The temporal interaction of modality specific and process specific neural networks supporting simple working memory tasks.

Several theories of brain function emphasize distinctions between sensory and cognitive systems. We hypothesized, instead, that sensory and cognitive systems interact to instantiate the task at the neural level. We tested whether input modality interacts with working memory operations in that, despite similar cognitive demands, differences in the anatomical locations or temporal dynamics of activations following auditory or visual input would not be limited to the sensory cortices. We recorded event-related brain potentials (ERPs) while participants performed simple short-term memory tasks involving visually or auditorily presented bandpass-filtered noise stimuli. Our analyses suggested that working memory operations in each modality had a very similar spatial distribution of current sources outside the sensory cortices, but differed in terms of time course. Specifically, information for visual processing was updated and held online in a manner that was different from auditory processing, which was done mostly after the offset of the final stimulus. Our results suggest that the neural networks that support working memory operations have different temporal dynamics for auditory and visual material, even when the stimuli are matched in term of discriminability, and are designed to undergo very similar transformations when they are encoded and retrieved from memory.

Deficits in automatically detecting changes in conjunction of auditory features in patients with schizophrenia.

Disturbances in processing simple acoustic changes in a stream of stimuli have been widely reported in patients with schizophrenia, but little is know about auditory feature conjunction in these individuals. This study was designed to examine the extent to which patients with schizophrenia automatically process changes in conjunction of auditory features by using event-related brain potentials. Seventeen patients and 17 age-matched controls were presented with frequent low pitch tones at 45 degrees to the left of center and frequent high pitch tones at 45 degrees to the right of center while performing a continuous visual serial-choice reaction time task. The sequence of auditory stimuli included rare conjunction-deviants comprised of a different combination of features (e.g., low pitch tone at 45 degrees right) and double-deviant tones that differed from the standard tones in both pitch and location (i.e., middle pitch at 0 degrees azimuth). Conjunction-deviant stimuli elicited an MMN wave that was maximum at frontocentral sites. Compared with controls, the MMN to conjunction-deviant was reduced in patients and was more centrally distributed. Double-deviant sounds generated a biphasic MMN followed by a P3a wave at central sites. Both MMN and P3a were reduced in patients compared with controls. These results show that patients with schizophrenia have difficulty in automatically detecting changes in a combination of auditory features as well as orienting to what "normally" would be considered salient by healthy individuals.