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Soo Hyun Park, Sang-Hun Lee; Feature-specific modulation of Gamma oscillations in visual detection. Journal of Vision 2007;7(9):1014. doi: 10.1167/7.9.1014.
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The visibility of a threshold-level target can be modulated by spatiotemporally neighboring stimuli (Kovacs & Julesz, 1994; Purushothaman et al., 2003). An abrupt-onset stimulus is known to elicit phase-locked gamma oscillations in the visual cortex (Juergens et al., 1999), suggesting that the visibility of a target might fluctuate as a function of time relative to a preceding stimulus in gamma frequency range.
To explore this possibility, we measured contrast thresholds for a brief (20 ms), small (s=.0916°) Gabor target, which followed a high-contrast Gabor stimulus (‘trigger’) at varying SOA (55 ∼ 170 ms) in fine-grained (5 ms) time scale using the 2IFC task. The 45°-oriented target appeared in a fixed location (2° eccentricity in the lower-left visual quadrant) whereas the trigger, the position and orientation of which was collinear to the target, appeared at different distances to the target (0°, 1° and 2°).
The threshold of the target oscillated as a function of SOA in a broad range of frequency (20–70 Hz). Significant oscillations were limited to a narrow window of SOA, with the ‘time to peak’ and ‘amplitude’ of oscillations being systematically delayed and reduced with increasing trigger-target distance. When the orientation of the trigger was tilted away from the collinear axis, the amplitude of oscillations was reduced only in gamma frequency range, with amplitude at other frequency ranges intact. The spatiotemporal dynamics of gamma oscillations was remarkably consistent across sessions for each of three observers.
Our findings provide psychophysical evidence supporting the idea that waves of cortical excitability modulation evoked by external visual input can account for trial-to-trial variability in detection of the target following the trigger. Furthermore, the feature-specific nature of gamma oscillations implies that the observed modulation of visibility is likely to be mediated by orientation-specific horizontal connections within early visual cortex (Bosking et al., 1997).
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