Title
FMRI AND FSE IMAGES INDICATE THAT CORTICAL AREAS ACTIVATED EXCLUSIVELY BY ILLUSORY CONTOURS AND STEREO DEPTH LIE OUTSIDE PRIMARY VISUAL CORTEX

Authors
J. Hirsch^1,2, R. DeLaPaz³, K. Kim1,², J. Victor², N. Relkin², K.-M. Lee1,², D. Moreno¹, N. Rubin⁴, and R. Shapley⁵ Memorial Sloan-Kettering Cancer Center1, Cornell University Medical College²³, Center for Neural Science, NYU⁴, New York, NY; Harvard University⁵,, Cambridge MA

Introduction:
The role of primary (striate) visual cortex in human visual perception is the subject of active investigation and diverse hypotheses. In this study we employ functional (fMRI), topographic, and myeloarchitectonic imaging techniques to identify human primary visual cortex, and show that although primary visual cortex is active during the presence of all visual stimuli, the cortical areas that we have found to be associated exclusively with the perception of illusory contours¹ and stereo-depth² are found primarily in extra-striate visual cortex.

Methods:
Images were acquired on a 1.5T scanner (GE) retrofitted for echo planar imaging (Advanced NMR) with a 5 inch surface coil centered approximately 2 cm above the inion. Eight contiguous slices, each 3.1 mm thick, were oriented orthogonal to the calcarine sulcus in order to optimize images of the sulcal morphology and myeloarchitecture³.

Topography and myeloarchitecture of primary visual cortex.
High resolution T1/T2 "balanced" images were acquired with a fast spin echo (FSE) sequence (TR=4000 ms, effective TE=58 ms, echo train length=16 ms, 8 nex; field of view = 10 x 10 cm; and array size = 256 x 256). Voxel size was 0.4 x 0.4 x 3.1 mm. The calcarine sulcus was identified on each of the slices. On some slices a thin ribbon of low signal intensity within the middle cortical laminae was observed to follow the calcarine sulcus, as illustrated on Fig. 1a for slice 4. This structure has previously been identified as the Stria of Gennari³.

Functional imaging of primary visual cortex. T2*-weighted images were also acquired on the same planes of section with a gradient echo sequence employed for functional studies (TR=3000 ms, TE=60 ms, and flip angle=30 deg). The subject wore light proof, patterned-flash stimulating goggles (Grass Instruments, model S10V5 B) with a 5 x 6 red LED display that subtended approximately 60 degrees of visual angle and alternated on and off at 4.0 Hz. These flashing lights provided no pattern or object cures and have been previously employed to stimulate primary visual cortex⁴. Voxels that were activated in response to the display followed the calcarine sulcus and included the presumptive Stria of Gennari (Fig 1b). Thus primary visual cortex was "tagged" by both functional and anatomical markers.

Illusory contour and stereo depth stimuli. Illusory contours were created by four "pacmen" aligned at the corners of a Kanizsa square. The connecting illusory contours subtended 4.5 deg, the diameters of each of the pacmen were 2 deg, and the stimulus field was approximately 6x6 deg. The non-connecting-contours (control stimuli) were locally similar to the illusory contour stimuli but the pacmen were not aligned; and there was no perception of illusory contours. The perception of stereo-depth was created with red-green random dot anaglyphs with about 13 min of disparity within the central 4.5 deg field that produced a stereo-defined square in front of the background dots when viewed through red and green filters placed over the right and left eyes respectively. The neutral random dot pattern was similar to the stereo-depth pattern but without the color disparity and there was no perception of depth. Voxel-by-voxel intensities were statistically compared during baseline (30 sec), visual stimulation (30 sec), and recovery (30 sec) periods, and all statistical criteria were replicated on a second run¹. On phantom data, this procedure yielded false-positive pixels at a rate of 0.0001. We also employed a variety of similar stimulation procedures with no change in the main findings for this and other subjects.

Results:
The cortical areas associated with the perceptions of the illusory contours and depth were identified as 1) those areas where activity was observed during the illusory contour condition and not during the non-connecting contour control condition, and 2) where activity was observed during the stereo-depth condition and not during the neutral random dot condition. These areas were found outside of the primary visual cortex as illustrated for a typical subject (EV) in Fig. 2a (slice 7) for illusory contours, and Fig. 2b (slice 6) for stereo depth.

Conclusion:
These findings suggest that the neural activity in human primary visual cortex is not sufficient to mediate the perceptions of illusory or stereo-defined contours. Rather, nearby extra-striate visual areas were activated.

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References:
1. Hirsch,J., et al, PNAS, 92, 6469-6473, 1995.
2. Hirsch,J., et al, Soc.for Neuroscience (abst), 21,18, 1995.
3. Clark, et al, Cer Cortex, 2, 417-424, 1992.
4. Belliveau, J.W., et al, Science, 254, 716-718, 1991.