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Biomedical subjects

L Hardage

Publications and source records attributed to L Hardage.

4 recordsLinked to original sources
PubMed1998

Long-range twinkle induction: an achromatic rebound effect in the magnocellular processing system?

After viewing a blank region surrounded by a dynamic noise stimulus, viewers report the perception of prolonged dynamic twinkle in the unstimulated blank region. This twinkle aftereffect may be induced over long ranges in the visual field, up to 10 degrees from the edge of the noise in central vision. Our previous studies of the properties of this aftereffect suggested mediation by the magnocellular processing system. We therefore evaluated the properties predicted by the magnocellular hypothesis by varying the coloring, the temporal and the spatial frequency of the stimulus. No aftereffect could be induced by an equiluminant color stimulus or by luminance noise below the temporal frequency of 5 Hz. The aftereffect obtained by luminance noise above 5 Hz was stronger for larger inducing elements. These results are consistent with known properties of the magnocellular processing system.

Color Perception↗
PubMed1995

Induced twinkle aftereffect as a probe of dynamic visual processing mechanisms.

After viewing a blank patch surrounded by a dynamic noise stimulus (a video "snowstorm"), viewers report the prolonged perception of twinkle in the unstimulated region of the blank patch. We compare this induced twinkle aftereffect to the filling-in phenomenon, which may be seen in a small blank region, under similar test conditions but during stimulation. We found that strong induced twinkle aftereffects were seen both centrally and peripherally for blank test regions from 0.5 deg to as large as 20 deg in diameter, whereas filling-in was seen centrally only for test patch diameters smaller than 0.75 deg, becoming stronger peripherally but still limited to test regions less than about 3 deg in diameter. Lower noise density and larger noise element size facilitated filling-in but had little effect on the induced twinkle aftereffect. Conversely, noise frame rate had little effect on filling-in but had to be faster than 10 frames/sec to produce a twinkle aftereffect. Induced twinkle showed binocular superiority but no interocular transfer. The binocular superiority was partially explained by monocular blankout of the dynamic noise by the blank field in the occluded eye. These results all imply a different mechanism for the induced twinkle aftereffect than for filling-in. We consider a model in which the induced twinkle aftereffect is produced by post-inhibitory rebound in complex cells.

Adaptation, Ocular↗
PubMed1995

Multiple mechanisms for the detection of mirror symmetry.

Symmetry detection was investigated for static and dynamic noise targets consisting of a field of approximately 0.3 million random dots on which was imposed a bilateral symmetry. The minimum duration for detection was 40 ms for static and 80 ms for dynamic symmetry. The exponents of the psychometric functions averaged about 4 for both static and dynamic tasks, as opposed to the value of 1 expected for such suprathreshold tasks, implying that there is some neural mechanism performing full temporal integration of the symmetry information up to durations of a second or more. Static symmetry was perceivable when information around the symmetry axis was masked up to 3 deg away from the symmetry axis, revealing extrafoveal symmetry detection in approximately 300 ms exposures. The static data were fitted with a model consisting of three mechanisms with Gaussian spatial profiles and mutual inhibition (two mechanisms were sufficient for the dynamic data). The profile of the widest mechanism was 20 times wider for static than for dynamic symmetry.

Dominance, Cerebral↗