. Nonword reading improved but remained inferior to word reading. Despite a residual semantic deficit naming improved. The Simultaneous Activation Hypothesis explains recovery from deep to phonological dyslexia and the continued dissociation between reading and naming errors. Partial recovery to all three reading routes increased constraints for word selection at the phonological output lexicon (POL) improving word reading. With recovery, the POL receives additional supportive information from the partially recovered direct oral reading route and grapheme-to-phoneme conversion (GPC) eliminating semantic errors in oral reading. Nonword reading also improved because of partial recovery to all three routes. Semantic errors in naming persisted because additional constraints were unavailable at the POL to activate a phonological entry. Phonological and derivational errors were more frequent in reading than in naming the result of incomplete GPC recovery. Residual nonword reading deficits resulted from incomplete GPC recovery, indicated by the persistence of neologisms in nonwords. The Simultaneous Activation Hypothesis readily accounts for the evolution from deep to phonological dyslexia.

Sensory and response interference by ipsilesional stimuli in tactile extinction.

Extinction is thought to be due to a pathologically limited attentional capacity in which multiple stimuli cannot be processed simultaneously to conscious awareness. Patients with tactile extinction are aware of being touched on a contralesional limb, but seem unaware of similar contralesional touch if touched simultaneously on their ipsilesional limb. The ipsilesional stimulus interferes and competes with the processing of the contralesional stimulus. Most theorists assume that the ipsilesional stimulus affects the sensory processing of the contralesional stimulus, although the precise functional level at which this interference occurs is not clear. We report a series of experiments using signal detection analyses to investigate tactile extinction in one patient (DC). These analyses revealed that ipsilesional stimuli, in addition to interfering with processing of contralateral sensations, also interfere with verbal reports of those sensations. This influence on responses suggests that interference in tactile extinction can occur at a post-perceptual level, further 'downstream' than previously thought.

The interaction of multiple routes in oral reading: evidence from dissociations in naming and oral reading in phonological dyslexia.

During oral reading we hypothesized that lexical representations are activated and selected for output by the simultaneous activation of the semantic, the direct lexical orthography to phonology, and the sublexical grapheme-to-phoneme conversion (GPC) routes (Southwood & Chatterjee, 1999). Serial models of reading argue that the semantic route governs oral reading with minimal influence from the nonlexical direct route and the sublexical GPC route. These models predict that semantic errors should occur in reading when the semantic route and GPC are both impaired. The Simultaneous Activation Hypothesis predicts few semantic errors in oral reading but many during picture naming. Semantic errors are infrequent in reading because information from all three reading routes constrains activation of a phonological entry. By contrast phonological selection in picture naming is constrained primarily by the semantic route and if damaged additional information is unavailable to select the appropriate phonological code. In agreement with the Simultaneous Activation Hypothesis five phonological dyslexics produced semantic errors during picture naming but not when reading aloud. Phonological errors were present during oral reading and minimal during picture naming.

Simultaneous activation of reading mechanisms: evidence from a case of deep dyslexia.

We report the performance of LC, a deep dyslexic. We investigated extensively her errors according to serial cognitive neuropsychological models of oral reading. Initial evaluation of her reading suggested impaired access to the phonological output lexicon (POL). Impaired grapheme-to-phoneme conversion (GPC) and semantic errors in reading suggested that LC read via an impoverished semantic route. However, a serial model of oral reading could not explain error differences in reading, picture naming, spontaneous speech, and repetition. Neologisms occurred in oral reading but not in spontaneous speech and repetition. Semantic errors in naming exceeded those in oral reading. To account for these different error patterns we propose that the semantic route, the direct route from the orthographic input lexicon to the POL, and GPC activate simultaneously during reading, converging at the POL to constrain phonological selection. These routes are modular but not functionally encapsulated. For LC, the POL receives ambiguous information due to degradation of all routes, causing reading errors.

Verbs, events and spatial representations.

Are concepts expressed in language also represented spatially? To pursue this question we investigated the structure of events. Events are defined as actions with spatial trajectories that can be perceived by our senses and described in language. Events are expressed linguistically in sentences containing verbs which determine the thematic roles of the arguments (e.g., who is doing what to whom, where). Because of previous observations we focused on whether events are represented spatially by location of thematic roles and direction of actions. Location and direction were dissociated by contrasting different kinds of verbs: 'push' vs 'pull' in which actions move toward or away from the agent. To control for spatial effects produced by the surface structure of a left to right written language, we kept the structure of sentences constant and sought for spatial biases produced by differences in the meaning of these sentences. From three experiments using drawing and sentence-picture matching reaction time tasks, we found that normal subjects located agents to the left of patients and represented actions with a left to right directionality. These results are not easily explained by features of the surface structure of language or properties of propositional representations. We suggest that events have spatial representations in addition to their propositional counterparts of verbs and thematic roles. The specific spatial properties observed may relate to functional properties of the left hemisphere.

Vocal reaction times of children with CAPD, age-matched peers, and young adults to printed words.

The purpose of this study was to determine if there were differences between children identified in a clinical setting as having Central Auditory Processing Disorder (CAPD), an age-matched peer group, and young adults when tested using a vocal reaction time (VRT) format. The children with CAPD were matched by gender and age to peers between the ages of 8 and 10 years. All speakers were presented visually with printed third-grade-level one- and two-syllable words (e.g., boy, mother) as well as the syllable "uh". Participants spoke each word according to the criteria of seven separate conditions, which included immediate naming tasks (0 s delay), a short delay before speaking (M = 1.5 s), and a longer delay before speaking (M = 4.0 s). Speakers VRTs were measured, and production errors were recorded. All speakers took longer to respond in the immediate-response conditions than the delayed-response conditions. Statistically significant differences were found for the immediate-response conditions, with means for the children with CAPD reflecting slower performance than that of their peers. The peer group was slower than the adults. For the delayed conditions, both groups of children responded with significantly longer VRTs than the adults. The two groups of children did not differ for these tasks. The children with CAPD produced a significantly greater number of errors than their peers, specifically for the long-delay conditions. The adults showed no performance differences across the immediate response conditions nor across the delayed conditions. These results suggested that children with CAPD may have processing difficulties with visual stimuli.

Cortical blindness and visual imagery.

Controversy exists concerning the neural basis underlying visual imagery. Some propose that visual images evoked from memory are mediated by primary visual cortices. Others argue that these primary visual areas perform computations on elementary visual features when constructing visual representations from retinal input but that they are not activated during recall of these representations. The visual imagery abilities of patients with cortical blindness may resolve this controversy. The proposition that primary visual cortex is necessary for visual imagery predicts a cortically blind subject's inability to perceive visual stimuli would be accompanied by an inability to image visually. Our investigations of three patients with cortical blindness provide strong evidence that primary visual cortices are not essential for the mediation of visual images recalled from memory.