Electrophysiological analysis of cortical mechanisms of selective attention to high and low spatial frequencies

Objectives: This study investigated whether short-latency (<100 ms) event-related potential (ERP) components were modulated during attention to spatial frequency (SF) cues. Methods: Sinusoidally modulated checkerboard stimuli having high (5 cycles per degree (cpd)) or low (0.8 cpd) SF content were presented in random order at intervals of 400-650 ms. Subjects attended to either the high or low SF stimuli, with the task of detecting targets of slightly higher or lower SF, respectively, than the above standards. ERPs were recorded from 42 scalp sites during task performance and spatio-temporal analyses were carried out on sensory-evoked and attention-related components. Results: Attended high SF stimuli elicited an early negative difference potential (ND120) starting at about 100 ms, whereas attended low SF stimuli elicited a positivity (PD130) in the same latency range. The neural sources of both effects were estimated with dipole modeling to lie in dorsal, extrastriate occipital areas. Earlier evoked components evoked at 60-100 ms that were modeled with striate and extrastriate cortical sources were not affected by attention to SF. Starting at 150 ms, attended stimuli of both SFs elicited a broad selection negativity (SN) that was localized to ventral extrastriate visual cortex. The SN was larger over the left/right cerebral hemisphere for attended stimuli of high/low SF. Conclusions: These results support the view that attention to SF does not involve a mechanism of amplitude modulation of early-evoked components prior to 100 ms. Attention to high and low SF information involves qualitatively different and hemispherically specialized neural processing operations.