Grapheme-color synesthesia subtypes: Stable individual differences reflected in posterior alpha-band oscillations.

Grapheme-color synesthesia is a condition in which seeing letters and numbers produces sensations of colors (e.g., the letter R may elicit a sky-blue percept). Recent evidence implicates posterior parietal areas, in addition to lower-level sensory processing regions, in the neurobiological mechanisms involved in synesthesia. Furthermore, these mechanisms seem to differ for "projectors" (synesthetes who report seeing the color "out there in the real world") versus "associators" (synesthetes who report the color to be only an internal experience). Relatively little is known about possible electrophysiological characteristics of grapheme-color synesthesia. Here we used EEG to investigate functional oscillatory differences among associators, projectors, and non-synesthetes. Projectors had stronger stimulus-related alpha-band (~10 Hz) power over posterior parietal electrodes, compared to both associators and non-synesthetes. Posterior alpha activity was not statistically significantly different between associators from non-synesthetes. We also performed a test-retest assessment of the projector-associator score and found strong retest reliability, as evidenced by a correlation coefficient of .85. These findings demonstrate that the projector-associator distinction is highly reliable over time and is related to neural oscillations in the alpha band.

Abnormal timing of visual feedback processing in young adults with schizophrenia.

BACKGROUND: Recent studies have shown that schizophrenia is characterized by visual perceptual deficits, especially in the ability to integrate stimulus details into a global percept. Also, several studies have found amplitude attenuation of the visual P1 component of the event-related brain potential (ERP), probably indicating impaired visual feedforward processing in schizophrenia. However, there is little knowledge on the role of feedbackward processing in this group. This question is of importance, as recent studies indicate that feedback processing is critical in stimulus integration. METHODS: In the present study we tested whether there is evidence for atypical recurrent processing in a group of 14 young adults with recent-onset schizophrenia (mean age 21.7 years, mean TIQ 92.7) and 17 age and IQ matched control subjects, all males. To achieve this aim, we used a texture segregation task and measured ERP activity concurrently. RESULTS: We found normal amplitudes, but longer latencies of activity related to feedbackward processing in the schizophrenia group. In addition, we found enhanced occipito-temporal activity around 160 ms that is probably the reflection of increased detail processing. DISCUSSION: We show for the first time evidence for abnormal timing in feedback activity related to visual perception in subjects with schizophrenia. It is hypothesized that this latency effect is the functional reflection of abnormal structural connectivity in this group, and might result in increased processing of stimulus detail.