Biomedical subjects

David A Medler

Publications and source records attributed to David A Medler.

5 recordsLinked to original sources

PubMed2006

Tuning of the human left fusiform gyrus to sublexical orthographic structure.

Neuropsychological and neurophysiological evidence point to a role for the left fusiform gyrus in visual word recognition, but the specific nature of this role remains a topic of debate. The aim of this study was to measure the sensitivity of this region to sublexical orthographic structure. We measured blood oxygenation (BOLD) changes in the brain with functional magnetic resonance imaging while fluent readers of English viewed meaningless letter strings. The stimuli varied systematically in their approximation to English orthography, as measured by the probability of occurrence of letters and sequential letter pairs (bigrams) comprising the string. A whole-brain analysis showed a single region in the lateral left fusiform gyrus where BOLD signal increased with letter sequence probability; no other brain region showed this response pattern. The results suggest tuning of this cortical area to letter probabilities as a result of perceptual experience and provide a possible neural correlate for the 'word superiority effect' observed in letter perception research.

Adult↗

PubMed2006

Syntactic and semantic modulation of neural activity during auditory sentence comprehension.

In previous functional neuroimaging studies, left anterior temporal and temporal-parietal areas responded more strongly to sentences than to randomly ordered lists of words. The smaller response for word lists could be explained by either (1) less activation of syntactic processes due to the absence of syntactic structure in the random word lists or (2) less activation of semantic processes resulting from failure to combine the content words into a global meaning. To test these two explanations, we conducted a functional magnetic resonance imaging study in which word order and combinatorial word meaning were independently manipulated during auditory comprehension. Subjects heard six different stimuli: normal sentences, semantically incongruent sentences in which content words were randomly replaced with other content words, pseudoword sentences, and versions of these three sentence types in which word order was randomized to remove syntactic structure. Effects of syntactic structure (greater activation to sentences than to word lists) were observed in the left anterior superior temporal sulcus and left angular gyrus. Semantic effects (greater activation to semantically congruent stimuli than either incongruent or pseudoword stimuli) were seen in widespread, bilateral temporal lobe areas and the angular gyrus. Of the two regions that responded to syntactic structure, the angular gyrus showed a greater response to semantic structure, suggesting that reduced activation for word lists in this area is related to a disruption in semantic processing. The anterior temporal lobe, on the other hand, was relatively insensitive to manipulations of semantic structure, suggesting that syntactic information plays a greater role in driving activation in this area.

Adult↗

PubMed2005

Neural substrates of phonemic perception.

The temporal lobe in the left hemisphere has long been implicated in the perception of speech sounds. Little is known, however, regarding the specific function of different temporal regions in the analysis of the speech signal. Here we show that an area extending along the left middle and anterior superior temporal sulcus (STS) is more responsive to familiar consonant-vowel syllables during an auditory discrimination task than to comparably complex auditory patterns that cannot be associated with learned phonemic categories. In contrast, areas in the dorsal superior temporal gyrus bilaterally, closer to primary auditory cortex, are activated to the same extent by the phonemic and nonphonemic sounds. Thus, the left middle/anterior STS appears to play a role in phonemic perception. It may represent an intermediate stage of processing in a functional pathway linking areas in the bilateral dorsal superior temporal gyrus, presumably involved in the analysis of physical features of speech and other complex non-speech sounds, to areas in the left anterior STS and middle temporal gyrus that are engaged in higher-level linguistic processes.

Acoustic Stimulation↗

PubMed2005

Functional localization and double dissociations: the relationship between internal structure and behavior.

Lesioning studies are often used in cognitive neuroscience to make inferences about the architecture of cognition. Recently, computational models have been used to address some of the underlying assumptions-such as modularity and locality-often implicitly used when interpreting lesion data. In this article, we explore the "functional localization" assumption and its role in interpreting lesioning data, especially from double dissociations. The functional localization assumption states that units or subunits within an information processing system become functionally specialized for dealing with specific aspects of the input environment. Networks were trained on one of two problems-an abstract "rules and sub-rules" problem, and a more concrete "logic classification" problem-and then systematically lesioned. Networks were analyzed in terms of their overt behavior, and more importantly, in terms of their internal structure. Performance deficits in both form and magnitude could be directly related to the ablated internal structure of the networks. That is, if an ablated area had little or no functional localization, then little or no behavioral dissociations were observed. If, however, the ablated area had very specific internal structure, then very specific behavioral dissociations were observed. It is important to note, however, that there was not a one-to-one correspondence between internal structure and behavioral dissociations, implying that cognitive neuroscientists must be careful when using lesioning data to theorize about the functional architecture of cognition.

Brain↗

PubMed2004

Neural correlates of sensory and decision processes in auditory object identification.

Physiological studies of auditory perception have not yet clearly distinguished sensory from decision processes. In this experiment, human participants identified speech sounds masked by varying levels of noise while blood oxygenation signals in the brain were recorded with functional magnetic resonance imaging (fMRI). Accuracy and response time were used to characterize the behavior of sensory and decision components of this perceptual system. Oxygenation signals in a cortical subregion just anterior and lateral to primary auditory cortex predicted accuracy of sound identification, whereas signals in an inferior frontal region predicted response time. Our findings provide neurophysiological evidence for a functional distinction between sensory and decision mechanisms underlying auditory object identification. The present results also indicate a link between inferior frontal lobe activation and response-selection processes during auditory perception tasks.

Acoustic Stimulation↗